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.

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.

Thesis (Ph. D. in Physics)--Vanderbilt University, Aug. 2008. / Title from title screen. Includes bibliographical references.

Thesis (Ph. D.)--University of Hong Kong, 2000. / Includes bibliographical references (leaves 280-300).

Thesis (Ph. D. in Physics)--Vanderbilt University, Dec. 2006. / Title from title screen. Includes bibliographical references.

We present the results of a numerical simulation tool for modeling the second-harmonic generation (SHG) interaction experienced by a diffracting beam. This code is used to study the simultaneous frequency and spatial profile conversion of a truncated Bessel beam that closely resembles a higher-order mode (HOM) of an optical fiber. SHG with Bessel beams has been investigated in the past and was determined have limited value because it is less efficient than SHG with a Gaussian beam in the undepleted pump regime. This thesis considers, for the first time to the best of our knowledge, whether most of the power from a Bessel-like beam could be converted into a second-harmonic beam (full depletion), as is the case with a Gaussian beam. We study this problem because using HOMs for fiber lasers and amplifiers allows reduced optical intensities, which mitigates nonlinearities, and is one possible way to increase the available output powers of fiber laser systems. The chief disadvantage of using HOM fiber amplifiers is the spatial profile of the output, but this can be transformed as part of the SHG interaction, most notably to a quasi-Gaussian profile when the phase mismatch meets the noncollinear criteria. We predict, based on numerical simulation, that noncollinear SHG (NC-SHG) can simultaneously perform highly efficient (90%) wavelength conversion from 1064 nm to 532 nm, as well as concurrent mode transformation from a truncated Bessel beam to a Gaussian-like beam (94% overlap with a Gaussian) at modest input powers (250 W, peak power or continuous-wave operation). These simulated results reveal two attractive features – the feasibility of efficiently converting HOMs of fibers into Gaussian-like beams, and the ability to simultaneously perform frequency conversion. Combining the high powers that are possible with HOM fiber amplifiers with access to non-traditional wavelengths may offer significant advantages over the state of the art for many important applications, including underwater communications, laser guide stars, and theater projectors.

We study the behavior of electric fields in and around dielectric and metal nanoparticles, and prepare the ground for their applications to a variety of systems viz. photovoltaics, imaging and detection techniques, and molecular spectroscopy. We exploit the property of nanoparticles being able to focus the radiation field into small regions and study some of the interesting nonlinear, and quantum coherence and interference phenomena near them. The traditional approach to study the nonlinear light-matter interactions involves the use of the slowly varying amplitude approximation (SVAA) as it simplifies the theoretical analysis. However, SVVA cannot be used for systems which are of the order of the wavelength of the light. We use the exact solutions of the Maxwell's equations to obtain the fields created due to metal and dielectric nanoparticles, and study nonlinear and quantum optical phenomena near these nanoparticles. We begin with the theoretical description of the electromagnetic fields created due to the nonlinear wavemixing process, namely, second-order nonlinearity in an nonlinear sphere. The phase-matching condition has been revisited in such particles and we found that it is not satisfied in the sphere. We have suggested a way to obtain optimal conditions for any type and size of material medium. We have also studied the modifications of the electromagnetic fields in a collection of nanoparticles due to strong near field nonlinear interactions using the generalized Mie theory for the case of many particles applicable in photovoltaics (PV). We also consider quantum coherence phenomena such as modification of dark states, stimulated Raman adiabatic passage (STIRAP), optical pumping in $4$-level atoms near nanoparticles by using rotating wave approximation to describe the Hamiltonian of the atomic system. We also considered the behavior of atomic and the averaged atomic polarization in $7$-level atoms near nanoparticles. This could be used as a prototype to study any $n-$level atomic system experimentally in the presence of ensembles of quantum emitters. In the last chapter, we suggested a variant of a pulse-shaping technique applicable in stimulated Raman spectroscopy (SRS) for detection of atoms and molecules in multiscattering media. We used discrete-dipole approximation to obtain the fields created by the nanoparticles.

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