Mohammad Mojahedi

Propagation characteristics of hybrid modes supported by metal-low-high index waveguides and bends

Hybrid-mode waveguides consisting of a metal surface separated from a high index medium by a low index spacer have attracted much interest recently. Power is concentrated in the low index spacer region for this waveguide. Here we investigate the properties of the hybrid mode in detail and numerically demonstrate the possibility of realizing compact waveguide bends using this wave guiding scheme.

Super mode propagation in low index medium

We investigate a novel waveguide geometry consisting of a high dielectric medium adjacent to a metal plane with a thin low dielectric spacer. The mechanism of operation is explained and simulation results are presented.

Compact hybrid TM-pass polarizer for silicon-on-insulator platform

Hybrid waveguides consisting of a metal plane separated from a high-index medium by a low-index spacer have recently attracted a lot of interest. TM and TE modes are guided in two different layers in these structures and their properties can be controlled in different manners by changing the waveguide dimensions and material properties. We examine the effects of different parameters on the characteristics of the two modes in such structures. We show that by properly choosing the dimensions, it is possible to cut off the TE mode while the TM mode can still be guided in a well-confined manner. Using this property of the hybrid guide, we propose a TM-pass polarizer. The proposed device is very compact and compatible with the silicon-on-insulator platform. Finite-difference time-domain simulation indicates that such a polarizer can provide a high extinction of the TE mode for a reasonable insertion loss of the TM mode.

Time-domain measurement of negative group delay in negative-refractive-index transmission-line metamaterials

We have simulated and constructed a one-dimensional metamaterial composed of a periodically loaded transmission line that exhibits both negative and positive group velocities in a band of effective negative index of refraction. The negative group velocity or, equivalently, the negative group delay, is demonstrated theoretically and experimentally in the time domain using modulated Gaussian pulses. Due to this negative delay, we can show an output pulse peak emerging from the loaded transmission line prior to the input peak entering the line, i.e., the output pulse precedes the input pulse. The fact that this surprising behavior does not violate the requirements of relativistic causality is illustrated with time-domain simulations, which show that discontinuities in the pulse waveforms are traveling at exactly the speed of light in vacuum. The pulse-reshaping mechanism underlying this behavior is also illustrated using time-domain simulations.

Periodically loaded transmission line with effective negative refractive index and negative group velocity

Media with negative refractive index (NRI) are expected to exhibit properties that are unusual compared to materials with a positive index of refraction. However, until recently, these properties were not experimentally observed, since no NRI material occurs naturally. Periodic structures with NRI have been constructed. Our group created artificial NRI materials by loading a cellular network of transmission lines with series capacitors and shunt inductors (Eleftheriades, G.V. et al., IEEE Trans. Microwave Theory and Techniques, vol.50, no.12, p.2702-12, 2002; Microwave and Wireless Component Lett., 2003). We have extended that work to design a medium that exhibits negative group velocity (NGV) in addition to NRI. To achieve the NGV, a resonant circuit is embedded within each loaded transmission line (LTL) unit cell. The resonance produces a region of anomalous dispersion in which the group delay, and thus the group velocity, is negative. The NGV means that the peak of the output pulse emerges from the LTL prior to the peak of the input pulse, though much reduced in magnitude. Note that the front of the output pulse does not precede the front of the input pulse; that is, the output pulse front suffers the usual positive delay. The proposed transmission line is fabricated using coplanar waveguide technology. Scattering matrix measurements verify the theoretical predictions.

Gain assisted surface plasmon polariton in quantum wells structures

In this paper we propose a structure to compensate the propagation loss of surface plasmons by using multiple quantum wells as a gain medium. We analyze the required gain for lossless surface plasmon propagation for different thicknesses and widths of the metallic guiding layer. We study the effects of the gain layers and a finite height superstrate on the surface plasmon mode and its propagation loss. It is shown that the gain required for lossless plasmon propagation is achievable with present technology.

The effects of an electromagnetic crystal substrate on a microstrip patch antenna

The effects of a two-dimensional (2D) electromagnetic bandgap substrate on the performance of a microstrip patch antenna are investigated. The microstrip patch antenna is placed on a defect in the electromagnetic bandgap substrate that localizes the energy under the antenna. Finite-difference time-domain calculations are employed to determine the effects of the substrate. The excitation frequency of the antenna near the resonance frequency of the defect mode can be used to control the coupling between antennas that are placed in an array.

Group velocity inversion in AlGaAs nanowires

We investigated the dispersion characteristics of submicron sized AlGaAs waveguides. Numerical simulations shows that the tight confinement of the optical waves in such nanowires leads to strong variations of the dispersion characteristics compared to classic, weakly guided waveguides of the same material system. We found numerically that the investigated structure has negative GVD for the TE mode provided the waveguide width is between 670 nm and 280 nm. Experimental data obtained from 300 mum - 1 mm long wires confirms the numerical results.

Frequency-domain detection of superluminal group velocity in a distributed Bragg reflector

Using a free-space configuration and a frequency-domain detection setup, group velocities of electromagnetic waves in a distributed Bragg reflector are investigated. Experimental data indicate that, near the regions of minimal transmission in our configuration, the group velocity is 2.1 times faster than the speed of light in vacuum. A transmission model based on diagonalization of the transfer matrix is used to compare the experimental data and the theoretical calculations, and good agreement is obtained. An overview of the experimental uncertainties and their effects on the measured quantities is provided.

Integration of a Microstrip Patch Antenna with a Two-Dimensional Photonic Crystal Substrate

ABSTRACT We have experimentally and computationally studied the integration of a microstrip patch antenna with a two-dimensional photonic crystal substrate. This antenna was fabricated on a defect in the two-dimensional photonic crystal lattice that localized the energy under the patch antenna. The finite-difference time-domain method was employed to study the characteristics of this antenna. Measurements are in excellent agreement with calculations. The effects of finite size ground planes were also studied. This work can lead to a new design tool for integrating patch antennas with photonic crystal substrates.