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

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.