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Thesis (Ph.D.)--University of Nebraska-Lincoln, 2009. / Title from title screen (site viewed January 5, 2010). PDF text: xiii, 116 p. : ill. (chiefly col.) ; 5 Mb. UMI publication number: AAT 3360164. Includes bibliographical references. Also available in microfilm and microfiche formats.

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This thesis describes the design and construction of a portable laser induced fluorescence sensor. The objective was to create a low-cost, versatile, and modular laser induced fluorescence sensor for agricultural remote sensing. The sensor module should able to be integrated with different pieces of hardware. The objective was successfully accomplished with the creation of a sensor module that met all of the requirements. The sensor module integrates with a handheld unit for reading and visualizing the data that was constructed and is described in this work. Performance testing and experiments were carried out with the sensor module in the handheld device with a focus on plant physiology. In particular, chlorophyll fluorescence related to stress and ripeness was studied and fungal toxins found in corn were detected with this device. Ongoing work consisting of mounting the sensor module to an unmanned aerial vehicle and testing in flight is described.

Laser Induced Breakdown Spectroscopy LIBS is a fast non-contact technique for the analysis of the elemental composition using spectral information of the emission from a laser-induced plasma. For the LIBS studies in this thesis the focus has been in using very low energy, microjoule pulses in order to give high spatial resolution and minimize the laser system requirements. This is a regime that we refer to as microLIBS. Under such conditions it is important to maximize the signal detected to give the lowest limit of detection LOD possible. One technique to improve the signal to noise ratios is by coupling LIBS with Laser Induced Fluorescence. This is a technique where the _rst pulse creates a vapor plume and the second pulse tuned to a resonant absorption line of the species of interest re-excites the plume. We term this technique as Laser ablation Laser Induced Fluorescence LA-LIF. We have been investigating the performance of LA-LIF at low pulse energies (_ 1 mJ for both pulses) for the detection of elemental contaminants in water. This technique allows reasonable performance compared to high energy singlepulse LIBS, but at a much reduced total energy expenditure. This allows LODs in the parts per billion range ppb range which typically cannot be obtained with low energy single pulse probing of the systems. This approach or exceeds the sensitivities which can be obtained with many shots using much larger energy systems. In this thesis we investigated the performance of LIBS at low pulse energies for the detection of Pb as a contaminant in water. An LOD of 70 ppb was obtained for an accumulation of 100 shots with the ablation laser pulse energy of 250 _J and an excitation laser pulse energy of 8 _J. A systematic study of the detector conditions was made for the system for the detection of Pb. Scaling laws for the LOD in terms of the pump and probe energies were measured and also the e_ect of detector gain, the gate delay and the gate width were studied. In this thesis LIBS and LA-LIF were also used to analyze ultralow volumes of analyte in liquids in microuidic geometries. LIBS was applied for the detection of Na in liquid droplets in a microuidic system. The detection of Na as low as 360 femtograms was demonstrated for 100 shots integrated in this system. An LOD of 7 ppm for Pb for 100 shot accumulation was demonstrated using the LA-LIF technique on an 18 _m diameter microdroplet. To study the laser interaction with the water targets the MEDUSA one dimensional hydrocode was used. The propagation of the shockwave and plume dynamics were studied using this modeling code. The expansion of the plume was studied and compared to experimentally measured values and to physical models for blast wave expansion and stagnation. Two preconcentration techniques were also studied, one of which used a wood-chip as a substrate to absorb the analyte liquid and wick the salt on to the surface for analysis and the other used an electroplating technique to plate the analyte metal as a thin _lm on a substrate metal used as a cathode. The electroplating method for preconcentration was also studied using a microchip laser and a LOD of 6.4 ppb for Pb in water was obtained for an accumalation of 200,000 shots. / Photonics and Plasmas

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The aim of this thesis was to discover the mechanism for non-photochemical laser-induced nucleation (NPLIN), which is a technique for inducing nucleation of crystals with laser light without absorption. The mechanism of the optical Kerr effect (OKE) was suggested by Garetz et al. [Physical Review Letters 77, 3475–3476 (1996)] to give an explanation for NPLIN. Since the feasibility of the OKE mechanism for NPLIN has been questioned, a series of experiments on NPLIN of aqueous supersaturated urea were carried out to quantify the relationship between crystal alignment and laser polarization. Digital imaging of crystal growth during laser irradiation showed that nascent needle-shaped crystals of urea were not aligned with the direction of the electric field of the laser. Additionally, work on glycine was aimed at verifying the possibility to control the polymorph of the obtained crystal via the laser polarization. However, our finding shows that the probability of γ-glycine is more likely to increase with increasing supersaturation; and the influence of laser polarization on the resulting morphologies is not strong as reported by Sun et al. [Crystal Growth & Design 6, 684–689 (2006)]. Furthermore, in another work on NPLIN of L-histidine, based on Sun et al. [Crystal Growth & Design 8, 1720–1722 (2008)], we were unable to reproduce the results as stated in Sun’s published paper. We find their results exhibit a large uncertainty when recalculated through the Wilson score interval for binomial distributions. On account of these revised uncertainties, it is unlikely that laser polarization gives polymorphism control. Comparison with the nucleation probability of unfiltered samples in the work of urea and glycine shows that the number of filtered samples nucleated as a result of NPLIN was largely decreased. Moreover, experiments on NPLIN of NaCl and KCl also exhibited that the number of filtered samples nucleated was significantly lower than that of unfiltered samples. This downward tendency in nucleation probability after filtration cannot be explained by Garetz’s OKE mechanism. On account of this, an alternative mechanism named particle-heating mechanism was proposed, and supported by experiments on NPLIN of sodium acetate. Sodium acetate experiments showed that the crystallization of sodium acetate can be induced by a single pulse of a nanosecond laser (1064 nm) with minimum power of 0.1 J cm−2. As discovered by Oliver et al. [D. Oliver, PhD Thesis, Edinburgh University, 2014], anhydrous or trihydrate sodium acetate can be formed under the effects of different organic and inorganic additives, such as poly- (methacrylic acid) and disodium hydrogen phosphate. We demonstrate that crystalline growth velocities and crystal morphology can be influenced by these additives. We find that high levels of additive cause only nucleation of bubbles. By counting the number of crystals, which is approximately consistent to the number of bubbles observed, video microscopy of laser-induced crystallization of sodium acetate has revealed that the general mechanism of NPLIN is most likely to be caused by a particle-heating mechanism. Chapter 8 of the thesis describes a number of solute molecules that were tested using NPLIN, but failed. In terms of improvements for future work or a perspective on NPLIN, detailed suggestions have been described in Chapter 9, which also gives a summary of all work presented.

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The objectives of this study were to implement a system to measure mixing in nonreacting flows and to study the mass transfer characteristics of two actively excited turbulent jets. This thesis describes the acquisition and analysis of phase-locked concentration field data using planar Mie scattering from smoke particles and planar laser-induced fluorescence of acetone. Both techniques were shown to be effective in providing information for the actively excited nozzles. However, the laser-induced fluorescence technique was superior for revealing detail in the flowfield structure. Spatial mode control techniques were applied to a triangular nozzle with vibrating actuators as the three sides and a swirl nozzle with pulsating tangential air jets. The effect of the different spatial modes on jet column development and the far fields of both nozzles is presented. Two- and three-dimensional iso-intensity contours, showing the relative intensity of light scattered by the nozzle fluid marker, were generated to show the flow structure. The areas inside the iso-intensity contours in the far field were also measured to determine relative effectiveness of nozzle fluid transport. Large scale structures were visible in the three-dimensional iso-intensity contours from both nozzles. In addition, the transport of seeded nozzle fluid was enhanced by the spatial mode excitation for both nozzles. Spatial mode excitation was also able to affect the shape of the far field contour. In particular, the first counterrotating helical mode, m=±1, generated the greatest effect on nozzle fluid transport and the most pronounced elliptical contour shape in the far field. / Master of Science