Saturation of the superconductor quasiparticle direct radiation detector

The saturation properties of the superconductor-insulator-superconductor (SIS) direct detector are calculated. It is shown that this device begins to saturate only at powers far larger than one would expect from a simple analogy with the SIS mixer. Since the SIS direct detector is known to have very low noise, this result should lead to its immediate practical exploitation for direct detection of millimeter wavelength radiation.