Browsing by Author "Sauerbrey, Roland A."
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Item Compression of femtosecond krypton fluoride excimer laser pulses(1993) Cote, Frederic; Sauerbrey, Roland A.Temporal characteristics of compressed femtosecond KrF pulses are measured with the help of a single-shot phase-sensitive autocorrelator for the UV. The performance of a prism compressor and the influence of higher order phase dispersion on the ultimate pulse width of compressed pulses is investigated. Self-phase modulation of high intensity femtosecond KrF pulses in MgF$\sb2$ is reported. The nonlinear refractive index n$\sb2$ (7.4 $\times$ 10$\sp{-16}$ $\pm$ 1.5 $\times$ 10$\sp{-16}$ cm$\sp2$/W) of MgF$\sb2$ is derived from the recorded broadened spectra. Implications of self-phase modulation in solids for compression of femtosecond KrF pulses are discussed.Item Excimer laser induced electrical conductivity and nanostructures in polymers(1993) Phillips, Harvey Monroe; Sauerbrey, Roland A.The generation of substantial electrical conductivity in high temperature polymers and thin film C$\sb{60}$ by means of KrF (248 nm) excimer laser irradiation has been investigated. Formation of both laser ablated surface structures and laser induced electrically conducting wires in polymers with nanometer dimensions is also demonstrated. The electrical conductivity of polymers has been changed by up to 18 orders of magnitude by laser irradiation, obtaining values exceeding 10 $\Omega\sp{-1}$ cm$\sp{-1}$. The conducting material consists of graphitized carbon clusters whose size varies from 2-50 nm. The conduction mechanism is phonon assisted variable range hopping. The large change in the electrical conductivity is an example of a three dimensional percolative metal-insulator phase transition. The critical volume fraction is determined to be $\Phi\sb{\rm c}$ = 0.30 $\pm$ 0.05 and the critical exponent is t = 2.2 $\pm$ 0.4. The electrical conductivity of thin film C$\sb{60}$ has been altered by more than seven orders of magnitude with laser irradiation, obtaining values of 1 $\Omega\sp{-1}$ cm$\sp{-1}$. The threshold for KrF laser ablation of C$\sb{60}$ is determined to be 20 $\pm$ 2 mJ/cm$\sp2.$ Modification of the surface morphology and the electrical conductivity of polymers with high spatial resolution using excimer lasers has also been achieved. Using holographic techniques with a KrF excimer laser, periodic lines structures with periods ranging from 166 nm to 950 nm were ablated into polyimide (Kapton$\sp{\rm TM})$ and polybenzimidazole (PBI). The nonlinear nature of laser ablation permits linewidths as small as 30 nm to be obtained, exceeding the resolution expected from linear optics. These experiments establish a new spatial resolution limit for laser ablation and illustrate the dependence of resolution on material properties. This technique was combined with the ability to modify the electrical conductivity of polymers to produce an array of permanently electrically conducting wires in polyimide with a 0.5 $\mu$m width and a 0.9 $\mu$m period. The electrical conductivity of these submicron wires exceeded 1 $\Omega\sp{-1}$ cm$\sp{-1}.$Item Excimer laser induced electrical conductivity in polyimide: Applications to engineering, physics, and applied science(1996) Ball, Zane Andrew; Sauerbrey, Roland A.Various aspects of the interaction of excimer laser pulses and polyimide were investigated, focusing on low fluences where laser induced electrical conductivity is observed. Multiple shot excimer ablation of polyimide was investigated, and at fluences between 20 mJ/cm$\sp2$ and 260 mJ/cm$\sp2,$ ablation would cease after a few hundred pulses. This cessation of ablation was found to be the result of a dramatic, fluence dependent increase in surface roughness accompanied by the formation of a conducting, carbonaceous layer on top the laser treated sample. The repetition rate of the laser could affect the electrical conductivity of the layer but had little effect on any other properties, implying that the electronic structure of laser induced conducting layer was highly temperature sensitive. The optical properties of the laser irradiated surface were also studied with regard to a transient decrease in reflectivity that is observed during and in the first few tens of nanoseconds after irradiation. Pump probe experiments using electronically delayed probe pulses were performed and measurements of reflectivity, transmissivity and scattering were made under identical conditions. The results, a decrease in reflectivity and scattering with a simultaneous increase in transmissivity, imply a transient refractive index phenomenon during the first 150 ns after excimer irradiation. A nonlinear current-voltage relation was observed near the critical point of the laser induced metal-insulator transition. The crossover behavior from nonlinear to linear regimes was studied in detail and interpreted in terms of a percolation theory of binary conductor-insulator mixtures. The nonlinearity and the crossover can, for the first time, be described simultaneously in terms of a new theory of space charge limited currents on percolation clusters.Item Generation and characterization of femtosecond vacuum ultraviolet pulses(1994) Qi, Zhangfen; Sauerbrey, Roland A.High power femtosecond VUV/XUV pulses covering new spectral regions have been generated through nonlinear interaction of a femtosecond KrF laser with xenon and argon. Conversion efficiencies as high as 10$\sp{-3}$ have been observed. A six-wave mixing process and several four-wave mixing processes under intense light field have been experimentally studied. The temporal duration of the femtosecond VUV pulse has been experimentally measured for the first time based on the defocusing caused by a femtosecond laser produced plasma.Item Percolative metal-insulator transition in UV excimer laser irradiated polyimide(1994) Ball, Zane Andrew; Sauerbrey, Roland A.The metal-insulator transition observed in krypton-fluoride laser irradiated polyimide is used as a method to study the three dimensional percolation theory of random resistor networks. Values for the percolation threshold and conductivity critical exponent were measured and found to be 0.31 $\pm$ 0.06 for the percolation threshold of a three dimensional continuum of interpenetrating spheres and 2.05 $\pm$ 0.05 for the critical exponent. Both values are in excellent agreement with theoretical predictions. The metal-insulator transition was observed to have a nonlinear current voltage characteristic near the percolation threshold when a small applied voltage was used for measurement. Under these conditions the conductivity critical exponent was measured and found to be 2.7 $\pm$ 0.4. The nonlinear case corresponds to theories for nonlinear random resistor networks and is the first experimental demonstration of the effect of nonlinear conduction on the critical exponent. The value is in agreement with theoretical calculations.Item Phase modulation by high density laser-produced plasmas(1994) Fure, Jan; Sauerbrey, Roland A.A model for simulation of laser produced plasmas that couples absorbed energy, plasma dynamics and radiation pressure was developed. The unusual experimental result of spectral narrowing of reflected laser pulses was attributed to deceleration of the critical (reflective) electron density surface due to the radiation pressure. A mechanism different from the Doppler effect that can cause spectral shifts was identified; a plasma reflectivity that changes rapidly in time can act as a frequency filter when the reflected laser pulse is chirped. The plasma expansion length and temperature that were calculated with the model was used to estimate the X-ray emission from the plasma with a simple scheme involving Kirchhoffs radiation law and Planck's black body radiation law.Item Plasma-induced self-phase and cross-phase modulation of femtosecond laser pulses(1994) Le Blanc, Stephen Paul; Sauerbrey, Roland A.The spectral, temporal, and spatial characteristics of plasma-induced self-phase and cross-phase modulation in rare gases have been investigated using a femtosecond KrF excimer laser focused to peak intensities of 10$\sp{14}$-10$\sp{15}$ W cm$\sp{-2}.$ The quiver energy of a free electron under these conditions is less than the ionization potential of all rare gases, ensuring that ionization occurs only by optical field-induced processes. Spectral blueshifts of up to 2 nm have been observed, and the blueshifted spectra show an oscillatory structure. The blueshifted spectra are shown to be the result of plasma-induced self-phase modulation and can be modeled by assuming tunneling ionization and one dimensional pulse propagation. The newly discovered oscillatory structure in the spectra is related to that observed in earlier experiments on self-phase modulation in optical fibers. To investigate the temporal behavior of the field ionization process, pump-probe experiments have been performed with a 100 fs probe pulse at 497 nm and a 400 fs pump pulse at 248 nm. Under conditions of weak ionization (Z $\ll$ 1), pump-probe experiments and theoretical calculations show that the ionization rate of the field ionized gas is maximum at the peak of the laser pulse and that the degree of ionization changes over a time equal to about half of the pump pulse width. By observing changes in the transmission of the probe pulse caused by plasma absorption, the electron temperature of a field ionized rare gas is determined to be on the order of 1 eV. The time varying electron density in the pump-probe experiments also causes plasma-induced cross-phase modulation, or spectral blueshifting of the probe pulse spectrum of up to 15 nm. The pump-probe experiments show that plasma defocusing causes the spectral blueshifting to be spatially dependent. Experimental results and a two dimensional pulse propagation model indicate that the most defocused beam components also show the maximum spectral blueshift. Plasma-induced cross-phase modulation has also been used to characterize the amplitude and phase of a 1 ps chirped pulse at 497 nm and the pulse width of a 400 fs pulse at 147 nm generated by four wave frequency mixing in xenon.Item Pulsed ultraviolet laser ablation: Theoretical considerations and applications in medicine(1990) Pettit, George H.; Sauerbrey, Roland A.Pulsed ultraviolet lasers can be used to induce clean etching in organic and some inorganic materials. This effect is called photoablation or ablative photodecomposition. Although the precise cause remains unknown, this phenomenon is becoming of vital importance in micromachining, materials processing, and medicine. To better understand the photoablation process, a theoretical description of the process has been developed. This description is based on a treatment of the radiation transport of intense ultraviolet light pulses through absorbing organic material. The theory predicts deviations from Beer's Absorption Law at high intensities which are in fact observed. These discrepancies are due to three main effects: saturation of the finite number of chromophores in the material, multiphoton absorption, and attenuation of laser light by ablation products. Using the analysis to model the ablation process it is possible to describe observed ablation behaviors for a variety of synthetic and biological substrates. Excimer laser photoablation has also been studied experimentally as a means of removing occlusive arterial thrombi. Thrombi induced in canine coronary arteries were removed with XeF excimer laser light (351 nm) delivered via flexible optical fiber. The results of this study indicated that it was possible to remove significant thrombi (27 mg mass) within 3 minutes, without causing injury to the adjacent arterial wall. In addition, the ablation products consisted of a minute volume of gas and small particulate debris ($>$100 $\mu$m), which would not be a cause of concern in a clinical procedure. Excimer laser induced autofluorescence signatures have been analyzed as possible diagnostic aids in laser angioplasty procedures. Spectra acquired from cadaver artery samples using low intensity laser light revealed distinct differences in fluorescence response between healthy and atherosclerotic arterial wall. Fluorescence spectra acquired during tissue ablation with intense laser pulses also exhibited variation between healthy and diseased sites. Spectral examination of tissue sites after ablative tissue removal showed changes in fluorescence response which coincided with penetration of diseased lesions. These findings indicated that it should be possible to use UV laser fluorescence spectroscopy to target atherosclerotic lesions and to monitor in real time the ablative removal of these vessel obstructions.Item Rare gas alkali ionic excimers(1989) Warwar, Gregory C.; Sauerbrey, Roland A.The rare gas alkali ionic excimers (RgA)$\sp+$& are investigated as possible VUV laser sources. The detection of emission from (XeRb)$\sp+$ at 165 nm using laser produced plasma soft x-ray excitation is reported. In addition the production of (XeCs)$\sp+$ with a radiative emission at 160 nm was observed using pulsed high energy electron beam excitation. A preliminary kinetic model for the formation and quenching processes of the ionic excimer (XeCs)$\sp+$ is presented. Fair agreement of the kinetic model with experiments is obtained. In order to reproduce the experimental results a relatively long radiative lifetime ($>$140 ns) and a small electron quenching rate ($<$3 $\times$ 10$\sp{-9}$ cm$\sp3$/s) have to be assumed. A quantum mechanical model based on previous single atom Hartree-Fock calculations is presented for calculating the potential energy curves for the rare gas alkali ionic molecules. Results are compared with previous calculations based on Rittner potentials.Item Rare gas alkali ionic excimers(1990) Millar, Pamela S.; Sauerbrey, Roland A.The rare gas alkali ionic molecules are established as a new class of ionic excimers which emit in the vacuum ultraviolet (VUV) regime of the electromagnetic spectrum. Temporal and spectral characteristics of these species with (XeRb)$\sp{+}$ at 164 nm and (XeCs)$\sp{+}$ at 160 nm have been investigated by soft x-ray excitation in a laser-produced plasma and by high energy pulsed electron beam pumping in this work. Soft x-ray pumping of XeF(B $\to$ X) and (XeRb)$\sp{+}$ yielded the first observation of excimer molecules formed by reactive kinetics in a laser-produced plasma. The spectrum of (XeRb)$\sp{+}$ was observed. The spectral structure could be assigned to 3 dipole allowed transitions originating from the 0$\sp{+}$(II), 1(II), 1(I) upper states ending in the 0$\sp{+}$(I) ground state. A kinetic study of electron beam pumped mixtures of Ar/Xe/Rb and Ar/Xe/Cs was performed. The observed temporal decays of (XeRb)$\sp{+}$ and (XeCs)$\sp{+}$ were analyzed. The results suggest that electron deactivation is surprisingly not a dominant quenching process for rare gas alkali ions. The radiative lifetimes are 150 $\pm$ 50 ns for (XeCs)$\sp+$ and 250 $\pm$ 50 ns for (XeRb)$\sp+$. In addition several quenching rate constants were extracted from the experimental results. These rate constants and lifetimes were incorporated into a kinetic model for these species. This kinetic model reproduced the experimental observations well providing the electron quenching rate coefficient is kept to a maximum of 5 $\times$ 10$\sp{-9}$ cm$\sp3$s$\sp{-1}$. Estimates for the upper state densities were computed using this model to be $\sim$4 $\times$ 10$\sp{15}$ cm$\sp{-3}$ for (XeCs)$\sp+$ and 2 $\times$ 10$\sp{15}$ cm$\sp{-3}$ for (XeRb)$\sp+$. Including absorption due to the photoionization of the alkali atoms, the net gain coefficients are computed to be on the order of 10$\sp{-2}$ cm$\sp{-1}$. Therefore the rare gas alkali ionic excimers appear to be a promising class of candidates as storage media for VUV lasers.Item Spectral and temporal characteristics of a subpicosecond krypton fluoride excimer laser (krypton fluoride, photon fluorescence)(1991) Le Blanc, Stephen Paul; Sauerbrey, Roland A.Spectral and temporal characteristics of a subpicosecond KrF excimer laser are determined with the use of a one meter spectrometer, a single shot intensity autocorrelator, and a single shot phase sensitive autocorrelator. Spectral analysis is shown to be a simple and sensitive method for aligning the laser system. Experimental results and model calculations show that the center of the laser spectrum is influenced by self phase modulation. Near the edge of the amplifier gain bandwidth, the spectrum is also influenced by the vibrational levels of the KrF upper state. To support the spectral analysis, temporal measurements are performed with single shot autocorrelators. Using a single shot autocorrelator based on three photon fluorescence of XeF, the pulse width was measure at 477 fs. To determine the frequency chirp of the laser, a single shot phase sensitive autocorrelator for ultraviolet lasers was designed, and preliminary tests were performed.Item Spectroscopic and kinetic studies of ionized helium excited by a laser-produced plasma soft x-ray source(1988) Chen, Jian; Sauerbrey, Roland A.The work of this thesis describes the experimental set-up and measurements for spectroscopic and kinetic studies of ionized helium gas. An injection-controlled KrF 248nm laser beam focussed to an intensity of 3 $\times$ 10$\sp{11}$ Wcm$\sp{-2}$ was used to generate highly ionized tantalum plasma. Helium gas was photoionized by the broadband soft x-ray flux from the laser-produced Ta plasma. The possibilities for realizing a short wavelength laser (below 2000A) have been investigated. The spectra of He$\sp+$ transitions 4 $\to$ 2 and 3 $\to$ 2 were experimentally measured by an optical multichannel analyzer, and the time evolution of the fluorescences were observed by a fast response photomultiplier. The effects of helium pressure on the fluorescence were also studied. In addition to the experiments, a kinetic model was developed. The properties of ionized helium predicted by this model are in relatively good agreement with the experimental results.Item Studies of ionic alkali halide excimer molecules excited by laser-produced plasmas(1991) Kubodera, Shoichi; Sauerbrey, Roland A.New ionic states of alkali halide molecules, which are isoelectronic to the neutral rare gas halide molecules, have been generated by direct photoionization using the soft x-ray flux from a laser-produced plasma. The fluorescence emission of three ionic excimer transitions has been observed at 185 nm for CsF, 208 nm for CsCl, and 130 nm for RbF. The emission wavelengths are in good agreement with those calculated using Rittner-type potentials. The vibrational structure in the emission spectrum of CsF has been calculated and excellent agreement with the experiment is found. A model for photoionization of these molecules has revealed that the vibrational population of the bound photoionized state is Boltzmann distributed if the neutral ground state has a thermal vibrational distribution. Molecular halogen transitions of Cl$\sb2$* at 258 nm and F$\sb2$* at 158 nm have been observed in CsCl and RbF, respectively, as a result of predissociation of the ionic states of these molecules. Rate constants for the molecular halogen formation and lifetimes for the precursor states have been experimentally evaluated. The predissociation probability of the ionic state by adiabatic curve crossing has been estimated to describe the observed spectral intensity. A new kinetic model has been developed to describe the photoionization of CsF vapor. The calculated number density of the Cs$\sp{2+}$F$\sp-$ ionic state of 6 $\times$ 10$\sp{13}$ cm$\sp{-3}$ is in good agreement with the measured one. Based on the kinetic model, it is concluded that a CsF ionic excimer laser is feasible when a short pulse excitation with a line focusing geometry is employed. The absorption spectra of CsF, CsCl, and RbF in the UV and VUV have been determined using a VUV plasma continuum probe light source. The observed spectral peaks have been identified as the transitions from the ground states to the neutral covalent states. The spectra have been analyzed using the reflection method, providing potential curves of the neutral covalent states. The absorption oscillator strengths of these transitions have been evaluated from the absorption spectra.