Picosecond photoresponse of epitaxial YBa2Cu3O7−δ thin films

Photoresponse signals as fast as 16 ps (full width at half-maximum) have been observed from current-biased bridge structures of epitaxial YBa2Cu3O7−δ thin films on LaAlO3 using 5 ps, 820 nm laser pulses. Operating at liquid-nitrogen temperature (77.4 K), the amplitude of the fast response was found to be linear with current at low bias currents. At higher bias currents, a slow component appeared in the signal with a decay over several nanoseconds which could be attributed to a resistive bolometric response. Fast transients about 20 ps wide have been observed in films with thicknesses ranging from 47 to 200 nm. We believe the fast response is primarily due to a kinetic inductive bolometric mechanism associated with heating of the film by the laser pulse. There is some evidence of a nonbolometric contribution but it is not yet conclusive.