On the response time of YBa2Cu3O7 superconducting bolometer films

Except at helium temperatures where the electron-phonon interaction may become very slow, the speed of response of metal film bolometers is generally limited by the rate of heat transfer to the substrate. Therefore, it is determined either by acoustic radiation at low T (Kapitza regime) and or by phonon diffusion at higher T. We propose here two independent measurements of the time of phonon escape tau(gamma). In the first one, one directly monitors the resistive response to a laser pulse; the second one is based on the delay before current-switching into the dissipative state. tau(gamma) for YBa2Cu3O7 on MgO was found to be proportional to film thickness (85 picoseconds per rim) and independent of T up to about 40 K. This is consistent with the mechanism of radiation with a "mismatch factor" eta approximate to 0.020, in surprisingly good agreement with data on other structures (Al on sapphire; PbBi on sapphire) given in the litterature.