Hot-electron direct detector for radioastronomy

A hot-electron transition-edge superconducting bolometer with adjustable thermal relaxation speed is proposed. The bolometer contacts are made from a superconductor with high critical temperature which blocks the thermal diffusion of hot carriers into the contacts. Thus electron-phonon interaction is the only mechanism for heat removal. The speed of thermal relaxation for hot electrons in a nanometre-size superconducting bolometer with T_c = 100-300 mK is controlled by the elastic electron mean free path l. The relaxation rate behaves as T⁴l at subkelvin temperatures and can be reduced by a factor of 10-100 by decreasing l. Then an antenna- or waveguide-coupled bolometer with a time constant approx. 10^-3-10^-4 s will exhibit photon-noise-limited performance at millimetre and submillimetre wavelengths. The bolometer will have a figure of merit NEPτ ^1/2 = 10^-23-10^-22 J^-1 at 100 mK which is 10³-10⁴ times better (i.e. smaller) than that of a state-of-the-art bolometer. A tremendous increase in speed and sensitivity will have a significant impact for observational mapping applications.