Short pulse (< 100 fs) tunable X-ray and VUV laser sources, based on the free electron laser (FEL) concept, will be a watershed for high energy density research in several areas. These new 4^th generation light sources will have extremely high fields and short wavelength (~0.1 nm) with peak spectral brightness -photons/(s/mrad²/mm²/0.1% bandwidth- 10¹⁰ greater than 3^rd generation light sources. We briefly discuss several applications: the creation of warm dense matter (WDM), probing of near solid density plasmas, and laser-plasma spectroscopy of ions in plasmas. The study of dense plasmas has been severely hampered by the fact that laser-based probes that can directly access the matter in this regime have been unavailable and these new 4^th generation sources will remove these restrictions. Finally, we present the plans for a user-oriented set of facilities that will incorporate high-energy, intense short-pulse, and x-ray lasers at the first x-ray FEL, the LCLS to be opened at SLAC in 2009.

The fundamental physics of high-field laser-matter interactions has driven ultrashort pulse generation to achieve record power densities of 10²² Watts per cm² in focal spot sizes (FWHM) of 0.8 μm¹. These enormous fields are generated by compressing longer, high energy pulses to ever shorter lengths using so-called CPA compressors. Great care has to be taken to achieve such record power densities by controlling the spatio-temporal shape during pulse compression. Despite these remarkable experimental achievements, there have been relatively few developments on the theoretical side to derive realistic physical optical material models coupled to sophisticated E.M propagators. Many of the theoretical analysis tools developed in this emerging field of extreme nonlinear optics are restricted to oversimplified 1D models that completely ignore the complex vector spatio-temporal couplings occurring within such small nonlinear interaction volumes. The advent of these high power ultra-short pulsed laser systems has opened up a whole new vista of applications and computational challenges. The applications space spans relatively short propagation lengths of centimeters to meters to a target up to many kilometers in atmospheric propagation studies. The high local field intensities generated within the pulse can potentially lead to electromagnetic carrier wave shocking so it becomes necessary to fully resolve the optical carrier wave within the 3D propagating pulse envelope. High local field intensities also lead to an explosive growth of the white-light supercontinuum spectrum and the intensities of even remote spectral components can be high enough to generate nonlinear coupling to the host material. For this reason, spectrally local models of light-matter coupling are expected to fail. In this paper, we will present a fully carrier-resolved E.M. propagator that allows for few meter long propagation lengths while fully resolving the optical carrier wave. Our applications focus will be on the relatively low intensity regime where critical self-focusing collapse in air or water can lead to very strong non-paraxial ultra-broadband excitations. One reason for this restriction is that we do not yet have computationally feasible robust physical models for ultra-broadband excitation of materials where nonlinear dispersion and absorption become dominant. The propagation of terawatt femtosecond duration pulses in the atmosphere can be qualitatively captured by physical models that include reliable linear dispersion/absorption while treating the nonlinear terms as spectrally local. We will review some recent experimental results by the German-Franco Teramobile team on atmospheric propagation, penetration through obscurants and remote laser induced breakdown spectroscopy. As a second application example will address the issue of strongly non-paraxial spectral superbroadening of femtosecond pulses while propagating in water - these latter nonlinear interactions generate so-called nonlinear X- and O-waves depending on the optical carrier wavelength of the initial pulse.

The use of high intensity ultraviolet (UV) and vacuum ultraviolet (VUV) radiation generated from decaying excimer complexes through dielectric barrier discharge (silent discharges) sources for the purposes of surface processing and modification is reviewed. Such sources provide a singular dominant narrow-band emission at various wavelengths(λ) between 126 - 354 nm. The remarkable simplicity of supplying these sources and flexibility of their geometric configurations allow them to be coupled in parallel thus providing high photon fluxes over large areas. The monochromatic selectivity allows for application to process and chemical pathway specific tasks by simple variation of the discharge gas mixture. These sources are an interesting addition to and as an alternative to lasers for large scale industrial applications and their unique characterisitics have led to their use in a number of low-temperature material modification techniques, some of which are reviewed here. These include the photo-induced low-temperature formation of oxynitride layers, high-κ thin film layers and the post-deposition annealing of pulsed laser deposited (PLD) thin films.

Ellipsometry is a very useful optical technique to probe the complex index of refraction of a material. We perform dynamic ellipsometry using ultrafast lasers to probe the complex index dynamics during passage of compressional shock waves through materials of interest. When used to measure passage of a shock wave through dielectric materials, dynamic ellipsometry provides a direct measure of the equation of state (except temperature). In addition, the changes in complex index can be used to measure phase transformations and their kinetics. Using two CCD cameras and two Wollaston prisms, ellipsometric data at two incidence angles and two polarizations can be obtained simultaneously. Adding a spectrograph in front of each CCD camera and using chirped probe pulses and frequency domain interferometry provides a few hundred ps of ellipsometric data from a single shock event.

An understanding of the timing and dynamics of hohlraum filling by laser-induced gold wall ablation is critical to the performance of indirectly-driven fusion ignition designs for the National Ignition Facility [E. Moses and C. Wuest, Fusion Science and Technology, 43, 420 (2003)]. Hohlraum wall ablation negatively affects ignition hohlraum performance by (1) reducing laser coupling by increasing backscatter by laser plasma instabilities, e.g., stimulated Brillouin scattering, (2) altering where lasers couple by moving the critical surface away from the walls and changing the refractive index, and (3), in the case of vacuum hohlraums, ablating directly into contact with the ablation layer of the fuel capsule. We report on measurements of gold-filling of hohlraums from a series of OMEGA laser [T.R. Boehly, R.L. McCrory, C.P. Verdon et al., Fusion Engineering and Design, 44, 35 (1999)] experiments involving vacuum and gas-filled hohlraums. On-axis x-ray imaging of gold self-emission shows delayed filling for gas-filled hohlraums, as expected. In addition, we present data on the hohlraum temperature penalty incurred with the use of a 1-atmosphere methane-fill. We discuss data and simulation predictions for 1-atmosphere neopentane filled hohlraums driven with a modified laser pulse.

The impact of intense femtosecond laser pulses on dielectric targets results in a non-equilibrium state of the surface. We consider the influence of this instability on ablation and surface relaxation phenomena. Important consequences of the laser-material coupling and energy dissipation are addressed such as transient and permanent modification of the surface. From experiments on ablation products kinetics, Coulomb explosion upon multiphoton surface ionization has been established as the initial mechanism for desorption of fast positive ions from dielectric surfaces. We refer to the role of surface defects responsible for ion yield enhancement and the nature of defects by detecting laser induced fluorescence. Additionally, observations point to a set-in of a thermal emission process at higher laser intensity. Investigating the dynamics of particle emission, we find ultra-short timescales for the coherence of electronic excitation and energy relaxation via transient phases, the latter related to the coupling strength of the various solids. The surface morphology after ablation is modified, with regular nano- and micro-structures of features originated from self-organization of surface instabilities.

At fluences well below the threshold for plasma formation, we have characterized the direct desorption of atomic ions from fused silica surfaces during 157-nm irradiation by time-resolved mass spectroscopy. The principal ions are Si⁺ and O⁺. The emission intensities are dramatically increased by treatments that increase the density of surface defects. Molecular dynamics simulations of the silica surface suggest that silicon ions bound at surface oxygen vacancies (analogous to E' centers) provide suitable configurations for the emission of Si⁺. We propose that emission is best understood in terms of a hybrid mechanism involving both antibonding chemical forces (Menzel-Gomer-Redhead model) and repulsive electrostatic forces on the adsorbed ion after laser excitation of the underlying defect.

The paper reviews recent results on modeling a viscous liquid flow driven by ablation pressure. Based on the analysis of the Navier-Stokes equation various strongly different manifestations of this phenomenon are explained. These are: (i) a "clean" laser ablation, when laser spot has a clean sharp spot border, free from a re-solidified melt dross; (ii) a new form of material removal in laser ablation - expulsion on a poly(methyl methacrylate) target of long (up to 1 mm) nanofibers with a radius about 150-200 nm to the exterior of the spot under the action of a single pulse of KrF excimer laser; and (iii) a new way of laser surface nanostructuring - the formation of a surface foam having a structure of micro-pores interconnected with nanofilaments of diameters about 100 nm as a result of single pulse KrF laser irradiation of biopolymer films.

Irradiation of metals with ultrashort laser pulses reveals a variety of versatile microscopic processes compared to longer pulses. In particular, the impact of some material-specific characteristics, such as the electron-phonon coupling, seems to get more significance in order to meet the machining requirements. Finding the optimal process parameter area has been therefore a dominating problem in materials processing with sub-picosecond laser radiation. Ablation of bulk metals (Al, Cu) has been investigated in-situ by means of high-resolution pump-probe photography using pump laser radiation of pulse duration t_p=80 fs, at wavelength of 820 nm. This technique enables direct visualization of laser-induced processes up to 1 μs after the interaction of a single laser pulse with material. Variation of the fluence of the laser radiation, behavior and time characteristics of melting and post-melting processes have been matter of research. Depending on metal-specific parameters, qualitatively different ablation phenomena have been observed. Structural analysis by electron and optical microscopies reveals rosette-like surface structures showing the morphology of the ablated regions. The temporal development of the ablation dynamics can be conditionally categorized into different characteristic time regions. Particularly, laser induced melt injection has been observed in the time range of 700 ns to 1.0 μs after the initial laser-metal interaction.

For many applications of pulsed laser ablation it is necessary to have an understanding of the expansion dynamics of the ablation plume both in vacuum and in low pressure gases. Knowledge of the ablation plume hydrodynamics can also contribute to the understanding of the laser ablation process. In this paper we will consider some of the existing theoretical models of laser ablation plume expansion and draw some conclusions as to which model is most appropriate for the low temperature plasmas which arise in pulsed laser deposition. For ablation plumes which are significantly ionised, Langmuir probes have proved to be a relatively simple and inexpensive tool for measuring the plume shape, ion energy distribution and electron temperature. We describe some recent work on the development of Langmuir probes for laser ablation plume diagnosis. Typically in laser ablation plasma the flow velocity is supersonic, which complicates the interpretation of the I-V probe characteristic. We describe some new work on the behaviour of a flat probe lying parallel to the plasma flow. For nanosecond ablation of silver, we also show how a planar Langmuir probe can be used to obtain a fairly comprehensive description of the expansion dynamics of the ionised part of the ablation plume, including plume shape, ion energy distribution and electron temperature.

Time- and space-resolved forward scattering detection was demonstrated as a suitable technique to characterize the dynamics of the ejected particles during dry laser cleaning. Silica particles with radii of 250 nanometers deposited on silicon substrates were irradiated by single nanosecond laser pulses with fluences above the particle removal threshold. The observation of different particle clouds propagating with different velocities was in support of the coexistence of at least two removal mechanisms. The ejection velocities were measured as a function of laser fluence in order to distinguish between the mechanisms involved in laser-assisted particle removal.

Dry and steam laser cleaning, DLC and SLC, of nano-and micro-contaminant particles from UV/vis opaque and transparent critical substrates has been studied in front-side laser illumination geometry with the help of time-resolved optical techniques and broadband photoacoustic spectroscopy using a nanosecond 10.6-μm TEA CO₂-laser and different absorbing energy transfer media (ETM) fluids. Corresponding basic DLC and SLC mechanisms for removal of nano- and micro-particles from opaque and transparent critical substrates as well as parameters of explosive removal of ETM fluids have been determined and discussed.

A requirement for efficient pulsed laser propulsion from ground to LEO is the achievement of a specific impulse of up to 800 s at a jet efficiency of at least 50%. With CO₂ laser radiation at pulse lengths in the range of 10 microseconds and polymers as propellant these numbers cannot be attained by classical laser ablation because the impulse formation by laser ablation is limited by the premature absorption of the incident laser radiation in the initially produced cloud of ablation products^1,2. The power fraction of a CO₂ laser pulse transmitted through a small hole in a POM sample has been compared with the incident power. It was found that the transmitted power fraction is directly proportional to the inverse of the pulse energy. The plasma formation in vacuum and in air of 3500 Pa and the spread of the shock wave with velocities of 1.6 to 2.4 km/s in the low pressure air was observed by Schlieren photography. A sharp edged dark zone with a maximum extension of 10 to 12 mm away from the target surface develops within 5 μs independently of the pressure and is assumed to be a plasma. In order to find out, if this is also the zone where the majority of the incident laser radiation is absorbed, a CO₂ probe laser beam was directed through the expansion cloud parallel to and at various distances from the sample surface. The front of the absorption zone is found to move rapidly away from the target surface with increasing speed. The absorption lasts twice as long as the laser pulse. It is not associated with a pressure rise that would increase the mechanical impulse. The radial motion of the absorption wave turned out to be faster than the shock wave seen in the Schlieren pictures.

The contactless removal of small particles from surfaces by irradiation with intense laser pulses - dubbed laser cleaning - has been used and studied for nearly two decades. Nevertheless, its applicability and the mechanisms involved are still under debate. Here we give first a brief overview on relevant processes, and then present measurements of the velocities of colloidal model particles after detachment under vacuum conditions. We also demonstrate a new Laser Cleaning approach, by which submicrometer particles are removed by laser irradiation of the rear side of the wafers. The particles are detached by an acoustic shock wave traveling to the wafer front side after laser ablation of the rear side. Not only is this promising approach capable of defect free surface cleaning, detailed studies of particle velocities versus laser fluence also allow insight into the different cleaning mechanisms involved. Furthermore, this technique could be applied to determine adhesion energies of particles in the future.

Crystal-liquid phase transitions induced in monocrystalline Si and GaAs surface layers by nanosecond ruby laser irradiation have been studied. The values of undercooling at crystallization stage were calculated on the basis of a nonequilibrium model of the phase transitions. The calculated values of undercooling are compared with experimental results obtained by a pyrometric method under irradiation of samples with (111) and (100) surface crystallography orientations. Calculated values of surface temperature at crystallization stage are in a reasonable agreement with experimental data. The revealed experimentally difference in melt undercooling at crystallization stage for (100) and (111) surface orientations is explained within the framework of the nonequilibrium model.

Analyzing existing theoretical models of laser-induced photo-ionization of non-metallic solids, we came to conclusion that they are not valid to describe ionization of wide band-gap materials by high-intensity radiation due to specific limitation on either intensity or laser wavelength. Attempting to improve the theory of photo-ionization, we combined the Keldysh approach and a realistic energy-band model to include the effect of Bragg-type electron reflections at edges of the first Brillouin zone. It was found that the effect of Bragg-type electron reflections produces a set of singularities on intensity dependence of ionization rate. Intensity at which the first singularity occurs is close to 10 TW/cm² for most transparent materials and visible or near-IR spectrum range. Further considerations show the singularities to be directly related to the effect of field-induced flattening of effective band structure under action of laser radiation. We discuss some recent experimental results from the view-point of the new ionization model.

The availability of compact table top amplified femtosecond lasers and the technical simplicity of experimental design have opened the way to many recent and fast developments towards thin film elaboration by Pulsed Laser Deposition (PLD) with ultra short laser pulses, with the aim of producing materials of high quality previously unattainable or attainable only through more complex means. The first developments of PLD using femtosecond lasers were made on Diamond-Like Carbon thin films elaboration, with the attempt to reach high sp³ content. PLD with ultra short pulses was used recently to deposit several systems such as quasicrystals or oxides with a transfer of the target composition to the deposited films, even for compounds with complex stoechiometry. Femtosecond laser ablation from solid targets has shown its capability in producing nanoparticles of different materials, even in high vacuum conditions. Nanostructured films of doped Diamond-Like Carbon were obtained recently, opening the way to large applications towards functional materials. The characteristics of the plasma are a well-suited signature of the physics of laser-matter interaction and plasma plume creation and expansion. Recent studies on the control of the film growth and femtosecond PLD processes will be reported. Emphasis on actual capability of the existing sources to elaborate high quality materials will be questioned in terms of energy per pulse, time width, repetition rates but also in the need for further source development and beam shaping improvement.

We study the formation of carbon nanoclusters created by MHz repetition rate - picosecond laser pulses. We show that the average size of a nanocluster is determined exclusively by single laser pulse parameters and is largely independent of the gas fill (He, Ar, Kr, Xe) and pressure, in a range from 20 mTorr up to 200 Torr. We provide evidence of the formation of large clusters at higher pressures in excess of 400 Torr, where the gas fill density is comparable or higher to the density of carbons in the ablated plume, and use simple kinetic theory to estimate cluster sizes, which are in qualitative agreement with the experimental data. We conclude that at pressures well below 400 Torr, the role of the buffer gas is to induce a transition between thin solid film formation on the substrate and nanofoam formation by diffusing the clusters through the gas, with no significant effect upon the average cluster size. At the higher pressure the buffer gas serves as a confiner for the carbon plume, increasing the collision frequency between the carbon atoms and resulting in cluster size growth. We also introduce preliminary ICCD imaging results investigating the temporal evolution of the laser plume.

We present the experimental and theoretical studies of the optical response from the single-crystal of bismuth to the excitation by the femtosecond laser pulse. The experimental results revealed a complex, first - positive and a few picoseconds later - negative, change in time-dependent reflectivity, which could not be explained in the light of the existing theories. It is shown that reflectivity oscillations are related to the excitation of coherent phonons by the pulse with duration shorter of all relaxation times. We demonstrate that swiftly heated electrons are responsible for the phonon excitation due to the fast modification of the attractive (electronic) part of inter-atomic potential. The electronic perturbation of potential is also responsible for the red shift of phonon frequency and for the increase in the amplitude of phonons. The coherent phonon excitation process as well as the change in the reflectivity is related mainly to the modification of the electron-phonon momentum exchange frequency. The comparison between the theory and experiments shows an excellent agreement. Moreover, the reflectivity measurements allow direct recovery of the electron-phonon coupling rate in bismuth crystal, which has not been measured before.

We report on the development of novel high-speed techniques to measure the surface topography and instantaneous velocity of ablatively launched thin metal layers with sub-nanosecond temporal resolution. Applications for laser detonator technology require the understanding of laser fiber optical energy deposition and ablative launch of a thin metal layer into an explosive. Characterization of the ablation process requires a time-resolved diagnosis of the ejected material state (topography, velocity, density, pressure, etc.). A pulsed Nd:YAG fibercoupled laser is used to ablate a 250 nm layer of titanium deposited on a 500 μm thick fused silica substrate at fluences below 10 J/cm². Time-resolved imaging of the free expansion of the metal surface is accomplished with Fourier plane imaging using a Shack-Hartmann lenticular array coupled to a fast framing camera. The imager performs topographical surface measurements by detecting changes in the optical wavefront of a reflected picosecond probe laser beam off the expanding surface. Consequently, single-event sub-nanosecond time-resolved "movies" of surface motion dynamics are captured. Crosscheck of the Shack-Hartmann imager is done using advanced velocimetry. A 1550 nm heterodyne laser-based Photonic Doppler Velocimeter is used to measure surface velocity. Using a 1550 nm single mode fiber laser, 10 GHz InGaAs detectors and telecom hardware, we directly record the resulting beat signal produced by the accelerated surface onto a fast digitizer. Free surface velocities as high as 6.5 μm/ns are recorded. Comparisons between the dynamic topography, surface velocimetry and laser hydrocode simulations are presented.

Using the Born-Oppenheimer approximation we calculate potential energy surfaces of photoexcited Bismuth-bulk. We determine phonon frequencies and potential anharmonicities for the case of high density of excited carriers. In particular, we focus on the phonon modes A_1g and E_g. We find strong softening of the A_1g-frequency for increasing excited carrier density. Furthermore, from the analysis of the lattice motion upon excitation we show that there is a coupling between the A_1g and E_g modes, which is consistent with recent experimental findings.

When transparent solids are irradiated with laser intensities above a certain threshold, strong absorption of laser energy occurs. The increasing absorptivity is caused by the formation of a free electron gas in the conduction band of the dielectric. The transient free electron density is a fundamental parameter for numerous theoretical and experimental investigations and applications. We study the mechanisms of free-electron generation in the frame of different approaches. A full kinetic treatment reveals a non-stationary behavior, which is neglected when applying the standard rate equation. A new model, the multiple rate equation, keeps track of the non-stationarity of the electron energy distribution while maintaining the conceptual and analytic simplicity of standard rate equation. We present the analytical asymptotic solution of the multiple rate equation which yields an expression for the avalanche coefficient and provides information about the validity of the standard rate equation. The numerical calculation shows the transient distribution of free electrons and the effect of the non-stationarity of its shape on the impact ionization probability. We study the role of different ionization processes and its dependence on laser pulse duration. The fraction of impact-ionized electrons is found to depend only on the product of pulse duration and intensity, i.e. on the fluence. A remarkable effect of the shape of the laserpulse on the total free electron density and the conditions for dielectric breakdown is found.

The past few years have seen a rapid growth in the development and exploitation of X-ray diffraction on ultrafast time-scales. One area of physics which has benefited particularly from these advances is the the field of shock-waves. Whilst it has been known for many years that crystalline matter, subjected to uniaxial shock compression, can undergo plastic deformation and, for certain materials, polymorphic phase transformations, it has hitherto not been possible to observe the rearrangement of the atoms on the pertinent timescales. We have used laser-plasma generated X-rays to study how single crystals of metals (copper and iron) react to uniaxial shock compression, and observed rapid plastic flow (in the case of copper), and directly observed the famous alpha-epsilon transition in Iron. These studies have been complemented by large-scale multi-million atom molecular dynamics simulations, yielding significant information on the underlying physics.

The temperature dependences of the electron heat capacity and the electron-phonon coupling factor are investigated for Au based on the electron density of states obtained from ab initio electronic structure calculations. Thermal excitation of d band electrons leads to a significant (up to an order of magnitude) increase in the electronphonon coupling factor and makes a considerable contribution to the electron heat capacity in the range of electron temperatures typically realized in femtosecond laser material processing applications. Simulations performed with a combined atomistic-continuum method demonstrate that the increase in the strength of the electron-phonon coupling at high electron temperatures leads to a faster lattice heating, generation of stronger thermoelastic stresses, and a significant decrease in the time of the onset of the melting process. The timescale of the melting process predicted in the simulation accounting for the thermal excitation of d band electrons is in excellent agreement with the results of recent time-resolved electron diffraction experiments. A simulation performed with commonly used approximations of a constant electron-phonon coupling factor and a linear temperature dependence of the electron heat capacity, on the other hand, significantly overpredicts the time of the beginning of the melting process, supporting the importance of the electron density of states effects and thermal excitation of lower band electrons for realistic modeling of femtosecond pulse laser processing.

Experiments on pulsed laser vaporization of many different kinds of polymers have demonstrated that it is possible to eject intact polymers into the ambient, whether air or vacuum, by resonant pulsed laser excitation, using both neat and matrix targets. Two recent studies of resonant infrared ablation - one on polystyrene, the other on poly(amic acid), the precursor for the thermoset polyimide - show moreover that the ablation process is both wavelength selective and surprisingly non-energetic, especially compared to ultraviolet laser ablation. We propose a wavelength-selective photothermal mechanism involving breaking of intermolecular hydrogen bonds that is consistent with these observations.

High Brightness and Power Diode Pumped Nd:YAG Laser is the engine for Laser Produced X-ray (LPX). The LPX system consists of a compact diode pumped Nd:YAG laser system which produces brightness in excess of 10¹⁵ W/cm² on target at up to 300 watts average power (4 parallel beams). The MOPA (Master Oscillator Power Amplifier) laser system delivers 250 mJ/pulse @ 300 Hz per beam (75 W average). 4 beams system at 300 W average power was demonstrated. The high brightness is achieved using ~800 ps pulse duration and near diffraction limited beam quality. Very high conversion efficiency (~9%) into 2π sr from 1064 nm laser output to 1 nm broadband x-ray was demonstrated using copper tape target. 1 nm x-rays were used for proximity x-ray lithography and demonstrated feature size resolution down to 75 nm. Narrow linewidth x-ray (λ/Δλ~600) has also been demonstrated with 1% conversion efficiency from 532 nm to 3.37 nm using Mylar tape target for compact high resolution and contrast x-ray tomography for biological cells. A unique mechanical shutter was developed to stop all the ablated debris from the target material that can damage and contaminate the fragile x-ray optics.

The non-equilibrium transformations induced by sub-picosecond lasers on space scale of nanometers and time scale of less than picosecond are considered in this presentation. We demonstrate that the fast (during the pulse time) change in the inter-atomic potential due to the electrons excitation is responsible for the swift coherent atomic displacement. We calculate the coherent displacement of atoms in non-equilibrium and compare it to that following from the familiar Lindemann approach to the melting in thermodynamic equilibrium. We compare our analysis with the experiments on non-equilibrium phase transformation of Gallium by 150 fs pulses at intensity well below the ablation threshold. The presented analysis and direct measurements indicate that the melting in its conventional sense either is not completed, or that, most probably, some transient state of matter has been created during the interaction even when the deposited energy exceeds three times the equilibrium enthalpy of melting. In conclusion we address some unresolved problems in understanding of ultra-fast phase transformations induced by ultra-short laser pulses in non-equilibrium conditions.

The laser-induced plasma-assisted ablation (LIPAA) process developed by our group, in which a single conventional pulsed laser is only used, makes it possible to perform high-quality and high-speed glass microfabrication. Up to the present, this process has been widely applied for micromachining of various transparent hard and soft materials. In this process, the laser beam first passes through the glass substrate since the laser beam has no absorption by the substrate. Then, the transmitted beam is absorbed by a solid target (typically a metal), located behind the substrate so that the target is ablated, resulting in plasma generation. Due to the interaction of the laser beam and the laser-induced plasma, significant ablation takes place at the rear surface of the substrate. Recently, we have developed the proto-type LIPAA system using a second harmonic of diode pumped Q-switched Nd:YAG laser for the practical use. In this paper, we demonstrate micromachining, crack-free marking and color marking of glass materials. Additionally, selective metallization of glass and polyimide by the LIPAA process followed by metal chemical-plating is investigated. A possible mechanism of LIPAA is also discussed based on the results from double pulse irradiation using near-IR fs laser, transient absorption measurement and plasma-conductivity measurement.

The micro laser plasma thruster (μLPT) is a micro propulsion device, designed for the steering and propelling of small satellites (1 to 10 kg). A laser is focused onto a polymer layer on a substrate to form a plasma. The thrust produced by this plasma is used to control the satellite motion. To understand the influence of the specific properties of the polymers, three different "high"- and "low"-energetic polymers were tested: poly(vinyl chloride) (PVC) as a low-energetic reference polymer that showed the best properties among commercial polymers, a glycidyl azide polymer (GAP), and poly(vinyl nitrate) (PVN) as high-energetic polymers. It was necessary to dope the polymers with carbon nanoparticles or an IR-dye to achieve absorption at the irradiation wavelength in the near IR. Decomposition into smaller fragmentation was measured for the energetic polymers than for PVC corresponding well to the higher momentum coupling coefficient of the energetic polymers, which indicates that more thrust can be gained from a chosen incident laser power. The measurements of the kinetic energies of selected decomposition fragments revealed no significant difference between the different carbon doped polymers. Only for GAP with the IR-dye a change in the ratio between ions with different kinetic energy was observed with increasing fluence. More C⁺ ions with higher kinetic energy were detected at higher fluences. No correlation between the kinetic energies of the ablation products and the specific impulse could be established for the obtained data.

Non-solid and non-rigid optics employ gas and liquid transmission and reflection, as well as flexible membranes to influence laser beams, laser driven particle beams and harmonic generators. Some examples are acoustic gratings, phase conjugate mirrors, Raman cells, gas-jets, gas and flame lenses, gas capillaries, plasma cones, mercury mirrors and rotating and aerodynamic windows. Industrial scale laser propulsion, laser fusion, laser accelerators, lithography and laser isotope separation will necessitate handling average beam powers very different from present day single shot demonstrations. Standard solid state optics may prove incapable of handling such conditions.

Properties of plasma produced in volume nanosecond high-pressure discharge and its formation conditions under elevated pressure in the gap with the cathode having small curvature radius have been investigated. Energy distribution of beam electrons and X-ray quanta of a volume nanosecond discharge in atmospheric pressure air have been studied. Several groups of runaway electrons were registered. It is shown that the basic contribution to beam current amplitude measured behind thin foils is given by the electrons having the energies of tens - hundreds of keV (smaller than amplitude of the maximal gap voltage). It has been confirmed that the fast electrons with the energies of units - tens of keV appear by 100-500 ps ahead of the main peak of the beam current that leads an increase of current pulse duration and amplitude. It is shown that the electrons with anomalous energies (above the maximal voltage amplitude in a gap) give minor contribution to beam current (less than 5%). Spectra of X-ray radiation generated in various gas diodes have been analyzed. Measuring methods of current amplitude of a subnanosecond electron beam and a formation mechanism of fast electrons and runaway electrons in volume discharges in high-pressure gases are being analyzed too.

Singlet delta oxygen O₂(a¹Δ_g) (SDO) production in a slab discharges ignited in oxygen gas mixtures was experimentally studied. An influence of gas mixture content and input electric power on SDO yield was analyzed. In self-sustained RF slab discharge SDO yield of 10±5% was measured by comparison luminescence intensity of SDO going from a chemical generator and SDO generated in electric discharge. SDO yield of 7.2% was measured by intracavity laser spectroscopy method. It was demonstrated that the choice of electrodes material is very important. Experiments on SDO production in slab non-self-sustained discharge with external ionization by repeating high-voltage pulses were carried out. SDO concentration was measured by the method of intracavity laser spectroscopy. The measured concentration of SDO was about 1.5 10¹⁶ cm^-3, with SDO yield of ~10.6%. A development of electric discharge oxygen-iodine laser with SDO production in long electrodes slab discharge is discussed.

We describe the physical principles and architecture of a multi-stage picosecond terawatt CO₂ laser system, PITER-I, operational at Brookhaven National Laboratory (BNL). The laser is a part of the DOE user's facility open for international scientific community. One of the prospective strong-field physics applications of PITER-I is the production of proton- and heavy-ion beams upon irradiating thin-film targets and gas jets. We discuss the possibilities for upgrading a CO₂ laser to a multi-terawatt femtosecond regime.

Laser micromachining has great potential as a MEMS (micro-electro-mechanical systems) fabrication technique because of its materials flexibility and 3D capabilities. The machining of deep polymer structures with complex, well-defined surface profiles is particularly relevant to microfluidics and micro-optics, and in this paper we review recent work on the use of projection ablation methods to fabricate structures and devices aimed at these application areas. In particular we focus on two excimer laser micromachining techniques that are capable of both 3D structuring and large-area machining: synchronous image scanning (SIS) and workpiece dragging with half-tone masks. The methods used in mask design are reviewed, and experimental results are presented for test structures fabricated in polycarbonate. Both techniques are shown to be capable of producing accurately dimensioned structures that are significantly deeper than the focal depth of the projection optics and virtually free from fabrication artifacts such as the steps normally associated with multiple-mask processes.

The demand on performance for displays and opto-electronics is ever increasing and the industry is looking for ways to produce large area microoptical films to help that cause. While conventional techniques are reaching their limits for large area structuring, earlier reports show that it is possible to structure a few m² polymer film with microoptical features (>20 μm) by direct laser ablation. By employing the same optics and hardware studies were carried out to find the minimal feature size possible without compromising the area that can be processed. Looking at the sub-resolution ablation behaviour of Polycarbonate enables us to modify the so-called Synchronised Image Scanning (SIS) mask design to control shape and form of 3D-features only a few times bigger than the resolution limit of the laser ablation mask projection system. Results of optical 10μm and 5μm features are shown and discussed. The findings show that it is realistic to direct laser cut well defined optical 3D-features into polymer film with an unprecedented feature-area-ratio in excess of 1:10¹⁰.

We describe here the modification of various polymers (polytetrafluoroethylene, polyethyleneterephthalate, and polyvinyl alcohol) by UV-irradiation with wavelengths below 200 nm in an inert or reactive atmosphere. The light sources employed are F_2- or excimer lasers and excimer lamps. The reactive gases include ammonia (NH₃), acetylene (C₂H₂), and oxygen (O₂). Photo-dissociated fragments of these gases can react with the polymers or be deposited thereon, resulting in new chemical groups at the surface. Special emphasis is put to improved adhesion of biological cells at these modified surfaces. Potentials applications include cell coated medical implants and prostheses as well as cell micro-arrays for high throughput screening.

Femtosecond laser pulses have the unique ability to deposit energy into a microscopic volume in the bulk of a material that is transparent to the laser wavelength without affecting the surface of the material. Here we review the use of this capability to disrupt specifically targeted structures in live cells and animals with the goal of elucidating function and modeling disease states. Particular attention will be paid to recent work that uses femtosecond laser disruption to injure cerebral blood vessels that lie below the brain surface in a live, anesthetized rat. By varying the degree of injury, the vessel can be made to leak blood plasma, to rupture, or to clot. This technique thus provides a versatile model of cerebrovascular disorders such as small-scale stroke.

Microstructuring of polymers and biopolymers is of application in medical technology and biotechnology. Using different fabrication techniques three-dimensionally shaped and micro structured constructs can be developed for drug release and tissue engineering. As an alternative method, laser microstructuring offers a series of advantages including high resolution capability, low heat deposition in the substrate and high level of flexibility. In this work we present evidence of laser microfoam formation in collagen and gelatine by nanosecond pulsed laser irradiation in the UV at 248 and 266 nm. Irradiation at 355 nm produces melting followed by resolidification of the substrate, whereas irradiation at 532 and 1064 nm induces the formation of craters of irregular contours. Single pulse irradiation of a collagen film with an homogenized KrF microbeam yields a 20 μm thick expanded layer, which displays the interesting features of a nanofibrous 3-dimensional network with open cells. In gelatine, irradiation at 248 and 266 nm produces similar morphological modifications. The effect of the structural properties of the substrate on the laser induced microfoam is studied by comparing gelatines differing in gel strength (Bloom values 225 and 75) and in crosslinking degree. While results are discussed on the basis of thermal and photomechanical mechanisms and of the role played by the water content of the substrates, it is thought that such structures could have a biomimic function in future 3D cell culture devices for research.

In dental hard tissue ablation, ultra-short laser pulses have proven sufficiently their potential for material ablation with negligible collateral damage providing many advantages. The absence of microcracks and the possibility to avoid overheating of the pulp during dental cavity preparation may be among the most important issues, the latter opening up an avenue for potential painless treatment. Beside the evident short interaction time of laser radiation with the irradiated tissue, scanning of the ultra-short pulse trains turned out to be crucial for ablating cavities of required quality and shape. Additionally, such a technique allows to treat larger areas like the ones dentists are used to work with, i.e. ~ 1 mm Ø.In this paper, an overview of different scanning methods together with the algorithms used and an assessment of their applicability is presented. A variety of pulse durations from ~100 fs up to several ps has been used by numerous authors over the last approximately ten years. Having employed 330 fs pulses, we present the corresponding ablation thresholds for dental hard tissue (enamel, dentine; human and bovine), for a number of dental restoration materials, as well as for different types of bovine bone. Dental implants frequently have to be cleaned from plaque being deposited around their necks in areas where the gums have already retreated. A preliminary investigation is presented on the applicability of ultra-short pulses with mentioned duration for the gentle cleaning of titanium implants focusing on the preservation of the special plasma-sprayed biocompatible implant surface.

We report on master-oscillator/power-amplifier (MOPA) systems featuring Yb-doped photonic crystal fiber (PCF) amplifiers and designed to generate high peak and average powers in beams of good spectral and spatial quality. In the first setup, a 1064nm-wavelength Q-switched Nd:YAG micro-chip laser emitting 500ps pulses at pulse repetition frequency (PRF) ~13.5-kHz is amplified in a 40μm-core, Yb-doped PCF to obtain peak power ~1.1 MW in a diffraction-limited beam (M² = 1.05) of peak spectral brightness ~ 10 kW/(cm² sr Hz). In the second setup, a 1ns-pulse Nd:LSB microchip laser (1062nm wavelength, single frequency, PRF = 9.6 kHz) is used to seed a dual-stage amplifier featuring the same 40μm-core Yb-doped PCF as the final stage. From this MOPA, we obtained diffraction-limited pulses of 1.1mJ energy, peak/average power ~1.1 MW/~10.2 W, and spectral linewidth ~9GHz. In the third setup, a single-polarization, single-transverse-mode Yb-doped PCF was used as the final amplifier, which permitted to obtain high-peak-power, spectrally narrow, 100:1 linearly-polarized pulses directly usable for wavelength conversion. The PCF output was frequency doubled, tripled, and quadrupled in nonlinear crystals to generate peak power in excess of 400 kW in the visible and ~200 kW in the UV. Finally, in the fourth setup, an Yb-doped, 70μm-core, intrinsically single-mode photonic-crystal rod (PCR) was used to generate near-diffraction-limited (M² ~ 1.1) ~1ns pulses of 2.25mJ energy and peak-power >2.2 MW.

Laser action at 1315 nm on the I(²P_1/2) → I(²P_3/2) transition of atomic iodine has been obtained by a near resonant energy transfer from O₂(a¹Δ) produced using a low-pressure electric discharge. In the electric discharge oxygen-iodine laser (ElectricOIL) the discharge production of atomic oxygen, ozone, and other excited species adds significantly higher levels of complexity to the post-discharge kinetics which are not encountered in a classic purely chemical O₂ (a¹Δ) generation system. In this paper, the discharge species output for laser operating conditions are discussed. Spatial measurements of O₂(a¹Δ) and O₂ (b¹Σ) are reported, and various methods for the determination of atomic oxygen levels are discussed and compared. The injection of NO_X into the system to benefit O₂(a¹Δ) production is investigated.

The generator of singlet oxygen (SOG) remains still a challenge for a chemical oxygen-iodine laser (COIL). Hitherto, only chemical generators based on the gas-liquid reaction system (chlorine-basic hydrogen peroxide) can supply singlet oxygen, O₂(¹Δ), in enough high yields and at pressures to maintain operation of the high power supersonic COIL facilities. Employing conventional generators of jet-type or rotating disc-type makes often problems resulting mainly from liquid droplets entrained by an O₂ (¹Δ) stream into the laser cavity, and a limited scalability of these generators. Advanced generator concepts investigated currently are based on two different approaches: (i)O₂(¹Δ) generation by the electrical discharge in various configurations, eliminating thus a liquid chemistry, and (ii) O₂(¹Δ) generation by the conventional chemistry in novel configurations offering the SOG efficiency increase and eliminating drawbacks of existing devices. One of the advanced concepts of chemical generator - a spray SOG with centrifugal separation of gasliquid phases - has been proposed and investigated in our laboratory. In this paper we present a description of the generator principle, some essential results of theoretical estimations, and interim experimental results obtained with the spray SOG.

The chemical oxygen-iodine laser (COIL) with a chemical method of atomic iodine generation was studied. Two methods of atomic iodine generation were proposed and developed. They are based on fast reactions of gaseous hydrogen iodide with chemically produced chlorine or fluorine atoms. Atomic iodine formation via Cl atoms we studied earlier by mixing of reaction gases directly in the primary O₂(¹Δg) flow in COIL. A revealed oxidation of HI by singlet oxygen and the O₂(¹Δg) quenching by some reaction product, however, reduced the attainable laser gain. This problem could be avoided by atomic iodine generation in separate reactors with following injection of atomic iodine into the primary O₂(¹Δg) flow. Gain measurements using this arrangement are presented in this paper. New experimental results on atomic iodine production via F atoms are also summarized. Using of reactive gases commercially available in pressure cylinders is the main advantage of this method.

Since the early 1970s ablative laser propulsion (ALP) has promised to revolutionize space travel by reducing the 30:1 propellant/payload ratio needed for near-earth orbit by up to a factor of 50, by leaving the power source on the ground. But the necessary sub-ns high average power lasers were not available. Dramatic recent progress in laser diodes for pumping solid-state lasers is changing that. Recent results from military laser weapons R&D programs, combined with progress on ceramic disk lasers, suddenly promise lasers powerful enough for automobile-size, if not space shuttle-size payloads, not only the 4 - 10 kg "microsatellites" foreseen just a few years ago. For ALP, the 1.6-μm Er:YAG laser resonantly pumped by InP diode lasers is especially promising. Prior coupling experiments have demonstrated adequate coupling coefficients and specific impulses, but were done with too long pulses and too low pulse energies. The properties of ions produced and the ablated surface were generally not measured but are necessary for understanding and modeling propulsion properties. ALP-PALS will realistically measure ALP parameters using the Prague Asterix Laser System (PALS) high power photodissociation iodine laser (λ = 1.315 μm, E_L ≤1 kJ, τ ~ 400 ps, beam diameter ~29 cm, flat beam profile) whose parameters match those required for application. PALS' 1.3-μm λ is a little short (vs. 1.53-1.72 μm) but is the closest available and PALS' 2ω / 3ω capability allows wavelength dependence to be studied.

A combination of techniques including launch ballistics, force sensing, and time-resolved ICCD imaging was applied to the study of the mechanisms of liquid ablation in the irradiance regimes from 10⁶-10⁸ W/cm². A TEA CO₂ laser (λ = 10.6 μm), 300 ns pulse width and 9 J pulse energy, was used for ablation of liquids contained in various quartz glass containers in order to examine dependencies on surface tension, absorption depth, etc. Dominant mechanisms of force generation were analyzed in order to determine their characteristics, and the evolution of the liquid surface was studied in depth. Net imparted impulse and coupling coefficient were derived from the force sensor data and ballistics experiments, and relevant results will be presented for various container designs and liquids used. The key differences between surface and volume absorbing liquids was observed. Various mechanisms including plasma formation, vaporization, bulk liquid flow, etc. will be critically examined and their relevance to force generation and propulsion will be determined.

A fundamental study on laser-electrostatic hybrid acceleration thruster was conducted, in which laser-induced plasmas were induced through laser beam irradiation on to a solid target and accelerated by an electrostatic field of an acceleration electrode. Also, a fundamental relativistic consideration of thrust performance for the high-specific-impulse thrusters was conducted. For thrust measurements, a calibrated torsion-balance type thrust-stand was developed and utilized. A time-of-flight measurement with a Faraday cup was also conducted for ion current and velocity measurements. It was shown that an acceleration electrode with positive potential was more effective than that with negative potential for positive-ion acceleration in the laser induced plasma, in which ions were induced with the Coulomb explosion.

Confined plasma ablation for shock physics, plate launch, and material dynamics has several unique advantages over direct-drive laser plasma that allows the freely expanding plasma away from the ablated surface, thus reducing energy coupling to the surface of the specimen. By confining the plasma with a laser-transparent, high-impedance window, energy coupling from the plasma to the specimen under test can be increased by several orders. However, for confined ablation, the laser power density and energy fluence must be kept below the break down threshold of the confining window material. Confined ablation to accelerate flyer plates decouples the laser parameters from the shock profile imparted in a target by the plasma-accelerated flyer plate.

A six degree-of-freedom (DOF) dynamic model was developed to provide insight into the flight behavior of Type 200 Lightcraft, and to serve as a research tool for developing future engine-vehicle configurations for laser launching of nano-satellites (1-10 kg). Accurate engine, beam, and aerodynamics models are included to improve the predictive capability of the 6-DOF code. The aerodynamic forces of lift, drag, and aerodynamic pitching moment were derived from Fluent ® computational fluid dynamics predictions, and calibrated against limited existing wind tunnel data. To facilitate 6-DOF model validation, simulation results are compared with video analysis of flights under comparable conditions. Despite current limitations of the 6-DOF model, the results compared well with experimental flight trajectory data.

In recent years Pulsed-Laser Deposition (PLD) has became increasingly popular as a viable deposition process for numerous materials. To overcome the main drawback of this method (macroscopic particles on the surface of the films), femtosecond (fs) lasers have been thought to be an ideal tool to obtain high quality thin films. However, it appeared that the nature of films grown by fs PLD strongly depends on the material and growth conditions. Indeed droplets are often observed evidencing the presence of violent thermal effects during the fs PLD process. In addition, the films are generally constituted by the random stacking of clusters in the 10-100 nm range that may be interesting for applications especially in the field of sensors and catalysis. In this paper, the experimental conditions leading to the formation of films composed of cluster piles without droplets will be presented. The results will be discussed as well as the possible explanations of the formation of clusters during fs PLD at the light of the literature produced in this field.

Reducing production time at high precision is key target for applying short pulse laser ablation technologies in the growing market of die and mould manufacturing. Required precision accuracy for laser ablation in this market is a dimensional resolution of IT 6, which means only ±4 μm deviation in reference to specified dimensions of 3mm. Dedicated investigations of different short pulse laser ablation systems (lamp and diode pumped Nd:YAG and diode pumped end pumped Nd:YVO4) indicate that cavities produced by these lasers can not match described specifications of precision. The process capability of standard industrial laser systems is insufficient. In this paper a model is introduced which describes the lateral accuracy of three dimensional mould cavities in dependency of pulse crater dimensions and their positioning accuracy. These influences were transferred to the statistical variance of following parameters: laser beam peak power and accuracy of scanner positioning. The stationary solution of the model indicates that pulse peak power stability according to known first pulse phenomena becomes a dominant influence in first order approximation. In second order approximation the infeed offset determines the dimensional lateral accuracy of cavities.

A high number of papers were published on the simulation of laser/surface interaction at the level of nanosecond scale. Several assumptions on thermal properties data, laser spot homogeneity, were assumed for describing as well as possible the boundary conditions, the mathematical writing and finally the numerical or the analytical results. A few tentative of surface temperature monitoring during laser processing were proposed for the numerical validation. Also, simulation of the melting kinetics is rarely directly compared to in situ experiments. It is very hard to determine the time duration of a melting pool by in situ experiments. It should be the same for the surface temperature. A new method to plot the thermal history of the surface by using a combination of the Time Resolved Reflectivity (TRR) and the Pulsed Photo-Thermal (PPT) or Infrared Radiometry (IR) methods is proposed in this paper. Surface temperature, melting kinetics, threshold of melting and threshold of plasma formation are determined in the case of KrF laser spot in interaction with several materials. In the first step, the experimental setup including fast detectors (IR, UV, Vis.) and related optical devices is described. In the second step, typical results (TRR and IR spectra) for monocrystaline silicon are presented and discussed. Namely, phase change transitions (melting and resolidification) are detected versus fluence change and number of laser shots change. TRR and IR spectra of metallic surfaces (Cu, Mo, Ni, Stainless steel 15330 and 17246, Sn, Ti), are measured. For each sample the surface temperature during heating, the threshold of melting, melting duration and the threshold of plasma formation are directly deduced.

Although significant work has been conducted in order to explore the optimum conditions of application of LSP treatments and to assess their capability to provide enhanced mechanical properties, only limited attempts have been developed in the way of predictive assessment of the characteristic physical processes and transformations with a specific consideration of the associated laser interaction dynamics. For this reason, additionally to the authors' efforts in the line of the numerical predictive assessment of the effects induced by the LSP technique from a predominantly mechanical point of view, the observation and analysis of the plasma dynamics following laser interaction have been envisaged as a means for the proper assessment of the shocking process relative to the material target and also as a way of plasma dynamics control in view of process optimization. In the present paper, the basis for the plasma spectroscopic observation of LSP experiments in combination to numerical dynamics simulations are proposed as a means for the extraction of relevant guidelines for process design optimization.

The aim of the present study is to clarify the time-dependent characteristics of the impulsive force generated by irradiating a laser pulse onto metallic and polymer materials. A Velocity Interferometer System for Any Reflector (VISAR) is employed to measure the acceleration driven by the laser ablation. The VISAR has two delay-lines that enable the velocity measurement in the range from 10 m/s to 100 m/s. The ablation impulse is inferred from the measured acceleration history. The influence of the ambient air on the ablation pressure is investigated for aluminum using a Nd:YAG laser (wavelength: 1064 nm, pulse energy < 1 J, pulse duration ~ 10 ns) and for a polymer material using a CO₂ laser (wavelength: 1.06 μm, pulse energy < 10 J, pulse duration ~ 2 μs). The results of the preliminary experiments revealed the promising potential of the VISAR measurement.

We report here experimental results on laser ablation of metals in air and in vacuum in similar irradiation conditions. The experiments revealed that the ablation thresholds in air are less than half those measured in vacuum. Our analysis shows that this difference is caused by the existence of a long-lived transient non-equilibrium surface state at the solid-vacuum interface. The energy distribution of atoms at the surface is Maxwellian-like but with its high-energy tail truncated at the binding energy. We find that in vacuum the time needed for energy transfer from the bulk to the surface layer to build the high-energy tail, exceeds other characteristic timescales such as the electron-ion temperature equilibration time and surface cooling time. This prohibits thermal evaporation in vacuum for which the high-energy tail is essential. In air, however, collisions between the gas atoms and the surface markedly reduce the lifetime of this non-equilibrium surface state allowing thermal evaporation to proceed before the surface cools. We found that ablation threshold in vacuum corresponds to non-equilibrium ablation during the pulse, while thermal evaporation after the pulse is responsible for the lower ablation threshold observed in air. This paper provides direct experimental evidence of how the transient surface effects may strongly affect the onset and rate of a solid-gas phase transition.

In order to control the technique of laser-induced forward transfer (LIFT) in ultrashort regimes, it is necessary to understand the different basic mechanisms involved during the three steps: ablation-transfer-deposition. Back ablation of Cr thin film has been studied and compared to the front ablation of the same film in the same conditions. Experiments have been performed using ultrashort laser pulses (800 nm, 100 fs). The dynamics of the plumes have been monitored with a gated intensified charge coupled device (ICCD) camera. Image analysis gave us indications on the velocity and the composition of the ejected material. A parametric study of the ablation thresholds and ablation dynamics has been carried out as a function of the incident laser fluence and the thickness of the metal layer. These results contribute to optimize a process of LIFT. Transfers of Cr on glass and Silicon were obtained with a good spatial resolution.

This paper concerns the study of growth and characterizations of Co₂YZ with Y an element of transition metal group and Z, a III-V group element deposited onto Si, GaAs and InAs substrates. Two PLD configurations have been explored, the conventional 1-Beam-PLD and the 2-Crossed-Beams-PLD one. We demonstrated that depending on the configuration we got Co₂YZ polycrystalline structure with unwanted droplets or droplet-free, single crystalline oriented thin films at substrate temperature as low as 353 K. Optical conductivity and magnetic properties are presented.

Interaction of TW-ps laser with plasma results in a skin layer mechanism for nonlinear (ponderomotive) force driven two dimensional plasma blocks (pistons) if a very high contrast ratio is provided for suppression of relativistic self-focusing. This Skin layer acceleration (SLA) [1] results in space charge neutral plasma blocks with ion current densities larger than 10¹⁰ Amp/cm² [1-3]. Using Ions in the MeV range results in 1000 times higher proton or DT current densities [3] than the proton fast igniter [4] is using and may result in better conditions of this fast ignitor scheme. Using ballistic focusing of the generated plasma blocks and a short time thermal expansion of these blocks for increasing their thickness while keeping the high ion current densities, results in conditions favourable for this option of fast ignition of a fusion target. Some details of the interaction processes are still to be analysed but the solutions studies to date are most encouraging.

Economic operation of the National Ignition Facility at the Lawrence Livermore National Laboratory depends on controlling growth of laser damage in the large, high cost optics exposed to UV light at 351 nm. Mitigation of the growth of damage sites on fused silica surfaces greater than several hundred microns in diameter has been previously reported by us using galvanometer scanning of a tightly focused 10.6 μm CO₂ laser spot over an area encompassing the laser damage. Further investigation revealed that fused silica vapor re-deposited on the surface as "debris" led to laser damage at unexpectedly low fluences when exposed to multiple laser shots at 351 nm. Additionally, laser power and spatial mode fluctuations in the mitigation laser led to poor repeatability of the process. We also found that the shape of the mitigation pit could produce downstream intensification that could damage other NIF optics. Modifications were made to both the laser system and the mitigation process in order to address these issues. Debris was completely eliminated by these changes, but repeatability and downstream intensification issues still persist.

A 20W 355nm DPSS Q-switched nanosecond pulse width laser, with external beam-splitting optics, was used to simultaneously ablate two 600μm deep, 140μm wide, 13.4mm long blind trenches in silicon using a five line wide cut strategy, achieving a 1.22x throughput improvement compared with a single-beam 20W laser configuration. Improved split-beam throughput was achieved because overhead time consisting of non-cut time during galvanometer retrace and turn-around movements and the time taken to ablate shoulder formations, were found to be approximately independent of laser power. With this split-beam approach, where two identical trenches are simultaneously cut, overhead time is split between the two trenches when cut time/die is calculated, halving the effective overhead time/die, and thereby improving throughput. Specific throughput improvement depends upon cut strategy, trench size, and insertion loss of the beam-splitting optics. Beam splitting optics consisted of a half-wave plate, Glan-laser polarizing prism, and mirror. Making use of the linear polarization characteristic of laser light, rotation of the half-wave plate was used to adjust the relative power in each beam, and thereby equalize the ablation rate of each beam. Adjustment of the mirror angle determined the separation between the two trenches.

Using a factorial design of experiments approach with ANOVA, laser drilling experiments were performed on the semiconductor mercury-cadmium-telluride (HgCdTe). A commercial CPA femtosecond laser system operating at 775nm was used for the experiments. The test variables include laser parameters such as pulse length, fluence, beam shaping using apertures, assist gas, vacuum, and others. The response variable examined for optimization include hole size, hole depth, and melt effects. The analysis yielded an empirical formula for predicting laser drilling effects.

RHINO, Real-time Histogram Interpretation of Numerical Observations, is a specialty algorithm and tool under development for the United States Air Force Office of Scientific Research (AFOSR). The intent is to provide real-time feedback for adaptive control of telescope pointing for ground-space-ground laser illumination experiments. Nukove together with New Mexico State University first established a proof-of-principle laboratory experiment using RHINO and, under a controlled environment, reduction of the pointing error known as boresight was demonstrated. RHINO is resilient to effects such as glints, speckle, and scintillation. The forthcoming commercially available version of RHINO will use real-time field data and provide adaptive control to the user. The utility of RHINO is evident in a realistic scenario: Consider a space asset that has been joined by a microsat, perhaps 0.5m in size. The microsat may have been launched to simply listen in from close proximity, monitor the asset, image the asset or most critically, cause damage to the asset. If the goal is to destroy the microsat by long-range illumination with a high power laser and the microsat is meters from the asset (μrads at 1Mm) laser pointing is of utmost importance as the goal is certainly not to damage the space asset. RHINO offers the capability to estimate key metrics of laser system pointing, known as jitter and boresight. The algorithms used have been under development for nearly a decade, have been established in a laboratory environment, and have been tested with field data.

The paper describes the theoretical and experimental investigation of high power Gas Dynamic Laser(GDL, λ~10.6μm) interaction studies with a pressurized hollow metal(MS) target. The design and development of such type of target which has been shown bursting as well as burning effect at the time of interaction have been carried out. It has been filled by gas mixture of H₂ and Air in the range of flammability limit. Various parameters like power density, target thickness, filling pressure, mixture ratio etc have been optimized. High mass flow GDL of power level about several KW in unstable mode provides power density about 3.2 KW/Cm² by a beam delivery system at distance 25m. Since target material is thin and heat diffuses through it rapidly, by maintaining the required power density, rupturing is accomplished by heating an area of the pressure vessel to a temperature at which it will fail under the pressure load. Rupture initiates a propagating crack which spreads the damage over a large fraction of the pressure vessel. The gas mixture ignites due to its contact with atmosphere and explodes with a massive sound level of the order of 130dB. The sound level was measured by a Decibel meter. Temperature distribution along radial and depth have been studied theoretically. Surface temperature during interaction has been measured. Experimental data has been validated with theory. These study shows a very attractive demonstration showing potentiality of scientific applications of High Power CO₂ Laser.

Various schemes of geometrical coupling between optical resonator and gain medium were investigated for a 10-kW class Chemical Oxygen-Iodine Laser (COIL). Starting from theory, different types of resonator layouts were designed and optimized for COIL with a rectangular gain medium and an output coupling of about 10%. Hybrid resonators match these coupling conditions more easily than concentric unstable resonators. Compared to the negative branch type, the positive branch hybrid resonator shows very high sensitivity to the optical alignment in the unstable direction but avoids a focal line within the resonator. The obtained output power of both hybrid resonators is compared to the output power of the COIL device in a conventional stable resonator configuration. Measured margins for the sensitivity of resonator setup and alignment were found in close agreement with numerical calculations. Power density distributions were measured in the near field and in the far field. The divergence of the emitted laser beam in the unstable direction was nearly diffraction limited.

It has become apparent in the last few years that the light ion surface contamination on short-pulse laser targets is a major impediment to the acceleration of heavier target ions. Mitigation strategies have been tested in experiments at the Los Alamos Trident Laser facility using one arm of the Trident laser at 150 ps to ablatively clean a large area of heated targets in a single short that are subsequently irradiated by the Trident 30 TW short-pulse arm to accelerate the bulk target ions to high energies. This process was used on targets consisting of 15 microns of vanadium. The 150 ps pulse rids the rear of the target of its omnipresent surface contamination layer, consisting mainly of water vapor and hydrocarbons, and allows the Trident 30 TW short-pulse arm to illuminate the target and accelerate ions via the Target Normal Sheath Acceleration (TNSA) mechanism. Because this mechanism relies on a laser generated electrostatic sheath, the ions with the lightest charge to mass ratio (i.e. protons) would be accelerated preferentially at the expense of heavier ions. However with the contamination layer removed, and hence the bulk of the available protons, the TNSA mechanism is able to accelerate the bulk material ions to high energies. Our experimental results are discussed and compared to the LASNEX rad-hydro code to validate and improve our predictive capabilities for future acceleration experiments.

Femtosecond (fs) ablation is mediated via electron avalanche and multiphoton ionization and is characterized by very precise cutting and undetectable thermal damage in biological tissues. We have used a 775nm, 150 fs, 1kHz laser system compared to two conventional bone cutting techniques using carbide and diamond tip burs in a mice calvarial wound healing model. Wound healing was evaluated using micro computerized tomographs and histological techniques. Good healing outcomes were found for fs laser surgery in comparison to the conventional surgical methods. However, the degree of healing was highly variable in all treatment groups. The realization of healing comparable to that observed for conventional surgical tools demonstrates the possible use of fs lasers for clinical surgery involving small bones where a much higher degree of precision is required than that possible with current methods.

This paper discusses the momentum coupling coefficient in the relativistic, collisionless realm as contrasted with the collisional, ablation dominated regime. It is shown the total momentum coupling coefficient is a result of the combined ion and photon momenta.

A preliminary study of combustion-augmented laser-ramjets was conducted, in which chemical propellant such as a gaseous hydrogen/air mixture was utilized and detonated with a focused laser beam in order to obtain a higher impulse compared to the case only using lasers. CFD analysis of internal conical-nozzle flows and experimental measurements including impulse measurement were conducted to evaluate effects of chemical reaction on thrust performance improvement. From the results, a significant improvement in the thrust performances was confirmed with addition of a small amount of hydrogen to propellant air, or in combustion-augmented operation.

Researchers have previously observed that tissue ablation with a free electron laser tuned to wavelengths between 6-7 μm is accompanied by remarkably little collateral damage. Attempts to explain these observations have invoked a wavelength-dependent loss of protein structural integrity; however, the molecular nature of this structural failure has been heretofore ill-defined. In this report, we evaluate several candidates for the relevant transition by analyzing the non-volatile debris ejected during ablation. Porcine corneas were ablated with a free electron laser tuned to either 2.77 or 6.45 μm - wavelengths that are equally well absorbed by hydrated corneas, but that respectively target water or protein as the primary chromophore. The ejected debris was characterized via gel electrophoresis, as well as FTIR, micro-Raman and ¹³C-NMR spectroscopy. We find that high-fluence (240 J/cm²) ablation at 6.45 μm, but not at 2.77 μm, leads to protein fragmentation. This fragmentation is accompanied by the accumulation of nitrile and alkyne species. Although these initial experiments did not detect significant protein unfolding, the loss of collagen triple-helix structure was evident using UV and vibrational circular dichroism. The candidate transition most consistent with all these observations is scission of the collagen protein backbone at N-alkylamide bonds. Identifying this transition is a key step towards understanding the observed wavelength-dependence of collateral damage.

Mechanisms of absorption wave of laser radiation and spectral characteristics of laser plasma were investigated at a laser breakdown in a normal atmosphere. Q-switched Nd:YAG laser operated at 1064 and 532 nm were used in experiment. Laser pulse consisted of prepulse and basic pulse for both laser wavelengths. Time interval between pulses was 15 ns, time duration (FWHM) was 4 ns for 532 nm wavelength, 5 ns for 1064 nm wavelength. Molecular emission and collapse of intensity of plasma continuum during the initial moments of laser plasma expansion were registered. It was carried out examination of colliding plasmas interaction which depended on absorption wave mechanism and distance between focal points of lens. The magnification of integrated intensity in case of plasmas interaction is registered. Decay time of continuous spectrum and lines emission was increased thus contrast of emission lines was also increased.

Laser propulsion has gained increasing attention in the recent years. Ultra-high average power laser systems have emerged and found applications in launching satellites to the space. The impulse generated by ablation can also be used to move small parts. This article describes laser-induced releasing of microelectronic components from its carrier material. The releasing mechanisms can be divided in: ablative and thermal releasing, depending from polymers, which are used as the component's carrier material and whether low or high laser fluence is used. The directional variation and speed variations under different operating conditions were studied and presented. Application of this technique as a fast microelectronics components assembly method is demonstrated.

We investigate the phase transitions of intense ultrashort laser-heated solids, from the cold solid to the hot dense plasma state, by measuring the complex electrical conductivity (or refractive index) transients at terahertz (1 THz = 10¹² Hz) frequencies. Using optical-pump, terahertz-probe spectroscopy, we measured the phase shifts and absorption of terahertz probe pulses that were reflected from the warm dense plasma. To characterize the THz field, we developed and used a single-shot, high-temporal-resolution THz diagnostic capable of measuring free-space electromagnetic pulse fields in time and space. Due to relatively large focal spot sizes of the THz probe (~mm), mainly limited by the diffraction properties of THz radiation, the optical pump pulse was weakly focused onto the target in order to overfill the THz probe spot size with a peak intensity of ~10¹³ W/cm². In contrast to the previous measurements of conductivities at optical frequencies, our THz non-contact probe method can directly measure quasi-DC electrical conductivities, providing insight into the transport nature of warm dense matter and any present discrepancies with the Drude model. In case of warm dense aluminum, we observe a noticeable deviation from the Drude model even in the ~10¹³ W/cm² laser intensity regime. In addition, we observe strong coherent THz emission produced by a current surge in the laser-produced plasma.

Nanohole processing of silicon substrate using surface plasmon polaritons of nano gold excited by femtosecond laser is described in comparison with the nanohole processing with transparent polystyrene (PS) nanoparticle template. Gold particles with diameters of 40, 80, or 200 nm are spin coated on the substrate, and a 100 fs, 820 nm laser pulse is used to irradiate the samples. The produced holes are analyzed by scanning electron microscopy and atomic force microscopy. A theoretical analysis of the experimental results is conducted by FDTD (Finite Difference Time Domain) simulation. The dependence of the laser fluence and particle size on the nanohole properties is studied. The nanohole profiles correspond to the field distributions on the Si substrate at low fluence region. A highest electric field enhancement factor of about 26 is obtained for gold particles with a diameter of 200 nm.

Sub- and microsecond relaxation dynamics of superheated surface layers of bulk water cavitating at near-spinodal conditions during heating sub-microsecond long TEA CO₂ laser pulses were studied using contact broad-band photoacoustic spectroscopy. Characteristic nanosecond pressure-tension cycles representing steam bubble oscillations were recorded by an acoustic transducer and corresponding oscillation frequencies were measured as a function of incident instantaneous laser fluence during the heating laser pulses. Fundamental oscillation frequencies-9-11 MHz-were found to remain nearly constant in a broad laser fluence range, corresponding to bubble diameters close to thickness of the superheated surface layers. Damped nanosecond and microsecond oscillatory pressure-tension cycles recorded by an acoustic transducer are related to oscillations of steam bubbles of different sizes exhibiting strong dissipative losses and collective (coalescence and percolation) phenomena. These observations demonstrate the apparent ultimate thermodynamic limit of superheating for bulk liquids near their liquid-vapor spinode curves and provide an important insight into basic thermodynamic parameters and spatiotemporal scales of explosive liquid/vapor transformations in absorbing fluids ablated by short laser pulses in the thermal confinement regime.

The threshold fluence for laser induced damage in wide band gap dielectric materials, such as fused silica and MgF₂, is observed to be lower by up to 20% for negatively (down) chirped pulses than for positively (up) chirped, at pulse durations ranging from 60 fs to 1 ps. This behavior of the threshold fluence for damage on the chirp direction was not observed in semiconductors, such as silicon and GaAs. Based on a model describing electron generation in the conduction band and Joule heating, it is suggested that the decrease in the damage threshold for negatively chirped pulse is related to the role of multiphoton ionization in wide gap materials.

Time-of-flight mass spectroscopy (TOF MS) spectra of ions ablated from BN-ceramic, amorphous carbon, graphite and fullerene-60 targets were recorded at different experimental conditions. It was found that the TOF MS spectra significantly simplify as the energy density of the laser radiation was increased, and show temporal evolution as the distance between the target surface and work area of TOF MS is increased. Moreover, the spectral line widths significantly increase with increasing laser radiation density and distance between target surface and work area of TOF MS. Analyses of these data allowed to estimate the average velocity of the ablated beam center of mass as well as the average energy of ions in the ablated plasma. Assignment of the spectral lines was also done. The data obtained can be used to optimize the synthesis nanostructured B-N and B-C-N materials by laser ablation.

Time-resolved force measurements and Intensified Charge-Coupled Device (ICCD) imaging techniques were applied to the study of force generation in the laser ablation of water and ice. A transversely excited atmospheric (TEA) CO₂ laser operated at 10.6 μm, 300 ns pulse width, and up to 20 J pulse energy was used to ablate water and ice held in various containers. Net imparted impulse and coupling coefficient were derived from force sensor data and relevant results will be presented for ice and water. ICCD imaging was used in conjunction with time-resolved force measurements in order to determine the dominating physical mechanism under which the thrust is produced. The effect of shock wave generation and propagation, as well as its contribution to the overall impulse imparted to the targets, was examined from the comparison of the timelines for the pertinent phenomena. The process of mass removal was investigated for each case, and specific impulse and efficiency were calculated from the data. Differences in the force-time curves for ice and water will be presented and discussed. Ballistic experiments were conducted in order to corroborate the force measurements.

We present estimates of the refractive-index change in waveguides in silica produced by focused femtosecond laser pulses. The estimates are based on the shift of the central frequency of ω₄ (TO) band (Si-O stretching mode) in micro-Raman spectra. These data were compared with the relation of this parameter to density and to refractive index changes in seen in glasses modified by high pressure or irradiation. We conclude that the measured refractive-index increase in the waveguides can be explained by densification of glass.

We successfully delivered a therapeutic vector construct, which carries hepatocyte growth factor (HGF) gene, to rat skin in vivo. After HGF expression vector had been intradermally injected to rat skin, LISWs were generated by irradiating the laser target put on the rat skin with nanosecond pulses from the second harmonics (532 nm) of a Q-switched Nd:YAG laser. Concentration of HGF protein increased by a factor of four by the application of LISWs when compared with that of control samples without LISW application. We also investigated the effects of LISWs on the integrity of plasmid DNA.

Pulsed-laser-based methods have been applied for post-implant annealing of p-type Al doped 4H-SiC wafers in order to restore the crystal structure and to electrically activate the doping species. The annealing was performed with the second (532nm) and third (355nm) harmonic of a Nd:YAG laser at 4ns pulse duration. The epilayers were characterized by micro-Raman spectroscopy under surface and cross sectional backscattering. Changes in the phonon mode-intensity were related to the laser annealing induced recrystallization of the implanted material. The results were compared with changes in the infrared reflectivity across the Restsrahlen band.

The extraordinary characteristics of excilamps (excimer and exciplex lamps) led to a great many of applications, which had been demonstrated in a number of previous studies. This review is principally dedicated to our advances in application of UV/VUV capacitive and barrier discharge excilamps in photoscience (photochemistry and photomedicine). In the present paper, the review of the basic results obtained at the Laboratory of Optical Radiation at High Current Electronics Institute SB RAS during 2003-2005 years is presented.

Laser and discharge parameters in mixtures of rare gases with halogens driven by a pre-pulse-sustainer circuit technique are studied. Inductive energy storage with semiconductor opening switch was used for the high-voltage pre-pulse formation. It was shown that the pre-pulse with a high amplitude and short rise-time along with sharp increase of discharge current and uniform UV- and x-ray preionization allow to form long-lived stable discharge in halogen containing gas mixtures. Improve of both pulse duration and output energy was achieved for XeCl-, XeF- and KrF excimer lasers. Maximal laser output was as high as 1 J at intrinsic efficiency up to 4%. Increase of radiation power and laser pulse duration were obtained in N₂-NF₃ (SF₆) and He-F₂ (NF₃) gas mixtures, as well.

Laser shock cleaning (LSC) has been proved an effective method to clean sub-micron and micron particles from solid surface during last five years. In this report, dynamics of the interaction between plasma shock wave and adhered spherical particles is analyzed in theory, considering the change of particle contact radius induced by the load of the shock wave. Analysis of the rolling mechanism at the initial contact of the shock wave with particles shows working gap has a serious influence to the cleaning and smaller diameter particles are more difficult to be removed with smaller cleaned area. Moreover, particle energy obtained from the shock wave is analyzed through which particle removal trace and cleaned area are studied combined reflection shock wave and irregular turnover of the particle into account. Removal of micron copper particles on a silica surface in air is experimented at different working gap. Results show that particles can be effectively removed within the suitable working gap, i.e., 0.8 mm for 150 mJ explosion energy, and higher working gap represents poorer cleaning efficiency. Moreover, the cleaning situation of the heavy contamination shows out an interesting phenomenon of the cleaned area (0.4cm²) profile that is an ellipse caused by the non-uniform pressure distribution of plasma shock wave.