Surface properties of polymers and competition adsorption of toluene and heptane on silica were studied using IR-visible sum frequency generation (SFG) vibrational spectroscopy. SFG is intrinsically surface sensitive because the second-order optical process is forbidden in media with inversion symmetry, such as bulk polymers and liquids. This nonlinear optical technique provides surface vibrational spectra under ambient conditions without the need of an ultra-high vacuum environment. Polymer surface properties, including surface relaxation temperature of poly(methyl methacrylate) (PMMA) and surface electronic states of poly[2-methoxy, 5-ethyl (2’-hexyloxy) para phenylenevinylene] (MEH-PPV), were investigated. It was found that there are significant differences between the surface and bulk properties for these polymers. For PMMA, a new surface structure relaxation was identified at 67°C, which does not match any known structure relaxation temperatures for bulk PMMA and is 40°C below the bulk glass transition temperature. For MEH-PPV, SFG electronic spectra, which were obtained by scanning the frequencies of incident visible and JR beams, indicated that the electronic states at the polymer/solid and air/polymer interfaces are red-shifted with respect to that of the bulk. Finally, SFG was employed to study the competition adsorption of toluene and heptane on silica surfaces. Experimental data showed that heptane adsorbed favorably compared to toluene. Using a Langmuir adsorption isotherm, the changes of Gibbs free energy for the adsorption processes were calculated to be —12.1 ± 1.8 (kJ/mol) for toluene and —16.5 ± 2.3 (kJ/mol) for heptane.

Surface properties of polymers and competition adsorption of toluene and heptane on silica were studied using IR-visible sum frequency generation (SFG) vibrational spectroscopy. SFG is intrinsically surface sensitive because the second-order optical process is forbidden in media with inversion symmetry, such as bulk polymers and liquids. This nonlinear optical technique provides surface vibrational spectra under ambient conditions without the need of an ultra-high vacuum environment. Polymer surface properties, including surface relaxation temperature of poly(methyl methacrylate) (PMMA) and surface electronic states of poly[2-methoxy, 5-ethyl (2’-hexyloxy) para phenylenevinylene] (MEH-PPV), were investigated. It was found that there are significant differences between the surface and bulk properties for these polymers. For PMMA, a new surface structure relaxation was identified at 67°C, which does not match any known structure relaxation temperatures for bulk PMMA and is 40°C below the bulk glass transition temperature. For MEH-PPV, SFG electronic spectra, which were obtained by scanning the frequencies of incident visible and JR beams, indicated that the electronic states at the polymer/solid and air/polymer interfaces are red-shifted with respect to that of the bulk. Finally, SFG was employed to study the competition adsorption of toluene and heptane on silica surfaces. Experimental data showed that heptane adsorbed favorably compared to toluene. Using a Langmuir adsorption isotherm, the changes of Gibbs free energy for the adsorption processes were calculated to be —12.1 ± 1.8 (kJ/mol) for toluene and —16.5 ± 2.3 (kJ/mol) for heptane.

Surface properties of polymers and competition adsorption of toluene and heptane on silica were studied using IR-visible sum frequency generation (SFG) vibrational spectroscopy. SFG is intrinsically surface sensitive because the second-order optical process is forbidden in media with inversion symmetry, such as bulk polymers and liquids. This nonlinear optical technique provides surface vibrational spectra under ambient conditions without the need of an ultra-high vacuum environment. Polymer surface properties, including surface relaxation temperature of poly(methyl methacrylate) (PMMA) and surface electronic states of poly[2-methoxy, 5-ethyl (2’-hexyloxy) para phenylenevinylene] (MEH-PPV), were investigated. It was found that there are significant differences between the surface and bulk properties for these polymers. For PMMA, a new surface structure relaxation was identified at 67°C, which does not match any known structure relaxation temperatures for bulk PMMA and is 40°C below the bulk glass transition temperature. For MEH-PPV, SFG electronic spectra, which were obtained by scanning the frequencies of incident visible and JR beams, indicated that the electronic states at the polymer/solid and air/polymer interfaces are red-shifted with respect to that of the bulk. Finally, SFG was employed to study the competition adsorption of toluene and heptane on silica surfaces. Experimental data showed that heptane adsorbed favorably compared to toluene. Using a Langmuir adsorption isotherm, the changes of Gibbs free energy for the adsorption processes were calculated to be —12.1 ± 1.8 (kJ/mol) for toluene and —16.5 ± 2.3 (kJ/mol) for heptane. / Science, Faculty of / Chemistry, Department of / Graduate

Optically pumped semiconductor vertical external cavity surface emitting lasers (VECSEL) were first demonstrated in the mid 1990's. Due to the unique design properties of extended cavity lasers VECSELs have been able to provide tunable, high-output powers while maintaining excellent beam quality. These features offer a wide range of possible applications in areas such as medicine, spectroscopy, defense, imaging, communications and entertainment. Nowadays, newly developed VECSELs, cover the spectral regions from red (600 nm) to around 5 µm. By taking the advantage of the open cavity design, the emission can be further expanded to UV or THz regions by the means of intracavity nonlinear frequency generation. The objective of this dissertation is to investigate and extend the capabilities of high-power VECSELs by utilizing novel nonlinear conversion techniques. Optically pumped VECSELs based on GaAs semiconductor heterostructures have been demonstrated to provide exceptionally high output powers covering the 900 to 1200 nm spectral region with diffraction limited beam quality. The free space cavity design allows for access to the high intracavity circulating powers where high efficiency nonlinear frequency conversions and wavelength tuning can be obtained. As an introduction, this dissertation consists of a brief history of the development of VECSELs as well as wafer design, chip fabrication and resonator cavity design for optimal frequency conversion. Specifically, the different types of laser cavities such as: linear cavity, V-shaped cavity and patented T-shaped cavity are described, since their optimization is crucial for transverse mode quality, stability, tunability and efficient frequency conversion. All types of nonlinear conversions such as second harmonic, sum frequency and difference frequency generation are discussed in extensive detail. The theoretical simulation and the development of the high-power, tunable blue and green VECSEL by the means of type I second harmonic generation in a V- cavity is presented. Tens of watts of output power for both blue and green wavelengths prove the viability for VECSELs to replace the other types of lasers currently used for applications in laser light shows, for Ti:Sapphire pumping, and for medical applications such as laser skin resurfacing. The novel, recently patented, two-chip T-cavity configuration allowing for spatial overlap of two, separate VECSEL cavities is described in detail. This type of setup is further used to demonstrate type II sum frequency generation to green with multi-watt output, and the full potential of the T-cavity is utilized by achieving type II difference frequency generation to the mid-IR spectral region. The tunable output around 5.4 µm with over 10 mW power is showcased. In the same manner the first attempts to generate THz radiation are discussed. Finally, a slightly modified T-cavity VECSEL is used to reach the UV spectral regions thanks to type I fourth harmonic generation. Over 100 mW at around 265 nm is obtained in a setup which utilizes no stabilization techniques. The dissertation demonstrates the flexibility of the VECSEL in achieving broad spectral coverage and thus its potential for a wide range of applications.

A coherent optical THz (1.5 THz, 200 µm) source was developed based on pulsed, near IR, fiber lasers, and frequency mixing in nonlinear crystals. The generated THz frequency is determined by the difference frequency of two high peak power pulsed fiber lasers at 1550 nm and 1538 nm. When incident to the crystal, the near IR lasers induce a polarization at their beat frequency which generates the THz radiation. The pulsed fiber lasers are single transverse mode, have high pulse energy and peak powers of 0.38 mJ and 128 kW respectively. They are transform limited at a few ns in duration with very good beam quality of M² ≈ 1.2. The pulse seed was created by modulating a constant laser beam with an electro-optic modulator. An arbitrary waveform generator was used to pre-shape these pulses to compensate for pulse distortion caused by pump gain depletion in the subsequent fiber amplifiers. Pre-amplifiers were constructed using commercial erbium doped silica fiber. Special, highly doped, large core, phosphate fiber was developed in-house to further amplify the pulses, while avoiding nonlinear scattering processes such as stimulated Brillouin scattering and stimulated Raman scattering. THz generation was achieved in both ZnGeP₂ and GaP which were chosen based on their low pump and THz absorption, as well as high nonlinear coefficient. Angle tuning was used to phase match all three optical frequencies in ZnGeP₂ thanks to its birefringence. Layers of GaP ~500 µm thick were pressed together alternately rotated 180° around the normal to quasi-phase match the pump and THz frequencies. To increase the efficiency of the THz generation an external optical cavity was used to enhance and recycle the IR pump pulses. The nonlinear crystal was placed inside the cavity and 151 times enhancement of THz power was observed.

This thesis describes the development of a new type of sensor for nitric oxide (NO) that can be used in a variety of combustion diagnostics and control applications. The sensor utilizes the absorption of ultraviolet (UV) radiation by the NO molecule to determine the concentration via optical absorption spectroscopy. UV radiation at 226.8 nm is generated by sum frequency mixing the outputs from a 395-nm external cavity diode laser (ECDL) and a 532-nm diode-pumped, intracavity frequency doubled Nd:YAG laser in a beta-barium borate (BBO) crystal. This radiation is used to probe the (v'=0, v"=0) band of the Α*Σ+ - Χ*π electronic transition of NO. The ECDL is tuned so that the UV radiation is in resonance with a specific energy level transition, and it is then scanned across the transition to produce a fully resolved absorption spectrum. Preliminary experiments were performed in a room-temperature gas cell in the laboratory to determine the accuracy of the sensor. Results from these experiments indicated excellent agreement between theoretical and experimental absorption line shapes as well as NO concentrations. Further experiments were performed at two actual combustion facilities to demonstrate the operation of the sensors in realistic combustion environments. Tests on a gas turbine auxiliary power unit (APU) at Honeywell Engines and Systems and on a well-stirred reactor (WSR) at Wright-Patterson Air Force Base produced excellent results despite the harsh temperatures and vibrations present. Overall, the sensitivity was estimated to be 0.8 parts per million (ppm) of NO (at 1000 K) for a 1 meter path length and the measurement uncertainty was estimated to be ±10%.

This thesis describes the development of a new type of sensor for nitric oxide (NO) that can be used in a variety of combustion diagnostics and control applications. The sensor utilizes the absorption of ultraviolet (UV) radiation by the NO molecule to determine the concentration via optical absorption spectroscopy. UV radiation at 226.8 nm is generated by sum frequency mixing the outputs from a 395-nm external cavity diode laser (ECDL) and a 532-nm diode-pumped, intracavity frequency doubled Nd:YAG laser in a beta-barium borate (BBO) crystal. This radiation is used to probe the (v'=0, v"=0) band of the Α*Σ+ - Χ*π electronic transition of NO. The ECDL is tuned so that the UV radiation is in resonance with a specific energy level transition, and it is then scanned across the transition to produce a fully resolved absorption spectrum. Preliminary experiments were performed in a room-temperature gas cell in the laboratory to determine the accuracy of the sensor. Results from these experiments indicated excellent agreement between theoretical and experimental absorption line shapes as well as NO concentrations. Further experiments were performed at two actual combustion facilities to demonstrate the operation of the sensors in realistic combustion environments. Tests on a gas turbine auxiliary power unit (APU) at Honeywell Engines and Systems and on a well-stirred reactor (WSR) at Wright-Patterson Air Force Base produced excellent results despite the harsh temperatures and vibrations present. Overall, the sensitivity was estimated to be 0.8 parts per million (ppm) of NO (at 1000 K) for a 1 meter path length and the measurement uncertainty was estimated to be ±10%.

Thorough understanding of interfaces requires an assessment of both the surface and bulk properties through the use of multiple techniques. In this thesis, infrared absorption, Raman scattering and sum frequency generation were used as vibrational probes of different features of interfacial systems including the ability to measure surface and bulk effects. Two-dimension correlation analysis was used to study the relationship between the spectral response of the different techniques. Attenuated total reflection absorption, bulk Raman scattering and sum frequency generation were used to study the adsorption of ethanol--water mixture on fused silica. With the use of two-dimension correlation analysis, interesting results were observed concerning the behavior of the surface in respect to the bulk. Surface concentration of ethanol were concluded to be higher than in the bulk indicative of competitive adsorption. Furthermore, at low concentration ethanol was shown to adsorb to the surface in dimers, to then form a bilayer of strongly oriented ethanol molecules at higher concentration. At highest concentration, this bilayer is disturbed, leaving only one layer at the surface of oriented ethanol molecules. The same spectroscopic techniques were applied to pressure sensitive adhesives of different composition while drying on a sapphire surface. The presence or absence of acrylic acid in the material was shown to alter the reorientation at the surface while drying. In the case where no acrylic acid is present, the orientation of the polymer at the surface was driven by the packing of the molecules at the surface. When acrylic acid was present in the pressure sensitive adhesive, reorientation occurred much faster and was caused by strong hydrogen bonding with the surface of the sapphire. An increase in acrylic acid composition, increased the rate of reorientation. An experimental set up was constructed to specifically study interfaces with a nonuniform distribution within the plane of the surface. This allows for concomitant measurement of polarized total internal reflection Raman scattering and sum frequency generation spectroscopy along with bright field imaging and cross polarized imaging. This set up was used to study the L-histidine crystal in situ adsorbed on fused silica. The polarized experiments along with calculations allowed for a more in-depth analysis of the crystal orientation effect on the birefringence, the Raman and the sum frequency generation. / Graduate

L'interface (0001) alpha-quartz hydroxylé/eau liquide est modélisée par dynamique moléculaire dans le formalisme de la théorie de la fonctionnelle de la densité (DFT-MD). Poursuivant une étude déjà publiée sur la structure et la réactivité acido-basique de cette interface, nous élargissons l'analyse aux effets d'ions simples et de pH sur la structure et les propriétés de l'interface avec une attention particulière aux phénomènes d'adsorption ainsi qu'aux modifications de l'eau interfaciale et des sites de surface. Nous caractérisons en particulier les changements dans la réactivité de surface dus à la présence d'ions (cations alcalins et halogénures) par calcul direct de pKa et détaillons la structure de la double couche électrique dans le cas de paires d'ions.Nous cherchons de plus à calculer les spectres SFG (Sum Frequency Generation) vibrationnels à ces interfaces, ce qui serait une première pour une interface solide/liquide traitée au niveau DFT-MD. L'approche théorique de cette spectroscopie vibrationnelle non linéaire du second ordre permettrait de proposer une interprétation claire des bandes d'élongation O-H, alors que les études expérimentales continuent à diverger sur ce sujet. / We use Density-Functional-Theory-based molecular dynamics simulations to investigate the hydroxylated (0001) alpha-quartz/liquid water interface. As a follow up of an already published study on the structural and acid/base properties of this interface, we now focus on simple ion and pH effects on these properties and characterize adsorption behaviors and interfacial changes on both solid and liquid sides. In particular, we directly calculate surface pKa's in presence of ions (alkaline cations and/or halide anions) and provide microscopic details on the structure of the electric double layer when ion pairs are concerned.Besides, we try to apply DFT-MD simulations to the computation of vibrational Sum-Frequency Generation spectra at a solid/liquid interface. Indeed, calculations would be necessary to provide a clear interpretation of the vibrational bands in the OH stretching region since experimental band assignment is still a matter of debate.

The molecular structure and charge on solid surfaces in aqueous environments is of fundamental importance to various scientific research and applications, yet remain not sufficiently understood. The research herein uses sum frequency generation spectroscopy to reveal the molecular structure of the mineral and polymer surfaces, and also probes the water molecules near the charged aqueous interfaces to get information about the surface charge. The application of visible-infrared sum-frequency generation spectroscopy to polymer thin-films requires a careful interpretation of the results, as the electric field magnitude and phase at each interface must be determined in a manner that takes thin film interference effects into account. A straightforward method that has a concise analytic solution in the case of a single thin film that exhibits interference effects was proposed. This method enabled selective probing of transparent thin-films using sum frequency generation spectroscopy, hence eliminated the ambiguity of the contribution of signal from two interfaces. The method was then extended to multiple polarization schemes, enabling easier and more comprehensive study of the molecular orientation on thin-films. Nonlinear vibrational spectroscopy has also been used to study the temperature-dependent surface structure of polydimethylsiloxane when exposed to water and a perfluorinated hydrophobic liquid. Quantitative analysis of the methyl plane orientation was performed using a combination of vibrational peak ratios and peak amplitudes that enable proposed structures to be identified. For both environments, the tilt and twist of the methyl plane was found to increase with temperature in a reversible manner. This has been attributed to be a consequence of the backbone reorganization due to temperature-dependent density changes. At charged aqueous interfaces, the structure of water adjacent to solid interface is sensitive to the surface potential. As a result, close inspection of signals originating from these water molecules can be used to reveal the surface charge density. Nonlinear vibrational spectroscopy was used to monitor the water O-H stretching band over a temperature range of 10-75°C to account for the increase in surface potential from deprotonation. It has been demonstrated that the behavior at the silica surface is a balance between increasing surface charge, and a decreasing contribution of water molecules aligned by the surface charge. Together with a model that accounts for two different types of silanol sites, the change in enthalpy and entropy for deprotonation at each site were reported. The surface charge density of untreated polydimethylsiloxane surface in water with various ionic strengths was also determined. It was found that the surface charge could be explained with an ion adsorption model. A relationship between the surface potential and measured nonlinear optics response that is valid at high potentials and low ionic strength was proposed. Finally, a universal method was demonstrated to derive the surface potential with nonlinear optics by modulating the coherence length. / Graduate / 2022-09-07