Abstract
Ultrafast optical modulation of the super carrier density inside of a YBa 2Cu 3O 7− δ dipole antenna structure can be used to generate single subpicosecond electromagnetic pulses, i.e. terahertz (THz) radiation. We investigate the dependence of the emitted THz radiation on an addition excitation of the material by means of a common pump-and-probe technique. After femtosecond laser excitation the pump induced changes of the signal are connected to the nonequilibrium quasiparticle dynamics. This process contains both the ultrafast generation and relaxation of hot quasiparticles and their recombination to Cooper pairs in superconducting YBa 2Cu 3O 7− δ thin films. The modulation of the emitted THz 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 can be explained by the avalanche Cooper pair breaking, by the supercurrent modulation followed by carrier acceleration and by the Cooper pair recombination which cause the deceleration of the supercurrent carriers.
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