Analysis of modes in a freestanding microsquare resonator by 3-D finite-difference time-domain

Modes in a microsquare resonator slab with strong vertical waveguide consisting of air/semiconductor/air are analyzed by three-dimensional (3-D) finite-difference time-domain simulation, and compared with that of two-dimensional (2-D) simulation under effective index approximation. Mode frequencies and field distributions inside the resonator obtained by the 3-D simulation are in good agreement with those of the 2-D approximation. However, field distributions at the boundary of the resonator obtained by 3-D simulation are different from that of the 2-D simulation, especially the vertical field distribution near the boundary is great different from that of the slab waveguide, which is used in the effective index approximation. Furthermore the quality factors obtained by 3-D simulation are much larger. than that by 2-D simulation for the square resonator slab with the strong vertical waveguide.

Mode analysis and Q-factor enhancement due to mode coupling in rectangular resonators

Modes in rectangular resonators are analyzed and classified according to symmetry properties, and quality factor (Q-factor) enhancement due to mode coupling is observed. In the analysis, mode numbers p and q are used to denote the number of wave nodes in the direction of two orthogonal sides. The even and odd mode numbers correspond to symmetric and antisymmetric field distribution relative to the midlines of sides, respectively. Thus, the modes in a rectangle resonator can be divided into four classes according to the parity of p and q. Mode coupling between modes of different classes is forbidden; however, anti-crossing mode coupling between the modes in the same class exists and results in new modes due to the combination of the coupled modes. One of the combined modes has very low power loss and high Q-factor based on far-field emission of the analytical field distribution, which agrees well with the numerical results of the finite-difference time-domain (FDTD) simulation. Both the analytical and FDTD results show that the Q-factors of the high Q-factor combined modes are over one order larger than those of the original modes. Furthermore, the general condition required to achieve high-Q modes in the rectangular resonator is given based on the analytical solution.

The strain distributions and carrier's confining potentials of self-organized InAs/GaAs quantum dot

On the basis of the finite element approach, we systematically investigated the strain field distribution of conical-shaped InAs/GaAs self-organized quantum dot using the two-dimensional axis-symmetric model. The normal strain, the hydrostatic strain and the biaxial strain components along the center axis path of the quantum dots are analyzed. The dependence of these strain components on volume, height-over-base ratio and cap layer (covered by cap layer or uncovered quantum dot) is investigated for the quantum grown on the (001) substrate. The dependence of the carriers' confining potentials on the three circumstances discussed above is also calculated in the framework of eight-band k (.) p theory. The numerical results are in good agreement with the experimental data of published literature.

Detect the sub-wavelength data by applying a very-small-aperture laser

We present a theoretical and experimental research about applying a very-small-aperture laser (VSAL) to detect sub-wavelength data. Near-field distribution of a VSAL, which is essential for the application of such near-field devices, will be affected by the sample or fiber posited in the near-field region of the aperture. When the device is applied to detect the sub-wavelength data, the real resolution depends on the near-field spot size, the divergent angle of the beam and the distance from the aperture to the sample. Experimental results, including the near-field detection of the spot and detection of the sub-wavelength data by using the VSAL, are presented in this paper. We realize the two dimensional scanning about the sub-wavelength data (with the width 600 nm) by employing a VSAL with a 300 nm x 300 nm aperture.

Mode characteristics for equilateral triangle optical resonators

Mode characteristics of equilateral triangle resonators (ETRs) are analyzed based on the symmetry operation of the point group C-3v. The results show that doubly degenerate eigenstates can be reduced to the A(1) and A(2) representations of C-3v, if the longitudinal mode number is a multiple of 6; otherwise, they form the E irreducible representation Of C-3v. And the one-period length for the mode light ray is half of the perimeter of the ETR. Mode Q-factors are calculated by the finite-difference time-domain (FDTD) technique and compared with those calculated from far-field emission based on the analytical near-field pattern for TE and TM modes. The results show that the far-field emission based on the analytical field distribution can be used to estimate the mode Q-factor, especially for TM modes. FDTD numerical results also show that Q-factor of TE modes reaches maximum value as the longitudinal mode number is a multiple of 7. In addition, photoluminescence spectra and measured Q-factors are presented for fabricated ETR with side lengths of 20 and 30 mu m, and the mode wavelength intervals are compared with the analytical results.

Radiation forces of a highly focused radially polarized beam on spherical particles

A radially polarized beam focused by a high-numerical-aperture (NA) objective has a strong longitudinal and nonpropagating electric field in the focal region, which implies that it is suitable for axial optical trapping. In this paper, we use the vectorial diffraction integral to represent the field distribution of the radially polarized beam focused by a high-NA objective and then employ the T-matrix method to compute the radiation forces on spherical particles. Effects of different parameters, such as the size of the sphere, the inner radius of the radially polarized beam, and the NA of the objective, on the radiation forces are presented.

Trapping of low-refractive-index particles with azimuthally polarized beam

Azimuthally polarized beams, focused by a high-numerical-aperture (NA) objective lens, form a hollow intensity distribution near the focus, which is appropriate for trapping low-refractive-index particles, in contrast to common linearly polarized or radially polarized beams. In this paper, the field distribution of the azimuthally polarized beam focused by a high-NA objective is described by the vectorial diffraction integral, and then the radiation forces on spherical particles with different parameters such as radius and refractive index are calculated by the T-matrix method. Numerical results show that the azimuthally polarized beam not only can steadily trap low-refractive-index particles at the focus center but also can trap multiple high-refractive-index particles around the focus center by virtue of the hollow-ring configuration. The range of the sizes of low-refractive-index particles that can be trapped steadily are presented, corresponding to different parameters such as the NA of the objective and the relative refractive index, based on which the NA of the objective can be selected to trap the appropriate size of particles. (C) 2009 Optical Society of America

A Planar InGaAs/InP Geiger Mode Avalanche Photodiode with Cascade Edge Breakdown Suppression

A Geiger mode planar InGaAs/InP avalanche photodiode （APD） with a cascade peripheral junction structure to suppress edge breakdowns is designed by finite-element analysis. The photodiode breakdown voltage is reduced to 54.3V by controlling the central junction depth, while the electric field distribution along the device central axis is controlled by adjusting doping level and thickness of the lnP field control layer. Using a cascade junction structure at the periphery of the active area, premature edge breakdowns are effectively suppressed. The simulations show that the quadra-cascade structure is a good trade-off between suppression performance and fabrication complexity, with a reduced peak electric field of 5.2 × 10~5 kV/cm and a maximum hole ionization integral of 1. 201. Work presented in this paper provides an effective way to design high performance photon counting InGaAs/InP avalanche photodiodes.

Eigenmode confinement in semiconductor microcavity lasers with an equilateral triangle resonator

We analyze the mode behaviors for semiconductor lasers with an equilateral triangle resonator by deriving the mode field distribution and the eigenvalue equation. The eigenvalue equation shows that the longitudinal mode wavelength interval is equivalent to that of a Fabry-Perot cavity with the cavity length of 1.5a, where a is the side length of the equilateral triangle resonator. The transverse waveguiding is equivalent to as a strip waveguide with the width of root 3a/ 2, and the number of transverse modes supported by the resonator is limited by the total reflection condition on the sides of the equilateral triangle. Semiconductor microcavity laser with an equilateral triangle resonator is suitable to realize single mode operation, and the mode wavelength can be adjusted by changing the side length.

两井条件下圆形封闭煤层中压力场分布研究

煤层气开采方式的本质是排水降压采气,了解煤层的降压效果对排采制度的确定有着积极的意义。通过建立圆形封闭煤层中考虑煤层气解吸作用的两井不定常渗流模型,利用有限元方法求得了其数值解,分析解吸作用对理论特征曲线的影响。计算结果表明,煤层气解吸具有减缓压力传播的作用;分析不同性质邻井对理论曲线特征的影响,给出存在邻井影响时的压力场发展规律。提出压力场的4种描述方法,分析邻井性质、邻井流量变化,以及外边界性质对煤层压力场分布的影响,得到不同条件下煤层气压力场的变化规律。

磁控溅射阴极靶磁场分布的定量评价

针对磁控溅射阴极靶磁场分布难以进行定量评价的问题,提出以磁场水平分量Bx的平行率Rk为量化指标,对磁场分布状态进行评价的新方法;采用有限元方法,模拟分析了磁控溅射阴极靶结构参数对磁场分布的影响规律,并利用Rk对结构参数的合理性进行了验证.结果表明,量化指标Rk可以有效地评价磁场分布的优劣,能够为磁场模拟及分析提供基础的科学判据

Study on the localization of the electric dipole sources

The research of dipole source localization has great significance in both clinical research and applications. For example, the EEG recording from the scalp is widely used for the localization of sources of electrical activity in the brain. This paper presents a closed formula that describes the electric field of dipoles at arbitrary position, which is a linear transformer called the transfer matrix. The expression of transfer matrix and its many useful characteristics are given, which can be used for the analysis of the electrical fields of dipoles. This paper also presents the closed formula for determining the location and magnitude of single dipole or multi-dipoles according to its electrical field distribution. A calculation result for a single dipole shows that the dipole will be located at the midpoint of a line segment if there are equivalent fields at its two ends.

Test and analysis of uniform magnetic fields for imaging of electrons produced in ion-atom collisions

In order to expand the solid angle for imaging of electrons in ion-atom collisions, we designed a complex Helmholtz coils composed of four single coils. Theoretical simulations were carried out to optimize the arrangement of the coils. The complex is constructed according to the theoretical analysis, and the magnetic fields were measured for interested regions. The measured results show that the relative uniformity of the magnetic fields is +/- 0.6%, which satisfies the requirements of collision experiments.

面向微粒操纵的介电泳芯片系统研究

针对目前在纳米器件及传感器的制造中尚无对大量粒子进行有效操纵的方法,我们利用介电泳方法对大量微粒进行定位和传输操纵,介绍了利用MEMS工艺进行介电泳芯片加工的过程以及整个观测与实验系统的建立,通过有限元软件对传统介电泳和行波介电泳中电极阵列的电场分布进行求解,并在该实验系统下实现了对微通道中的悬浮高度和微粒的运动速度的测量.该实验系统的研究为液体环境下微纳颗粒的装配和分离提供了一条有效的技术路径.