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 similar to 10(-20) W/Hz(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 similar to 0.1-1.0 ms that is too short for integrating a flux of THz background photons arriving at a rate of < 100 s(-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, 10-20 W/Hz(1/2) at >= 1 THz while operating at 300 mK. With the sensor time constant of a few microseconds, the dynamic range is similar to 30-40 dB. A compact array of the antenna-coupled counters can be fabricated on a silicon wafer without membranes.