Femtosecond Carrier Dynamics in Semiconductors and Metals
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FEMTOSECOND CARRIER DYNAMICS IN SEMICONDUCTORS AND METALS WEI ZHU LIN, ROBERT W. SCHOENLEIN, STUART D. BRORSON, ERICH P. IPPEN AND JAMES G. FUJIMOTO Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139 ABSTRACT We describe investigations of carrier dynamics in GaAs and AlGaAs semiconductors and thin gold films using femtosecond optical techniques. Transient absorption saturation measurements in semiconductors permit an investigation of the fundamental scattering processes of optically excited carriers. Measurements of energy relaxation provide evidence for transient nonthermal carrier distributions. Studies in metals allow an investigation of nonequilibrium electron and phonon effects since the electrons are decoupled from the lattice on a femtosecond time scale. Nonequilbrium temperatures and energy transport in the electron gas have been observed. INTRODUCTION Semiconductors and metals provide complementary systems for the investigation of carrier dynamics. Recent advances in femtosecond laser generation and measurement techniques permit the investigation of ultrafast phenomena in these materials on time scales as short as a few tens of femtoseconds. We will discuss studies of carrier dynamics in GaAs and AlGaAs. Femtosecond absorption saturation measurements are applied to investigate carrier scattering and energy relaxation. Using pulses as short as 35 fs at 2 eV, initial scattering times on the ten femtosecond time scale have been observed. Pump and continuum probe measurements permit an investigation of the energy relaxation of the excited carriers. Evidence for spectral hole burning and transient nonthermal carrier distributions is observed. In contrast to semiconductors, the inherent high density of metals results in the rapid thermalization of excited electrons. Pump and continuum probe measurements of transient reflectivy in gold metal films provide evidence for nonequilibrium electron and lattice temperatures. Nonequilibrium energy transport has been observed at velocities comparable to the Fermi velocity using time of flight measurements in thin films. FEMTOSECOND CARRIER DYNAMICS IN GaAs AND A1GaAs The dynamics of excited carriers in GaAs and AIGaAs have been the subject of 1 extensive investigation for several years using a variety of experimental techniques . Transient absorption spectroscopy provides a powerful approach for investigating carrier relaxation phenomena directly in the time domain. Subpicosecond absorption saturation spectroscopy provided the first direct observation of ultrafast band gap renormalization in GaAs2' 3 . Recent advances in femtosecond generation and measurement techniques allow the extension of these types of studies to the femtosecond domain. Equal pulse correlation as well as noncollinear pump-probe transient absorption saturation measurements have been used to investigate the initial scattering mechanisms of optically excited carriers 4 9 . Pump and continuum probe sp
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