High-Q TM whispering-gallery modes in three-dimensional microcylinders

We present a generation condition for realizing high-Q TM whispering-gallery modes (WGMs) in semiconductor microcylinders. For microcylinders with symmetry or weak asymmetry vertical waveguiding, we show that TM WGMs can have a high Q factor, with the magnitude of 10(4) at the radius of the microcylinder of 1 mu m, by three-dimensional numerical simulation. The Q factor of TE WGMs is much less than that of TM WGMs in the semiconductor microcylinders due to a vertical radiation loss caused by mode coupling with the vertical propagating mode. The results open up a possible application of TM WGMs in semiconductor microcylinders for efficient current injection microlasers and single photon sources.

Electro-optic coefficients of Si0.75Ge0.25/Si/Si0.5Ge0.5 asymmetric superlattice measured by polarization-maintaining fiber-optic Mach-Zehnder interferometer

Ten-period 5.5 nm Si0.75Ge0.25/10.3 nm Si/2.5 nm Si0.5Ge0.5 trilayer asymmetric superlattice was prepared on Si (001) substrate by ultrahigh vacuum chemical vapor deposition at 500 degrees C. The stability of Mach-Zehnder interferometer was improved by utilizing polarization-maintaining fibers. According to the electro-optic responses of the superlattice with the light polarization along [110] and [-110], respectively, both electro-optic coefficients gamma(13) and gamma(63) of such asymmetric superlattice were measured. gamma(13) and gamma(63) are 2.4x10(-11) and 1.3x10(-11) cm/V, respectively, with the incident light wavelength at 1.55 mu m. (c) 2006 American Institute of Physics.

Study on in-plane optical anisotropy of Si0.75Ge0.25/Si/Si0.5Ge0.5 asymmetric superlattice by reflectance difference spectroscopy - art. no. 071908

Si0.75Ge0.25/Si/Si0.5Ge0.5 trilayer asymmetric superlattices were prepared on Si (001) substrate by ultrahigh vacuum chemical vapor deposition at 500 degrees C. The nonlinear optical response caused by inherent asymmetric interfaces in this structure predicted by theories was verified by in-plane optical anisotropy in (001) plane measured via reflectance difference spectroscopy. The results show Si0.75Ge0.25/Si/Si0.5Ge0.5 asymmetric superlattice is optically biaxial and the two optical eigen axes in (001) plane are along the directions [110] and [-110], respectively. Reflectance difference response between the above two eigen axes can be influenced by the width of the trilayers and reaches as large as similar to 10(-4)-10(-3) in 15-period 2.7 nm-Si0.75Ge0.25/8 nm-Si/1.3 nm-Si0.5Ge0.5 superlattice when the normal incident light wavelength is in the range of 500-1100 nm, which is quite remarkable because the optical anisotropy does not exist in bulk Si.

High Q-factor TM modes in three-dimensional semiconductor microresonators

We have investigated the mode characteristics for three-dimensional (3D) semiconductor microresonators by finite-difference time-domain (FDTD) technique. The results show that the quality-factors (Q-factors) of TM-like modes are much larger than those of TE-like modes as the vertical waveguidng formed by semiconductor materials.

Evaluating 0.53 eVGaInAsSbthermophotovoltaic diode based on an analytical absorption model

Radiant heat conversion performance dominated by the active layer of Ga0.84In0.16As0.14Sb0.86 diode has been systematically investigated based on an analytic absorption spectrum, which is suggested here by numerically fitting the limited experimental data. For the concerned diode configuration, our calculation demonstrates that the optimal base doping is 3-4 x 10(17) cm(-3), which is less sensitive to the variation of the external radiation spectrum. Given the scarcity of the alloy elements, an economical device configuration of the 0.2-0.6 mu m emitter and the 4-6 mu m base would be particularly acceptable because the corresponding conversion efficiency cannot exhibit discouraging degradation in comparison to the one for the optimal structure, the thickness of which may be up to 10 mu m. More importantly, the method we suggested here to calculate alloy absorption can be easily transferred to other composition, thus bringing great convenience for design or optimization of the optoelectronic device formed by these alloys.

Prediction of simultaneously large and opposite generalized Goos-Hanchen shifts for TE and TM light beams in an asymmetric double-prism configuration

It is predicted that large and opposite generalized Goos-Hanchen (GGH) shifts may occur simultaneously for TE and TM light beams upon reflection from an asymmetric double-prism configuration when the angle of incidence is below but near the critical angle for total reflection, which may lead to interesting applications in optical devices and integrated optics. Numerical simulations show that the magnitude of the GGH shift can be of the order of beam's width.

基于项目驱动的藉河流域土地利用时空变化动态分析

根据1997年、2001年和2005年三期TM影像资料,运用遥感非监督分类的方法提取土地利用信息。利用地理信息系统空间分析和数据统计的方法,分析藉河流域1997~2005年间土地利用数量变化和空间变化特征,研究该地区主要土地利用类型数量变化、空间变化以及在不同坡度的动态变化特征。结果表明:耕地、林地和草地三种主要土地利用类型占该研究区土地利用面积的97%以上,是该研究区主要的土地利用类型。以1997年为基数,到2001年和2005年耕地减少幅度分别为1.2%和11.1%,因此1997~2001年的退耕程度小于2001~2005年;在不同坡度上,土地利用变化主要发生在5°~40°,而且在15°~25°之间变化最大。

无机材料的结构与物理性能：第一性原理计算

本论文的研究方向是通过密度泛函理论研究各种材料的物理化学性质。近年来相关理论和数值算法的飞速发展，使得基于密度泛函理论的第一性原理方法成为凝聚态物理、量子化学和材料科学中的常规计算研究手段。本论文对无机材料进行第一性原理研究，研究涉及材料物性包括几何构型、电子结构、磁性能、和力学性质等等。 第一章简要地介绍了密度泛函理论的基本框架和近年来的理论发展。密度泛函理论的发展以及寻找合适的交换相关能量泛函为主线。从最初的局域密度近似（LDA）、广义梯度近似（GGA）到现在的非局域泛函、自相互作用修正，多种泛函形式的出现使得密度泛函理论可以提供越来越精确的计算结果。除了改进交换相关泛函，近年来密度泛函理论向动力学平均场和含时理论等方面扩展也很活跃。这些扩展式的密度泛函理论的应用领域不断扩大。在本章的最后，我们介绍一些密度泛函理论的应用程序。 第二章我们通过第一性原理从头算系统的研究了5d过渡金属二硼化物TMB2 (TM = Hf, Ta, W, Re, Os and Ir, Pt)在假想的Pmmn空间群中的结合能、生成焓、力学常数和电子能带结构。我们的计算结果表明在力学常数和价电子浓度之间存在一种关系：即当价电子浓度在6.8到7.2之间时，体模量和剪切模量达到最大值。再者，这种力学常数与价电子浓度之间的关系可以通过电子能带结构分析，如费米面附近价电子的占据情况，决定了体系的结合能和力学性能。最大的体模量和剪切模量的获得归因于TM d-B p成键态的几乎完全占据和反键态的未占据。依据上述这种关系，我们预测了在正交结构OsB2合金化W和Re将比合金化Ir元素更容易和更硬。事实上，我们的进一步计算证实了我们的期望。 通过第一性原理计算研究了ReB2和WB2的结构、弹性力学，和电子性质。计算结果表明：ReB2的平衡态结构参数和报导的实验结构一致。在常温常压下，WB2在P63/mmc空间群比在P6/mmm更稳定。依据我们计算的多晶聚集体的体模量、剪切模量，ReB2和WB2可以被看作是具有前景的低压缩率和硬材料。再者，化合物的力学各项异性通过计算得到的弹性力学常数来详细的分析讨论。态密度和电子密度分析揭示过渡金属和硼原子之间的共价键是材料具有高的体模量、剪切模量和小的Poisson比率的原因。 第四章，采用WIEN2k程序包中的缀加平面波+局域轨道方法的第一性原理计算，研究了层状钙钛矿化合物Cs2AgF4的结构、电子结构和磁性等性质。我们的计算结果表明Cs2AgF4的基态为正交相，能量比四方相低。我们同时发现Cs2AgF4应该表现出一种强的二维铁磁性，铁磁层之间为弱的反铁磁性耦合，这一结果与实验观察相一致。更主要的是，通过分析态密度图和自旋电子密度等密度面的分析，可以清楚地看出体系中存在 和 轨道的反铁电弹性有序现象。 第五章中，采用WIEN2k程序包中的缀加平面波+局域轨道方法的第一性原理计算，研究了YBaFe2O5的晶体结构、电子和磁性等方面的物理性能，特别是有关电荷和轨道有序的情况。尽管总的3d电荷不均衡程度很小，以Fe2+ 和Fe3+阳离子t2g轨道占据数的差别所定义的轨道有序序参量相当大（0.73），无可非议的显示YBaFe2O5中存在电荷和轨道有序。O 2p和Fe eg轨道之间的强的杂化作用使得Fe2+ 和Fe3+阳离子之间总电荷差别几乎完全消失。此外，我们讨论了轨道有序和电荷有序以及磁有序之间的关系。dxz轨道有序决定了G型反铁磁性自旋有序的稳定性和电荷有序模式。