Angie C. Lin

Order within disorder: the atomic structure of ion-beam sputtered amorphous tantala (a-Ta2O5)

Amorphous tantala (a-Ta2O5) is a technologically important material often used in high-performance coatings. Understanding this material at the atomic level provides a way to further improve performance. This work details extended X-ray absorption fine structure measurements of a-Ta2O5 coatings, where high-quality experimental data and theoretical fits have allowed a detailed interpretation of the nearest-neighbor distributions. It was found that the tantalum atom is surrounded by four shells of atoms in sequence; oxygen, tantalum, oxygen, and tantalum. A discussion is also included on how these models can be interpreted within the context of published crystalline Ta 2O5 and other a-T2O5 studies.

Epitaxial growth of GaP/AlGaP mirrors on Si for low thermal noise optical coatings

GaP/AlGaP multilayers were grown directly on Si to form a single crystalline mirror with very low mechanical loss. The effects of growth initiation, nucleation layers, and growth variations on antiphase domains and overall film quality were investigated. Using the conditions which yielded smooth nucleation layers and fewer antiphase domains, GaP/AlGaP mirror pairs were grown. These epitaxially-integrated mirrors on Si have potential use in gravitational wave detection, relying on precision interferometric sensing, which requires extremely low mechanical loss in the optical cavities.

Investigating the medium range order in amorphous Ta2O5 coatings

Ion-beam sputtered amorphous heavy metal oxides, such as Ta2O5, are widely used as the high refractive index layer of highly reflective dielectric coatings. Such coatings are used in the ground based Laser Interferometer Gravitational-wave Observatory (LIGO), in which mechanical loss, directly related to Brownian thermal noise, from the coatings forms an important limit to the sensitivity of the LIGO detector. It has previously been shown that heat-treatment and TiO2 doping of amorphous Ta2O5 coatings causes significant changes to the levels of mechanical loss measured and is thought to result from changes in the atomic structure. This work aims to find ways to reduce the levels of mechanical loss in the coatings by understanding the atomic structure properties that are responsible for it, and thus helping to increase the LIGO detector sensitivity. Using a combination of Reduced Density Functions (RDFs) from electron diffraction and Fluctuation Electron Microscopy (FEM), we probe the medium range order (in the 2-3 nm range) of these amorphous coatings.

Optical characterization of orientation-patterned GaP structures by micro reflectance difference spectroscopy

The integration of zincblende semiconductors on silicon demands for a real-time control of the crucial steps of epitaxial growth process at a microscopic level. Optical probes, being non-invasive, are very useful in monitoring such processes at a microscopic level. By using the reflectance anisotropy technique with microscopic resolution (μ-RD/RA), which detects the difference in reflectance for two orthogonal crystal directions, we measured the optical anisotropies below and above band gap of orientation-patterned GaP structures deposited on both Si(100) and GaP(100) vicinal substrates. We have developed a physical model to describe the line shape of the spectra below and above the fundamental gap of GaP. By using this model, we have successfully analyzed μ-RD/RA spectra, and we were able to do anisotropy topographic maps of the surface and buried interface, which are consistent to those measured with scanning electron microscopy.

Two-dimensional III-V nucleation on Si for nonlinear optics

The nucleation of AlGaP and GaP on offcut (100) Si was studied by molecular beam epitaxy for the purpose of improving GaP films on Si. Atomic force microscopy and high-resolution transmission electron microscopy were used to characterize film quality. Significant reduction in surface rms roughness and antiphase domain annihilation within 30 nm of the III-V/Si interface was observed when using AlGaP as a nucleation layer. This technique was optimized for development of orientation-patterned GaP on Si for nonlinear optics but could also be applied to III-V integration on Si.

Investigating the atomic structure and properties ofTa 2 O 5 coatings

The atomic structure and properties of Ta2O5 coatings are studied using a combination of X-ray absorption spectroscopy, transmission electron microcopy and atomic modeling, which is then correlated to macroscopic material properties.

Pumping a GaAs Optical Parametric Oscillator with Circularly Polarized and Depolarized Light

We demonstrate an optical parametric oscillator based on GaAs pumped with both a circularly polarized and a pseudo-depolarized pump. High symmetry in the nonlinear susceptibility tensor of GaAs enables pumping with “exotic” polarizations.

Low loss orientation-patterned AlGaAs waveguides for quasi phase matched second harmonic generation

We have demonstrated all-epitaxially fabricated orientation-patterned AlGaAs waveguides with reduced waveguide core corrugation for the quasi-phase-matched second harmonic generation (SHG) pumped at 1.55 μm. The attenuation coefficient is measured to be ~4.5 dB/m at 1.55 μm, and ~9.7 dB/cm at 780 nm. The conversion efficiency at continuous wave operation is 43%W-1 with an 8-mm long waveguide.