The effect of interdiffusion on the intersubband optical propertiesin a modulation-doped quantum-well structure

The linear and nonlinear (based on optical field intensity) intersubband absorptions in conduction band, and its change in refractive index in AlGaAs-GaAs interdiffused quantum wells (QWs) are presented. The calculation of the electron energy levels and the envelope wave functions in a modulation doped interdiffused QWs with screening effects are considered. QW interdiffusion shows a wavelength tunability of the intersubband absorption peaks and refractive index dispersions. This shifting of the transition energies is also demonstrated here to be a useful technique for broad-band and multicolor photodetector application. In addition, it can serve to remove noise, such as minor peaks and dispersions, in the optical spectra     

n型半导体量子阱红外探测器的正入射吸收机制

基于有效质量理论，从导带子带间光跃迁矩阵元的表达式出发，推出ｎ型半导体量子陆红外探测器的正入射吸收条件，用一些简单的数学手段，把正入射的吸收系数表达为量子阱生长方向的解析函数，进而讨论正入射吸收的优化、极限及与平行吸收的比较。     

Macroscopic defects in GaN/AlN multiple quantum well structures grown by MBE on GaN templates

We have used MBE to grow in AlN/GaN superlattices, with different number of periods, on 2.5-μm-thick MOVPE-GaN templates to study the development of defects such as surface deformation due to strain. After growth the samples were studied by atomic force microscopy (AFM), transmission electron microscopy (TEM), XRD and Fourier transform infrared spectroscopy (FT-IR). The strain increased with the number of quantum wells (QWs) and eventually caused defects such as microcracks visible by optical microscopy at four or more QW periods. High-resolution TEM images showed shallow recessions on the surface (surface deformation) indicating formation of microcracks in the MQW region. The measured intersubband (IS) absorption linewidth from a four period structure was 97 meV, which is comparable with the spectrum from a 10 period structure at an absorption energy of ∼700 meV. This indicates that the interface quality of the MQW is not substantially affected by the presence of cracks.     

Understanding the ultra-low intersubband saturation intensity in InGaAs-AlAsSb quantum wells

We report a novel intersubband material system based on strained InGaAs-AlAs-AlAsSb quantum wells that exhibits a very low, 3 fJ//spl mu/m/sup 2/, saturation intensity and about 2-ps carrier-relaxation time at 1.68 /spl mu/m. We performed a density matrix calculation to estimate the saturation intensity by simulating the pulsed excitation conditions of the experiment. These studies indicate slow dephasing time and reduced inhomogeneity as the possible mechanisms for the observed large nonlinearity. A detailed dephasing time calculation shows that rather than the reduction in the electron effective mass in these strained quantum wells (QWs), the effect of enhanced concentration of doped carriers in these QWs with reduced inhomogeneity could be the origin of slow dephasing and the observed large nonlinearity.     

Intersubband absorption of light in heterostructures with double tunnel-coupled GaAs/AlGaAs quantum wells

Intersubband absorption of mid-IR light was studied in heterostructures with asymmetrical tunnel-coupled quantum wells in equilibrium conditions and under high-power pumping by picosecond pulses of light. The energy spectrum of electrons in tunnel-coupled quantum wells was found from an analysis of equilibrium and nonequilibrium intersubband absorption spectra. The dynamics of intersubband absorption under high-power optical pumping was studied using the pump-and-probe picosecond technique. The experimental data are compared with the results of calculations based on solving rate equations. The intersubband relaxation times are determined.     

High-speed mid-IR modulator using Stark shift in step quantum wells

We show in calculations that there is a capability for high speeds with a low applied voltage in modulators based on intersubband transitions in step quantum wells (QWs). A waveguide based on surface plasmons is assumed to achieve the necessary tight confinement of the optical field. In a structure with 8 GaInAs-AlGaInAs-AlInAs step QWs, we obtain a device capacitance of 14 fF corresponding to a RC limitation of electrical f_{3 dB}=190 GHz. The extinction ratio of 6.6-μm light is 10 dB at an applied voltage of 0.9 V and T=300 K. By simple reasoning, we find that the device capacitance is approximately proportional to the absorption linewidth cubed when the linewidth is considered in the device design. Thus, the linewidth is very decisive for the modulation speed. We propose to place the dopants asymmetrically in the barriers in order to reduce broadening caused by doping induced potential fluctuations. In addition, the doping levels in the outermost barriers of the multi-QW structure are proposed to be reduced and asymmetrical, in order to achieve a uniform electric field over the step QWs, which is shown to increase the achievable f_{3 dB} very markedly     

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.     

SiGe／Si多量子阱中垂直方向红外吸收及共振色散效应

用垂直入射的中红外光束调制非掺杂SiGe/Si量子阱中光致子带间吸收，氩离子激光器作为子带间跃迁的光泵浦源在阱中产生载流子，红外调制光谱用步进式傅立叶变换光谱仪记录，实验中观察到明显的层间干涉效应与子带间跃迁有关的色散效应，理论和实验分析认为样品折射率变化造成的位相调制可以补偿吸收所造成的幅度调制。     

Hole mixing and strain effects in the conduction intersubband transitions of undoped quantum wells

We study the effects of hole dispersion and mixing in the conduction intersubband transitions and infrared dressing of undoped quantum wells (QWs). This is done considering transitions of photo-excited electrons from one conduction subband (e1) to another (e2) in the presence of Coulomb interaction with the photo-excited holes. We show that, when the dispersion of the hole subband hh1 (lh1) is mainly parabolic, these transitions occur mainly between the s-states of e1-hh1 (e1-lh1) and e2-hh1 (e2-lh1) excitons with the same principal quantum numbers (allowed transitions). When the hole subbands have nonparabolic dispersions, however, such transitions are suppressed while another type of intersubband transitions in which the initial and final exciton states have different principal quantum numbers (nonallowed transitions) are enhanced. We show the enhancement and suppression processes reach their maxima when hh1 and lh1 are about to cross over, allowing multilevel mixing of excitons to occur when the undoped QW interacts with a single intense infrared field polarized along its growth. We associate these results with spinor mixing of hh1 and lh1 and illustrate how via changing the spinor contributions in these subbands one can employ strain to manipulate the dipole moments of the intersubband transitions.     

A multi-color CdS/ZnSe quantum well photodetector for mid- and long-wavelength infrared detection

In this paper, we report on the design and characterization of a quantum well based infrared photodetectors covering simultaneously infrared radiation within mid- and long-infrared spectral regions. The proposed infrared photodetectors rely on intersubband transitions in asymmetric ZnSe/CdS double quantum wells. The three-energy-level and the wavelengths of the intersubband transitions in the asymmetric double quantum wells are obtained by solving the Schrödinger and Poisson equations self consistently, the influence of the right well width on the absorption coefficient is studied. The peak positions of intersubband absorption coefficients in the structure are found at 3.31, 4.4 and 13.5 µm for a 1 nm right well width while the absorption peak positions are located at 3.33, 6.43 and 6.95 µm for a 1.4 nm right well thickness. Then, the electro-optic performances of the infrared photodetector are evaluated; the dark current dependence with the applied voltage and temperature is discussed. This work demonstrates the possibility of detection of widely separated wavelength bands using intersubband transitions in quantum wells with a low dark current.     

1.55-μm picosecond all-optical switching by using intersubbandabsorption in InGaAs-AlAs-AlAsSb coupled quantum wells

We report on the first all-optical switching operation of intersubband absorption at an optical communication wavelength (~1.55 μm). The 1.55-μm intersubband absorption was achieved by InGaAs-AlAs-AlAsSb coupled quantum wells. A switching operation on an ultrafast signal (equivalent to 1 THz) was successfully demonstrated with a control pulse energy as low as 27 pJ     

Intersubband transitions at 1.3 and 1.55 μm in a novel coupledInGaAs-AlAsSb double-quantum-well structure

We report the first observation of intersubband absorption at 1.3 μm in a coupled InGaAs-AlAsSb double-quantum-well structure lattice matched to InP substrate. The novel structure exhibits intersubband transitions concurrently at 1.3 and 1.55 μm due to the strong coupling of the intersubband states of the adjacent wells. A transient analysis of optical response in the multilevel conduction subband states for the designed structure indicates ultrafast optical switching behavior with a subpicosecond response     

Intersubband Absorption in Terahertz Lasers Based on Optically Pumped Quantum Well Structures

We study the effects of microscopic dynamics of electrons on the intersubband optical absorption in terahertz lasers based on optically pumped quantum wells (THz-OPQW). The ensemble Monte Carlo method is used to include different electron scattering mechanisms in the calculation, so that the dependence of intersubband absorption lineshape on microscopic dynamics of electrons can be easily investigated. We find that electron-electron scattering is the dominant factor to determine the absorption linewidth. The real distributions of hot electrons are included to study the temporal changes of the spectra lineshape. Our findings show that the dependence of spectral lineshape on electron population essentially results from the electron scattering and the non-Fermi electron energy distribution.     

Analysis of reduced interband absorption mechanisms insemiconductor quantum wells

Mechanisms for reduced interband absorption using intersubband coherence or intersubband coupling in semiconductor quantum wells are analyzed by exactly solving the density matrix equation. By placing some mild limitations on the diagonal density matrix elements, general analytic solutions are obtained without weak-field restrictions. Our solutions demonstrate reduced absorption, various nonlinear processes, and coupling-field-modulation of interband transitions. Our results agree well with previous results for three-level atomic systems, give insights into nonlinear mechanisms, and point out the importance of nonperturbative approaches to this class of problems     

半抛物量子阱中的线性与非线性光学吸收系数

采用密度矩阵与迭代处理的方法,给出了计算施加电场的半抛物量子阱(QWs)系统中的线性与非线性光学吸收系数的详细过程.并且推导了此系统中的简单解析的线性与非线性光学吸收系数.对GaAs材料构成的QWs进行了数值计算.结果显示,系统中的线性与非线性光学吸收系数敏感地依赖于外加的电场强度、QWs的受限势频率以及入射光的强度.     

Effect of the number of quantum wells in the active region on the linearity of the light-current characteristic of a semiconductor laser

The light-current characteristic of a semiconductor laser with multiple quantum wells (QWs) is calculated, with the delayed capture of charge carriers from the waveguide region into the wells taken into account. It is shown that increasing the number of QWs is a more effective way to improve the power characteristics of a laser, compared with an increase in the velocity of carrier capture into each of the wells. For example, using two QWs as the active region leads to a substantial increase in the internal quantum efficiency of stimulated emission and to a significantly better linearity of the light-current characteristic of the laser, compared with a single-well structure. At the same time, using three or more QWs only slightly improves the power characteristics of the laser, compared with the double-well structure. Thus, a double-well structure is the optimal as regards high output power and simplicity of growth.     

Strain effect on intersubband transitions in rolled-up quantum well infraredphotodetectors

Pre-strained nanomembranes with four embedded quantum wells (QWs) are rolled up into threedimensional (3D) tubular QW infrared photodetectors (QWIPs),which are based on the QW intersubband transition (ISBT).A redshift of~0.42 meV in photocurrent response spectra is observed and attributed to two strain contributions due to the rolling of the pre-strained nanomembranes.One is the overall strain that mainly leads to a redshift of~0.5 meV,and the other is the strain gradient which results in a very tiny variation.The blue shift of the photocurrent response spectra with the external bias are also observed as quantum-confined Stark effect (QCSE) in the ISBT.     

Comparison of hole and electron intersubband absorption strengthsfor quantum well infrared photodetectors

It is well known that the hole intersubband absorption of normally incident (TE polarized) radiation is nonzero for p-doped quantum well infrared photodetectors (p-QWIP's) which have been fabricated without an optical grating. This present paper shows from k&oarr;·p&oarr; theory that, for typical p-QWIP designs, this hole intersubband absorption of TE polarized radiation (without the help of an optical grating) is significantly smaller than the electron intersubband absorption of TE polarized radiation in those n-doped QWIP's (n-QWIP's) fabricated with an optical grating. A second result of this present work is that, even when there is significant mixing of the light and heavy hole states, the p-QWIP absorption of TE polarized radiation (without the help of an optical grating) is still much smaller than the n-QWIP absorption of TE polarized radiation (with the help of an optical grating). The reason is that the mixing of light and heavy hole states never increases the amount of |S〉-symmetry in the mixed hole wave function beyond the amount of |S〉-symmetry which was present in the unmixed, purely light hole state. Finally, this present paper shows from k&oarr;·p&oarr; theory that strained layer growth on an (001) substrate does not significantly affect the strength of the hole intersubband absorption. The reason is that the Hamiltonian describing uniaxially strained quantum wells has precisely the same (tetragonal) symmetry as the Hamiltonian describing carrier confinement in unstrained quantum wells. All of these results are important in choosing a QWIP device design     

The effect of resonant sublevel coupling on intersubband transitions in coupled double quantum wells

The effect of resonant sublevel coupling on intersubband transitions in double quantum wells is investigated using far-infrared spectroscopy. We study widely tuneable parabolic double quantum wells in which potential spikes of different energetic height and thickness provide tunnel barriers for the electron systems on either side of the barrier. The use of gate electrodes enables us to tune both the carrier densities as well as the respective sublevel spacings and allow for a manifold degeneracy scheme like in an artificial molecule where the atomic number of both partners can be intentionally changed. Depending on the actual experimental condition, we observe pronounced level anticrossings into a symmetric and antisymmetric state. This single-particle sublevel coupling manifests itself in a rich spectrum of the observed collective intersubband transitions which occur at the depolarization shifted intersubband energy.     

Intersubband absorption characteristics in OMVPE grown delta-doped GaAs/AlGAs multiple quantum well structures

A study was carried out using a series of multiple quantum well structures grown by organometallic vapor phase epitaxy (OMVPE) to evaluate the intersubband absorption characteristics, thus enabling a direct correlation of the delta doping parameters to device performance. Significant improvements in the absorption characteristics were obtained by adopting a localized delta-doping profile in the absorption quantum wells as opposed to homogeneously doped structures. The transition linewidths were observed to decrease from 40 meV in the uniformly doped sample to 20 meV in devices in corporating delta-doped quantum wells. In addition, the measured absorption strengths of the intersubband resonance grew in a linear fashion as the carrier sheet density in the well was increased to approximately 2 × 10¹² cm⁻².