Optimization of planar, multilayer waveguides

Thesis (M.S.)--University of Rochester. College of Engineering and Applied Science. Institute of Optics, 1991. Missing pages 57, 58

Investigation of radio frequency sputtered high refractive index glasses for planar optical waveguides

Thesis (M.S.)--University of Rochester. College of Engineering and Applied Science. Institute of Optics, 1990.Sputter deposited high refractive index glasses were investigated for fabricating optical waveguides. It was found that a titania-silicate glass with a titanium modifier content of 20 mole percent could be sputtered and form a glass thin film. The stoichiometry and optical properties of these films could be varied with the sputtering process parameters to achieve glass compositions that are not possible in the bulk form. Initially scattering losses were extremely high with these films. Annealing reduced the losses to 8 dB/cm and this value could not be improved upon using conventional anneal methods. The films proved to be extremely durable. Laser damage testing was done on a single film and a two layer AR coating using this glass. Damage thresholds were comparable to simple traditional oxides used for high energy laser coatings

Gradient-index profile control by ion exchange in glass

Thesis (Ph. D.)--University of Rochester. College of Engineering and Applied Science. Institute of Optics, 1987.Gradient-Index (GRIN) parameters are useful in the design of optical imaging systems, but fabrication technology has limited the application of these designs in the past. The control of large geometry index profiles using ion exchange in single alkali glass is investigated, with special attention paid to quantitative prediction of index profiles. The study focuses on a simple aluminosilicate glass and silver for sodium ion exchange from various chloride salt melts. The research strategy consists of two approaches. The first is characterization of the glass and salt melts and the resulting dopant distributions using well known methods. These include radiotracer diffusions, microprobe and wet chemical composition analysis and interferometry of the final index profiles. In the second approach, the index profiles are measured as they form during diffusion using a new instrument, an oven interferometer, designed and built for this purpose. Useful methods for controlling the maximum change in refractive index (Δn) and the profile distribution were discovered. Additives in the salt melt can be used to reduce fabrication costs while maintaining high Δn's. Various methods, including anneals and field-assisted ion exchange, were shown to be useful in controlling the dopant distribution. The results of the characterization of the glass and salts showed that the ion exchange model works for small index changes (Δn GRIN materials must be made under conditions which lead to high An's and large diffusion depths. The current single alkali glass model does not apply to these conditions. It is shown that realtime measurement of the index profile evolution is useful for process control and improving the understanding of ion exchange in glass

Upconversion in Erbium-doped transparent glass ceramics

Thesis (Ph. D.)--University of Rochester. Institute of Optics, 2005.Transparent glass ceramics (TGCs) are a class of materials that are composed of a robust glass matrix which is densely embedded with nanometersized fluoride crystals. In bulk, fluoride materials tend to have poor handling and mechanical properties, and can be expensive to produce. In contrast, the forming and handling properties of the TGC are similar to those of the precursor glass, and are engineered to be robust and mechanically stable. Rare earth ions can be incorporated into the TGC during manufacture and can become partially segregated into the crystalline phase. There they experience the low-phonon energy environment of the fluoride nanocrystallite, which induces long energy level lifetimes and enhanced frequency upconversion. Therefore, rare earth doped TGCs can have the spectroscopic properties of a crystal with the durability of an aluminosilicate glass. Upconversion fluorescence is studied for an aluminosilicate TGC containing LaF₃ nanocrystallites and doped with an erbium density of 1.7 x 10²⁰ cm⁻³. Time gated fluorescence and excitation spectra as well as photoluminescence decays are used to find the nature and origin of this fluorescence. It is determined that energy transfer upconversion occurs only in the nanocrystallite phase and sequential two-photon absorption upconversion occurs in both glass and crystal phases

Group III-vacancies mediated disordering of intrinsic and n-type AlGaAs/GaAs superlattices

Thesis (Ph. D.)--University of Rochester. College of Engineering and Applied Science. Institute of Optics, 1993.The mechanisms of superlattice disordering, a useful technique for optoelectronic device fabrication and integration, have been investigated. Al-Ga interdiffusion coefficients in AlGaAs/GaAs superlattices were determined using photoluminescence spectroscopy (PLS) and secondary ion mass spectrometry (SIMS). Al-Ga interdiffusion in intrinsic AlGaAs heterostructures is mediated by second nearest-neighbor hopping of group III vacancies throughout the 700 to 1050 °C range and in both Ga- and As-rich annealing ambients. The predicted increase of two orders of magnitude in the Al-Ga interdiffusion coefficient with As pressure between the Ga- and As-rich solidus limits was observed. The depth profiles of AlGa interdiffusion are consistent with group III-vacancy diffusion between the crystal surface and bulk, and yielded a single, reasonable vacancy diffusion coefficient. The activation energy of interdiffusion agreed with that predicted for second nearest-neighbor hopping of vacancies based on theoretical estimates of vacancy formation and migration energies. Additionally, the annealing of silicon nitride encapsulated superlattices indicated very limited Al-Ga interdiffusion even with an As overpressure. This is attributed to the lack of group III vacancies. Conversely, silicon dioxide caps appeared to be very permeable. A quantitative test of the Fermi-level effect on Al-Ga interdiffusion in n-type superlattices was performed using PLS, SIMS, and C-V profiling. A significant variation was observed in the enhancement with annealing ambient in quantum wells that were Si-doped during growth. This attributed to electrical compensation and the As overpressure's effect on group III-vacancy formation at the crystal surface. The predicted Fermi-level enhancement of a factor of forty was observed only when neither excess Ga or As were included. Disordering by the indiffusion of a variety of group IV and VI donors was also investigated. An enhancement in Al-Ga interdiffusion was observed in each case with the disordering being attributed to group III vacancies. However, important differences have been observed in the interdiffusion characteristics induced by Si or Ge, and that by S or Se. Additionally, the depth profiles of deep levels associated with group III vacancy-donor complexes were obtained using cathodoluminescence

Modal expansions in transparent and nontransparent planar waveguides

Thesis (Ph. D.)--University of Rochester. College of Engineering and Applied Science. Institute of Optics, 1994.Eigen-Mode expansions provide a convenient means for modeling and understanding the behavior of optical waveguides. Recent increase interest in waveguide that incorperate nontranaparent materials, e.g. detector stuctures amplifies have brought about a need for the adaptation of the methods being applied to model and understand these waveguides. Here a variety of planar eigentechniques are evaluated and adapted for use in understanding propagation in nontransparent structures and structures supporting partially bound "leaky" fields. A variety of mathematical and numerical improvements to these techniques are developed and discussed