Asymmetric Light Absorption and Reflection in Freestanding Nanostructured Metallic Membranes

Resonant light absorption in metallic nanostructures results from highly localized electric fields that are crucial for many processes such as Raman scattering, photoluminescence, hot-carrier creation, and photovoltaics. In recent years, there have been a substantial amount of studies related to the design and realization of resonant optical absorbers from microwave to optical frequencies. However, there has been little thought that went into investigation of direction-dependent absorption and reflection characteristics of optical materials. In this study, we introduce and realize an absorber that is capable of absorbing light asymmetrically depending on the illumination direction. By designing an asymmetrical dielectric permittivity profile along the propagation direction, we have been able to control the optical resonance strength of the absorber, resulting in asymmetric light absorption and reflection at resonance wavelengths. The proposed structure consists of a square hole lattice etched into a free-standing Si3N4/Ag bilayer of total thickness 200 nm. A 9-fold front to back absorption contrast was measured with the fabricated structures, whereas a 13-fold contrast was achieved in finite difference time domain simulations. Moreover, mode profiles of observed resonances were discussed in detail to understand the physical mechanism of the asymmetric light absorption. Our analyses indicate that the same resonance modes were excited regardless of the illumination direction; however the coupling strength is greatly reduced when the structure is illuminated from the dielectric side. Asymmetric light absorbers and reflectors introduced here could find applications in energy-harvesting applications, particularly for photovoltaics and thermophotovoltaics.