Planar Dual-Layer System for Ultra-Broadband Absorption and Hot-Carrier Photodetection in Longwave Near-Infrared band

Ultra-broadband and perfect optical absorption is usually realized by introducing metallic nanostructures, which however have stringent requirements on fabrication. It would be highly useful if this can be realized by planar systems, which do not normally have such a capability in manipulating the light. Here, we theoretically deduce the critical optical coupling conditions in a simple planar dual-layer system consisted of a lossless dielectric film on an absorptive substrate to enable the desired broadband and strong optical absorption. We numerically predict and experimentally observe an ultra-broadband, strong, polarization-insensitive, and wide-angle absorption across the longwave near-infrared (LW-NIR, 1.1 – 2.5 μm) band from the planar dual-layer system. The application of the dual-layer system in broadband hot-carrier photodetection is further explored from both theory (by combining electromagnetic simulations, first-principles calculations and Monte Carlo approach) and experiment perspectives. Results show that the responsivity and detectivity of the planar device can even be an order of magnitude higher than those based on the conventional metallic nanostructures. Such a simple planar system shows great potentials in large-area and lithography-free thermo-photovoltaics, photodetections, thermal emitters, etc .