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

Thermal detectors are a cornerstone of infrared (IR) and terahertz (THz) technology due totheir broad spectral range. These detectors call for suitable absorbers with minimal thermal mass.Often, this is implemented with plasmonic antennas, which ensure high absorptivity but only fora narrow spectral band. Alternatively, a common approach is based on impedance-matching thesheet resistance of a thin metallic film to half the free-space impedance. Thereby, it is possibleto achieve a wavelength-independent absorptivity of up to 50 %. However, existing absorber filmstypically require a thickness of the order of tens of nanometers, which can significantly deterioratethe response of a thermal transducer. Here, we present the application of ultrathin gold (2 nm) ontop of a surfactant layer of oxidized copper, as an effective IR absorber. An almost wavelength-independent and long-time stable absorptivity of 47(3) %, ranging from 2μm to 20μm, could beobtained and is further discussed. The fabricated gold thin-film absorber represents a promisingapproach towards an ideal impedance-matched absorber, allowing for a significant improvement ofstate-of-the-art thermal detector technology.