Robert John Strutt, (1 875 — 1 947), fig. I was an experimental physicist of such exceptional skill and ingenuity in optics that, had he lived today, he would undoubtedly have made significant contributions to the fields of laser physics and laser spectroscopy. Eldest son of the Nobel laureate John William Strutt, third Baron Rayleigh (1 842 — I 91 9, Nobel prize I 904 together with Sir William Ramsay for the isolation of argon), he enrolled for the Natural Science Tnpos at Cambridge in 1894, completing both parts with first class honours before commencing research at the Cavendish Laboratory under Sir J. J. Thomson (Nobel prize I 906 for the discovery of the electron). Rapid progress and publication in his earliest fields of research, electrical discharges in gases and radioactivity, ensured his election as a Fellow of the Royal Society in I 905, 'at the exceptionally early age of 29', and he was one of the first (1 904), after Rutherford, to publish a book on the new field of radioactivity, i.e. Nuclear Physics at its inception1'2. A striking demonstration of the charge carried by /3 -rays was invented by Strutt, and shown at the family home (Terling Place in Essex, figs. 2 and 3) to both Ernest Rutherford and Lord Kelvin, the latter being 'wildly enthusiastic' at seeing a process which appeared to him to violate the Second Law of Thermodynamics1. Later dubbed Rayleigh's radioactive clock, this was an ingenious modification of the gold-leaf electroscope to incorporate radioactive samples of different concentration. In these early years, Strutt also provided the spectroscopic evidence that the radioactivity released in the natural springs in Bath in fact yielded the newly isolated atmospheric constituent helium (1904). A later discovery (1906), that the radium embedded in the earth's crust released sufficient thermal energy to account for the temperature gradient observed near the surface, was instrumental in overturning Lord Kelvin's calculation (1862) of the age of the Earth, and in bnnging the physical picture much closer to that provided by geological evidence (some 4 billion years).

Physical principles of ablation, that is a phenomenon of fast superficial evaporation of the solid or liquid due to energy deposition into a thin layer near a surface of the specimen, are discussed. The opportunity of studying ablation at a laboratory is directly related to invention of a laser, which is very intensive source of radiation energy. For the first time, the laser ablation was demonstrated by Maiman in 1960 just by using his ruby laser for perforation of a razor blade. Since then there has been demonstrated, that the laser ablation has very complicated physics and can be used for a lot of applications. This fact results from a tremendous variety of laser parameters. Variations of laser intensity in time, radiation spectrum, geometry of specimen irradiation, its chemical composition, etc., result in unique possibility of changing physical parameters of ablation and, hence, its physical features, which allow us to use laser radiation both in science and technology beginning from laser material processing up to high-temperature plasma formation for laser fusion.

The paper deals with some basic lightning properties and processes causing its ability of attraction to the high terrestrial structures. Condition of the vital upward leader and counterleader inception is formulated. Mechanism of the long-distance leader interaction is cleared out. Initiation of lightning by means of laser plasma channel as well as current demands in lightning science and protection practice discussed. This discussion is connected with current activities and recent results in the field of lightning initiation by laser. Conditions for such effects at high altitude are considered.

The recent progress of high power diode laser is opening new fields of laser and its application. We are developing high average power diode pumped solid state laser DPSSL for laser fusion power plant, for space propulsion and for various applications in industry. The common features or requirements of our High Average-power Laser for Nuclear-fusion Application (HALNA) are large pulse energy with relatively low repetition of few tens Hz, good beam quality of order of diffraction limit and high efficiency more than 10%. We constructed HALNA 10 (10J X 10 Hz) and tested the performance to clarify the scalability to higher power system. We have obtained in a preliminary experiment a 8.5 J output energy at 0.5 Hz with beam quality of 2 times diffraction limited far-field pattern.

Ultrashort laser pulses interacting with brittle dielectrics in the mid-infrared region of the spectrum produce a number of novel effects which are potentially useful in materials processing and analysis. These include the texturing of the surface, the generation of hydrodynamic instabilities, and a surprisingly efficient and gentle ablation behavior. Nevertheless, the mechanism of infrared laser ablation remains somewhat mysterious. Here we present evidence for a mechanism of explosive vaporization in fused silica, initiated by picosecond pulses from a tunable free-electron laser operating in the wavelength region from 2 - 10 micrometer. The unusual pulse structure of the free-electron laser -- which produces 1-ps micropulses at intervals of 350 ps in a macropulse lasting up to 4 microseconds -- makes it possible to test separately the effects of intensity and fluence. We show in particular that thermal descriptions of the ablation process fail in the regime where there is high vibrational excitation density in the solid due to resonant absorption of mid-infrared laser light.

The condensation of vapor within the expanding plume produced by ns-laser ablation is discussed in the framework of Zeldovich and Raizer theory of condensation. The spherical plume expansion is described by numerical solution of hydrodynamic equations by CIP method. This permits to analyze the role of initial distributions in density and pressure onto the size distribution function for nanoclusters. With parabolic distributions (for the plume expansion into vacuum), calculations reproduce the same results, as those found with particular solutions of gas-dynamic equations. When initial distributions deviate from parabolic, the size distribution function varies. For the rectangular plume, size distribution function for nanoclusters demonstrates to maximums. The similar effect can be found due to the ambient gas influence.

There are considered non-thermal and thermal processes of femtosecond laser-induced damage and ablation of wide band-gap transparent materials. Dominating of one or other of them depends on radiation and material parameters among which pulse repetition rate, focal spot size and absorption play key role. Non-thermal mechanisms of damage and ablation can dominate at initiating stage and at low repetition rates (below 10 kHz). They are attributed to nonlinear electrodynamical processes such as higher harmonic generation and formation of shock electromagnetic waves. Considering interaction of shock electromagnetic wave with a particle (single charged particle and a dipole) placed in potential well, we derive expression for threshold of laser-induced ionization and delocalization. Thermal mechanisms can dominate at later stages of damage and ablation at repetition rates above 10 kHz. It is considered possibility of their description within modified two-temperature model. There are also discussed after- heating and non-equilibrium non-thermal processes taking place between initiating and thermal stages. There are considered several mechanisms of laser-induced ionization -- multiphoton, tunneling, avalanche ionization, also ionization by higher harmonics and by shock-wave front. Estimations of ionization rates show that the latter two of them can dominate at the stage of initiating of femtosecond damage and ablation and determine critically following ionization processes. Obtained results are compared with experimental data.

There is considered influence of absorption on process of formation and evolution of shock electromagnetic wave (SHEW) in solids with different levels of absorption (dielectric, semiconductor, metal, plasma). Main aims of this work are (1) to show possibility of SHEW formation in absorbing materials, (2) to estimate value of absorption required for preventing formation of SHEW, (3) to study characteristic features of higher harmonic generation in transparent and absorbing materials during formation and propagation of SHEW. Analytical calculations and numerical modeling are based on full wave equation with specific terms describing plasma optical response, absorption and nonlinear dielectric response. Electromagnetic shock is shown to appear in absorbing materials with relatively low absorption but absorption changes dynamics of shock-wave front significantly as compared to the case of ideal nonabsorbing medium. Obtained results show abrupt increasing of threshold laser intensity required for formation of SHEW with increasing of absorption. If absorption exceeds certain level, formation of SHEW becomes impossible at reasonable laser intensity and focusing geometry.

The National Ignition Facility (NIF) will consist of 192 laser beams that have a total energy of up to 1.8 MJ in the third harmonic ((lambda) equals 0.35 micrometer) with the amount of second harmonic and fundamental light depending on the pulse shape. Material near best focus of the third harmonic light will be vaporized/ablated very rapidly, with a significant fraction of the laser energy converted into plasma x rays. Additional plasma x rays can come from the imploding/igniting capsule inside Inertial Confinement Fusion (ICF) hohlraums. Material from outer portions of the target, diagnostic components, first-wall material, and optical components, are ablated by the plasma x rays. Material out to a radius of order 3 cm from target center is also exposed to a significant flux of second harmonic and fundamental laser light. Ablation can accelerate the remaining material to high velocities if it has been fragmented or melted. In addition, the high velocity debris wind of the initially vaporized material pushes on the fragments/droplets and increases their velocity. The high velocity shrapnel fragments/droplets can damage the fused silica shields protecting the final optics in NIF. We discuss modeling efforts to calculate vaporization/ablation, x-ray generation, shrapnel production, and ways to mitigate damage to the shields.

Pulsed laser ablation appears as a promising technique for depositing thin films. A large variety of successful experimental results were obtained in this field, including the growth of high- temperature superconducting films, ferroelectric films, oxides, semiconductors, diamonds, etc. One of the main advantages of this technology is the relative simplicity of the experimental set-up and the possibility to get good homogeneity, complex stoichiometry materials and well adhesive dense layers. The main drawback seems to be the production of macroparticles, their transfer to the growing film inducing inhomogeneity and roughness onto the surface, lowering the properties of the thin films. In a common configuration, the laser-generated flux is collected on a planar substrate positioned parallel to the irradiated surface. In order to improve the stoichiometry and the quality of the films (particle free) several modifications were proposed like simultaneous generation of two plumes from different targets (double ablation) with different laser beams. In this paper, we present some results concerning the production of cryolite thin films using the conventional pulsed laser deposition technique (C-PLD) and the dual crossed beam pulsed laser deposition technique DCB-PLD). Plasma plumes expanding in vacuum and interacting together are visualized. The different species ejected in the plumes are detected through narrow-band filters in order to determine their kinetic energies. The morphology and the composition of the films are compared with the thermal evaporation technique.

If production technology is divided into three groups -- namely processes with material removal, processes with material addition and processes without a change of workpiece mass, it points out immediately, that the first two groups have been conquered to a large extent by the laser, but the third group suffers from an essential lack of laser application. The reason is, that for these processes, -- mainly forming -- rather strong mechanical forces are needed, that cannot be generated by a deliberation of optical energy. Nevertheless, the strong dependence of the yield strength, that must be reached for plastic deformations, on the temperature shows a way to reduce mechanical forces in forming and to obtain thus a beneficial effect of laser assistance. Moreover, brittle or hard materials as most recent high strength steels or titanium, that cannot withstand deformations at room temperature without cracks or rupture, can be formed successfully with the help of lasers. Compared to conventional hot working, laser heating can be applied only to those regions of the workpiece, where strong deformations take place, thus beneficially reducing production time. Good examples for the above concept are laser assisted deep drawing (LADD) as well as dieless wire drawing (DLWD) and similar forming processes, that have been investigated by the author and his coworkers experimentally and theoretically. The latter studies, that will be presented in the actual paper, show clearly, that the above predictions are true, whereas a reduction of the drawing force up to 50% seems to be feasible and that the difficult materials mentioned above can be formed with high quality without any cracks. Nevertheless, relatively high beam powers in the order of magnitude of 10 Kilowatts must then be used, thus giving rise to a promising application of very high power lasers.

This paper describes gasdynamic regimes of high-power pulsed laser interaction with solid targets in the ambient air of normal and reduced densities in the moderate intensity range 0.1 - 1000 MW/cm², typical four numerous laser applications. The experiments have been performed with a set of CO, CO₂ and KrF laser installations with output energy up to 100 J and pulse durations of 0.1 - 200 microsecond(s) . Supersonic erosion jet formation during target evaporation as well as different types of laser- supported absorption waves, i.e. laser-supported detonation waves, subsonic and supersonic radiation waves have been identified for the variety of radiation wavelength 0.25, 5.0 - 5.6, 10.6 (9.6) micrometer. The results on the gasdynamic regimes determination, being obtained for sufficiently big irradiated spots approximately 1 cm, might be extended for much higher energies available for these types of lasers in large-scale environments.

Femtosecond laser ablation has been shown to produce well-defined cuts and holes in metals with minimal heat effect to the remaining material. Ultrashort laser pulse processing shows promise as an important technique for materials processing. We will discuss the physical effects associated with processing based experimental and modeling results. Intense ultra-short laser pulse (USLP) generates high pressures and temperatures in a subsurface layer during the pulse, which can strongly modify the absorption. We carried out simulations of USLP absorption versus material and pulse parameters. The ablation rate as function of the laser parameters has been estimated. Since every laser pulse removes only a small amount of material, a practical laser processing system must have high repetition rate. We will demonstrate that planar ablation is unstable and the initially smooth crater bottom develops a corrugated pattern after many tens of shots. The corrugation growth rate, angle of incidence and the polarization of laser electric field dependence will be discussed. In the nonlinear stage, the formation of coherent structures with scales much larger than the laser wavelength was observed. Also, there appears to be a threshold fluence above which a narrow, nearly perfectly circular channel forms after a few hundred shots. Subsequent shots deepen this channel without significantly increasing its diameter. The role of light absorption in the hole walls will be discussed.

Micromachining experiments were performed with Ti:sapphire laser pulses (130 fs - 150 fs, 800 nm, approximately 10 Hz) in air. Employing the direct focusing technique, highly absorbing titanium nitride (TiN) and weakly absorbing polyimide (PI) and polymethylmethacrylate (PMMA) served as target materials. The lateral and vertical precision of the laser ablation and morphological features were characterized by scanning force (SFM), scanning electron (SEM) and optical microscopy. For TiN, incubation can be observed, i.e. the single-pulse surface damage threshold (0.26 J/cm²) is by a factor of two greater than the threshold for 100 pulses. Ablation rates below 10 nm per pulse can be achieved. The evolution of sub-wavelength ripples is presented in dependence on pulse number and laser fluence, respectively. The incubation behavior of the polymers can be described by an accumulation model as for TiN. Experiments on PI with varying focal lengths result in the same modification thresholds. Different polarization states of light (linear, circular) lead to a variation of the ablation rate and to various morphological patterns in the ablation craters (wavelength ripples, cones). Swelling of PMMA occurred at fluences below the ablation threshold.

We report the first study of laser ablation and the demonstration of plasma interferometry with a tabletop soft x-ray laser. A capillary discharge pumped Ne-like Ar laser (46.9 nm) was focused using multilayer optics to significantly exceed the energy density necessary for the ablation of metals. Ablation in brass, stainless steel and aluminum samples is reported. The ablation patterns on brass were used in combination with ray tracing computations to characterize the focused soft x-ray laser beam. The radiation intensity within the 2 micrometer diameter central region of the focal spot is estimated to be approximately 10¹¹ W/cm², with an energy density of approximately 100 J/cm². In a separate experiment we performed soft x-ray interferometry of a laser-created plasma using a table-top capillary discharge laser operating at 46.9 nm in combination with a novel amplitude division interferometer. The soft x-ray interferometer utilizes diffraction gratings as beam splitters in a Mach-Zehnder configuration to generate high contrast interferograms over a large field of view. This table-top system was used to probe a large-scale (3 mm long) plasma created by a Nd:YAG laser. The short wavelength of the probe laser has allowed mapping of the electron density in plasma regions with density gradients steeper than those that could be probed with the fourth harmonic of Nd:YAG for a plasma of this length.

Conversion efficiencies concerning the transfer of laser energy into thermal or mechanical specific internal energy on targets, both depend on material parameters and on the spectral and temporal irradiation characteristics. Investigations to be reported refer to processes governing small signal absorption at low power densities up to saturation levels, due to nonlinear optical effects at medium or high energy density values. The paper gives a survey of present achievements in this field of research. It includes results of recent investigations, carried out at the German-French Research of Saint-Louis (ISL). Particular interest in these studies was focused on the behavior of optically passive and active dielectric materials, such as those used in optronical devices, subject either to in-band, or to out-of-band laser radiation across large area surfaces. Experimental results are presented, as obtained by the use of a repetitively pulsed high average power CO₂-laser (P_average up to 15 kW) with pulsed peak powers up to about 100 MW. Suitability chosen focusing conditions provide a high flexibility concerning the available range of achievable power densities, correspondingly. Numerical simulations based on the experimental CO₂-laser results (10.6 micrometer) allow an extrapolation towards processes and responsible mechanisms to be expected in other wavelength ranges of actually interesting high-power lasers.

The new material processing characteristic of aluminum nitride (AlN) ceramic is compared with microsecond, nanosecond and femtosecond laser ablation. The conventional laser material processing technology with longer pulsewidth laser such as TEA CO₂ laser, Q-switched YAG laser, and excimer lasers leads to the thermal shock or lateral damage on target material, and those thermal effect causes the surface modification of AlN ceramic target. The comparative study of the laser ablation with microsecond TEA CO₂ laser pulse, nanosecond KrF excimer laser pulse, and femtosecond Ti:sapphire laser pulse is performed in time domain. Using intense ultrashort titanium sapphire laser two-photon laser ablation of TiO₂ photo-catalyst was also investigated experimentally and theoretically aiming at the enhancement of photo-catalyst reaction. The black-surfacing of the TiO₂ photo- catalyst crystal was successfully achieved by drilling a large number of conical micro-holes with two-photon laser ablation. The ablated surface has a roughness of sub micrometer order, and no heat-affected zone was observed. The simple equation is developed to explain two-photon ablation process of the TiO₂ photo- catalyst and the dependence of the ablation characteristic on the pulsewidth.

The optical resonance within the transparent particle with size a approximately (lambda) (radiation wavelength) strongly influences the intensity distribution in the contacted area (substrate surface). As a result, the efficiency of dry laser cleaning is a strong non-monotonous function of the particle size. Another important effect is related to the interaction between scattered and reflected light of the particle on the surface (near-field effect). Examination of these effects is done within the frame of exact solution of the light scattering problem. The distribution of absorbed intensity is the basic input characteristic for the laser cleaning model. It is used during solution of heat equation and equations of theory of the elasticity (thermal expansion and acoustic wave generation). This paper concentrates on the calculations of intensity distributions as a function of typical parameters in laser cleaning.

Laser cleaning was demonstrated both theoretically and experimentally to be an effective cleaning technique for removing particulate and film-type contaminants from solid surfaces in this paper. The laser-induced removal of film-type contaminants from IC (Integrated Circuit) packages, IC mould, Printed Circuit Board and flexible circuit was studied. The cleaning models were established for removal of particles from substrate surfaces without and with a thin liquid layer by taking adhesion forces and cleaning force into account. The models not only explain the influence of laser fluence on cleaning efficiency, but also predict the cleaning thresholds. Laser-induced removal of particles from magnetic sliders and disk surfaces was investigated. Monitoring of laser cleaning by acoustic wave, electric field signal and particle counter was also addressed.

The cleaning of silicon surfaces from submicron dust particles has been studied by means of the 'Steam Laser Cleaning' (SLC) process and compared to 'Dry Laser Cleaning' (DLC) which is used nowadays in many applications. For SLC a thin liquid layer (e.g. a water- alcohol mixture) is condensed onto the substrate, and is subsequently evaporated by irradiating the surface with a short laser pulse. The DLC process, on the other hand, only relies on the laser pulse, without application of a vapor jet. We have systematically investigated the efficiency of these two processes for the removal of well-characterized polymer, silica and alumina particles of various sizes down to 60 nm in diameter, and have also studied the influence of light wavelength and laser pulse duration for nanosecond and picosecond pulses. The results demonstrate that for the gentle cleaning of silicon wafers SLC is a very efficient method and is superior to DLC. An effect which so far has only rarely been taken into account for laser cleaning is the field enhancement under the particles, which can give rise to serious surface damage, in particular when cleaning pulses in the picosecond and femtosecond range in the DLC are applied.

Laser processing of siliceous materials becomes increasingly important. Analogous to the laser processing of conventional materials there are applications in the fields of cleaning, surface processing, cutting, etc. The present paper concerns the state of the art and new applications: (1) Laser cleaning of natural stone surfaces. The good disability allows restoration work to be carried out conveniently, as for example the complete removal of crusts or the removal to such degree that moisture is not trapped beneath. (2) Non-slip finish of polished natural stone surfaces: The excellent focusing of laser beams on spots as small as 100 micrometer and below can be exploited to produce macroscopically invisible structures on the surfaces of different materials. This permits microscopically small craters and lentil shaped depressions to be generated on the stone surface. Therefore it is possible to provide a non-slip finish to polished natural stone surfaces without noticeably impairing the gloss. (3) Concrete cutting: In Europe, and particularly in Germany, there is a growing demand for redevelopment of concrete apartment buildings, involving the removal of non-bearing walls and the cutting of openings. The temporal relocation of residents due to the noise and moisture from the use of diamond tools could be avoided by applying a laser cutting technology. With a 3 kW-Nd-YAG-laser, 70 mm concrete can be cut with rates up to 25 mm/min.

High-powered, pulsed CO₂ coherent ladar systems and their potential application to space debris tracking and characterization.

The Nd³⁺, Cr⁴⁺ co-doped GGG epitaxial thin films for self Q-switched waveguide laser has been fabricated by a two-target pulsed laser deposition (PLD) method. The concentrations of Nd and Cr ion in the co-doped GGG thin films are well controlled by changing respective KrF laser ablation fluence and repetition rate for Nd:GGG and Cr,Ca:GGG sintered targets. The structure of Nd,Cr:GGG thin films on YAG substrate shows a planar waveguide structure with high numerical aperture. It is confirmed that Nd³⁺ and Cr⁴⁺ ions are optically active as laser active ions and saturable absorbers respectively at 1.06 micrometer to be used for monolithic self-Q-switched waveguide laser to generate high peak power and short pulse output.

We have fabricated hollow glass fibers with an internal metal and polymer film. The fibers are fabricated by a simple, liquid phase technique or metal-organic CVD process. These fibers show low transmission losses and high power capability for ultraviolet to infrared lasers that are useful for industrial and medical applications. We review the fabrication process and transmission characteristics of hollow fibers for high-power Nd:YAG and excimer lasers.

Adaptive optics at astronomical telescopes aims at correcting in real time the phase corrugations of incoming wavefronts caused by the turbulent atmosphere, as early proposed by Babcock. Measuring the phase errors requires a bright source located within the isoplanatic patch of the program source. The probability that such a reference source exists is a function of the wavelength, of the required image quality (Strehl ratio), of the turbulence optical properties, and of the direction of the observation. It turns out that the sky coverage is disastrously low in particular in the visible wavelength range where, unfortunately, the gain in spatial resolution brought by adaptive optics is the largest. Foy and Labeyrie have proposed to overcome this difficulty by creating an artificial point source in the sky in the direction of the observation relying on the backscattered light due to a laser beam. This laser guide star (hereinafter referred to as LGS) can be bright enough to allow us to accurately measure the wavefront phase errors, except for two modes which are the piston (not relevant in this case) and the tilt. Pilkington has emphasized that the round trip time of the laser beam to the mesosphere, where the LGS is most often formed, is significantly shorter than the typical tilt coherence time; then the inverse-return-of-light principle causes deflections of the outgoing and the ingoing beams to cancel. The apparent direction of the LGS is independent of the tilt. Therefore the tilt cannot be measured only from the LGS. Until now, the way to overcome this difficulty has been to use a natural guide star to sense the tilt. Although the tilt is sensed through the entire telescope pupil, one cannot use a faint source because $APEX 90% of the variance of the phase error is in the tilt. Therefore, correcting the tilt requires a higher accuracy of the measurements than for higher orders of the wavefront. Hence current adaptive optics devices coupled with a LGS face low sky coverage. Several methods have been proposed to get a partial sky coverage for the tilt. The only one providing us with a full sky coverage is the polychromatic LGS (hereafter referred to as PLGS). We present here a progress report of the R&D program Etoile Laser Polychromatique et Optique Adaptative (ELP-OA) carried out in France to develop the PLGS concept. After a short recall of the principles of the PLGS, we will review the goal of ELP-OA and the steps to get over to bring it into play. We finally shortly described the effort in Europe to develop the LGS.

A brief description and use of two LIDAR (Acronym for LIght Detection And Ranging) systems in the measurements of atmospheric aerosols and vertical temperature profiles above Durban are presented. Early local aerosol profiles for low medium and high altitudes from the old LIDAR are shown. With the recent installation of the new LIDAR, vertical temperature measurements in the troposphere and stratosphere are made possible. A first validation of the new LIDAR has been carried out showing atmospheric wave activity above the Southern African continent for the first time. It is envisaged in the future to correlate the results obtained with the new LIDAR, especially for the low altitude, with those of the old LIDAR. Plans are also going ahead to implement an additional channel on the new LIDAR which will measure ozone concentration in the troposphere.

Industrial, military, medical, and research and development applications of lasers frequently require a beam with a specified irradiance distribution in some plane. A common requirement is a laser profile that is uniform over some cross-section. Such applications include laser/material processing, laser material interaction studies, fiber injection systems, optical data/image processing, lithography, medical applications, and military applications. Laser beam shaping techniques can be divided in to three areas: apertured beams, field mappers, and multi-aperture beam integrators. An uncertainty relation exists for laser beam shaping that puts constraints on system design. In this paper we review the basics of laser beam shaping and present applications and limitations of various techniques.

Microscopic collision processes in solids occur on femtosecond time scales. A description of materials response to laser irradiation on this time scales should take these processes explicitly into account. Averaging descriptions are not applicable from the first. We calculate the distribution function of free electron gas in metals and insulators for the case of irradiation with a laser pulse of moderate intensity. A microscopical description on the basis of time-dependent Boltzmann equations is used. For the metal, photon absorption by free electrons, electron-electron collisions and electron-phonon collisions are considered each by a corresponding collision integral. In dielectrics, additional terms for two ionization processes (strong-electric-field ionization and impact ionization) are included. With this model, describing explicitly materials transient behavior, we are able to check the applicability of common averaging equations. For metals irradiated with sufficient high intensities about and above damage threshold the energy exchange between electrons and phonons can be described with the two temperature model, whereas for low excitation the non- equilibrium in the electron gas affects the electron-phonon coupling. For dielectrics we show that the commonly used rate equation for collisional ionization is not applicable for pulse durations below hundred femtoseconds. We propose an extended system of two rate equations taking the effect of energy dependence of impact ionization into account. This averaging approach can reproduce the evolution of free electron density in SiO₂ with reasonable accuracy.

The nonequilibrium properties of the electrical and thermal currents in metals in the transient regime are investigated. The transition range, by definition, lies between the time necessary for establishing the electron temperature and the time that justifies a description by the standard steady state equations. Using a second order expansion of the Boltzmann equation, we derive the relaxation functions for the electrical and thermal cases and determine the relaxation times related to them. It is shown that the relaxation time for the electrical transport corresponds to Drude's momentum scattering time whereas the corresponding time for the heat flow is identified as the electron temperature relaxation time. Consequently, Ohm's law should remain a good approximation in most cases whereas the Fourier equation must be supplemented by a relaxation term leading to the hyperbolic heat conduction equation. In addition, we discuss the changed properties of the electrical and thermal conductivity on short time scales and show that both quantities becomes explicit functions of time. Moreover, the thermal conductivity shows a dependence on the laser frequency.

The goal of this report is to discuss (in addition to review) the independent literature works which come to our attention in 2 last years with experimental or theoretical proofs the existence of the solitonic type Wave of Change in Reflection and Conduction (WCRC). WCRC presents a new variety of transfer phenomena in condensed matter. It was excited by a single IR laser pulse with a threshold of more than 10 kW/cm² and consists of a series of about 30 solitary pulses with propagation velocity of each subsequent pulse decreasing two times comparing with that of preceding one in the range from sound velocity to less than about micron/s. Each pulse has the following solitary wave features: (1) it is all the time of one sign, (2) its velocity U_i is nearly constant, (3) it reflects from sample surfaces without noticeable velocity change. So far the systematic WCRC study was made in Lebedev Physical Institute, grate deal in collaboration with group of Marseilles University (prof. M. Autric) and also with some others groups. Literature analysis showed independent works where WCRC can be seen and which was made in different institutions with different own goals in mind. As example, work on optical monitoring of laser damage in IR materials or thermocouple measurements of temperature non stability in water cooled copper shield stopped the high power e-beam, etc. We will discuss also some details of theoretical work connected with development of Frenkel-Kontorova (1937) topological soliton model. WCRC is rather universal phenomenon, it appears in many laser-condensed matter interactions and so it should be studied for the WCRC mechanism understanding and its effect evaluation for different applications.

In the present report, plasma formed on the surface of metals and dielectric materials under the action of XeCl and KrCl lasers is studied. Plasma formation thresholds and plasma expansion velocities are measured. It was shown that long-lived plasma objects are formed near a target in the atmosphere of Xe or air at pressure 0.1 - 1 atm. Duration of luminescence of the plasma objects was maximal in xenon. Non-symmetric current passing through the laser plasma and the current interruption were obtained. The interruption time was as short as 10 ns while the plasma resistance increases with the rate up to 10¹⁰ Ohm/s. It is shown that the interruption rate increases when plasma of elements with small atomic weight is used and at higher rice-time of the current before its interruption. It was found that the plasma ionization degree is saturated before the interruption. The interruption effect was used for development of generators with inductive energy storages. Generator with a plasma erosion opening switch with opening current of 10 kA operating at pulse repetition rate up to several Hz was used for excitation of pulsed lasers. Using the generator, laser action on nitrogen and XeCl molecules and atomic transition of Ne was obtained.

Material processing with lasers has grown greatly in the previous decade, with annual sales in excess of $1 B (US). In general, the processing consists of material removal steps such as drilling, cutting, as well as joining. Here lasers that are either cw or pulsed with pulsewidths in the microsecond(s) time regime have done well. Some applications, such as the surface processing of polymers to improve look and feel, or treating metals to improve corrosion resistance, require the economical production of laser powers of the tens of kilowatts, and therefore are not yet commercial processes. The development of FELs based on superconducting RF (SRF) linac technology provides a scaleable path to laser outputs above 50 kW, rendering these applications economically viable, since the cost/photon drops as the output power increases. Such FELs will provide quasi-cw (PRFs in the tens of MHz), of ultrafast (pulsewidth approximately 1 ps) output with very high beam quality. The first example of such an FEL is the IR Demo FEL at the Thomas Jefferson National Accelerator Facility (Jefferson Lab), which produces nearly 2 kW of high average power on a routine basis. Housed in a multilaboratory user facility, we as well as members of our user community have started materials process studies in the areas mentioned earlier. I will present some of the first results of these studies. I will also briefly discuss the status of our DOD-funded project to upgrade the FEL to 10 kW in the mid IR.

Applications of pulsed laser ablation to the manufacture of micro- electro-mechanical systems (MEMS) and micro-opto-electro-mechanical systems (MOEMS) devices are presented. Laser ablative processes used to manufacture a variety of microsystems technology (MST) components in the computer peripheral, sensing and biomedical industries are described together with a view of some future developments.

Laser ablation of some metals in gases induced by nanosecond Nd:YAG laser has been studied by both photoacoustic and fast imaging techniques. Photoacoustic technique using piezoelectric polymer film revealed the change of coupling among laser radiation, ablated matter, plasma and target as a function of the laser fluence. Nanosecond imaging technique showed surface phenomena during and immediately after the ablating laser pulse. Photoacoustic signal intensity as a function of laser fluence was measured at constant pulse energy. It is constant at low fluence, starts to increase with fluence at certain threshold, reaches the maximum and then decreases gradually with increasing fluence. At fluence higher than about 7 J/cm², there appeared jet-like plasma growing toward incident laser beam at velocities of as high as 10⁵ ms^-1 in addition to the laser induced plume. The jet grew during the laser pulse and when the pulse terminated, its rapid growth stopped. At lower fluence, laser induced plasma expanding at about 10⁴ ms^-1 was observed. The growth speed of the jet-like plasma depended on laser fluence and gas atmosphere but did not change for different metals.

Vertical flight and pendulum experiments have been carried out with a simple paraboloid type lightcraft in the air-breathing mode. Pulsed laser energy of up to 240 J/pulse was delivered from a highly reproducible e-beam sustained CO₂-laser at repetition rates up to 45 Hz. The lightcraft mass was varied in the range between 22 and 55 g. An average thrust of 1.1 N has been derived from the flight data and the highest impulse coupling coefficient found in the pendulum experiments was 33.3(DOT)10^-5 Ns/J. A double shock wave was detected that leaves the thruster exit and an attempt was made to model the thrust, using a modification of Sedov's similarity solution for a blast wave. Finally, the propulsion requirements for the launch of a 10 kg mass into low Earth orbit are presented.

Laser-generated shocks can and have been used to study their effects on single crystal materials during shock compression. While a crystal undergoes shock compression and release, the transient x- ray diffraction (TXD) of the Bragg and Laue signals is indicative of the change in the crystal lattice spacing. The lattice spacing directly relates to the strain in the crystal. From the dynamic lattice data, strain, strain rate, and/or phase change in a material may be determined. Confined ablation plasmas can efficiently launch a flyer plate for direct impact on a target material imparting a well-characterized shock input and generate kilobar to megabar pressure pulses over a wide range of pulse duration (<EQ 1 - >= 20 ns). The laser-launched flyer plates are analogous to those launched by gas guns, but the smaller size provides an experimental method not easily accessible by larger gas gun experiments. With lasers, diagnostic equipment can be easily synchronized to study dynamic material parameters, i.e., single crystal shock dynamics, interfacial bond strengths of thin coatings, grain-interfaces, texture, and high strain rates (10⁶ - 10⁹ sec^-1).

Flat top shocks generated reproducibly by short pulse lasers are useful in studies of shock compression phenomena and may have applications in materials science, biology, and medicine. We have found the fluence profiles of Gaussian spatial mode 120 - 400 fs duration incident laser pulses are reproducibly flattened via surface optical breakdown in dielectric substrates at fluences just about the breakdown threshold. These flat top laser profiles have been used to produce shocks flat to 0.7 nm RMS over a 75 - 100 micrometer diameter.

Generation of thrust by laser propulsion has gained reality and much attention due to the recent development of high average-power lasers and demonstrations of sizeable object launching. Generating thrust requires a large amount of energy or high average power, but the question is how it is provided. This study deals with effectiveness of highly repetitive ultrashort laser pulses on generating high momentum coupling coefficient, C_m in vacuum condition. Two laser parameters, pulse width and repetition rate, have been studied in terms of the enhancement of C_m. It was found that with pico- and femtosecond pulse ablation, higher C_m is generated compared with longer pulse duration although it is on the same scaling as longer pulse duration. With high repetition rate pulses (80 MHz), more than one order of magnitude enhancements in C_m have been observed compared with single pulse interaction.

Approximately ideal flight paths to low-Earth orbit (LEO) are illustrated for laser-driven flights using a 1-MW Earth-based laser, as well as sensitivity to variations from the optima. Different optima for ablation plasma exhaust velocity V_E, specific ablation energy Q*, and related quantities such as momentum coupling coefficient C_m and the pulsed or CW laser intensity are found depending upon whether it is desired to maximize mass m delivered to LEO, maximize the ratio m/M of orbit to ground mass, or minimize cost in energy per gram delivered. A notional, cone-shaped flyer is illustrated to provide a substrate for the discussion and flight simulations. Our flyer design conceptually and physically separates functions of light collection, light concentration on the ablator, and steering. All flights begin from an elevated platform. Flight simulations use a detailed model of the atmosphere and appropriate drag coefficients for sub- and supersonic flight in the continuum and molecular flow regimes. A 6.2-kg payload is delivered to LEO from an initial altitude of 35 km with launch efficiencies approaching vacuum values of about 100 kJ/g.

The laser interaction in water confinement regime has been studied experimentally by reflectivity and ablated thickness measurements with a 3 ns squared laser pulse and 15 Gaussian laser pulse at 1.064 micrometer. Depending on laser pulse duration, the absorption of the confined plasma is 80 - 90%. The ablated thickness equals to 1.1 micrometer and 0.75 micrometer, for the 3 ns squared pulse at 20 GW/cm² and the 15 ns Gaussian pulses at 1 GW/cm² respectively. The plasma parameters have been evaluated from experimental results and new developments of the description of the confined interaction. The density range of heavy particles is 3.4 - 4.6 10²¹ cm^-3, the degree of ionization 0.39 - 1.3, the temperature 1.7 - 4.0 eV. Numerical results agree with previous experimental results. The range of coupling parameter of these confined plasmas, which is 0.2 - 1, allows to classify them in strongly coupled plasmas.

Laser-boosted light sail experiments were carried out on 13 - 20 Dec. 1999 with the 150 kW LHMEL II carbon dioxide CW laser at Wright Patterson Air Force Base, using a 2.74-m long, 2.13-m diameter vacuum chamber evacuated to 36 - 44 microTorr. The 5-cm diameter laser sail discs (i.e., the test articles) were fabricated from an ultralight carbon microtruss fabric that was sputter-coated with molybdenum on one side only, to improve its reflectivity to 10.6 micrometer laser radiation. Four laser sails discs with three different areal densities (one at 6.6 g/sq.m., two at 27 g/sq.m., and one at 28 g/sq.m.) were tested as magnetically-supported pendulums with an overall length of 18 cm. Pendulum deflections for the three heavier sails, ranged from 2.4 to 11.4 degrees, measured as a function of incident laser powers from 7.9 to 13.9-kW. These pendulum sails had masses of 83.7, 87.3, and 88 milligrams each; their center-of-mass was located at 11.5, 11.7, and 11.9 cm (respectively) below the magnetic bearing. Laser photon thrust ranged from 3.0 to 13.8 dynes, as calculated from pendulum deflections. Seven of the 10 data points fell in the feasible range of 3.3 to 6.67 N/GW for photon propulsion physics; the other three (higher laser power) data points exceeded the 6.67 N/GW limit by as much as 50%. From this data set, the onset for significant ablation was clearly identified to be 12.9-kW. Laser sail temperature was monitored with an optical pyrometer, and fell in the range of 2270- K to above 2823-K for laser powers from 8-kW to 20.8-kW, respectively. The experiments are the first known measurements of laser photonic thrust performance with real candidate light sail materials.

Study on focus characteristics of the laser beam for long distance propagation in atmosphere is very important for many laser applications, especially for lightcraft vehicle, laser weapon. In this paper, the influence of atmospheric turbulence and thermal blooming on focus characteristics of laser beam is given for several typical lasers with different wavelength. The excursion of the laser beam axis for long distance propagation in atmosphere is also obtained for several laser beam axes. The calculations coincide with the experimental result.

Laser-induced ablation and surface processing are investigated in GaN epitaxial layers irradiated by 130 - 150 fs pulses at wavelength of 398 and 795 nm. These layers are important materials in optoelectronics and microelectronics. GaN is an inert and hard substance with very limited possibility of wet etching. Laser processing seems to be suitable for different steps of fabrication of nitride-based devices (mesa shaping, grooving, scribing, mirror and surface grating preparation, etc.). Atomic force microscopy is used for a high-resolution investigation of initial and irradiated GaN surface. Imperfections of the mirror-like as-grown (0001) surface are identified (monolayer steps, open-core dislocations). The laser-induced damage threshold (LIDT) is determined at 398 and 795 nm. The dependence of LIDT(N) on the number of shots, N, is tested in accordance with the phenomenological relation LIDT(N) equals LIDT(1)N^((gamma -1)) in order to reveal the defects accumulation in an ablation process of GaN surface ((gamma) equals 1 corresponds to an accumulation-free ablation). The damage of GaN is found to be free of accumulation effect at low pulse energy below 0.6 X LIDT(1) at 795 nm and some effect is found at higher energy with (gamma) equals 0.86 at multi-shot irradiation conditions. A successful surface processing of GaN epitaxial layer (grown by MOCVD technique) is demonstrated for a single-pulse laser ablation of a typical energy 4 - 6 times higher than LIDT(1). The ablation depth up to 500 nm is achievable. The edge is almost vertical and the rim of a laser-ablated channel is free of debris and peel-off. There is an experimental evidence of a successful laser processing by two-photon absorption of femtosecond pulses at GaN surface with photon energy in a transmission region of the material. Photoluminescence (PL) of the defect-related Y-band at 550 - 600 nm was not enhanced at the rim of ablated region when energy of ablated pulses was 3 X LIDT(1). The PL of GaN was excited by two-photon absorption of 110 +/- 10 fs illumination at 726 nm.

The typical uniform rectangular beam produced by excimer laser is attractive for surface treatment. These process are developed for to change crystalline structure, surface morphology and chemical composition by doping or to remove a contamination layer. The simple modeling of mono-dimensional heat flow equation is often used to describe these processes. Depending on the deposited laser power density, different physical and chemical processes occur, from transformation in solid or liquid phase to vapor production and plasma formation. Below laser ablation fluence, the crystalline and chemical surface modifications of polymer surface are achieved for metallization and treatment on silicon surfaces are carried out for microelectronics applications and thin film transistor devices. Over laser ablation threshold, a plasma is created in the vapor induced by laser-material interaction. At low presure (mbar) this plasma can transport species from target to a substrate for thin film growth. For high pressure ambient gas (1 bar) the plasma stays confined on the target surface and react. As chemical reactions occur there is a chemical composition transform of the target surface.

Transient surface deformations in dielectric materials induced by laser irradiation were investigated with time-resolved interferometry. Deformation images were acquired at various delay times after exposure to single pulses (100 ps at 1.064 micrometer) on fresh sample regions. Above the ablation threshold, we observe prompt ejection of material and the formation of a single unipolar compressional surface acoustic wave propagating away from the ablation crater. For calcite, no deformation -- either transient or permanent -- is discernable at laser fluences below the threshold for material ejection. Above and below-threshold behavior was investigated using a phosphate glass sample with substantial near infrared absorption (Schott filter KG3). Below threshold, KG3 exhibits the formation of a small bulge roughly the size of the laser spot that reaches its maximum amplitude by approximately 5 ns. By tens of nanoseconds, the deformations become quite complex and very sensitive to laser fluence. The above-threshold behavior of KG3 combines the ablation-induced surface acoustic wave seen in calcite with the bulge seen below threshold in KG3. A velocity of 2.97 +/- 0.03 km/s is measured for the KG3 surface acoustic wave, very close to the Rayleigh wave velocity calculated from material elastic parameters. Details of the transient interferometry system are also given.

A major problem in the regular maintenance of aerospace systems is the removal of paint and other protective coatings from surfaces without polluting the atmosphere or endangering workers. Recent research has demonstrated that many organic coatings can be removed from surfaces efficiently using short laser pulses without the use of any chemical agents. The lasers employed in this study were repetitively-pulsed neodymium YAG devices operating at 1064 nm (15 - 30 ns, 10 - 20 Hz). The efficiency of removal can be cast in terms of an effective heat of ablation, Q* (kJ of laser energy incident per g of paint removed), although, for short pulses, the mechanism of removal is believed to be dominated more by thermo- mechanical or shock effects than by photo-ablation. Q* data were collected as a function of pulse fluence for several paint types. For many paint types, there was a fairly sharp threshold fluence per pulse near 1 J/cm², above which Q* values dropped to levels which were a factor of four lower than those observed for long- pulse or continuous laser ablation of paint. In this regime, the coating is removed in fairly large particles or, in the case of one paint, the entire thickness of the coating was removed over the exposed area in one pulse. Hardware for implementing short-pulse laser paint stripping in the field is under development and will be highlighted in the presentation. Practical paint stripping rates achieved using the prototype hardware are presented for several paint types.

Experiments on laser-rock-fluid interaction have been carried out by using pulsed CO and CO₂ lasers which irradiated rocks typical for oil field: sandstone, limestone, shale and granite. Energy fluence and laser intensity on rock surface were up to 1.0 kJ/cm² and 10⁷W/cm², respectively. The dependencies of specific energy consumption (i.e. energy per volume needed for rock excavation) on energy fluence, the number of pulses, saturated fluid, rock material and irradiation conditions have been obtained for various rock samples. The dependencies of momentum transferred to the rock on energy fluence for dry rocks and rocks with surface saturated by water or mineral oil have been measured. High-speed photography procedure has been used for analyzing laser plasma plume formation on a rock surface. Infrared spectra of reflectivity and absorption of rocks before and after irradiation have been measured.

The interaction of a high power (infrared) laser beam with samples of rock encountered in hard-rock metal mining operations was experimentally investigated. These tests were intended to explore the feasibility of using high power lasers to improve the speed, performance, accuracy, and safety of rock cutting and drilling in mining operations. The current results were compared to similar tests, performed with the same laser, of materials typically encountered in gas and oil well drilling. Suggestions are made for the next steps in exploring how laser systems could possibly revolutionize drilling and cutting operations in the mining and oil/gas industries, particularly by augmenting other drilling hardware and techniques.

When a single-pulse high-power laser irradiates a surface at atmospheric pressure, a laser supported detonation (LSD) wave can form above the target surface. The high-pressure gas behind the LSD wave transfers momentum to the target. The laser target coupling is substantially reduced in vacuum, the coupling coefficient typically being an order of magnitude less than that when an atmosphere is present. Another pressure enhancement technique is to confine the laser-target interface. Confinement or 'tamping' also can substantially increase the momentum coupling to the target. Experiments tend to differ from one another based on the target size (thickness) and confinement geometry. This work describes and compares some experimental results for metallic targets irradiated by 1054 nm radiation in the GW/cm² range and interprets them in terms of simple models. As will be discussed in this paper, such models predict a weak sensitivity to target materials but results are likely to be different for inhomogeneous materials as has been seen in recent experiments on iron-nickel and stony meteorites.

This paper summarizes recent progress that has occurred in several research areas related to the development of a repetitively-pulsed, frequency-shifted chemical oxygen iodine laser (COIL). COIL gain- switch experiments at 10 kHz pulse rates are described using a novel solid state pulsed magnetic field system. Raman conversion experiments in hydrogen using a pulsed photolytic iodine laser as a COIL surrogate are also described.

A compact, short pulse photolytic iodine laser (PIL) system designed for use as a source in Raman conversion experiments is described. The single-shot, flashlamp-pumped laser outputs 10 Joules in a 3 microsecond(s) FWHM pulse at a wavelength of 1.315 micrometer and uses n-C₃F₇I as the renewable laser fuel. Laser design and performance characteristics are presented.

We report the operation of a short pulse photolytic iodine laser (PIL) using an unstable resonator under long-pulse and injection seeded operation. The laser is designed as a surrogate source to replicate the output from a q-switched chemical oxygen iodine laser (COIL). Under long pulse conditions the single shot laser produces up to 10 Joules in a 3 microsecond(s) pulse. When seeded with the output from a narrow pulse (10 ns) broadband KTP Optical Parametric Oscillator (OPO), the temporal output is composed of a train of 10 ns pulses separated by the round trip time of the cavity. The enhanced peak power in the individual pulses is more attractive for subsequent efficient Raman conversion. A description of the laser performance with the unstable resonator and with seeding is presented.

We have investigated rotational Raman conversion experiments in hydrogen using a single-shot photolytic iodine laser (PIL) at 1.315 micrometer as a pump laser. The total output energy of the PIL is between 5 - 7 J per pulse distributed in a train of approximately 120 pulses each with a FWHM of 6 or 9 nsec and a temporal spacing of 33 nsec. The energy distribution within the pulse train is characterized by high energy pulses in the gain switched spike followed by lower energy pulses in the tail of the laser pulse. Stimulated Raman scattering (SRS) experiments were performed with (1) a focused beam geometry in a single Raman cell, (2) two Raman cells, whereby the pump focus was reimaged into a second Raman cell, and (3) a Stokes resonator specifically suited for an annular pump beam. Thermal distortions in the laser beam made it necessary to lower the peak intensity of the pump laser beam by adjusting the focusing conditions. With a long focus mirror we demonstrated (1) a conversion efficiency of up to 70% for the high-energy pulses of the gain switched spike of the PIL micropulses and (2) lowering of the threshold with a Stokes resonator.

We present a Raman laser based on rotational scattering in H₂, synchronously pumped by a Q-switched, mode locked Nd:YAG laser. Appropriate choice of optics allows us to operate as either a first or second Stokes laser, while remaining below the threshold for extra-cavity scattering. We model the Raman scattering using a computer simulation based on higher order Stokes and anti-Stokes waves in the transient regime.

The conception of a high power laser system based on new physical principles (direct nuclear-to-optical conversion) has been proposed in the Institute for Physics & Power Engineering (Russia, Obninsk). Energy model of system mentioned above is constructed at IPPE. It consists of a thermal subcritical (in neutron aspect) laser module (LM) controlled by the neutron flux from a two-core fast burst reactor. The laser module is designed as a cylindrical structure with a longitudinal cavity for two cores of the pulse reactor and reactivity modulator and consists of the laser active elements (LAEL) with U-235, imitators of LAEL and neutron moderator elements. The laser active element is filled with an Ar-Xe or He- Ar-Xe laser active medium. The flanks of the laser-active elements are manufactured from an optically transparent material and provide the input and output of the laser beam. Generation of the laser irradiation was produced on the transitions with the wavelengths 1.73 and 2.03 micrometer. This paper is devoted to the analysis of the preliminary neutron and laser experimental results received during the energy start-up of the system. The main neutron and laser parameters were measured by using special elaborated methods. Comparative analysis for computed and experimental neutron data is performed. Approaches to the increase of output laser energy of a reactor-pumped laser system are also discussed.

A large volume of experimental study has been carried out with industrial self-heating sealed-off metal vapor active elements in superradiation regime and with stable and unstable telescopic resonators. Such laser output radiation has been showed to have multibeam structure. The beams partly overlap both in time and space, so conning during their formation in concurrence each other in power. While the resonator beams of small divergences and high coherency are easy to be isolated from the superradiance background, e.g. by diaphragm in the focal plane, spatial separation of high quality beams from each other is, on the contrary, rather difficult, especially at large magnification factors. For example, at the resonator magnification 100 the beams divergences differ 3 times, at magnification 200 they differ 2 times, and 1,5 at magnification 300. In this paper an analysis is carried out targeted to reveal the possible contribution of the resonator different beams into diffractional laser output formation.

Due to pump source restrictions, Nuclear-Pumped Lasers (NPLs) typically have relatively long (micro- to milli-second) pulse lengths with only modest peak powers but with very high total energy. These pump power restraints seriously limit the choice of laser media. One way to avoid this problem is to employ a Nuclear Driven Flashlamp (NDF) for the primary pumped element in the system. The fluorescence from this NDF can then be used for pumping a laser or for other high intensity light applications. The first experimental example of this approach was a ³He-XeBr₂ NDF employed by Williams and Miley (1993) to pump a small iodine laser. The present paper discusses issues involved in scaling such an NDF up to high power levels. Possible optimum configurations include use of microsphere or fiber pump elements dispersed in the NPF media. Analysis of such possibilities is presented along with consideration of special reflecting surface designs.

Simple way to increase total laser power at the target is combining of multiple beams from a laser array. For incoherent operation of N lasers, an on-axis intensity in the far-field can be increased N times. By establishing coherence across the N lasers in the array, additional N-times gain in the on-axis intensity can be achieved. Known techniques for optical coupling arrangement between lasers in the array are overviewed with analysis of their advantages and shortcomings. Principal causes for reduction of far-field on-axis intensity from laser arrays are discussed demonstrating necessity to seek compromise conditions between high laser array efficiency and optical quality of a combined beam.

High-energy oscillators, generating femtosecond pulses at 800 nm and 1250 nm, are demonstrated. These pulses are used for fast-speed laser ablation and material modification of transparent dielectric materials. Sub-micron size features are produced in fused silica by tight focusing of these pulses.

Pulsed laser capabilities at the Laser Hardened Material Evaluation Laboratory are described relevant to optical coupling, impulse generation and laser propulsion. Capabilities of the Nd:Glass laser are presented as well as supporting test systems.

An overview of theoretical and experimental results on first- overtone (FO) CO laser is presented. A special attention is paid to the quite new results jointly obtained at the Lebedev Physics Institute (Russia), TRINITI (Russia) and Air Force Research Lab (USA). Efficient multiwavelength pulsed FO CO lasing with output efficiency up to 11% and specific output energy up to 50 J/l Amagat is observed within the spectral range of 2.5 - 4.1 micrometer corresponding to the vibrational transitions V + 2 yields V from 6 yields 4 up to 37 yields 35. Single-line frequency tunable FO CO lasing on the wavelengths from approximately 2.7 up to 4.2 micrometer with the maximum output efficiency up to approximately 1% is also observed. Multiline and single-line lasing on 430 ro- vibrational lines is obtained. Theoretical calculations based on the experimental data predict that multiline FO CO laser efficiency can be enhanced up to 20%. FO CO laser with output efficiency and specific output energy comparable with those of CO₂ laser and spectral region of lasing covering that of HF and DF lasers can be considered as a potential source of high-power IR radiation for frequency selective laser ablation.

We are developing rod-type all solid-state laser with average power more than 10 kW as a tool for high speed and highly precise material processing such as cutting and welding. In this year, we developed rod-type all solid-state laser system with average power of 3-kW level and succeeded in attaining an average power of 5.1 kW with CW operation and 3.3 kW with combined mode of CW and QCW operation. We also obtained electrical-optical efficiency of 22% in CW operation and that of 26% in QCW operation with average power of 2 kW.

Performance limits of 10 kHz, 15 nsec-pulse diode-pumped solid- state laser in microdrilling of metals and ceramics were studied. Average drilling rates approaching 1 cm/sec with micrometer-range accuracy of micro-holes are achievable in steel and ceramic samples up to 1 mm thick. We found that shielding effects of plasma plume can be minimized by proper selection of laser intensity. In samples 1 mm and thicker, using third harmonic output allows drilling of high aspect ratio holes at almost an order of magnitude higher ablation rates as compared to IR laser output.

The micro laser plasma thruster ((mu) LPT) is an efficient, long- life, low-thrust pulsed rocket engine which uses a high-brightness semiconductor or glass fiber laser as a source of energy. It uses a simple, low-voltage semiconductor switch to drive the laser, resulting in zero off-state electrical power. Results are presented of the first experimental demonstration of uLPT's. We measured single impulses covering 5 orders of magnitude from 40 micro dyn-s [< 1 nano newton-s] to 2 dyn-s, specific impulse up to 1,800 seconds and coupling coefficients up to 25 dyne-s/J. Several target materials were studied. Initial applications are orientation and re-entry at end of life for micro- and nanosatellites. Anticipated lifetime output of the prototype engine now under development is about 5E7 dyn-s [500 newton-s], sufficient to re-enter a 5 kg LEO satellite.

The trial for world first oscillation of oxygen iodine laser using high frequency discharge was conducted. Maximum excited oxygen efficiency was recorded up to 21% by the microwave (MW) discharge and 32% by the radio frequency (RF) discharge. The highest efficiency of 32% singlet oxygen was achieved by producing plasma jet through the hollow cathode of RF discharge. Laser oscillation test was carried out connecting with RF discharge singlet oxygen generator (DSOG) to a resonator which was arranged longitudinal to the gas flow. Spontaneous emission of its wavelength 1.315 micrometer from iodine electronic transition was detected by the spectra-analyzer from laser output mirror. The laser oscillation was confirmed by detecting an amplification of the emission when laser mirrors were aligned.

The possibility of creation of capacitive discharge excilamps with short pulse duration was studied. There were three types of pulsed excilamps in experiments: cylindrical glow discharge lamp, capacitive discharge lamp, high pressure volume discharge planar lamp with UV-preionization of discharge gap. In capacitive discharge cylindrical KrCl-excilamp, at (lambda) approximately 222 nm the radiation pulse power through the end face of a lamp up to 2.5 kW has been obtained. Powerful radiation pulses of 50 ns in duration were obtained at pulse repetition rate of 1 kHz. In case of high pressure volume discharge at operating pressure of several atmospheres the radiation peak power density values were as much as 5 kW/cm² at (lambda) approximately 250 nm, and 3.5 kW/cm² at (lambda) approximately 222 nm and at (lambda) approximately 308 nm. In cylindrical longitudinal excilamp with inner electrodes with Xe- I₂ mixture the total pulse power of 75 kW has been obtained. The first experiments with harnessing of inductive energy stores for excilamp excitation have been carried out.

Results of the experimental investigation of e-beam sustained cryogenic CO-laser operating in a repetition rate mode are presented. The action of CO- and CO₂-laser radiation on the surface of some metals have been compared. The rate of matter removal from the surface irradiated by CO-laser was shown to be several times higher that observed in experiments with CO₂ laser pulses.

The theory of defect-strain instability with formation of periodic surface relief in semiconductors irradiated by ultrashort ((tau) _p equals 10^-13 s) laser pulse is developed. The period and time of formation of surface relief are calculated. Regimes of multipulse laser ablation leading to formation of either smooth surface or arrays of surface relief spikes are considered and corresponding experimental results are interpreted from the viewpoint of the developed theory.

A new technique for surface modification of polymer fibers is introduced by irradiating with UV an excimer laser, this technique can be used to modify the chemical and physical properties of fibers surface. Under certain conditions the irradiation of polymer fibers induced a characteristic morphology on the polymer fiber surface. The original smooth surface of polymer fibers changes its morphology to a rather regular roll-like structure perpendicular to the fiber axis after this treatment. The dependence of characteristic surface data on laser fluency and pulse number are studied. The mechanism for formation periodic surface is also discussed.

In lightcraft vehicle system, a ringy laser beam is focused by a 90 degree off-axis annular parabolic mirror. In this paper, methods are applied combining the geometrical optics and the physical optics. The effects of the angular misalignment of the optical axis and divergent angle on the focal power density are analyzed. Ray tracing shows that a pair of vertical focal lines is produced due to the angular misalignment. Basing on this, and from the theory of diffraction optics, we can know that the incident Gaussian beam will become the beam that is similar to the elliptical Gaussian mode for the misalignment. Moreover, the focal power density is decreased due to the angular misalignment. The parabolic mirror is very sensitive to misalignment of the incident beam, and the misalignment angle on the order of magnitude of mrad magnitude is enough to decrease peak power density down to the half. The tolerance of the misalignment angle down rapidly with the incident beam diameter increasing. When the incident angle has a divergent angle, coma is produced that also decrease the power density at focal plane.

With the launch of the South African National Laser Centre, new programs will need to be defined. Medical, environmental and industrial laser applications must obviously take top priority -- as opposed to the uranium isotope separation and military applications of the past. We argue however, that a small effort in laser ablation for space propulsion is justifiable, since a few very large CO₂ lasers are available and since two tentative propulsion experiments have already been conducted in South Africa. We attempt to give LISP (Laser Impulse Space Propulsion) an equatorial and a Southern dimension.

In this study, the new material processing characteristic of aluminum nitride (AlN) ceramic is compared with microsecond, nanosecond and femtosecond laser ablation. The conventional laser material processing technology with longer pulsewidth laser such as TEA CO₂ laser, Q-switched YAG laser, and excimer lasers leads to the thermal shock or lateral damage on target material, and those thermal effect causes the surface modification of AlN ceramic target. The comparative study of the laser ablation with microsecond TEA CO₂ laser pulse, nanosecond KrF excimer laser pulse, and femtosecond Ti:sapphire laser pulse is performed in time domain.

In a general way, study on the distributions of laser modes is under the condition that the modes are regularly distributed. In fact, because of the misalignment of resonator, sometimes the output laser modes are irregular, and because of the superposition of laser modes, and the disturbance of the atmosphere in the process of propagation, laser beams output are partially coherent. In this paper, with the aid of the computer, the far field distributions of some laser modes and the influence of spatial coherence on the distributions are discussed.

One of possible approaches to description of laser-induced heating of transparent materials by femtosecond pulses is discussed. There are considered (1) traditional equation of heat transfer, (2) two- temperature model, (3) modified two-temperature model including relaxation terms and diffusion equation for electron plasma. It is discussed validity of those models for description of early stages of laser-induced heating (within few picoseconds after laser pulse has gone) when non-equilibrium laser-induced electron plasma co- exists with non-equilibrium phonon system. It is proposed to use modified two-temperature model for that early stage while two- temperature model is proposed to be applied for later moments of time when relaxation processes within both electron and phonon subsystems are close to finish and the systems are quasi- equilibrium. Formation of abrupt space variations of temperature similar to shock-wave front is discussed as one of important properties of solutions to modified two-temperature model. That process depends critically on value of linear absorption coefficient and temperature-induced variations of material parameters. Obtained results are discussed from the viewpoint of experimental data on morphology of ablation crates in transparent materials.

There are presented results of theoretical investigation of self- depolarization effect in transparent isotropic materials resulting in variation of initial space distribution of polarization-ellipse parameters of focused high-power laser beam. Qualitative results are obtained within non-paraxial approximation while detailed calculations are done within paraxial approximation. In particular, both linear and circular initial polarizations are shown to change and turn into elliptic polarization with inhomogeneous distribution of polarization ellipses in focal area. Detailed calculations are presented for particular case of low-order Gaussian beams (TE₀₀, TE₀₁, TE₁₀ and TE₁₁). Bearing in mind obtained results, we discuss specific symmetry structures of self- depolarization effect allowing experimental checking of described phenomenon.

Metal ablation taking into account the hydrodynamics of a dense ablated material with ion temperature close to critical is considered. An extended two-temperature model taking into account hydrodynamic plasma expansion and degeneracy of the electron gas is developed. The new version of the RAPID code is used to perform calculations of ablation rates for several metal targets under conditions where the electron degeneracy is important.

We present the key features of design and performance of PROGRESS-P CPA Nd:YLF/Nd:glass laser facility capable of producing 1.5-ps pulses and a power up to 30 TW at the wavelength 1053 nm for laser- plasma experiments in ultrahigh irradiance on the target up to 10¹⁹ W/cm². We describe voltage pulse drivers based on drift step recovery diodes which produce output voltage up to 15 kV, rise time approximately 1 ns, jitter of 100 ps and repetition rate up to 10 kHz to electro-optical devices.

The applicability of hydrodynamic models for theoretical description of UV laser ablation of polymers is studied. The plume formation is considered as a first-kind phase transition. In case of strongly absorbing polymers this phase transition occurs as a surface evaporation, and in case of weakly absorbing polymers as a bulk evaporation. The vapor plume is assumed to be transparent for laser radiation, and its expansion is described by the isoentropic hydrodynamic equations. New analytical expressions for ablation (etch) depths per pulse are obtained, which are in a good agreement with the available experimental data.

Standard anodized aluminum samples with about 20 micrometer thick oxide layer were investigated for cleaning study by means of laser pulses. The removal mechanism was studied for short laser pulses at 1064 nm, 532 nm and 248 nm wavelength in air and water. The samples were examined using scanning electron, optical microscopy. Photoacoustic, shock wave measurements and high-speed visualization have been set-up. The experiments were carried out in a new cleaning set-up, which allows a liquid surrounding medium. We investigate the influence of the surrounding medium and the dependence of the laser cleaning efficiency on energy densities and number of pulses. The ablation threshold and ablation mechanisms vary due to the different absorptance of the oxide and aluminum at different wavelength. The surroundings influence has been shown.

We report laser-induced phase transformations in CdS nanocrystals under radiation of high-power nanosecond laser pulses with wavelength of 532 nm and 355 nm. The observed changes in size distribution of nanocrystals as well as the changes of their crystal structure from orthorhombic to cubic are uncovered by absorption spectroscopy, high-resolution transmission electron microscopy and selected area electron diffraction.

AlN nitride films are grown by reactive pulsed laser ablation of aluminum target in N₂ atmosphere. The influence of process parameters such as N₂ pressure and laser fluence is investigated. Films are characterized by Rutherford Backscattering Spectroscopy, Nuclear Reaction Analysis, X Ray Diffraction and X Ray Photoelectron Spectroscopy. O contamination appears in the film and its origin is discussed. To enhance N₂ dissociation, a RF discharge device is coupled to the deposition chamber. Its effect on thin film composition is studied. Emission spectroscopy is performed in order to find the best RF working point for N₂ molecule dissociation and to understand species transport from the target towards the substrate as a function of process parameters. Thin film with a stoichiometry near to Al₁N₁ can be obtained with low O contamination working with 6 J/cm² laser fluence, 0.01 mbar N₂ with RF discharge added.

This paper discusses some questions about transition radiation effect in foil stack, analyses stimulated amplification of optical wave in foil stack. In this paper, authors research two possible models about x-ray or soft x-ray FEL (free-electron lasers). First model is transition radiation effect and circulating crystal resonator apply to x-ray FEL, second model is to produce x-ray FEL by means of high-energy electron beam, undulator and circulating crystal resonator.

A 200-watts average power excimer laser with excellent output energy stability has been developed. The improved laser oscillator can keep an output of 200 watts (667 mJ/pulse, 300 Hz) with servo operation over 25 million pulses. During the operation, the output stability is less than 1.5% for 1-(sigma) and less than 10% for peak to peak value. We have just started to improve the pulsed power circuit for excitation. At first the numerical simulation is executed about this pulsed power circuit using PSpice simulator. The experimental results are in good agreement with the simulation results. The special capacitance transfer method is used for this pulsed power circuit. The result shows that this energy transfer is not enough at the high input voltage. The improvement of the magnetic assist circuit by making full use of the simulation result leads to the complete energy transfer. It is possible for this laser to produce almost 300 watts (1 J, 300 Hz) in the largest output power. Experimental and theoretical studies of various parameters governing the laser performance are used to improve further. It is expected that higher power laser output is produced to be applied for practical use.

A problem of accelerating pellets of significant mass with a CO₂-laser pulse (or a pulse train) is under consideration. As it is known, the highest magnitudes of the accelerated pellet velocity of about 100 km/s were observed in the experiments on accelerating flat foils with a nanosecond Nd-laser pulse. The acceleration efficiency achieved was 5 - 10%. However the accelerated target usually turned into a cloud of superdense low-temperature plasma in these experiments. To avoid pellet destruction and to achieve maximum acceleration it is necessary, depending on the task stated, to meet certain requirements to the laser wave-length, power density and pulse duration. So, for instance, to accelerate pellets of frozen hydrogen only long-wave lasers can be used. When pellets of other materials are to be accelerated the wave length range used can be broadened. However, the laser pulse duration must be large enough to avoid shock wave formation. The regime of laser-driven rocket traction seems to be the most acceptable. Difficulties in attaining this regime in the experiment mainly concern formation of a uniform and extended in the atmosphere laser beam. Acceleration of frozen hydrogen pellets for fuel injection in thermo-nuclear setups with magnetic confinement are discussed. It is shown that on the basis of laboratory CO₂-lasers available pellet velocities up to 10 - 100 km/s can be obtained.

Results are presented of the first measurements of laser-ablation impulse on structured targets of the modified Fabbro type in which impulse coupling coefficients C_m up to 500 dyne-s/J were obtained for 85-ns duration laser pulses by direct measurement with momentum pendula. Our target design generated these C_m values for single-pulse laser fluence of 1.2 J/cm² and peak intensity 14 MW/cm², an order of magnitude below the intensity at which similar coupling has been observed before. This result is important for the ORION demonstration, since it effectively closes a factor- of-20 deficit between presently available projected intensity at 300 km range and the intensity required for optimum coupling to standard materials. The Nd:glass laser employed in these measurements had 1.05 micrometer wavelength and pulse duration from 25 to 100 ns. Ambient pressure was less than 10 millitorr. Impulse coupling data on water ice, stone, carbon phenolic, PMMA and other materials were also obtained, for cross-calibration and because of interest in applications to 'uncooperative' natural bodies in space. We discuss the significance of these results for planning a laser propulsion demonstration in space, as well as possible extensions which could yield appropriate C_m for repetitively- pulsed propulsion of objects into low Earth orbit (LEO).

Drillings in zirconia coated Ni-superalloys is done by melt extraction with pulsed laser radiation provided by a Nd:YAG slab laser with microsecond pulse duration. This laser system distinguishes itself by a high beam quality and offers the possibility to investigate drilling of holes with a diameter of 200 micrometer by percussion drilling and trepanning. The quality of drilled holes, e.g. the heat affected zone (HAZ), the recast layer and the conicality, are presented. During drilling different process gases are used. The results in drilling velocities, melt thickness and chemical composition of the melting zone are shown for oxygen, argon and nitrogen by SEM and EDX. A numerical simulation of the trepanning process will be presented. The different time scales of the contributing physical processes related, for example, to the small melt film layer during trepanning are described. A coating is distributed on the multilayer system to protect the blade from recast. Aim of the investigation is the production of holes in a multilayer system, consisting of CMSX-4, VPS-MCrAlY and EB-PVD-zirconia. With this used laser system inclined holes up to 60 degrees in this layer system can be drilled. No recast layer and no spalling of the zirconia-layer are observed.

We have succeeded for the first time to simulate dynamic phase transition from metal to vapor. This success is due to the CIP (Cubic-Interporated Pseudoparticle/Propagation) method that can treat solid, liquid and gas together and can trace a sharp interface with almost one grid. We report here the application to laser-induced evaporation and welding process. In the former case, aluminum is evaporated well after the laser beam ended and evaporation occurs with a large angle to the target normal leading to large debris. In the latter case, a deep penetration welding of SUS304 by YAG laser has been successfully replicated the experiments and the simulation clarifies the formation mechanism of keyhole.

We discuss application of dynamic holography in optically addressed liquid crystal spatial light modulators to correction for thermo- optical distortions in high average power solid-state pulse- repetitive picosecond lasers. Possible scheme of multipass laser and possible schematics of dynamic hologram-corrector record, including the novel technique of blazed dynamic holograms are discussed.

Laser cutting of textiles with excimer lasers was undertaken for the Amtex program at Los Alamos. These studies were carried out in tandem with laser cutting studies at YAG and copper vapor laser (Green) wavelengths at Lawrence Livermore National Laboratory and at CO₂ wavelengths at the Argonne National Laboratory. Laser ablation through the process of photo chemical bond breaking at UV (XeCl at 308 nm) wavelengths proved to be at least a factor of 24 more efficient than thermal ablation at the longer YAG wavelength (1.06 (mu) ). We project that a diffraction limited 100 watt XeCl laser is capable of cutting at a rate of 500 cm/sec (200 in/sec) for all cloths tested with the exception of denim and air bag material.

The effect of 'spectrum alternation' (i.e. an alternation of strong and weak vibrational bands) in multiline pulsed first-overtone CO laser spectrum, which is determined by two sets of cascade transitions operating independently, is experimentally observed. The influence of laser mixture composition and gas density on the 'spectrum alternation' is discussed. Lasing on the highest experimentally observed vibrational transition 38 yields 36 can be obtained with a nitrogen free laser mixture or with a little nitrogen admixture only. This phenomenon can be explained by quasi- resonance collisional interaction between the 38 yields 36 vibrational transition of CO molecule and the 0 yields 1 vibrational transition of N₂. A formation of first-overtone CO laser spectrum corresponding to the highest vibrational transitions is also discussed.

We describe several spectroscopic techniques for space-resolved diagnostics of inhomogeneous plasmas from the line and continuous XUV spectra of multiply charged ions, involving measurements of temperature, density, ion composition, ablation velocity, etc. To this end, we have developed a family of stigmatic high-throughput spectroscopic instruments ranging in resolution from 500 to over 20000. The new instruments harness separation of the focusing and dispersion functions: the XUV radiation is dispersed by plane grazing-incidence gratings or transmission diffraction gratings while the focusing is transferred to concave normal-incidence multilayer mirrors or grazing-incidence toroidal mirrors. We have obtained medium-to-high resolution spectra of laser-produced plasmas to infer the plasma parameters (density profiles, expansion velocity, etc). The operating range of a spectrograph which incorporates periodic multilayer mirrors is confined to the resonance reflection band of the mirrors. To meet the demand for broadband stigmatic instruments, a panoramic (110 - 300 angstrom) medium-resolution spectrograph was made employing a Mo/Si multilayer mirror with a lateral gradient of the period structure (0.9 Angstrom/mm) and a transmission diffraction grating (5000 lines/mm). An alternative way to obtain panoramic stigmatic spectra involves the development of broadband aperiodic multilayer mirrors. We have developed a numerical technique based on the fast calculation of derivatives, which allows us to determine aperiodic structures with prescribed reflectance spectra and augmented integral reflectivity. This technique has proved to be efficient both in the soft and hard X-ray ranges.

Theory of the shock wave initiation and propagation under the action of the powerful plasma-producing laser beam on a solid target is developed. The limiting depth of the material destruction which is the distance where the pressure at the front of shock wave initiated by the laser pulse decays to the value corresponding to the yield point of the material is analytically determined. It is shown that for laser pulse more intensive than 10¹⁰ - 10¹¹ W/cm² the depth of the material destruction due to a shock wave which decays in the material after the end of laser pulse significantly exceeds the depth of the material destruction during the period of laser pulse. The found dependence of the limiting destruction depth on the laser pulse parameters and strength characteristics of the material allows to calculate the crater depth in a thick target undergoing the laser pulse action or the maximum thickness of a target which can be perforated by the laser beam.

Diode lasers [(lambda) equals 1480 nm] are used with in-vitro fertilization [IVF] as a promoter of embryo hatching. A focused laser beam is applied in vitro to form a channel in the zona pellucida (shell) of the pre-embryo. After transfer into the uterus, the embryo hatches: it extrudes itself through the channel and implants into the uterine wall. Laser-assisted hatching can result in improving implantation and pregnancy success rates. We present examples of zone pellucida ablation using animal models. In using the laser it is vital not to damage pre-embryo cells, e.g. by overheating. In order to define safe regimes we have derived some thermal side-effects of zona pellucida removal. The temperature profile in the beam and vicinity is predicted as function of laser pulse duration and power. In a crossed-beam experiment a HeNe laser probe detects the temperature-induced change in refractive index. We find that the diode laser beam produces superheated water approaching 200 C on the beam axis. Thermal histories during and following the laser pulse are given for regions in the neighborhood of the beam. We conclude that an optimum regime exists with pulse duration <EQ 5 ms and laser power approximately 100 mW.

Current established solid state Raman laser (SSRL) materials tend to be oxides or tungstates, which have low thermal conductivity and therefore inherently limited power scaling potential. We have tested the Raman material bulk undoped gallium phosphide (GaP), which has excellent thermal and mechanical properties, and assessed its ability to power scale. Pumping GaP with 1.06 and 1.3 micron Q- Switched Nd:YAG lasers has resulted in outputs of up to 14 mJ, the highest pulse energy GaP Raman laser known to date. Conclusions from laboratory tests and finite element modeling indicate that this Raman laser material can scale to kW average output power levels. We are currently investigating Raman lasers that will improve the pump laser spatial beam quality during Raman conversion. This could be developed into an add-on 'kit' that would improve beam quality in Nd:YAG industrial lasers with power output levels of over 100 W. We will present our latest GaP Raman laser laboratory results and discuss power scaling performance estimates.

CW laser action has been demonstrated on the electronic I* (²P_1/2) yields I(²P_3/2) transition of atomic iodine at 1.315 micrometer from the NCl (a¹(Delta) ) + I(²P_3/2) energy transfer reaction. The stimulated emission was generated in a transverse subsonic flow device when hydrogen azide, HN₃ was injected into a flow of iodine and chlorine atoms. The measured laser output power was 180 mW.