Theoretical and experimental study of KrF fluorescence in amultimicrosecond longitudinal discharge

Multimicrosecond KrF (B→X) fluorescence characteristics were measured using KrF laser mixtures pumped by a low-pressure longitudinal stable discharge to make clear the mechanism of the premature termination of the KrF laser. Using the experimental setup with a seven-stage pulse-forming network (PFN), a KrF fluorescence pulse with a pulse duration of 2.0 μs was obtained at a total pressure of 75 torr, whose gas mixture was F₂/Kr/He=0.1/7.5/92.4(%). The pulse width (FWHM) of KrF fluorescence was decreased from 1400 ns to 500 ns with increasing F₂ concentration from 0.1 to 1.5%. The theoretical analysis made it clear that this termination mechanism was strongly due to the KrF fluorescence kinetics     

Influence of laser fluence and irradiation timing of F2 laser on ablation properties of fused silica in F2-KrF excimer laser multi-wavelength excitation process

A collinear irradiation system of F₂ and KrF excimer lasers for high-quality and high-efficiency ablation of hard materials by the F₂ and KrF excimer lasers’ multi-wavelength excitation process has been developed. This system achieves well-defined micropatterning of fused silica with little thermal influence and little debris deposition. In addition, the dependence of ablation rate on various conditions such as laser fluence, irradiation timing of each laser beam, and pulse number is examined to investigate the role of the F₂ laser in this process. The multi-wavelength excitation effect is strongly affected by the irradiation timing, and an extremely high ablation rate of over 30 nm/pulse is obtained between -10 ns and 10 ns of the delay time of F₂ laser irradiation. The KrF excimer laser ablation threshold decreases and its effective absorption coefficient increases with increasing F₂ laser fluence. Moreover, the ablation rate shows a linear increase with the logarithm of KrF excimer laser fluence when the F₂ laser is simultaneously irradiated, while single KrF excimer laser ablation shows a nonlinear increase. The ablation mechanism is discussed based on these results. Received: 16 July 2001 / Accepted: 27 July 2001 / Published online: 2 October 2001     

Kinetic study of a short-pulse electron-beam pumped Kr/F2 laser amplifier medium at very low pressure operation

A numerical investigation has been performed for very low pressure (200 Torr) buffer-free KrF laser-amplifier medium pumped by a short pulse (10 ns FWHM) electron beam with low excitation rate operation (200 kW/cm³). The small-signal-gain coefficient (g ₀) and absorption coefficient () have been estimated for this new operational mode. The formation and quenching processes are also discussed kinetically.     

Subpicosecond UV pulse generation for a multiterawatt KrF laser

A 180-fs UV pulse has been generated based on a hybrid synchronously pumped mode-locked dye laser for a multiterawatt KrF laser system. The pulse width was measured by the single shot autocorrelation technique with the three-photon fluorescence of the XeF C-A transition. The pulse width broadening due to dispersive media was investigated. The results show that the observed pulse width broadening from 210 fs to 390 fs through the entire system is explained mostly by the linear dispersion of the optical elements for near-transform-limited input pulses.     

Influence of the laser wavelength on the epitaxial growth and electrical properties of La0.8Sr0.2MnO3 films grown by excimer laser-assisted MOD

Epitaxial La_1−xSr_xMnO₃ (LSMO) films were prepared by excimer laser-assisted metal organic deposition (ELAMOD) at a low temperature using ArF, KrF, and XeCl excimer lasers. Cross-section transmission electron microscopy (XTEM) observations confirmed the epitaxial growth and homogeneity of the LSMO film on a SrTiO₃ (STO) substrate, which was prepared using ArF, KrF, and XeCl excimer lasers. It was found that uniform epitaxial films could be grown at 500 °C by laser irradiation. When an XeCl laser was used, an epitaxial film was formed on the STO substrate at a fluence range from 80 to 140 mJ/cm² of the laser fluence for the epitaxial growth of LSMO film on STO substrate was changed. When the LaAlO₃ (LAO) substrate was used, an epitaxial film was only obtained by ArF laser irradiation, and no epitaxial film was obtained using the KrF and XeCl lasers. When the back of the amorphous LSMO film on an LAO substrate was irradiated using a KrF laser, no epitaxial film formed. Based on the effect of the wavelength and substrate material on the epitaxial growth, formation of the epitaxial film would be found to be photo thermal reaction and photochemical reaction. The maximum temperature coefficient of resistance (TCR) of the epitaxial La_0.8Sr_0.2MnO₃ film on an STO substrate grown using an XeCl laser is 4.0%/K at 275 K. XeCl lasers that deliver stabilized pulse energies can be used to prepare LSMO films with good a TCR.     

13C-selective infrared multiple-photon decomposition study of CBrClF2

The infrared multiple-photon single-frequency decomposition (IRMPD) of CBrClF₂ was examined as functions of laser wavenumber, laser fluence, and partial pressure of CBrClF₂. The initial step was the scission of a C-Br bond. In the presence of O₂ the carbon-containing product was CF₂O and its subsequent hydrolysis gave CO₂. The initial dissociation was highly ¹³C selective at wavenumbers below 1014 cm^–1. CBrClF₂ decomposed at relatively low fluences as compared to CHClF₂. However, the decomposition yield rapidly decreased with increasing pressure. In the large-scale irradiation experiment using about 8 J pulse at 1 Hz, we obtained a carbon yield of 0.41 mol per pulse at a ¹³C-atom fraction of 17% for a mixture of 10 Torr CBrClF₂ and 10 Torr O₂, and a carbon yield of 0.17 mol per pulse at a fraction of 29% for a mixture of 20 Torr CBrClF₂ and 20 Torr O₂. The IRMPD of CHClF₂ gave a carbon yield of 0.18 mol per pulse at 48% for 10 Torr neat CHClF₂ and yield of 0.25 mol at 52% for 20 Torr CHClF₂. The large-scale irradiation experiment was also carried out for mixtures of CBr₂F₂ and O₂. CHClF₂ is the most productive of ¹³C.     

Peculiarities of KrF excimer vibrational relaxation in low-pressure Kr/F2 mixtures excited by a short pulse

The importance of excimer vibrational relaxation is manifested once again by the example of the low-pressure Kr/F₂ gain medium excited by a short pulse. The pressure is determined at which a sharp fall of the population efficiency of low KrF(B) vibrational levels should appear. Time peculiarities of the gain are investigated analytically for short-pulse excitation operation.     

Simultaneous single-shot imaging of OH and O2 using a two-wavelength excimer laser

A technique is described for simultaneous single-shot imaging of OH and O₂. Laser-induced fluorescence of both molecules is excited by a tunable KrF laser, which is operated simultaneously on two wavelengths. By using two CCD detectors with image intensifiers and suitable filters, separate images of OH and O₂ distributions in H₂/O₂ and hydrocarbon/air flames were obtained.     

On the performance of an ArF and a KrF laser as a function of the preionisation timing and the excitation mode

An X-ray preionised ArF and KrF excimer laser has been studied with three different spiker-sustainer excitation circuits. We observed large differences in the laser performance, when the preionisation delay timing was varied on a nanosecond timescale. The behaviour of both lasers was found to be equivalent. The observations can be understood by considering the effect of the discharge excitation technique on the preionisation process. An excitation mode with a prepulse well above the steady-state voltage V_SS with a subsequent reversed overshoot voltage for initiating the discharge, in combination with a well-timed preionisation pulse is found to give the best results. Optimum output energies of 50 mJ with ArF and 175 mJ with KrF were obtained from an active volume of 60×1.5×1.2 cm in the so-called swing mode, with the preionisation applied 60 ns before the discharge breakdown. Received: 23 February 1999 / Revised version: 4 June 1999 / Published online: 16 September 1999     

248 nm laser etching of GaAs in chlorine and ozone gas environments

KrF laser etching of GaAs in Cl₂ and O₃ gas ambients by direct laser illumination is reported. The etch depth per pulse in Cl₂ was found to be linear versus the laser fluence on the sample in the 0.2–1.1 J/cm² range. It increased as a function of the Cl₂ pressure up to 6 Torr and slightly decreased for pressures above this value. It also decreased as a function of the laser repetition rate. Very smoothly etched surfaces were obtained after irradiation using the Cl₂ and O₃ etching gases. Auger analysis of the etched GaAs surfaces shows almost no traces of chlorine after etching in Cl₂, whereas a thick oxide layer of about 1500 Å thickness was found after etching in ozone.     

Studies on excimer laser doping of GaAs using sulphur adsorbate as dopant source

Excimer laser doping of GaAs using sulphur adsorbate as a dopant source is demonstrated. Box-like n-type layers of depths of about 100 nm with carrier concentration as high as (23)×10¹⁹ cm^–3 are formed. Passivation of GaAs using a (NH₄)₂S_x solution for 40 min followed by sublimation of the excess sulphur atoms in high vacuum result in an effective dopant for controllable n-type doping. The samples are irradiated using a KrF excimer laser in a N₂ gaseous environment. Secondary ion mass spectrometry (SIMS) measurements show that sulphur is successfully incorporated in the GaAs. The sheet resistance is controlled by adjusting the laser energy fluence and number of laser pulses. Rutherford backscattering spectrometry with channeling (RBS/C) alignment measurement indicates that lattice damage is undetectable for N₂ gas pressures of 760 Torr.     

Photochemical effects of KrF excimer excitation in laser-induced fluorescence measurements of OH in combustion environments

This paper presents experimental evidence that using the KrF excimer laser for quantitative laser-induced fluorescence (LIF) studies of the OH A-X (3,0) system is highly problematic if the effects of both photobleaching and photochemistry are not included for laser spectral irradiances greater than 20 MW/cm² cm^-1. Pump-probe and time-resolved measurements of the OH LIF signal in an atmospheric pressure, premixed CH₄-air flame at low- and high-laser-spectral-irradiance conditions show that a significant amount of OH is produced from photofragments resulting from the simultaneous 2-photon predissociation of H₂O molecules in the C-X system. A 5+2-level rate-equation model that includes the effects of both photobleaching and photochemical OH production is shown to satisfactorily predict the data using a single adjustable parameter given by the effective, spectrally integrated 2-photon cross-section of H₂O near 248 nm. The time-integrated OH LIF signal was found to depend on both the laser spectral irradiance and the local concentration of H₂O. Additionally, use of the KrF excimer laser for 2-line rotational thermometry can produce temperature errors as great as +550 K at high laser-pulse energies. Received: 21 August 2000 / Revised version: 30 October 2000 / Published online: 21 February 2001     

Gas density measurement of pulsed gas jets using XeF four-photon fluorescence induced by a KrF laser

The gas densities of two pulsed gas jets were measured together with spatial and temporal distributions by the XeF fluorescence induced by a KrF laser. The B-X and C-A transitions of XeF showed a biquadratic dependence on laser intensity when SF₆ was used as the F donor instead of F₂, and quadratic and cubic dependences on gas density, respectively.     

Single picosecond UV pulse generation by mode-locking of an excimer laser-pumped dye laser

A single picosecond ultraviolet pulse has been generated based on mode-locking of a dye laser pumped by a long pulse XeCl laser to serve as the input source for a high-power ps KrF laser system. A short-pulse uv dye laser (BBQ) pumped by an additional XeCl laser was used to selectively amplify a single pulse from a mode-locked pulse train with the pulse separation of 3.2 ns. The amplified single pulse was frequency-doubled to 248 nm with the pulse duration of 20 ps.     

Analyses of surface coloration on TiO2 film irradiated with excimer laser

TiO₂ film of around 850 nm in thickness was deposited on a soda-lime glass by PVD sputtering and irradiated using one pulse of krypton-fluorine (KrF) excimer laser (wavelength of 248 nm and pulse duration of 25 ns) with varying fluence. The color of the irradiated area became darker with increasing laser fluence. Irradiated surfaces were characterized using optical microscopy, scanning electron microscopy, Raman spectroscopy and atomic force microscopy. Surface undergoes thermal annealing at low laser fluence of 400 and 590 mJ/cm². Microcracks at medium laser fluence of 1000 mJ/cm² are attributed to surface melting and solidification. Hydrodynamic ablation is proposed to explain the formation of micropores and networks at higher laser fluence of 1100 and 1200 mJ/cm². The darkening effect is explained in terms of trapping of light in the surface defects formed rather than anatase to rutile phase transformation as reported by others. Controlled darkening of TiO₂ film might be used for adjustable filters.     

Optical and waveguiding properties of Nd:KGW films grown by pulsed laser deposition

 Nd: KGd(WO₄)₂ thin films were deposited by KrF laser ablation on MgO, YAP, YAG and Si substrates at temperatures up to 800 °C. Film crystallinity, morphology, stoichiometry (WDX, RBS and PIXE), excitation spectra, fluorescence, refractive index and waveguiding properties were studied in connection with deposition conditions. The best films were crystalline and exhibited losses of approximately 5 dB cm^-1 at a wavelength of 633 nm. Received: 8 January 2001 / Accepted: 7 November 2001 / Published online: 11 February 2002     

Detailed study of the effect of a short prepulse on soft X-ray spectra generated by a high-intensity KrF* laser pulse

The effect of a short prepulse (0.5 ps) on soft X-ray spectra from a plasma generated by a high intensity KrF* laser pulse (main pulse: 0.5 ps, intensity I _main=5.3×10¹⁵ W/cm²) on flat targets of Al and Cu has been studied in detail. The spectra have been measured as a function of the pulse separation t between the two pulses and the prepulse intensity I _pre. It was found that both the overall emission and the line emission increased with t (at constant I _pre) and with I _pre (at constant t). In particular, lines in the shorter wavelength region had higher intensity. The influence of the prepulse on the line emission of specific transitions in the Al spectra was investigated systematicly. An explanation for the observed effects is given.     

Comparison of the influence of the fictive and the annealing temperature on the UV-transmission properties of synthetic fused silica

′ and NBOH). Samples with high OH content exhibit gradual recovery from the absorption band within several minutes after exposure to the KrF laser radiation. The formation of the KrF laser-induced 210 nm absorption band depends on the fictive temperature and on the OH content. Low fictive temperature, as a measure for the number of intrinsic defects, retards E^′ generation at the beginning of intense KrF excimer laser irradiation when the majority of defects are generated from precursor defects. However, for longer irradiation periods with pulse numbers of the order of 10⁵ pulses, a high OH content is the beneficial parameter. The accompanying atomic hydrogen is essential for the suppression of the 210 nm absorption band. This happens by transformation of the E^′ centers into Si-H defects. In contrast to a generally held view, annealing (decreasing of the fictive temperature) of fused silica does not always reduce UV induced defect generation. For example, annealing of the samples in an argon atmosphere causes a significantly higher 210 nm absorption increase during KrF excimer laser irradiation (240000 pulses) compared to nonannealed samples. Two spectroscopic methods to determine the OH content of fused silica were applied: Raman and infrared spectroscopy, which in this work lead to differing results. The energetics of the 210 nm absorption band generation and bleaching is summarized by a diagram explaining the interaction of the 248 nm laser radiation with fused silica. Received: 2 June 1997/Accepted: 13 June 1997     

Generation of femtosecond vacuum-ultraviolet pulses

High power femtosecond pulses in the Vacuum Ultra Violet (VUV) have been generated through the nonlinear interaction of femtosecond KrF pulses with xenon and argon gas. Under near resonant two photon excitation of xenon by a femtosecond KrF laser, parametric four wave mixing processes lead to VUV pulses at 147 and 108 nm with pulse energies in the 10 µJ range. Tuning is demonstrated by mixing the KrF pulse with a 500 fs dye laser pulse at 497 nm, resulting in 165 nm emission. In argon, a three photon resonance leads to third harmonic generation at 83 nm and micro joule level pulses near 127 nm generated by a six wave mixing process. Since the spectra of the VUV pulses show an ionization-induced blue shift with increasing KrF laser intensity, the VUV pulses can be shown to have temporal duration less than the pulse width (450 fs) of the KrF laser. Blue shifting of the third harmonic of the KrF laser in argon is dominated by a reduction in the neutral gas density rather than by an increase in the electron density.     

Laser-induced fluorescence detection of singlet CH2 in low-pressure methane/oxygen flames

Methylene, CH₂, is a chemically important intermediate in hydrocarbon combustion but has previously eluded optical detection in a combustion environment. The CH₂ signal as a function of height above the burner surface in a premixed, laminar, methane/oxygen flame (5.6 Torr and fuel equivalence ratio 1.05) is measured by laser-induced fluorescence (LIF) in the B ₁ – ã¹ A ₁ electronic system. The ã state which lies 3165 cm^–1 above the ground state is populated at the high temperatures of the flame (800–1800 K). Although less than one photon for each laser pulse is detected, we can unambiguously attribute the LIF features in the region 450 to 650 nm to CH₂ by both scanning the excitation laser and dispersing fluorescence. LIF temperatures and CH and OH LIF concentration profiles are also obtained for the flame. The CH₂ radical concentration maximum occurs closer to the burner than that of either OH or CH, as expected from models of methane combustion chemistry.