Hot-electron direct detectors: feasibility of NEP H 10-20 WHz at submillimeter waves

Recently, we have presented a concept for a hot-electron direct detector (HEDD) capable of counting single millimeter-wave photons [1]. Such a detector meets the needs of future space far-infrared missions (NEP £ 10-19W / Ö Hz) and can be used for background-limited detector arrays on the Space InfraRed Interferometric Telesope, the 10-meter space telescope and Submillimeter Probe of the Evolutionof Cosmic Structure [2]. The detector is based on a microbridge (1-µm-long) transition edge sensor fabricated from an ultra-thin film of a superconductor with the critical temperature Tc = 0.1-0.3 K. A very strong temperature dependence of the electron-phonon coupling allows adjustment the electron-phonon scattering time, te-ph, to the desired time constant of the detector (t =10-4-10-3 s) at T = 0.1 K. Further adjustment of t eph is possible due to the electron-mean-free-path dependence of t e-ph. The microbridge contacts are made from a superconductor with a higher critical temperature (Nb); these contacts will block the thermal diffusion of hot carriers into the contacts because of the Andreev reflection (see Fig. 1). The low electron-phonon heat conductance, high thermal resistance of the contacts, and small heat capacity of electrons in a micron-size bridge determine the noise equivalent power of ~ 10 20 – 1021 W / Ö Hz at T = 0.1 K, which is 102 to 103 times better than that of state-of-the-art bolometers. By exploiting the negative electro-thermal feedback, the detector time constant can be made as short as 10-5-10-4 s without sacrificing sensitivity