Light-trapping sheet and rod, and light-receiving device and light-emitting device using the same

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 at a first and second distance 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 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; and

the light-trapping sheet further comprises a transparent cover sheet opposing at least one of the first and second principal surfaces of the light-transmitting sheet with a gap interposed therebetween.

2. The light-trapping sheet of claim 1 , wherein surfaces of the first and second light-transmitting layers that are located opposite to the third light-transmitting layer are parallel to the first and second principal surfaces of the light-transmitting sheets.

3. The light-trapping sheet of claim 1 , wherein the third light-transmitting layer of the first light-coupling structure and the third light-transmitting layer of the second light-coupling structure are continuous with each other.

4. 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 a first distance or more and a second distance or more from the first and second principal surfaces, respectively.

5. The light-trapping sheet of claim 1 , wherein a material having a lower refractive index than a refractive index of the light-transmitting sheet is arranged in the gap.

6. The light-trapping sheet of claim 1 , wherein a pitch of the diffraction grating is 0.1 μm or more and 3 μm or less, and

wherein 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.

7. The light-trapping sheet of claim 6 , 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, and

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.

8. The light-trapping rod comprising:

a light-transmitting rod having a principal surface and a circular or elliptical cross section; and

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:

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 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 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 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; and

the light-trapping rod further comprises a transparent cover sheet opposing the principal surface of the light-transmitting rod with a gap interposed therebetween.

9. The light-trapping rod of claim 8 , 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.

10. The light-trapping rod of claim 8 , wherein a material having a lower refractive index than a refractive index of the light-transmitting sheet is arranged in the gap.

11. The light-trapping rod of claim 8 , wherein a pitch of the diffraction grating is 0.1 μm or more and 3 μm or less, and

wherein 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, and

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.

13. A light-receiving device comprising: a light-trapping sheet of claim 1 ; 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.

14. The light-receiving device of claim 13 , 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.

15. A light-receiving device comprising: a light-trapping sheet of claim 1 ; 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.

16. A light-receiving device comprising: a light-trapping sheet of claim 1 ; and a protrusion/depression structure provided on a portion of the first principal surface or the second principal surface of the light-trapping sheet.

17. 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.

18. A light-emitting device comprising: a light-trapping rod of claim 8 ; and at least one light source provided adjacent to the first principal surface of the light-transmitting rod.

19. The light-emitting device of claim 18 , wherein further comprising a prism sheet or a protrusion/depression structure provided on a portion of the first principal surface of the light-transmitting rod.