Efficient VUV xenon excimer laser (λ = 172 nm) AT 6 atm

Experiments on a VUV xenon excimer laser working at medium pressure (3–7 atm) are reported. Xenon gas was excited by an intense electron beam (550 keV, 13 kA, 20 ns) from a 520 Apex machine (S³). Dielectric mirror dammage of intracell cavity and gaz pollution were studied. At 6 atm, we obtained typically 10 MW, 10 ns pulse, with 4 nm spectral width and 1% medium efficiency. This value obtained at 6 atm instead of 12 atm (Hughes) is in agreement with our calculations from kinetics.     

Co-axially pumped,narrow band,continuously tunable,high power VUV xenon laser

High efficiency spectral narrowing to 0.13 nm and frequency tuning from 169 nm to 176 nm has been produced with a new design of coaxial-pumped xenon laser employing a single intra-cavity prism. The laser peak power is 3 MW (9 mJ) and becomes 0.7 MW (power density 10 MW cm^-2) when frequency narrowed.     

Luminescence of rare gas crystals at high excitation densities for VUV laser applications

Exciton densities of the order of 10¹⁸ cm^–3 are generated in 0.1–0.3 mm thick surface layers in an area of 10×20 mm² of optically clear rare gas crystals. The quantum efficiencies at 126 nm (Ar), 145 nm (Kr), and 172 nm (Xe) remain near 0.5 even for the highest excitation densities. The corresponding gain coefficients of 2.6 cm^–1 (Ar) to 18 cm^–1 (Xe) exceed those of high pressure gas lasers by a factor of 20. Stimulated emission is inferred by observing the line narrowing, the dependence of intensities and time courses on excitation density and amplification measurements. The net gain coefficient is reduced however to 0.5–1 cm^–1 by transient absorption of excited centers and scattering by irradiation induced defects. The results are analysed by a system of rate equations for the excitation, relaxation, quenching, and amplification processes. A peculiar temperature dependence of the quantum efficiencies and time courses is attributed to electron trapping at grain boundaries.     

VUV laser ablation of insulators

We present theoretical and experimental investigations of factors that may influence the surface quality of insulators patterned by 157 nm F₂ laser ablation. Aspects covered include modelling of beam spatial coherence and diffraction effects for proximity mask exposure, and the role of melt relaxation on surface microstructures. Experimental results for micron-scale patterning of N-BK7 glass and NaCl crystals using proximity masks are presented and discussed within the framework of the modelling. PACS 79.20.Ds; 42.25.Fx; 47.20.Gv; 61.82.Ms     

One- and two-barrier excilamps on xenon dimers operating in the VUV range

Spontaneous emission sources in the VUV range based on one- and two-barrier pulsed-periodic gas (Xe) discharges are experimentally investigated. Volt-coulomb figures, as well as typical waveforms of current pulses and discharge voltage are presented. The time dependences of the voltage drop across the gas-discharge plasma, excitation power, and excitation energy are calculated. The radiation power versus xenon pressure, exciting pulse repetition rate, and average excitation power is determined. The radiation power average densities for one- and two-barrier excilamps are found to be 20–30 and ～40 mW/cm², respectively.     

Polarization effects in resonance-enhanced frequency tripling and multiphoton ionization in xenon under excitation by crossed laser beams

Generation of resonance-enhanced third harmonic and multiphoton ionization in xenon have been studied for different polarizations of crossed laser beams. Polarization control over the harmonic generation and resonant ionization processes in crossed beams is analyzed and discussed.     

VUV laser photoionization of laser-stimulated desorbed species

We report here first results about single-photon VUV laser photoionization of desorbed species from a silicon surface irradiated by a pulsed and tunable UV laser (290-300 nm). The combination of VUV photoionization at 10 eV with laser-induced surface desorption offers a largely non-destructive and sensitive method for quantitative analysis. Indeed it allows mass spectrometry measurements with uniform sensitivity and without breaking the chemical bonds in the probed species. The energy of the VUV photons (9.91 eV) is above the ionization limits of a number of molecules and fragments. Moreover, adjustment of the delay between the desorbing and the probe lasers allows the measurement of the time-of- flight distribution of the ejected species. Data extracted from these measurements are fundamental for a better understanding of laser-surface interaction phenomena.     

Xenon clusters in intense VUV laser fields

A simple model is developed that quantitatively describes intense interactions of a vacuum ultraviolet (VUV) laser pulse with a xenon cluster. We find good agreement with a recent experiment [Nature (London) 420, 482 (2002)]]. In particular, the large number of VUV photons absorbed per atom, at intensities significantly below 10(16) W/cm(2), is now understood.     

High-resolution VUV spectroscopy using pulsed laser sources

The resonance spectroscopic techniques - level-crossing, optical double resonance and quantum-beat spectroscopy - have been employed following pulsed laser excitation. The use of pulsed lasers has enabled spectroscopic studies of atomic structures and radiative lifetimes in the UV/VUV spectral region.     

VUV generation by frequency tripling the third harmonic of a picosecond kHz Nd:YLF laser in xenon and mercury vapour

uv ^vac=351.165 nm) of a ps 1 kHz Nd:YLF laser system is frequency tripled in xenon and mercury vapour. About 4×10⁴ photons per pulse, i.e. 4×10⁷ photons/s, are generated in xenon yielding a conversion efficiency of η=3×10^-10. The unusual frequency tripling in xenon takes place in a positive dispersive wavelength region. It is shown that Kerr-induced dispersion in the atomic system and a fifth-order process rather than a third-order process can explain the frequency tripling. For comparison a four-wave mixing process is investigated in negative dispersive mercury vapour. Due to absorption of the generated VUV radiation in the autoionization region of mercury the observed effective efficiency is, in our experimental arrangement, even lower than in xenon. An analysis of the VUV generation with respect to absorption is given. Received: 1 September 1997     

Multiphoton excitation of visible and VUV luminescence in MgO crystals

Irradiation of pure MgO crystals with intense XeCl laser radiation yields VUV emission associated with the decay of bi-excitons. This emission exhibits a 4-photon dependence on incident laser intensity. Collisional excitation of luminescence from Cr³⁺ and Ni²⁺ impurities has been detected in doped samples. This emission also shows a multiphoton dependence on incident laser intensity.     

Cone evolution on VUV laser ablated polymers

We report on conical microstructures formed on polycarbonate and polyallyl di-glycol carbonate (CR39) ablated using 157-nm F₂ laser radiation. These are distinguished by having smooth, straight walls and, for CR39, sharp tips, together with interference and diffraction fringes around their base. Etch-rate modelling of a non-planar surface and diffraction-modulated fluence is used to explore how cones may evolve, and an estimate is made of thermal loading on microcones.     

Intrinsic luminescence in oriented BeO crystals under VUV and inner-shell excitation

Beryllium oxide (BeO) crystals were investigated by time-resolved low temperature VUV-spectroscopy at the SUPERLUMI station and BW3 beam line of HASYLAB (DESY, Hamburg). Photoluminescence spectra (3–10.5 eV), luminescence decay kinetics upon selective photoexcitation, as well as luminescence excitation (50–650 eV) and reflectivity (9–35 eV) spectra were measured and analyzed for oriented BeO crystals. It was shown that study of oriented crystals makes the traditional time-resolved spectroscopy method essentially more informative. Formation of the self-trapped exciton excited states of different multiplicity was found to sensitively depend on excitation energy and mutual orientation of the crystal's C optical axis and electric vector E of exciting polarized synchrotron radiation.     

A magnetically stabilized coaxial laser discharge

A new magnetic discharge stabilization technique for coaxial laser systems is described. The approach utilizes crosses electric and magnetic fields to create and maintain a large and rapidly rotating plasma volume which does not experience glow-to-arc transitions. Very high cw specific discharge power loadings have been achieved even without the benefit of external gas cooling or circulation.Performance is insensitive to gas composition and pressure such that high power coaxial discharges have been run in CO₂ laser gas mixtures up to several hundred torr. Stable cw discharges have also been obtained in mixtures containing several torr of SF₆.The technique appears to be readily scalable to give very large excited volumes in systems with comparatively small overall physical dimensions.     

Temporal evolution of the xenon laser pulse

An experimental investigation was made of the xenon excimer laser. Several laser cavities have been employed with excitation by electron beams of cross section (15×2) cm² and (55×4) cm² and current density 100–150 A cm^-2 injected transverse to the cavity axis. A numerical model of the xenon laser is compared with experimental results. The laser pulse width was found to be dependent on laser intensity and cavity mirrors. Early termination of the laser pulse was observed consistent with changing reflectivity of the cavity mirrors. An uncoated MgF₂ retroreflector produced a laser output of significantly longer pulse width. Quasi-cw laser action was observed.     

A subpicosecond dye laser pumped by a xenon ion laser

A pulsed xenon ion laser has been used to pump a rhodamine 6G dye laser utilizing a ring resonator. The dye laser has been passively mode-locked; a pulsewidth of 0.5 ps and a 50 kW peak power have been obtained.Work partly supported by Italian C.N.R.