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-T(c) supercondu ctors. Using Y-Ba-Cu-O thin films of thickness less than 15 nm, we hav e observed voltage pulses as fast as 40 ps. The temporal photoresponse signal was found to depend strongly on the sample's thickness. A syst ematic 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) relaxatio n process. The role of electron-phonon coupling in the QP decay proces s has also been studied with 500-fs and 40-ps laser pulses.