Tunable short- to mid-infrared perfectly absorbing thin films utilizing conductive zinc oxide on metal

Abstract

A simple thin-film perfect absorber structure is shown that can achieve greater than 99.9% absorption and is tunable throughout the short-wave and mid-wave infrared. This is attained by use of the tunable mobility and carrier concentration, which in turn tunes the complex refractive indices, of a gallium-doped zinc oxide (GZO) thin film, and by choice of the GZO film thickness. The structure takes advantage of a metal substrate with large k, i.e. is opaque, with silver shown to be one suitable choice. The metal layer supporting GZO can be deposited on any practical substrate. An experimental deposited GZO film underwent subsequent etch steps and demonstrated 99% absorption at a wavelength of 2.1 μm. Finally, designs are shown that enable near perfect absorption in the range of 1.5-4.7 μm, with similar structures also likely possible extending beyond this wavelength range by further tailoring the GZO optical parameters and layer thickness. The presented structure, which is polarization insensitive at normal and near-normal incidence, has potential applications in reflection band filters, infrared scene generators, photodetectors and photovoltaics.