Discharge pumped rare gas halide lasers have been demonstrated to have high efficiency and to operate reliably but the single pulse energies attainable from these devices have been limited by the difficulties associated with scaling to larger volumes. In this paper we discuss the preliminary results obtained from a twelve liter active volume device. This laser system, which is capable of exciting up to 3-0 liters, uses x-rays to preionize the gas. The design and operation of the system to date is presented.
The novel operational regime of high-efficiency KrF lasers is presented. The excitation sources are intense electron beams and intense proton beams. Theoretical evaluation of electron-beam-excited KrF laser using one atmospheric-pressure, argon-free mixtures showed that by using high-excitation-rate and shorter-excitation-pulsewidth pumping, higher intrinsic laser efficiency than conventional argon-rich, high-pressure mixtures is attainable. The experimental results are also presented.
The generation of high power, ultrashort pulses in the UV wavelength region has important implications for nonlinear optics, photochemistry, plasma X-ray sources and dye laser pumping. Excimer lasers have sufficient bandwidth for the amplification of pulses having a duration in the picosecond range and a significant effort has been devoted to the development of short pulse sources based on these devices. The application of conventional mode-locking techniques has been investigated by several groups but has not been particularly successful in exploiting the wide spectral bandwidths available in the laser media. This has been due in large part to the limited number of cavity round-trips permitted by the relatively short duration of the gain provided by discharge-excited lasers. Thus rare gas halide lasers have been used to amplify picosecond pulses to power levels of tens of gigawatts but have depended on fairly sophisticated external sources to generate the initial ultrashort pulse. Recent experiments on stimulated Brillouin scattering from liquids have demonstrated that subnanosecond pulses can be generated from a narrow linewidth XeC1* source, and the use of this pulse shortening process may permit the development of less complex ultrashort pulse excimer systems in the future.
In this paper the factors controlling the performance of electron beam-pumped, broadband diatomic and tri-atomic rare gas-halide excimer lasers are discussed. Particular attention is directed toward the blue/green in the XeF(C>A) laser centered at 485 nm, for which significantly improved performance has been obtained by selective tailoring of mixture kinetic processes. Maximum XeF(C>A) laser output energies in excess of 1.5 J/liter have been achieved, corresponding to an intrinsic efficiency of approximately 1%.
A multidimensional model of a transverse discharge HgBr laser has been developed. Laser energy scaling is examined as a function of preionization electron density distribution, electrode shape, and stored electrical energy. Maintaining preionization electron density profiles and electrode contours to a maximum variation of 10% will minimize constriction of the discharge and maximize laser intensity uniformity.
The collisional coupling between the green (A - 505 nm) and UV (342 nm) bands of 12 has been examined in electron beam-pumped mixtures of Ar and hydrogen iodide (HI). The strong interaction between the two is evidenced by the large suppression (> 40%) of the 342 nm D' 4. A' fluorescence that is observed when a green dye laser pulse (A - 503 nm) of I > 4 MW - cm-2 is injected into the medium.
Experimental results of a triatomic noble gas halide excimer Kr2F laser, pumped by an electron beam, are reported. Such a laser has been operated using high pressure mixtures composed of optimized ratios of Ar (o; Ne, Kr, and He), Kr, and NF3 (or F2), and N2. Furthermore, the effect of gas temperature upon the Kr2F emission characteristics wasinvestigated.
The temporal behaviour of gain and absorption coefficients for electron beam pumped gas mixtures containing helium, C12, CC14 and F2 are investigated. From the transient absorption in pure helium photoionization cross sections for He(23S) and He (a ,3 ) in the UV and VUV are determined. In He/C12 mixtures a strong absorption around 220 nm was observed that is ascribed to C12(A' )-Cl2(D') transitions. The small signal gain coefficient of the Cl2(D' -A' ) laser at 258 nm was increased con-siderably in magnitude and temporal duration by the addition of CC14 to a He/C12 gas mixture. The observed effects are explained by a very efficient electron quenching of the D. -state.
Multiphoton absorption of picosecond ArF (193 nm) laser radiation has resulted in the production of highly charged ions and in the observation of stimulated emission in the vacuum ultraviolet (VUV) and extreme ultraviolet (XUV).
Ultraviolet photoemission provides a powerful tool for studying both the bulk and surface electronic structure of crystalline solids. While all of the previous photoemission studies of solids have concentrated on the time independent properties of filled electronic levels, i.e. the valence bands of semiconductors and the filled conduction bands of metals, the extension of this technique to study the dynamic transient processes experienced by electrons excited into otherwise unoccupied states has only begun to be exploited. In this paper we describe an experimental system which utilizes the generation of short pulsewidth coherent vacuum ultraviolet light to perform photoemission studies at solid surfaces and in the bulk. Furthermore, the nature of the short time duration of the light pulses, approximately 10 picoseconds, provides the opportunity to study the transient dynamics of electrons which are excited into unoccupied levels in the solid.
Third harmonic conversion of radiation from a XeF laser is described and processes that limit the conversion efficiency are examined. Proposed experiments at the 200 mJ pump level, designed to produce VUV pulses near the mJ level, are discussed.
Applications of rare gas halide phase conjugate mirrors to wavefront correction, automatic target alignment and image projection are discussed. Particular emphasis is placed on potential applications of these mirrors to high-power excimer laser development, laser fusion and photolithography.
Amplification of a diffraction-limited Stokes beam in a hydrogen Raman amplifier pumped by a severely aberrated XeC1 laser has been studied experimentally. Spatial quality of the amplified Stokes beam and conversion efficiency from the pump laser were measured. Numerical study of this process using a two-dimensional propagation code that includes pump depletion was performed. The simulation shows that the quality of the amplified Stokes beam depends critically on the absence of any near-axial components in the pump laser. These components of the pump can cause phase matched four-wave mixing interactions with the Stokes, leading to increased angular divergence of the amplified Stokes beam and the development of secondary sidebands in the far field. An optical integrator was used to focus the poor quality pump beam into the amplifier and to remove all near-axial components in the pump field. The diffraction-limited nature of the input Stokes beam was then preserved in the amplified Stokes beam. A power conversion from the aberrated pump to the Stokes beam of the order of 30% was observed. The far field intensity of the Stokes beam increased by a factor of 5000 over that of the original pump.
The use of short pulse lasers to dope silicon is discussed. Results of the process when used to fabricate silicon solar cells and p+-n- diode structures are reviewed. The mechanism of doping is discussed and electrical and structural data presented. Successful fabrication of very shallow, high concentration junctions by this technique makes it a viable process for silicon VLSI processing.
InP and In203 have been deposited on GaAs, and InP ,ubstrates by excimer laser induced photodecomposition of (CH3)3InP(CH3)3 and P (CH3) 3 vapors at 193 nm. Phosphorus incorporation in the films was greatly enhanced by focusing the laser beam to promote multiple-photon dissociation processes. These conditions also lead to enhanced carbon inclusion in the films, due to formation of species such as CH and CH2 in the gas-phase. However, this carbon inclusion could be suppressed by focusing the beam onto the surface at normal incidence. The technique offers several potential advantages over conventional metal-organic chemical vapor deposition (MOCVD), including lower temperature, enhanced rates, safer gases, and three-dimensional film composition control. Strong atomic In emission is observed in the gas-phase above the depositing film, due to a multiple photon dissociation process. Gas-phase fluorescence from P, CH, and C was also observed. These emissions give insight into the photodecomposition mechanism and also serve as a monitor of metal organic precursor concentrations. In addition, excimer laser excitation has been used to detect PH3, P2, AsH3, As2, InCI, and GaC1 in a reaction tube designed to simulate a conventional CVD growth reactor. These are the primary reactants participating in the chemical vapor deposition (CVD) of InP/InGaAsP epitaxial layers. Detection limits for all these species are well below those levels typically employed during layer growth.
A brief survey of the fundamental interaction mechanisms of pulsed lasers with materials at intensities less than 1010 W/cm2 is presented, with emphasis given to the effects of wave-length and ambient pressure on impulse coupling phenomena.
The results of experiments and theoretical analyses are presented for the response of aluminum and titanium alloy surfaces irradiated in vacuum by an XeF laser pulse (X=0.35μm, pulse time = 10-6s). Thermal coupling measurements at low irradiance indicate effective optical absorptance values of -0.4 for as-received aluminum alloys, -0.2 for polished or "laser-cleaned" aluminum alloys, and 0.4-0.5 for Ti6AX4V (as-received or polished). The onset of measurable impulse is shown to result from bulk target vaporization at fluences >5 J/cm2 for titanium and >20 J/cm2 for 'polished' aluminum alloys. Plasma formation thresholds are obtained both theoretically and from experimental data, and good agreement is found. The results indicate that an important absorption mechanism for laser-induced plasma ignition at this wavelength is photoionization of excited electronic states in the metal vapor.
There is a bewildering array of processes that can be used for the production of thin films. A list that is certainly far from complete is given in Table 1 simply with the intention of demonstrating this fact. Many of the processes listed there have not been or are not normally used for optical purposes and so their potential is largely unknown. Those that are used are not necessarily suitable for all types of optical coatings. An exhaustive survey is impossible within the constraints of this paper and so we deal with only a few of them. We choose those that are most commonly employed together with some that although currently uncommon appear to have considerable future potential. Although this conference is aimed at excimer lasers, it would be too restrictive to limit the discussion to only those processes that have been already used for the production of excimer laser coatings. It must be recognized that much pioneering work on the processes to be described has been carried out for applications other than optics. This account is heavily biased towards optical applications and so optical references have been selected and cited wherever possible in preference to other, even prior, work.
This paper offers a status report of photothermal-deflection microscopy applied to defect analysis in UV dielectric thin films. Absorption maps obtained by this technique from such films are presented and detection limits are discussed. A comparison with other relevant observations such as laser damage is being undertaken.
The results of damage threshold measurements made at LLNL using ultraviolet wavelength laser pulses are reviewed. Measurements were made with pulses from a krypton fluoride laser with wavelength of 248 nm and pulse duration of 20 ns and with Nd-glass laser pulses converted to the third harmonic wavelength of 355 nm with duration of 0.6 ns. Measurements are presented for transparent window materials, crystals for harmonic generation, single layer dielectric films of oxide and fluoride materials and multilayer high reflectivity and antireflective coatings.
Experiences at Pacific Northwest Laboratory in the selection, deposition and characterization of oxide optical coatings for use with excimer lasers are reviewed. Fabrication results for all-dielectric high reflectors at 248 and 308 nm are presented. Glassy or amorphous coating materials such as Si02 and Ta205 appear to be more promising for ultraviolet use than polycrystalline materials such as Y203, A1203, Zr02 and Hf02 because of reduced surface roughness. Glassy materials, formed by ion bombardment or by mixing oxides according to the method of Sanders, Farabaugh and Haller, appear to be a promising direction for future ultraviolet coating development.