Light acquisition sheet and rod, and light receiving device and light emitting device each using the light acquisition sheet or rod

1. A light-trapping sheet comprising:

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, the inner portion being located 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 arranged therebetween;

refractive indecies of the first and second light-transmitting layers are smaller than a refractive index of the light-transmitting sheet;

a refractive index of the third light-transmitting layer is larger than the refractive indecies 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 plurality of light-coupling structures include a first light-coupling structure and a second light-coupling structure arranged next to each other on a surface parallel to the first and second principal surfaces, and the first light-transmitting layer of the first light-coupling structure is spaced apart from the first light-transmitting layer of the second light-coupling structure and the second light-transmitting layer of the first light-coupling structure is spaced apart from the second light-transmitting layer of the second light-coupling structure.

2. The light-trapping sheet of claim 1 , wherein the plurality of light-coupling structures are arranged three-dimensionally in an inner portion of the light-transmitting sheet at the first distance or more and the second distance or more from the first and second principal surfaces, respectively.

3. The light-trapping sheet of claim 2 , wherein:

a pitch of the diffraction grating is 0.1 μm or more and 3 μm or less;

surfaces of the first and second light-transmitting layers are each sized so as to circumscribe a circle having a diameter of 100 μm or less; and

the plurality of light-coupling structures each have a thickness of 3 μm or less.

4. The light-trapping sheet of claim 3 , wherein at least two of the plurality of light-coupling structures are different from each other in terms of a direction in which the diffraction grating extends.

5. The light-trapping sheet of claim 3 , wherein at least two of the plurality of light-coupling structures are different from each other in terms of a pitch of the diffraction grating.

6. The light-trapping sheet of claim 1 , wherein:

the light-transmitting sheet includes:

a first region being in contact with the first principal surface and having a thickness equal to the first distance;

a second region being in contact with the second principal surface and having a thickness equal to the second distance;

a third region arranged between the first and second regions; and

at least one fourth region provided in the third region and connecting the first region and the second region to each other;

the plurality of light-coupling structures are arranged only in the third region excluding the at least one fourth region; and

an arbitrary straight line passing through the fourth region is 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.

7. The light-trapping sheet of claim 1 , wherein in at least one of the plurality of light-coupling structures, thicknesses of the first and second light-transmitting layers are decreased toward an outer edge side away from a center of the light-coupling structure.

8. The light-trapping sheet of claim 1 , wherein 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 is 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.

9. A light-trapping rod comprising:

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, the inner portion being located at a first distance or more from the principal surface, 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 arranged therebetween;

refractive indecies of the first and second light-transmitting layers are smaller than a refractive index of the light-transmitting rod;

a refractive index of the third light-transmitting layer is larger than the refractive indecies 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 plurality of light-coupling structures include a first light-coupling structure and a second light-coupling structure arranged next to each other on a surface parallel to the first and second principal surfaces, and the first light-transmitting layer of the first light-coupling structure is spaced apart from the first light-transmitting layer of the second light-coupling structure and the second light-transmitting layer of the first light-coupling structure is spaced apart from the second light-transmitting layer of the second light-coupling structure.

10. The light-trapping rod of claim 9 , wherein the plurality of light-coupling structures are each arranged three-dimensionally in an inner portion of the light-transmitting rod at the first distance or more from the principal surface.

11. The light-trapping rod of claim 10 , wherein:

a pitch of the diffraction grating is 0.1 μm or more and 3 μm or less;

surfaces of the first and second light-transmitting layers are each sized so as to circumscribe a circle having a diameter of 100 μm or less; and

the light-coupling structures each have a thickness of 3 μm or less.

12. The light-trapping rod of claim 11 , wherein at least two of the plurality of light-coupling structures are different from each other in terms of a direction in which the diffraction grating extends.

13. The light-trapping rod of claim 11 , wherein at least two of the plurality of light-coupling structures are different from each other in terms of a pitch of the diffraction grating.

14. The light-trapping rod of claim 9 , wherein 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 is 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.

15. A light-receiving device comprising:

a light-trapping sheet of claim 1 ;

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.

16. The light-receiving device of claim 15 , further comprising:

another light-trapping sheet of claim 1 , wherein:

the photoelectric conversion section is provided on the first principal surface of the light-trapping sheet; and

an end face of the other light-trapping sheet is connected to the second principal surface of the light-trapping sheet.

17. A light-receiving device comprising:

a light-trapping sheet of claim 1 ; and

a protrusion/depression structure or a prism sheet provided on at least a portion of the first principal surface or the second principal surface of the light-trapping sheet.

18. A light-emitting device comprising:

a light-trapping sheet of claim 1 ;

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.

19. A light-emitting device comprising:

a light-trapping rod of claim 9 ; and

at least one light source provided adjacent to the first principal surface of the light-transmitting rod.

20. The light-emitting device of claim 19 , further comprising a prism sheet or a protrusion/depression structure provided on a portion of the first principal surface of the light-transmitting rod.