Design of surface emitting distributed feedback quantum cascade laser with single-lobe far-field pattern and high outcoupling efficiency

A 7.8-mu m surface emitting second-order distributed feedback quantum cascade laser (DFB QCL) structure with metallized surface grating is studied. The modal property of this structure is described by utilizing coupled-mode theory where the coupling coefficients are derived from exact Floquet-Bloch solutions of infinite periodic structure. Based on this theory, the influence of waveguide structure and grating topography as well as device length on the laser performance is numerically investigated. The optimized surface emitting second-order DFB QCL structure design exhibits a high surface outcoupling efficiency of 22% and a low threshold gain of 10 cm(-1). Using a pi phase-shift in the centre of the grating, a high-quality single-lobe far-field radiation pattern is obtained.

构建度量区间时序逻辑的时间自动机

实时系统的安全性至关重要，使用模型检测工具对其进行形式化分析是提高 其安全性的重要手段。我们已有的实时系统模型检测工具CTAV 目前可以验证 LTL 描述的性质。本文工作的最终目的是让CTAV 能够验证实时时序逻辑，即， 使得CTAV 同样可以直接验证由实时时序逻辑描述的连续时间域上的实时性质。 度量区间时序逻辑MITL 是常用的一种关于实时系统的性质规范语言。已有 不少文献讨论过MITL 到时间Büchi 自动机的转换，但由于过程较复杂，大多数 仅限于理论研究，并没有与时间Büchi 自动机的空性判定过程结合成为一个可用 的关于MITL 的模型检测工具。 为了开发可用的关于MITL 性质的模型检测工具，本文采用与CTAV 中待验 证系统模型一致的pointwise 语义模型，选取了MITL 的一个可以较高效的进行模型检测的子类——MITL≤/<。在表达能力方面，这个子集能够描述实时系统模 型检测实际过程中常见的实时性质。 本文给出了将MITL≤/<公式转化为等价的基于迁移的扩展时间Büchi 自动机 的算法过程，并讨论了转化中逻辑公式重写规则的正确性和完备性问题。基于迁 移的扩展时间Büchi 自动机是时间Büchi 自动机的一种变体。二者最主要的区别在于它们采用了不同类型的Büchi 接受条件。文中给出的构造算法是将原先应用于非实时领域的Tableau 方法扩展调整，并应用于实时连续领域。Tableau 方法的关键之一是公式重写规则。而为了保证结果自动机仅存在有限多个状态，在重写规则中定义不动子公式是至关重要的。本文给出的在pointwise 语义模型下的公式重写规则，借用了此前在continuous 语义模型下已有的将时钟变量作为时序算子的约束直接引入公式中来保证重写规则中存在不动子公式的思想，并对其进行了修改，使原先性质自动机的时钟递减变为时钟递增，与系统自动机中的时钟行走方式一致。这省去了下一步将性质自动机和系统自动机相乘判空时，因为时钟行走方式不一致而引起的麻烦。另外，本文将MITL≤/<中的某些公式扩展为包含时钟约束的新的公式。这样，在定义重写公式时，不仅保证重写规则中存在不动子公式，也保证结果自动的前进性。 本文还根据构造算法实现了转化工具MITLCon。与Marc Geilen 和Dennis Dames 实现的逻辑转化工具相比，MITLCon 显著地减少了结果自动机节点和迁 移的数量，从而降低了结果自动机的大小，有利于进一步的模型检测过程。

Nonlinear optical properties of semiconductor quantum wells under intense terahertz radiation

The nonlinear optical properties of semiconductor quantum wells driven by intense in-plane terahertz electric fields are investigated theoretically by employing the extended semiconductor Bloch equations. The dynamical Franz-Keldysh effect of the optical absorption near the band edge is analyzed with Coulomb correlation among the carriers included. The in-plane terahertz field induces a variety of behavior in the absorption spectra, including terahertz replicas of the (dark) 2p exciton and terahertz sidebands of the 1s exciton. The dependence of these interesting features on the intensity, frequency, and phase of the terahertz field is explored in detail.

Franz-Keldysh effect and dynamical Franz-Keldysh effect of cylindrical quantum wires

We investigate the interband optical absorption spectra near the band edge of a cylindrical semiconductor quantum wire in the presence of a static electric field and a terahertz electric field polarized along the axis. Optical absorption spectra are nonperturbatively calculated by solving the low-density semiconductor Bloch equations in real space and real time. The influence of the Franz-Keldysh (FK) effect and dynamical FK effect on the absorption spectrum is investigated. To highlight the physics behind the FK effect and dynamical FK effect, the spatiotemporal dynamics of the polarization wave packet are also presented. Under a reasonable static electric field, substantial and tunable absorption oscillations appear above the band gap. A terahertz field, however, will cause the Autler-Townes splitting of the main exciton peak and the emergence of multiphoton replicas. The presented results suggest that semiconductor quantum wires have potential applications in electro-optical devices.

Intersubband optical absorption in a step asymmetric semiconductor quantum well driven by a terahertz field

The nonlinear optical absorption in a three-subband step asymmetric semiconductor quantum well driven by a strong terahertz (THz) field is investigated theoretically by employing the intersubband semiconductor-Bloch equations. We show that the optical absorption spectrum strongly depends on the intensity, frequency, and phase of the pump THz wave. The strong THz field induces THz sidebands and Autler-Townes splitting in the probe absorption spectrum. Varying the pump frequency can bring not only the new absorption peaks but also the changing of the energy separation of the two higher-energy levels. The dependence of the absorption spectrum on the phase of the pump THz wave is also very remarkable.

SELF-CONSISTENT TREATMENT OF 3-DIMENSIONAL - 2-DIMENSIONAL AND 2-DIMENSIONAL - 2-DIMENSIONAL RESONANT-TUNNELING IN DOUBLE-BARRIER STRUCTURES

By using a transfer-matrix method on the basis of two-dimensional (2D) Bloch sums in accordance with a tight-binding scheme, a self-consistent calculation on the resonant tunneling in asymmetric double-barrier structures is presented, in which contributions to resonant tunneling from both three-dimensional (3D) electrons in the contacts and 2D electrons in the spacer or accumulation layers are considered simultaneously. The charge buildup effect on the current versus voltage (I-V) curves is evaluated systematically, showing quantitatively how it results in the I-V bistability and enhanced differences between I-V curves for positive and negative bias in an asymmetric double-barrier structure. Special attention is focused on the interaction between 3D-2D and 2D-2D resonant-tunneling processes, including the suppression of 2D-2D resonant tunneling by the charge buildup in the well accompanying the 3D-2D resonant tunneling. The effects of the emitter doping condition (doping concentration, spacer thickness) on the presence of two types of quasi-2D levels in the emitter accumulation layers, and on the formation of a potential bulge in the emitter region, are discussed in detail in relation to the tunneling process.

BAND DISPERSION IN THE RECURSION METHOD

The advantages of the supercell model in employing the recursion method are discussed in comparison with the cluster model. A transformation for changing complex Bloch-sum seed states to real seed states in recursion calculations is presented and band dispersion in the recursion method is extracted with use of the Lanczos algorithm. The method is illustrated by the band structure of GaAs in the empirical tight-binding parametrized model. In the supercell model, the treatment of boundary conditions is discussed for various seed-state choices. The method is useful in applying tight-binding techniques to systems with substantial deviations from periodicity.

Theoretical and experimental investigations of flexural wave propagation in periodic grid structures designed with the idea of phononic crystals

The propagation characteristics of fiexural waves in periodic grid structures designed with the idea of phononic crystals are investigated by combining the Bloch theorem with the finite element method. This combined analysis yields phase constant surfaces, which predict the location and the extension of band gaps, as well as the directions and the regions of wave propagation at assigned frequencies. The predictions are validated by computation and experimental analysis of the harmonic responses of a finite structure with 11 × 11 unit cells. The fiexural wave is localized at the point of excitation in band gaps, while the directional behaviour occurs at particular frequencies in pass bands. These studies provide guidelines to designing periodic structures for vibration attenuation.

平面阵列光学波导模式特性及其光强传输分析

分析了平面阵列波导的模式特性。其导波模式为周期性调幅的平面波(Bloch波)。阵列波导输入/输出波导结构是影响其传输特性的重要因素。采用扇形过渡波导结构以提高阵列波导输入/输出端口芯区宽度与波导周期宽度的比值，可降低阵列波导光强的传递损耗。并可改善布里渊区入射/出射波的损耗非均匀性。采用低折射率差的波导芯区/包层结构可降低波导对光波的限制，提高光强的传输效率。

Mode theory and analysis of planar array waveguides

Bloch modes can be excited in planar array due to its periodic lateral refractive index. The power coupled into each eigenmode of the array waveguides is calculated through the overlap integrals of the input field with the eigenmode fields of the coupled infinite array waveguides projected onto the x-axis. Low losses can be obtained if the transition from the array to the free propagation region is adiabatic. Due to the finite resolution of lithographic process the gap between the waveguides will stop abruptly, however, when the waveguides come into too close together. Calculation results show that losses will occur at this discontinuity, which are dependent on the ratio of the gap between the waveguides and grating pitch and on the confinement of field in the array waveguides. Tapered waveguides and low index contrast between the core and cladding layers can lower the transmitted losses.

Flat Surface Plasmon Polariton Bands in Bragg Grating Waveguide for Slow Light

The formations of the surface plasmonpolariton (SPP) bands in metal/air/metal (MAM) sub-wavelength plasmonic grating waveguide (PGW) are proposed. The band gaps originating from the highly localized resonances inside the grooves can be simply estimated from the round trip phase condition. Due to the overlap of the localized SPPs between the neighboring grooves, a Bloch mode forms in the bandgap and can be engineered to build a very flat dispersion for slow light. A chirped PGW with groove depth varying is also demonstrated to trap light, which is validated by finite-difference time-domain (FDTD) simulations with both continuous and pulse excitations.