FAR-INFRARED KINETIC-INDUCTANCE DETECTORS

Extremely sensitive far-infrared detectors suitable for both direct detection and heterodyne applications are possible based upon mu-m-sized thin films with thickness less than a superconducting penetration depth. The penetration depth of such a film, and therefore its inductance, varies with temperature and with quasiparticle population (described by an effective temperature T*), resulting in both bolometric and non-equilibrium "photoinductive" responses. Incident radiation is coupled into the small-area kinetic inductor by a lithographic antenna, and the resulting inductance changes are amplified and converted to a voltage signal by an integrated microstrip DC SQUID. The device is sensitive because, unlike junction-based devices with large capacitive reactances, the kinetic inductor is naturally well matched to the antenna impedance at the far-IR frequency (nu > 2-DELTA/h), and to the pre-amplifier (SQUID) impedance at microwave or video frequencies (nu << 2-DELTA/h). The best kinetic inductor materials are those with low electronic mean free path, large penetration depth, and high critical current density. Thus, common magnet alloys such as NbTi are the natural choice for liquid-He temperature operation. A detailed analysis predicts a (phonon-limited) NEP of 4x10(-17) W/square-root Hz for a bolometer with an iridium kinetic inductor operated at 0.1 K. A heterodyne noise temperature of 2250 K (single-sideband) at 3 THz, with 200 MHz bandwidth, is predicted for a Nb-Ti mixer with a operated at 4 K.