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 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.

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

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

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

the plurality of light-coupling structures 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 are spaced apart from one another between the first light-coupling structure and the second light-coupling structure.

5 . The light-trapping sheet of claim 4 , wherein:

the light-transmitting sheet and the third light-transmitting layer of the plurality of light-coupling structures are 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 are continuous with each other via a portion of the light-transmitting sheet therebetween.

6 . The light-trapping sheet of claim 5 , wherein a pitch of the diffraction structure 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.

7 . The light-trapping sheet of claim 6 , wherein in the plurality of light-coupling structures, the two-dimensional diffraction grating is formed by concentric or concentric elliptical rings.

8 . 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 pitch of the two-dimensional diffraction grating.

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

the light-transmitting sheet includes:

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 are arranged only in the third area excluding the at least one fourth area; and

an arbitrary straight line passing through the fourth area 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.

10 . 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.

11 . The light-trapping sheet of claim 1 , wherein 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 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.

12 . 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 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 two-dimensional diffraction grating parallel to a central axis of the light-transmitting rod.

13 . The light-trapping rod of claim 12 , 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.

14 . The light-trapping rod of claim 13 , 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.

15 . The light-trapping rod of claim 14 , wherein in the plurality of light-coupling structures, the two-dimensional diffraction grating is formed by concentric or concentric elliptical rings.

16 . The light-trapping rod of claim 14 , wherein at least two of the plurality of light-coupling structures are different from each other in terms of a pitch of the two-dimensional diffraction grating.

17 . The light-trapping rod of claim 12 , 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.

18 . 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.

19 . A light-receiving 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.

20 . A light-emitting device comprising:

a light-trapping rod of claim 12 ; and

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