Patent Application
20080007852
The present invention relates to an optical element capable of changing the reflectance or absorptivity of light according to the wavelength.
Various kinds of optical storage media for storing or reproducing digital information, such as CDs and DVDs, are widely prevalent. Since the amount of information recently has been significantly increased as a result of the advancement of a variety of technologies, there is a demand for optical storage media that has a larger capacity for storing more information.
For the purpose of increasing the storage density of optical storage media, various relating techniques have been developed. For example, Patent Document 1 discloses a multilayer film structure obtained by laminating, via a transparent resin film layer in between, a plurality of island-like metal thin films having different spectral characteristics each consisting of fine metal powders of 100 nm or less in diameter, in which each of the island-like metal thin films serves as an optical storage layer, and the multilayer film structure is used as multiwavelength optical storage medium. Irradiation of a laser light having a high energy density near the resonance wavelength of the respective metal layers makes the metal powders absorb light and generate heat, causing local melting or deformation of the peripheral transparent resin media, and as a result of this the reflectance around the laser-irradiated portion is changed so that a mark is recorded.
In this kind of technique utilizing the reflection or transmittance of light, in order to reliably store or reproduce information, it is important that, when storing information, the reflectance or absorptivity of the media forming the recording layer be changed as much as possible. Regarding this technology, Patent Document 2 discloses a light transmissive material-ablation type, three-layered optical storage media including a light-reflecting material, a layer of light transmissive material on the light-reflecting material, and a layer of light-absorbing material on the light transmissive layer. According to this technique, by appropriately setting the thickness of the light transmissive layer or the thickness of the light absorbing material layer, it is possible to reduce the light reflectance of the light-absorbing material layer. On the other hand, by forming an opening by ablating the light-absorbing layer so as to expose the underlying light-reflecting layer, it is possible to perform optical storage utilizing a difference between the high reflectance of the light-absorbing layer and the low reflectance of the light-absorbing material layer.
Patent Document 3 discloses a technique for modifying the three-layered optical storage media disclosed in Patent Document 2, where the outermost layer consists of a so-called “island film” having a configuration in which metal particles having a particle diameter of approximately 10-30 nm are independently present with an interval of about 5-20 nm in between. In Patent Document 3, gold, which has an excellent stability especially in air, is used as the metal particles. According to this technique, irradiation of laser light to the island film causes thermal aggregation of the peripheral portions of the irradiated part in a convex shape, increasing void spaces, and as a result, the light absorptivity of the above-mentioned section is reduced so that it is possible to perform optical storage by utilizing the optical change.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2002-11957
[Patent Document 2] U.S. Pat. No. 4,329,697
[Patent Document 3] International Publication Pamphlet WO83/04332
The island films using gold, disclosed in Patent document 3, have a problem that, when the particles forming the island films are agglomerated and granulated by irradiation of a laser beam, the granulated particles have a strong Plasmon absorption in the visible region, limiting the wavelength at which light absorptivity is reduced, as well as increasing light absorption at some wavelengths. Further, there is a problem with the island films composed of gold that, despite their high stability against oxidation, they do not have enough stability for use as an optical element, because its light absorption characteristics can often change with a lapse of time.
On the other hand, an optical element having a laminated structure capable of significantly modulating the reflectance has been known as disclosed in Patent Document 2 and Patent Document 3. However, what is known is only a basic structure, and such a structure is far from being optimal for sufficiently taking advantage of the optical characteristics of the optical element.
To solve the above-mentioned problems, the present invention provides a reflectance control optical element capable of causing a change in the reflectance of light according to the wavelength, including:
a substrate including a material having a high reflectance;
a transparent film including a material having a light transmissivity formed on the surface of the substrate; and
an ultrathin film including a material with a predetermined light absorptivity formed on the surface of the transparent film, where:
the aforementioned ultrathin film is a metal thin film, which is made of metal nanoparticles having an average particle diameter of 10 nm or less, the metal nanoparticles being adjacent to or in contact with each other, and
As another mode of the reflectance control optical element, the present invention provides a light absorption enhancing element in an ultrathin film, which is characterized by the surface of the substrate being formed with a light scattering reflection film in the previously described reflectance control optical element.
In the present specification, “light” includes not only visible light but also any electromagnetic wave.
The reflectance control optical element of the present invention can cause a change in reflectance according to wavelength, and thus it is possible to dramatically improve the credibility of an optical storage medium, which stores and reproduces digital information by the reflectance change. Moreover, since it is possible to freely control the wavelength at which the reflectance peaks and the wavelength at which the reflectance is minimized by properly setting the materials and the thickness of the transparent film and ultrathin film, the reflectance control optical element of the present invention can be applied to a broad range of fields. Furthermore, since the ultrathin film has a metal thin film structure in which platinum group metal nanoparticles having an average particle diameter of 10 nm or less are in a state adjacent to or in contact with each other, it is possible to accurately control the materialistic existence or non-existence of the ultrathin film by pulsed laser irradiation. Therefore, in using the reflectance control optical element of the present invention as an optical storage medium or the like, high density storage can be achieved by taking advantage of the high resolution.
In addition, the basic structure of the element is a simple structure consisting of three layers, which is advantageous in that the production cost is minimal.
In the light absorption enhancing element, which is another embodiment of the light reflectance control element of the present invention, the light absorbing effect of the ultrathin film can be enhanced ten times or more with a very simple structure. Therefore, it is possible to form an ultrathin film which is very thin and yet also has an excellent light absorption capability.
A schematic diagram of the reflectance control optical element of the present invention is shown in
Materials forming the substrate are not particularly limited; however, in consideration of achieving a large reflectance difference, it is of course desirable that the materials should be those having as high a reflectance level as possible. Examples of such materials include metals such as aluminum, gold, and silver. Moreover, in the present invention, the thickness of the substrate is not limited; it may be a thin film or bulk.
Materials forming the transparent film may be any, including any kinds of glass and polymer, as long as the material has light transmissivity. However, with the objective of achieving high reflectance, the material is desirably as transparent as possible (i.e. its light absorptivity should be low.) Also, a transparent electrode such as ITO (Indium Tin Oxide) may be used as a transparent film when utilizing the light absorption enhancing effect of the reflectance control optical element of the present invention.
As will be described later, the wavelength-dependent reflectance of the reflectance control optical element of the present invention changes depending on the thickness and the refractive index of the transparent film.
The ultrathin film is formed on the surface of the aforementioned transparent film, and the thickness thereof is normally several tens of nm or less. According to the optical element of the present invention, incident light is scarcely absorbed in any of the substrate and the transparent film. Therefore, it is presumed that the presence of the ultrathin film should mainly cause a large change in reflectance. Materials forming the ultrathin film are not particularly limited; however, the materials desirably have high light absorption (i.e. its light absorptivity should be higher than reflectance in the case of the preferable thickness according to the present invention described later) in order to cause a large change in reflectance.
The ultrathin film is desirably a metal thin film composed of metal nanoparticles, in which each particle is densely located adjacent to or in contact with each other, and a single layer or several layers of the metal thin film are formed in a thickness direction and distributed almost evenly in a plane direction. In this metal thin film, an average particle diameter of the metal nanoparticles is desirably in the range of 3 to 10 nm. It is possible to produce a metal thin film having a structure of this kind by, for example, DC sputtering method.
The aforementioned metal thin film is optically equivalent to a complete continuous film, and thus it can be simulated using optical constants of a bulk, which is advantageous for designing the element. On the other hand, since the metal thin film is thermally and electrically discontinuous, thermal diffusion along the film rarely occurs, and the electrical conductivity is low.
The ultrathin film having the aforementioned characteristics is advantageous when the element of the present invention is utilized as optical storage medium. For example, suppose that a laser is irradiated to predetermined areas of the ultrathin film. The metal nanoparticles existing in those areas absorb energy of the irradiated light, and generate heat and then dissolve, and then a plurality of the metal nanoparticles coalesce with one another so that they are agglomerated and granulated. Since the agglomerated and granulated areas do not absorb light for the reasons described later, it is possible to form areas where practically no ultrathin film is present. Above all, due to the characteristic of rare thermal diffusion occurrence, energy is concentrated only at areas where laser has been irradiated in the ultrathin film. Accordingly, it is possible to accurately control the materialistic existence of the ultrathin film, and therefore high density storage with an excellent resolution can be realized. Herein, for further improving the resolution, a pulsed laser is preferably used at the time of laser irradiation so as to reduce the thermal diffusion to a minimum level.
The present inventor has found that platinum group elements such as platinum and palladium are particularly preferable as the metal capable of relatively easily realizing the aforementioned ultrathin film. Platinum group elements have a thermal conductivity as low as approximately one fifth of gold, silver, copper or the like, and are also excellent in chemical stability and thermal stability. In the cases of gold, silver, copper or the like, upon producing the ultrathin film, individual particles tend to have a large particle size, making it difficult to obtain the metal nanoparticles having a particle diameter of 10 nm or less. Furthermore, since independent spherical nanoparticles of platinum have almost no absorptivity in the visible region, when they are irradiated by a laser and thus agglomerated and granulated while being used as the materials for the ultrathin film (absorption in the visible region is caused by adjacence or contact of the nanoparticles), the granulated part becomes transparent in the visible region.
In the element of the present invention, the platinum metal group element may be a simple substance or an alloy. Further, for the purpose of increasing the mechanical strength of the ultrathin film or its adhesive strength to the transparent film, it is possible to use an alloy with a hard material such as nickel, as materials forming the ultrathin film.
The following description will discuss the detailed structure of the reflectance control optical element and the ultrathin film light absorption enhancing element of the present invention.
<Thickness of Transparent Film>
The wavelength-dependent reflectance of the reflectance control optical element of the present invention changes depending on the thickness of the transparent film 2.
A graph of the reflectance obtained when the thickness of the transparent film was set to 0.5 μm in the aforementioned condition is shown in
<Refractive Index of Transparent Film>
A simulation was performed to examine the influence of the refractive index of the transparent film on the wavelength-dependent reflectance change. Silver is used as the substrate, and an ultrathin film of platinum having a thickness of 5 nm was used, and the refractive index and the thickness of the transparent film were changed to lower the refractive index over the entire visible region.
<Thickness of Ultrathin Film>
Next, an experiment was performed to examine how the thickness of the ultrathin film affected the reduction of the reflectance. A substrate composed of silver and a transparent film composed of spin-on glass (refractive index n≈1.3 to 1.5) having a thickness of 80 nm were used, and the thickness of an ultrathin film (platinum) was changed in a range of 3 to 10 nm. The results are shown in
<Dye Ultrathin Film>
A dye can be used as a material for the ultrathin film, instead of metal nanoparticles. In the present invention, the dye is not limited to those which are generally called a dye, but refers to any material having a property of absorbing light of a specific wavelength spectrum. A composite material containing a dye as a main component is also included. The dye alone does not noticeably change the absorptivity of light, even if the thickness of a dye film is increased. On the other hand, when a dye is used in the ultrathin film of the element of the present invention, it becomes possible to significantly increase its light absorptivity.
It is also possible to form the ultrathin film using a plurality of dyes having different light absorption characteristics. In this case, the dyes may be mixed, or the ultrathin film may be formed by laminating layers of each dye. In consideration of the application as a multiple recording layer, the layer structure, like the latter, in which each dye layer functions independently is desirable. Supposing that the thickness of one dye layer is approximately 10 nm, even the lamination of three layers results in a thickness of only approximately several tens of nanometers, and thus causes no problem for the transparency of the ultrathin film. As one example of this, the graph in
[Ultrathin Film Light Absorption Enhancing Element]
As described above, the reflectance control optical element of the present invention can extremely reduce the reflectance by the appropriate design of its structure. That is to say, it is definitely possible to significantly increase the light absorptivity in the ultrathin film.
Meanwhile, many of the optical function devices such as an optical sensor and a photoelectric transducer have a laminated structure including a photoexcitation layer (light absorption layer). The energy transfer or the material transfer due to the crossing of nonequilibrium energy generated in the light absorption layer or charge carriers (electron or hole) across layer boundaries have a very important role, and therefore the thickness of the light absorption layer is desirably as small as possible. Otherwise, those carriers will be deactivated inside the light absorption layer, and the desired function will not be initiated. There are quite a few devices in which the thickness of the light absorption layer is set to the monomolecular layer level. A typical example of this kind of device in which the light absorption layer is thinly formed includes a dye-sensitized solar cell utilizing the light absorption of a dye adsorbed on the titanium oxide surface. However, the light capturing (absorption) efficiency of a dye layer of a monomolecular level decreases from a high level to approximately several to 10 percent due to the small level of thickness. As a technique for compensating this situation, some techniques are applied. For example, in the dye-sensitized solar cell, titanium oxide is formed into an aggregate of nanoparticles or a porous body so as to obtain a larger effective surface area for dye absorption. This technique, however, cannot always be extensively used in general applications, and the system is naturally complicated. In addition, the technique is expensive.
On the other hand, if the light absorptivity of a thin layer of a monomolecular layer level can be enhanced by more than ten times, the light capturing efficiency of approximately 100% should be obtained, and as a result, it becomes possible to realize an optical functional device with much more simple element structure as compared with the conventional ones.
As a structure capable of dramatically enhancing the absorption efficiency of the ultrathin film, by which the aforementioned problems can be solved, the present inventor thought of the structure of the reflectance control optical element of the present invention, in which the surface of the substrate is formed into a light scattering reflection film. That is, as shown in
With this structure, as shown on the lower side of
In this structure, an optimal roughness of the light scattering reflection film 1S depends on the thickness of the transparent film. As mentioned above, for reducing the reflectance as much as possible, or in other words for achieving a high absorption, in the reflectance control optical element of the present invention, the refractive index of the transparent film 2 is desirably as low as possible, and for such refractive index the optimal thickness of the transparent film 2 is approximately 100 nm (description below). This indicates that the upper limit of the roughness of the light scattering reflection film 1S is approximately 100 nm in terms of a ten point height of irregularities (Rz) value. More preferably, the roughness of the light scattering reflection film 1S is set to approximately 20% of the thickness of the transparent film 2. Moreover, for efficient light scattering, in the light scattering reflection film 1S, a cycle of a high-low formation is desirably almost equal to the wavelength of the incident light. A reflective film having such roughness can be produced in a relatively simple manner by, for example, DC sputtering method.
Further, in order to generate light scattering more efficiently inside the transparent film 2 to increase the absorptivity, it is preferable that the surface of the transparent film 2 should have approximately the same roughness as that of the light scattering reflective substrate as shown in
The following describes the experiment performed by the present inventor for confirming the light absorption enhancing effect of the reflectance control optical element having the aforementioned structure. In this example, as the ultrathin film (absorption layer), a fluorescent organic dye was used and was thinly formed with a thickness of approximately several nanometers. In this case, the light absorptivity of the ultrathin film itself is only less than several percent.
Specifically, rhodamine B (RhB), which is a fluorescent organic dye, was dissolved in a 0.1% polyvinyl alcoholic solution at a concentration of 0.05 mM, and the solution was spin coated on the transparent film at 3000 rpm. The resulting ultrathin film had a thickness of approximately 3 nm, and the amount of supported RhB dye contained therein, as the molecular number per unit projected area, was 1.3-2.0×1013/cm2. The support amount fell within the above-mentioned range in both cases where the surface of the transparent film was smooth and rough. The light absorptivity of the ultrathin film itself was approximately 1% at a maximum absorption wavelength. In the experiment, in order to confirm the absorptivity enhancing effect, the fluorescence intensity upon photoexcitation under the same condition was measured, instead of directly measuring the absorptivity.
Samples with the following structures (a) to (c) (
Excitation light was made incident on each of the above-mentioned samples (a) to (c) in a direction perpendicular to the substrate, and the fluorescence was measured at approximately 40 degrees from the perpendicular on the ultrathin film side. The measurement results are shown in
With the installation of the reflective film, the fluorescence intensity was enhanced by four times (a→b). The results correspond to the expectation that the fluorescence directed toward the reflective film, among the fluorescence generated in the ultrathin film, is reflected on the reflective film, and also the incident light passing through the ultrathin film without being absorbed therein is reflected on the reflective film and absorbed in the ultrathin film, which leads to approximately four times the enhancement.
Furthermore, when the transparent film was formed into an ultrathin film having a thickness of approximately 100 nm, the fluorescence intensity was enhanced by approximately three times (b→c). This result demonstrates that the basic structure of the reflectance control optical element of the present invention is effective for enhancing the absorptivity as well as controlling the reflectance.
Herein, the present inventor examined the relationship between the fluorescence intensity and the thickness of the transparent film (material: SOG, refractive index: up to 1.4). As shown in
Next, a sample with the structure mentioned below was prepared as the ultrathin film absorption enhancing element of the present invention, and the fluorescence was measured in the same manner as the comparative example.
The light scattering reflection film was produced by DC sputtering under the condition that a silver thin film being deposited on a glass substrate was strongly exposed to plasma irradiation. The substrate was naturally heated to a temperature range from 50 to 100 degrees Celsius by plasma irradiation during the film formation, even without intentional heating. An excessive heating at this stage may cause excessive surface roughness, and thus precaution is required.
<Influence of Surface Roughness of Substrate>
The following description will discuss the characteristics of two kinds of light scattering reflection films having a roughness significantly different from one another.
Hereinafter, the sample with a higher roughness is referred to as Ag-SS, and the sample with a lower roughness is referred to as Ag-S.
The aforementioned roughness measurement results demonstrate that:
As for the Ag-S (
From the foregoing description, it can be said that the Ag-S has a higher fluorescence intensity than the AG-SS.
In addition, as expected, the fluorescence intensity of the Ag-SS was up to approximately 700 at a maximum, showing a lower enhancing effect than the Ag-S.
The reflectance control optical element of the present invention is directly applicable to high density ROM storage, because the reflectance can be significantly changed by the presence or absence of the ultrathin film. Further, large changes of the reflectance indicate that intensity of the reproducing light can be lowered. Moreover, the reflectance does not change so much even in the case where the angle of the incident light leans approximately up to 40 degrees. In other words, even though the storage medium slightly leans relative to the incident light, there is almost no influence on the reflectance, and accordingly it is possible to considerably simplify a media inclination control mechanism in a reproducing apparatus.
It is also possible to produce digital data media by making use of not the presence or absence of the ultrathin film, but the reflectance changes associated with the changes of the transparent film thickness. For example, as shown in
The element which causes a large change in the reflectivity in a relatively narrow band of wavelengths (e.g. the element shown in
Furthermore, with the use of the reflectance control optical element of the present invention, it is possible to obtain a well defined interference pattern with excellent resolution, and accordingly a holographic storage media suitable for multiplexed digital hologram can be obtained.
Above all, in order to obtain an interference pattern with excellent resolution, the ultrathin film of the reflectance control optical element of the present invention should have a thickness of 20 nm or less, or desirably 10 nm or less.
Furthermore, when the thickness of the ultrathin film is increased, the thermal diffusion in the film increases, and therefore it is not possible to obtain a favorable interference pattern.
In the element of the present invention including a substrate (vapor-deposited silver film), a transparent film (refractive index n=up to 1.4, thickness: approximately 90 nm) and an ultrathin platinum film, interference patterns of approximately 1000 lines/mm were recorded by irradiation of a pulsed laser of 532 nm on the ultrathin platinum film and their first-order diffraction efficiency was calculated. With regard to a plurality of elements in which the thickness of the ultrathin film was gradually changed (thickness of the ultrathin film was changed in a range approximately 2 nm to 20 nm), a graph showing the relationship between the reflectance (horizontal axis) before recording of the interference pattern and the first-order diffraction efficiency (vertical axis) after recording of the interference pattern was created, as shown in
The aforementioned intensification of the first-order diffraction efficiency was presumably caused by the mutual intensification between the phase of the reflection diffraction light from the portions where the ultrathin film was present (referred to as “portion A”) and the phase of the diffraction light from the intensified electric field of the portions where the ultrathin film was practically removed (referred to as “portion B”) in the first-order diffraction direction. In order to establish such a relation, desirably the phase of complex reflection coefficient of the portions A differs from that of positions B by approximately 180°.
Therefore, by appropriately designing the thickness of the ultrathin film in the reflectance control optical element of the present invention, it becomes possible to maintain the high spatial resolution during recording of the interference pattern, as well as to obtain a high diffraction efficiency of approximately 10%. Since the diffraction efficiency is up to approximately 2% in a normal structure, it is proved that this increase of the diffraction efficiency is quite significant. With the increase of the ultrathin film thickness, it becomes gradually difficult to maintain the spatial resolution of the interference pattern, while the diffraction efficiency is further increased. In the case of creating a holographic storage media for some applications, such as a personal identification card, a diffraction grating or dispersion, it is possible to increase its diffraction efficiency as high as 20% because those applications do not require a very high resolution.
Moreover, as mentioned above, the ultrathin film light absorption enhancing element is provided with an ultrathin film having an extremely small thickness and a high light absorptivity, and therefore it is directly applicable as an efficient optical functional device, such as a solar cell.
The reflectance control optical element and applications thereof according to the present invention have been described with reference to examples; however, the applications are of course not limited to the ones previously described, and it is possible to freely add modification or changes within the idea of an element having a controllable reflectance.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-033358 | Feb 2005 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP06/02029 | 2/7/2006 | WO | 8/1/2007 |
