THz Hot-Electron Photon Counter

We discuss an implementation of a hot-electron transition-edge sensor (TES) capable of counting THz photons. The main need for such a THz calorimeter is spectroscopy on future space telescopes with a background limited NEP/spl sim/10/sup -20/ W/Hz/sup 1/2/. The micromachined bolometers will unlikely reach such sensitivity at temperatures above 10 mK. The hot-electron TES with sufficient sensitivity will still have a time constant /spl sim/0.1-1.0 ms that is too short for integrating a flux of THz background photons arriving at a rate of <100 s/sup -1/. The Hot-Electron Photon Counter based on a submicron-size superconducting Ti bridge with Nb Andreev contacts will be able to detect individual photons above 170 GHz due to its very low heat capacity. A discrimination of the low energy fluctuations with a threshold device would allow for realization of an NEP/spl sim/10/sup -20/ W/Hz/sup 1/2/ at /spl ges/1 THz while operating at 300 mK. With the sensor time constant of a few microseconds, the dynamic range is /spl sim/30-40 dB. A compact array of the antenna-coupled counters can be fabricated on a silicon wafer without membranes.