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Chinese Physics

Enhancement of absorptive optical nonlinearity of asymmetrically coupled quantum well structures

胡振华, 黄德修

Published Mar 28, 2005• 2 citations
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Abstract

The third-order nonlinear susceptibility X^(3) of asymmetrically coupled quantum well (ACQW) structure is calculated by employing a V-type three-level model. The efficiency of absorptive optical nonlinearity, which we define as the ratio of the third-order nonlinear susceptibility X^(3) to the linear absorptive coefficient αo, is analysed for different electronic coherence oscillation frequencies. We show that the efficiency is enhanced with the increase of the electronic coherence oscillation frequency between the two wells but is degenerated as that of a single quantum well when the electronic coherence oscillation frequency is zero. Compared with single quantum well, the ACQW designed has the property of nonlinear absorption and dispersion depending strongly on the external electric field along the growth direction. We predict that ACQW structures can provide both high efficiency limiters and controllable optical Kerr switch in future communication systems.

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Chinese Journal of Luminescence•Jan 1, 2001•1 citation
The third-order nonlinear susceptibility X^(3) of asymmetrically coupled quantum well (ACQW) structure is calculated by employing a V-type three-level model. The efficiency of absorptive optical nonlinearity, which we define as the ratio of the third-order nonlinear susceptibility X^(3) to the linear absorptive coefficient αo, is analysed for different electronic coherence oscillation frequencies. We show that the efficiency is enhanced with the increase of the electronic coherence oscillation frequency between the two wells but is degenerated as that of a single quantum well when the electronic coherence oscillation frequency is zero. Compared with single quantum well, the ACQW designed has the property of nonlinear absorption and dispersion depending strongly on the external electric field along the growth direction. We predict that ACQW structures can provide both high efficiency limiters and controllable optical Kerr switch in future communication systems.
The third-order nonlinear susceptibility χ(3) of asymmetrically coupled quantum well (ACQW) structure is calculated by employing a V-type three-level model. The efficiency of absorptive optical nonlinearity, which we define as the ratio of the third-order nonlinear susceptibility χ(3) to the linear absorptive coefficient α0, is analysed for different electronic coherence oscillation frequencies. We show that the efficiency is enhanced with the increase of the electronic coherence oscillation frequency between the two wells but is degenerated as that of a single quantum well when the electronic coherence oscillation frequency is zero. Compared with single quantum well, the ACQW designed has the property of nonlinear absorption and dispersion depending strongly on the external electric field along the growth direction. We predict that ACQW structures can provide both high efficiency limiters and controllable optical Kerr switch in future communication systems.
In the past few years advance in nanofabrication techniques have made it possible to make well-like compound semiconductor structures of low nanometer size.In these structures,the state of the electron in the direction perpendicular to the length of the well is quantized with sufficiently large separation in energy levels and the motion of the electron parallel to the length is that of a quasi-free electron.One of the most remarkable properties of these one-dimensional electron system is that the optical transitions between the size-quantized subbands are feasible.The result shows there is a large optical nonlinearities in asymmetric quantum wells.The problem of optical saturation of intersubband absorption has recently attracted considerable attention.The problem is very important to the development of superlattice devices such as fast infrared detectors and(modulators) because one needs to know the range of input optical intensities for which linear operation is(possible.) The analytical expression of the linear and the third-order nonlinear optical intersubband absorption coefficients are obtained in Pschl-Teller wells using the density matrix formalism taking into account the(intrasubband) relaxation.Since there are two adjustable parameters κ and λ in Pschl-Teller wells,the shapes of the wells will change with the change of the parameters κ and λ,accordingly the linear and the third-order nonlinear optical absorption coefficients will change with the parameters κ and λ.It is shown that the optical absorption coefficient depends on the parameter,and the optical intensity strongly. The absorption saturation will occur with I increase.Numerical results are presented for a typical AlGaAs/GaAs Pschl-Teller well.
The absorption and dispersion behaviors of an asymmetrically coupled quantum well (ACQW) structure are studied based on V-type system. We show that the quantum-confined Stark shift due to an electric field applied along the growth direction of ACQW leads to the detuning absorption, and the corresponding linear refraction is dramatically decreased in a smaller negative-bias voltage region. This means that refraction quenching induced by an external electric field can occur in an ACQW structure. However, the absorption and the dispersion behaviors become the same as those of the lowest excitonic state in a single quantum well structure as the negative-bias voltage further increases due to the disappearance of quantum-confined Stark effect. This means that the ACQW structure has controllable dispersion by using an external field.
The analytic forms of the linear and the third-order nonlinear optical intersubband absorption coefficients were obtained for general asymmetric quantum well systems using the density matrix formalism.Based on the model,the linear and the third-order nonlinear intersubband optical absorptions in electric-field biased hyperbolic quantum wells were studied.The numerical results were presented for a typical AlxGa1-xAs/GaAs electric-field biased hyperbolic quantum wells.The results show that the contributors to the nonlinear absorption coefficient are due to the electric field,the shape of quantum well and the optical intensity.
Based on the density matrix approach and iterative method, we calculated the analytic forms of linear and third-order nonlinear optical absorption coefficients and refractive index changes for symmetric coupled quantum wells (SCQWs) considering a two-level system, and gave the corresponding numerical results. The calculated results show that the optical properties of SCQWs are significantly better than those of asymmetrically coupled quantum wells, and the optical absorption coefficient and refractive index change are affected by the structure parameters of the SCQWs and the incident optical intensity. Their physical mechanisms are elucidated.
In recent years,the subject of optical nonlinearity in semiconductor quantum confined structures such as quantum wires and quantum dots has attracted a considerable attention because of their relevance for studying practical applications and as a probe for the electronic structure of media.It has been shown that the low dimensional quantum systems present very large nonlinear optical susceptibilities .Theoretically,most of the studies on quantum dots are focused on spherical quantum dots for simplicity.The shape of quantum dot depends on growth conditions.In fact,quantum dots are better described as thin disk or cylinder.In the present work third order nonlinear optical susceptibilities in disk shaped quantum dots due to resonant intersubband transition are analyzed by a compact density matrix approach.It has rarely been discussed in literatures.Firstly,we consider an electron moving in a quantum disk whose radius R is very large compared to its thickness d .For simplicity,we assume that the isotropic effective mass approximation is valid within the disk and that the inner potential is null while the outer confining potential is approximately infinite.Using the adiabatic approximation we solve the three dimensional Schrdinger equation and obtain the analytical expressions of wave functions and energy levels in cylindrical coordinates.Secondly,we consider the system is excited by an electromagnetic field with frequency ω. The electromagnetic field is incident with a polarization vector normal to the disk.We treat H′=ezE(t) as a perturbation term.By using the compact density matrix method,we derive a general formula for third order nonlinear optical susceptibilites employing a four level model in quantum disk.Finally,numerical results are illustrated for a typical GaAs quantum disk.We calculate the third order nonlinear optical susceptibilities as a function of the disk thickness.It is shown that the third order nonlinear optical susceptibilities in quantum disk are 10～100 times larger than that in quantum well for various disk thickness.Furthermore,if we choose an optimized thickness of quantum disk,we could obtain a quite large refractive index with a comparatively small absorption coefficient.Therefore,we suggest that the quantum dots would be an excellent system in the field of optical communications and optical switching.