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Ultrafast and Large Nonlinear Optical Response due to Spatial Structure of Internal Electric Field in Semiconductor Thin Film

Toshiro Isu, K Akiyama, N Tomita, T Nishimura, Y. Nomura, Hajime Ishihara, K Cho

Published Sep 30, 2002• 0 citations
Publisher

Abstract

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.

DOI
10.1201/9781482268980-61

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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.
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.
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.
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 nonlinear optical response of quantum well excitons is investigated experimentally using polarization resolved four wave mixing, optical-pump optical-probe, and optical-pump Terahertz-probe spectroscopy. The four-wave mixing data reveal clear signatures of coherent biexcitons which concur with straight-forward polarization selection rules at the Γ point. The type-I samples show the well-established time-domain beating signatures in the transients as well as the corresponding spectral signatures clearly. The latter are also present in type-II samples; however, the smaller exciton and biexciton binding energies in these structures infer longer beating times which, in turn, are accompanied by faster dephasing of the type-II exciton coherences. Furthermore, the THz absorption following spectrally narrow, picosecond excitation at energies in the vicinity of the 1s exciton resonance are discussed. Here, the optical signatures yield the well-established redshifts and blueshifts for the appropriate polarization geometries in type-I quantum well samples also termed “AC Stark Effect”. The THz probe reveals intriguing spectral features which can be ascribed to coherent negative absorption following an excitation into a virtual state for an excitation below the 1s exciton resonance. Furthermore, the scattering and ionization of excitons is discussed for several excitation geometries yielding control rules for elastic and inelastic quasiparticle collisions.
The vectorial four-wave mixing response of an individual strongly confined exciton–biexciton system with fine-structure splitting in a GaAs/AlGaAs quantum dot is measured by dual-polarization heterodyne spectral interferometry. The results are compared with theoretical predictions based on the optical Bloch equations. The system is described by a four-level scheme, which is a model system of the nonlinear excitonic response in low-dimensional semiconductors. We measure its coherence properties and determine the underlying dephasing mechanisms. An impact of the inhomogeneous broadening by spectral wandering on the coherent response is investigated. We further discuss the different four-wave mixing pathways, polarization selection rules, the time-resolved polarization state, the vectorial response in two-dimensional four-wave mixing and ensemble properties.