Curt A. Flory

Thermal noise and radiation pressure in MEMS Fabry-Perot tunable filters and lasers

In this paper, we examine thermal noise and radiation-pressure effects in MEMS tunable Fabry-Perot etalons. We show that thermal noise causes a jitter in the center wavelength in very high finesse etalons. In turn, the jitter causes an effective increase in the time-averaged filter bandwidth. Radiation pressure is of little consequence in conventional Fabry-Perot etalons, but it can give rise to nonlinearities and hysteresis in the tuning response of high-finesse MEMS filters. We develop models of noise and optical nonlinearities and compare the models with a series of measurements on commercial tunable high-finesse MEMS Fabry-Perot etalons.

Modeling of GaN optoelectronic devices and strain-induced piezoelectric effects

Modeling of nitride-based LEDs and laser diodes requires a fast modular tool for numerical simulation and analysis. It is required that the modeling tool reflects the primary physical processes of current injection, quantum well (QW) bound-state dynamics, QW capture, radiative, and nonradiative transitions. The model must also have the flexibility to incorporate secondary physical effects, such as induced piezoelectric strain fields due to lattice mismatch and spontaneous polarization fields. A 1-D model with a phenomenological well-capture process, similar to that developed by Tessler and Eisenstein, has been implemented. The radiative processes are calculated from first principles, and the material band structures are computed using k/spl middot/p theory. The model also features the incorporation of such effects as thermionic emission at heterojunctions. Shockley-Read-Hall recombination, piezoelectric strain fields, and self-consistent calculation of the QW bound states with dynamic device operation. The set of equations underlying the model is presented, with particular emphasis on the approximations used to achieve the previously stated goals. A sample structure is analyzed, and representative physical parameters are plotted. The model is then used to analyze the effects of incorporation of the strain-induced piezoelectric fields generated by lattice mismatch and the spontaneous polarization fields. It is shown that these built-in fields can accurately account for the blue-shift phenomena observed in a number of different GaN LEDs.

Analysis of directional grating-coupled radiation in waveguide structures

Radiation scattered from diffraction gratings on the surface of waveguides is analyzed using the volume current method. This technique allows the interpretation of the grating elements as scattering centers of the underlying waveguide mode, and radiating sources for the diffracted fields. The framework allows separation of the effects of the grating array global periodicity and the effects of the specific shape of the individual grating elements. A straightforward analogy between the effects of the grating element shape and the behavior of phased-antenna array systems allows a clear and intuitive understanding of the effects of blazed gratings on the directionality of grating-coupled radiation. The analysis is applied to some specific geometries.

Analysis of super-radiant Smith-Purcell emission

Smith-Purcell emission is studied for a system with no incident field or electromagnetic feedback mechanism other than what is provided by the grating itself. A model is developed that analyzes the interaction between the electron beam and the grating electromagnetic modes. The system of equations is solved numerically, demonstrating linear emitted power versus current at low current levels and super-radiant power emission at higher currents. These results are compared to the previously published experimental data, and the model is used to gain some understanding of the physical mechanisms at work.

Improved method for designing a cylindrical Zhang–Enke ion mirror

Abstract A series solution for the potential within a cylindrical, three-element Zhang–Enke ion mirror has been obtained. The only simplification is that the small gaps between mirror elements are ignored. Automated function minimization by the simplex method has been used to optimize the mirror region lengths and voltages. Flight times only along the central axis were used here for convenience, but the potential can be calculated fully in three dimensions. The mirror designed by this method has good focusing characteristics. Additional refinement could be obtained by optimization of three-dimensional ion trajectories starting from the one-dimensional parameters obtained here.

Add-drop photonic crystals filters

The transmission spectra of photonic crystal add-drop filters created in the microwave region are measured. The photonic crystal is a two dimensional square lattice of dielectric rods. The add-drop filters consist of two waveguides formed by removing rods along a line and a cavity lying between the two waveguides. The cavity is formed by removing rods or by replacing them with smaller diameter rods. Depending on the cavity geometry, certain wavelengths can be dropped from one waveguide and added to the other waveguide via resonant coupling through the cavity. A systematic study of the add-drop characteristics is performed as the cavity region is modified. Theoretical results, obtained from finite difference time domain calculations, are in good agreement with measurements.

Modeling of gallium nitride optoelectronic devices

The nature of our work in the area of research and development of nitride-based LEDs and laser diodes requires a fast modular tool for numerical simulation and analysis. It is required that the modeling tool reflects the primary physical processes of current injection, quantum well bound state dynamics, quantum well capture, radiative and non-radiative transitions. The model must also have the flexibility to incorporate secondary physical effects, such as induced piezoelectric strain fields due to lattice mismatch. A one-dimensional model with a phenomenological well capture process, similar to that developed by Tessler and Eisenstein, has been implemented. The radiative processes are calculated from first principles, and the material band structures are computed using k (DOT) p theory. The model also features the incorporation of such effects as thermionic emission at heterojunctions, Shockley- Read-Hall recombination, piezoelectric strain fields, and self-consistent calculation of the quantum well bound states with dynamic device operation. The set of equations underlying the model is presented, with particular emphasis on the approximations used to achieve the previously stated goals. A sample structure is analyzed, and representative physical parameters are plotted. Finally, as an example of the modeled secondary physical processes, results showing the effects of incorporating strain-induced piezoelectric fields due to lattice mismatch are given.