Ph.D. Defense: Gregory Cohoon

    Thursday, December 17, 2015 - 3:00pm
    Franken Conference Room (Meinel 821)

    Fabrication, Characterization, and Application of Microresonators and Resonant Structures.


    Optical resonators are structures that allow light to circulate and store energy for a duration of time. This work primarily looks at the fabrication, characterization, and application of whispering gallery mode microresonators and the analysis of organic photonic crystal-like structures and simulation of their resonant effects.

    Whispering gallery mode (WGM) microresonators are a class of cylindrically symmetric optical resonator which light circulates around the equator of the structure. These resonators are named after acoustic whispering galleries, where a whisper can be heard anywhere along the perimeter of a circular room. These optical structures are known for their ultra high Q-factor and their low mode volume. Q-factor describes the photon lifetime in the cavity and is responsible for the energy buildup within the cavity and sharp spectral characteristics of WGM resonators. The energy buildup is ideal for non-linear optics and the sharp spectral features are beneficial for sensing applications. Characterization of microbubble resonators is done by coupling light from a tunable laser source via tapered optical fiber into the cavity. The fabrication of quality tapered optical fiber on the order of 1-2 µm is critical to working on WGM resonators. The measurement of Q-factors up to 2x108 and mode spectra are possible with these resonators and experimental techniques.

    This work focuses on microdisk and microbubble WGM resonators. The microdisk resonators are fabricated by femtosecond laser micromachining. The micromachined resonators are fabricated by ablating rotating optical fiber to generate the disk shape and then heated to reflow the surface to improve optical quality. These resonators have a spares mode spectrum and display a Q factor as high a 2x106. The microbubble resonators are hollow microresonators fabricated by heating a pressurized capillary tube which forms a bubble in the area exposed to heat. These have a wall thickness of 2-5 µm and a diameter of 200-400 µm. Applications in pressure sensing and two-photon fluorescence of dye in microbubble resonators is explored.

    Photonic crystals can have engineered resonant properties by tuning photonic band gaps and introducing defects to create cavities in the photonic structure. In this work, a natural photonic crystal structure is analyzed in the form of diatoms. Diatoms are a type of phytoplankton which are identified by unique ornamentation of each species silica shell, called a frustule. The frustule is composed of a quasi-periodic lattice of pores which closely resembles manmade photonic crystals. The diatom frustules are analyzed using image processing techniques to determine pore-to-pore spacing and identify defects in the quasi-periodic structure which may contribute to optical filtering and photonic band gap effects. The data gathered is used to simulate light propagation through the diatom structure at different incident angles and with different material properties and to verify data gathered experimentally.

    Speaker Bio(s): 

    Gregory Cohoon's committee is composed of Robert Norwood, Masud Mansuripur and Khanh Kieu.