Nonperturbative rates of harmonic generation or frequency mixing and rates of multiphoton ionisation were obtained for atomic hydrogen and for a one-electron model of the negative hydrogen ion, using the Floquet-Sturmian method. The following cases were investigated: elliptically polarised, monochromatic incident field, two-colour incident field with incommensurable frequencies, coherent super position of an incident field and its third harmonic, and superposition of a laser field and a static electric field. For elliptical polarisation, the ellipticity angle of the harmonics differ from the ellipticity of the incident laser and we established that there is an offset angle between the major axis of the incident laser's polarisation ellipse and that of the harmonics generated. The variation is greatest in the plateau region. Resonance-enhanced ionisation was studied when a high frequency field was applied in addition to a fundamental field. In contrast to these changes in the ionisation rate only small variations have been obtained in the harmonic generation rate. Results for frequency-mixing exhibit a difference in the strength of the harmonics, although the behaviour is the same in terms of the length of the plateau and the cut-off order. Also we found a marked change between sum-frequency and difference frequency processes, which depends on the harmonic order. In addition to a strong production of even harmonics, it was found that the variation of the rate of ionisation and harmonic generation in the presence of a static field oscillates as a function of the strength of the latter. In the case of hydrogen, we observed dc-stark shift induced resonance enhancements. The results obtained for H(^-) are in very good agreement with those obtained for two-colour (w, 3w) mixing. We concluded that the application of a static field in general leads to a reduction in the conversion efficiency.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

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Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 178-187). Available also in a digital version from Dissertation Abstracts.

The ability of asphalt binders (bitumen) in road surfaces to self-heal after cracking is important to developing a robust transportation system that can tolerate heavy traffic and varying weather conditions. In order to develop improved binders, there is a need for noninvasive, in-situ, interface-specific methods of monitoring the kinetics, physics and chemistry of self-healing bitumen interfaces. Here the feasibility of using optical second-harmonic generation (SHG) by focused femtosecond laser pulses to monitor bitumen surfaces is demonstrated. SHG signals are observed in transmission through a sample composed of bitumen spin-coated onto a borosilicate microscope coverslip. The SHG signals are absent from uncoated coverslips, demonstrating that they originate from the bitumen layer. Further tests demonstrate that the bitumen-air surface makes the dominant contribution to the SHG signal. The SHG signal is observed to decay on a time scale comparable to typical self-healing times because of sample heating by the incident laser irradiation. Methods to control this effect by translating the sample during data acquisition are developed. Although the present results were obtained with a single incident wavelength (800 nm), they demonstrate the feasibility of probing bitumen interfaces spectroscopically with tunable light sources in order to monitor bond-specific chemical kinetics. / text

Thesis (Ph. D.)--University of Texas at Austin, 2005. / Vita. Includes bibliographical references.

We investigate ionization and high harmonic generation (HHG) from the interaction of a mid infra-red laser pulse with a solid state system confined to nano-dimensions. The theory of strong field processes in solids is developed for confined quantum systems in general. Here it is applied to two-dimensional quantum wires with a driving field linearly polarised along the axis of the wires. Our findings indicate that that we are able to control the ionization and high-harmonic output by altering the width of the wire. Control of ionization leads to an increased damage threshold which has important implications for nano-engineering and realizing all solid state coherent XUV sources.

In this study we demonstrate the use of third harmonic (TH) and second harmonic (SH) generation in imaging dental sections. Teeth are the hardest and most indestructible part in human body. The TH and SH greatly facilitate observation of porous structures and collagen within the dental sections, respectively. Strong SH has been found on various biological specimens, such as collagen, potato starch, and skeletal muscles. These materials all possess periodical nano-structures that are often referred as (nonlinear) bio-photonic structures. In particular, collagen is an extra-cellular structural protein and is a major component of bone, cartilage, skin, and other tissues. Collagen fibrils have a triple-helical structure and it is believed that this structure enables collagen to generate SH signal from a wide range of wavelengths in the infrared region. For comparison, microtubule structures within dentin, due to its large index mismatch with surrounding, can be clearly seen with THG imaging. The THG also facilitate observation of prismatic structures in enamel. The successful construction of a multi-photon laser scanning microscope that can operate in both reflection and transmission modes is the key for this study. A femtosecond, sync-pumped optical parametric oscillator (OPO) is used to generate second and third harmonics from dental sections. Dental sections have large index of refraction¡]n~1.68¡^and scatter visible light severely. The employment of excitation wavelength at 1260 nm greatly reduces scattering and absorption within the sample. Its corresponding SH and TH wavelengths are at 630 nm and 420 nm, respectively. Additionally, 3-D structural views are also reconstructed from the optically sectioned images by the use of specialized 3D image processing software.