Nanorectenna spectrally-selective plasmonic hot electron response to visible-light lasers.

Active metasurfaces with novel visible and infrared (vis/IR) functionalities represent an exciting, growing area of research. Rectification of vis/IR frequencies would produce needed direct current (DC) with no inherent frequency limitation (e.g. no semiconducting bandgap). However, controlling the materials and functionality of (nano)rectennas for rectifying 100 s of THz to the visible regime is a daunting challenge, because of the small features and simultaneously the need to scale up to large sizes in a scalable platform. An active metasurface of a planar array of nanoscale antennas on top of rectifying vertical diodes is a 'nanorectenna array' or 'microrectenna array' that rectifies very high frequencies in the infrared, or even higher frequencies up to the visible regime. We employ a novel strategy for forming optical nanorectenna arrays using scalable patterning of Au nanowires, demonstrate strong evidence for spectral-selective high-frequency rectification, characteristic of optical antennas. We discover a previously unreported out-of-equilibrium electron energy distribution, i.e. hot electrons arising from plasmonic resonance absorption in an optical antenna characterized by an effective temperature, and how this effect can significantly impact the observed rectification.

Enhanced coupling of broadband light into amorphous silicon via periodic nanoplasmonic arrays.

Achieving enhanced coupling of solar radiation over the full range of the silicon absorption spectrum up to the bandgap is essential for increased efficiency of solar cells, especially thin film versions. While many designs for enhancing trapping of radiation have been explored, detailed measurements of light scattering inside silicon cells is still lacking. Here, we demonstrate experimentally and computationally that plasmonic-assisted localized and traveling modes can efficiently couple red and infrared radiation into ultrathin amorphous silicon (a-Si) layers. Utilizing patterned periodic arrays of aluminum nanostructures on thin a-Si, we perform specular and diffuse reflectivity and transmission measurements over a broad spectrum. Based on these results, we are able to separate parasitic absorption in aluminum plasmonic arrays from enhanced light absorption in the 200 nm thick amorphous silicon layer, as compared to a blank silicon layer. We discover a very efficient near-infrared a-Si absorption mechanism that occurs at the transition from the radiative to evanescent diffractive coupling, analogous to earlier surface-enhanced infrared studies. These results represent a direct demonstration of enhanced radiation coupling into silicon due to large angle scattering and show a path forward to improved ultrathin solar cell efficiency.