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Physical Review Letters

Large Four-Wave Mixing of Spatially Extended Excitonic States in Thin GaAs Layers

Hajime Ishihara, Kikuo Cho, Koichi Akiyama, Nobuyuki Tomita, Yoshinori Nomura, Toshiro Isu

Published Jun 18, 2002• 77 citations
Publisher

Abstract

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.

DOI
10.1103/physrevlett.89.017402

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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.
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In nanostructures with a high excitonic coherence, the interplay between the spatial structure of the excitonic wavefunction and that of the radiation field plays an essential role, causing drastic size effects in the optical response, such as size-resonant enhancement of the radiative decay rate and the nonlinear response and the interchange of quantized levels. This article discusses such peculiar optical effects based on the theoretical treatment beyond the long-wavelength approximation and demonstrates the possibility of designing novel photofunctions with the explicit use of the spatial structures of the radiation field.
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.