Photon-noise-limited direct detector based on disorder-controlled electronheating

We present a concept for a hot-electron direct detector capable of counting single millimeter-wave photons. The detector is based on a microbridge (1 mu m size) transition edge sensor made from a disordered superconducting fi lm. The electron-phonon coupling strength at temperatures of 100-300 mK is proportional to the elastic electron mean free path l and can be reduced by over an order of magnitude by decreasing l. The microbridge contacts are m ade from a different superconductor with higher critical temperature Nb, wh ich blocks the thermal diffusion of hot carriers into the contacts. The low electron-phonon heat conductance and the high thermal resistance of the co ntacts determine the noise equivalent power of similar to 10(-20)-10(-21) W /root Hz at 100 mK, which is 10(2)-10(3) times better than that of state-of -the-art bolometers. Due to the effect of disorder, the electron cooling ti me is similar to 10(-1)-10(-2) s at 0.1 K. By exploiting negative electroth ermal feedback, the detector time constant can be made as short as 10(-3)-1 0(-4) s without sacrificing sensitivity. (C) 2000 American Institute of Phy sics. [S0021-8979(00)00310-8].