Excitonic signatures in the photoluminescence and terahertz absorption of a $\mathrm{Ga}\mathrm{As}∕{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ multiple quantum well

Measurements of the THz absorption and the time-resolved photoluminescence have been performed on the same GaAs quantum well sample. The strength of the absorption at the internal $1s\text{−}2p$ exciton transition frequency is used as a measure of the density of excitons in the sample. When the interband pump laser is resonant with the $1s$ exciton frequency, induced absorption at the $1s\text{−}2p$ frequency is clearly seen. If the same density of carriers is created pumping in the continuum, no significant $1s\text{−}2p$ absorption is seen in a time window of $450\mathrm{ps}$. Complementary time-resolved photoluminescence experiments, detecting the emission at the exciton energy under the same pump conditions, show the PL intensity in resonant and nonresonant cases to be similar. The counter-intuitive existence of luminescence at the exciton energy simultaneously with the absence of the $1s\text{−}2p$ absorption is consistent with the recent theoretical predictions of Kira et al., Phys. Rev. Lett. 81, 3263 (1998).

Figure 1

Linear absorption spectrum of the sample (upper solid curve). Also shown are the spectra of the interband pump pulses. The dotted line depicts the spectrum of the bandpass filter used in the TRPL.

Figure 2

TRPL counts at the exciton frequency as a function of time and the instrumental response profiles. The dashed line is a biexponential least squared fit as described in the text.

Figure 3

Theoretical THz absorption spectrum of an ensemble of GaAs quantum well $1s$ excitons for a range of different broadenings. The linewidth quoted refers to the FWHM of a single absorption line.

Figure 4

Transmission change of the FIR probe pulse as a function of pump-probe delay. The pump is tuned at (a) $1.560\mathrm{eV}$ (below resonance), (b) $1.596\mathrm{eV}$ (on resonance), (c) $1.658\mathrm{eV}$ (above resonance). The traces have been offset vertically for clarity.