1. Technical Field
The present application relates to a light-trapping sheet and rod for allowing light intake utilizing diffraction, and to a light-receiving device and a light-emitting device using the same.
2. Description of the Related Art
Where light is propagated between two light-propagating media of different refractive indices, since there is transmission and reflection of light at the interface, it is typically difficult to transfer, with a high efficiency, light from one light-propagating medium to the other light-propagating medium and maintain this state. A conventional grating coupling method shown in Non-Patent Document No. 1 (Ohmsha Ltd., “Optical Integrated Circuits”, p 94, p 243, Hiroshi Nishihara, et al.), for example, can be mentioned as a technique for taking light into a transparent sheet from an environmental medium such as the air.
With the conventional techniques described above, however, the amount of light that can be coupled to guided light is small. A non-limiting example embodiment of the present invention provides a light-trapping sheet and rod capable of taking in a larger amount of light than with the conventional techniques, and a light-receiving device and a light-emitting device using the same.
A light-trapping sheet according to one aspect of the present application includes: a light-transmitting sheet having first and second principal surfaces; and a plurality of light-coupling structures arranged in an inner portion of the light-transmitting sheet at a first distance or more and a second distance or more from the first and second principal surfaces, respectively, wherein: each of the plurality of light-coupling structures includes a first light-transmitting layer, a second light-transmitting layer, and a third light-transmitting layer sandwiched therebetween; a refractive index of the first and second light-transmitting layers is smaller than a refractive index of the light-transmitting sheet; a refractive index of the third light-transmitting layer is larger than the refractive index of the first and second light-transmitting layers; and the third light-transmitting layer has a diffraction grating parallel to the first and second principal surfaces of the light-transmitting sheet. The diffraction grating is a two-dimensional diffraction grating so as to efficiently take in light from every direction.
A light-trapping rod according to one aspect of the present application includes: a light-transmitting rod having a principal surface and a circular or elliptical cross section; and a plurality of light-coupling structures arranged in an inner portion of the light-transmitting rod at a first distance or more from the principal surface, wherein: the at least one light-coupling structure includes a first light-transmitting layer, a second light-transmitting layer, and a third light-transmitting layer sandwiched therebetween; a refractive index of the first and second light-transmitting layers is smaller than a refractive index of the light-transmitting rod; a refractive index of the third light-transmitting layer is larger than the refractive index of the first and second light-transmitting layers; and the third light-transmitting layer includes a diffraction grating parallel to a central axis of the light-transmitting rod. The diffraction grating is a two-dimensional diffraction grating so as to efficiently take in light from every direction.
A light-receiving device according to one aspect of the present application includes: a light-trapping sheet set forth above; a protrusion/depression (or diffraction) structure or a prism sheet provided on the first principal surface or the second principal surface of the light-trapping sheet; and a photoelectric conversion section for receiving light output from the protrusion/depression structure or the prism sheet.
A light-emitting device according to one aspect of the present application includes: a light-trapping rod set forth above; and at least one light source provided adjacent to the first principal surface of the light-transmitting rod.
With a light-trapping sheet and a light-trapping rod according to one aspect of the present application, light incident on the light-transmitting sheet and the light-transmitting rod enters a light-coupling structure arranged in an inner portion thereof, and is converted by the two-dimensional diffraction grating of the third light-transmitting layer in the light-coupling structure to light that propagates in the direction along the third light-transmitting layer to be radiated from the end face of the light-coupling structure. Since the light-coupling structure is in such a positional relationship that it is parallel to the light-transmitting sheet surface or the rod central axis, and the surface of the light-coupling structure is covered by a low-refractive-index environmental medium such as the air, light that is once radiated is repeatedly totally reflected between the surface of the light-transmitting sheet, the surface of the light-transmitting rod, and surfaces of other light-coupling structures, to be confined within the light-transmitting sheet or the light-transmitting rod. Since the two-dimensional diffraction grating in the light-coupling structure has an equal period in two or more directions, it is possible to couple with the light-coupling structure with two or more azimuthal angles even for light beams with different azimuthal angles of incidence on the surface of the light-coupling structure, thereby allowing light beams entering the light-trapping sheet from various directions to be more uniformly confined within the light-trapping sheet. By varying the pitch of the two-dimensional diffraction grating for the plurality of light-coupling structures, it is possible to take in light over a wide area, over a wide wavelength range, e.g., over the entire visible light, for every angle of incidence.
Additional benefits and advantages of the disclosed embodiments will be apparent from the specification and Figures. The benefits and/or advantages may be individually provided by the various embodiments and features of the specification and drawings disclosure, and need not all be provided in order to obtain one or more of the same.
The present inventors have made an in-depth study on conventional grating coupling methods. As a result, it was found that according to the method disclosed in Non-Patent Document No. 1, only light that satisfies predetermined conditions can be taken into a light-transmitting layer 20, and light that falls out of the conditions is not taken in.
Induced guided light has modes such as zeroth, first, second, and so forth, which have different characteristic curves as shown in
where q is the diffraction order represented by an integer. At an angle of incidence other than θ defined by Expression 1, light is not coupled into the light-transmitting layer 20. Even with the same angle of incidence A, light is not coupled for different wavelengths.
Note that as shown in
The guided light 23B, while propagating through the grating area, radiates light 23b′ in the same direction as reflected light of the incident light 23a. Therefore, even if light is incident at a position far away from an end portion 20a of the grating and propagates through the light-transmitting layer 20 as the guided light 23B, it attenuates by the time it reaches the end portion 20a of the grating. Therefore, only the light 23a that is incident at a position close to the end portion 20a of the grating can propagate through the light-transmitting layer 20 as the guided light 23B without being attenuated by the radiation. That is, even if the area of the grating is increased in order to couple a large amount of light, it is not possible to allow all the light incident on the grating to propagate as the guided light 23B.
In view of such problems, the present inventors have arrived at a novel light-trapping sheet and rod capable of efficiently taking in large amounts of light, and a light-receiving device and a light-emitting device using the same. One aspect of the present invention is outlined as follows.
A light-trapping sheet according to one aspect of the present invention includes: a light-transmitting sheet having first and second principal surfaces; and a plurality of light-coupling structures arranged in an inner portion of the light-transmitting sheet at a first distance or more and a second distance or more from the first and second principal surfaces, respectively, wherein: each of the plurality of light-coupling structures includes a first light-transmitting layer, a second light-transmitting layer, and a third light-transmitting layer sandwiched therebetween; a refractive index of the first and second light-transmitting layers is smaller than a refractive index of the light-transmitting sheet; a refractive index of the third light-transmitting layer is larger than the refractive index of the first and second light-transmitting layers; and the third light-transmitting layer has a two-dimensional diffraction grating parallel to the first and second principal surfaces of the light-transmitting sheet.
The plurality of light-coupling structures may be arranged three-dimensionally in an inner portion of the light-transmitting sheet at a first distance or more and a second distance or more from the first and second principal surfaces, respectively.
Surfaces of the first and second light-transmitting layers located opposite to the third light-transmitting layer may each be parallel to the first and second principal surfaces of the light-transmitting sheet.
The plurality of light-coupling structures may include a first light-coupling structure and a second light-coupling structure arranged on a surface parallel to the first and second principal surfaces; and at least either the first light-transmitting layers or the second light-transmitting layers may be spaced apart from one another between the first light-coupling structure and the second light-coupling structure.
The light-transmitting sheet and the third light-transmitting layer of the plurality of light-coupling structures may be made of the same material; and the third light-transmitting layer of the first light-coupling structure and the third light-transmitting layer of the second light-coupling structure may be continuous with each other via a portion of the light-transmitting sheet therebetween.
A pitch of the diffraction structure may be 0.1 μm or more and 3 μm or less.
Surfaces of the first and second light-transmitting layers may each be sized so as to circumscribe a circle having a diameter of 100 μm or less; and the plurality of light-coupling structures may each have a thickness of 3 μm or less.
In the plurality of light-coupling structures, the two-dimensional diffraction grating may be formed by concentric or concentric elliptical rings.
At least two of the plurality of light-coupling structures may be different from each other in terms of a pitch of the two-dimensional diffraction grating.
The light-transmitting sheet may include: a first area being in contact with the first principal surface and having a thickness equal to the first distance; a second area being in contact with the second principal surface and having a thickness equal to the second distance; a third area sandwiched between the first and second areas; and at least one fourth area provided in the third area and connecting the first area and the second area to each other; the plurality of light-coupling structures may be arranged only in the third area excluding the at least one fourth area; and an arbitrary straight line passing through the fourth area may be extending along an angle greater than a critical angle, which is defined by the refractive index of the light-transmitting sheet and a refractive index of an environmental medium surrounding the light-transmitting sheet, with respect to a thickness direction of the light-transmitting sheet.
In at least one of the plurality of light-coupling structures, thicknesses of the first and second light-transmitting layers may be decreased toward an outer edge side away from a center of the light-coupling structure.
In at least one light-coupling structure of the plurality of light-coupling structures, a protrusion/depression structure whose pitch and height are ⅓ or less of a design wavelength may be formed on one of surfaces of the first and second light-transmitting layers that are in contact with the light-transmitting sheet, the first principal surface, and the second principal surface.
The refractive index of the first and second light-transmitting layers may be equal to a refractive index of the environmental medium.
A light-trapping rod according to one aspect of the present invention includes: a light-transmitting rod having a principal surface and a circular or elliptical cross section; and a plurality of light-coupling structures arranged in an inner portion of the light-transmitting rod at a first distance or more from the principal surface, wherein: the at least one light-coupling structure includes a first light-transmitting layer, a second light-transmitting layer, and a third light-transmitting layer sandwiched therebetween; a refractive index of the first and second light-transmitting layers is smaller than a refractive index of the light-transmitting rod; a refractive index of the third light-transmitting layer is larger than the refractive index of the first and second light-transmitting layers; and the third light-transmitting layer includes a two-dimensional diffraction grating parallel to a central axis of the light-transmitting rod.
The plurality of light-coupling structures may each be arranged three-dimensionally in an inner portion of the light-transmitting rod at the first distance or more from the principal surface.
A pitch of the diffraction grating may be 0.1 μm or more and 3 μm or less.
Surfaces of the first and second light-transmitting layers may each be sized so as to circumscribe a circle having a diameter of 100 μm or less; and the light-coupling structures may each have a thickness of 3 μm or less.
In the plurality of light-coupling structures, the two-dimensional diffraction grating may be formed by concentric or concentric elliptical rings.
At least two of the plurality of light-coupling structures may be different from each other in terms of a pitch of the two-dimensional diffraction grating.
In at least one of the plurality of light-coupling structures, a protrusion/depression structure whose pitch and height are ⅓ or less of a design wavelength may be formed on one of surfaces of the first and second light-transmitting layers that are in contact with the light-transmitting rod, and the principal surface.
The refractive index of the first and second light-transmitting layers may be equal to a refractive index of an environmental medium surrounding the light-transmitting rod.
A light-receiving device according to one aspect of the present invention includes: any of the light-trapping sheets set forth above; and a photoelectric conversion section provided on one of the first principal surface of the light-trapping sheet, the second principal surface thereof, and end faces adjacent to the first principal surface and the second principal surface.
The light-receiving device may further include any other one of the light-trapping sheets set forth above, wherein: the photoelectric conversion section may be provided on the first principal surface of the light-trapping sheet; and an end face of the other light-trapping sheet may be connected to the second principal surface of the light-trapping sheet.
A light-receiving device according to another aspect of the present invention includes: any of the light-trapping sheets set forth above; and a protrusion/depression structure or a prism sheet provided on the first principal surface or the second principal surface of the light-trapping sheet; and a photoelectric conversion section for receiving light output from the protrusion/depression structure or the prism sheet.
A light-receiving device according to another aspect of the present invention includes: any of the light-trapping sheets set forth above; and a protrusion/depression structure provided on a portion of the first principal surface or the second principal surface of the light-trapping sheet.
A light-emitting device according to one aspect of the present invention includes: any of the light-trapping sheets set forth above; a light source provided adjacent to one of the first principal surface and the second principal surface of the light-trapping sheet; a protrusion/depression structure provided on the other one of the first principal surface and the second principal surface of the light-trapping sheet; and a prism sheet arranged so as to receive light output from the protrusion/depression structure.
A light-emitting device according to another aspect of the present invention includes: any of the light-trapping rods set forth above; and at least one light source provided adjacent to the first principal surface of the light-transmitting rod.
The light-emitting device may include three of the light sources; and the three light sources may output red, blue and green light.
The light-emitting device may further include a prism sheet or a protrusion/depression structure provided on a portion of the first principal surface of the light-transmitting rod.
A first embodiment of a light-trapping sheet according to the present invention will be described. FIG. LA is a schematic cross-sectional view of a light-trapping sheet 51. The light-trapping sheet 51 includes a light-transmitting sheet 2 having a first principal surface 2p and a second principal surface 2q, and at least one light-coupling structure 3 provided in the light-transmitting sheet 2.
The light-transmitting sheet 2 is formed by a transparent material that transmits light of a desired wavelength or within a desired wavelength range determined according to the application.
For example, it is formed by a material that transmits visible light (wavelength: 0.4 μm or more and 0.7 μm or less). The thickness of the light-transmitting sheet 2 is about 0.03 mm to 1 mm, for example. There is no particular limitation on the size of the first principal surface 2p and the second principal surface 2q, and they each have an area determined according to the application.
As shown in
The light-coupling structures 3 are three-dimensionally arranged in the third area 2c of the light-transmitting sheet 2. The light-coupling structures 3 may be two-dimensionally arranged on a surface parallel to the first principal surface 2p and the second principal surface 2q, and a plurality of sets of the two-dimensionally-arranged light-coupling structures 3 may be layered together in the thickness direction of the light-transmitting sheet 2. The term “parallel” as used in the present specification is not limited to strict positional relationships as defined mathematically, but refers to positional relationships where two planes, two straight lines, or a plane and a straight line are at an angle of 10 degrees or less with respect to each other.
The light-coupling structures 3 are arranged with a predetermined density in the x,y-axis direction (in-plane direction) and the z-axis direction (thickness direction). For example, the density is 10 to 103 per 1 mm in the x-axis direction, 10 to 103 per 1 mm in the y-axis direction, and about 10 to 103 per 1 mm in the z-axis direction.
In order to efficiently take in light illuminating the entirety of the first principal surface 2p and the second principal surface 2q of the light-transmitting sheet 2, the density with which the light-coupling structures 3 are arranged in the x-axis direction of the light-transmitting sheet 2, that in the y-axis direction and that in the z-axis direction may be independent of one another and uniform.
Note however that depending on the application or the distribution of light illuminating the first principal surface 2p and the second principal surface 2q of the light-transmitting sheet 2, the arrangement of the light-coupling structures 3 in the light-transmitting sheet 2 may not be uniform and may have a predetermined distribution.
In the light-coupling structure 3, the two-dimensional diffraction grating 3d of the third light-transmitting layer 3c is arranged in the light-transmitting sheet 2 so as to be parallel to the first principal surface 2p and the second principal surface 2q of the light-trapping sheet 51. Herein, the two-dimensional diffraction grating being parallel to the first principal surface 2p and the second principal surface 2q means that the reference plane, which is a predetermined plane on which the grating is provided, is parallel to the first principal surface 2p and the second principal surface 2q.
Where a plurality of light-coupling structures 3 are arranged on a surface parallel to the first principal surface 2p and the second principal surface 2q, at least either the first light-transmitting layers 3a or the second light-transmitting layers 3b are spaced from each other between adjacent light-coupling structures 3. That is, at least either the first light-transmitting layers 3a and the second light-transmitting layers 3b are spaced apart from one another between any two of three or more light-coupling structures arranged in two dimensions on the same surface parallel to the first principal surface 2p and the second principal surface 2q, e.g., the first light-coupling structure and the second light-coupling structure. At least either the first light-transmitting layers 3a or the second light-transmitting layers 3b may be spaced apart from one another, and they may be both spaced apart from one another. In other words, in a plurality of light-coupling structures 3 arranged on a surface parallel to the first principal surface 2p and the second principal surface 2q, either the first light-transmitting layers 3a or the second light-transmitting layers 3b may be continuous with one another between adjacent light-coupling structures 3.
Where a plurality of light-coupling structures 3 are arranged in the thickness direction of the light-transmitting sheet 2, they are arranged to be spaced apart from each other in the thickness direction. That is, in any two of three or more light-coupling structures arranged in one dimension in the thickness direction of the light-transmitting sheet 2, e.g., the first light-coupling structure and the second light-coupling structure located above the first light-coupling structure, the first light-transmitting layer 3a of the first light-coupling structure and the second light-transmitting layer 3b of the second light-coupling structure are spaced apart from each other.
The thicknesses of the first light-transmitting layer 3a, the second light-transmitting layer 3b and the third light-transmitting layer 3c are a, b and t, respectively, and the step (depth) of the two-dimensional diffraction grating of the third light-transmitting layer 3c is d. The surface of the third light-transmitting layer 3c is parallel to the first principal surface 2p and the second principal surface 2q of the light-transmitting sheet 2, and surfaces 3p and 3q of the first light-transmitting layer 3a and the second light-transmitting layer 3b that are located on the opposite side from the third light-transmitting layer 3c are also parallel to the first principal surface 2p and the second principal surface 2q of the light-transmitting sheet 2.
As will be described below, in order to be able to take in light of different wavelengths incident on the light-trapping sheet 51, the light-trapping sheet 51 may include a plurality of light-coupling structures 3, the plurality of light-coupling structures may differ from one another in terms of the pitch A of the two-dimensional diffraction grating.
The first light-transmitting layer 3a, the second light-transmitting layer 3b and the third light-transmitting layer 3c of the light-coupling structure 3 are each formed by a transparent material that transmits light of a desired wavelength or within a desired wavelength range determined according to the application. For example, it is formed by a material that transmits visible light (wavelength: 0.4 μm or more and 0.7 μm or less).
The refractive index of the first light-transmitting layer 3a and the second light-transmitting layer 3b is smaller than the refractive index of the light-transmitting sheet 2, and the refractive index of the third light-transmitting layer 3c is larger than the refractive index of the first light-transmitting layer 3a and the second light-transmitting layer 3b. The refractive index of the light-transmitting sheet 2 may be equal to the refractive index of the third light-transmitting layer 3c.
As long as the refractive index satisfies these relationships, the light-transmitting sheet 2, the first light-transmitting layer 3a, the second light-transmitting layer 3b and the third light-transmitting layer 3c of the light-coupling structure 3 may be formed by any of various materials, and may be formed by materials of the same type having different refractive indices. Where the refractive index of the light-transmitting sheet 2 and the refractive index of the third light-transmitting layer 3c are set equal to each other, the light-transmitting sheet 2 and the third light-transmitting layer 3c may be formed by different materials having an equal refractive index, or the light-transmitting sheet 2 and the third light-transmitting layer 3c may be formed by the same material.
Where the light-transmitting sheet 2 and the third light-transmitting layer 3c are formed by the same material, the light-transmitting sheet 2 and the third light-transmitting layer 3c of the light-coupling structure 3 may be formed integrally. That is, in such a case, the light-transmitting sheet 2 is formed by a portion that serves as the third light-transmitting layer 3c, and a portion that surrounds a plurality of light-coupling structures 3. In such a case, for a plurality of light-coupling structures 3 arranged on the same surface parallel to the first principal surface 2p and the second principal surface 2q of the light-transmitting sheet 2, the third light-transmitting layer 3c of a light-coupling structure 3 (the first light-coupling structure) is connected to the third light-transmitting layer 3c of an adjacent light-coupling structure 3 (the second light-coupling structure) via a portion of the light-transmitting sheet 2 formed by the same material. Therefore, the third light-transmitting layers 3c of a plurality of light-coupling structures 3 arranged on the same plane can be formed by an integral member, thus simplifying the manufacturing process.
Hereinbelow, it is assumed that the first light-transmitting layer 3a and the second light-transmitting layer 3b are the air, and the refractive index thereof is 1. It is also assumed that the third light-transmitting layer 3c is formed by the same medium as the light-transmitting sheet 2, and they have an equal refractive index.
The surfaces 3p and 3q of the first light-transmitting layer 3a and the second light-transmitting layer 3b of the light-coupling structure 3 are each a rectangular of which two sides are the lengths W and L, for example, and W and L are 3 μm or more and 100 μm or less.
That is, the surfaces of the first light-transmitting layer 3a and the second light-transmitting layer 3b of the light-coupling structure 3 are each sized so as to circumscribe a circle having a diameter of 3 μm or more and 100 μm or less.
The thickness (a+t+d+b) of the light-coupling structure 3 is 3 μm or less. While the surface (plane) of the light-coupling structure 3 has a rectangular shape as shown in
The light-trapping sheet 51 is used while being surrounded by an environmental medium. For example, the light-trapping sheet 51 is used in the air. In this case, the refractive index of the environmental medium is 1. Hereinbelow, the refractive index of the light-transmitting sheet 2 is assumed to be n5.
Light 4 from the environmental medium enters the inside of the light-transmitting sheet 2 through the first principal surface 2p and the second principal surface 2q of the light-transmitting sheet 2. An AR coat or anti-reflective nanostructures may be formed on the first principal surface 2p and the second principal surface 2q in order to increase the transmittance of the incident light 4.
The anti-reflective nanostructures include minute protrusion/depression structures, such as moth-eye structures, whose pitch and height are ⅓ or less the design wavelength. The design wavelength is the wavelength of light used when designing the various elements so that the light-trapping sheet 51 exhibits a predetermined function. Note that with anti-reflective nanostructures, Fresnel reflection is reduced but total reflection is present.
Hereinbelow, of the light present inside the light-trapping sheet 51, light that satisfies sin θ<1/ns will be referred to as the narrow-angle light (light with propagation angle which is lower than critical angle) and light that satisfies sin θ1/ns as the wide-angle light (light with propagation angle which exceeds critical angle), regarding the angle θ (hereinafter referred to as the propagation angle) formed between the propagation azimuth thereof and the normal to the light-transmitting sheet 2 (a line perpendicular to the first principal surface 2p and the second principal surface 2q). In
On the other hand, where wide-angle light 6a is present inside the light-trapping sheet 51, a portion thereof is totally reflected by the surface of a light-coupling structure 3 to be wide-angle light 6b, and this light is totally reflected by the first principal surface 2p to be wide-angle light 6c that stays inside the sheet. A portion of the remaining light of the light 6a becomes wide-angle light 6b′ that passes through the third area 2c where the light-coupling structures 3 are provided, and this light is totally reflected by the second principal surface 2q to be wide-angle light 6c′ that stays inside the light-trapping sheet 51.
Although not shown in the figure, there is also wide-angle light that stays inside the sheet while being totally reflected between different light-coupling structures 3 and between the first principal surface 2p and the second principal surface 2q, i.e., light that propagates through, while staying in, the first area 2a, the second area 2b or the third area 2c.
In this case, there may occur a deviation in the distribution of light propagating through the first area 2a and the second area 2b. Where the deviation in the distribution of light in the light-trapping sheet 51 is problematic, it is preferred that one or more fourth area 2h is provided, in the third area 2c in the light-transmitting sheet 2, where no light-coupling structure 3 is provided, as shown in
That is, the light-coupling structures 3 are arranged only in the third area 2c excluding the fourth area 2h. In the light-transmitting sheet 2, the fourth area 2h connects between the first area 2a and the second area 2b. The fourth area 2h extends from the first area 2a to the second area 2b, or in the opposite direction, and the azimuth of an arbitrary straight line passing through the fourth area 2h is along a larger angle than a critical angle that is defined by the refractive index of the light-transmitting sheet and the refractive index of the environmental medium around the light-transmitting sheet. That is, assuming that the refractive index of the environmental medium is 1 and the refractive index of the light-transmitting sheet 2 is ne, the angle θ′ of the direction 2hx in which the arbitrary straight line passing through the fourth area 2h extends with respect to the normal to the light-transmitting sheet 2 satisfies sin θ≧1/ns. Herein, a straight line passing through the fourth area 2h refers to the straight line penetrating the surface at which the fourth area 2h is in contact with the first area 2a and the surface at which the fourth area 2h is in contact with the second area 2b.
As shown in
The coupling to the guided light 5B is the same as the principle of the conventional grating coupling method. Before the guided light 5B reaches an end face 3s of the third light-transmitting layer 3c, a portion thereof is radiated in the same direction as the narrow-angle light 5r to be narrow-angle light 5r′, and the remainder is guided to be radiated from the end face 3s of the third light-transmitting layer 3c to be the wide-angle light 5c. On the other hand, the wide-angle light 6a is totally reflected at the surface 3q of the second light-transmitting layer 3b, and it entirely becomes the wide-angle light 6b. Thus, wide-angle light incident on the surface of the light-coupling structure 3 (the surface 3p of the first light-transmitting layer 3a and the surface 3q of the second light-transmitting layer 3b) is reflected, as it is, as wide-angle light, while a portion of narrow-angle light is converted to wide-angle light.
Note that if the length of the two-dimensional diffraction grating 3d of the third light-transmitting layer 3c is too long, the guided light 5b is entirely radiated before reaching the end face 3s. If it is too short, the efficiency of coupling to the guided light 5b is insufficient. How easily the guided light 5B is radiated is represented by the radiation loss coefficient α, and the intensity of the guided light 5B is multiplied by a factor of exp(−2αL) at a propagation distance of L. Assuming that the value of α is 10 (1/mm), the light intensity will be multiplied by a factor of 0.8 after propagation over 10 μm. The radiation loss coefficient α is related to the depth d of the two-dimensional diffraction grating 3d, and it monotonously increases in the range of d≦dc while being saturated in the range of d>dc. Where the wavelength of light is λ, the equivalent refractive index of the guided light 5B is neff, the refractive index of the light-transmitting layer 3c is n1, and the duty of the diffraction grating 3d (the ratio of the width of the protruding portion with respect to the pitch) is 0.5, dc is given by Expression 2 below.
For example, dc=0.107 μm if λ=0.55 μm, neff=1.25, and n1=1.5. In the monotonous increase region, the radiation loss coefficient α is in proportion to d squared. Therefore, the length of the two-dimensional diffraction grating 3d, i.e., the length of the third light-transmitting layer 3c (the dimensions W and L) is determined by the radiation loss coefficient α, and is dependent on the depth d of the two-dimensional diffraction grating 3d. Assuming that by adjusting the depth d, the value of α is set in the range of 2 to 100 (1/mm) and the attenuation ratio to 0.5, W and L will be about 3 μm to 170 μm. Therefore, if W and L are 3 μm or more and 100 μm or less, as described above, it is possible to suppress the radiation loss to obtain a high coupling efficiency by adjusting the depth d.
Table 1 shows the visible light wavelength (λ=0.4 to 0.7 μm) of light that is coupled for the pitch Λ and the angle of incidence θ based on Expression 1, where the equivalent refractive index neff of the guided light 5B is set to 1.25. Each section of a dotted line is the range for coupling. For example, where the pitch is 0.4 μm, light having a wavelength of 0.4 μm is coupled at θ=−14° and light having a wavelength of 0.7 μm is coupled at θ=30°, thereby giving a visible light coupling range from θ=−14° to θ=30°.
The polarity of the angle of incidence θ is relevant to the light coupling direction. Therefore, if one focuses only on the presence/absence of coupling while ignoring the light coupling direction, covering either the range of angles of incidence from 0 to 90° or from −90 to 0° means that coupling is achieved for every angle of incidence.
Therefore, it can be seen from Table 1 that in order for light to be coupled for every visible light wavelength and for every angle of incidence, light-coupling structures 3 including two-dimensional diffraction gratings 3d having pitches A from 0.18 μm to 0.56 μm (from 0° to 90°), or from 0.30 μm to 2.80 μm (from −90° to 0°), may be used in combination. Taking into consideration changes in the equivalent refractive index and manufacturing errors occurring when forming the waveguide layer and the diffraction grating, the pitch of the two-dimensional diffraction grating 3d may be generally 0.1 μm or more and 3 μm or less.
As shown in
Note however that where the two-dimensional diffraction grating 3d is formed by concentric rings, the pitch of the two-dimensional diffraction grating 3d is independent of the azimuthal angle φ and is constant. Therefore, where light beams of different wavelengths are to be coupled to the light-coupling structure 3 of the light-trapping sheet 51, the pitch of the two-dimensional diffraction grating 3d needs to be varied. Specifically, where the two-dimensional diffraction grating 3d is formed by concentric rings, and light of an angle of incidence θ of 0° to 90° is coupled to the light-coupling structure 3, it can be seen from Table 1 that the two-dimensional diffraction grating 3d having a pitch Λ of 0.18 μm or more and 0.56 μm or less, or 0.30 μm or more and 0.56 μm or less, may be provided. By combining together light-coupling structures 3 having such two-dimensional diffraction gratings 3d of different pitches, light can be taken in for every visible wavelength and for every angle of incidence. In this case, the pitch of the two-dimensional diffraction grating 3d may be varied between a plurality of light-coupling structures 3 arranged in two dimensions on a surface parallel to the first principal surface 2p and the second principal surface 2q, or the pitch of the two-dimensional diffraction grating 3d may be varied between a plurality of light-coupling structures 3 arranged together in a direction perpendicular to the first principal surface 2p and the second principal surface 2q, or both of these may be used. Note however that the pitch Λ may be constant within each two-dimensional diffraction grating 3d of the light-coupling structure 3 in order to obtain a sufficient diffraction intensity.
Next, light at end faces 3r and 3s perpendicular to the surfaces 3p and 3q of the light-coupling structure 3 (surfaces extending along the normal direction to the light-transmitting layer 3b) will be discussed. As shown in
For reference,
When the narrow-angle light 5a is incident on the surface 3q of the light-coupling structure 3, if the position of incidence is close to the end face 3s, it is output through the end face 3s as the wide-angle light 5a′ as a result of refraction.
When the narrow-angle light 5a is incident on the end face 3r of the light-coupling structure 3, it is totally reflected by the end face 3r. When the wide-angle light 6a is incident on the end face 3r of the light-coupling structure 3, it is output from the surface 3p as the narrow-angle light 6a′ as a result of refraction, irrespective of the position of incidence. When the wide-angle light 6a is incident on the surface 3q of the light-coupling structure 3, it is totally reflected by the surface 3q.
Thus, for light that is incident on the end faces 3r and 3s of the light-coupling structure 3, the behavior is complicated, and even if wide-angle light is incident on the end face, it is not always output as wide-angle light. However, if the size of the surface (W, L) is set to be sufficiently (e.g., 4 times or more) larger than the size of the end face (a+t+d+b), the influence at the end face will be sufficiently small, and then the transmission or the reflection of light at the surfaces 3p and 3q can be regarded as being the transmission or reflection behavior of light for the entire light-coupling structure 3.
Specifically, if the size of the surface 3p of the first light-transmitting layer 3a and the surface 3q of the second light-transmitting layer 3b is 4 times or more of the thickness of the light-coupling structure 3, it is possible to sufficiently ignore the influence of light at the end faces 3r and 3s of the light-coupling structure 3. Therefore, the light-coupling structures 3 exhibit a function of irreversibly converting narrow-angle light to wide-angle light while maintaining wide-angle light as wide-angle light, and if the density of the light-coupling structures 3 is set to a sufficient density, it is possible to convert all the light incident on the light-trapping sheet 51 to wide-angle light (i.e., light confined within the sheet).
As shown in
The first principal surface 2p of the light-transmitting sheet 2 is located at a position of 2.5 μm from the surface of the first light-transmitting layer 3a. The positions of the first light-transmitting layer 3a, the second light-transmitting layer 3b and the third light-transmitting layer 3c are shifted side to side based on the angle θ so that a plane wave having a polarization at an angle of 45° with respect to the drawing sheet is output from the light source S at an azimuth forming the angle of θ with respect to the normal to the second principal surface 2q, and the center of the incident light passes through the center of the surface of the second light-transmitting layer 3b.
The thickness a of the first light-transmitting layer 3a was set to 0.3 μm, the thickness c of the second light-transmitting layer 3b to 0.3 μm, the thickness t of the third light-transmitting layer 3c to 0.4 μm, the depth d of the two-dimensional diffraction grating to 0.18 μm, and the pitch Λ of the diffraction grating to 0.36 μm. The refractive index of the light-transmitting sheet 2 and the third light-transmitting layer 3c was assumed to be 1.5, and the refractive index of the environmental medium, the first light-transmitting layer 3a and the second light-transmitting layer 3b to be 1.0.
The transmittance was measured while it was stable, and was defined by the ratio of the integrated value of the Poynting vectors passing through the bottom surface (z=0 μm) and the top surface (z≈8 μm) of the analysis area with respect to the integrated value of the Poynting vectors passing through a closed curved surface surrounding the light source.
While there are some calculation results exceeding 100%, it is because of slight errors in the measurement of the Poynting vectors of the light source.
A comparison between the results obtained in a case where the light-coupling structures 3 are present but the depth d of the two-dimensional diffraction grating is d=0 and the results (Nothing) obtained in a case where there is no light-coupling structure shows that the former has a lower transmittance than the latter in a range within the critical angle (41.8°), and they are both substantially zero for angles greater than or equal to that. The reason why the former has a lower transmittance within the critical angle is because light incident on the surface 3q of the second light-transmitting layer 3b is refracted and a portion thereof is output from the end face 3s as wide-angle light, as described above with reference to
Note however that for the former, wide-angle light entering through the end face 3r of the light-coupling structure 3 is refracted through this surface, and is then refracted through the surface 3p of the first light-transmitting layer 3a to be narrow-angle light inside the light-transmitting sheet 2, as described above again with reference to
On the other hand, a comparison between the results for a case where the depth of the grating is d=0.18 μm and the results for a case where d=0 shows that although the transmittance of the former is substantially close to that of the latter, the transmittance drops at positions indicated by arrows a, b, c, d and e.
For the conditions and the angles of incidence shown in
Also for the conditions and the angles of incidence shown in
Note that in
However, in the region above the critical angle, the latter comes to the vicinity of zero whereas the former is substantially floating. This is because light of an angle of incidence above the critical angle diffracts through the two-dimensional diffraction grating of the light-coupling structure 3, and a portion thereof is converted to narrow-angle light in the sheet.
A comparison between analysis results of
Thus, with the light-trapping sheet of the present embodiment, light incident on the first principal surface and the second principal surface of the light-transmitting sheet at various angles becomes narrow-angle light and enters a light-coupling structure arranged inside the light-transmitting sheet, and a portion thereof is converted by the two-dimensional diffraction grating in the light-coupling structure to guided light that propagates inside the third light-transmitting layer and is radiated from the end face of the light-coupling structure to be wide-angle light. By varying the pitch of the two-dimensional diffraction grating between a plurality of light-coupling structures, this conversion can be achieved for every azimuth over a wide wavelength range, e.g., over the entire visible light range.
Since the two-dimensional diffraction grating in the light-coupling structure has an equal period in two or more directions, it is possible to couple with the light-coupling structure with two or more azimuthal angles even for light beams with different azimuthal angles of incidence on the surface of the light-coupling structure, thereby allowing light beams entering the light-trapping sheet from various directions to be more uniformly confined within the light-trapping sheet.
Since the length of the diffraction grating is short, it is possible to reduce the radiation loss of the guided light. Therefore, narrow-angle light present inside the light-transmitting sheet is all converted to wide-angle light by a plurality of light-coupling structures. Since the refractive index of the first and second transmission layers of the light-coupling structure is smaller than the refractive index of the light-transmitting sheet, the wide-angle light is totally reflected by the surface of the light-coupling structure, and the light is repeatedly totally reflected between the surfaces of other light-coupling structures and the surface of the light-transmitting sheet, thus being confined within the light-transmitting sheet.
Thus, the light-coupling structure irreversibly converts narrow-angle light to wide-angle light, while maintaining wide-angle light in the out-of-critical-angle state. Therefore, if the density of the light-coupling structures is set to a sufficient density, it is possible to convert all the light incident on the light-trapping sheet to wide-angle light, i.e., light confined within the sheet.
Note that while the two-dimensional diffraction grating is formed by concentric rings in the present embodiment, it may be a two-dimensional diffraction grating having any other shape as long as it is periodic with an equal period in at least two directions different from each other. For example, the two-dimensional diffraction grating may be formed by concentric elliptical rings. Also in such a case, the two-dimensional diffraction grating is periodic with an equal period at any azimuthal angle φ about the center 5C on a surface parallel to the principal surface of the light-coupling structure. Alternatively, the two-dimensional diffraction grating may have a polygonal shape.
For example, as shown in
The light-trapping sheet 51 can be manufactured by the following method, for example.
In
As shown in
As shown in
As shown in
As shown in
As shown in
Thereafter, the resin sheets 24, 24a, 24′ and 24′a are replaced by the resin sheets 24′ and 24′a of
Resin sheets to be the first area 2a and the second area 2b of the light-transmitting sheet 2 are bonded to the front surface and the reverse surface of the third area 2c of the light-transmitting sheet 2, thereby completing the light-trapping sheet 51 shown in
While an adhesive is used for the bonding between resin sheets in the present embodiment, the surfaces of the resin sheets may be heated so as to weld together the resin sheets, instead of using an adhesive. Anti-reflective nanostructures may be formed in advance on the surface of the resin sheet 24a and the resin sheets to be the first area 2a and the second area 2b.
A second embodiment of a light-trapping sheet according to the present invention will be described. A light-trapping sheet 52 of the present embodiment is different from the light-coupling structure of the first embodiment in terms of the structure at the end face of the light-coupling structure. Therefore, the description hereinbelow will focus on the light-coupling structure of the present embodiment.
A comparison between the results obtained in a case where the light-coupling structures 3′ are present but the depth of the two-dimensional diffraction grating is d=0 and the results (Nothing) obtained in a case where there is no light-coupling structure shows that the former is smaller than the latter in a range within the critical angle (41.8°), and they are both zero for angles greater than or equal to that. The reason why the former is smaller within the critical angle is because light incident on the surface 3q of the second light-transmitting layer 3b is refracted and a portion thereof is output from the right side face (the right side face of the third light-transmitting layer 3c) as wide-angle light, as described above with reference to
On the other hand, a comparison between the results for a case where the depth of the grating is d=0.18 μm and the results for a case where d=0 shows that although the transmittance of the former is substantially close to that of the latter, the transmittance drops at positions indicated by arrows a, b, c, d and e. These positions correspond to conditions under which light is coupled to the guided light.
With any wavelength, the take-out efficiency decreases as d increases (at least for the comparison between d=0 and d=0.18). This represents the light-trapping sheet by a single light-coupling structure, as with the analysis results in the first embodiment. This effect can be accumulated, and by increasing the number of light-coupling structures, it is possible to confine all the light. Note that while this analysis is a result obtained by a calculation model based on a two-dimensional linear diffraction grating, similar effects are obtained for light in the 0 direction and there is always incident light that satisfies Expression 1, which is the coupling condition, for an arbitrary azimuthal angle φ shown in the plan view of
The drops at positions of arrows b, c, d and e are smaller as compared with those of the analysis results of the first embodiment because the length of the grating (coupling length) is made smaller in the analysis model of this embodiment.
A comparison between the results for the model of the second embodiment and the results (Nothing) obtained in a case where there is no light-coupling structure shows that they substantially coincide with each other in both cases within the critical angle (41.8° or less), but the latter is substantially zero and the former substantially floats from zero outside the critical angle (41.8° or more). The former floats outside the critical angle because, as described above with reference to
In contrast, in the analysis results for the model of the second embodiment, the floating outside the critical angle is partially suppressed. This is because the first light-transmitting layer 3a and the second light-transmitting layer 3b account for no area on the end face of the second embodiment, and the refraction at the end face is somewhat suppressed.
Therefore, the second embodiment is a configuration such that the influence at the end face (the phenomenon that wide-angle light is converted to narrow-angle light) can be suppressed more than in the first embodiment, and can be said to be a configuration having a greater light-trapping sheet. Note that in
A third embodiment of a light-trapping sheet according to the present invention will be described. A light-trapping sheet 53 of the present embodiment is different from the light-coupling structure of the second embodiment in terms of the structure at the end face of the light-coupling structure. Therefore, the description hereinbelow will focus on the light-coupling structure of the present embodiment.
A comparison between the results obtained in a case where the light-coupling structures are present but the depth of the grating is d=0 and the results (Nothing) obtained in a case where there is no light-coupling structure shows that the former is smaller than the latter in a range within the critical angle (41.8°), and the latter is zero for angles greater than or equal to the critical angle, whereas floating remains for the former in the range up to 55°. The reason why the former is smaller within the critical angle is because light incident on the surface 3q of the second light-transmitting layer 3b is refracted and a portion thereof is output from the right side face (the right side face of the third light-transmitting layer 3c) as wide-angle light, as described above with reference to
Second, the thickness of the second light-transmitting layer 3b is too small in the outer edge portion, and a portion of light exceeding the critical angle passes into the inside of the light-coupling structure in the form of evanescent light, and this light diffracts through the grating to be narrow-angle light.
On the other hand, a comparison between the results for a case where the depth of the two-dimensional diffraction grating is d=0.18 μm and the results for a case where d=0 shows that although the transmittance of the former is substantially close to that of the latter, the transmittance drops at positions of arrows a, b, c, d and e. These positions correspond to conditions under which light is coupled to the guided light, and the light is guided, after which it is radiated from the end face of the third light-transmitting layer 3c to be wide-angle light. This radiated light falls within the range of about ±35° about a propagation angle of 90° (x-axis direction) (see
In
In contrast, with the results for the model of the third embodiment, the floating is significantly suppressed to be substantially zero in the range where the angle of incidence is 55° or more. This is because the first light-transmitting layer 3a and the second light-transmitting layer 3b account for no area on the end face of the third embodiment, and a component that is supposed to refract through the end face is totally reflected at the sloped surface 3q of the second light-transmitting layer 3b.
Therefore, the third embodiment is a configuration such that the influence at the end face (the phenomenon that wide-angle light is converted to narrow-angle light) can be ignored more than in the first embodiment or the second embodiment, and can be said to be a configuration having a greater light-trapping sheet.
The light-trapping sheet 53 can be manufactured by the following method, for example.
The rectangular minute structures 25B and 25B′ are two-dimensionally arranged also on the surfaces of the molds 25b and 25b′ of
As shown in
As shown in
In this process, the diffraction grating is all buried to disappear in the attached portion, and remains only in the area where the resin sheet 24a is raised. Raising the resin sheet 24a forms an air layer (or a vacuum layer) between the resin sheet 24a and the resin sheet 24. As shown in
As shown in
Thereafter, these attached sheets are bonded together via an adhesive layer therebetween, and the process is repeated, thereby producing the third area 2c of the light-transmitting sheet 2 shown in
An embodiment of a light-receiving device according to the present invention will be described.
A reflective film 11 may be provided on end faces and 2r of the light-trapping sheet 51. The photoelectric conversion section 7 is provided adjacent to the second principal surface 2q of the light-trapping sheet 51. If the light-transmitting sheet 2 has a plurality of end faces, the reflective film 11 may be provided on all of the end faces. In the present embodiment, a portion of the second principal surface 2q and a light-receiving portion of the photoelectric conversion section 7 are in contact with each other. The photoelectric conversion section 7 may be provided in a portion of the first principal surface 2p of the light-trapping sheet 51.
By covering the end faces 2r and 2s of the light-trapping sheet 51 with the reflective film 11, light that has been taken and enclosed in the light-trapping sheet 51 will circulate in the light-trapping sheet 51.
The photoelectric conversion section 7 is a solar cell formed by a silicon. A plurality of photoelectric conversion sections 7 may be attached to one sheet of light-trapping sheet 51. Since the refractive index of silicon is about 5, even if light is made incident perpendicularly on the light-receiving surface of a solar cell, around 40% of the incident light is normally lost through reflection without being taken in the photoelectric conversion section 7. The reflection loss further increases when the light is incident diagonally. Although an AR coat or anti-reflective nanostructures are formed on the surface of a commercially-available solar cell in order to reduce the amount of reflection, a sufficient level of performance has not been achieved. Moreover, a metal layer is present inside the solar cell, and a large portion of light that is reflected by the metal layer is radiated to the outside. With an AR coat or anti-reflective nanostructures, the reflected light is radiated to the outside with a high efficiency.
In contrast, the light-trapping sheet of the present embodiment takes in and encloses light for every visible light wavelength and for every angle of incidence in the light-trapping sheet. Therefore, with the light-receiving device 54, light entering through the first principal surface 2p of the light-trapping sheet 51 is taken into the light-trapping sheet 51 and circulates in the light-trapping sheet 51. Since the refractive index of silicon is larger than the refractive index of the light-transmitting sheet 2, the wide-angle light 5b′ and 6b′ incident on the second principal surface 2q are not totally reflected but portions thereof are transmitted into the photoelectric conversion section 7 as refracted light 5d′ and 6d′ and are converted to electric current in the photoelectric conversion section.
After the reflected wide-angle light 5c′ and 6c′ propagate inside the photoelectric conversion section 7, they enter again and are used in photoelectric conversion until all the enclosed light is gone. Assuming that the refractive index of the transmissive sheet 2 is 1.5, the reflectance of light that is incident perpendicularly on the first principal surface 2p is about 4%, but the reflectance can be suppressed to 1 to 2% or less, taking into account the wavelength dependency and the angle dependency, if an AR coat or anti-reflective nanostructures are formed on the surface thereof. Light other than this enters to be confined within the light-trapping sheet 51, and is used in photoelectric conversion.
With the light-receiving device of the present embodiment, most of the incident light can be confined within the sheet, most of which can be used in photoelectric conversion. Therefore, it is possible to significantly improve the energy conversion efficiency of the photoelectric conversion section. The light-receiving area is determined by the area of a first principal surface p, and all of the light received by this surface enters the photoelectric conversion section 7. Therefore, it is possible to reduce the area of the photoelectric conversion section 7 or reduce the number of photoelectric conversion sections 7, thereby realizing a significant cost reduction of the light-receiving device.
Another embodiment of a light-receiving device of the present invention will be described.
The light-receiving device 55 is different from the light-receiving device 54 of the fourth embodiment in that a protrusion/depression structure 8 is provided on the second principal surface 2q, with a gap between the protrusion/depression structure 8 and the photoelectric conversion section 7. The protrusion/depression structure 8 provided on the second principal surface 2q includes depressed portions and protruding portions whose width is 0.1 μm or more and which may be in a periodic pattern or a random pattern. With the protrusion/depression structure 8, the wide-angle light 5b′ and 6b′ incident on the second principal surface 2q are not totally reflected, and portions thereof travel toward the photoelectric conversion section 7 as output light 5d′ and 6d′ to undergo photoelectric conversion. Light that are reflected by the surface of the photoelectric conversion section 7 are taken inside through the second principal surface 2q of the light-trapping sheet 51 and propagates inside the light-trapping sheet 51, after which the light again travel toward the photoelectric conversion section 7 as the output light 5d′ and 6d′.
Therefore, also with the light-receiving device of the present embodiment, most of the incident light can be confined within the light-trapping sheet, most of which can be used in photoelectric conversion. As in the fourth embodiment, it is possible to reduce the area of the photoelectric conversion section 7 or reduce the number of photoelectric conversion sections 7. Therefore, it is possible to realize a light-receiving device having a significantly improved energy conversion efficiency and being capable of cost reduction.
Another embodiment of a light-receiving device of the present invention will described.
The light-receiving device 56 is different from the light-receiving device 54 of the fourth embodiment in that the prism sheet 9 is provided between the second principal surface 2q and the photoelectric conversion section 7. Tetrahedron prisms 10 are arranged adjacent to one another inside the prism sheet 9. The prism sheet 9 may be formed by layering together two triangular prism array sheets orthogonal to each other. Since the refractive index of the prism 10 is set to be larger than the refractive index of the prism sheet 9, the wide-angle light 5b′ and 6b′ incident on the surface of the prism sheet 9 are refracted by the prism surface to be 5d′ and 6d′ and travel toward the photoelectric conversion section 7. Since the angle of incidence of light to the photoelectric conversion section 7 is close to perpendicular, it is possible to reduce the reflection at the light-receiving surface of the photoelectric conversion section 7 and to reduce the number of light circulations within the light-trapping sheet 51 as compared with the fourth embodiment.
Also with the light-receiving device of the present embodiment, most of the incident light can be confined within the light-trapping sheet, most of which can be used in photoelectric conversion. As in the fourth embodiment, it is possible to reduce the area of the photoelectric conversion section 7 or reduce the number of photoelectric conversion sections 7. Therefore, it is possible to realize a light-receiving device having a significantly improved energy conversion efficiency and being capable of cost reduction. Since the number of light circulations within the sheet is smaller than the fourth embodiment, it is less influenced by the light-trapping capacity of the light-trapping sheet.
Another embodiment of a light-receiving device of the present invention will be described.
The light-receiving device 57 is different from the light-receiving device 54 of the fourth embodiment in that the end faces 2s and 2r are covered by the photoelectric conversion section 7 instead of the reflective film 11. If the light-transmitting sheet 2 has a plurality of end faces, the photoelectric conversion section 7 may be provided on all of the end faces. In the present embodiment, the fourth area 2h may be absent in the light-trapping sheet 51.
When the photoelectric conversion section 7 is provided on the end faces 2s and 2r, the wide-angle light 5c, 6c, 5c′ and 6c′ enter the photoelectric conversion section 7 along the normal to the light-receiving surface of the photoelectric conversion section 7, as opposed to the fourth embodiment. Therefore, there is less reflection at the surface of the photoelectric conversion section 7, and it is possible to reduce the number of light circulations within the light-trapping sheet 51.
Also with the light-receiving device of the present embodiment, most of the incident light can be confined within the light-trapping sheet, most of which can be used in photoelectric conversion. Therefore, it is possible to realize a light-receiving device having a significantly improved energy conversion efficiency. Since the area of the photoelectric conversion section 7 can be reduced as compared with the fourth embodiment, it is possible to significantly reduce the cost. Since the number of light circulations within the sheet is smaller than the fourth embodiment, it is less influenced by the light-trapping capacity of the light-trapping sheet.
Another embodiment of a light-receiving device of the present invention will be described.
The light-receiving device 58 is different from the fourth embodiment in that the attachment is such that the end face 2s of the light-trapping sheet 51 is in contact with the first principal surface 2p of the light-receiving device 54 of the fourth embodiment. The light-trapping sheet 51′ may be attached orthogonal to the light-trapping sheet 51. In the light-trapping sheet 51′, the reflective film 11 may be provided on the end face 2r, and a reflective film 11′ may be provided on a first principal surface 2p′ and a second principal surface 2q′ in the vicinity of the end face 2s which is attached to the light-trapping sheet 51. The reflective film 11′ serves to reflect the light 6b so as to prevent the wide-angle light 6b from the light-trapping sheet 51 from leaking out of the light-trapping sheet 51′.
The light 4 incident on the first principal surface 2p of the light-trapping sheet 51 is taken into the light-trapping sheet 51. On the other hand, light 4′ incident on the first principal surface 2p′ and the second principal surface 2q′ of the light-trapping sheet 51′ is taken into the light-trapping sheet 51′. Light taken into the light-trapping sheet 51′ becomes guided light 12 propagating toward the end face 2s, since the end face 2r is covered by the reflective film 11, and merges with the light inside the light-trapping sheet 51. Since a portion of the second principal surface 2q in the light-trapping sheet 51 is in contact with the surface of the photoelectric conversion section 7, and the refractive index of silicon is larger than the refractive index of the light-transmitting sheet 2, the wide-angle light 5b′ and 6b′ incident on the second principal surface 2q are not totally reflected but portions thereof are incident on the photoelectric conversion section 7 as the refracted light 5d′ and 6d′ and are converted to electric current in the photoelectric conversion section 7. The reflected wide-angle light 5c′ and 6c′ propagate inside the light-trapping sheet 51, are incident again on the light-receiving surface of the photoelectric conversion section 7, and are used in photoelectric conversion until the enclosed light is mostly gone.
Since the light-receiving device of the present embodiment includes the light-trapping sheet 51′ perpendicular to the light-receiving surface of the photoelectric conversion section 7, even light that is incident diagonally on the first principal surface 2p of the light-trapping sheet 51 is incident, at an angle close to perpendicular, on the first principal surface 2p′ and the second principal surface 2q′ of the light-trapping sheet 51′. This makes it easier to take in light of every azimuth.
Also with the light-receiving device of the present embodiment, most of the incident light can be confined within the light-trapping sheet, most of which can be used in photoelectric conversion. As in the fourth embodiment, it is possible to reduce the area of the photoelectric conversion section 7 or reduce the number of photoelectric conversion sections 7. Therefore, it is possible to realize a light-receiving device having a significantly improved energy conversion efficiency and being capable of cost reduction.
An embodiment of a lighting plate according to the present invention will be described.
The protrusion/depression structure 8 is formed on a portion of the first principal surface 2p, forms a random pattern of depressed portions and protruding portions whose width is 0.1 μm or more. Light taken into the light-trapping sheet 51 propagates inside the light-trapping sheet 51, and portions of the propagating light are radiated outside as the output light 5d′ and 6d′ by the protrusion/depression structure 8.
The lighting plate 59 is provided on a window for lighting of a building such as a house so that the first principal surface 2p with the protrusion/depression structure 8 provided thereon is facing the room side. During the day, the lighting plate 59 takes in the light of the sun 13a through the second principal surface 2q, and radiates it into the room through the protrusion/depression structure 8. Thus, it can be used as an indoor lighting in which light is radiated from the protrusion/depression structure 8. During the night, the lighting plate 59 takes in light from an indoor lighting 13b through the first principal surface 2p, and radiates the light through the protrusion/depression structure 8. Thus, the lighting plate 59 can be used as an auxiliary to an indoor lighting. Thus, with the lighting plate of the present embodiment, it is possible to confine most of the incident light within the sheet, and reuse it as a lighting, thereby realizing an efficient use of energy.
An embodiment of a light-emitting device according to the present invention will be described.
The light source 14, such as an LED, is provided adjacent to one of the first principal surface 2p and the second principal surface 2q of the light-trapping sheet 51, with the protrusion/depression structure 8 provided on the other. In the present embodiment, the light source 14 is provided adjacent to the first principal surface 2p, and the protrusion/depression structure 8 is provided on the second principal surface 2q. The reflective film 11 is provided on the end faces 2s and 2r of the light-trapping sheet 51. The protrusion/depression structure 8 includes depressed portions and protruding portions whose width is 0.1 μm or more and which may be in a periodic pattern or a random pattern.
The prism sheet 9 is arranged with a gap from the second principal surface 2q so as to oppose the protrusion/depression structure 8. The tetrahedron prisms 10 are arranged adjacent to one another inside the prism sheet 9. The prism sheet 9 may be formed by layering together two triangular prism array sheets orthogonal to each other.
The light 4 output from the light source 14 is taken in through the first principal surface 2p of the light-trapping sheet 51 to be the light 12 that propagates inside the light-trapping sheet 51. Portions of this light are radiated outside as the output light 5d′ and 6d′ by the protrusion/depression structure 8. The radiated light is condensed through the prisms 10 inside the prism sheet 9 to be light 4a having a substantially parallel wave front.
With the light-emitting device of the present embodiment, it is possible, with a simple and thin configuration, to confine light output from a point light source into a light-trapping sheet, and take out the light as a surface light source.
An embodiment of a light-trapping rod according to the present invention will be described.
The light-transmitting rod 2′ has a circular or elliptical cross-sectional shape on a plane that is perpendicular to the central axis C. The light-transmitting rod 2′ is formed by a transparent material that transmits therethrough light of a desired wavelength or light within a desired wavelength range determined according to the application, as in the first embodiment.
Where the cross section of the light-transmitting rod 2′ is circular, the diameter D of the light-transmitting rod 2′ on a cross section perpendicular to the central axis C is about 0.05 mm to 2 mm, for example. One or more light-coupling structures 3 are provided at a distance of d3 or more from a surface 2u, which is the principal surface of the light-transmitting rod 2′, in the direction toward the central axis C. The light-trapping rod 61 includes a plurality of coupling structures 3. The light-transmitting rod 2′ has a circular cross-sectional shape, and the light-coupling structures 3 are arranged within a core region 2A that has a circular shape having a diameter of d=D−2×d3 centered about the central axis C on a plane that is perpendicular to the central axis C of the light-transmitting rod 2′ and that is extending along the central axis C direction.
The light-coupling structures 3 are arranged within the core region 2A at a predetermined density in the axial direction, the radial direction and the circumferential direction. The density at which the light-coupling structures 3 are arranged is, for example, 10 to 103 per 1 mm in the axial direction, 10 to 103 per 1 mm in the radial direction, and 10 to 103 per 1 mm in the circumferential direction. The cross-sectional shape of the core region is circular or elliptical, and may be a shape with two or more rings.
The light-coupling structures 3 have the same structure as that of the light-coupling structures 3 of the first embodiment. The light-trapping rod 61 may include the light-coupling structures 3′ of the second embodiment or the light-coupling structures 3″ of the third embodiment, instead of the light-coupling structures 3.
The light-coupling structures 3 is arranged within the core region 2A so that the diffraction grating of the third light-transmitting layer 3c is parallel to the central axis C of the light-transmitting rod 2′. The length L of the light-coupling structure 3 in the central axis C direction is 3 μm to 100 μm, and the length W thereof in the direction orthogonal thereto is about ⅓ to 1/10 of L.
In
First, light vectors on a cross section parallel to the central axis C of the light-transmitting rod 2′ will be discussed. In this cross section, a portion of the narrow-angle light 5a inside the light-transmitting rod 2′ is converted by a light-coupling structure 3 to the wide-angle light 5b, and this light is totally reflected by the surface 2u to be the wide-angle light 5c which stays inside the light-transmitting rod 2′.
A portion of the remaining narrow-angle light 5a′ of the narrow-angle light 5a is converted by another light-coupling structure 3 to the wide-angle light 5b′, and this light is totally reflected by the surface 2u to be the wide-angle light 5c′ which stays inside the rod.
In this manner, all of the narrow-angle light 5a is converted to the wide-angle light 5b or 5b′ within the core region 2A where the light-coupling structures 3 are provided. On the other hand, a portion of the wide-angle light 6a inside the light-transmitting rod 2′ is totally reflected by the surface of a light-coupling structure 3 to be the wide-angle light 6b, and this light is totally reflected by the surface 2u to be the wide-angle light 6c which stays within the rod. A portion of the remaining light of the narrow-angle light 6a passes through the core region 2A where the light-coupling structures 3 are provided, and this wide-angle light 6b′ is totally reflected at the surface 2u to be the wide-angle light 6c′ which stays within the light-transmitting rod 2′. Although not shown in the figure, there is also wide-angle light that stays within the sheet while being totally reflected between different light-coupling structures 3 and between the surfaces 2u.
As described above with reference to
Next, light vectors on a cross section orthogonal to the central axis of the rod will be discussed. On this cross section, light entering inside the rod are classified into three types. These are light 15a passing through the core region 2A, light 15b passing through the outer edge of the core region 2A, and light 15c passing through the outside of the core region 2A. The light 15a is converted to wide-angle light which stays within the rod on the cross section along the central axis of the rod as described above. On the other hand, the light 15b is light that is incident at an angle of ψ on the surface 2u of the rod, where ψ satisfies Expression 3.
Naturally, the angle of incidence of the light 15c on the surface 2u is greater than ψ. Therefore, if Expression 4 holds true, the light 15b is totally reflected by the first principal surface 2p of the rod, and the light 15b and 15c become wide-angle light which stays within the light-transmitting rod 2′ on the cross section orthogonal to the central axis.
Therefore, satisfying Expression 4 for both the cross section parallel to the central axis C of the light-transmitting rod 2′ and the cross section orthogonal thereto is the condition for all the light inside the light-transmitting rod 2′ to stay within the light-transmitting rod 2′.
An embodiment of a light-emitting device according to the present invention will be described.
The reflective film 11 is provided on the end face 2r of the light-trapping rod 61. A tapered portion 2v is provided on the surface 2u of the light-trapping rod 61 on the side of the end face 2s, and a waveguide 18 having a smaller diameter than the light-transmitting rod 2 is connected thereto.
The light sources 14R, 14G and 14B are formed by LDs and LEDs, and output red, green and blue light, respectively, for example. Light output from these light sources are condensed through lenses to radiate light 4R, 4G and 4B toward the surface 2u of the light-transmitting rod 2′. These light are confined inside the light-transmitting rod 2′ by the light-coupling structures 3 in the core region 2A, and since the end face 2r is covered by the reflective film 11, it as a whole becomes the guided light 12 which propagates in one direction inside the rod. The guided light 12 is narrowed with no loss through the tapered portion 2v over which the diameter of the rod 2′ decreases gradually, and it becomes guided light which propagates inside the waveguide 18 having a narrow diameter. Thus, the light 19, which is close to a point light source, is output from the end face of the waveguide 18.
Where the light sources are lasers, the light 4R, 4G and 4B are coherent light, but since the light are radiated from the individual light-coupling structures 3 in varied phases, the guided light 12 obtained by synthesizing the radiated light together will be incoherent light. Therefore, the output light 19 is also incoherent light. By adjusting the light amounts of the light 4R, 4G and 4B, the output light 19 can be made white light. At present, red and blue semiconductor lasers have been realized, and a green laser is also available by using SHG. Synthesizing white light from these light sources typically requires a complicated optical configuration, and results in glaring light due to the coherence characteristic of laser light. However, with the light-emitting device 62 of the present embodiment, it is possible to provide a more natural, white-light point light source with no glare with a very simple configuration.
In the case of the present embodiment, what needs an adjustment is the positional adjustment between the convergent light formed by the incident light 4R, 4G and 4B and the rod 2′.
Another embodiment of a light-emitting device according to the present invention will be described. FIG. schematically shows a cross-sectional structure of a light-emitting device 63 of the present embodiment. The light-emitting device 63 includes the light-trapping rod 61, the light source 14, and the prism sheet 9. The light-trapping rod 61 has such a structure as described above in the eleventh embodiment.
The reflective film 11 is provided on the end face 2r of the light-trapping rod 61. A portion of the light-trapping rod 61 where the light-coupling structures 3 are absent functions as the waveguide 18. The prism sheet 9 is provided on the surface 2u of the waveguide 18.
The light source 14 is formed by an LD, an LED, or the like, and emits visible light. The light output from the light source is condensed through a lens to be the light 4 passing through the light-transmitting rod 2′. These light are confined inside the light-transmitting rod 2′ by the light-coupling structures 3 in the core region 2A, and since one of the end faces is covered by the reflective film 11, it as a whole becomes the light 12 which propagates in one direction inside the light-transmitting rod 2′, and becomes guided light which propagates inside the waveguide 18.
The prism sheet 9 is provided in contact with the waveguide 18. The tetrahedron prisms 10 are arranged adjacent to one another inside the prism sheet 9. It may be formed by triangular prism array sheets orthogonal to each other that are bonded together. Since the refractive index of the prism 10 is larger than the refractive index of the prism sheet 9, light leaking out of the waveguide 18 to be incident on the prism sheet 9 refracts and is output from the prism sheet 9 to be the parallel output light 19. Note that the prism sheet 9 may be separated from the waveguide 18, in which case a protrusion/depression structure is formed on one side of the surface of the waveguide 18 that is opposing the prism sheet 9 for outputting light therethrough. Where the light source is a laser, the light 4 is coherent light, but since the light are radiated from the individual light-coupling structures 3 in varied phases, the guided light 12 obtained by synthesizing the radiated light together will be incoherent light. Therefore, the output light 19 is also incoherent light. At present, red and blue semiconductor lasers have been realized, and a green laser is also available by using SHG. Using these light sources, red, green and blue linear light sources are obtained. For example, by bundling together these linear light sources, it is possible to provide a color backlight for a liquid crystal display with a very simple configuration.
Sheets and rods according to one aspect of the present invention are capable of taking in light over a wide area, and over a wide wavelength range (e.g., the entire visible light range) for every angle of incidence; therefore, light-receiving devices using the same are useful as high-conversion-efficiency solar cells, or the like, and light-receiving and light-emitting devices using the same provide a new form of a lighting or a light source, and are useful as a recycle lighting using the sunlight or light from a lighting, a high-efficiency backlight, and an incoherent white light source.
While the present invention has been described with respect to preferred embodiments thereof, it will be apparent to those skilled in the art that the disclosed invention may be modified in numerous ways and may assume many embodiments other than those specifically described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention that fall within the true spirit and scope of the invention.
Number | Date | Country | Kind |
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2011-260611 | Nov 2011 | JP | national |
This is a continuation of International Application No. PCT/JP2012/007608, with an international filing date of Nov. 28, 2012, which claims priority of Japanese Patent Application No. 2011-260611, filed on Nov. 29, 2011, the contents of which are hereby incorporated by reference.
Number | Date | Country | |
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Parent | PCT/JP2012/007608 | Nov 2012 | US |
Child | 14013727 | US |