Differential absorption spectra of high quality InGaAs quantum wells were measured by means of pump–probe spectroscopy as a function of pump–probe delay, for different polarizations of the pulses, and different spectral detunings of the pump beam with respect to the 1s heavy-hole exciton. A clear redshift of the heavy-hole resonance is observed for large detunings and opposite circularly polarized pulses. Microscopic calculations that include not only the Hartree-Fock contribution but also higher-order Coulomb correlations reproduce the experimental findings very well. It is shown that the redshift is caused by Coulomb memory effects and is further enhanced due to the presence of the bound biexciton.
The intensity dependence of pump-induced excitonic absorption changes is studied in high-quality
In0.04Ga0.96As quantum wells for different detunings and polarization configurations. For resonant excitation with increasing pump intensity a saturation of both the exciton bleaching and the induced absorption due to two-exciton resonances is observed, whereas for pumping below the exciton a characteristic polarization dependence of higher-order contributions to the optical Stark effect is found. Microscopic calculations including Coulomb correlations show that the experimental observations can be described by the coherent dynamics of single- and two-exciton resonances up to fifth order in the optical field.
Summary form only given. Since the original discovery of the excitonic optical Stark effect, substantial progress has been made in the understanding of this ultrafast nonlinear optical effect. Recently, a red shift of the excitonic absorption line has been reported using low intensity excitation detuned 4.5 meV below the 1s-heavy-hole (hh) resonance and cross-circularly-polarized pulses. This was attributed to higher-order Coulomb correlations, interpretable as bound and unbound two-exciton states and memory effects. For co-circularly polarized pulses the well-known blue shift was found. Here we present a related effect occurring in the higher-intensity regime. Standard pump-probe experiments were performed on high-quality 8.5 nm-thick In/sub 0.04/Ga/sub 0.96/As multiple quantum well (MQW) samples.
Pump-probe experiments on high-quality ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum wells are used to investigate the influence of light-hole excitons on the optical Stark effect. For anticircular polarization of pump and probe pulses and a moderate negative detuning of the pump energy, a redshift of the heavy-hole resonance is observed. However, with increasingly negative detuning a transition from this redshift to a blueshift is found. Microscopic calculations that include both heavy holes and light holes reproduce the experimental results. The theoretical analysis shows that the observation of the redshift depends very sensitively on the detuning of the pump pulses and the heavy-hole to light-hole splitting.
Summary form only given. Recently a lot of interest has been focused on higher-order Coulomb correlations. It was shown, that even in the /spl chi//sup (3)/-limit higher-order excitonic correlations, i.e., bound and unbound two exciton states, influence the coherent optical response. While /spl chi//sup (3)/-contributions to the excitonic Stark effect are well understood, very little is known about the influence of /spl chi//sup (5)/-contributions. Here we investigate the dependence of coherent pump-induced excitonic absorption changes on the pump intensity and relate our results to fifth-order contributions. Pump-probe experiments are performed on high-quality InGaAs quantum wells dominated by a heavy-hole exciton resonance.
We perform femtosecond spectrally resolved pump-probe differential reflectivity measurements on excitons in GaAs quantum wells in both coherent and incoherent regimes. The spectral signal is nonzero and oscillatory at $\ensuremath{-}\mathrm{ve}$ delay up to a few ps indicating persistent coherence between excitonic polarization induced by the probe pulse and the delayed pump pulse. The signal for small
+ve and $\ensuremath{-}\mathrm{ve}$ delay $(\ensuremath{\tau})$ shows modulation caused by quantum beats due to coexcitation of heavy and light hole excitons. The rise of the signal with $\ensuremath{\tau}$ for $\ensuremath{\tau}<0$ is rather nonexponential and is sensitive to detuning of the detection energy with respect to the exciton energy. The signal decay with $\ensuremath{\tau}$ for small
+ve $\ensuremath{\tau}$ shows dependence on pump-probe relative polarization. The excitonic reflectivity shows a large reduction caused by excitonic absorption saturation for both
+ve and $\ensuremath{-}\mathrm{ve}$ $\ensuremath{\tau}$ when the pump pulse intensity is increased to large values. To compare the experimental results with theory, we obtain an expression for the PPDR signal in terms of the third order excitonic polarization. This has an additional term not considered earlier to describe pump-probe experiments. The calculations are performed using optical Bloch equations, modified to incorporate exciton inhomogeneous broadening, quantum beats, and many body effects such as local field, excitation induced energy shift, and dephasing. These effects are found essential in explaining the observed PPDR spectral data. The delay dependence of the signal near the exciton energy is mainly controlled by inhomogeneous broadening for $\ensuremath{-}\mathrm{ve}$ $\ensuremath{\tau}.$ The signal decay for large
+ve $\ensuremath{\tau}$ has an exponential behavior determined by the lifetime of incoherent excitons. Experiments for collinear polarization reveal an additional exponential component for small
+ve delay. Theoretically, this is found to be related to exciton dephasing and is present only when many body effects are included.