<title>Internal thermalization of the electron-hole plasma in the subpicosecond regime in GaAs</title>

Abstract

We report on new experimental results on the internal thermalization of the electron-hole plasma (EHP) in bulk GaAs. Time-resolved differential transmission spectra have been systematically studied for different pump photon energies (1 .45 eV<ha(1 .5 1 eV) and different excitation intensities. The experimental data demonstrate that the plasma thermalization is achieved within a stable interval of 150-200 fs for plasma densities ranging from 2x1&7 to 2x1018/cm3. Experimental results are in good agreement with the thermalization times deduced from plasma dynamics calculations using dynamic screening ofthe interactions in the framework ofBoltzmann's equations. U-INTRODUCTIONThe absorption of a short laser pulse with a central frequency above the band gap in a semiconductor creates free electrons and holes with an initial energy distribution that is essentially determined by the laser. spectrum. On a very short time scale, this nonthermal energy distribution is transformed by carrier-carrier scattering into Fermi-Dirac distributions, i.e., a quasi-equilibrium is established that can be characterized by carrier temperatures and chemical potentials for the various bands. The time needed for this internal thermalization is of fundamental physical interest because it is closely related to the strength of the carrier-carrier interaction and determines critically non-linear optical properties. Therefore, the development of ultrashort laser pulses has triggered much effort to clarify the initial stages of carrier relaxation and to detect nonthermal carrier distributions and the subsequent transition into the thermal regime. The temporal evolution of carrier-induced optical nonlinearities has been intensively studied in GaAs, A1GaAs, InP and quantum well structures mainly by pump-probe [1-6] and time-resolved photoluminescence measurements [7-Il]. Carrier-carrier scattering in bulk GaAs and quantum wells has also been studied by four-wave-mixing measurements of interband polarization dephasing [12][13][14]. However, experiments on GaAs with 2 eV excitation pulses are complicated by concurrent ultrafast intervalley scattering [15][16]. Photoluminescence experiments with time resolutions down to 100 fs [ 7-10] failed to show nonthermal distributions, except in the case of extremely low excitation density [11]. 110 / SPIE Vol. 2142 0-8194-1437-9/94/$6.00 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/25/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx