Experimental Demonstration\nof Improvement in Near-Infrared\nPhotodetection Efficiency by Plasmonic Diffraction

Near-infrared\n(NIR) photodetectors are crucial to various\napplications,\nincluding face recognition, night vision, and laser detection and\nranging (LiDAR). However, conventional silicon (Si)-based photodetectors\nexhibit poor sensitivity in the NIR region (λ = 750–1060\nnm) because of low photoabsorption within the photoabsorption layer.\nTo overcome this limitation, we proposed a plasmonic diffraction approach\nthat can improve photosensitivity in the NIR regime by properly designing\nthe period of the metal nanograting required to diffract the incident\nlight at large angles in Si, thereby extending the effective propagation\nlength in the photoabsorption layer. In addition, the metal nanograting\ncan transmit the specific wavelength and polarization while enhancing\nphotosensitivity through optimized geometric design. It can be highly\nadvantageous for active sensing applications such as LiDAR, which\noffers the distinction between signal and noise by selectively transmitting\nspecific wavelengths and polarizations. However, the effectiveness\nof plasmonic diffraction has never been experimentally demonstrated\nbecause it requires the fabrication of a metal nanograting structure\nwith a fine gap. In this study, we successfully fabricated a gold\nnanograting array on a photodetector and demonstrated a significant\nimprovement (1.79×) in its photosensitivity while employing it\nas a bandpass filter as well as a polarization filter, even with a\nsingle thin gold layer. In addition, providing highly reflective trenches\nat the border of each pixel will allow the diffracted light to be\nconfined within the pixel, leading to the expansion of image sensors\nwhile increasing photosensitivity. This breakthrough will usher in\nfurther advances in nanophotonic devices that will enable the development\nof active sensing technologies with high signal-to-noise ratios.