Time-resolved photoresponse in the resistive flux-flow state in Y–Ba–Cu–O superconducting microbridges

We report our studies on ultrafast voltage transients of optically thick superconducting YBa2Cu3O7−x microbridges biased with nanosecond supercritical current pulses (I > Ic) and, simultaneously, illuminated with 100 fs optical pulses (810 nm wavelength) from a Ti:sapphire laser. The pulsed current bias created a resistive flux-flow state in the superconductor, while the laser pulse transferred it into a state of more intensive flow of vortices within a time of less than 100 ps. The light-enhanced flux state remained constant until the end of the biasing pulse. The amplitude of the photoresponse signal increased rapidly with the increase of laser fluence in the range from 8 × 106 to 2 × 108 photons per pulse, as well as with the current-bias increase up to 2Ic, exhibiting the maximum, when the light illuminated the entire area of the microbridge. Maximal repetition rate of optically excited YBa2Cu3O7−x photoresponse signals was found to be in the GHz range, appropriate for applications of YBa2Cu3O7−x thin-film microbridges as high-power ultrafast switches.