Nonthermal mixing mechanism in a diffusion-cooled hot-electron detector

We present an analysis of a diffusion-cooled hot-electron detector fabricat ed from clean superconducting material with low transition temperature. The distinctive feature of a clean material, i.e., material with large electro n mean free path, is a relatively weak inelastic electron scattering that i s not sufficient for the establishment of an elevated thermodynamic electro n temperature when the detector is subjected to irradiation. We propose an athermal model of a diffusion-cooled detector that relies on suppression of the superconducting energy gap by the actual dynamic distribution of exces s quasiparticles. The resistive state of the device is caused by the electr ic field penetrating into the superconducting bridge from metal contacts. T he dependence of the penetration length on the energy gap delivers the dete ction mechanism. The sources of the electric noise are equilibrium fluctuat ions of the number of thermal quasiparticles and frequency dependent shot n oise. Using material parameters typical for A1, we evaluate performance of the device in the heterodyne regime at terahertz frequencies. Estimates sho w that the mixer may have a noise temperature of a few quantum limits and a bandwidth of a few tens of GHz, while the required local oscillator power is in the mu W range due to ineffective suppression of the energy gap by qu asiparticles with high energies. (C) 2000 American Institute of Physics. [S 0021-8979(99)01424-3].