Scattering-parameter measurements of laser diodes

An accurate and simple technique for measuring the input reflection coefficient and the frequency response of semiconductor laser diode chips is proposed and demonstrated. All the packaging parasitics could be obtained accurately using a calibrated probe, and the impedance of the intrinsic diode chip is deduced from the directly measured reflection coefficient. The directly measured impedance of a laser diode is affected strongly by the short bond wire. In the frequency response (S(2)1) measurements of semiconductor laser diode chips, the test fixture consists of a microwave probe, a submount, and a bond wire. The S-parameters of the probe could be determined using the short-open-match (SOM) method. Both the attenuation and the reflection of the test fixture have a strong influence on the directly measured frequency response, and in our proposed technique, the effect of test fixture is completely removed.

Tuning of exciton-photon coupling in a planar semiconductor microcavity by applying hydrostatic pressure

By means of hydrostatic pressure tuning, we have observed the strong-coupling exciton-polariton mode in a planar microcavity with an InGaAs/GaAs quantum well embedded in it, over a pressure range from 0.37 to 0.41 GPa. The experimental data can be fitted very well to a corresponding theoretical formula with a unique value of the vacuum Rabi splitting equal to 6.0 meV. A comparison between pressure tuning and other tuning methods is made as regards to what extent the intrinsic features of the exciton and cavity will be influenced during the tuning procedure.

Tuning the resonant wavelength of long period fiber gratings by etching the fiber's cladding

In this paper the resonant wavelength of a long period fiber grating (LPG) is tuned toward longer wavelength by etching the fiber, For LP04 and LP05 cladding modes', the tuning ranges of 23 and 81 nm are achieved, respectively. Also the dependence of the resonant wavelength on the cladding radius of LPG is theoretically simulated. (C) 2001 Elsevier Science B,V. All rights reserved.

Tuning of conduction intersublevel absorption wavelengths in (In, Ga)As/GaAs quantum-dot nanostructures

We demonstrate that by increasing the amount of (In, Ga)As deposit in a quantum dot layer, the intersublevel absorption wavelength for (In, Ga)As/GaAs quantum-dot infrared photodetectors can be blue-shifted from 15 to 10 mu m while the photoluminescence peak is red-shifted. We directly compare the measured energy spacing between intersublevels obtained from infrared absorption spectroscopy with those obtained from photoluminescence spectroscopy. We find that the intersublevel energy spacing determined from absorption measurements is much larger than that obtained from the photoluminescence measurements. (C) 2000 American Institute of Physics. [S0003-6951(00)04524-1].

Wavelength tuning in GaAsAlGaAs quantum wells by InAs submonolayer insertion

Wavelength tuning of exciton emissions has been achieved simply by inserting an InAs submonolayer at the centre of GaAs quantum wells during molecular beam epitaxy growth. Photoluminescence measurements show that the emission energy can be effectively tuned from the quantum-well-determined energy down to less than the band gap of GaAs, depending on the well width as well as the InAs layer thickness. Using the effective-mass approximation, the tuning effect can be well predicted theoretically The results reported here may provide an alternative way to tune the wavelength in optoelectronic devices.

Experimental Demonstration on Structure-Parameter Dependence of Photonic Crystal Optical Spectrum

We present fabrication and experimental measurement of a series of photonic crystal waveguides and coupled structure of PC waveguide and PC micro-cavity. The complete devices consist of an injector taper down from 3 mu m into a triangular-lattice air-holes single-line-defect waveguide. We fabricated these devices on a silicon-on-insulator substrate and characterized them using tunable laser source. We've obtained high-efficiency light propagation and broad flat spectrum response of photonic-crystal waveguides. A sharp attenuation at photonic crystal waveguide mode edge was observed for most structures. The edge of guided band is shifted about 31 nm with the 10 nm increase of lattice constant. Mode resonance was observed in coupled structure. Our experimental results indicate that the optical spectra of photonic crystal are very sensitive to structure parameters.

Growth Parameter Dependence of Structural Characterizations of Diluted Magnetic Semiconductor (Ga, Cr)As

We report the influence of growth parameters and post-growth annealing on the structural characterizations and magnetic properties of (Ga, Cr)As films. The crystalline quality and magnetic properties are sensitive to the growth conditions. The single-phase (Ga, Cr)As film with the Curie temperature of 10 K is synthesized at growth temperature T-s = 250 degrees C and with nominal Cr content x = 0.016. However, for the films with x > 0.02, the aggregation of Cr atoms is strongly enhanced as both T. and x increase, which not only brings strong compressive strain in the epilayer, but also roughens the surface. The origin of room-temperature ferromagnetism in (Ga, Cr)As films with nanoclusters is also discussed.

Tuning of emission wavelength of InAs/GaAs quantum dots sandwiched by combination layers

We investigate about controlling of photoluminescence (PL) wavelengths of InAs/GaAs self-assembled quantum dots (QDs) sandwiched with combination strained-buffer layer (CSBL) and combination strained-reducing layer (CSRL). The emission peak position of QDs is red-shifted to 1.37 mu m. The density of the QDs is increased to 1.17x10(10) cm(-2). It is indicated that optical properties of QDs could be improved by optimizing of the buffer and covering layers for the QDs. These results may provide a new way to further developing GaAs-based 1.3 mu m light sources.

Period Continuous Tuning of an Efficient Mid-Infrared Optical Parametric Oscillator Based on a Fan-out Periodically Poled MgO-Doped Lithium Niobate

We report a period continuously tunable, efficient, mid-infrared optical parametric oscillator (OPO) based on a fan-out periodically poled MgO-doped congruent lithium niobate (PPMgLN). The OPO is pumped by a Nd:YAG laser and a maximum idler output average power of 1.65 W at 3.93 mu m is obtained with a pump average power of 10.5 W, corresponding to the conversion efficiency of about 16% from the pump to the idler. The output spectral properties of the OPO with the fan-out crystal are analyzed. The OPO is continuously tuned over 3.78-4.58 mu m (idler) when fan-out periods are changed from 27.0 to 29.4 mu m. Compared with temperature tuning, fan-out period continuous tuning has faster tuning rate and wider tuning range.

Tuning the Exciton Binding Energies in Single Self-Assembled InGaAs/GaAs Quantum Dots by Piezoelectric-Induced Biaxial Stress

We study the effect of an external biaxial stress on the light emission of single InGaAs/GaAs(001) quantum dots placed onto piezoelectric actuators. With increasing compression, the emission blueshifts and the binding energies of the positive trion (X+) and biexciton (XX) relative to the neutral exciton (X) show a monotonic increase. This phenomenon is mainly ascribed to changes in electron and hole localization and it provides a robust method to achieve color coincidence in the emission of X and XX, which is a prerequisite for the possible generation of entangled photon pairs via the recently proposed "time reordering'' scheme.

An Enhanced Particle Swarm Optimization Algorithm

Particle Swarm Optimization (PSO) algorithm is often used for finding optimal solution, but it easily entraps into the local extremum in later evolution period. Based on improved chaos searching strategy, an enhanced particle swarm optimization algorithm is proposed in this study. When particles get into the local extremum, they are activated by chaos search strategy, where the chaos search area is controlled in the neighborhood of current optimal solution by reducing search area of variables. The new algorithm not only gets rid of the local extremum effectively but also enhances the precision of convergence significantly. Experiment results show that the proposed algorithm is better than standard PSO algorithm in both precision and stability.

循环优化序列自动定制方法研究

现代计算机在体系结构和应用场景复杂性的增长使得程序性能的增长、保持程序性能的可移植性以及程序开发效率的提升越来越困难，程序自动调优(auto-tuning) 是解决此问题的一个可行途径。本文研究一种通过为循环嵌套自动定制循环优化序列来进行程序自动调优的方法，提高了被编译程序的性能和可移植性，也有助于提升开发效率。 在综述相关工作的基础上，本文首先分析了循环优化序列自动定制研究需要解决的几个关键问题：循环优化的形式化描述及实施、循环优化序列的性能评估、循环优化序列的自动定制方法以及相关的原型工具。本文围绕这几个问题进行研究。 本文采用了Polyhedron模型来描述循环优化，并基于此实施了循环优化的仿射变换。这一工作基于URUK/WRaP-IT开源软件包实现，并作出了若干重要改进。借助Polyhedron模型，本文还给出了部分常见循环优化之间的使能(enable)关系。循环优化之间的相互影响较为普遍，一个循环优化的执行可能会破坏后续优化的实施条件，也可能不破坏甚至创造相应条件。这一关系可以用于过滤循环优化序列定制过程中产生的无效序列从而加快定制速度。本文分析了两个循环优化以先后次序执行时，循环嵌套内所有依赖距离向量均得以保持的条件，给出了它们之间的使能关系。并且给出了使能关系在Polyhedron模型下的计算方法，以及在序列搜索过程中通过维护依赖距离向量状态计算使能关系的方法。 对于循环优化序列性能评估方法，本文采用了高速缓存失效率简化方程(CMES方程)以动态给出循环嵌套中的高速缓存失效率数据，提出了LoopCost模型以静态分析方式发现循环嵌套中对性能影响最大的维度，这两种评估方法分别适用于不同的序列定制方法。 给出了Cache 失效率简化方程(CMES, Cache Miss Equation Simplified)在Polyhedron 模型下的求解方法。CMES方程可在编译时给出循环嵌套中的Cache失效率数据，不需要实际编译运行目标程序，从而避免相关文献中使用运行时信息来评价优化序列带来的巨大编译开销。给出了CMES方程各个参数从循环嵌套的Polyhedron模型中提取的方法，并给出了具体计算过程。 提出了一种可发现循环嵌套中对性能影响最大维度的循环优化性能评估模型LoopCost。相关文献中所使用的性能评估模型可给出一个循环嵌套的整体性能，但无法给出哪个维度对其性能影响最大；而大多数循环优化实际上是对某一个维度所做的变换，因此这些评估模型不能直接指导优化序列的搜索，只能是在一个迭代过程中提供性能信息。LoopCost模型基于数据依赖关系对一个循环嵌套估算出其每个维度使用的cache行数，作为衡量局部性优劣的指标，适用于紧嵌套循环和非紧嵌套循环，并且其复杂度与循环嵌套个数和引用个数成线性关系。通过对SPEC CPU 2006中5,703个循环嵌套的实验，LoopCost模型对其中98.3%的循环嵌套都能给出正确的性能评估数据。 对于循环优化序列自动定制方法，本文首先提出了一种基于CMES方程的Custom方法，使用迭代方式搜索可行优化序列；为了进一步提升优化效率，又提出了一种基于LoopCost模型的快速定制方法FastCustom。 Custom方法使用CMES方程评估优化序列，使用遗传算法作为序列搜索方法。在遗传算法中的各个算子中利用循环优化的enable关系加速搜索速度。该方法可以避免使用运行时刻信息评估性能所带来的巨大编译开销，又可避免相关工作中利用Polyhedron模型中语句实例执行次序矩阵的性质来搜索带来的局限性，适用的循环优化范围广，编译效率高，同时能取得与相关工作基本相当的加速比。 FastCustom方法优化效率高，并且与Custom方法不同，适用于所有场合。它基于LoopCost模型寻找对性能影响最大的维度，并根据一个启发式策略给出一个循环优化序列。该方法编译速度快，优化效果与Custom方法基本相当。与Custom相比，该方法需要给出启发式策略，因此适用的循环优化范围不如Custom，适用于迭代编译无法进行的情形。 对于循环优化序列自动定制原型工具，本文设计并实现了原型工具CPOLO。该工具改进了URUK/WRaP-IT开源软件包，使其能够支持较为复杂的程序；增强了代码重生成的效率；并且实现了本文所提出的优化序列性能评估模型和自动定制方法。 最后，总结了全文并探讨了进一步的工作方向。

Wavelength multiplexing and tuning in nano-Ag/dielectric multilayers

Multicolored optical active planes have been fabricated with magnetron sputter method coupled with selective masking technique. The plane is multilayer structured with Ag nanoparticles and TiO2 thin layer as the building blocks. It was found that the formed multilayer can be readily wavelength multiplexed by simply overlapping several nano-Ag/TiO2 layered structures, each of which may have different surface plasmon resonance wavelength. Unlike high order multiple resonances of large particles each of the multiplexing wavelengths in such a system is separately tunable. Importantly, it reveals that modification of the TiO2 layer thickness generates a fine tuning of the resonance wavelength.