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     

Calculation of linear and nonlinear intersubband optical absorptions in a quantum well model with an applied electric field

Analytic forms of the linear and the third-order nonlinear optical intersubband absorption coefficients are obtained for general asymmetric quantum well systems using the density matrix formalism, taking into account the intrasubband relaxation. Based on this model, we calculate the electric field dependence of the linear and the third-order nonlinear intersubband optical absorption coefficients of a semiconductor quantum well. The energy of the peak optical intersubband absorption is around 100 meV (wavelength is 12.4 μm). Thus, electrooptical modulators and photodetectors in the infrared regime can be built based on the physical mechanisms discussed here. The contributors to the nonlinear absorption coefficient due to the electric field include 1) the matrix element variation and 2) the energy shifts. Numerical results are illustrated.     

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.     

Analysis of transient interband light modulation by ultrashortintersubband resonant light pulses in semiconductor quantum wells

We explore the transient characteristics of an interband resonant light modulation process by ultrashort intersubband resonant light pulses in semiconductor quantum wells (QW's). The modulation characteristics in a three-level semiconductor QW system, including the effects due to in-plane momentum, have been investigated by a numerical analysis of the coupled Bloch equations using a density-matrix approach. We have studied the effect of the carrier density and the nature of the intersubband coupling light on the transient absorption features of the interband light. The modulation process has been compared in doped and undoped QW's. The switching behavior of the strong interband light field in presence of a train of intersubband light pulses has also been discussed     

基于非对称量子阱的太赫兹波调制器

主要研究了一个特殊的GaAs/AlGaAs非对称量子阱中的线性和三阶非线性太赫兹波吸收系数和介质折射率的相对改变。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和相对折射率改变的表达式,然后以典型的阶梯型量子阱材料为例做了数值计算。计算表明,基于泵浦光场和偏置电压控制的太赫兹波调制器不仅可以做太赫兹波开关,还可以灵活地调制太赫兹波信号的强度和相位,方便实用。     

Modulation of intersubband light absorption and interband photoluminescence in double GaAs/AlGaAs quantum wells under strong lateral electric fields

The effect of a lateral electric field on the mid-infrared absorption and interband photoluminescence spectra in double tunnel-coupled GaAs/AlGaAs quantum wells is studied. The results obtained are explained by the redistribution of hot electrons between quantum wells and changes in the space charge in the structure. The hot carrier temperature is determined by analyzing the intersubband light absorption and interband photoluminescence modulation spectra under strong lateral electric fields.     

Saturation of interband absorption in semiconductors

The nonlinear absorption factors in a direct-gap semiconductor are calculated both for the pump and probe waves using the density matrix formalism. The key feature of the calculation is the consideration of the carrier dispersion. It is shown for the first time that the dispersion law defines the saturation behavior of the absorption factor. The intensity and frequency dependences of the absorption factors calculated differ significantly from those obtained previously. It is also shown that the conventional theory of nonlinear interband absorption based on the kinetic equations for carriers leads to the correct results only in the limit of small pump intensities.     

First demonstration of room temperature intersubband-interbanddouble-resonance spectroscopy of GaAs/AlGaAs quantum wells

Intersubband-interband double-resonance experiments in undoped GaAs/Al_0.33Ga_0.67As multiple quantum well (MQW) structures at room temperature are discussed. The well width is 78 Å. A Ti:sapphire laser is used to pump the interband transitions, while the first intersubband transition is probed with a CO₂ laser. The intersubband absorption is found to peak at 10.6 μm, and a 10-meV linewidth is measured. The absorption signal is also recorded at a fixed CO₂ tuning while varying the pump laser wavelength from 700 to 850 nm. A high-resolution spectrum is obtained, reflecting the steplike density of states with sharp peaks at the exciton resonances     

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。     

Large Stark effects for transitions from local states to globalstates in quantum well structures

The electric-field dependence of the optical absorption for the type of quantum well structure in which one or more small wells are embedded in a big well is discussed. In such structures, local energy states confined by the small well(s) and global states confined by the big well have different electric-field dependences while their wave functions remain overlapped. Thus, a large Stark effect (large energy shift and oscillator strength) can be achieved for the optical transition from a local state to a global state. This concept of using the local and global states can be applied to both interband and intersubband transitions. For intersubband transitions, a typical 10-20 meV shift is predicted, compared to a reported 1.1 meV blue shift at the field 30 kV/cm. For interband transitions, the Stark shift is larger than that in the single quantum wells     

Intersubband optical absorption in strained double barrier quantumwell infrared photodetectors

A systematic theoretical investigation of intersubband optical absorption in AlGaAs-AlAs-InGaAs strained double barrier quantum well is presented for the first time. Electron states are calculated within the effective mass approximation which includes the effects of subband nonparabolicity and strain, and found to be in good agreement with experiments. Intersubband optical absorption is investigated using the density matrix formalism with the intrasubband relaxation taken into account. Analytical formulas are given for electron energies, absorption coefficient, and responsivity. Subband nonparabolicity and elastic strain are found to significantly influence both electron states and intersubband optical absorption. The peak absorption wavelength is found to decrease linearly if the In composition is increased, and an approximate formula is given. Electron states and optical absorption are affected by the inner barrier thickness if it is less than 40 Å. The results are useful for design and improvement of the performance of quantum well infrared photodetectors operating in the important wavelength region between 1.5 and 4 μm     

Intersubband optical absorption coefficients and refractive index changes in spherical hydrogenic antidots

In this work, we have studied the optical absorption coefficients and refractive index changes in spherical quantum antidots with hydrogenic donor impurity at the center. For this purpose, the energy spectrum and wavefunctions are first determined, analytically. Then, we have used analytical expressions for the intersubband absorption coefficients and refractive index changes obtained by the compact density matrix formalism. The results show that i—total absorption coeficient increase with increasing size of antidot, ii—the refractive index changes decrease with decreasing antidot size.     

Electroabsorption modulator using intersubband transitions in GaN-AlGaN-AlN step quantum wells

We calculate the high-speed modulation properties of an electroabsorption modulator for /spl lambda/=1.55 /spl mu/m based on Stark shifting an intersubband resonance in GaN-AlGaN-AlN step quantum wells. In a realistic simulation assuming an absorption linewidth /spl Gamma/=100 meV we obtain an RC-limited electrical f/sub 3dB//spl sim/60 GHz at an applied voltage swing V/sub pp/=2.8 V. We also show that a small negative effective chirp parameter suitable for standard single-mode fiber is obtained and that the absorption is virtually unsaturable. The waveguide is proposed to be based on the plasma effect in order to simultaneously achieve a strong confinement of the optical mode, a low series resistance, and lattice-matched cladding and core waveguide layers. Extrapolated results reflecting the decisive dependence of the high-speed performance on the intersubband absorption linewidth /spl Gamma/ are also given. At the assumed linewidth the modulation speed versus signal power ratio is on a par with existing lumped interband modulators based on the quantum confined Stark effect.     

Optically stimulated intersubband absorption in undoped amorphous Si/SiO2 qiantum well

Intersubband absorption has been observed in undoped amorphous multiple-quantum-well (MQW) strucctures under interband excitation. The transitions take place between the first and second subbands of the conduction band and involve non-equilibrium electrons excited into the first subband by optical pumping. The absorption band FWHM of 0.1 eV is much larger than for crystalline MQWs, a fact which reflects the relaxation of the electron momentum conservation in the QW plane. The high joint density of states and oscillator strength for the intersubband transitions in amorphous QWs allow one to observe the absorption at low electron concentrations (N_e ≈ 10¹³ cm^?3) in the ground conduction subband.     

Intersubband absorption of TE and TM waves in p-type semiconductorsuperlattice including the effects of continuum states

We present theoretical results for the absorption coefficients for both TE and TM waves due to intersubband transitions within the valence subbands of a p-type superlattice. Both the bound and continuum states are included in our calculation. The continuum states are discretized by applying a periodic boundary condition. Numerical results are illustrated for different well and barrier widths. The results show that the intersubband transitions among the bound states result in sharp resonance absorption while the intersubband transitions between bound states and continuum states give a broad maximum at a photon energy larger than the barrier height. The TE and TM absorption coefficients are comparable in magnitude. Our theoretical results are in agreement with recent experimental observations on bound states to continuum states transitions     

The role of intersubband optical transitions on the electrical properties of InGaAs/GaAs quantum dot solar cells

We report on an experimental study of the intersubband optical response of an In_0.5Ga_0.5As/GaAs quantum dot solar cell (QDSC). By calculating the quantum dot absorption cross section, the strength of the intersubband optical transitions is gauged, and their importance and influence on the electrical properties of the solar cell can be compared with those of other physical processes such as thermal intersubband and optical interband transitions. The temperature‐dependent photocurrent and dark current characteristics of the QDSC have also been analyzed in detail, leading to an understanding of the specific effects reducing the open‐circuit voltage. The deviation of QDSCs from idealized models is discussed, and some conditions required for an improved open‐circuit voltage are suggested. Copyright © 2012 John Wiley & Sons, Ltd.     

Intersubband gain and stimulated emission in long-wavelength(λ=13 μm) intersubband In(Ga)As-GaAs quantum-dotelectroluminescent devices

The dynamics of injected carriers and the conditions for intersubband gain and population inversion in In(Ga)As-GaAs self-organized quantum dots have been studied. Direct femtosecond pump-probe spectroscopy as a function of temperature and excitation density confirms earlier results and shows a long (>100 ps) electron relaxation time between the excited states and ground state in the dots. Intersubband gains as high as 170 cm^-1 are calculated in the dots. Far-infrared spontaneous emission centered around 13 μm is observed in edge-emitting light-emitting diodes. Stimulated emission, with a distinct threshold around 1.1 kA/cm² in the light-current characteristics, is observed in plasmon-enhanced waveguide devices. The intersubband threshold occurs after a threshold is observed for interband lasing (~1 μm) in the same device     

Intersubband absorption saturation in InGaAs-AlAsSb quantum wells

We present intersubband absorption saturation studies in InGaAs-AlAsSb quantum wells. We carried out a density matrix calculation to simulate the pulsed excitation condition after including the dephasing time and the short pulse profile to estimate the saturation intensity (I/sub S/). We compare the calculated results with measurements for both resonant and nonresonant excitation. We also present our results on the effect of pulsewidth on Is estimation.     

Efficient modeling of the optical properties of MQW modulators onInGaAsP with absorption edge merging

The optical properties of quantum wells on Ga_xIn_1-xAs_1-yP_y are investigated. The dielectric function ϵ(ω) is calculated with a density matrix formalism valid for excitonic transitions as well as for the interband absorption including band mixing. With two simple approximations, the required number of overlap integrals is greatly reduced, allowing a fast and efficient exciton calculation. The calculated results are compared with measurements at 77 K and at a room temperature of 300 K. Furthermore, we present a field-induced heavy and light hole absorption merging for a Ga_xIn_1-xAs_1-yP_y-based modulator for the first time. It can he operated at a wavelength λ=1.55 μm, showing a very large absorption change and a small negative chirp factor, which is recommended for a low bit error rate     

Doping effects on intersubband and interband optical transitions inGaSb-InAs superlattices

Normal incidence intersubband and interband absorptions of a novel type II GaSb-InAs superlattices can be obtained by utilizing the various doped-type cap and buffer layers. Moreover, the types and intensities of the absorptions could also be modulated by changing doping concentration. The intersubband transition can occur due to the strong mixing of the heavy-hole band and the light-hole band for InAs n-type cap and buffer layers. But the interband transition is a result of coupling between the wave-functions of the first conduction subband and the first heavy-hole subband for GaSb p-type cap and buffer layers. Both the intensities of intersubband can be modulated by changing doping concentration, and the corresponding wavelengths are in the ranges of 3-5 μm and 8-14 μm, respectively. Hence, it shows the potential application as an infrared photodetector