Time-resolved study of intervalence band thermalization in a GaAs quantum well

Abstract

The thermalization of optically excited cold holes in a GaAs quantum well is investigated by femtosecond two-color pump-probe measurements. Clear evidence is found for scattering from heavy-holes into the lowest light-hole band due to LO-phonon absorption. We obtain firm data on scattering times which depend strongly on lattice temperature. They vary from 230 fs at room temperature to 900 fs at Tϭ105 K. The experimental data are well reproduced by numerical calculations. © 1996 American Institute of Physics. ͓S0003-6951͑96͒03021-5͔Firm experimental data on ultrafast carrier dynamics in semiconductors and semiconductor heterostructures are of prime importance for the development of novel ultrafast opto-electronic devices. The development of ultrashort-pulse lasers has stimulated a large number of time-resolved studies addressing the picosecond and subpicosecond dynamics of nonequilibrium carriers in GaAs and other III-V semiconductors. 1,2 Since the density of states in the conduction band is generally much lower than that of the valence band, in most studies the experimental signals are dominated by the dynamics of optically excited electrons. 3 Only in a few specific experiments could information on the hole relaxation dynamics be obtained in special sample structures. [4][5][6][7][8] In these studies, holes were excited with a certain excess energy and subsequent cooling processes were observed. In contrast, optical excitation of excitons close to the band edge will create initially cold holes. In this case, the hole dynamics at elevated lattice temperatures is expected to be dominated by the absorption of longitudinal optical ͑LO͒ phonons that leads to a heating of the initial hole distribution. 3,7 In this letter, we investigate the intervalence band thermalization of optically excited holes. Femtosecond timeresolved measurements of the bleaching of the heavy-hole ͑HH͒ and light-hole ͑LH͒ exciton transition in a GaAs multi quantum well ͑MQW͒ sample are performed. The nonlinear absorption of the excitonic transitions is determined by two contributions, namely the reduction of the oscillator strength and the broadening of the exciton line. 9 It has been shown previously that the reduction of the oscillator strength strongly depends on the subband occupation number, while the intrasubband carrier distribution does not play a significant role. 9,10 In the same work it was found that broadening is not affected by changes of the subband occupation. Furthermore, measurements of absorption changes at the lowenergy edge of the E1H1 transition-which are dominated by broadening-under various excitation conditions only show an ''instantaneous'' signal without any further dynamics on a picosecond time scale, indicating that this signal contribution is not sensitive to the exact intrasubband carrier distribution, either. 11 These facts make it possible to measure changes of the hole subband occupation by choosing sample and excitation parameters that prevent any electron intersubband dynamics.Our experiments are performed...

Bloch oscillations

“…The observed dephasing time is attributed to the ultrafast thermalization of the excited distribution due to the absorption of LO phonons. Theoretical calculations for electron thermalization times due to LO phonon absorption are in the range of 600 fs (Lam & Singh 1995) and 200 fs are determined experimentally for heavy holes (Kim et al 1996). These electronic thermalization times are close to the observed value.…”

Quantum Coherence of Continuum States in the Valence Band of GaAs Quantum Wells

“…The observed dephasing times are more than an order of magnitude smaller than previously reported hole-spin relaxation times at lower densities [17] and calculated values based on the D'Yakonov-Perel'-like hole scattering mechanism [18]. We therefore attribute the observed dephasing times to hole-hole scattering [19] and to LO-phonon assisted intervalenceband thermalization [15] at elevated temperatures.…”

“…Although the amplitude of the beats decreases with increasing excess energies by nearly 2 orders of magnitude, the dephasing time decreases by less than a factor of 2 between resonant and OR excitation at 10 K. For higher excess energies, the dephasing time only slightly decreases. This decrease is mainly due to the intervalence band thermalization of the holes by LO phonon absorption at higher temperatures [15]. Although the total optical excess energies are more than a LO phonon energy, the emission of LO phonons plays a minor role as a scattering mechanism in the valence band, since most of the excess energy is transferred to the conduction band.…”

Direct experimental observation of different diffusive transport regimes in semiconductor nanostructures

“…At room temperature, the latter occurs in about a picosecond if carriers are injected at the band edge of GaAs/Al x Ga 1Ϫx As quantum wells. 25,26 Therefore, Eqs. ͑1͒ and ͑2͒ are used for time delays of 2 ps and longer.…”