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Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 72-82). Available also in a digital version from Dissertation Abstracts.

Thesis (Ph. D.)--University of Chicago, Department of Chemistry, 2002. / Includes bibliographical references. Also available on the Internet.

Thesis (Ph.D)--Physics, Georgia Institute of Technology, 2010. / Committee Chair: Rick Trebino; Committee Member: Ahmet Erbil; Committee Member: John Buck; Committee Member: Stephen Ralph; Committee Member: Zhigang Jiang. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Nonlinear microscopy has become an indispensable tool in the study of biological systems. It includes many nonlinear contrast mechanisms, each sensitive to different biological structures. However, interpretation of the images generated in nonlinear microscopy is a complex matter due to factors such as the structural complexity of the sample, phase relationships between the excitation beam and the detected signal and the nonlinear interactions in the focal volume of the microscope. This thesis contains a new theoretical and numerical framework that describes the focusing of an excitation beam in a nonlinear microscope, the nonlinear optical interactions with the material in the focal volume, and the resulting nonlinear optical signal in the far field. The framework is the first to include reflection and refraction of the excitation beam and nonlinear signals by an arbitrary number of interfaces in the focal volume, which is especially significant for the interpretation of third harmonic generation (THG). It also uses the chirp-z transform to speed up calculations by orders of magnitude compared to numerical integration techniques. The framework is used to investigate second harmonic generation (SHG) by collagen. Focusing effects alter polarization-dependent SHG measurements of collagen properties compared to the plane wave approximation, and this is verified experimentally. Furthermore, a technique of imaging the far field SHG radiation from collagen fibres is proposed, which can be used to extract the orientation of collagen fibres unambiguously. The framework is then applied to analyze the influence of interfaces on THG. Reflection effects at interfaces significantly affect THG, which leads to the development of a new super-resolution THG imaging technique based on backward-propagating THG. This super-resolution technique is experimentally demonstrated by imaging surface profiles with tens of nanometers resolution, which is the first time that such resolution is obtained in coherent nonlinear microscopy. Therefore, this imaging technique shows promise to become an important tool in high-resolution imaging of (biological) samples. The theoretical and numerical framework provides a foundation for future research on the origin of nonlinear microscopy signals. The new imaging techniques based on this framework have great potential in quantifying fibrillar structures and interfaces in biological samples.

Nonlinear microscopy has become an indispensable tool in the study of biological systems. It includes many nonlinear contrast mechanisms, each sensitive to different biological structures. However, interpretation of the images generated in nonlinear microscopy is a complex matter due to factors such as the structural complexity of the sample, phase relationships between the excitation beam and the detected signal and the nonlinear interactions in the focal volume of the microscope. This thesis contains a new theoretical and numerical framework that describes the focusing of an excitation beam in a nonlinear microscope, the nonlinear optical interactions with the material in the focal volume, and the resulting nonlinear optical signal in the far field. The framework is the first to include reflection and refraction of the excitation beam and nonlinear signals by an arbitrary number of interfaces in the focal volume, which is especially significant for the interpretation of third harmonic generation (THG). It also uses the chirp-z transform to speed up calculations by orders of magnitude compared to numerical integration techniques. The framework is used to investigate second harmonic generation (SHG) by collagen. Focusing effects alter polarization-dependent SHG measurements of collagen properties compared to the plane wave approximation, and this is verified experimentally. Furthermore, a technique of imaging the far field SHG radiation from collagen fibres is proposed, which can be used to extract the orientation of collagen fibres unambiguously. The framework is then applied to analyze the influence of interfaces on THG. Reflection effects at interfaces significantly affect THG, which leads to the development of a new super-resolution THG imaging technique based on backward-propagating THG. This super-resolution technique is experimentally demonstrated by imaging surface profiles with tens of nanometers resolution, which is the first time that such resolution is obtained in coherent nonlinear microscopy. Therefore, this imaging technique shows promise to become an important tool in high-resolution imaging of (biological) samples. The theoretical and numerical framework provides a foundation for future research on the origin of nonlinear microscopy signals. The new imaging techniques based on this framework have great potential in quantifying fibrillar structures and interfaces in biological samples.

Optical imaging has been developed quickly in the past decades because it has become an important research tool in biology, biochemistry, and biomedical sciences. Coherence domain optical imaging is one of the well developed optical imaging modalities, as it provides high resolution and long penetration depth. In this dissertation, we will report our work on development of molecular contrast in coherence domain optical imaging. In order to image important molecules which are poor fluorophores, we developed a high resolution molecular imaging technique, pump-probe optical coherence microscopy (PPOCM), which does not rely on fluorescent tags. PPOCM is the fusion of Pump-Probe spectroscopy and optical coherence microscopy (OCM). We have demonstrated the prototype system on a fixed human skin sample containing a nodular melanoma. The results indicate that PPOCM can clearly provide strong contrast between the melanotic and amelanotic regions. This technique can be applied to early diagnosis of melanoma and the mapping of tumor margins during excision. It also can be extended to any biological chromophore with a known absorption spectrum and sufficient concentration. In order to differentiate further multiple chromophores, we developed a spectrally resolved two color pump-probe Optical Coherence Microscopy (SRPPOCM). We showed the prototype system on a red hair and a black hair. Our preliminary results show that the SRPPOCM technique could provide a contrast between pheomelanin and eumelanin. This technique could be used potentially as a clinical tool for diagnosing different progression stages of melanoma. This technique could also be applied to differentiate other mixed chromophores. Second harmonic optical coherence tomography (SHOCT) is non-linear high resolution optical molecular imaging modality which is widely used in non-centrosymmetric material. However, depth ambiguity is associated with SHOCT in tissue sample because forward generated second harmonic signal does not correctly report where the second harmonic signal is generated. We studied the feasibility of collecting the backward generated second harmonic signal from nanocrystals through a Second Harmonic Optical Coherence Tomography in Fourier domain. The preliminary result shows that we can collect backward generated second harmonic signal from nanocrystals which indicates that this technique could suppress the depth ambiguity.

High Harmonic Generation (HHG) enables the production of bursts of coherent soft x-rays with attosecond pulse duration. This process arrises from the nonlinear interaction between intense infrared laser pulses and an ionizing gas medium. Soft x-ray photons are used for spectroscopy of inner-shell electron correlation and exchange processes, and the availability of attosecond pulse durations will enable these processes to be resolved on their natural time scales. The maximum or cutoff photon energy in HHG increases with both the intensity as well as the wavelength of the driving laser. It is highly desirable to increase the harmonic cutoff as this will allow for the generation of shorter attosecond pulses, as well as HHG spectroscopy of increasingly energetic electronic transitions. While the harmonic cutoff increases with laser wavelength, there is a corresponding decrease in harmonic yield. The first part of this thesis describes the experimental measurement of the wavelength scaling of HHG efficiency, which we report as lambda^(-6.3) in xenon, and lambda^(-6.5) in krypton. To increase the HHG cutoff, we have developed a 1.8 um source, with stable carrier envelope phase and a pulse duration of <2 optical cycles. The 1.8 um wavelength allowed for a significant increase in the harmonic cutoff compared to equivalent 800 nm sources, while still maintaing reasonable harmonic yield. By focusing this source into neon we have produced 400 eV harmonics that extend into the x-ray water window. In addition to providing a source of photons for a secondary target, the HHG spectrum caries the signature of the electronic structure of the generating medium. In krypton we observed a Cooper minimum at 85 eV, showing that photoionization cross sections can be measured with HHG. Measurements in xenon lead to the first clear observation of electron correlation effects during HHG, which manifest as a broad peak in the HHG spectrum centred at 100 eV. This thesis also describes several improvements to the HHG experiment including the development of an ionization detector for measuring laser intensity, as well as an investigation into the role of laser mode quality on HHG phase matching and efficiency.

The linear and nonlinear optical response of plasmon hybridized systems is the subject of study of this thesis. Plasmonic silver nanoprisms are able to confine light to a sub-wavelength volume, which provides local field enhancement. This confined field is promising for achieving an enhanced nonlinear optical response. For many of plasmon nanoparticles, however, the plasmonic resonance is not at the near-infrared wavelengths of a Ti:Sapphire laser, the most common source used for ultra-fast measurements. To achieve resonance at these wavelengths, a tuning mechanism is required. The plasmon hybridization between silver nanoprisms and a thin gold film provides this tuning mechanism, which allows for enhanced optical second harmonic generation. Overlapping the plasmon resonance of the system with excitation source, by varying the spacer layer between the nanoprisms and the gold film, enhances the second harmonic counts by approximately three orders of magnitude. The finite-difference time-domain calculations agree to within a factor of two with the experimental findings in terms of the predicted enhancement factor. This plasmon hybridization approach is promising for future applications, including enhanced multi-photon lithography and nonlinear sensing using metal nanoparticles. / Graduate

This Thesis is concerned with the prediction, synthesis, characterization and testing of inorganic materials for Second Harmonic Generation (SHG). Chapter One describes the fundamentals of non-linear optics, and poses the problems, and some of their solutions which confront the synthetic chemist and the theoretical prediction of the second order hyperpolarizability constant β using CNDOVSB calculations. Chapter Two describes the design, implementation and calibration of an apparatus for measurements of the second harmonic generating efficiency of solids based on the Kurtz powder technique, and a solvatochromic method for the determination of β. Novel compounds with potential chirality due to atropisomerism, asymmetric octahedral structures, and asymmetric tetrahedral symmetry of metal centers are discussed in Chapters Three to Five. Chapter Three surveys the use of pentane-2,4-dionato- ligands and their coordination compounds as possible NLO active materials. The single crystal X-ray structures of bis(triphenylphosphine)(4-nitrobenzoylacetonato)palladium(II) tetrafluoroborate and tris(triphenylphosphine)[3-(2,4-dinitrophenyl)-pentane-2,4-dionato]palladium(II) tetrafluoroborate were determined. Chapter Four describes the syntheses, characterization and SHG properties of trans-β-ionylidenecyanoacetic acid (2-cyano-3-methyl-5-(2,6,6-trimethyl-l-cyclohexen-1- yl)-2,4-pentadienoic acid) and some of its metal and non-metal salts. Chapter Five describes the synthesis, characterization and second harmonic generation properties of some platinum(II) and palladium(II) complexes of β- ionylidenecyanoacetic acid. Chapter Six describes the use of conventional asymmetric carbon centers to introduce chirality into centrosymmetric compounds. The chiral compound (L)-N-[2-cyano- 3-methyl-5-(2,6,6-trimethyl-1 -cyclohexene-1 -yl)-2,4-pentadiene-1 -one]-L-proline and some of its salts were synthesized from β-ionylidenecyanoaeetic acid and tested for SHG.