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Journal of Physics D Applied Physics

Pulse modulation towards low-power operation based on the quantum beat of excitons in a GaAs/AlAs multiple quantum well

Osamu Kojima, Ken Hayashii, Takashi Kita, Kouichi Akahane

Published Feb 19, 2014• 5 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3727/47/10/105101

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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.
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.
In this paper, we report the manipulation of coherent exciton in single- and coupled-quantum-wells using phase locked pulses generated by pulse shaping techniques instead of Michelson interferometer. The pulse shaping system with double liquid crystal spatial light modulator, and the Ti3+-sapphire laser with pulse width of about 120 fs are used to generate phase-locked pulses. GaAs/AlGaAs quantum-wells (exciton energy: E0 equals 1.5470 eV) and asymmetric double coupled quantum-well structures (exciton energy: E1 equals 1.5348 eV, E2 equals 1.5422 eV) are used in the experiments. Manipulation of coherent exciton are demonstrated by observation of both the reflectivity-change in the pump-probe and the diffracted power in the degenerate-four-wave-mixing (DFWM) measurements using phase- locked twin-pulse (k1) and single-pulse (k2). Coherent destruction in single quantum-well is observed by the rapid decrease of reflectivity-change after the arrival of the second pulse (pulse interval: 500 fs) and good coherent control with 87% of coherent carrier destruction is demonstrated at low excitation power of 0.15 mW. In case of coupled quantum-wells, the modulation of SLM is carefully set to the twin-pulse having the spectrum with a minimum at both exciton wavelengths for destructive condition. After arrival of the second pulse, coherent control of the quantum beats are observed in DFWM signal.
We demonstrate the possibility of ultrafast optical control of excitons confined in GaAs thin films. The intensity of a two-beam degenerate four-wave-mixing signal changes with an increase in the power of each pulse. The signal intensity monotonically decreases after reaching the maximum. Peaks appear at different excitation powers for k 1 and k 2 pulses, which suggests that the decrease in the signal intensity is caused by excitonic Rabi oscillations. Our results demonstrate the possibility of ultrafast control of the exciton population by using optical pulses.
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.
In this paper, we report on the ultrafast coherent control of excitons in quantum wells using a phase-locked pulse sequence generated by pulse-shaping techniques. The pulse-shaping system with a double liquid-crystal spatial light modulator (SLM), and a Ti3+-sapphire laser is used to generate phase-locked pulses. The ultrafast coherent control of the exciton population and polarization is demonstrated by the observation of the reflectivity change in the pump-probe and the diffracted power in the degenerate-four-wave-mixing (DFWM) measurements. In single quantum wells, good coherent control characteristics with 87% coherent carrier destruction are demonstrated at the low excitation power of 0.15 mW (∼1.1×1010 photons/cm2). In addition, the preliminary manipulation of the exciton population is demonstrated by the phase-locked triple pulses. In the case of coupled quantum wells, the modulation of the SLM is carefully controlled and the coherent control of both the exciton population and the polarization is demonstrated in the two different coupled quantum-well samples.