Analytical solution of the subbands and absorption coefficients ofAlGaAs-GaAs hyperbolic quantum wells

A hyperbolic function is used to model the carrier confinement potential profile of a disordered Al_0.3Ga_0.7As-GaAs single quantum well produced by interdiffusion of group III atoms across the well-barrier interfaces. The confined states are determined analytically for the hyperbolic profile, and their wavefunctions and energies compare well with the results obtained numerically using an error function profile. The subband edge energy in the hyperbolic quantum well increases initially for a small diffusion length and then decreases for larger diffusion lengths. The absorption coefficient is calculated for different interdiffusion lengths to demonstrate the model. The first absorption peak of the TM polarization has a larger magnitude than does the corresponding peak of the TE polarization, which is in agreement with measurements performed on quantum-well waveguide structures. A similar trend is observed for the higher-order transitions. The magnitude of the second absorption peak also varies with the amount of interdiffusion in the case of TM polarization     

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     

A physical optics solution for the backscattering by triangularly shaped trihedral corners

In this paper, the backscattering by a trihedral corner, with perfectly conducting and triangularly shaped faces, has been considered and a closed form solution for its evaluation has been developed. This solution has been obtained by applying physical optics to evaluate the backscattered field and geometrical optics to take into account the lighting of each face by direct, singly and doubly reflected rays. The comparison with both the few experimental results available in the open literature and the measurements carried out by the authors has been found satisfactory, thus confirming the validity of the proposed approach.     

A high-speed intersubband modulator based on quantum interferencein double quantum wells

Calculations on a modulator based on quantum interference in AlGaAs/GaAs asymmetric double quantum wells (QWs) are performed. The modulation of the absorption is based on the anti-crossing behavior of the two lowest states in the coupled wells. At anti-crossing, the oscillator strengths of the transitions from these two lowest states to a higher state are changed in opposite directions. The width of the barrier between the wells should be thick enough to allow a large change in oscillator strength with applied field, yet thin enough so that the absorption peaks of the transitions are resolved. The QWs are designed so that one absorption peak has only a small energy shift for the transition used for modulation while the absorption varies rapidly with the applied voltage. A complete structure including a surface plasmon waveguide is proposed enabling calculations of modal absorption. Parameters important for the performance of the modulator are then determined. An extinction ratio of 10 dB at a wavelength of 8.4 μm is predicted for a device length of 18 μm and a peak-to-peak voltage of 0.9 V. The resistance-capacitance-limited 3-dB bandwidth is 130 GHz. The predicted performance compares very favorably with present interband modulators based on the quantum-confined Stark effect     

任意三维表面微型光器件的灰度光刻技术

使用传统的微加工技术,如各向异性或各向同性干刻蚀、湿刻蚀只能加工有限形貌的表面,为了克服这一缺点,发展了多层掩模技术、激光三维立体光刻、电子束直接写入技术等许多三维微加工技术。灰度光刻最被看好,它通过灰度掩模把加工光束能量密度分布调制成不同的形状,对光刻胶进行曝光,微型器件一次成形,不需要移动掩模或移动加工晶片,也不需要对光刻胶进行热处理,只需要对掩模版进行一定的编码和标准的光刻设备,容易和其他IC工艺相兼容,实现系统芯片结构的制作。本文分析了它的物理机制、掩模类型、编码过程、约束条件和优化方法。     

Electromagnetic scattering from arbitrary shaped conducting bodiescoated with lossy materials of arbitrary thickness

A simple and efficient numerical technique is presented to solve the electromagnetic scattering problem of coated conducting bodies of arbitrary shape. The surface equivalence principle is used to formulate the problem in terms of a set of coupled integral equations involving equivalent electric and magnetic surface currents which represent boundary fields. The conducting structures and the dielectric materials are modeled by planar triangular patches, and the method of moments is used to solve the integral equations. Numerical results for scattering cross sections are given for various structures and compared with other available data. These results are proved accurate by a number of representative examples     

Strained quantum wells for polarization-independent electroopticwaveguide switches

A polarization-independent quantum well waveguide switch is demonstrated. By altering the composition and hence the degree of built-in strain, the bandgap of In_1-xGa_xAs/InP quantum wells is engineered to produce equal field-induced refractive-index change in TE and TM polarizations. At the same time, the enhanced electrooptic effects characteristic of unstrained quantum wells are maintained, such that the voltage-length product for switching is only 3 V-mm     

On the properties of InSb quantum wells

A brief survey of the properties of InSb quantum wells is reported. The energy level scheme is described on the basis of a square-well model taking into account the pronounced non-parabolicity of the conduction band. A simple model for electron transport with polar optical mode scattering is presented. The temperature dependence of the low-field electron mobility is calculated for various well-widths. Threshold fields for negative differential resistance and for energy runaway are estimated. Threshold energies for impact ionization, including resonance effects involving the electronic sub-bands are also obtained. Novel features expected of InSb quantum wells are summarized and discussed.     

A hybrid method for the solution of scattering from inhomogeneousdielectric cylinders of arbitrary shape

A hybrid moment method/edge-element method (MM/EEM) is presented. The formulation is quite general; however, the method is applied to two-dimensional scattering problems. Such a hybrid formulation unites the advantages of finite and integral-equation methods and is able to handle unbounded problems in which complex inhomogeneities are present. The edge-element method is easily coupled to the moment method, and it doesn't introduce spurious modes. The equivalence principle is used to divide the original problem into two separate problems: an unbounded homogeneous one in which the moment method is used and a bounded inhomogeneous one in which the edge-element method is used. Several examples involving two-dimensional scattering with TE and TM plane wave excitation are presented. The RCS is computed and compared to results obtained by other numerical techniques     

多量子位量子Fourier变换的仿真实现研究

量子Fourier变换(QFT)是许多量子算法的关键子例行程序,核磁共振系统(NMR)是目前最有希望实现量子计算的物理系统之一.在介绍QFT,并对其进行进一步分析,得到了多量子位QFT的实现逻辑线路后,应用多量子算符代数理论,给出了基本量子逻辑门的核磁共振实现,分解了QFT各相应的逻辑操作,设计了相应的核磁共振脉冲序列.并通过量子计算仿真程序进行了实现多量子位QFT的实验验证,证明了所设计的NMR脉冲序列的合理性和实用性.     

Optical properties of metallic quantum wells

The authors model the optical properties of metallic quantum wells with thicknesses in the range of a few nanometers. A simple picture that includes only single electron transitions predicts strong absorption lines at frequencies associated with allowed interband transitions. This strong absorption feature can be in the near infrared for metal films several monolayers thick. The model is extended to include collective electron interactions by solving simultaneously Schroedinger's and Poisson's equations. It is found that the single electron picture does not change appreciably for frequencies above the bulk plasma frequency of the metal. For frequencies at or below the plasma frequency, however, the absorption is reduced in strength and becomes nearly featureless     

Gain characteristics of InGaN-GaN quantum wells

A self-consistent approach is used to examine gain and radiative lifetimes in InGaN-GaN quantum-well structures. The effect of high built-in electric fields due to spontaneous polarization and the piezoelectric effect and the screening of these fields by carrier injection is examined. We study how the peak gain and radiative lifetime vary with injection density and well size. We also examine the blue shift in the peak gain energy with carrier injection. Implications for laser design are discussed     

A novel Fourier series time function for modeling and simulation of PWM

In this paper, a novel time function, called a modulated Fourier series (MFS), is presented for accurate modeling and rapid simulation of natural sampled pulsewidth modulated (PWM) systems in power electronic applications. This analytical technique eliminates the necessity for an iterative search, employed in currently available simulation packages, of modulated pulse edge transitions during accurate PWM waveform simulation. PWM simulation results are presented for MFS model validation, which are indistinguishable from those for an analog comparator modulator employing arbitrary nonperiodic signals. Additional simulation traces are presented of the fidelity with which the MFS accurately replicates, with only two carrier harmonics, the effects of PWM inverter switching on motor drive current as high-frequency harmonic ripple when compared with experimental test data. Substantial savings of up to fiftyfold in motor drive simulation time are presented without loss of simulation accuracy.     

Optical characterization of AlGaN-GaN-AlGaN quantum wells

High quality AL _x Ga_1−x N-GaN-Al _x Ga_1−x N quantum wells of different thicknesses andx values were grown by low pressure metalorganic chemical vapor deposition (LPMOCVD). The change in their emission energies (measured at 77 K by photoluminescence) as a function of both well width andx value was typical of a type I heterojunction. The experimental data was compared to theoretical calculations based on the finite square well model and the confined particle transitions were identified. The experimentally observed energy shifts differed from calculated values of then = 1 electron to heavy hole transition by a constant amount (for a givenx value) attributed to strain in the AlGaN-GaN system. Also, an estimate of the critical thickness in the AlGaN-GaN system was determined based on the Matthews and Blakeslee force balance model.     

A bird's-eye view on the evolution of semiconductor superlattices and quantum wells

Following the past seventeen-year developmental path in the research of semiconductor superlattices and quantum wells, significant milestones are presented with emphasis on experimental investigations in the device physics of reduced dimensionality performed in cooperation with the materials science of heteroepitaxial growth.     

A NETWORK MODEL DECOMPOSITION METHOD FOR THE CALCULATION OF CUT-OFF FREQUENCIES OF AN ARBITRARILY SHAPED WAVEGUIDE WITH ARBITRARY FILLING

A new method, termed network model decomposition method (NMDM), is pre-sented in this paper for the evaluation of cut-off frequencies of an arbitrarily shaped waveguidewith arbitrary filling. Through discretizing the cross section of the waveguide, a topologicalmodel is first constructed, and then the corresponding network model is established based on thedifferential forms in electromagnetic field theory. A general algorithm for evaluating the cut-offfrequencies of an arbitrarily shaped waveguide with arbitrary filling has also been developed byusing the diakoptic node analysis in electrical network theory. Finally the algorithm is applied toseveral typical waveguides, as a demonstration of the validity of the method.     

On the reciprocal of a Fourier series

The inverse or reciprocal of a complex periodic time function (expressed as a complex Fourier series) is another complex Fourier series. An algorithm for finding this inverse Fourier series is presented.     

MBE growth of tensile-strained Ge quantum wells and quantum dots

Germanium (Ge) has gained much interest due to the potential of becoming a direct band gap material and an efficient light source for the future complementary metal-oxide-semiconductor (CMOS) compatibl...     

Intermixing of InP-based multiple quantum wells for integrated optoelectronic devices

The intermixing characteristics of three widely used combinations of InP-based quantum wells (QW) are investigated using the impurity-free vacancy disordering (IFVD) technique. We demonstrate that the bandgap energy shift is highly dependent on the concentration gradient of the as-grown wells and barriers, as well as the thickness of the well, with thinner wells more susceptible to interdiffusion at the interface between the barrier and well. According to our results, the InGaAsP/InGaAsP and InGaAs/InP are well suited for applications requiring a wide range of bandgap values within the same wafer. In the case of the InGaAs/InGaAsP system, its use is limited due to the significant broadening of the photoluminescence spectrum that was observed. The effect of the top InGaAs layer over the InP cladding is also investigated, which leads to a simple way to obtain three different bandgaps in a single intermixing step.