Ultrafast carrier dynamics in optically excited YBa/sub 2/Cu/sub 3/O/sub 7-(delta)/ thin films diagnosed by a pump-and-probe terahertz experiment

A pump-and-probe terahertz emission technique is reported to investigate the nonequilibrium process of ultrafast generation and relaxation of hot quasiparticles and their recombination to form Cooper pairs in superconducting YBa/sub 2/Cu/sub 3/O/sub 7-(delta)/ thin films following femtosecond laser excitation. Subpicosecond electromagnetic pulses emitted by an YBa/sub 2/Cu/sub 3/O/sub 7-(delta)/ microbridge change after an excitation of the material due to an additional laser pulse, the so-called pump pulse. The modulation of the emitted terahertz pulse amplitude is used to measure the time resolved evaluation of the hot carrier excitation and relaxation after a time delay to the pump laser pulse. The results are compared to terahertz pump-and-probe experiments on GaAs photoswitches and can be explained by avalanche Cooper pair breaking, supercurrent modulation followed by carrier acceleration and Cooper pair recombination accompanied with deceleration of the supercurrent carriers.