Efficient light-trapping sheet for the entire visible spectrum by using stacked concentric grating couplers

We confirmed the effectiveness of light-trapping by forming concentric grating couplers (CGCs) inside a transparent sheet. A portion of the light incident to the CGCs is input-coupled and converted to the guided mode and is radiated from neighboring CGCs at a wider angle than the incident angle or even over the critical angle for both surfaces of the sheet. We demonstrated that three stacked sheets with multi-layered CGCs can trap an average of 19%–29% of the incident light at angles of 0° to ±70° in the visible spectrum.