Capacitively coupled hot-electron nanobolometer as far-infrared photon counter

We show theoretically that hot-electron nanobolometers consisting of a small piece of normal metal, capacitively coupled to a superconducting antenna through a pair of normal metal--insulator--superconductor (NIS) tunnel junctions may be used as far-infrared photon counters. To make the device most effective at high counting rates, we suggest the use of the bolometer in the simplest configuration, when the NIS tunnel junctions are used as both an electron cooler and thermometer. The absorption of the photon in the normal metal produces a pulse in the electron temperature, which is measured by the NIS junctions. The counter may resolve photons up to 0.3--0.4 mm wavelength and has a typical re-equilibration time constant of about 20 ns.