We report measurements characterizing the performance of a kinetic inductance detector array designed for a wavelength of 25 microns and very low optical background level suitable for applications such as a far-infrared instrument on a cryogenically cooled space telescope. In a pulse counting mode of operation at low optical flux, the detectors can resolve individual 25-micron photons. In an integrating mode, the detectors remain photon noise limited over more than six orders of magnitude in absorbed power from 70 zW to 200 fW, with a limiting NEP of 4.6 x 10^-20 W/rtHz at 1 Hz. In addition, the detectors are highly stable with flat power spectra under optical load down to 1 mHz. Operational parameters of the detector are determined including the efficiency of conversion of the incident optical power into quasiparticles in the aluminum absorbing element and the quasiparticle self-recombination constant.
@article{arxiv.2404.10246,
title = {A 25-micron single photon sensitive kinetic inductance detector},
author = {Peter K. Day and Nicholas F. Cothard and Christopher Albert and Logan Foote and Elijah Kane and Byeong H. Eom and Ritoban Basu Thakur and Reinier M. J. Janssen and Andrew Beyer and Pierre Echternach and Sven van Berkel and Steven Hailey-Dunsheath and Thomas R. Stevenson and Shahab Dabironezare and Jochem J. A. Baselmans and Jason Glenn and C. Matt Bradford and Henry G. Leduc},
journal= {arXiv preprint arXiv:2404.10246},
year = {2024}
}