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.     

Analysis on interband-resonant light modulation byintersubband-resonant light in n-doped quantum wells

The authors describe a theoretical analysis on interband-resonant light modulation by intersubband-resonant light in the n-doped quantum well. The modulation principle is briefly explained. The theoretical formulation for the modulation is derived by using density matrix theory. The modulation characteristics concerning the absorption coefficient and refractive index are calculated for various parameters. It is shown that the absorption coefficient for the interband-resonant light can be greatly changed by the incidence of intersubband-resonant light     

基于波长调制技术的吸收谱线线型函数测量

在可调谐二极管激光吸收光谱技术中,直接吸收法通过对透射光强拟合可直接得到线型函数,但目前具有较高信噪比的波长调制法不能对其进行有效测量。提出一种基于多次谐波的线型函数测量理论及方法,通过谐波通项表达式推导出谱线中心频率处二次与四次谐波比值仅与线型函数和调制系数有关,当调制系数m为2.492 8时,无论线型函数中Gauss线宽和Lorentz线宽所占比例如何,二次与四次谐波比值均为2.186 2,根据该点的特征可首先得到谱线半宽,然后将其应用于弱吸收条件下2f/1f气体浓度免标法测量。实验中采用2 326.82 nm处谱线对CO浓度进行了测量,其结果与传统直接吸收法测量结果误差小于2%。文中为波长调制法中线型函数的精确测量提供了重要的理论依据,进一步完善了2f/1f免标法。     

Optical bandwidth considerations in p-i-n multiple quantum-wellmodulators

We investigate the optical bandwidth of p-i(multiple quantum well [MQW])-n modulators employing various MQW designs. The optical bandwidth translates directly into an operating temperature range due to the shift of the band gap with temperature. We find that although greater maximum modulation may be obtained with narrow (~90 Å) quantum wells operating below the band edge (absorption increases with field), uniform large performance may be obtained over a larger bandwidth using wider (~110 Å) quantum wells operating at the exciton (absorption decreases with field). We obtain a usable bandwidth of 7.7 nm, which translates into a operating temperature range of 27°C     

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     

基于偶次谐波吸收率函数复现方法

TDLAS中波长调制光谱具有灵敏度高、信噪比高的优点,然而由于其无法有效得到绝对吸收率函数一般需要采用二次谐波峰值结合标定实验确定待测气体参数。考虑到这一问题,本文基于波长调制光谱策略,在时域上对经过高频调制的激光透过率函数于谱线中心频率处进行泰勒级数展开,在此基础上得到谐波表达式由透过率及其导数和频率调制幅度等组成,通过分析各次谐波间的关系特征建立了一种基于偶次谐波信号的吸收率函数高精度复现方法。随后采用CO2分子6976.203 cm^-1谱线对该方法进行了数值模拟和实验验证,并将实验结果与传统直接吸收法测得的吸收率函数进行对比。模拟和实验结果表明:波长调制光谱中偶次谐波信号可复现TDLAS核心参数—吸收率函数,其测量精度随着所采用谐波阶次的增加而提高,预期可为谱线参数高精度标定、复杂工业现场中痕量气体浓度监测提供新的测量方法。     

用调制光谱研究半导体体材料及微结构的非线性极化率

提出了用调制光谱信号强度表征半导体体材料及微结构的非线性极化率,研究了测量的原理和方法．对玻璃中量子点电反射调制光谱信号强度随团簇颗粒尺寸的不同而产生几个数量级变化的原因作出了解释     

新型声学结构的吸声特性研究

提出了赫姆霍兹共振吸声结构附加吸声材料的新型吸声结构,在插入长管的赫姆霍兹共振吸声结构表面敷贴吸声材料,对其吸声特性进行了理论分析与仿真计算。结果表明,吸声材料与吸声结构的有机结合显著提高了结构的吸声效果,拓宽了吸声频带宽度。进一步研究了不同吸声材料、吸声结构的不同组合方式等因素对该新型结构吸声特性的影响,结果表明:组合结构、优化材料有助于提高吸声系数、拓宽吸声频带。     

纳米结构太阳电池的研究与展望

纳米结构太阳电池在未来第三代太阳电池中具有潜在应用价值。首先,介绍了各种纳米结构光伏材料的优异物理特性。然后,着重评述了不同纳米结构太阳电池近年的研究进展。这些太阳电池包括具有带隙可调谐特性的量子阱太阳电池、具有良好光吸收特性的纳米薄膜太阳电池、具有低反射率特性的纳米线太阳电池和基于多基子产生效应的量子点太阳电池。最后,提出了发展纳米结构太阳电池的若干技术对策,包括合理选择适宜纳米结构的材料、制备高质量的纳米光伏材料、优化设计太阳电池的组态结构以及揭示与阐明太阳电池中光生载流子的输运机制。     

利用K-K关系计算量子点玻璃的介电函数

为了得到PbSe量子点(QD)玻璃的光学常数,为 实验制备QD材料提供定性和定量参考,本文利用 实验中测量的吸收光谱,结合米氏散射理论和克拉默斯-克勒尼希(K-K)关系,通过多次迭 代运算,从吸收光 谱中分离出相应的介电函数的实部和虚部,最终获得材料的光学常数。为了使矩阵迭代运算 能够快速收敛, 设计了一种快速、不产生奇异矩阵的简便方法。计算得给出的PbSe QD玻璃的介电函数与 实验测量得到 的胶体QD介电函数分布非常相似,表明本文方法能够用于快速测定QD玻璃的介电 函数。本文方法可 以推广到其他液体、固体和气体材料光学常数的测量中,为纳米材料制备提供一定的参考。     

Quantum confined light modulators

A general framework for the operation of a large class of quantum confined light modulators is offered, supported by a discussion of the theoretical background. The design of a quantum confined modulator for a specific host system allows a wide choice of modulation mechanism, geometrical design configuration and field-controlled optical parameter. In this short review, we discuss specific combinations of optical property, semiconductor material system and geometry suitable for selected digital and analogue applications. Excitonic optical effects are emphasised as providing electric-field control of both optical absorption and refractive index in multiple quantum well structures. Geometrical aspects are also discussed, including phase modulation in a horizontal waveguide configuration and absorption modulation in transmission and reflection-mode vertical modulators. Self electrooptic effect devices (SEEDs) figure prominently across the range of optoelectronic system components and are used to illustrate various modulator functions in lightwave logic operations and neural networks.     

Electric-field dependence of linear optical properties in quantum well structures: Waveguide electroabsorption and sum rules

We summarize the electric-field dependence of absorption and luminescence in quantum wells for fields perpendicular to the layers, present extended discussion of electroabsorption spectra and devices in waveguide samples, and derive sum rules for electroabsorption. Optical bistability, self-linearized modulation, and optical level shifting are demonstrated in self-electrooptic effect device configurations, with good modulation contrast and polarization-dependent properties. The electroabsorption spectra enable quantitative comparison of theory and experiment for absorption strengths in quantum wells with field. The sum rules enable excitonic effects to be included in the comparison, and good agreement is seen. One sum rule is also more generally applicable to electroabsorption in semiconductors.     

PbS量子点同质P-N结光电探测器

PbS胶体量子点因其带隙可调、可溶液加工、吸收系数高等优异特性而广泛应用于光电探测器领域。然而基于光电二极管结构的PbS量子点光电探测器通常会使用不同的材料来制备N型层,从而增加了器件设计和工艺的复杂性,不利于这类光电探测器未来在面阵成像芯片中的应用。为简化制备工艺,提出了一种PbS量子点同质P-N结光电探测器,仅通过一种工艺过程实现了器件P型层和N型层的制备。经测试,探测器对不同入射光强度的探测表现出了良好的线性响应;在0.5 V反向偏压作用下,器件在700 nm处的响应度为0.11 A/W,比探测率为3.41×1011 Jones,展现出了其对弱光探测的优异能力。结果表明文中提出的PbS量子点同质PN结光电探测器有助于推动其在面阵成像领域中的发展。     

CdTe/CdS、CdTe/ZnS核壳结构半导体量子点制备及荧光特性

核壳半导体量子点材料因其在修复单量子点表面缺陷方面的特殊性能,极大地提高了量子点的光学性能而受到人们的研究。改进了CdTe核心的制作方法,使用小型三口瓶替代传统的小烧瓶作为反应容器,制备碲氢化钠,合成了不同核心尺寸、不同壳层厚度与不同壳层材料的10种CdTe/CdS、CdTe/ZnS核壳结构半导体量子点。对10种核壳结构半导体量子点材料进行紫外可见吸收光谱及荧光光谱测试,并分析其荧光特性。量子点在紫外可见波段的吸收光谱表明随着量子点尺寸的增大,吸收峰发生红移。通过实验结果与分析可推断出CdTe/CdS量子点荧光寿命和强度的不同是由于核心和壳层尺寸的不同量子点在I型和II型中相互转换;CdTe/ZnS的壳层厚度增加时,由于ZnS的壳层降低了核心外表的悬空键和表面缺陷态的数量,使电子空穴对复合机率加大,使得荧光峰位产生了红移。     

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     

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.     

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。     

新的电调制反射谱技术及其应用

提出一种微接触电调制反射谱（LCER）测试方法,该方示与无接触电调制反射（CER）谱相比较,可极大降低调制电压。给出了自制的详细样品架结构,测量了InGaAs／GaAs量子阱样品,并与光调制反射谱（PR）相比较,结果证实了此方法的可行性和高光谱灵敏度,表明样品与电极的轻微接触既对测量结果没有明显的影响,又可简化测试条件,降低对测量环境的要求,是研究半导体材料和微结构一种方便而又有效的方法。     

Saturable absorber intensity modulator

We propose and demonstrate a reflection-type optical modulator, with surface-normal architecture that exploits saturation of absorption in semiconductor quantum wells pumped optically. The modulation section, composed of quantum wells placed within a Fabry-Perot cavity, is exposed to an intensity-modulated beam generated by an in-plane laser integrated monolithically on the same wafer. Since the modulation section and the in-plane laser share the same medium, an efficient coupling between the control beam and the signal beam is achieved. Design guidelines and device performance are presented. We demonstrate that the modulator provides an efficient light modulation mechanism that is sufficient to actively mode-lock an erbium-doped fiber laser.     

Gain-coupled distributed feedback semiconductor lasers with anabsorptive conduction-type inverted grating

A gain-coupled (GC) distributed feedback (DFB) semiconductor laser with an absorptive conduction-type-inverted grating is proposed. Devices based on GaAlAs/GaAs materials are fabricated using two-step OMVPE. By inverting the conduction type of the absorptive region, threshold current is lowered by 10 mA, which is to compensate for the threshold increase due to extra absorption. In addition, nonlinear output property associated with the saturable nature of the absorption is eliminated. An ultralow chirping capability under gain switching high speed modulation and the narrow linewidth nature of this laser are experimentally studied