Ultra-sensitive mid-infrared emission spectrometer using a WSi superconducting nanowire single-photon detector
Varun B. Verma, Martin J. Stevens, Richard P. Mirin, Sae Woo Nam, Li Chen, Dirk Schwarzer, Jascha A. Lau
We evaluate the performance of a mid-infrared emission spectrometer operating at wavelengths between 2 and 7 υm based on an amorphous tungsten silicide (a-WSi) superconducting nanowire single-photon detector (SNSPD). To demonstrate the spectrometers capabilities, we perform laser induced fluorescence spectroscopy of surface adsorbates with sub-monolayer sensitivity and sub-nanosecond temporal resolution. The a-WSi SNSPD shows saturated internal quantum efficiency at wavelengths between 1.5 υm and 2.5 υm, where the absolute internal quantum efficiency is found to be close to unity. The internal quantum efficiency remains above 10 % at wavelengths as long as 6 υm. We determine the noise equivalent power (NEP) of the current SNSPD detector system from 2 υm (4×10^-16^ W/√Hz) to 6 υm (5×10^-15^ W/√Hz) - these results are not yet limited by detector noise but rather from background radiation, which is largest near 4.5 υm (NEP=1×10^-14^ W/√Hz). This is nevertheless several orders of magnitude lower than semiconductor-based detectors typically used in this wavelength range. The temporal response is also markedly better. We discuss possible future improvements of the SNSPD-based infrared emission spectrometer and its potential applications in molecular science.