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Scientific Reports

Effects of non-exciton components excited by broadband pulses on quantum beats in a GaAs/AlAs multiple quantum well

Osamu Kojima, Yuki Iwasaki, Takashi Kita, Kouichi Akahane

Published Jan 27, 2017• 5 citations
Publisher

Abstract

In this study, we report the effect of the excitation of non-exciton components caused by broadband pulses on quantum beat oscillation. Using a spectrally controlled pump pulse, a long-lived oscillation is clearly observed, and the pump-power dependence shows the suppression of the dephasing rate of the oscillation. Our results from incoherent carrier generation using a continuous wave laser demonstrate that the non-exciton components behaving as free carriers increase the oscillation dephasing rate.

DOI
10.1038/srep41496

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In this study, we report the effect of the excitation of non-exciton components caused by broadband pulses on quantum beat oscillation. Using a spectrally controlled pump pulse, a long-lived oscillation is clearly observed, and the pump-power dependence shows the suppression of the dephasing rate of the oscillation. Our results from incoherent carrier generation using a continuous wave laser demonstrate that the non-exciton components behaving as free carriers increase the oscillation dephasing rate.
We report pulse modulation caused by the exciton quantum beat in a GaAs/AlAs multiple quantum well.The modulation was evaluated by measuring the cross-correlation signal which is the second harmonic light generated by the probe pulse reflected from the sample and the gate pulse.The intensity of the correlationsignal decreases owing to the generation of the exciton quantum beat, and recovers with dephasing of the quantum beat oscillation.Moreover, we found that the decrease caused by the quantum beat is larger than that by changing the refractive index due to the exciton generation.These results indicate that quantum beats can be a potential mechanism to enable low power operation of ultrafast optical switches.
We discuss the dephasing of quantum beat oscillation related to the intersubband transition between heavy-hole and light-hole excitons in a GaAs/AlAs multiple quantum well as measured by a pump-probe technique. We investigate the dependence of dephasing on temperature and pump energy. The analysis of the time-domain signals reveals that a strong quantum beat induces the rapid dephasing related to intersubband transition; the dependencies of the dephasing rate correspond to those of the amplitude. This implies that a quantum beat is useful in applications utilizing an ultrafast optical response.
We show the coherent control of the non-Markovian oscillation of exciton-folded acoustic phonon scattering in GaAs multiple quantum wells by using four-wave mixing measurements. The extent of non-Markovian scattering depends on the delay time ${t}_{{1}^{\ensuremath{'}}1}$ between the phase-locked pulses. The phenomenological dephasing time in the long-time (Markovian) region does not depend on the scattering. The dephasing process is suppressed when ${t}_{{1}^{\ensuremath{'}}1}$ corresponds to half the period of the non-Markovian oscillation. The dephasing suppression is the result of a fifth-order nonlinear optical effect (a six-wave mixing process).
Using the renormalized wavefunction approach, we examined the excitonic quantum beat in a three-level system in disk-shaped quantum dots. The non-stationary electron wave function and the time-dependent exciton absorption intensity under the effect of an intensity pump laser that resonates with two excited electron states were also provided. Our numerical findings demonstrate that the time-dependent exciton absorption intensity takes the shape of a periodic oscillation when the quantum dot is subjected to a powerful resonant pump laser. This is clear evidence that quantum beats appears in the quantum disk. Additionally, the dot radius and the pump field detuning significantly affect the beat characteristics, such as frequency and amplitude.
The degenerate four-wave mixing technique in the self-diffraction geometry is employed to investigate a novel GaAs/AlAs quantum well microstructure. Time-integrated transient measurements are carried out to study the coherent dynamics of excitons in a GaAs quantum well with AlAs islands inserted at the center of the well (IIQW) in excitation densities from 6×109 to 1.5×1011 cm-2. The decay of the time-integrated self-diffracted signal shows slower dephasing for the IIQW compared to that of a normal quantum well (NQW) without island insertion. The dephasing rate is found to increase slightly in the IIQW, while it is found to increase sharply in the NQW with the excitation density. A deep beat with a period of 1.45 ps, which disappears for co-circular polarization configuration, is observed in the decay of the self-diffracted signal in the IIQW. The detection-energy-resolved measurement reveals that the modulation of the beat increases up to full depth at low detection energy with respect to the peak energy of the self-diffracted signal. The dependence on detection energy and on polarization configuration shows that the beat is a quantum beat due to biexcitonic effect, which is enhanced by localization due to island insertion.