Michael Cada

Simple model for calculating the ratio of the carrier capture and escape times in quantum-well lasers

We have developed a simple model for the carrier capture and escape processes in quantum-well (QW) lasers, which yields an analytical expression for the ratio of the carrier capture and escape times. It predicts a decrease in the escape time with injected carrier density due to the state filling effect. It also shows an exponential dependence of the escape time on the effective barrier height and on the inverse of the temperature. A comparison between experimental and calculated values for InGaAs/GaAs QW lasers is presented showing a good agreement.

Two-dimensional metallo-dielectric photonic crystals embedded in anodic porous alumina for optical wavelengths

A theoretical study is presented for hexagonal lattice metallic pillar photonic crystals in anodic porous alumina with a lattice constant of 500nm. The objective of the investigation is to design a two-dimensional metallo-dielectric photonic crystal with an anodic porous alumina template. Optical responses are calculated for silver pillars of radii 100nm, and 200nm in porous alumina. The nature of their stop bands and attenuation in the near-infrared region is investigated. Calculations reveal that two-dimensional photonic band gaps for the TM polarization exist at visible wavelengths when the radius is 200nm.

Modeling the Response of a Long-Period Fiber Grating to Ambient Refractive Index Change in Chemical Sensing Applications

The transmittance spectra of a long-period fiber grating element immersed in different mixtures of water and dimethyl sulfoxide were recorded. The obtained data were compared with a theoretical model based on linearly polarized modes treated with a coupled-mode approach. Excellent agreement between the measurements and theoretical results was found over a wide wavelength range.

The possible use of Fiber Bragg Grating based accelerometers for seismic measurements

Over the last two decades, extensive studies and research have opened a new era for Fiber Bragg Grating (FBG) based sensing technologies. One of the new applications is the FBG based accelerometers. In various field applications, FBG based accelerometers are replacing conventional electronic sensors, due to their long term stability, high accuracy and low power consumption. In this paper, theoretical analysis on FBG is introduced followed by the sensing principles. In-depth studies are made on three existing FBG based accelerometers for their unique transducer designs and signal interrogation techniques, and most importantly their possible use for seismic measurements.

Quadratic Electro-Optic Kerr Effect: Applications to Photonic Devices

We propose novel applications of the quadratic electro-optic Kerr effect to photonic devices. Specifically in this work, two new illustrative examples are described, namely an electrically controlled multistable switch (ECMS), and an electrically tunable Bragg grating (ETBG). Their functionality is based on the third-order nonlinearity in an isotropic medium. On one hand, we note that the first key feature is the all-optical as well as electro-optical control/tunability. This can be achieved only in the third-order nonlinear material as opposed to a more frequently used linear electro-optic effect exploited in optical crystals. On the other hand, the second important key feature is the availability of integrated and compatible materials that show third order nonlinearity. In the first application proposed here, ECMS, the interplay between the quadratic electro-optic and all-optical Kerr effects is crucial for its tunable operation and leads to an interesting feature of storing an electrical information optically. In the second example, ETBG, employing the quadratic electro-optic effect makes it attractive thanks to the existence of the third-order nonlinearity in many interesting isotropic materials that are suitable for device integration. Devices such as modulators, switches, mixers, variable attenuators or optical limiters can be designed.

Imperfectly geometric shapes of nanograting structures as solar absorbers with superior performance for solar cells

The expectation of perfectly geometric shapes of subwavelength grating (SWG) structures such as smoothness of sidewalls and sharp corners and nonexistence of grating defects is not realistic due to micro/nanofabrication processes. This work numerically investigates optical properties of an optimal solar absorber comprising a single-layered silicon (Si) SWG deposited on a finite Si substrate, with a careful consideration given to effects of various types of its imperfect geometry. The absorptance spectra of the solar absorber with different geometric shapes, namely, the grating with attached nanometer-sized features at the top and bottom of sidewalls and periodic defects within four and ten grating periods are investigated comprehensively. It is found that the grating with attached features at the bottom absorbs more energy than both the one at the top and the perfect grating. In addition, it is shown that the grating with defects in each fourth period exhibits the highest average absorptance (91%) compared with that of the grating having defects in each tenth period (89%), the grating with attached features (89%), and the perfect one (86%). Moreover, the results indicate that the absorptance spectrum of the imperfect structures is insensitive to angles of incidence. Furthermore, the absorptance enhancement is clearly demonstrated by computing magnetic field, energy density, and Poynting vector distributions. The results presented in this study prove that imperfect geometries of the nanograting structure display a higher absorptance than the perfect one, and provide such a practical guideline for nanofabrication capabilities necessary to be considered by structure designers.

Investigation of optical absorptance of one-dimensionally periodic silicon gratings as solar absorbers for solar cells

A rigorous design using periodic silicon (Si) gratings as absorbers for solar cells in visible and near-infrared regions is numerically presented. The structure consists of a subwavelength Si grating layer on top of an Si substrate. Ranges of grating dimensions are preliminary considered satisfying simple and feasible fabrication techniques with an aspect ratio defined as the ratio of the grating thickness (d) and the grating lamella width (w), with 0 < d/w < 1.0. The subwavelength grating structure (SGS) is assumed to comprise different lamella widths and slits within each period in order to finely tune the grating profile such that the absorptance is significantly enhanced in the whole wavelength region. The results showed that the compound SGS yields an average absorptance of 0.92 which is 1.5 larger than that of the Si plain and conventional grating structures. It is shown that the absorptance spectrum of the proposed SGS is insensitive to the angle of incidence of the incoming light. The absorptance enhancement is also investigated by computing magnetic field, energy density, and Poynting vector distributions. The results presented in this study show that the proposed method based on nanofabrication techniques provides a simple and promising solution to design solar energy absorbers or other energy harvesting devices.

Dark and antidark diffraction-free beams

We present dark and antidark diffraction-free beams and discuss their properties. We show that all such beams must be partially spatially coherent. The new beams can be used for optical trapping of atoms.

A Global Optimization Approach to Laser Design

The objective of this work is to develop and validate the basis of a novel laser modeling and design methodology that incorporates a global optimization approach. Classical modeling techniques typically involve design evaluations that are conducted at the lasers threshold injection current. This is the point where the laser is just “turning on”, and the (standard practice) numerical challenge is minimal. The fundamental difference offered by the proposed new methodology is the possibility of developing laser designs directly at the injection current (power level) of interest.The effectiveness of the new methodology is verified by considering the computationally difficult problem of maximizing a lasers internal (cavity) field “flatness” over a range of above-threshold injection currents, while also considering the boundary condition error of the lasers internal field solution. Global optimization is then used to find an optimally flat field solution in terms of the lasers structural design parameters. The favorable comparison between our results and the results obtained by the extrapolation of threshold designs to the same injection current indicate the self-consistency and fundamental capabilities of the new methodology.

All-optical bistable switching dynamics in 1.55-/spl mu/m two-segment strained multiquantum-well distributed-feedback lasers

All-optical bistable switching dynamics of 1.55-/spl mu/m two-segment strained multiquantum-well (MQW) distributed-feedback (DFB) lasers were systematically studied both experimentally and theoretically. Some fundamental optical functionalities, including all-optical set-reset (flip-flop) operations, were demonstrated. The switching responses of these bistable lasers were studied, for the first time, with optical injection from a single-mode DFB laser, indicating that the switching dynamics based on gain quenching and absorption saturation are inherently different. A theoretical model including optical injection was developed to study these all-optical bistable switching characteristics in segmented bistable lasers. It was found that the nonuniform distribution of the photon density in the bistable laser cavity induced by optical injection was essential to perform the time-domain switching operations. Simulations showed a good agreement with experimental observations and indicated design improvements. Although the switching responses in the range of tens of picoseconds can be obtained with these bistable lasers, the maximum repetition frequency of the bistable systems would still be limited to the hundreds of megahertz due to the slow carrier recovery time (5 ns) of the lasers.