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PhD Defense: Paul Burkins

Friday, July 21, 2017
1:00 PM - 3:30 PM
Physics : 401
ADVISOR: Dr. Anthony Johnson


For many years nonlinear optics has focused on discovering and fashioning optical limiters and all optical switches. Optical limiting engages nonlinear constituents by means of nonlinear absorption, nonlinear scattering, and defocusing nonlinear refraction designed for multispectral protection of sensors and the human eye. All optical processing necessitates an ultrafast nonlinear refractive medium to act as an optical switch. Both components of the complex third order susceptibility can be determined from the Z-scan technique. In this dissertation work, a modification called Thermally Managed Z-scan was implemented to resolve instantaneous and cumulative thermal and other effects contributions to the nonlinearity.
   Graphene, and its derivative graphene oxide (GO) proved to have a negative nonlinear refractive index of one order of magnitude higher than silica glass. Nonlinear refraction of nanosized GO (n2 ~ -2.7 x 10-15 cm2/W) were greater in scale than the microsized (n2 ~ -1.6 x 10-15 cm2/W) counter parts, while analogous nonlinear absorption was observed. The existence of oxidative ligands and imperfections in lattice structure at the boundaries of GO sheets can augment electron polarization and phonon–phonon scattering. The volume encapsulated by the laser beam contains more particles of which have smaller diameter, leaving nanoparticles with a pronounced nonlinear refraction.
   TiO2 thin films grown by atomic layer deposition (ALD) unveiled significant saturable absorption at 800 nm making them nascent optical limiters. ALD growth produces films and nanolaminates that are inherently more nonlinear than any other growth method (n2 ~ 1.2 x 10-11 to 7.8 x 10-10 cm2/W) . Deposition and annealing temperature dependence of the third order polarization were present for TiO2 and nanolaminate thin films. Amorphous film structure packs more nonlinearly active molecules in a constricted dimension, allowing for stronger interaction and a greater χ(3) polarization component.