Fast photoresponse from infrared-laser-induced flux motion in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ films

A fast photosignal has been obtained from epitaxial superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ thin films when a small alternating magnetic field was applied in the film plane. No bias current was required to observe this response. Pulsed infrared radiation at a wavelength of 10.6 μm and of 150-ns duration yielded nonlinear signals: the voltage pulses had approximately half the duration of the laser output. Signal polarity was found to depend on the sign of dH/dt. For an applied magnetic field amplitude of 1600 A/m and a frequency of 20 Hz, a maximum signal amplitude of 0.5 mV was observed at 74 K for an absorbed energy of 1 μJ. The response originates from the motion of radiation depinned fluxoids across the plane of the film. These fluxoids are subject to a driving force arising from the flux-line density gradient established in the sample by the alternating magnetic field. The resulting net flux movement generates the photosignal by Faraday’s law.