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AIP conference proceedings

Selective Generation Of Quantum Beats Of Weakly Confined Excitons

Osamu Kojima, Toshiro Isu, Junko Ishi‐Hayase, Masahide Sasaki, Masahiro Tsuchiya

Published Jan 1, 2007• 1 citation
Publisher

Abstract

We report the selective generation of quantum beats of weakly confined excitons in GaAs thin films. The oscillatory structure is observed in the degenerate four‐wave‐mixing (DFWM) signals, and the frequency varies with excitation energy. Comparing with the DFWM spectrum, we concluded that the origin of the oscillation is the quantum beat of the weakly confined excitons. Our results indicate the frequency selectivity of the excitonic quantum beats.

DOI
10.1063/1.2729963

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We report the selective generation of quantum beats of weakly confined excitons in GaAs thin films. The oscillatory structure is observed in the degenerate four‐wave‐mixing (DFWM) signals, and the frequency varies with excitation energy. Comparing with the DFWM spectrum, we concluded that the origin of the oscillation is the quantum beat of the weakly confined excitons. Our results indicate the frequency selectivity of the excitonic quantum beats.
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.
We report the effects of depolarization on the excitonic Rabi oscillation in GaAs thin films measured in the 2k1-k2 direction of a degenerate four-wave-mixing signal. The Rabi frequency, measured by changing the k2 pulse intensity, varies with the k1 pulse intensity; the Rabi frequency decreases with an increase in the k1 power. This decrease originates from the reduction in the field intensity of the k2 pulse, which consists of the local field and depolarization terms. Cancellation of the k2 field due to depolarization leads to the decrease in the Rabi frequency.
We observe ultrafast and large nonlinear optical responses of a semiconductor thin film where excitons are weakly confined and their center-of-mass motion is quantized. The observed degenerate four-wave mixing (DFWM) signal of a GaAs double-hetero-structure layer exhibits an anomalous thickness-dependence, and is much enhanced at the thickness of 110nm. The decay time of the response is the order of ps, which depends on the thickness of the layer and on the number of the layer. These phenomena are elucidated by the theory of the nonlocal interaction between excitonic states and the light beyond the long wavelength approximation regime. The enhancement at the thickness is due to the size-resonant enhancement of the internal field with a nano-scale spatial structure, where the second quantized level of the exciton center-of-mass motion mainly contributes to the large response at the thickness. We, furthermore, demonstrate ultrafast response in a reflective type nonlinear polarization rotation switch with the 110nm thick three double-hetero. The signal intensity of 20% for the reflective pulse was obtained at a pump intensity of 10nJ/cm2, and the decay time is 1.5ps at low temperatures. These results provide a novel design of a semiconductor structure in order to obtain both large and ultrafast nonlinear response that is suitable for device application.
Multiple frequency quantum beats of a system of the coherently excited quantum confined exciton states in a high-quality semiconductor structure containing a wide InGaAs/GaAs quantum well are experimentally detected by the spectrally resolved pump-probe method. The beat signal is observed both at positive and at negative delays between the pump and probe pulses. Several quantum beat (QB) frequencies are observed in the experiments, which coincide with the interlevel spacings in the exciton system. A theoretical model is developed, which allows one to attribute the QBs at negative delay to the four-wave mixing (FWM) signal detected at the nonstandard direction. The beat signal is strongly enhanced by the interference of the FWM signal with the secondary emission induced by the probe pulse. At positive delays, the QBs are due to the interference of the quantum confined exciton states. The decay time for QBs is of the order of several picoseconds both at positive and negative delays. This is close to the relaxation time of the exciton population that allows one to consider the exciton depopulation as the main mechanism of the coherence relaxation in the system under study.
We study the size dependence of the nonlinear response of weakly confined excitons for the size region beyond the long wavelength approximation regime. The observed degenerate-four-wave mixing signal of GaAs thin layers exhibits an anomalous size dependence, where the signal is resonantly enhanced at a particular thickness region. The theoretical analysis elucidates that this enhancement is due to the size-resonant enhancement of the internal field with a spatial structure relevant to the nondipole-type excitonic state. These results establish the formerly proposed new type of size dependence of nonlinear response due to the nonlocality induced double resonance.