High speed hot-electron superconducting bolometer

Physical limitation of response time of a superconducting bolometer as well as the nature of non-equilibrium detection of radiation have been investigated for Al, Nb and NbN thin films in spectral range from submillimeter to near-infrared wavelengths. In the case of ideal heat removal from the film with the ≅ 100 A thickness the detection mechanism is an electron heating effect that is not selective to radiation wavelength in a very broad range. The response time of an electron heating bolometer is determined by an electron-phonon interaction time. This time is of about 10 ns, 0.5 ns and 20 ps for Al, Nb, and NbN correspondingly near the critical temperature of the superconducting film. The sensitive area of the bolometer consists of a number of narrow strips (with a width of 1 μm) connected in parallel to contact pads; these pads together with a sapphire substrate and a ground plate represent the microstrip transmission line with an impedance of 50 Ω. Detectivity of the electron heating bolometer is as nigh as 5 · 1011 W-1cm·Hz1/2 for Al, 1011 W-1cm·Hz 1/2 for Nb and 1010 W-1cm·Hz1/2 for NbN films. The sensitivity can be enhanced by several orders of magnitude in a microbolometer design. In the case of electron heating bolometer the superconducting strip in the resistive state serve as a resistive load for the infrared or a submillimeter current. In contrast to a conventional microbolometer the noise equivalent power (NEP) of the electron heating one diminishes not only due to a reduction of a sensitive area but because of the decreasing of its thickness also. The principal term in NEP is determined by a electron temperature fluctuation arising from the heat exchange with phonons