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Applied Physics Express

Ultrafast All-Optical Control of Excitons Confined in GaAs Thin Films

Osamu Kojima, Ayumi Miyagawa, Takashi Kita, Osamu Wada, Toshiro Isu

Published Nov 14, 2008• 6 citations
Publisher

Abstract

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.

DOI
10.1143/apex.1.112401

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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.
In this paper, we experimentally demonstrate an ultrafast and large nonlinear optical response by controlling the thickness of the GaAs layers in a double hetero-structure through a degenerate four-wave mixing (DFWM) experiment. We show that the polarization corresponding to the second quantized level of the CM state (n = 2) induces a large nonlinear response although this is optically forbidden for ordinal treatment using LWA. We prepare very high quality samples of GaAs layers in order to properly measure the nonlinear optical response of excitons, which extend coherently over the sample. A calculation based on a nonlocal response theory is performed to explain the experimental result.
An excitonic approach to the ultrafast optical response of confined semiconductors at elevated densities below the Mott transition is presented. The theory is valid from the coherent regime, where coherent excitonic transitions and biexcitons dominate, to the incoherent regime, where excitonic occupations dominate. Numerical simulations of the 1s1s1s exciton dynamics during intense circularly polarized pump pulses in two different Coulomb-interaction regimes are performed for two-dimensional semiconductors: Moderate Coulomb interaction is compared with dominating Coulomb interaction with respect to the light-matter interaction strength. The different many-body contributions are disentangled and it is found, that excitonic Rabi oscillations in the Coulomb-dominated regime are considerably less strong. By also comparing circular and linear excitation in a MoSe2_22​ monolayer, it is found, that linear excitation creates a regime, where excitonic Rabi oscillations are almost completely suppressed.
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.