Ultrathin 2 nm gold as ideal impedance-matched absorber for infrared light

TL;DR

This paper demonstrates that a 2 nm ultrathin gold film, combined with a copper oxide seed layer, acts as an almost wavelength-independent, impedance-matched IR absorber with high stability, significantly enhancing thermal detector performance.

Contribution

The study introduces a 2 nm ultrathin gold film as an effective, stable, and impedance-matched IR absorber, outperforming thicker existing films for thermal detector applications.

Findings

Achieved 47% absorptivity from 2 to 20 μm wavelength range.

Ultrathin gold film maintains long-term stability as an IR absorber.

Significantly improves thermal detector efficiency with minimal thermal mass.

Abstract

Thermal detectors are a cornerstone of infrared (IR) and terahertz (THz) technology due to their broad spectral range. These detectors call for suitable broad spectral absorbers with minimalthermal mass. Often this is realized by plasmonic absorbers, which ensure a high absorptivity butonly for a narrow spectral band. Alternativly, a common approach is based on impedance-matching the sheet resistance of a thin metallic film to half the free-space impedance. Thereby, it is possible to achieve a wavelength-independent absorptivity of up to 50 %, depending on the dielectric properties of the underlying substrate. However, existing absorber films typicallyrequire a thickness of the order of tens of nanometers, such as titanium nitride (14 nm), whichcan significantly deteriorate the response of a thermal transducers. Here, we present the application of ultrathin gold (2 nm) on top of a 1.2 nm copper oxide seed layer as an effective IR absorber. An almost wavelength-independent and long-time stable absorptivity of 47(3) %, ranging from 2 $\mu$m to 20 $\mu$m, could be obtained and is further discussed. The presented gold thin-film represents analmost ideal impedance-matched IR absorber that allows a significant improvement of state-of-the-art thermal detector technology.