Picosecond photoresponse in Y-Ba-Cu-O ultrathin films

We have applied a double-bridge voltage-correlation technique to study the nonequilibrium superconducting properties in high-Tc superconductors. Using Y-Ba-Cu-O thin films of thickness less than 15 nm, we have observed voltage pulses as fast as 40 ps. The temporal photoresponse signal was found to depend strongly on the sample's thickness. A systematic trend in the superconductivity decay time as a function of film thickness was obtained. The phonon-trapping effect has been verified to be the major limiting mechanism in the quasiparticle (QP) relaxation process. The role of electron-phonon coupling in the QP decay process has also been studied with 500-fs and 40-ps laser pulses.