Phillip Sewell

Accurate simulation of two-dimensional optical microcavities with uniquely solvable boundary integral equations and trigonometric Galerkin discretization

A fast and accurate method is developed to compute the natural frequencies and scattering characteristics of arbitrary-shape two-dimensional dielectric resonators. The problem is formulated in terms of a uniquely solvable set of second-kind boundary integral equations and discretized by the Galerkin method with angular exponents as global test and trial functions. The log-singular term is extracted from one of the kernels, and closed-form expressions are derived for the main parts of all the integral operators. The resulting discrete scheme has a very high convergence rate. The method is used in the simulation of several optical microcavities for modern dense wavelength-division-multiplexed systems.

Cell biology: Non-thermal heat-shock response to microwaves

Exposure limits set for microwave radiation assume that any biological effects result from tissue heating: non-thermal effects have been reported but remain controversial. We show here that prolonged exposure to low-intensity microwave fields can induce heat-shock responses in the soil nematode Caenorhabditis elegans. This effect appears to be non-thermal, suggesting that current exposure limits set for microwave equipment may need to be reconsidered.

Modeling Electromagnetic Emissions From Printed Circuit Boards in Closed Environments Using Equivalent Dipoles

In this paper, a method for representing electromagnetic emissions from a printed circuit board (PCB) using an equivalent dipole model deduced from near-field scanning is proposed. The basic idea is to replace the PCB with a set of infinitesimal dipoles that generate the same radiated fields. Parameters of the equivalent dipoles are determined by directly fitting to the measured magnetic near fields. In closed-environment simulations, the equivalent method is extended to a dipole-dielectric conducting plane model to account for the interactions between the PCB and enclosure by including the basic physical features of the PCB. The electromagnetic emissions can then be predicted by solving the equivalent model with numerical methods, thereby, significantly reducing the simulation time and storage costs. A basic test board and a more complex practical telemetry PCB are modeled in different configurations and compared with measurements and full-field simulations, confirming the validity and efficiency of the model.

Directional Emission, Increased Free Spectral Range, and Mode

Achieving single-mode operation and highly directional (preferably unidirectional) in-plane light output from whispering-gallery (WG)-mode semiconductor microdisk resonators without seriously degrading the mode Q-factor challenges designers of low-threshold microlasers. To address this problem, basic design rules to tune the spectral and emission characteristics of microscale optical cavity structures with nanoscale features by tailoring their geometry are formulated and discussed in this paper. The validity and usefulness of these rules is demonstrated by reviewing a number of previously studied cavity shapes with global and local deformations. The rules provide leads to novel improved WG-mode cavity designs, two of which are presented: a cross-shaped photonic molecule with introduced asymmetry and a photonic-crystal-assisted microdisk resonator. Both these designs yield degenerate-mode splitting, as well as Q-factor enhancement and directional light output of one of the split modes

Q

The nonlinear phenomena accompanying the process of light generation in high-power tapered semiconductor lasers are studied using a combination of simulation and experiment. Optical pumping, electrical overpumping, filamentation, and spatial hole burning are shown to be the key nonlinear phenomena influencing the operation of tapered lasers at high output powers. In the particular tapered laser studied, the optical pumping effect is found to have the largest impact on the output beam quality. The simulation model used in this study employs the wide-angle finite-difference beam propagation method for the analysis of the optical propagation within the cavity. Quasi-three-dimensional (3-D) thermal and electrical models are used for the calculation of the 3-D distributions of the temperature, electrons, holes, and electrical potential. The simulation results reproduce key features and the experimental trends.

-Factors in 2-D Wavelength-Scale Optical Microcavity Structures

Results from detailed numerical analyses of the modal characteristics of large-cross-section silicon-on-insulator-based rib waveguides are presented. They highlight for the first time that satisfying widely used design criteria is not sufficient to ensure single-mode behavior. In particular, the geometries that the design formulas predict should be single-mode are shown to support higher order vertical modes that do not couple (leak) into the outer slab region and are thus low loss in nature. Fortunately, a wide range of practical rib geometries still remains for which the leakage loss of modes other than those of EH/sub 00/ and HE/sub 00/ is sufficiently high to make the waveguides effectively single mode for each polarization.

Nonlinear properties of tapered laser cavities

We examine the extension of a simple and versatile model of the electromagnetic fields in an equipment enclosure with an aperture to include the effects of loading the enclosure with conducting planes or printed circuit board structures (PCB). Modeling results are compared with experimental measurements of the shielding effectiveness in a cuboidal enclosure loaded with both grounded and ungrounded conducting planes and/or PCBs with a range of grounded and ungrounded tracks. Measurement results are compared with full electromagnetic simulations and the simple model to demonstrate the accuracy and range of validity of the simple model.

The single-mode condition for silicon-on-insulator optical rib waveguides with large cross section

Transmission-line modeling (TLM) is an established numerical simulation method for electromagnetics that has been employed in a wide variety of application areas. To date, TLM has been formulated primarily for structured (usually Cartesian) meshes. This paper presents and illustrates the application of a new TLM algorithm suitable for use with unstructured two-dimensional meshes based upon triangular elements.

Model of the electromagnetic fields inside a cuboidal enclosure populated with conducting planes or printed circuit boards

Lasing modes in cyclic photonic molecules (CPMs) composed of several identical thin semiconductor microdisks in free space are studied in a linear approximation. Maxwells equations with exact boundary conditions and the radiation condition at infinity are considered as a specific eigenvalue problem that enables one to find natural frequencies and threshold gains. It is demonstrated that careful tuning of the distance between the disks in CPMs is able to drastically reduce the lasing thresholds of the whispering-gallery modes having small azimuth indices.

Transmission-line modeling using unstructured triangular meshes

Efficient spot-size convertors are an essential component of modern integrated optoelectronics, providing a simple and reliable interface with fibers. One recently proposed design functions by forcing the field from a small spot rib waveguide into a large spot one by using a taper. Accurate models of these structures, and a clear understanding of the coupling mechanisms they rely upon, are essential if optimized designs are to be produced. Furthermore, it is necessary to develop analysis techniques which are accurate yet not computationally intensive, suitable for use within an iterative design environment. Here, the well known and proven spectral index method is extended to efficiently analyze this class of vertically coupled rib waveguide spot-size transformer, providing both the guided modes and for the first time, the substrate radiation modes of the structure. These modes can then subsequently be used to design and determine the performance of the full tapered structure.