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IEEE Journal of Quantum Electronics

Ultrafast optical nonlinearity by virtual charge polarization in DC-biased quantum well structures

M. Yamanishi, M. Kurosaki

Published Feb 1, 1988• 9 citations
Publisher

Abstract

An optical nonlinearity due to virtual charge polarization in a quantum-well structure biased by a DC electric field is proposed and discussed. The switching time of the nonlinearity is expected to be extremely short, on the order of 100 fs. An effective degenerative four-wave chi /sup 3/ parameter is estimated to be 1.0*10/sup -9/ (esu) for a graded gap AlGaAs quantum well and a detuning energy of pump light, 35 meV. The nonlinearity is observable and quite useful for designing an all-optical and ultrafast optical gate.>

DOI
10.1109/3.129

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jun 2, 2003•1 citation
An optical nonlinearity due to virtual charge polarization in a quantum-well structure biased by a DC electric field is proposed and discussed. The switching time of the nonlinearity is expected to be extremely short, on the order of 100 fs. An effective degenerative four-wave chi /sup 3/ parameter is estimated to be 1.0*10/sup -9/ (esu) for a graded gap AlGaAs quantum well and a detuning energy of pump light, 35 meV. The nonlinearity is observable and quite useful for designing an all-optical and ultrafast optical gate.>
A new concept on optical nonlinearity due to virtual transitions in quantum well structure with electric field, named virtual charge-induced optical nonlinearity, is proposed, with a showing of some examples of theoretical result on the nonlinearity. The switching time of the nonlinearity is expected to be extremely short, 100 fs. The nonlinearity seems to be observable and quite useful for the design of an ultrafast optical gate. A possibility of optical bistability without external optical feedback, based on the proposed nonlinearity, is pointed out.
It has recently been shown that the dipole matrix elements between subbands in semiconductor quantum wells are extremely large.1 Third-order nonlinear susceptibilities are proportional to the fourth power of these matrix elements. Large nonlinearities have thus been predicted for these systems, in particular an intensity-dependent index of refraction suitable for use in all-optical logic elements.1 Second-order nonlinearities vanish identically in symmetric wells, suggesting the use of asymmetrically graded quantum wells for quadratic interactions.2 External bias electric fields also break the inversion symmetry, resulting in Stark shifts and large quadratic susceptibilities whose sign depends on the direction of the applied field. The magnitude of the nonlinearity depends on the carrier density, the dc bias field, and the detuning of the fundamental and second harmonic frequencies from resonances.
A new mechanism for enhancing optical nonlinearities in quantum well structures, subject to an electric field normal to the layers, is investigated. It is found that large nonlinearities are induced by virtual transitions of Wannier excitons with "giant dipole moments." In contrast to other mechanisms, this enhancement mechanism is independent of correlations between excitons, for any order of nonlinearities. This fact results in a resonant enhancement at half the excitonic energy, and an ultrafast response, even faster than other virtual-process-induced mechanisms.
Optical nonlinearities of Al(x)Ga(1-x)As/GaAs asymmetric coupled quantum wells within a p-i-n structure have been observed by using time-resolved spectroscopy. Under photoexcitation, exciton absorption lines exhibit spectral shifts of as much as approximately 1 meV. The recovery times of these spectral shifts are of the order of hundreds of picoseconds when the excitation photon is above the lowest exciton state but become less than 10 psec when excitation is below the lowest exciton state, indicating a virtual process. The behavior of these spectral shifts is consistent with the presence of a polarization induced by optical pumping. The polarization is believed to be due to the excitons that have nonvanishing electric dipole moments along the axis of the asymmetric coupled quantum wells.
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.