Dissertation Title: “First and Third Susceptibility of Organic Molecule”
Abstract:
Illuminating a material with intense laser excitation may change its properties and result in nonlinear absorption (NLA) and nonlinear refraction (NLR). In this dissertation we study the nonlinear absorption of organic compounds, the effect of extremely nondegenerate NLR in semiconductors, and the linear refractive index of organic solvents. In liquids, the refractive index has been studied for decades and different kinds of refractometers have been proposed. However, most of the reported values are in the visible region and only for commonly used solvents. We proposed a new interferometer-based refractometer that allows us to measure the refractive index from the visible to the near-infrared (NIR). We characterized 24 organic solvents over 7 wavelengths. A refractive index database was developed that displays our results and also allows comparison with values in the literature. The corresponding Cauchy equation is given according to different wavelengths and temperature regions. A group of solvents that do not have C-H, O-H, or N-H bonds in their chemical structure show a wide transparent spectral window in the NIR region. This group of solvents generates great interest in several research areas such as supercontinuum generation and optical parametric amplification in liquid-core optical fibers and for the design of optofluidic devices. In organic compounds, tailoring their molecular structure can change their linear and nonlinear optical properties. We performed a comprehensive linear and nonlinear spectroscopic characterization of two groups of organic compounds: gold dithiolenes and boron-dipyrromethene (BODIPY) dyes. The gold dithiolenes show a wide excited-state absorption band across the visible region with an approximate 10 ps excited-state lifetime, with no corresponding fluorescence or singlet oxygen yield observed. The BODIPY dyes have both two-photon absorption and excited-state absorption in the NIR region. The excited-state lifetime and the fluorescence quantum yield depend on the solvent polarity. The brominated counterpart shortens the excited-state lifetime instead of forming the triplet state. Optical gain is observed for non-fluorescence dyes in the polar solvent. These properties make them potential candidates for many applications such as optical power limiting and all-optical switching. In semiconductors, the NLR in the sub-gap region is enhanced under the nondegenerate case, which means the refractive index at one frequency is enhanced with a presence of a beam with another wavelength. This enhancement is not just in the refractive index, but also in group index and group velocity dispersion. We calculated how to pulse evolution with different arrival times between two pulses. The enhanced NLR shifts the spectrum of the pulse and shows a potential application in all-optical switching.
Educational Career:
BS: 2011, Mechanical Engineering, National Taiwan University
MS: 2014, Applied Mechanics, National Taiwan University
Committee in Charge:
David Hagan, Chair
Eric Van Stryland
Kyle Renshaw
Eloy Hernandez, External
Approved for distribution by David Hagan, Committee Chair, on June 6th, 2022.
The public is welcome to attend.
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