Fs Spectroscopy of Phonon Emission and Absorption for A Cold Plasma in Gallium Arsenide
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21 Mat. Res. Soc. Symp. Proc. Vol. 397 ©1996 Materials Research Society
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2.5 delay ps
1.5
3.5
4.5
Figure 1. Reflectivity versus time at 829 nmn (open squares) and 788 nm (full squares) of bulk
GaAs crystal after irradiation with a =60 fs laser pulse at the same wavelength. The lines are the results of a non-linear fit of the data. The integral of autocorrelation signal of pump and probe is also shown. Table I. Excess Energies and Observed Time Constants for LO and Acoustic Phonon Emission at the Two Probing Wavelengths 788 nm3 and 829 nm at an Initial Carrier Density of 3 x 1017 cm" Wavelength
788 nm
Electrons excess energy Holes excess energy LO phonon time constant Topt Acoustic phonon time constant Tac Relative amplitudes ARopt, ARac, ARt
122 mneV 15 meV 140 fs 1.9 ps 3 8,
21,
829 nm
55 meV 7 meV 180 fs 2.0 ps 2, 9, 14,
The two exponentials describe the contributions of LO and acoustic phonons, respectively, whereas the third contribution, which is constant in this time scale, accounts for the thermalized carriers which have relaxed to the bottom of the band. They contribute to the optical response both for filling of the optically coupled states and bandgap renormalization. Time constants extracted with a nonlinear fitting procedure are given in Table I. The deduced values are in agreement with previous observations performed at higher excitation energies [5] . Moreover, they exhibit the correct dependence upon the related energy levels in the conduction band. Further infonnation on carrier dynamics in this regime is obtained by observing the fluence dependence of reflectivity traces at long wavelengths, i.e. close to the threshold for LO phonon emission. As an example, Figure 2 illustrates the relative reflectivity changes at 829 nm versus time for three different carrier injection regimes. All three curves exhibit a rise temporally coincident with the exciting laser pulse, as illustrated by the integral of the autocorrelation function. The decay of the signals is again mnade up by two contributions, the first being a fast decay in the = 200 fs time scale. Its amplitude progressively decreases with initial injection density. For the two higher fluence traces, the slow component shows a decay towards a constant
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delay ps Figure 2. Relative changes in reflectivity versus delay time for 829 nmn at the three carrier densities indicated.The integral of the cross-comrelation signal between pump and probe is shown with a thick line. Thin lines are nonlinear fits of the decaying parts of the data according to Eq. 1. value as explained previously. A striking feature occurs instead at the smaller excitation level, where, in contrast with the measurements at higher fluences, a reflectivity increase with time is observed. Table II illustrates the characteristic times as deduced via a non
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