非对称量子阱中线性和三阶非线性光吸收系数的研究

主要研究了一个特殊非对称量子阱中的线性和三阶非线性光吸收系数。首先利用量子力学中的密度矩阵算符理论和迭代方法导出了线性和三阶非线性光吸收系数的表达式，然后以典型的非对称量子阱GaAs／AIGaAs材料为例作了数值计算。数值结果表明，入射光强以及系统的非对称性对总的光吸收系数有比较大的影响，从而为实验研究提供理论依据。     

Spin-orbit splitting dependent resonant third-order nonlinear optical susceptibility in InGaN/GaN multiple quantum wells

For the transition between valence band and conduction band, the third-order nonlinear optical susceptibility χ⁽³⁾ for degenerated four-wave mixing in In_xGa_1−xN/GaN multiple quantum wells (MQWs) has been calculated. The contributions of spin-orbit split-off energy to the resonant third-order nonlinear optical susceptibility of the modes, whose polarization is vertical to the [0 0 1] direction of the MQWs, are discussed in detail. The correlations between the peaks of χ⁽³⁾, which are due to the transitions from the spin-orbit split-off energy level to first conduction subband, and the width of the quantum well and the constituents of the semiconductor material are obtained.     

Growth and optical characterization of strained CdZnTe/ CdTe quantum wells

We have grown strained Cd_1-xZn_xTe(x ≈ 0.2)/CdTe single and multiple quantum wells by molecular beam epitaxy. GaAs was used as a substrate. The well widths were systematically increased until the critical thickness was exceeded. Low-temperature (liquid helium) photoluminescence (PL) spectroscopy was used to characterize the films. Two prominent PL peaks were observed: one arising from the quantum well and the other from the barrier material. The energy of the quantum well luminescence is consistent with theory when strain is included. The critical layer thickness for the CdTe quantum wells was found to be between 150 and 175 å, in agreement with the model of Matthews and Blakeslee.     

Auger recombination in strained quantum wells

A nonthreshold mechanism for Auger recombination of nonequilibrium carriers in quantum wells with strained layers is investigated theoretically. It is shown that the dependence of the Auger recombination rate on the magnitude of the strain and the height of the heterobarriers for electrons and holes can be analyzed only by calculating the overlap integrals between initial and final particle states microscopically. In quantum wells with strained layers the presence of strain affects qualitatively and quantitatively the electron-hole overlap integral. The dependence of the Auger recombination rate on the quantum well parameters, the magnitude of the stress, and temperature are analyzed for heterostructures based on InGaAsP/InP and InGaAlAs/InP. Fiz. Tekh. Poluprovodn. 31, 358–364 (March 1997)     

Optical investigations of strained InGaAs quantum wells

InGaAs/GaAs MOCVD-grown quantum wells have been investigated. Photoluminescence (PL) measurements have shown heavy-hole-related excitonic transitions within the temperature range from 10 to 100 K for all samples. In room-temperature photoreflectance (PR), sharp heavy- and light-hole excitonic transitions in the quantum wells have been observed. The transition energies obtained have been compared with values derived from theoretical considerations using the envelope function model including lattice-mismatch-related stress. The heavy- and light-hole transitions have been identified as excitonic transitions of types I and II respectively. © 1997 John Wiley & Sons, Ltd.     

Interband nonlinear optical generation in presence of intersubbandlight in asymmetric quantum wells

The interband sum frequency generation process due to three-wave interaction of interband and intersubband coupling lights has been investigated in a semiconductor quantum well using the perturbational density matrix approach. The origin of the nonlinear process lies in the second-order susceptibility χ_c2-h1⁽²⁾ arising due to the optical transition between the second conduction subband and the first heavy hole state. Both the sign and the magnitude of the second-order susceptibility of the well may be controlled by the carrier density level and the frequency of the intersubband field     

An ultra-high level second-order nonlinear optical susceptibility in strained asymmetric GaN–AlGaN–AlN quantum wells: Towards all-optical devices and systems

In this paper an asymmetric structure for enhancement of second-order nonlinear optical susceptibility in the strained asymmetric GaN–AlGaN–AlN quantum well (QW) is proposed. In this structure, the strain-induced spontaneous and piezoelectric effects have been taken into account, and the second-order optical susceptibility of the δ-doped step QW structure is analyzed by considering the Shrödinger–Poisson self-consistent for different Al mole fraction x, step position, and pump photon energy hω. The magnitude of the second-order susceptibility of the simulated results for the proposed structure show an enhancement more than 400 times compared traditional strained QWs. Our simulations show that with increasing well width in the case of constant step width, peak of the second-order susceptibility is increased as well as resonant wavelength is decreased. Also, it is shown that for a given well and step widths there is an optimum mole fraction corresponding to maximum second-order susceptibility. Finally in the case of constant well width and mole fraction of the step, with increasing step width the second-order susceptibility is decreased.     

Single-pulse pump-probe measurement of optical nonlinear propertiesin GaAs/AlGaAs multiple quantum wells

Single-pulse ps-pump and ns-probe nonlinear transmission measurements provide carrier-density-dependent optical nonlinear spectra in GaAs/AlGaAs multiple quantum wells grown by metalorganic chemical vapor deposition. The use of the ps pump eliminates the need to know carrier lifetime to determine carrier density. The saturation behavior of changes in absorption coefficient and refractive index are modeled by a simple saturation equation to obtain saturation carrier density. The saturation spectra for different well thicknesses are obtained. The minimum saturation carrier density appears around 150 Å     

Theoretical studies of optical modulation in lattice matched and strained quantum wells due to transverse electric fields

Electrooptical modulators based on quantum well structures have become an important area of research due to potential applications in high-speed optical modulation and image processing. In this paper, we examine the physics of a quantum well modulator within the generalized Kohn-Luttinger Hamiltonian. Issues of importance for the modulator structure are the excitonic absorption shift, exciton binding energy, line broadening, tunneling rates for electrons and holes in the presence of a transverse electric field, and changes in optical absorption coefficients as a function of electric field. A formalism to study these effects for both lattice matched and nonlattice matched quantum well structures is provided and the potential of material tailoring for specific optical response is discussed. It is shown that the reliability of this technology is critically related to the fabrication of high-quality interfaces and alloys since even a one-monolayer variation in quantum well size can have a substantial effect on the modulation properties.     

Electroabsorption enhancement in tensile strained quantum wells viaabsorption edge merging

Electroabsorption in quantum wells under biaxial tension is investigated theoretically. It is found that enhanced electroabsorption due to a field-induced merging of the light and heavy hole absorption edges can be achieved in these structures at moderate operating fields. Calculations showing this merging and electroabsorption enhancement for In_xGa_1-xAs-InP and GaAs_xP_1-x-Al_0.35Ga_0.65As quantum well structures are described. Trade-offs involving the advantages of merged absorption edges are identified through comparisons of tensile strained modulators utilizing the merging effect of analogous lattice matched structures. Optimal structures for operation at 1.55 μm in In_xGa_1-xAs-InP and 0.77 μm in GaAs_xP_1-x-Al_0.35Ga_0.65As are identified, and the sensitivities of their electroabsorption characteristics to material and structural parameters are examined     

Calculation of the size-quantization levels in strained ZnCdSe/ZnSe quantum wells

The ZnSe and CdSe parameters required to calculate levels in ZnCdSe/ZnSe quantum wells are determined by fitting to published data. The model is shown to be adequate for the example of structures with a collection of quantum wells whose thickness and composition were determined by independent methods. Fiz. Tekh. Poluprovodn. 31, 939–943 (August 1997)     

Enhanced optical polarization anisotropy in quantum wells under anisotropic tensile strain

Anisotropic in-plane strain in quantum wells leads to an optical polarization anisotropy that can be exploited in optoelectronic devices such as modulators. A theoretical model shows that the behavior of the polarization anisotropy with increasing strain anisotropy is radically different for quantum wells under anisotropic tensile and compressive strains of equal magnitude. This strikingly different behavior arises from the different valence-subband mixing that occurs in the cases of anisotropic tensile and compressive strain. Specifically, the mixing of the first heavy- and light-hole subbands that occurs only under anisotropic tensile strain is central to the polarization anisotropy.     

Band structure of strained quantum wells grown on novel index surfaces

We present a theoretical study of the in-plane valence subband structure of unstrained GaAs---AlGaAs, compressively strained InGaAs---AlGaAs and tensile strained GaAsP---AlGaAs quantum wells grown along the (001), (111), (011) and (113) directions. From numerical solution of the 6 × 6 Luttinger k · p hamiltonian we find that confinement energies, warping and effective in-plane masses strongly depend on the direction of confinement and on strain. Piezoelectric effects further affect the dispersion for the (111) and (113) directions. Besides, we give analytic expressions for the heavy and light hole in-plane and perpendicular effective masses for any (hkl) growth direction in the limit of uncoupled subbands.     

Terahertz luminescence of GaAs-based heterostructures with quantum wells under the optical excitation of donors

Spontaneous emission from selectively doped GaAs/InGaAs:Si and GaAs/InGaAsP:Si heterostructures is studied in the frequency range of ～3–3.5 THz for transitions between the states of the two-dimensional subband and donor center (Si) under the condition of excitation with a CO₂ laser at liquid-helium temperature. It is shown that the population inversion and amplification in an active layer of 100–300 cm^?1 in multilayered structures with quantum wells (50 periods) and a concentration of doping centers N _D ≈ 10¹¹ cm^?2 can be attained under the excitation-flux density 10²³ photons/(cm² s).     

Analytical formulas for the optical gain of quantum wells

Analytical expressions for the quantized energy levels in quantum wells, the optical gain, the differential optical gain, and the linewidth enhancement factor are presented based on a simple parabolic-band gain model. Explicit formulas show clearly the dependence of these factors on well width, doping, and photon energy. The optical gain in the form of g=g₀ In(N/N₀) is derived using explicit approximations in the Fermi functions, where g₀ is the proportionality constant, N is the injected carrier density, and N ₀ is the transparency carrier density. The approximate formulas are shown to provide not only an efficient way of computing the gain-related parameters but also a convenient way of getting physical insights into the overall interplay of quantum well parameters     

双曲型量子阱中非线性光学吸收率的计算

量子阱中的非线性光学效应因其潜在的实用价值而引起了人们的广泛的关注,而量子阱内带间的光学吸收问题对研究远红外光学探测器等光电子器件具有重要的理论指导意义.用量子力学中的密度矩阵算符理论导出了双曲型量子阱中的线性与三阶非线性光学吸收系数的表达式.因双曲型量子阱中有一个可调参数,随着参数的增加,阱宽将相应增加,因此势阱的形状以及阱内的非线性光学吸收率将随参数变化而发生规律性的变化,并且当入射光强增强到一定程度会出现较强的饱和吸收现象,通过对这些规律的研究从而为实验研究提供了必要的理论依据.     

Carrier heating in quantum wells under optical and current injection of electron-hole pairs

Carrier heating in GaAs/AlGaAs quantum wells (QWs) under optical interband pumping in the spontaneous-emission mode has been studied. The electron temperature was determined as a function of the pumping intensity. The effect of the electric field on the photoluminescence spectrum was examined. The change in the carrier concentration with the drive current in the spontaneous- and stimulated-emission modes in InGaAsSb/InAlGaAsSb QWs was determined from electroluminescence spectra. The rise in the temperature of hot carriers, which results in the increase in the carrier concentration with the drive current, was roughly estimated.     

Parameter-dependent third-order nonlinear susceptibility of parabolic InGaN/GaN quantum dots

The electron states confined in wurtzite In_xGa_1−xN/GaN-strained quantum dots (QDs) have been investigated in the effective-mass approximation by solving the Schrödinger equation, in which parabolic confined potential and strong built-in electric field effect due to the piezoelectricity and spontaneous polarization have been taken into account. The third-order nonlinear susceptibility of the QDs in various directions (both parallel to z direction and vertical to z direction) have been calculated, and the magnitude reaches 10⁻¹⁴ m²/V². It has been shown from the results that the order of the built-in electric field in the strained QD is of MV/cm. Furthermore, the results of how the third-order nonlinear susceptibility depend on the radius R of QDs, the height L of QDs, the In content x of QDs and the relaxation rate Γ₁₀ have been given.     

Mid-IR optical limiter based on type-II quantum wells

We show that strong optically induced intervalence band transitions in type-II InAs/GaSb/AlSb quantum wells lead to reverse saturable absorption, and propose to apply it to optical limiting in the mid-infrared spectral region. A salient feature of the proposed limiter is the flexibility of the design for different wavelengths and threshold powers. We develop a rigorous theoretical model of the proposed limiter, and use it to estimate the relationship between the key figures of merit: insertion loss, threshold, optical bandwidth and dynamic range. We investigate tradeoffs involving the dynamic range, optical bandwidth and thickness of the limiter, and show that one can attain a favorable combination of these.