Patent Application
20060055993
1. Field of the Invention
The present invention relates to a hologram element able to transmit, reflect, diffract, or scatter an incident light beam, and more particularly, to a hologram element which is dependent on the polarization state of the incident beam (thus referred to as a “polarization hologram element”), and is useful for reducing the size of an optical header for recording and reproducing data in an optical disk or a magneto-optical disk, or a hologram element which is able to improve light utilization efficiency of illumination light in an image display unit (for example, display units for reflection display by means of light transmission and scattering, or color display by means of light interference, or three-dimensional image display) or a projection display device, or a hologram element which is applicable to an optical switch for switching the light path of the incident beam depending on the polarization plane of the incident beam. In addition, the present invention relates to a method of producing the hologram element, and an optical header using the hologram element.
2. Description of the Related Art
In the related art, the following methods of producing hologram elements are disclosed.
In Japanese Laid Open Patent Application No. 10-78503 (hereinafter, referred to as “reference 1”), the applicant of the present application proposed a method of fabricating a diffraction grating used in two-beam interference exposure on a photo resist, in which a photo mask having a transmission region during exposure is used to shield a portion of an interference pattern to define an exposure region.
In addition, Japanese Laid Open Patent Application No. 2005-11478 (hereinafter, referred to as “reference 2”), proposes a method of fabricating a diffraction grating used in two-beam interference exposure, in which a grating portion of an element is divided into plural divisions, the diffracted light beams from these divisional regions are received by separate photo detection regions, divergent light or converged light emitted from positions equivalent to light emission spots of a light source interferes with divergent light or converged light emitted from positions equivalent to light receiving spots corresponding to the photo detection regions, and the thus obtained interference pattern is exposed on a recording material.
In Japanese Laid Open Patent Application No. 11-52825 (hereinafter, referred to as “reference 3”), a method of fabricating a hologram element is proposed, in which a hologram image of an object is recorded on a first recording material to make a master copy; the master copy is superposed on a second material having a lower exposure sensitivity than the first recording material; then the recording area of the stacked structure is divided into plural divisions, and the hologram image is duplicated on the recording area by contact copying.
Japanese Laid Open Patent Application No. 11-202743 (hereinafter, referred to as “reference 4”) discloses a method of fabricating a hologram element, in which positions of plural divergent points of hologram object light beams and reference light beams of different diffraction characteristics are set to be approximately the same, and light from this divergent point is exposed for duplication during contact duplication.
Japanese Laid Open Patent Application No. 11-212435 (hereinafter, referred to as “reference 5”) discloses a method of producing a hologram, in which a master hologram, on which plural identical holograms are formed, is used to perform contact duplication; according to this method, plural identical holograms can be formed by whole-area exposure at one time.
Japanese Laid Open Patent Application No. 9-138632 (hereinafter, referred to as “reference 6”) discloses a duplication method of a master diffraction grating using a 4f relay optical system including plural lenses. According to this method, even for a grating pattern of narrow pitches, a diffraction grating cell is formed which is obtained by faithfully reproducing the grating pattern from the master grating on a photosensitive material; hence, it is relatively easy to fabricate a diffraction grating array with high precision.
On the other hand, the following polarization selective hologram elements (polarization hologram elements) are disclosed in the related art.
Japanese Laid Open Patent Application No. 7-287117 (hereinafter, referred to as “reference 7”) discloses a polarized beam splitter fabricated by forming a diffraction grating shape on an optical anisotropic substrate, and burying materials having specified diffractive indexes into grooves of the diffraction grating shape.
Japanese Laid Open Patent Application No. 10-92004 (hereinafter, referred to as “reference 8”) discloses an optical anisotropic diffraction element fabricated by forming a diffraction grating shape on an optical isotropic substrate, and burying optical anisotropic materials into grooves of the diffraction grating shape.
Japanese Laid Open Patent Application No. 10-74333 (hereinafter, referred to as “reference 9”) discloses an optical anisotropic diffraction element. To fabricate the optical anisotropic diffraction element, using a liquid crystal cell, which includes a photo-polymerized liquid crystal held by a transparent substrate having a periodic transparent electrode pattern, a voltage is applied on the transparent electrode pattern to align the liquid crystal periodically in the vertical direction for photo-polymerization, and at the same time, a portion of the liquid crystal, on which the voltage is not applied, is aligned in the horizontal direction for photo-polymerization, thus forming the optical anisotropic diffraction element having a periodic structure including horizontal alignment regions and vertical alignment regions.
Japanese Laid Open Patent Application No. 11-271536 (hereinafter, referred to as “reference 10”) discloses a hologram element using the above photo polymerized liquid crystal. To fabricate the hologram element, exposure with interference light is performed on the photo-polymerized liquid crystal with the liquid crystal being horizontally aligned; after the exposed portion of the liquid crystal is periodically polymerized and solidified, an external electric field is applied on the unexposed portion of the liquid crystal, and the photo-polymerized liquid crystal reacts and solidifies while being vertically aligned.
Japanese Laid Open Patent Application No. 2000-221465 (hereinafter, referred to as “reference 11”) discloses a diffractive optical element in which liquid crystals are aligned in a uniform direction relative to a fine periodic structure. To construct the fine periodical structure, an optical medium including the liquid crystals and polymer molecules is adjusted in a specific temperature range corresponding to a liquid crystal N-I (Nematic-to-Isotropic) transition temperature, and then two-beam interference exposure is performed.
In recent years and continuing, in order to reduce the size of an optical header (also referred to as an “optical pickup device” below), a laser diode and a photo detector (light receiving element) are arranged to be close to each other, and a polarization selective hologram element (that is, a polarization hologram element) is used to efficiently condense light emitted from the light diode (light source) on a disk without diffracting the light; after the light is reflected on the disk, only the retuning light, whose polarization plane is rotated by 90 degrees, is diffracted and efficiently directed to the light receiving element.
In an optical drive (also referred to as an “optical disk device”) including the optical header, in order to increase intensity of the device, development is made of a light source having a shortened wavelength. When the above hologram element is used, since the diffraction angle is dependent on the wavelength, a diffraction grating having rather short pitches is required in order to realize a compact layout to obtain necessary refractive angles. However, with the wavelength of the light source being shortened, detection sensitivity of the photo detector (light receiving element) declines; hence, when the light source having a shortened wavelength is used, a highly efficient optical system is required.
Further, improvement of efficiency of the optical system is also required in order to increase write and read speed. Concerning the polarization selective hologram element (that is, a polarization hologram element), it is required to obtain high diffraction efficiency with short pitches.
The aforesaid reference 7 and reference 8 disclose techniques of periodic structures like the grating pitches, and in reference 7 and reference 8, for example, the diffractive grating has to be fabricated by dry etching. In the structures obtained by techniques disclosed in reference 7 and reference 8, in order to obtain high diffractive efficiency, grooves thereof have to be made deep, and this processing is difficult. Further, it is necessary to bury materials into the deep grooves uniformly.
In the technique disclosed in reference 9, the pitch of the grating is determined by the pitch of the transparent electrode. However, with reduced size of the electrode, if the thickness of the liquid crystal film is increased to increase the diffraction efficiency, the liquid crystal film may be even thicker than the pitch of the transparent electrode, and due to influence from neighboring electrodes, a desired electric field cannot be imposed on the liquid crystal film. In addition, with a short pitch, the alignment state of the vertically aligned liquid crystal region may influence the adjacent horizontally aligned liquid crystal region, thus, desired alignment states cannot be obtained.
In the technique disclosed in reference 6, although it is possible to shorten the pitches of exposure, it is difficult to form short pitches as being exposed because of thermal diffusion of reactive active seeds.
In the technique disclosed in reference 7, with two-beam interference exposure, a periodic structure of short grating pitches can be fabricated easily by utilizing phase separation of a polymer and a liquid crystal. Further, when fabricating the periodic structure with two-beam interference exposure, it is easy to obtain short grating pitches compared to the aforesaid etching process.
In addition, as for the diffraction efficiency, since a hologram element, which has a binary structure, exhibits the first order (±1st order) or higher order diffraction, even if the second order (±2nd order) or higher order diffraction is suppressed, since only light due to the +1st order diffraction or the −1st order diffraction is utilized, utilization of the diffracted light is only 50% or lower.
It is disclosed in the related art, for example, in Japanese Laid Open Patent Application No. 9-50642 (hereinafter, referred to as “reference 12”), to arrange light receiving elements for the +1st order diffracted light or the −1st order diffracted light, respectively. However, this arrangement makes the structure of the device complicated, and causes a rise in the cost.
Ideally, a polarization separation element like a blazed grating is desirable which is capable of obtaining the +1st order diffracted light or the −1st order diffracted light only with high efficiency. But, as described above, it is difficult to obtain a periodic structure as desired by etching processing.
In contrast, with the two-beam interference exposure, by forming an inclined interference pattern of the two beams used for exposure, it is possible to obtain an element with high refraction efficiency of one of the +1st order diffracted light and the −1st order diffracted light.
The two-beam interference exposure method is useful in fabrication of a polarization hologram element which has high refraction efficiency with short pitches. As described above, a method of fabricating the polarization hologram element is disclosed in reference 3. As disclosed in reference 3, a master hologram allowing an interference pattern to be exposed thereon is fabricated, and with this master hologram, the polarization hologram element duplicates a hologram image. In addition, as disclosed in reference 1, there are other methods of fabricating the polarization hologram element, in which a photo mask having a transmission region is used to shield a portion of an interference pattern during the two-beam interference exposure to define the exposure region.
However, because of the recording materials used in the above techniques of the related art, development processing of the photo resist is required; hence, a large number of fabrication steps are needed, and the productivity is low.
In addition, concerning the close-contact duplication exposure with a master substrate and a recording material being in close contact, as disclosed in reference 11, a composite film including liquid crystals and polymer molecules can be used as the recording material. In this method, the development processing is not necessary, and the productivity is relatively high. However, when using the polymerization reaction in liquid crystals (including the phase separation process), there exist problems in the spread of the reaction and in leakage of light due to multiple reflection.
It is known that holograms are widely used in publishing, even ceremonies, and in industrial and medical fields. For example, holograms can be used to fabricate a mark for preventing counterfeiting of credit cards or bills for purposes of security, for package design for purposes of decoration, in a spectroscopic diffraction grating in an optical header (optical pickup device) or a projector, as a display element in a head-up display or a three-dimensional display and so on, and as an optical element in optical communications, scanners, and optical ICs (integrated circuits). Particularly, security-related techniques are attracting attention, and in these techniques, holograms which are more delicate and have higher resolution are required.
In optical recording techniques, it is known that usage of short-wavelength light enables high density recording, and for this purpose, diffraction gratings having narrow pitches and high diffraction efficiency are required. As described, when using such diffraction gratings in an optical header, the polarization characteristics thereof are important.
In the related art, hologram elements can be fabricated by various methods such as a general exposure with a mask by using a stepper, electron beam direct writing, laser beam direct writing, and two-beam interference exposure. However, resolutions of mask exposure and laser beam direct writing are not high. With the method of electron beam direct writing, although relatively high resolutions are obtainable, fabrication equipment is quite expensive, and thus the fabrication cost is high.
With the above methods, it is difficult to fabricate a single hologram having complicated characteristics by only one exposure, while by multiple exposures, it is possible to record (fabricate) plural holograms having different diffraction characteristics in a single hologram element. However, when fabricating a single hologram having desired characteristics, for example, diffraction wavelength, diffraction angle, and focal length, it is necessary to precisely position and fix a photosensitive material for recording the hologram relative to a laser in order to perform exposure. In order to fabricate plural holograms the same as the above hologram, one has to repeat the positioning and fixing steps as many times as the number of the holograms to be fabricated; thus, a large number of fabrication steps and a large amount of fabrication time are needed. Further, when laminating hologram elements having different characteristics, it is necessary to position the hologram elements precisely relative to each other, and fix the hologram elements with an adhesive layer. Because multiple hologram photosensitive materials are used, the cost is high, and hence not suitable for mass production.
To solve this problem, a method is proposed (for example, in reference 5), in which a master hologram having multiple characteristics is fabricated, and a hologram photo-sensitive material is set in close contact with the master hologram. Then, light is irradiated from the side of the hologram photo-sensitive material or from the side of the master hologram to duplicate the hologram. According to this method, it is not necessary to repeat the bothersome positioning step, and plural duplications of the same hologram can be fabricated successively.
A method is proposed (for example, in reference 6), for duplicating a master hologram image on the photosensitive material by exposure with interference light. For example, from the side of the exposure light source, a master hologram, a first lens, a second lens, and the photo-sensitive material are arranged in order, the distance from the master hologram to the first lens is set to be the focal length f of the first lens, the distance from the first lens to the second lens is set to be the sum (2f) of the focal lengths of the first lens and the second lens, and the distance from the second lens to the photo-sensitive material is set to be the focal length of the second lens. That is, the master hologram, the first lens, the second lens, and the photosensitive material constitute a 4f relay optical system. According to this method, the master hologram can be duplicated with very high precision. In addition, with a light shielding mask being arranged in the relay optical system, it is possible to reduce noise light generated from the master hologram.
As for methods of producing a master hologram, in order that the hologram exhibit desired characteristics, computers are employed to make calculations, and patterns are written on a photo mask blank plate according to the calculation results by using an electron beam writing device to produce a first master hologram; independent from this process, resin for forming a volume hologram is applied on a glass substrate to prepare a volume hologram substrate; the first master hologram produced in advance is superposed on the resin layer applied on the volume hologram substrate so that the mask surface of the master hologram is in contact with the resin layer on the volume hologram substrate; then a laser beam is irradiated to expose the structure from the side of the master hologram. After the exposure, ultraviolet light irradiation processing (decomposition of the photo polymerization initiator) and heating treatment (diffusion movement of photo polymerizable compounds) are performed; as a result, the master hologram is duplicated on the resin layer applied on the volume hologram substrate.
Here, the hologram recording material used in fabrication and duplication of the above hologram (including the diffraction grating) can be photo sensitive materials like dichromate gelatin, photopolymer, photo-resist, photo-polymerized liquid crystal polymer, and polymer dispersed liquid crystal (a composite film of a non-polymerized liquid crystal and a polymerized polymer). When fabricating holograms by means of exposure with interference light using such kinds of materials in which polymerization reactions occur, depending on the size of the element to be fabricated, the shape of the periodical structure, the temperature during exposure, quantity of exposure, and other conditions, polymerization diffusion may occur; due to this, characteristics in the element being exposed may influence other regions or neighboring elements. Further, similarly, in the course of forming the desired structure in exposure with the interference light, scattering, multiple reflections on the substrate interface along with a change of the refractive index of materials, noise light included in the master hologram, and other unnecessary light may influence the element being exposed or neighboring elements.
It is a general object of the present invention to solve one or more of the problems of the related art.
A more specific object of the present invention is to provide a hologram element, which is fabricated by exposure with interference light on a photo-sensitive recording material, able to prevent reaction spread when forming a periodic structure by a polymerization reaction in the photo-sensitive recording material, so as to prevent light leakage or occurrence of other unnecessary interference light due to multiple reflection during exposure with interference light, and thus be able to improve productivity in mass production; to provide a method of producing the hologram element by exposure with interference light on the photo-sensitive recording material, which is able to prevent influence of the spread of the polymerization reaction in the photo-sensitive recording material on the hologram element and other neighboring hologram elements, and to prevent light leakage or occurrence of other unnecessary interference light due to multiple reflection during exposure with interference light, and thus be able to improve productivity in mass production; and to provide an optical header using such a polarization hologram element.
According to a first aspect of the present invention, there is provided a hologram element able to transmit, reflect, diffract, or scatter incident light, comprising a pair of substrates; an isolation member that is provided between the substrates and forms an isolated region; and a recording material sealed in the isolated region, said recording material being a photo-sensitive material, wherein said hologram element includes a periodic structure formed by exposing the recording material to interference light.
As an embodiment, the interference light is generated by two or more light beams. Alternatively, the interference light is generated by using a master hologram.
As an embodiment, the recording material is formed from a composite material including a polymerized polymer. Alternatively, the recording material is formed from a composite material including a polymerized liquid crystal. Alternatively, the recording material is formed from a mixed composite material including a non-polymerized liquid crystal and a polymerized polymer. Alternatively, the recording material is formed from a mixed composite material including a polymerized polymer and at least one of a non-polymerized liquid crystal and a polymerized liquid crystal.
As an embodiment, the periodic structure is formed by exposing the recording material to the interference light to induce the polymerization reaction and phase separation of the composite material.
As an embodiment, a refractive index modulation of the periodic structure varies along with a polarization direction of the incident light.
As an embodiment, the hologram element further comprises plural device portions each having the periodic structure, wherein the composite material forming the recording material is held between the substrate, each of the device portions is isolated by the isolation member, and the isolation member is arranged in such a way that the device portions are arranged in a matrix manner.
As an embodiment, the isolation member also acts as a spacer for controlling a film thickness of the recording material.
As an embodiment, the isolation member is formed from a material capable of absorbing light of a wavelength of the light for exposure.
As an embodiment, the recording material is sealed in the isolated region by One Drop Fill (ODF) process.
As an embodiment, the isolation member is formed from a conductive material.
According to a second aspect of the present invention, there is provided a method of producing a hologram element with interference light from a photo-sensitive recording material, comprising the steps of forming a film of the photo-sensitive recording material; forming an isolated region on the recording material by using an isolation member; and exposing the isolated region on the recording material to interference light.
As an embodiment, the isolated region isolates one hologram element in plural areas.
As an embodiment, the isolated region corresponds to at least the area of one hologram element. Further, the method includes the step of cutting out at least one hologram element at a position of the isolated member corresponding to the area of the at least one hologram element.
As an embodiment, the recording material is held between a pair of substrates, each device portion is isolated by an isolation member, and the isolated region is formed in such a way that the device portions are arranged in a matrix manner. The method further includes the step of cutting the device portions at a position of the isolated member to divide the device portions into separate hologram elements.
As an embodiment, the interference light is generated by two or more light beams.
As an embodiment, the interference light is generated by using a master hologram. Further, a separation layer is formed on the master hologram; the interference light is irradiated by using a relay optical system.
As an embodiment, the photosensitive recording material film is formed between a pair of substrates, one of said substrates being thinner than the other one of said substrates.
As an embodiment, the recording material is formed from a composite material including a polymerized polymer.
As an embodiment, the recording material is formed from a composite material including a polymerized liquid crystal.
As an embodiment, the recording material is formed from a mixed composite material including a non-polymerized liquid crystal and a polymerized polymer.
As an embodiment, the recording material is formed from a mixed composite material including a polymerized polymer and at least one of a non-polymerized liquid crystal and a polymerized liquid crystal.
As an embodiment, the recording material is exposed to the interference light to induce a polymerization reaction and phase separation of the composite material to form a periodic structure. In addition, a refractive index modulation of the periodic structure varies along with a polarization direction of incident light.
As an embodiment, the isolation member also acts as a spacer for controlling a film thickness of the recording material.
As an embodiment, the isolation member is formed from a material capable of absorbing light of a wavelength of the light for exposure.
As an embodiment, the film of the recording material is formed in the isolated region by One Drop Fill (ODF) process.
As an embodiment, the isolation member is formed from a conductive material.
According to a third aspect of the present invention, there is provided a hologram element able to transmit, reflect, diffract, or scatter incident light, produced by a method comprising the steps of: forming a film of the photo-sensitive recording material; forming an isolated region on the recording material by using an isolation member; and exposing the isolated region on the recording material to interference light.
According to a fourth aspect of the present invention, there is provided an optical header that condenses light from a light source on a recording medium, detects reflected light from the recording medium with a photo detector, and records or reproduces information in the recording medium. The optical header has an optical element arranged on a light path from the recording medium to the photo detector for deflecting the reflected light from the recording medium to the photo detector. The optical element has a hologram element that is able to transmit, reflect, diffract, or scatter incident light. The hologram element includes a pair of substrates; an isolation member that is provided between the substrates and forms an isolated region; and a recording material sealed in the isolated region, the recording material being a photosensitive material, wherein the hologram element includes a periodic structure formed by exposing the recording material to interference light.
According to the present invention, the hologram element of the present invention includes a pair of substrates, an isolation member that is provided between the substrates and forms an isolated region, and a photosensitive recording material sealed in the isolated region, and the hologram element includes a periodic structure formed by exposing the recording material to interference light. In addition, in the hologram element of the present invention, the recording material (photosensitive recording material) is isolated by an isolation member prior to exposure with the interference light. The isolation member is able to prevent unnecessary scattered light, stray light, and spread of a polymerization reaction in the photosensitive recording material during exposure with interference light.
As a result, exposure of one isolated device region can be performed without being affected by unnecessary light, and without being affected by the polymerization reaction in neighboring regions. This can improve productivity of the hologram element.
According to the present invention, when fabricating the hologram element of the present invention, prior to exposure with the interference light, the recording material is separated into plural regions with the isolation member in each hologram element. The isolation member is able to prevent unnecessary scattered light, stray light, and spread of a polymerization reaction in the photosensitive recording material during exposure with interference light. As a result, exposure of one isolated device region can be performed without being affected by unnecessary light, and without being affected by the polymerization reaction in neighboring regions. This can improve productivity of the hologram element when the hologram element has multiple regions of different characteristics.
According to the present invention, when fabricating the hologram element of the present invention, prior to exposure with the interference light, the recording material is isolated with the isolation member in the area of each hologram element. The isolation member is able to prevent unnecessary scattered light, stray light, and spread of a polymerization reaction in the photosensitive recording material during exposure with interference light. As a result, exposure of one isolated device region can be performed without being affected by unnecessary light, and without being affected by the polymerization reaction in neighboring regions. This can improve productivity of the hologram element.
According to the present invention, prior to exposure with the interference light, the recording material is isolated with the isolation member in the area of each hologram element, and one isolated region is in correspondence to one hologram element. Because in the isolated region, there is no hologram region, with the isolated region as a cutting position, the hologram element can be cut out without degrading the characteristics of the hologram. This can improves productivity of the hologram element.
Specifically, in the hologram element, there is one or more device portions each having a periodic structure, the composite material forming the recording material is held between a pair of substrates, each of the device portions is isolated by the isolation member, the isolation member is arranged in such a way that the device portions are arranged in a matrix manner, and the device portions are cut at a position of the isolated member to divide the device portions into separate hologram elements. In this way, the hologram elements are mass-produced with one hologram element being fabricated in one isolated region.
In addition, the periodic structure of the hologram element is duplicated by exposing the recording material to interference light to form an interference pattern on the hologram element, and the periodic structure is formed by the polymerization reaction and phase separation of the recording material. Because each device region is isolated by the isolation member, the isolation member can prevent influence from the polymerization reaction on the neighboring elements. Therefore, it is possible to narrow the interval between the neighboring elements and improve productivity and light utilization.
In addition, the refractive index modulation of the periodic structure varies along with the polarization direction of the incident light, and hence, the periodic structure is able to transmit, reflect, diffract, or scatter the incident light according to the polarization direction of the incident light.
In the present invention, the interference light used for exposure is generated by a number of beams. For example, the hologram element is fabricated by two-beam interference exposure. Or, the interference light is generated by using a master hologram.
When exposure with interference light is performed without using the master hologram, it is necessary to use a half mirror or a beam splitter to split a light beam into two or more beams, and to adjust the interference pattern of these beams. In this case, although depending on optical elements to be used, the exposure optical system requires a relatively large space because of limitations of specifications and arrangement, and the device becomes large.
In contrast, when exposure with interference light is performed by using the master hologram, because the interference light is generated by using the master hologram, it is not necessary to split the light beam and to adjust the interference pattern, the exposure optical system does not require a large space, noise occurring during exposure in adjustment of the interference pattern can be reduced, and this can improve productivity of the hologram element.
In the present invention, because exposure with interference light is performed by using a master hologram, the exposure optical system can be simplified. By narrowing the interval between the master hologram and the recording material (photosensitive material), it is possible to increase the reproduction accuracy of the interference pattern. Specifically, a separation layer may be formed on the master hologram to be in close contact with the photosensitive material, the reproduction accuracy of the interference pattern can be increased, and this can improve productivity of the hologram element.
When exposure with interference light is performed by using the master hologram, because the interference light is generated by using the master hologram, the hologram element fabricated with the interference light is largely affected by the master hologram. Due to this, the reproduction accuracy of the master hologram is important, and when the master hologram itself generates noise light, it is required to further reduce the noise light.
In the present invention, because the interference light is processed by a relay optical system, and the noise light generated by the master hologram is reduced, only the interference pattern of necessity is duplicated by exposure with interference light, and the exposure can be performed with high reproduction accuracy of the master hologram. This can improve productivity of the hologram element.
In the present invention, because exposure with interference light is performed by using the master hologram, the exposure optical system can be simplified. But, when the master hologram is used in exposure with interference light, the region of the generated interference pattern becomes small when it is moved far from the master hologram, and the reproduction accuracy lowers. For this reason, if the interval between the master hologram and the recording material (photosensitive material) is small, the exposure region of the interference pattern becomes broad, it is possible to increase the reproduction accuracy. Especially, this is more effective when the recording material is divided into plural regions inside the hologram element. Therefore, with a thin substrate, which determines the interval between the master hologram and the photosensitive material, the productivity of the hologram element can be improved.
In the hologram element of the present invention, the recording material (photo-sensitive material) of the hologram is formed from (1) a composite material including a polymerized polymer, (2) a composite material including a polymerized liquid crystal, (3) a mixed composite material including a non-polymerized liquid crystal and a polymerized polymer, or (4) a mixed composite material including a polymerized polymer and at least one of a non-polymerized liquid crystal and a polymerized liquid crystal. Specifically, use can be made of (a) a polymerized polymer, (b) a polymerized liquid crystal, (c) a mixture of a non-polymerized liquid crystal and a polymerized polymer, (d) a mixture of a polymerized polymer and a polymerized liquid crystal, or (e) a mixture of a polymerized polymer, a non-polymerized liquid crystal, and a polymerized liquid crystal.
For example, the composite material constituting the recording material may be a photo polymerized polymer, such as a photopolymer, a photo resist, and a polymer liquid crystal; thereby, it is possible to select the resolution, exposure sensitivity, and photo-sensitive wavelength band in a wide region, and it is possible to fabricate a hologram element superior in environment tolerability, and of many degrees of freedoms in selection of the film thickness and size. In addition, because the granularity is small, high diffraction efficiency and high transparency, which are features of a hologram, are obtainable. In addition, when using a polymer dispersed liquid crystal, which is a mixture of a non-polymerized liquid crystal and a polymerized polymer, as the composite material constituting the recording material, it is possible to fabricate a polarization hologram element, which has polarization dependence caused by phase separation during exposure with the interference light.
In the present invention, as described above, the photo-sensitive recording material of the hologram element may be formed from one of a polymerized liquid crystal, a non-polymerized liquid crystal, and a polymerized polymer (a polymerized monomer, or prepolymer), or a mixture of some of these materials, and by a polymerization reaction and phase separation of the composite material during exposure with the interference light, it is possible to easily form a periodic structure of the hologram, and obtain a polarization hologram element in which the refractive index modulation varies along with a polarization direction of the incident light.
In the present invention, because the isolation member also acts as a spacer, it is possible to increase the accuracy of the film thickness of the hologram element (that is, the gap between substrates). This can improve productivity of the hologram element.
In the present invention, because the isolation member is formed from a material capable of absorbing light of a wavelength of the light for exposure, it is possible to prevent unnecessary scattered light occurring during exposure with the interference light, unnecessary light due to multiple reflections between the substrates, which may degrade the hologram element. As a result, exposure of one isolated device region can be performed without being affected by unnecessary light, and the isolated region can be made small. This can improve productivity of the hologram element.
In the present invention, when fabricating the hologram element, specifically, when forming a film of the photo-sensitive recording material, the One Drop Fill (ODF) process is used, which allows a tiny quantity of the photo-sensitive recording material to be applied, for example, by inkjet. Therefore, the material can be applied to the isolated region in an appropriate amount, thereby forming a uniform film. This can reduce the number of processing steps compared to vacuum injection, and can improve productivity of the hologram element.
In the present invention, the isolation member may be formed from a conductive material, thus it is possible to apply an electric field between isolated regions. For example, when a dielectric-anisotropic liquid crystal is used as the photosensitive recording material, the liquid crystal alignment between the isolated regions can be controlled by the applied electric field; hence, it is possible to enlarge the birefringence of the liquid crystal in the hologram element. This large birefringence can improve the incidence angle dependence and wavelength dependence, and improve polarization dependence.
With the above hologram element of the present invention, it is possible to make an optical device compact, and improve overall performance of the device. For example, it is possible to make an optical header compact, reduce noise of an optical switch, and increase the brightness of a display.
In an optical header of the present invention, because the hologram element of the present invention is used as an optical element for deflecting the reflected light from the recording medium to the photo detector, by making the hologram element polarization dependent, the hologram element is able to transmit, reflect, diffract, or scatter the incident light according to the polarization direction of the incident light; therefore, on the light path of the light emitted from the light source, the light is more effectively condensed on the recording medium while diffraction essentially does not occur. On the returning light path (the polarization plane is rotated by 90°), light having information can be diffracted with a high diffraction efficiency. Therefore, it is possible to obtain a compact optical header having high light utilization efficiency.
These and other objects, features, and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments given with reference to the accompanying drawings.
Below, preferred embodiments of the present invention are explained with reference to the accompanying drawings.
Below, descriptions are made of a configuration and operations of a hologram element of the present embodiment, and a method of producing the hologram element.
In the present embodiment, a polarization hologram element is used as an example, which has a periodic structure, the refractive index modulation of which changes with the polarization direction of incident light, and a method of producing such a polarization hologram element is described. More particularly, it is assumed that the polarization hologram element is fabricated by duplicating the hologram region of the periodic structure on a recording material by using a master hologram.
As illustrated in
Light having a specific wavelength for exposure is irradiated on a hologram region 11 of the master hologram 10 via a collimator 30; light transmitting through the master hologram 10 and light diffracted from the master hologram 10 interfere with each other and generate an interference pattern. This interference pattern can be duplicated on the recording material 21, thereby producing the hologram element 20.
In
As illustrated in
The structures shown in
It should be noted that the present embodiment is not limited to this configuration. A master hologram can be used which generates divergent or parallel transmission light and diffraction light, generally, master holograms suitable for intended applications can be used.
The hologram region 11 of the master hologram 10 can be fabricated by general hologram production methods, such as two-beam interference exposure, electron beam lithography or photolithography based on an interference pattern calculated by a computer.
It is preferable that the ratio of strength of the transmission light and the diffraction light be roughly 1:1 at the exposure wavelength.
For example, the photo sensitive recording material 21 of the hologram element 20 can be formed from a polymerized liquid crystal, a non-polymerized liquid crystal, a polymerized polymer (a polymerized monomer or a prepolymer), or a mixed composite material including some of these materials. When necessary, a photo polymerization initiator can be added.
For example, the polymerized liquid crystal may be a liquid crystalline monofunctional acrylate monomer, a liquid crystalline meta-acrylate monomer, a liquid crystalline difunctional diacrylate monomer, or a liquid crystalline dimeta-acrylate monomer. These materials may include a methylene chain between the functional group of acryloyloxy and the liquid crystalline skeleton.
In addition, the non-polymerized liquid crystal may be any liquid crystal exhibiting diffractive anisotropy, having a phase structure of any one of the Nematic phase, cholesteric phase, and smectic phase. For example, well known liquid crystals can be used, for example, which have skeletons formed from one of biphenyl, tert-phenyl, phenyl-cyclohexane, biphenyl-cyclohexane, benzoic acid phenyl ester, cyclohexane carboxylic acid phenyl ester, phenyl-pyrimidine, phenyl-dioxane, tolan, 1-phenyl-2-cyclohexylethane, 1-phenyl-2-biphenylethane, 1-cyclohexylethane-2-biphenylethane, biphenyl carboxylic acid phenyl ester, or 4-cyclohexyl-benzoic acid phenyl ester, and have an alkyl group, an alkoxy group, or a cyano group acting as a polarity assigning group for assigning dielectric anisotropy, and have a halogen group as a substituent group.
Preferably, the polymerized monomer or the prepolymer thereof is formed from materials having large polymerization curing shrinkage. For example, a photo-polymerizable compound having an ethylene unsaturated bond can be used as the polymerized monomer, such as a monomer, an oligomer, a prepolymer, and mixtures of them, each of which includes at least one ethylene unsaturated double bond in one molecule, and can be photo-polymerized and photo bridged.
In addition, the monomer and copolymer may also be an unsaturated carboxylic acid and unsaturated carboxylates, or an ester of the unsaturated carboxylic acid and an aliphatic polyalcohol compound, an amide of the unsaturated carboxylic acid and an aliphatic polyamine compound. Particularly, a polyfunctional monomer is preferable because it has large polymerization curing shrinkage.
The unsaturated carboxylic acid polymer may be an acrylic acid, a meta-acrylic acid, an itaconic acid, a crotonic acid, an isocrotonic acid, a maleic acid, and halogen-substituted unsaturated carboxylic acid thereof, such as chlorinated unsaturated carboxylic acid, brominated unsaturated carboxylic acid, or fluorinated unsaturated carboxylic acid.
The unsaturated carboxylate may be sodium salts and potassium salts of the above acids, and may be urethane acrylates, polyester acrylates, polyfunctional acrylates or methacrylates of epoxy resins and an acrylic acid.
It should be noted that a thermal polymerization inhibitor or a plasticizer can be added to the above materials.
The photo polymerization initiator may be any well known ones, for example, biacetyl, acetophenone, benzophenone, Michler's ketone, benzyl, benzoinalkylether, benzyldimethylketol, 1-hydroxy-2-methyl-1-phenylpropane-1-one, 2-chlorothioxanthone, methylbenzoylformate, 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropane-1-one, diethoxyacetophenone, 1-hydroxy-cyclohexyl-phenylketone, 2,2-dimethoxy-1,2-diphenyl-ethan-1-one, α-aminoalkylphenone, bis-acylphoshinoxide, and metallocene.
The quantity of the photo polymerization initiator to be added depends on the absorption capacity of the material at the wavelength of the irradiation light, and can be appropriately adjusted according to exposure conditions in duplication.
When duplicating the master hologram 10, by appropriately adjusting the exposure conditions, the interference pattern corresponding to the master hologram 10 can be formed on the recording material 21 by exposure, which is formed from composite materials as described above, and thereby forming a periodic structure on the recording material 21 having a refractive index modulation changing with the polarization direction of incident light, that is, modulation of the refractive index in the periodic structure depends on the polarization direction of the incident light.
For example, the composite materials forming the recording material 21 may be HPDLC (Holographic Polymer Dispersed Liquid Crystal), which is obtained by dispersing a non-polymerized liquid crystal and a photo polymerization initiator in a polymerized polymer, or may be PPLC (Photo-Polymerized Liquid Crystal), which is obtained by mixing a polymerized liquid crystal and a photo polymerization initiator.
Below, a description is made of formation of the interference pattern on the recording material 21 by exposure.
For example, in the interference pattern formed on HPDLC, the monomer, which is a component of the composite material forming the recording material 21, is moved to the bright portion of the interference pattern (that is, phase separation between the polymer and the liquid crystal occurs), and is polymerized and cured.
The liquid crystal, which is a component of the composite material forming the recording material 21, remains at the dark portion of the interference pattern, and is drawn by the polymer cured in the bright portion; thereby, the liquid crystal is aligned in a specific direction. Because of alignment of the liquid crystal, if the incident light is linearly-polarized with the polarization directions thereof being perpendicular to each other, for one polarization direction of the incident light, the refractive index of the liquid crystal is small, and the incident light is not affected by the variation of the refractive index of the liquid crystal; hence, substantially all of the incident light transmits through the recording material 21. On the other hand, for the other polarization direction of the incident light, the refractive index of the liquid crystal is large, and the incident light is affected by the variation of the refractive index of the liquid crystal; hence, nearly all of the incident light is diffracted. As a result, the HPDLC functions as a polarization hologram device.
When the recording material 21 is formed from PPLC, the liquid crystal having a functional group for photo-polymerization is sealed into the space between transparent electrodes (for example, formed from ITO) and a substrate carrying an alignment layer for aligning the liquid crystal, and the liquid crystal is aligned there. When the recording material 21 is exposed to interference light, an interference pattern is formed on the recording material 21, and in the bright portion of the interference pattern on the recording material 21, the liquid crystal molecules are polymerized and cured. Meanwhile, in the dark portion of the interference pattern on the recording material 21, the state of the liquid crystal molecules remain un-cured.
A light beam is irradiated on the liquid crystal layer with a voltage being applied on the transparent electrodes sandwiching the liquid crystal layer. Due to the applied voltage, the liquid crystal in the dark portion of the interference pattern is aligned in a direction perpendicular to the substrate, and is cured due to light irradiation.
As a result, in correspondence to the distribution of the bright and dark portions of the interference pattern, a periodic structure of the liquid crystal is formed, in which the liquid crystal is aligned in the vertical direction and in the horizontal direction alternately.
When linearly-polarized incident light with the polarization directions thereof being perpendicular to each other is incident on a diffraction grating (a hologram) recorded in this way, if the incident light has a polarization direction the same as a short axis direction of the liquid crystal molecules aligned in the horizontal direction, regardless of the periodic alignment of the liquid crystal in the vertical direction and in the horizontal direction, the incident light is not affected by the variation of the refractive index of the liquid crystal, and nearly all of the incident light transmits through the recording material 21. On the other hand, if the incident light has a polarization direction perpendicular to the aforesaid polarization direction, that is, the same as a long axis direction of the liquid crystal molecules aligned in the horizontal direction, the incident light is affected by the variation of the refractive index of the liquid crystal because of the periodic alignment of the liquid crystal in the vertical direction and in the horizontal direction, so that nearly all of the incident light is diffracted.
As described above, because of the phase separation occurring together with the polymerization reaction in the composite material constituting the recording material 21, or because of the polymerization reaction and the change of the alignment due to the external electric field, it is possible to produce a polarization hologram having polarization selectivity.
As illustrated in
As illustrated in
As illustrated in
In the method shown in
In this sense, methods involving multiple times of exposure, as shown in
In the case of multiple times of exposure, however, the recording material 21 is influenced by spread of the polymerization reaction.
As shown in
As illustrated in
For example, each isolation member 24 is a light curable adhesive agent or a heat curable adhesive agent, which are commonly used in liquid crystal displays. For example, the isolation members 24 can be formed by screen-printing or the like, but they can also be formed by other methods.
As illustrated in
For example, the isolation members 24 may be spherical spacers or fiber spacers used in liquid crystal displays.
As illustrated in
Because of the isolation members 24, it is possible to prevent influence of the spread of the polymerization reaction on the neighboring hologram elements.
Meanwhile, during exposure with interference light, light leakage occurs due to multiple reflections between the substrates 22, 23, which hold the recording material 21.
As illustrated in
The isolation members of the present embodiment can be formed from conductive materials, such as chromium, aluminum, or others.
In
As described above, because the liquid crystalline alignment of the composite material forming the recording material 21 changes depending on the direction of the applied electric field, with the voltage applied to the conductive isolation members 24D, the liquid crystalline alignment of the recording material 21 can be controlled, and it is possible to enlarge the birefringence of the liquid crystal in the recording material 21, thereby improving polarization selectivity. The strength of the electric field can be appropriately set to obtain desired birefringence.
When the isolation members 24, 24B, 24C, 24D as described above are provided, however, it becomes difficult to hold the recording material 21 between the substrates 22, 23 by vacuum injection, which is a common technique for sealing a liquid crystal.
To solve this problem, in the present invention, when sealing the recording material 21, a One Drop Fill (ODF) technique is used, which allows a tiny quantity of the liquid crystal to be applied to form the recording material 21, for example, by inkjet.
As illustrated in
As illustrated in
To solve this problem, as illustrated in
Below, descriptions are made of examples of the hologram element of the present embodiment and methods of producing the hologram element, and examples for comparison.
When producing the master hologram 10 shown in
As illustrated in
Below, a single hologram element 20 is also referred to as a “cell”.
The recording material 21 is formed from a composite material including a mixture of the following materials (1) through (5).
(1) Nematic liquid crystal (manufactured by Merck & Co., product name: TL216, Δε>0), 30 parts by weight (or 30 w/t parts).
(2) phenyl glycidyl ether acrylate hexamethylene diisocyanate urethane prepolymer (manufactured by Kyouei Chemistry Co., product name: AH600), 75 parts by weight (or 75 w/t parts).
(3) dimethylol-tricyclodecane-diacrylate (manufactured by Kyouei Chemistry Co., product name: DCP-A), 10 parts by weight (or 10 w/t parts).
(4) hydroxyethyl methacrylate (manufactured by Kyouei Chemistry Co., product name: H0), 5 parts by weight (or 5 w/t parts).
(5) a photo polymerization initiator based on bis-acylphoshinoxide (manufactured by Ciba-Geigy Co., product name: Irgacure 819), 1 parts by weight (or 1 w/t parts).
Specifically, the above composite material is injected into the cell by a capillary method while being heated and maintained at 60° C., forming a composite material film about 5 μm in thickness. Because the above composite material is reactive with respect to light having a wavelength shorter than green light, it is handled with red light in a dark room.
After the composite material is injected into the cell, the composite material is isotropic at room temperature.
Next, using a He—Cd laser having output power of 80 mW and emitting a laser beam of a wavelength of 442 nm, the laser beam is emitted to the master hologram with the diameter of the laser beam being enlarged. An ND filter is adjusted so that the intensity of the laser beam transmitting through the master hologram is about 11.1 mW/cm2.
As illustrated in
As shown in detail in
In the course of exposure, the cell substrate is attached to a heater to heat and maintain the cell 20 at 60° C., each exposure is executed for 5 minutes, and the exposure is executed twice; hence, a polarization hologram element having two hologram regions is produced.
The thus produced polarization hologram element 20 was evaluated as below.
A linear-polarized laser beam of a wavelength of 442 nm and including a P polarization component and an S polarization component was emitted to one duplicated polarization hologram element in a direction perpendicular to the substrate surface of the polarization hologram element. When the laser beam was incident to the hologram region formed in the first exposure, diffracted light depending on the polarization state of the incident light was observed, but in the hologram region formed in the second exposure, diffracted light was not observed, no matter whether the incident light is a P wave or an S wave.
The method shown in the present example is basically the same as that in the first example for comparison, except that an isolation member is provided.
As illustrated in
Except that two isolated regions are formed with the isolation member 24, fabrication of the cell and the exposure for duplication in the present example are the same as those in the first example for comparison.
The obtained polarization hologram element was evaluated in the same way as in the first example for comparison. In the test, diffracted light having selectivity of the polarization state of the incident light was observed in the hologram regions formed in both the first and the second exposure. That is to say, with the isolation member 24 to demarcate the recording material to form plural hologram regions (device regions), it is possible to duplicate plural polarization hologram elements with one cell.
In the hologram element produced in the example 1, the cell gaps in the hologram regions formed in the first and the second exposure were both 3.2 μm, which were different from a target value of 5 μm. In the example 1, the cell gaps were obtained by fitting measurement results of incident angle dependence of diffraction efficiency with theoretical values calculated by coupled wave theory.
In the present example, bead-like spacers having a diameter of 5 μm are mixed in the adhesive agent used to fix the isolation members 24, and then, fabrication of the cell and the exposure for duplication are performed in the same way as in the example 1, and the polarization hologram element 20 was produced and evaluated.
Similar to the example 1, diffracted light having selectivity of the polarization state of the incident light was observed in the hologram regions formed in both the first and the second exposure. The cell gap was 4.5 μm, which was close to the target value of 5 μm. That is, the accuracy of the cell gap was improved.
As illustrated in
The obtained hologram element was evaluated in the same way as in the previous examples. In the test, diffracted light was observed in the hologram regions formed in the second and the third exposure, but not observed in the hologram region formed in the first exposure.
The cell in the present example is the same as that shown in
The cell 20 is fabricated in the same way as in the second example for comparison, and exposure and duplication are also performed in the same way as in the second example for comparison. The thus produced polarization hologram element 20 was evaluated, and diffracted light having selectivity of the polarization state of the incident light was observed in all of the hologram regions formed in the first, the second and the third exposure. It reveals that by including materials able to absorb the light for exposure in the isolation members 24, even when the intervals between neighboring hologram elements in the cell 20 are small, the polarization hologram elements can be duplicated correctly.
In the present example, similar to the example 1, the isolation member 24 is provided on one of the substrates holding the recording material 21 of the cell 21, forming two isolated regions. Then, a dispenser robot capable of variable delivery (manufactured by SONY Co.) is used to apply the recording material 21 in each of the isolated regions demarcated by the isolation member 24, as shown in
In addition, a separation layer 26 made from a fluorine-based material is formed on a surface of the master hologram 10 having the hologram regions by a tape. On the separation layer 26, a UV curable region (manufactured by Three Bond Co.) for molding lenses thereon is applied by spin-coating to a thickness of about 0.5 μm. As shown in
After the exposure and duplication process, the master hologram 10 and the cell 20 (hologram element) are separated, and the polarization hologram element 20 was evaluated. Similar to the example 1, diffracted light having selectivity of the polarization state of the incident light was observed.
A micrometer was used to move the duplicated hologram element 20 to measure the size of one hologram area. It was found that a large hologram region was duplicated by exposure compared to the example 1.
In the present example, instead of the adhesive agent in which bead-like spacers are mixed, a conductive isolation member 24 made from an aluminum film having a thickness of 5 μm is used, and the conductive isolation member 24 also serves as a spacer. The present example is the same as the example 1 except for the conductive isolation member 24. The cell holding the recording material is fabricated in the same way as in the example 1. The exposure and duplication are performed in the similar way as shown in
The thus produced polarization hologram element 20 was evaluated, and it was found that the polarization selectivity was increased by 10% compared to example 1.
In the above, descriptions are made of examples of the hologram element of the present embodiment and methods of producing the hologram element, and examples for comparison. According to the present embodiment, it is possible to produce a polarization hologram element having good polarization selectivity, high light utilization efficiency, and good productivity in mass production.
It is quite effective to use the polarization hologram element of the present embodiment as a polarization splitting element of an optical header (optical pickup device), the polarization splitting element requiring a large diffraction angle because of its small size.
Application of Polarization Hologram Element
The optical pickup device shown in
For example, the semiconductor laser 101 is a laser diode (LD). The polarization hologram element 102 is duplicated by the method of the present embodiment, for example, those shown in one of examples 1 through 5, and as shown in
In
In the optical header illustrated in
The polarization plane of the returning light from the recording layer of the optical disk 106 is rotated by 90° by the ¼ wave plate 103, and then the returning light is incident on the polarization hologram element 102. Therefore, if the refractive index of the optical anisotropic region is different from the refractive index of the optical isotropic region of the polarization hologram element 102, and the film thickness of the polarization hologram element 102 is set beforehand so as to result in maximum diffraction efficiency relative to the refractive index difference, it is possible to improve the diffraction efficiency. In this case, if the separation angle of the polarization hologram element 102 is greater than or equal to 15°, the polarization hologram element 102, the semiconductor laser 101, and the light receiving elements 107 can be brought into proximity with each other, and this reduces the optical path length. Here, when the separation angle is 20°, and the wavelength is 405 nm, the grating pitch of the desired diffraction grating (hologram) is approximately 1 μm.
According to the present embodiment, in the polarization hologram element 102, the grating pitch can be made very small, and on the other hand, high diffraction efficiency can be obtained.
Below, descriptions are made of a configuration and operations of a hologram element of the present embodiment, and a method of producing the hologram element.
The interference exposure device shown in
For example, a photosensitive recording material 21 is fixed on a temperature controlling stage 58, which is used to heat the photosensitive recording material 21.
The semiconductor laser 51 is a coherent light source, for example, use can be made of a Krypton (Kr) Ion Laser having an oscillation wavelength of 407 nm, a Helium-Cadmium (He—Cd) laser having an oscillation wavelength of 442 nm, an Argon (Ar) Ion Laser having an oscillation wavelength of 488 nm or 514 nm, a Helium-Neon (He—Ne) laser having an oscillation wavelength of 633 nm, a Ti: Sapphire laser having an oscillation wavelength of 870 nm, or any other coherent light source.
When the semiconductor laser 51 is a laser operating in a single longitudinal mode, the coherent length is long, and a hologram with less noise can be produced.
The filter 52 is not always necessary in the interference exposure device shown in
In
Above, a basic structure of a two-beam interference exposure device is described as an example, but the present embodiment is applicable to an interference exposure device using three or more light beams for interference and exposure, and such an interference exposure device can be constructed by arranging plural sets of optical elements corresponding to the number of the light beams.
In the above two-beam interference exposure device, the half mirror 56, or other beam splitters, is used to split the incident beam into plural light beams (here, two light beams), and mirrors 57a and 57b or other lenses are used to adjust the interference pattern of these light beams. In this case, although depending on optical elements to be used, the exposure optical system requires a relatively large space because of limitations of specifications and arrangement, and the interference exposure device becomes large.
In a single hologram element as shown in
When performing exposure with interference light by using the master hologram as shown in
As for the method of producing the master hologram, in order that the hologram exhibits desired characteristics, computers are employed to make calculations, and patterns are written on a photo mask blank plate according to the calculation results by using an electron beam writing device to produce a first master hologram; independent from this process, resin for forming a volume hologram is applied on a glass substrate to prepare a volume hologram substrate; the first master hologram produced in advance is superposed on the resin layer applied on the volume hologram substrate so that the mask surface of the first master hologram is in contact with the resin layer on the volume hologram substrate; then a laser beam is irradiated to expose the structure from the side of the first master hologram. After the exposure, ultraviolet light irradiation processing (decomposition of the photo polymerization initiator) and heating treatment (diffusion movement of photo polymerizable compounds) are performed; as a result, the first master hologram is duplicated on the resin layer applied on the volume hologram substrate. The thus duplicated hologram is used as a master hologram.
Here, it is described that electron beam lithography is used to produce the first master hologram, but other methods, such as two-beam interference exposure may be used. In addition, concerning the characteristics of the master hologram, it is preferable that the interference pattern have a high contrast ratio, and it is preferable that the master hologram for producing a diffractive gratings be designed so that the ratio of strength of the transmission light and the diffraction light, which produce the interference pattern, be roughly 1:1 at the exposure wavelength.
When producing the hologram of the present embodiment in large volume, as illustrated in
Alternatively, as illustrated in
Alternatively, as illustrated in
In the method shown in
As for characteristics of the duplicated hologram produced by exposure, high diffraction efficiency, high transparency, and good environment tolerability are desired.
Concerning the environment tolerability, it is preferable to provide protection films in the hologram regions, but formation of the protection films can be omitted by overlapping a pair of glass substrates or a pair of plastic substrates to form cells to perform exposure for producing hologram elements.
In the aforesaid two-beam interference exposure or exposure with the master hologram in close contact, as shown in
For example, in order to produce a grating having a pitch of 1 μm, assume one of the two light beams, which are at 442 nm, for producing the interference pattern is incident on the substrate on the incident side perpendicularly, the other beam is incident at an angle of 26° relative to the substrate, and the refractive index of the substrate is 1.5; then, the axis of the incident light in the cell shifts by 0.55 mm, 0.20 mm, 0.13 mm, and 0.03 mm, when the thickness of the substrate on the incident side is 3 mm, 1.1 mm, 0.7 mm, and 0.15 mm, respectively. That is, the axis of the incident light changes according to the thickness of the substrate on the incident side, and the shift decreases when the thickness of the substrate becomes thin.
For a hologram diffractive element used in an optical header (optical pickup device), for example, a hologram diffractive element has a small active area (approximately 2 mm) and narrow pitches (approximately 3 μm). Because there are plural hologram regions having different characteristics in a single hologram element, due to limitations of the hologram regions, it is preferable that the thickness t1 of the incident side substrate be 0.7 mm or less, more preferably, 0.15 mm or less. These values may change depending on the pitches and area of the grating to be produced, the size of the element as a whole, or the applications of the element.
Meanwhile, it is normal to set the other substrate in the same cell but not on the incident side to have the same thickness as the thickness t1 of the substrate on the incident side (assume the thickness of the other substrate not on the incident side is t2). However, if both of the two substrates are made thin, the curing shrinkage of the photo-sensitive material 21 when duplicating the hologram becomes a problem; furthermore, durability and strength of the supplicated hologram decline. For this reason, it is desirable that the thickness t2 of the other substrate be 3 mm to 0.5 mm, and this is a typical range of the thickness of a substrate used in common optical elements or liquid crystal displays.
To solve both the problem in shift of the incident light axis and the problem in the cell strength, in the present embodiment, as shown in
As illustrated in
As illustrated in
This deficiency can be effectively prevented by reducing the thickness of the substrate as shown in
When exposure with interference light is performed by using the master hologram, because the interference light is generated by using the master hologram, the hologram element fabricated with the interference light is largely affected by the master hologram. Due to this, the reproduction accuracy of the master hologram is important, and thus, when the master hologram itself generates noise light, it is required to further reduce the noise light.
Generally, a master hologram having a more complicated shape is more likely to generate noise light, and it is regarded to be difficult to eliminate the noise light completely (although sometimes it is dependent on the shape of the periodical structure dictated by the desired hologram characteristics).
To eliminate the noise light when using the master hologram for exposure with interference light, a relay optical system as illustrated in
In
The relay optical system in
As shown in
When using interference light to form an interference pattern on the photo-sensitive material by means of two-beam interference exposure, interference exposure with the master hologram being in close contact, or interference exposure with the 4f relay optical system, as described in the previous embodiment, spread caused by the polymerization reaction in the recording material 21 influences the exposure and duplication processes, although the behavior of the spread depends on the exposure area (that is, the size of the hologram element to be fabricated), the interference pattern (that is, the shape of the periodic structure), exposure conditions (such as the exposure temperature, exposure quantity), and other conditions.
As shown in
As shown in
When the exposed region on the recording material 21 is broader than the actual exposure area on the master hologram 10, or when different exposed areas overlap each other in a single hologram element, satisfactory hologram characteristics cannot be obtained in the expanded exposed region and in the overlapping exposed region, and this causes degradation of the hologram characteristics of the hologram element; thus, these areas are not desired.
As illustrated in
For example, the isolation members 24 are light curable adhesive agents or heat curable adhesive agents, which are commonly used in liquid crystal displays or other devices. For example, the isolation members 24 can be formed by screen-printing or the like. For example, the printing plate used in the screen-printing may be a relief printing plate, a planographic printing plate, an intaglio printing plate, or a stencil printing plate; the screen printing plate can be a mimeographic plate, or a stencil plate. Specifically, silk (screen) woven with polyester or other fibers is used as the screen printing plate. This screen is stretched and fixed on a frame, a thin film (serving as a resist) of the printing plate is formed on the screen, and then with unnecessary streak eyes being blocked, the screen printing plate is completed. Printing ink (such as an ultraviolet curable one, or a thermal curable one,) is applied within a frame of the screen printing plate. When a sliding force is applied to the printing ink by a squeegee, the ink passes through a portion of the screen where the resist is not present, and is transferred to an object to be printed. By such screen printing, it is possible to print on materials of various shapes and sizes, various inks (isolation member) can be used, and further, relatively thick films (about 100 μm) can also be formed.
In this way, isolation members can be formed on the hologram substrate beforehand. It should be noted that the method of forming the isolation members of the present embodiment is not limited to the aforesaid screen-printing.
As illustrated in
Below, the isolation members 24 for isolating the hologram regions are described in detail.
As described above, generally, the isolation members 24 may be light curable adhesive agents or thermal curable adhesive agents, which are common adhesive agents. For example, spherical spacers or fiber spacers used in liquid crystal displays can be mixed in the isolation members 24; thereby, the isolation members 24 can also serve as spacers to control the gap between the substrates.
For example, the isolation members 24 having functions of spacers can be formed from plastic films which can be controlled to have a certain thickness, such as polyvinyl chloride, polyimide, polystylene, polyethylene, polyethylene naphthalate, polycarbonate, and polypropylene.
For example, the isolation members 24 may be projections on the surface of the substrate fabricated by photolithography, etching, molding, or other techniques. Specifically, as for photolithography, which is generally used in semiconductor processes, first, through a mask on which a desired pattern of the isolation member is formed (for example, the mask can be a glass substrate on which a shielding pattern is formed from Cr or Al), visible light or ultra violet rays are irradiated to a substrate for exposure, on which a photo resist photo-sensitive layer is formed beforehand (this is the hologram substrate before exposure). Alternatively, without using the mask, an electron beam is used to directly write the pattern of the isolation member. Due to the exposure process, the solubility of the photo resist relative to a developing solution changes, so that in the developing step, the pattern on the mask is transferred to the photo resist. In order to ensure that the resolution of the transferred pattern will be in the order of sub-microns, it is preferable to use light of a relatively short wavelength.
The thus obtained photo resist can be used as the isolation member 24; however, from the point of view of durability or reliability, it is preferable to transfer the pattern on the photo resist to the substrate by dry etching or wet etching to form an isolation pattern on the substrate directly.
When forming the isolation members 24 by molding, by an electron beam or cutting operations, projections can be formed on the hologram substrate before exposure to serve as the isolation members. Alternatively, a mold having projections used for forming the isolation members is formed from stainless steel, nickel, or aluminum by an electron beam or cutting operations, then by means of transfer, extraction, or injection, the hologram substrate with the isolation members thereon can be formed.
A schematic cross-sectional view of the isolation members having the function of spacers is the same as that shown in
Because of the isolation members 24, it is possible to prevent influence of the spread of the polymerization reaction on neighboring hologram elements. However, during exposure with interference light, light leakage occurs due to multiple reflections between the substrates holding the recording material, and this leakage imposes adverse influence on the hologram element.
As described above with reference to
For example, the material of the isolation members 24B can be appropriately selected according to an object wavelength. For example, it may be chromium or chromium oxide, resins with carbon or pigments being dispersed therein, or resists with pigments being dispersed therein. The pigments may be anthraquinone-based dyes at the red wavelength region, or phthalocyanine-based pigments at the green or blue wavelength region. In addition, the material for forming a resist may be a radical polymerized photo polymer formed from a polyfunctional acrylic ester monomer, a photo polymerization initiator from trihalomethyl triazine, and a copolymer of acrylic acid and acrylic ester. But the material of the resist is not limited to these.
When the isolation members 24 are formed from conductive materials, such as chromium, aluminum, or others, when an alternating voltage is applied on the conductive isolation members 24D, a uniform electric field can be applied in each of the isolated regions.
When the photosensitive material includes compositions such as the polymer crystal or polymer dispersed crystal, which have dielectric anisotropy, the liquid crystal alignment of the liquid crystal molecules changes along the direction of the applied electric field. Therefore, with the voltage applied to the conductive isolation members 24D, the liquid crystalline alignment of the recording material 21 can be controlled to obtain desired birefringence of the liquid crystal in the recording material. The birefringence can enlarge the refractive index modulation of the periodic structure in the hologram region. As in the polarization hologram element of the first embodiment, this can improve polarization selectivity.
Next, a description is made of the photosensitive material used as the recording material. For example, the hologram recording material can be common photo sensitive materials, for example, dichromate gelatin, photo clock materials, photo thermoplastic, electro-optic crystals (for example, ferroelectric oxides, LiNbO3, BaTiO3 crystals), photopolymer, photo-resist, polymer liquid crystals, and polymer dispersed liquid crystals.
Dichromate gelatin can be used to record an amplitude hologram and a phase hologram. But dichromate gelatin needs developing processing, and has low environment tolerability, for this reason, it is not convenient to store a hologram made from dichromate gelatin. On the other hand, because dichromate gelatin is not granular, it causes little scattering noise, and enables very high resolution.
Typically, the photo resist is used in a relief hologram with projections and depressions on the surface, but it can also be used to produce volume holograms. General positive materials or negative materials developed in semiconductor integrated circuit fabrication can be used as materials of the photo resist. In addition, materials developed to obtain high exposure sensitivity and long-wavelength photosensitive bands can also be used, such as methacrylate polymer, acryl-based polyfunctional monomer, photo polymerization initiators, and photo-polymerizing resists composed of sensitizing dyes. With these materials, in recording by exposure, it is possible to obtain high resolution, to select exposure sensitivity and the photosensitive wavelength band in a wide region, and to obtain volume holograms having high diffraction efficiency.
The photo polymer can be used to record a refractive index modulating hologram, and a high resolution and low noise hologram can be produced with the photo polymer because the photo polymer enables high diffraction efficiency and essentially does not have granularity. The photo polymer is composed of various materials, and can be generally classified into photo-polymerized photo polymer and photo bridged photo polymer.
The photo-polymerized photo polymer includes ones requiring development, and ones not requiring development. In terms of compositions, the photo-polymerized photo polymers not requiring development can be classified into (a) polymer, monomer, (b) monomer, monomer, and (c) inactive components (low molecular), monomer. Compositions of photo polymers belonging to (a) and (c) are selected so that the difference of the refractive indexes is large between monomer polymerized materials and polymers or low molecular compounds. Photo polymers belonging to (b) are composed of two kinds of monomers having different refractive indexes, for example, the photo polymers belonging to (b) can be formed from the following combinations, such as (1) monomers having different photo polymerization capability, (2) photo polymerized monomer and thermal polymerized monomer, (3) photo radical polymerized monomer and optical cation polymerized monomer.
In the above materials, the photo polymerizable polymers, such as photopolymer or the photo resist, enable selection of resolution, exposure sensitivity, photo-sensitive wavelength band in a wide region, and superior environmental tolerability; and offer many degrees of freedom in selection of the film thickness and size. In addition, because the granularity is small, high diffraction efficiency and high transparency are obtainable, which are features of a hologram.
As for a refractive index modulating hologram, which has high diffraction efficiency, when the difference of the refractive index modulation of the periodic structure in the hologram region is large, an optimum film thickness that results in high efficiency becomes small; accordingly, the variation of the diffraction efficiency, which depends on a wavelength change or an incidence angle, can be reduced. In other words, it is possible to produce a hologram enabling high diffraction efficiency in wide ranges of wavelength and incidence angle.
As for photosensitive materials able to enlarge the difference of the refractive index modulation of the periodic structure, there are polymer liquid crystals and polymer dispersed liquid crystals having large birefringence.
The polymer liquid crystals and polymer dispersed liquid crystals may be a composite material including a polymerized liquid crystal, or a mixed composite material including a non-polymerized liquid crystal and a polymerized polymer, or a mixed composite material including a polymerized polymer and at least one of a non-polymerized liquid crystal and a polymerized liquid crystal. Specifically, the above composite material may be one of (1) a polymerized liquid crystal, (2) admixture of a non-polymerized liquid crystal and a polymerized polymer, (3) a mixture of a polymerized polymer and a polymerized liquid crystal, and (4) a mixture of a polymerized polymer, a non-polymerized liquid crystal, and a polymerized liquid crystal. When necessary, a photo polymerization initiator may be added.
For example, the polymerized liquid crystal may be a liquid crystalline monofunctional acrylate monomer, a liquid crystalline meta-acrylate monomer, a liquid crystalline difunctional diacrylate monomer, or a liquid crystalline dimeta-acrylate monomer. These materials may include a methylene chain between the functional group of acryloyloxy and the liquid crystalline skeleton.
The non-polymerized liquid crystal may be any liquid crystal exhibiting diffractive anisotropy, having a phase structure of any one of the Nematic phase, cholesteric phase, and smectic phase. For example, well known liquid crystals can be used, for example, which have skeletons formed from one of biphenyl, tert-phenyl, phenyl-cyclohexane, biphenyl-cyclohexane, benzoic acid phenyl ester, cyclohexane carboxylic acid phenyl ester, phenyl-pyrimidine, phenyl-dioxane, tolan, 1-phenyl-2-cyclohexylethane, 1-phenyl-2-biphenylethane, 1-cyclohexylethane-2-biphenylethane, biphenyl carboxylic acid phenyl ester, or 4-cyclohexyl-benzoic acid phenyl ester, have an alkyl group, an alkoxy group, or a cyano group acting as a polarity assigning group for assigning dielectric anisotropy, and have a halogen group as a substituent group.
Preferably, the polymerized monomer or the prepolymer thereof is formed from materials having large polymerization curing shrinkage. For example, a photo-polymerizable compound having an ethylene unsaturated bond can be used as the polymerized monomer, such as a monomer, an oligomer, a prepolymer, and mixtures of them, each of which includes at least one ethylene unsaturated double bond in one molecule, and can be photo-polymerized and photo bridged.
In addition, the monomer and copolymer may also be an unsaturated carboxylic acid and unsaturated carboxylates, or an ester of the unsaturated carboxylic acid and an aliphatic polyalcohol compound, an amide of the unsaturated carboxylic acid and an aliphatic polyamine compound. Particularly, a polyfunctional monomer is preferable because it has large polymerization curing shrinkage.
The unsaturated carboxylic acid polymer may be an acrylic acid, a meta-acrylic acid, an itaconic acid, a crotonic acid, an isocrotonic acid, a maleic acid, and halogen-substituted unsaturated carboxylic acid thereof, such as chlorinated unsaturated carboxylic acid, brominated unsaturated carboxylic acid, and fluorinated unsaturated carboxylic acid.
The unsaturated carboxylate may be sodium salts and potassium salts of the above acids, and may be urethane acrylates, polyester acrylates, polyfunctional acrylates or methacrylates of epoxy resins and an acrylic acid.
Further, a thermal polymerization inhibitor or a plasticizer can be added into the above materials.
The photo polymerization initiator may be any well known one, for example, biacetyl, acetophenone, benzophenone, Michler's ketone, benzyl, benzoinalkylether, benzyldimethylketol, 1-hydroxy-2-methyl-1-phenylpropane-1-one, 2-chlorothioxanthone, methylbenzoylformate, 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropane-1-one, diethoxyacetophenone, 1-hydroxy-cyclohexyl-phenylketone, 2,2-dimethoxy-1,2-diphenyl-ethan-1-one, a -aminoalkylphenone, bis-acylphoshinoxide, and metallocene.
The quantity of the photo polymerization initiator to be added depends on the absorption capacity of the material at the wavelength of the irradiation light, and can be appropriately adjusted according to exposure conditions in duplication.
When duplicating a master hologram by exposure with interference light, by appropriately adjusting exposure conditions, an interference pattern corresponding to the master hologram can be formed on the recording material by exposure, which is formed from composite materials as described above, and thereby forming a periodic structure on the composite material having a refractive index modulation changing with the polarization direction of incident light, that is, modulation of the refractive index in the periodical structure changes with the polarization direction of the incident light.
For example, the composite materials forming may be HPDLC (Holographic Polymer Dispersed Liquid Crystal), which is obtained by dispersing a non-polymerized liquid crystal and a photo polymerization initiator in a polymerized polymer, or may be PPLC (Photo-Polymerized Liquid Crystal), which is obtained by mixing a polymerized liquid crystal and a photo polymerization initiator.
Below, a description is made of formation of an interference pattern on the composite material by exposure.
For example, in an interference pattern formed on HPDLC, the monomer, as a component of the composite material forming the recording material, is moved to the bright portion of the interference pattern (that is, phase separation between the polymer and the liquid crystal occurs), and is polymerized and cured.
The liquid crystal, which is a component of the composite material forming the recording material, remains at the dark portion of the interference pattern, and is drawn by the polymer cured in the bright portion; thereby, the liquid crystal is aligned in a specific direction. Because of alignment of the liquid crystal, if the incident light is linearly-polarized with the polarization directions thereof being perpendicular to each other, for one polarization direction of the incident light, the refractive index of the liquid crystal is small, and the incident light is not affected by the variation of the refractive index of the liquid crystal;, hence, substantially all of the incident light transmits through the composite material of the recording material; on the other hand, for the other polarization direction of the incident light, the refractive index of the liquid crystal is large, and the incident light feels the variation of the refractive index of the liquid crystal, hence, nearly all of the incident light is diffracted. As a result, the HPDLC functions as a polarization hologram device.
When the composite material is formed from PPLC, the liquid crystal having a functional group for photo-polymerization is sealed into the space between transparent electrodes (for example, formed from ITO) and a substrate carrying an alignment layer for aligning the liquid crystal, and the liquid crystal is aligned there. When the composite material of the recording material is exposed to interference light, an interference pattern is formed on the recording material, and in the bright portion of the interference pattern on the composite material, the liquid crystal molecules are polymerized and cured. Meanwhile, in the dark portion of the interference pattern on the composite material, the state of the liquid crystal molecules remain un-cured.
A light beam is irradiated on the liquid crystal layer with a voltage being applied to the transparent electrodes sandwiching the liquid crystal layer. Due to the applied voltage, the liquid crystal in the dark portion of the interference pattern is aligned in a direction perpendicular to the substrate, and is cured due to light irradiation.
As a result, in correspondence to the distribution of the bright and dark portions of the interference pattern, a periodic structure of the liquid crystal is formed, in which the liquid crystal is aligned in the vertical direction and in the horizontal direction alternately.
When linearly-polarized incident light with the polarization directions thereof being perpendicular to each other is incident on a diffraction grating (a hologram) recorded in this way, if the incident light has a polarization direction the same as a short axis direction of the liquid crystal molecules aligned in the horizontal direction, regardless of the periodic alignment of the liquid crystal in the vertical direction and in the horizontal direction, the incident light is not affected by the variation of the refractive index of the liquid crystal, and nearly all of the incident light transmits through the composite material; on the other hand, if the incident light has a polarization direction perpendicular to the aforesaid polarization direction, that is, the same as a long axis direction of the liquid crystal molecules aligned in the horizontal direction, the incident light is affected by the variation of the refractive index of the liquid crystal because of the periodic alignment of the liquid crystal in the vertical direction and in the horizontal direction, the nearly all of the incident light is diffracted.
As described above, because of the phase separation occurring together with the polymerization reaction in the composite material, or because of the polymerization reaction and the change of the alignment due to the external electric field, it is possible to produce a polarization hologram having selective polarization.
The photo-sensitive material for recording a hologram can be formed by general methods of film formation with solutions, specifically, by spin-coating or taping on a single substrate, or by vacuum injection or capillary injection into a cell formed by a pair of substrates. In fact, as long as a uniform film can formed, which ensures that desired hologram characteristics will be obtained, the method of film formation is not limited to the above ones.
After the film of the photo-sensitive material is formed, it is preferable to form the isolated regions according to the arrangement of the isolated regions.
After the film of the photo-sensitive material is formed, the isolated regions can be formed by exposing frames of the regions to be isolated before or at the same time as the interference exposure. However, in this case, the isolation member and the photo-sensitive material have the same composition, and it is difficult to form the aforesaid spacers, light absorbing members, and the conductive members. Hence, in the present embodiment, before forming the film of the photo-sensitive material, the isolation members are used to form the isolated regions. In doing so, the accuracy of the film thickness controlled by the isolation member having functions of a spacer member is improved, and it is possible to suppress stray light caused by the light absorbing members and to apply an electric field with conductive members.
When providing the isolation members as illustrated in
Below, descriptions are made of examples of the hologram element of the present embodiment, methods of producing the hologram element, and examples for comparison.
Photo-Sensitive Material
In the following examples, polymer dispersed liquid crystals are used as the photo-sensitive material. The photo-sensitive material is formed from a composite material including a mixture of the following materials (1) through (5), and the composite material is heated to about 85° C. on a hot plate with a stirrer and is stirred.
(1) Nematic liquid crystal (manufactured by Merck & Co., product name: TL216, Δε>0), 30 parts by weight (or 30 w/t parts).
(2) phenyl glycidyl ether acrylate hexamethylene diisocyanate urethane prepolymer (manufactured by Kyouei Chemistry Co., product name: AH600), 75 parts by weight (or 75 w/t parts).
(3) dimethylol-tricyclodecane-diacrylate (manufactured by Kyouei Chemistry Co., product name: DCP-A), 10 parts by weight (or 10 w/t parts).
(4) hydroxyethyl methacrylate (manufactured by Kyouei Chemistry Co., product name: H0), 5 parts by weight (or 5 w/t parts).
(5) a photo polymerization initiator based on bis-acylphoshinoxide (manufactured by Ciba-Geigy Co., product name: Irgacure 819), 1 parts by weight (or 1 w/t parts).
Cell for Recording Hologram
<Cell of the Related Art (Without Isolation Member)>
A blue light reflection prevention film is formed on one surface of each of two glass substrates, each of which is 40 mm in height, 30 mm in width, and 0.7 mm in thickness. The glass substrates are bonded to each other by using a UV curable adhesive agent in which bead-like spacers each having a diameter of 5 μm are mixed. The adhesive agent is applied at two sites on the other surface of each of the glass substrates opposite to the reflection prevention film.
Specifically, the photo-sensitive material is injected into the cell by a capillary method while being heated and maintained at 60° C. on the hot plate, forming a photo-sensitive material film having a thickness of about 4 to 5 μm. After being injected into the cell, the photo-sensitive material is isotropic at room temperature. Because the photo-sensitive material is reactive with respect to light having a wavelength shorter than green light, it is handled with red light in a dark room. This cell has the same structure as that shown in
<Cell of Present Embodiment (With Isolation Member)>
As illustrated in
Specifically, the photo-sensitive material 21 is injected into the cell by a capillary method while being heated and maintained at 60° C. on a hot plate, forming a photo-sensitive material film having a thickness of about 4 to 5 μm. After being injected into the cell, the photo-sensitive material exhibits isotropy at room temperature. Because the photo-sensitive material is reactive with respect to light having a wavelength shorter than green light, it is handled with red light in a dark room. The structure of the cell is shown in
Exposure with Interference Light
<Two-Beam Interference Exposure>
Next, using a He—Cd laser having output power of 80 mW and emitting a laser beam of a wavelength of 442 nm, a two-beam interference exposure device as shown in
A laser beam from the semiconductor laser 51 is magnified by the filter 52 including the object lens 53 (magnification: 40) the aperture 54 (diameter φ5 μm), and the collimator lens 55 (achromatic lens, focal length: 100 mm), after the incident light beam is converted to a parallel beam at power of about 10 mW/cm2, the light beam is split into two beams. The mirrors 57a and 57b are arranged to deflect the two light beams, respectively, and render the two deflected light beams to intersect each other at an angle of 26°. With this wavelength and this intersecting angle, an interference pattern having a pitch of 1 μm is formed in the intersecting area of two beams. Due to the two-beam interference exposure, the grating formed inside a hologram is inclined by about 81.7° relative to the surface of the substrate.
The cell for recording a hologram, in which the recording material 21 is sealed, is mounted on a heater, is heated to a specified temperature, and at this temperature the two-beam interference exposure is performed for about five minutes, thereby producing a hologram element. In order to prevent influence from dust and air flow, the two-beam interference exposure device is enclosed by an acryl plate. During the exposure, the ambient temperature is 25° C.
The exposure temperature during the two-beam interference exposure is set to be an optimum temperature that results in a maximum refractive index modulation (ΔnH) of the periodic structure in a certain polarization plane. Here, the exposure temperature is 65° C.
The refractive index modulation (ΔnH) of the hologram is calculated by a method described below in evaluation of the performance of the hologram element.
<Interference Exposure with Master Hologram>
A master hologram produced with a photo resist and two-beam interference exposure is obtained, which has the same structure as that in
With a laser beam of 442 nm being incident perpendicularly, the diffraction efficiency of the master hologram with respect to the +1st order light is about 30%, and the ratio of strength of the transmission light (0-th order light) and the +1st order diffracted light is about 1.5:1. The remaining 25% of light utilization efficiency corresponds to scattering, absorption, reflection and other loss of light, and noise light such as the −1st order light and higher order diffracted light. In the intersecting area of the transmission light (0-th order light) and the +1st order diffracted light from the master hologram, an interference pattern having a pitch of 1 μm is formed.
In this example, the exposure is performed with the optical system shown in
The same as the two-beam interference exposure, the exposure time is about five minutes, and the exposure temperature is 65° C. The master hologram 10 and the cell are brought into close contact with each other by using plate springs for interference exposure.
<Exposure with 4f Relay Optical System>
A 4f relay optical system as shown in
Evaluation of Hologram Element
When evaluating the hologram element produced as described above, a linearly-polarized laser beam at 442 nm is incident on the hologram element, and the strength of the +1st order diffracted light is measured.
An ND filter is adjusted so that the strength of the incident light is about 10 mW, and incident angle dependence of the diffraction efficiency is measured with the substrate surface of the hologram element being able to rotate in a range of ±20° relative to the normal direction of the substrate surface of the hologram element. Comparing the measurement results of the incident angle dependence of diffraction efficiency with theoretical results calculated by coupled wave theory by Kogelnik, the refractive index modulation ΔnH of the periodical structure of the hologram element is calculated.
Further, a linear-polarizing plate and a half-wave plate are provided in the incidence light path to rotate the optical axis of the half-wave plate by 45°, thereby, changing the polarization direction (p polarization component and s polarization component) of the light incident on the hologram element. With this configuration, polarization selectivity of the +1st order diffracted light is measured. In this case, the p polarization component is perpendicular to the direction of the stripes of the interference pattern formed in the interference exposure, and the s polarization component is along the direction of the stripes of the interference pattern.
The hologram elements produced as described so far in the present embodiment have similar characteristics, which are summarized below.
With the film thickness of the photo-sensitive material being in the range from 3.8 μm to 4.6 μm, correspondingly, the refractive index modulation (ΔnH) is in the range from 0.078 to 0.11, the diffraction efficiency of the +1st order light (p polarization component) is in the range from 78.0% to 81.0%, the diffraction efficiency of the 0-th order light (s polarization component) is in the range from 94.0% to 96.0%, and the polarization selectivity is in the range from 0.0% to 0.4%.
Productivity of Hologram Element
Plural hologram regions are formed in the cell for recording a hologram in order to improve productivity of the hologram element. The method illustrated in
Using the cell of the related art as shown in
The thus produced polarization hologram element was evaluated as below. A linear-polarized laser beam at 442 nm and including a P polarization component and an S polarization component was emitted to the hologram element in a direction perpendicular to the substrate surface of the element. When the laser beam was incident on the hologram region formed in the first exposure, diffracted light depending on the polarization state of the incident light was observed, but in the hologram region formed in the second exposure, diffracted light was not observed, no matter whether the incident light is a P wave or an S wave.
Using the cell of the present embodiment as shown in
The thus produced polarization hologram element was evaluated. A linear-polarized laser beam at 442 nm and including a P polarization component and an S polarization component was emitted to the hologram element in a direction perpendicular to the substrate surface of the element. Diffracted light having selectivity of the polarization state of the incident light was observed in the hologram regions formed in both the first and the second exposures. That is to say, with the isolation member to demarcate the recording material to form plural hologram regions (device regions), plural hologram regions can be formed in one cell.
In addition, the cell is cut by dicing at positions of the isolation members to divide the plural hologram regions in the cell into two separate hologram elements. These two hologram elements were evaluated by irradiating the same linear-polarized laser beam. It was found that diffracted light having polarization selectivity was observed in both of the hologram elements. Hence, the productivity of the hologram elements is improved.
In the hologram elements produced in the example for comparison and example 1, the cell gaps in the hologram region formed in the first exposure were deduced by fitting measurement results of incident angle dependence of diffraction efficiency with theoretical values calculated by the coupled wave theory. It was found that in the hologram element produced in the example for comparison, the cell gap was about 3.8 μm, while the cell gap in the example 1 was about 4.6μm. Because the beads mixed in the adhesive agent used for fixing the isolation member are 5 μm in diameter, the target value of the cell gap is 5 μm. It reveals that the accuracy of the cell gap was improved when isolation regions are formed.
As illustrated in
In the present example, the opening area of the opening mask 40 is reduced corresponding to the isolated regions. Exposure and duplication are performed with two-beam interference exposure in each isolated region, that is, exposure and duplication are performed three times.
The thus obtained hologram element was evaluated in the same way. It was found that the diffraction efficiencies of the hologram regions formed in the first, second, and third exposure were different, and the hologram regions formed later showed low diffraction efficiency. This could be ascribed to scattering light during the interference exposure or multiple reflection on the interface of the substrate.
To improve performance of the hologram element, black dye is mixed in the adhesive agent used for fixing the isolation members. That is, the isolation members in the present example can absorb light. Then, the cell is fabricated again and two-beam interference exposure is performed again.
The three hologram regions were evaluated again. It was found that the diffraction efficiencies of the hologram regions formed in the first, second, and third exposure were nearly the same, and performance of the hologram regions is stable regardless of the exposure timing.
In the present example, because the isolation member is formed from a material capable of absorbing the light used for exposure, it is possible to prevent unnecessary light; as a result, exposure of one isolated region can be performed without affecting neighboring isolated (un-exposed) regions. Due to this, the isolated region can be made small, and this can improve productivity of the hologram element.
In the present example, similar to the example 1, a hologram recording cell is fabricated which includes isolated regions the same as the example 2, and the photo-sensitive material is injected into the cell by vacuum injection. This process of fabricating the cell with the photo-sensitive material being sealed therein took about 45 minutes.
As another way of sealing the photo-sensitive material, the isolation member is arranged on one of the substrates holding the photo-sensitive material of the cell, and forms isolated regions. Then, a dispenser robot capable of variable delivery (manufactured by SONY Co.) is used to apply the recording material in each of the isolated regions demarcated by the isolation member, as shown in
This process of fabricating the cell with the photo-sensitive material being sealed therein took about 15 minutes.
That is, the fabrication time can be greatly shortened by applying the recording material in the isolated regions appropriately to form a film between the substrates.
In interference exposure with the master hologram 10, after the aforesaid cell is exposed with the interference light, as shown in
To solve this problem, a separation layer made from a fluorine-based material is formed on a surface of the master hologram 10 with the hologram regions being disposed by a tape. On the separation layer a UV-curable material (manufactured by Three Bond Co.) for molding lenses is applied by spin-coating to a thickness of about 2 μm.
As shown in
A stage with a micrometer was used to move the duplicated hologram element to measure the size of one exposed and duplicated hologram region. It was found that a large hologram region was duplicated by exposure when the separation layer was provided.
The thus exposed and duplicated hologram regions were evaluated, and diffracted light having polarization selectivity was observed in all of the hologram regions.
In the present example, instead of the adhesive agent in which bead-like spacers are mixed, a conductive isolation member made from an aluminum film having a thickness of 5 μm is used. The present example is the same as the example 3 except for the conductive isolation member. Exposure and duplication are performed in the way as shown in
The thus produced hologram element was evaluated, and it was found that the refractive index modulation increased compared to the hologram element of example 3.
According to the present embodiment, because the hologram element is produced by exposure of an interference pattern by using two or more light beams, or by using a master hologram, it is possible to produce a hologram element with good productivity in mass production, and it is possible to produce a hologram element having good polarization selectivity and high light utilization efficiency.
In addition, the hologram element of the present embodiment is applicable to an optical header (optical pickup device) for recording or reproducing information in a recording medium such as an optical disk or a magneto-optical disk, and preferably, the hologram element of the present embodiment can be used as a polarization splitting element which has polarization selectivity and is used to make an optical header compact. Further, the hologram element of the present embodiment can be used as a polarization splitting element in a projection display device for improving light utilization efficiency of illumination light, or can be used as an optical switch for switching the light path of the incident light beam depending on the polarization plane of the incident beam.
While the present invention is described above with reference to specific embodiments chosen for purpose of illustration, it should be apparent that the invention is not limited to these embodiments, but numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.
This patent application is based on Japanese Priority Patent Applications No. 2004-263489 filed on Sep. 10, 2004, No. 2005-117123 filed on Apr. 14, 2005, and No. 2005-132854 filed on Apr. 28, 2005, and the entire contents of which are hereby incorporated by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-263489 | Sep 2004 | JP | national |
| 2005-117123 | Apr 2005 | JP | national |
| 2005-132854 | Apr 2005 | JP | national |
