有强内电场的GaN-基阶梯量子阱中的线性与非线性光吸收（英文）

采用密度矩阵方法,考察了带强内建电场GaN-基阶梯量子阱中的线性与非线性光吸收系数.基于能量依赖的有效质量方法,在考虑了带的非抛物性情况下,推导了结构中的精确解析的电子本征态,给出了系统中简单解析的线性与非线性光吸收系数表达式.以AlN/GaN/AlxGa1-xN/AlN阶梯量子阱为例进行了数值计算.结果发现阶梯量子阱的阱宽Lw、阶梯垒宽Lb、阶梯垒的掺杂浓度x的减小将提高体系的吸收系数.而且,随着Lw,Lb和x减小,吸收光子的能量有明显的蓝移,总吸收系数的半宽度及饱和吸收强度均减小.计算获得的部分结果与最近的实验观察完全一致.     

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。     

正切平方势阱中线性和三阶非线性光学吸收系数的计算

研究了正切平方势阱中的子带光吸收,用量子力学中的密度矩阵算符理论和迭代法推导出了正切平方势阱中线性和三阶非线性光学吸收系数的解析表达式,并以典型的GaAs/AlGaAs正切平方量子阱为例进行数值计算。计算结果表明,该势阱中的势阱宽度b、势阱深度V₀和入射光强I对吸收系数有很大影响。随着势阱宽度b的增加和势阱深度V₀的减小,总吸收系数的峰值减小并且向低能方向移动。随着入射光强I的增加,总吸收系数会减小,出现了光饱和吸收现象,同时吸收谱线的线宽随着入射光强的增大而增大。     

电场驱动HgCdTe/CdTe量子阱拓扑相变引起的光吸收增强研究

HgTe/CdTe量子阱是研究拓扑绝缘体新奇物性的一个很好载体。采用Kane八带k·p模型,对电场驱动Hg1-xCdxTe/CdTe量子阱拓扑相变及其相变前后的光吸收性质进行了研究,并使用BHZ模型对吸收系数进行了解析计算和分析。结果表明:在电场能够驱动Hg1-xCdxTe/CdTe量子阱拓扑相变后继续增大电场,其能带可变为墨西哥帽形状,联合态密度将会增强,导致光吸收相比于无电场时显著增强,与解析计算结果相吻合。对于平行界面偏振光(TE)吸收曲线在带边还形成了双峰结构。文章结果可用于新型红外光电探测器、激光器以及频率选择器等量子阱器件的研究和设计。     

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.     

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。     

GaN基量子阱激子结合能和激子光跃迁强度

采用变分法,计算了GaN基量子阱中激子结合能和激子光跃选强度。计算结果表明,GaN基量子阱中激子结合能为10－55meV,大于体材料中激子结合能,并随着阱宽减小而增加,在临界阱宽处达到最大。结间带阶同样对激子结合能有着较大的影响,更大带阶对应更大的结合能。同时量子限制效应增加了电子空穴波函数空间重叠,因此加强了激子光跃迁振子强度,导致GaN/AlN量子阱中激子光吸收明显强于体材料中激子光吸收。     

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.     

基于量子阱带间跃迁的红外探测器原型器件（英文）

近期,实验发现PN结中局域载流子具有极高提取效率,并导致吸收系数的大幅度增加.文中报道基于上述现象的新型量子阱带间跃迁红外探测器原型器件的性能.利用含有InGaAs/GaAs多量子阱的PIN二极管,在无表面减反射膜的实验条件下,利用仅100 nm的有效吸收厚度,实现了31%的外量子效率.基于该数值推算得到,量子阱的光吸收系数达3.7×10~4cm~(-1),该数值高于传统透射实验测量结果一个数量级.上述实验结果指出,利用量子阱带间跃迁工作机制,有望实现新颖的器件结构设计和提高现有器件性能.     

一种特殊的非对称量子阱中的光整流效应

运用量子力学密度矩阵算符理论，推导出在一种特殊非对称量子阱中的光整流系数的解析表达式．最后利用典型的GaAs／A1GaAs非对称量子阱进行数值计算，得到了系统的非线性光整流效应和量子阱的非对称性以及入射光子能量之间的变化规律，并在此特殊量子阱中得到了较大的光整流系数，从而为实验上制作较好的非线性材料提供了一种可行的途径。     

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.     

Electron nonelastic scattering by confined and interface polar optical phonons in a modulation-doped AlGaAs/GaAs/AlGaAs quantum well

The calculations of electron scattering rates by polar optical (PO) phonons in an AlGaAs/GaAs/AlGaAs quantum well (QW) with a different width and doping level are performed. The electron-and PO-phonon scattering mechanisms which are responsible for the alternate dependence of electron mobility on a QW width, as well as for the decrease of conductivity in the QW with increasing sheet electron concentration, are determined. It is shown that the enhancement of the scattering rate by PO-phonon absorption when the lower subband electron gas is degenerated is responsible for the decrease of QW conductivity with increasing sheet electron concentration. The competition between the decrease of the intrasubband scattering and the increase of the intersubband scattering by PO-phonon absorption is responsible for the alternate changes of the mobility with a QW width.     

Electrons and phonons in quantum wells

The modulation of electron and polar optical phonon states in an AlGaAs/GaAs/AlGaAs quantum well (QW) with an inserted thin AlAs barrier is considered. The OW width dependence of electron-phonon scattering rates are estimated. The large contribution to the change of the electron subband population, the photovoltaic effect, and the electron mobility in the QW accounts for the resonant intersubband scattering of electrons by interface phonons. The decrease of electron mobility limited by polar optical phonon scattering with increasing carrier concentration in the QW is established. The conditions for the increase of mobility in the QW by inserting the AlAs barrier are found. Fiz. Tekh. Poluprovodn. 33, 1049–1053 (September 1999) This article was published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Optimized Asymmetric Double Quantum Well for High Electric-Field-Sensitivity Electroabsorption: Excitonic Mixing Effects

Asymmetric double quantum wells (ADQWs) are optimized to exhibit maximum optical modulation sensitivity by varying the barrier width, barrier position, and well width. Anticrossing of the two lowest excitons in ADQWs significantly enhances the modulation sensitivity. Consideration of exciton mixing is crucial to obtain accurate estimates of the effects. For a given linewidth of the excitonic peaks, we find optimum structural parameters that exhibit maximum modulation sensitivity. Values dalpha / dE ~ 6.51 x 10⁴ kV^-1of of at 4.2 K and ~1.25 x 10³ kV^-1of at 298 K are predicted in GaAs-based ADQWs. The result provides new design guidelines in fabricating high-sensitivity QW electroabsorption modulators.     

Electronic band structures and optical gain spectra of strainedwurtzite GaN-AlxGa1-xN quantum-well lasers

The electronic band structures, density-of-states, and optical gain spectra for wurtzite GaN-Al_xGa_1-xN quantum wells are studied theoretically based on the Hamiltonian derived using the k.p method. We investigate the dependence of the optical gain and transparent current density on the well width, barrier height, and strain using a numerical approach with high accuracy. The mole fraction of Al in the barrier material is progressively increased to study the effects of quantum confinement and compressive strain. A higher Al mole fraction in the barrier leads to improvement of the TE optical gain and suppression of the TM optical gain. Furthermore, we demonstrate that a reduction of the well width offers improved modal gain over all radiative current densities. We also predict a transparent current density of 250 A/cm² for the GaN-Al_xGa_1-x N single quantum-well (QW) structure. Our results suggest that a suitable combination of thin well width and large barrier height should be selected in improving the TE optical gain in wurtzite GaN-Al_x Ga_1-xN single QW     

Characterization of excitons in wurtzite GaN quantum wells undervalence band mixing, strain, and piezoelectric field

Low-dimensional excitons in general, and quantum-well (QW) excitons in particular, are important for linear and nonlinear semiconductor optics applications. The recent observation of the high binding energy of bulk excitons in gallium nitride samples being the main impetus, we undertake a theoretical work to characterize QW excitons in wurtzite semiconductors. In our formulation, we take into account valence band mixing, strain, and piezoelectric field effects. The in-plane behavior of excitons is treated variationally, whereas the finite-element method is used for the dependence along the growth direction. The formulation is applied to GaN-Al_x Ga_1-x N QW's. The presence of the piezoelectric field leads to the well-known quantum-confined Stark effect. We deduce from an oscillator strength analysis that the quantum-confined Franz-Keldysh effect is operational for QW's of width around 45 Å for an aluminum content of x=0.15. Our results further indicate that, for very clean samples, QW excitons should not ionize at room temperature even in the presence of the piezoelectric field for sufficiently narrow QW's. We determine the fractional dimensionality of the QW excitons in the absence of the piezoelectric field, which can in principle be cancelled by introducing delta-doped ionized layers on either side of the QW. The absorption spectra associated with the low-lying 1s excitons are also presented for several well widths     

用Z扫描技术研究Pd(mpo)2的非线性光学特性

为了研究一种新型功能配合物材料Pd(mpo)2的非线性光学特性,把二甲基甲酰苯胺(DMF)溶剂稀释的Pd(mpo)2溶液(浓度为0.75×10-4mol/L)作为待测样品,采用脉冲宽度为7ns,波长为532nm的激光束,对该样品进行了Z-扫描实验,测量了Pd(mpo)2材料的非线性吸收和非线性折射系数。实验发现,当入射光强很小时,其非线性吸收系数β=(1.916±0.016)cm/Gw,非线性折射系数γ0=-(3.153±0.048)×10-5cm2/Gw,且β和|γ|随入射光强的增大而线性减少。合理地解释了Pd(mpo)2的非线性光学特性产生机理:非线性吸收来自于近共振双光子吸收,而非线性折射是由于近共振双光子吸收造成的折射加强。非线性吸收和非线性折射系数随光强的增大而线性减少是近共振双光子吸收引起粒子数重新布居的结果。     

Strain-dependence of the gain saturations in InGaAsP/InP quantum-well gain media

Numerical analyses of polarization-dependent optical gain saturations are given for quantum-well (QW) lasers in the presence of strain in the well regions in order to investigate the strain dependence of polarization-bistable operations. Gain saturation coefficients are obtained from nonlinear susceptibilities calculated in the perturbative analyses of density matrices. Band dispersions and dipole matrix elements, which are put into the density matrices, are calculated by diagonalizing Luttinger's Hamiltonian, including valence band mixing. The strain induces a change in band dispersions and wavefunctions, leading to strain-dependent saturation coefficients. The self-saturation coefficients and the cross-saturation coefficients (with orthogonal optical polarizations) pertinent to InGaAsP/InP QW vertical-cavity surface-emitting lasers are calculated. We find that the relative magnitudes of self- and cross-saturation coefficients are strongly dependent on the strain; in the presence of compressive strain, the cross-saturation coefficients are larger than the self-saturation in the wide range of the linear gain spectra, especially in the vicinity of the gain peak, indicating that the compressively strained structure is more favorable for the polarization-bistable operations.