Optical Science and Engineering ETDs

Reed Weber

Publication Date

1-31-2013

Abstract

Making use of femtosecond laser sources, nonlinear microscopy provides access to previously unstudied aspects of materials. By probing third order nonlinear optical signals determined by the nonlinear susceptibility chi(3), which is present in all materials, we gain insight not available by conventional linear or electron microscopy. Third-harmonic (TH) microscopy is applied to supplement laser-induced damage studies of dielectric oxide thin film optical coatings. We present high contrast (S/N> 100 : 1) TH imaging of ~17 nm nanoindentations, individual 10 nm gold nanoparticles, nascent scandia and hafnia films, and laser induced material modification both above and below damage threshold conditions in hafnia thin-films. These results imply that TH imaging is potentially sensitive to laser-induced strain as well as to nanoscale defects or contamination in oxide films. Compared to other sensitive imaging techniques such as Nomarski and dark field, TH imaging exhibits dramatically increased sensitivity to typical material modifications undergone during the formation of optical damage as evidenced by a dynamic range 10^6 : 1. Four-wave mixing (FWM) microscopy is employed to investigate delay dependent FWM signals and their implied characteristic resonant response times in multiple solvents. Mathematical modeling of resonant coherent anti-Stokes Raman scattering (CARS), coherent Stokes Raman scattering (CSRS) and stimulated parametric emission (SPE) processes supplement the FWM studies and suggest a resonant CARS process that accounts for ~95% of the total visible FWM signal which probes a characteristic material response time ~100 fs. This signal enhancement likely indicates the net effects of probing several Raman active C-H stretch bands near 2950 cm^-1. This FWM technique may be applied to characterize the dominant resonant response of the sample under study. Furthermore this technique presents the newfound capability to provide estimates of characteristic material dephasing times in combination with potential spatial resolution ~1 micron. In addition to TH and FWM microscopy, a genetic algorithm is developed and implemented that allows for the synthesis of arbitrary temporal waveforms to maximize the generation of nonlinear optical signals in the focal plane of a microscope without any prior knowledge of the experiment. This algorithm is demonstrated to compensate high order optical dispersion and thereby increase TH microscopy signals ~10x in a fused silica sample.

Degree Name

Optical Science and Engineering

Level of Degree

Doctoral

Department Name

Optical Science and Engineering

First Committee Member (Chair)

Thomas, James

Second Committee Member

Lidke, Keith

Third Committee Member

Grey, John

Document Type

Dissertation

Language

English

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