Time-resolved optical spectroscopy of quasiparticle dynamics

J. Orenstein 
Department of Physics, University of California, Berkeley and 
Materials Science Division, Lawrence Berkeley National Laboratory, 
Berkeley, CA 94720, USA

We present an overview of time-resolved pump-probe experiments on the
nonequilibrium state of superconductors.  In each study the superconductors
are excited by a 100 fs pulse of 1.5 eV photons.  The resulting
nonequilibrium population of quasiparticles is probed by a second pulse,
either a second visible pulse, or a synchronous optoelectronically generated
terahertz transient.  By contrasting the results of the same experiment on
NbN and BSCCO, we show that thermalization and recombination are
qualitatively different in the high-Tc cuprates.  We show that the decay of
the excited state in BSCCO and YBCO (Ortho II) proceeds without limitation
imposed by the phonon bottleneck.  This allows direct measurement of the
microscopic or 'bare' recombination rate of superconducting quasiparticles
in these materials.