Experimental study of superconducting hot-electron sensors for submm astronomy

Relaxation, noise, and spectral properties of micron-size hot-electron sensors made from thin Ti film are studied. Due to the small heat capacity of electrons, the devices are sensitive to single quanta of submm radiation. The sensors can be used for both hot-electron direct detectors (HEDD) and hot-electron photon-counters (HEPC) depending whether electron-phonon relaxation or electron outdiffusion is a dominating cooling mechanism. In an HEDD, the diffusion is blocked by Andreev contacts and the cooling rate is determined by the electron-phonon relaxation. The electron-phonon time in disordered films is long (τ_e-ph≈0.16×T^-4 μs) providing an NEP≈10^-19 W/√Hz at 0.3 K and NEP≈10^-20 W/√Hz at 0.1 K. The output noise in micron-size bridges follows the predictions of the hot-electron model. In the diffusion mode, the relaxation time of 3 ns has been measured in a 3 μm-long device. Smaller size HEPC´s would be able to operate with the spectral resolution of 300 GHz at 0.3 K and 100 GHz at 0.1 K and with the photon counting rate in the GHz range. The spectral response of a prototype antenna-coupled Nb HEDD device has been measured and shown to be flat over the range 250-900 GHz.