The strength of empires and civilizations has often depended on novel forms of transportation: the Viking long boat, the Roman road, Iberian galleons, French and British steam ships, Indian trains, the car of the early twentieth century, the plane of the middle and the rocket of late. But Space has now come up against a barrier: the enormous and barely affordable expense of putting things into orbit and the unaffordable energy required to travel to the stars. The recent advent of very energetic lasers may reduce the cost. The pioneering ideas of the mid sixties appear less fanciful. Laser space propulsion is about to become such an important topic that its scientific origin and engineering roots need to be investigated. This is by no means an exhaustive survey. We review here the laser propulsion work of four eminent experts: Eugen Saenger, George Marx, Wolfgang Moeckel and Arthur Kantrowitz.

Laser-induced material processing is reviewed with special emphasis on recent achievements mainly obtained by the Linz group. Among those are investigations using optical fiber tips for nanoscale photophysical etching, laser-induced ablation using self-assembled microspheres, the pulse-laser deposition of thin films of high-temperature superconductors (HTS) and polytetrafluoroethylene (PTFE), and the modification and cleaning of surfaces.

At the Laser Zentrum Hannover investigations of possibilities to use femtosecond laser pulses for direct ablative writing and microstructuring of solid materials have been started in 1995. Since then considerable progress in understanding and in the application of different femtosecond technologies has been obtained. At present, we are able to produce high quality microstructuring and large area patterning of solids with structure sizes between one and ten micrometers. By using tightly focused femtosecond pulses it is possible to produce even sub-micrometer structures. In this paper we pursue the goal to find and characterize the limits of femtosecond laser micromachining. Detailed investigations of possibilities to use femtosecond lasers for the sub-wavelength microstructuring of metals and for fabrication of periodic structures in transparent materials with the scale length of the order of several hundreds nanometers are reported.

The US Air Force Airborne Laser (ABL) is an airborne, megawatt-class laser system with a state-of-the-art atmospheric compensation system to destroy enemy ballistic missiles at long ranges. This system will provide both deterrence and defense against the use of such weapons during conflicts. This paper provides an overview of the ABL weapon system including: the notional operational concept, the development approach and schedule, the overall aircraft configuration, the technologies being incorporated in the ABL, and the risk reduction approach being utilized to ensure program success.

Excimer laser polymer ablation has been an active field of research and development for some twenty years now. We briefly review basic mechanistic aspects of the interaction, practical considerations related to polymer processing by ablation and applications in micromachining.

The electric-discharge-excited carbon monoxide laser is one of the most efficient laser sources known that is scalable to very high continuous wave powers. We review work at Ohio State where such lasers are used to excite flowing molecular gas plasmas, in mixtures of CO and other diatomic gases, including air. These plasmas are stable, diffuse, and can be operated at high gas pressures and low gas kinetic temperature. They are being employed for various plasma chemistry applications. Recent results are presented in which such an optically pumped plasma reactor is used to synthesize single-walled carbon nanotubes, which show surprising order and alignment.

Time resolved studies using femtosecond laser pulses at 800 nm illuminate the distinctions in the dynamics of ultrafast processing of dielectrics compared to semi-conductors and metals. Dielectric materials are strongly charged at the surface on the sub-ps time scale and undergo an impulsive Coulomb explosion prior to thermal ablation. Provided the laser pulse width remains in the ps or sub-ps time domain this effect can be exploited for processing. Otherwise, the high localization of energy accompanied by ultrafast laser micro structuring is of great advantage also for high quality processing of thin metallic or semi-conductive layers, where the surface charge is effectively quenched.

The ablation characteristics of various polymers were studied at low and high fluences. The polymers can be divided into three groups, i.e. polymers containing triazene groups, designed ester groups, and reference polymers, such as polyimide. The polymers containing the photochemically most active group (triazene) exhibit the lowest threshold of ablation (as low as 25 mJ cm^-2) and the highest etch rates (e.g. 250 nm/pulse at 100 mJ cm^-2), followed by the designed polyesters and then polyimide. Neither the linear nor the effective absorption coefficients reveal a clear influence on the ablation characteristics. The different behavior of polyimide might be explained by a pronounced thermal part in the ablation mechanism. The laser-induced decomposition of the designed polymers was studied by nanosecond interferometry and shadowgraphy. The etching of the triazene polymer starts and ends with a laser pulse, clearly indicating photochemical etching. Shadowgraphy reveals mainly gaseous products and a pronounced shockwave in air. The designed polymers were tested for applications ranging from microoptical elements to polymer fuel for laser plasma thrusters.

Ultrafast time resolved microscopy of femtosecond laser irradiated surfaces reveals a universal feature of the ablating surface on nanosecond time scale. All investigated materials show rings in the ablation zone, which were identified as an interference pattern (Newton fringes). Optically sharp surfaces occur during expansion of the heated material as a result of anomalous hydrodynamic expansion effects. Experimentally, the rings are observed within a certain fluence range which strongly depends on material parameters. The lower limit of this fluence range is the ablation threshold. We predict a fluence ratio between the upper and the lower fluence limit approximately equal to the ratio of critical temperature to boiling temperature at normal pressure. This estimate is experimentally confirmed on different materials (Si, graphite, Au, Al).

High quality thin films (100nm to several 1000nm) of many complex material systems such as the high temperature superconductor YBCO, have been deposited using pulsed laser ablation. Long lengths of high quality YBCO on metallic conductors are needed to meet industrial and government requirements. Although, pulsed laser deposition (PLD) grows the best YBCO films on meter lengths of metallic conductors, it has not been proven to be a consistent manufacturing process. PLD is simple in concept but has quite complicated plume and growth dynamics which are not well understood. In this study we use in-situ time-resolved mesurements of collision-driven plume emissions to characterize critical deposition parameters and to develop a computational model which includes these dynamics in a reacting PLD plume.

Presently, from the world data available on interaction of laser radiation with metal and dielectric surfaces and development of experimental diagnostic techniques by itself it is possible to raise a reverse question, namely, restoration of laser radiation energy spatial distribution through surface imprint. With this in mind, we have made the detailed morphology of imprint of a pulse HF-laser interaction with carbon steel surface through atomic force microscope. Interaction of the radiation of a relatively short pulse of ArXe-laser ((tau) _½equals400 ns, (lambda) equals1,73 micrometers , Q ~ 20 J/cm+2)) with carbon steel surface of 40X (4140 (USA)) type besides the surface layer hardening leaded to decrease of steel hardness at below original value on depth of about 10 - 12 micrometers . This effect can be related to occurrence of deformation and thermal residual stresses in the subsurface layer. At multiple-pulse interaction (~500 pulses) of UV-laser focused radiation ((lambda) equals0,222 or 0,308 micrometers , (tau) _½equals 12 or 20 ns) with the steel 40X (4140 (USA)) surface a distinct interference imprint of laser beam is formed after diffraction at outer diaphragm being determined by its form.

The present paper deals with the impact of pulsed laser radiation on dielectric and semi-conducting materials. Energy trans-fer leads to fast heating and thermal ablation. Mechanical stresses additionally cause deteriorations, cracks, fragmentation or perforation. These processes develop quasi-explosively even at low energy levels. Laser effects are discussed using microsecond(s) - to ps- pulses, acting both on optically passive and active materials (optical sensors). Evaluations of plasma-sustained en-ergy transfer by the impact of ns-pulses in comparison to ps-pulses are included as well. In the medium energy range by contrast, target effects are considered which require three to four orders of magnitude higher energies per pulse and corre-spondingly higher average powers which in the burst mode are determined by the repetition rate. The repetitively pulsed CO₂-laser at ISL provides energies up to 150 J, repetition rates up to100 pps, an average power of 15 kW corresponding to peak power values of 75 MW for 2 microsecond(s) pulses. This laser provides a valuable tool, particularly suited for large area out-of-band target studies. Thermo-mechanical processes, as experimentally observed are discussed and compared with results of numerical simulation. Transparency changes of optics due to ablation as quantitatively measured by the modulation trans-fer function are further included in the discussions.

Laser ablation of transparent materials is induced by non- linear absorption and some laser-induced damages are introduced in the bulk as well as on the surface. This process is used in laser marking and other applications such as refractive index modification of optical materials and 3- D data storage. We have observed the laser ablation dynamics in inside of bulk transparent materials by nanosecond time- resolved imaging technique. Output of fundamental radiation (1064nm) from a Q-switched Nd:YAG laser was focused at the inside of bulk PMMA and soda glass. Second harmonic radiation (532nm) from the same laser was used as illuminating light and images were taken by a CCD camera with a band-pass filter at 532 nm. Series of images were taken at different intervals between the fundamental and the second harmonic light, which was controlled by the optical delay line. In observation at longer intervals than 50ns, another laser was used as illuminating source. When the laser was focused at inside of the bulk PMMA, damages occurred simultaneously at several independent points without the ablation at the surface. They located along laser incident axis. Propagation of shock waves, which started from these points, was clearly observed in the bulk. In the glass, absorbing point in the bulk formed a continuous line and its end-point moved from inside outward to the surface along the laser beam. Laser induced damages (cracks) continued to develop until some microseconds after laser pulse in PMMA.

The excimer laser nitriding and carburizing process reported is developed to enhance the mechanical and chemical properties of aluminum alloys. An excimer laser beam is focused onto the alloy surface in a cell containing 1 bar nitrogen or propylene gas. Vapor plasma expands from the surface then dissociates and ionizes ambient gas. Nitrogen or carbon atoms from plasma in contact with the surface penetrate in depth due to plasma recoil action onto the target surface heated by the plasma. It is thus necessary to work with a sufficient laser fluence to create the plasma, but this fluence must be limited to prevent laser-induced surface roughness. The nitrogen or carbon concentration profiles are determined from nuclear analysis. Crystalline quality is evidenced by X Ray Diffraction (XRD) technique. Transmission Electron Microscopy (TEM) gives the in-depth microstructure. Fretting coefficient measurements exhibit a satisfying behavior for some experimental conditions. The polycrystalline nitride or carbide layer obtained is several micrometers thick and composed of pure A1N or Al₄C₃ (columnar microstructure) top layer standing on a diffusion layer.

A significant improvement in the quality of ultrafast laser micromachining of brittle dielectrics is demonstrated by using temporally shaped pulse trains with sub-ps separation, synchronized with the material specific relaxation times. The individual material response to laser radiation depends on the efficiency of electron generation and on the ability to release the energy into the lattice. Loss mechanisms in the electron population, surface charging, as well as the strength of the electron-phonon interactions control the effectiveness of the energy deposition into the lattice. Knowledge of the response times of materials establishes a guideline for using temporally shaped pulses or pulse trains in order to optimize the structuring process with respect to the efficiency of material removal and reduction of the residual damage. The sequential energy delivery induces a material softening during the initial steps of excitation changing the energy coupling for the subsequent steps. This leads to lower stress, cleaner structures, and provides a material-dependent optimization process.

Laser annealing, laser surface processing and laser lift-off procedure are reviewed as applied to semiconductor nitride- based structures (GaN films and InGaN/GaN optoelectronic device structures grown on sapphire substrates). Data on laser ablation of composite GaN/sapphire material are reviewed with more detailed consideration of the ablation rate under subpicosecond laser pulses and under long-pulse irradiation in the IR range (wavelengths of 5.0-5.8 and 9.6 micrometers ).

The CIP method is used to calculate macroscopic plume expansion combined with the Zeldovich-Raiser theory for cluster formation process such as nucleation and growth. The effect of background gas and latent heat is examined in one-dimensional case. The latent heats keep the plume temperature at 2500K for a long period and this explains the delayed photoluminescence. Two contradicting experiments on the size dependence on ambient pressure are clearly explained and are attributed to the difference of laser energy. In two dimensional simulation, mushroom-like plume shape is replicated consistent with experimental results.

Laser-induced surface structuring of silicon was studied using fluences close to the metering threshold and He gas background atmosphere. The effects of an initial surface microstructured region and of light polarization on the evolution of the surface topography were investigated. The microstructured surface topology consisted of an array of microholes surround by microcones of 2-3 micrometers tip-diameter and over 20 micrometers high. Pulsed laser irradiation of laser- microstructured silicon induces the formation of nanostructures. Nanocolumns having a diameter of 100 to 200 nm and reaching a height of up to 3 micrometers upon cumulative laser pulses grow on top of every microcone. The mechanisms of nanocolumn origin and growth are analyzed. Periodic undulations approximately 10 nm-high are formed when flat silicon substrates are irradiated with polarized laser light. These periodic structures have a wavelength that is a function of the light wavelength and the angle of incidence of the laser beam. At a slightly higher laser fluence, approximately 30 nm-diameter nanoparticles form on the surface of laser irradiated flat silicon specimens. Linear arrays of silicon nanoparticles with fairly uniform size that extend up to a millimeter are formed if the irradiation is performed using polarized light or the irradiated area contains a microstructured region. These nanostructures are analyzed within the frame of the theory of laser induced surface periodic structures.

We use short-pulse high-power lasers to selectively modify the structure of nanolaminates and nanocrystals. It is demonstrated that femtosecond pulses can achieve superior results for microscopic thin film removal. Laser pulses can also be used to modify the crystal structure of thin films. It is also demonstrated that coherent laser excitation promotes a selective modification of nanocrystals, resulting in the change of size, shape and crystal structure of nanocrystals.

We present studies of the consequences of simultaneous exposure of inorganic single crystals to radiation and water. The first case consists of a biomineral namely CaHPO ₄ 2H₂O (brushite) which is a wide band gap, hydrated inorganic single crystal. We examine the laser induced ion and neutral emissions accompanying 248-nm excimer laser radiation. Both types of emission are several orders of magnitude higher following exposure to 2 keV electrons at current densities of 200 μA/cm 2 and doses of 10²-10³ mC/cm². We show the that the enhancements in emission are strongly correlated with e-beam induced morphology changes (including recrystallization) on this unusual surface. We then examine similar effects on dry crystals such as NaCl and NaNO₃ which are exposed to 10^-5 Pa partial pressures of H₂O. Again dramatic enhancements in radiation induced emissions are exhibited along with the generation of unique morphological structures with nanometer scale dimensions.

For 10 previous years dry laser cleaning has been analyzed in the frame of 1D model with homogeneous surface heating. This model gave qualitative description of the process and was sufficient for initial studies. Nevertheless further examinations show that 1D model is in one-two order magnitudes discrepancy with experiment. The problem is that the particle on the surface produces non-homogeneous distribution of laser intensity. For example, a small transparent particle can work as a near-field lens. This produces nonstationary 3D distribution of temperature and nonstationary 3D thermal deformations of the surface. 3D model is qualitatively different from the 1D model (the latest does not permit the inward motion of the surface). In some region of parameters 3D model predicts a result close to the experimental one (for small particles, typically smaller than 1 micrometers ). With higher particle size intensity under the surface becomes so strong that the particle is removed by the flux of evaporated material.

Laser microprocessing has been extensively studies with applications in microelectronics, data storage and photonics. In addition to the fundamental aspects of laser materials interactions, we have investigated various applications of laser microprocessing in different areas. Laser cleaning has been studies systematically both theoretically and experimentally for dry surface cleaning and steam surface cleaning. This technology has been applied for cleaning magnetic head, magnetic sliders, suspension, laser mold cleaning and laser deflash for IC packages. Laser texturing and related processes such as laser bumping, laser tagging have been studied for magnetic recording applications. The other laser works include real-time monitoring of laser surface processing, laser-induced controllable periodic structures, laser nanopatterning by scanning probe microscope tip-enhanced laser irradiation. The further prospects of using laser microprocessing for applications in formation of ultrashallow (less than 50 nm) pn junction for next-generation MOS devices, laser generation of Si nanoparticles for quantum-dot flash memory and light emission devices are addressed.

In the present work, the laser ultrasonics technique has been applied as diagnostic of the laser induced decohesion of coating. A modeling system consisting in a 304 stainless steel coated by a plasma sprayed layer was used to describe the basic measurements. These acoustic measurements were performed using an interferometric probe. It is concluded that this contactless technique can be used to follow the cleaning of oxidized materials.

Hybrid laser processing for precision microfabrication of glass materials, in which the interaction of a conventional pulsed laser beam and another medium on the material surface leads to effective ablation and modification, is reviewed. The main role of the medium is to produce strong absorption of the nanosecond laser beam by the materials. Simultaneous irradiation of the vacuum ultraviolet (VUV)laser beam, which possesses extremely small laser fluence, with the ultraviolet (UV) laser greatly improves the ablation quality and modification efficiency for fused (VUV-UV multiwavelength excitation processing). Metal plasma generated by the laser beam effectively for assists high- quality ablation of transparent materials, resulting in microstructuring, cutting, color marking, printing and selective metallization of glass materials (laser-induced plasma-assisted ablation (LIPAA)). The detailed discussion described in this paper includes the ablation mechanism of hybrid laser processing.

Femtosecond and nanosecond laser ablation and delamination of anodic oxide layers on an aluminium alloy were investigated. Laser wavelengths of 800 nm (125 fs pulse duration) and 532 nm (7 ns), respectively, and two oxide layers with thicknesses of 70 micrometers (opaque) and 30 micrometers (transparent) were considered. Laser-induced shock waves measured by means of a microphone provided an in-situ diagnostic tool for the monitoring of the coating removal process. The transparent anodic oxides showed contrasting ablation mechanisms in the nanosecond and femtosecond pulse duration regime. Nanosecond pulses led to spallation, whereas femtosecond treatment resulted in ablation.

Pulsed laser deposition of complex, chemically sensitive polymers using tunable, picosecond infrared laser excitation has shown great promise for producing films of these materials appropriate for a wide variety of sensor and coating applications. Fourier-transform infrared spectra of the bulk starting polymers and those of the deposited thin films are nearly identical, verifying that the short-range order and chemical functionality of the polymers are preserved during the process. Gel permeation chromatography and mass spectrometry have been used to characterize the polydispersity of the mass distribution; here the results are mixed, with the mass distributions of some poly-mers being preserved while others show significant bond scission. Most recently, we have demonstrated that it is possible to coat cantilever structures with the polymer SFXA; the deposited polymer responds as desired when 'challenged' by appropriate chemicals.

Nanosecond-interferometry and shadowgraphy is used to observe the dynamic behavior of the etching process during and after the irradiation pulse. Commercially available polymers exhibit quite often poor laser ablation properties for irradiation wavelengths >=248nm. At these wavelengths the absorption is due to the quite photostable aromatic groups. A photolabile triazene polymer was selected to compare the influence of a photolabile group on the laser ablation process. The photochemical active triazene reveals a strong absorption band at 332 nm and is responsible for the observed high etch rates and the low threshold for 308 nm irradiation. The absorption coefficients at 193 nm and at 308 nm are comparable, allowing to study the influence of the different absorption sites by ns-interferometry and shadowgraphy measures. The etching of the triazene polymer starts and ends with the laser beam. No surface swelling, which is assigned to photothermal ablation, is detected for fluences above the threshold of the ablation. The expansion of the laser ablation induced shockwave was measured for the photolabile triazene polymer and the photostable polyimide. The speed of the shockwave increases with fluence and is higher for irradiation with 193 nm than for 308 nm. A shockwave with equal or higher velocity is observed for the triazene polymer than for the polyimide.

The technology of laser sintering is introduced. Based on the researching of laser heating property, power thermal physics parameters, as well as laser sintering process, a numerical model of the temperature field during length- alterable line-scanning laser sintering of polymer-coated metal powder has been constructed. The calculations of numerical model have been finished by the using of Finite Element Method (FEM). The temperature field during laser sintering was measured to verify the calculated results. Experimental results show well conformity between them.

The study examines chemical and structural modifications effected in the UV ablation of polymers. For the study of the chemical processes, aromatic photosensitive compounds with well-defined photochemistry are employed as dopants and their reactivity is examined as a function of laser parameters (fluence, wavelength and laser pulse width). A 'pump-probe' scheme based on laser-induced fluorescence is employed for monitoring photoproduct formation in the polymeric substrate following UV irradiation. Ablation is shown to result in a change of the photolysis degree of the dopant and in the efficient formation of bi-aryl compounds, indicative of a high species mobility. Furthermore, kinetics of photoproduct formation in the ablative regime is shown to differ distinctly from that in the irradiation at low laser fluences. However, the quantitative extent of these changes is critically affected by the absorptivity of the substrate at the irradiation wavelength. On the other hand, structural modifications induced in polymer films are probed via holographic interferometry. Deformations are shown to be induced at distances far away (approximately 2-3 cm) from the irradiation spot. The implications for UV laser material processing schemes are briefly discussed.

Two new laser mask projection techniques Synchronized Image Scanning (SIS) and Bow Tie Scanning (BTS) have been developed for the efficient fabrication of dense arrays of repeating 3D microstructures on large area substrates. Details of these techniques are given and examples of key industrial applications are shown.

Titanium carbide (TiC) thin films have very desirable properties that make them ideally suited for applications to MEMS devices. TiC has been one of the preferred coatings for improving the performance of macroscopic moving mechanical components due to its established wear-resistance. Pulsed laser deposition (PLD) has been an excellent method for the deposition of TiC because unlike any other deposition process for TiC, PLD offers the capability of producing high-quality films even at room-temperature. Using a patented PLD technique, especially designed and optimized for the deposition of high-hardness, particulate-free films, we have deposited TiC coatings on a variety of surfaces, including Si and several MEMS compatible film-layers. Our results have demonstrated that TiC coatings also offer a high wear-resistance to Si surfaces. This, together with the excellent chemical and mechanical properties, and thermal stability of our PLD TiC coatings, has led to our application of TiC to moving MEMS devices fabricated from Si. The fabrication of Si MEMS devices is quite well established, however, the reliability and performance of MEMS devices such as microgears, micromotors and microactuators, which involve sliding Si surfaces, remain an open question. The short functional life of these devices is attributed to the excessive wear rate of Si induced by high friction. The work presented here describes a hybrid process whereby PLD is used in conjunction with the Aerospace MEMS fabrication process (AIMMOS) for inserting TiC coatings into critical interfaces in MEMS devices that would involve sliding contact between two Si surfaces. The PLD of TiC, the spectroscopy of the plume, and the properties and applications of PLD-TiC for MEMS will be discussed.

Pulsed laser ablation is a well-known technique used for thin film deposition, extending from oxydes to hard and wear resistant Diamond-Line Carbon (DLC) films. Most of the previous studies devoted to DLC thin films elaboration have used pulsed duration in the nanosecond range. The present study concerns femtosecond (10^-15 s range) laser ablation of a graphite target for the elaboration of Diamond-Like Carbon. Compared to conventional nanosecond laser ablation, femtosecond laser pulses allow the production of high energy (up to a few keV) ions in the plasma, which may strongly affect the structure and properties of the deposited films. DLC films have been deposited under vacuum onto (100) p-type silicon substrates at room temperature, by ablating graphite targets with femtosecond laser pulses. The nature and properties of the film have been characterized by various techniques, including Raman, XPS and AFM. Discussion will be focused on the comparison between present results obtained using femtosecond laser pulses, with previously published results related to DLC films deposited using nanosecond laser pulses. Especially, Raman spectra of DLC films obtained by nanosecond laser ablation always show the two well-known D and G bands (located respectively at around 1350 cm^-1 and 1550 cm^-1), whereas some DLC films obtained when using femtosecond laser pulses exhibit an intense peak at 1140 cm^-1, which may be attributed to nanocrystalline diamond.

We have studied the shock induced pressures due to the interaction of soft X-ray thermal radiation with foam-layered metal targets. Thermal X-ray radiation was produced by focusing a high energy laser inside a small size hohlraum. Experiments were performed using a 3ω converted (λ=0.44 μm) Iodine laser of τ≈450 ps (FWHM) duration. An increase in shock pressure was observed with the foam layered targets as compared to bare metal targets. Results are analyzed on the basis of the role of foam density and thickness.

We have used femtosecond laser pulse pairs to measure the positive ion yield, from wide band-gap single crystals, as a function of time-delay between pulses. Two pulse correlation allows direct observation of solid state and surface dynamics on an ultrafast timescale. The ion yield, from 265 nm irradiated MgO and KBr, depends critically on the time delay between two sub- threshold pulses. For example, the Mg⁺ desorption yield displays three distinct features; a coherence peak, followed by rise, and decay features. In constrast, the yield of K⁺ from KBr displays only the coherence peak and picosecond decay features. The data suggest, that although the nanosecond ion desorption mechanism is dominated by defect photoabsorption, significant electron-hole pair production may contribute to the desportion mechanism following femtosecond excitation. Nanosecond photoexcitation of KBr near 64 eV leads to desorption of hyperthermal neutral bromine atoms without a significant thermal velocity component. Two-photon femtosecond excitation at 3.2 eV produces very similar results. Multiphoton femtosecond excitation provides an efficient excitation mechanism of the wide-gap material. There results are likely general for ionic crystals and are consistent with a recently described theoretical model.

Semiconductor materials are still the material of choice also in many microsystems applications. This is mainly justified by the availability of mature processes and equipment originally developed for microelectronics fabrication. However, for microsystems more flexible requirements have to be fulfilled and new tools have to be developed especially with regard to smaller part numbers than in microelectronics. But also in microelectronics, conventional machines have often reached their limits in semiconductor processing which leads to the requirement of new processes. Lasers are generally able to ablate semiconductor materials. Especially ultrafast lasers are able to perform processes with high efficiency and accuracy. One of the most challenging conditions is not to influence the bulk material. Within this paper, the general interaction of ultrafast lasers with semiconductors is investigated. The ablation process is outlined and beam parameters influencing the removal and especially the cutting process are described, and potential applications are shown.

These are discussed microscopic physical processes underlying early phases of femtosecond laser-induced damage and ablation of wide band-gap solid dielectrics. They can result in 1) variations of energy-level structure of electron subsystem (in particular band-gap), and 2) instability of crystal structure. Those two microscopic processes are local and can be detected through different effects (variations of linear and nonlinear optical response and x-ray diffraction) each having its own characteristic time. There is analysed and estimated possibility of developing of instability of crystal structure and structure of electron energy levels of the solids under action of positive feedbacks. The feedbacks results from laser-induced ionization, straight action of electric field of laser radiation on ions, both resulting in increasing of density of laser-induced point defects. Estimations are obtained on the basis of the simplest relations describing laser-induced microscopic processes. Obtained results are compared with experimental data and shown to be capable of explaining some features of femtosecond laser interaction with transparent media.

The advent of table top high repetition rate regeneratively amplified femtosecond lasers has opened the way to many recent and fast developments towards applications of economical interest. The most well known are microprocessing, thin film elaboration, waveguide photoinscription, surface treatment, dentistry, ophthalmology. Recent studies on microprocessing and laser-matter interaction using femtosecond lasers are reported. This is done using largely presently performed work in Saint-Etienne including investigations on Heat Affected Zone (HAZ), plume expansion characterization, and thin film elaboration. Indeed specific characters appear as compared to what is obtained using multipicosecond/nanosecond laser pulses : Typical submicronic HAZ lengths have been evidenced and particle energies of plasma plume ranging up to a few KeV (carbon target) using typical pulse energies of 1 mJ (150 fs, 800 nm, 1 KHz), creating specific conditions for deposition. The concept of the often vocabled 'athermal' interaction is discussed. Emphasis on actual microprocessing capability of the existing sources to approach industrial applications are questioned in terms of energy per pulse, timewidth, repetition rates and the need for further source development and control beam improvement stressed. A brief review of the progresses under way in these fields and their capability to answer to actual large scale commercial applications are given.

We have developed an automatic pulsewidth tunable femtosecond Ti:sapphire laser system that can generate an output of 50fs-1ps and sub-mJ/pulse at a repetition rate of 1 kpps. The automatic pulse compressor enables on to control the pulsewidth in the range of 50fs-1ps by the use of a personal computer (PC). We describe our recent results of tailored ablation processing of advanced functional materials such as GaN, BN, and hydroxyapatite. By use of the femtosecond laser pulse tailored for a specific material, we have demonstrated precise processing without chemical composition change and heat affected zone.

From both a theoretical and an experimental point of view, the properties of fs laser pulses are fascinating. For a deeper understanding and for the prediction of the experimental results, the knowledge is necessary of absorption and of the optical penetration depth on the fs scale. For the description of the interaction of a fs-laser pulse with a solid, the stan-dard values of the properties of a solid are not suited because they have been experimentally determined or theoretically derived under the assumptions of a steady state and local thermal equilibrium. Depending on the property considered and the laser pulse duration either one or both of these conditions may be violated. In this paper, we derive equations for the optical properties of metals in the case of local thermal nonequilibrium be-tween the electron and phonon system. For a given laser intensity, we calculate, as an example, the absorption and opti-cal penetration depth for gold. For this purpose, we need the time dependent temperatures of the electrons and phonons. They are evaluated by means of our extended two temperature model. Finally, we compare the results with the standard equilibrium behavior as well as with the experimental findings and close with a short discussion.

There is a need in many scientific and manufacturing processes to drill to small diameter holes with high aspect ratios in both brittle materials as well as metals. Femtosecond lasers operating at 795 nm or frequency doubled to 400 nm provide a unique tool for carrying out these processes. In this work, the femtosecond laser nanomachining facilities at the University of Nebraska is discussed to drill 1 micrometers holes in Si/SiO₂ with aspect rations > 8. The quality of the cut and the small nanoparticles are discussed.

Fabrication of deep high-quality holes in different materials is required for many industrial applications. In this presentation we provide results of detailed investigations on deep drilling of metals by femtosecond laser pulses. We identify most important parameters which should be fulfilled for the fabrication of high-quality holes and efficient femtosecond laser ablation, discuss the role played by the laser-induced optical breakdown, debris, and laser pulse repetition rate.

Damage experiments of absorbing filters (Schott BG18 and BG36) were performed with Ti:sapphire laser pulses with durations from 30 fs to 340 fs (800 nm, 1 kHz) in air. The direct focusing technique was employed under single- and multi-pulse irradiation conditions. Ablation threshold fluences were determined from a semi-logarithmic plot of the ablation crater diameter vs. laser fluence. The damage threshold fluence decreases for shorter pulse durations. In the investigated pulse duration range, the measured multi-pulse ablation threshold fluences are practically similar to those of undoped glass material (~1 Jcm^-2). That means that the multi-pulse ablation threshold is independent on the doping level of the filters. For more than 100 pulses per spot and all pulse durations applied, the threshold fluence saturates. This leads to technically relevant damage threshold values in the femtosecond laser pulse duration domain.

We describe several scenarios of basic femtosecond machining and materials processing that should lead to practical applications. Included are results on high through-put deep hole drilling in polymers and glasses in ambient air, and precision high speed micron-scale surface modification of materials.

Femtosecond laser ablation shows promise in machining energetic materials into desired shapes with minimal thermal and mechanical effects to the remaining material. We will discuss the physical effects associated with machining energetic materials and assemblies containing energetic materials, based on experimental results. Interaction of ultra-short laser pulses with matter will produce high temperature plasma at high-pressure which results in the ablation of material. In the case of energetic material, which includes high explosives, propellants and pyrotechnics, this ablation process must be accomplished without coupling energy into the energetic material. Experiments were conducted in order to characterize and better understand the phenomena of femtosecond laser pulse ablation on a variety of explosives and propellants. Experimental data will be presented for laser fluence thresholds, machining rates, cutting depths and surface quality of the cuts.

The results of theoretical studies are reported for threshold characteristics of a metal ablation by picosecond and femtosecond laser pulse. Two possible mechanisms of the laser ablation at laser fluence F ≤ F_th are considered: thermal mechanism of ablation connected with a kinetics of a metal-vacuum surface evaporation and the mechanism of ablation connected with a hydrodynamics of a dense matter. The analysis has been made within the framework of a two-temperature model of metals for femtosecond and picosecond region of laser pulse duration and the extended of a two-temperature model of the metal in the case when the surface temperature T_i more than the critical temperature of metals. Analytical expressions for the ablation-threshold fluency F_th as well as the threshold values of the lattice temperature and the characteristic time of lattice temperature decay t_d(F_th) are obtained. This analytical description is in satisfactory agreement with particular numerical calculations.

Laser ablation has proven to be an important technology in an increasing number of applications. The fundamental mechanisms underlying laser ablation processes are quite complicated, and include laser interactions with the target as well as plasma development off the target. While substantial progress has been achieved in understanding laser ablation on the nanosecond and picosecond time scales, it remains a considerable challenge to elucidate the underlying mechanisms during femtosecond laser ablation. We present experimental observations of plasma development inside silica glass during single femtosecond laser pulse (100 fs, 800 nm) irradiation. Using a femtosecond time-resolved imaging technique, we measured the evolution of a laser-induced plasma inside the glass that has an electron number density on the order of 10¹⁹ cm^-3. Additionally, we observed an air plasma outside the target which forms long before the explosion of a material vapor plume.

A series of experiments has been performed to investigate the interaction of intense laser pulses with cryogenic noble gas droplets. Understanding of the time scales for this interaction is important for optimization of extreme ultraviolet (EUV) sources for next-generation lithography that utilize laser-produced plasmas. The temporal character of the plasma formed by the irradiation of micron-sized argon and krypton droplets with intense 200-mJ, 100-ps laser pulses was investigated using a pump-probe scheme. The evolution of the droplet plasma was assessed by monitoring delay-dependent x-ray and EUV emission, and by imaging frequency-doubled probe light scattered from the interaction region. Depending on the spectral region of interest and the droplet characteristics, the effective plasma lifetime extends from a few hundred picoseconds to several nanoseconds. These results are explained in terms of the plasma expansion, excitation emission, and recombination emission time scales.

Within the PREUVE project, the GAP of CEA Saclay has developed an EUV source that should meet (alpha) -tool specifications by the end of this year. In particular, a laser-produced plasma source has been developed that uses a dense and confined xenon jet target. Our technical solution is based on a specific target injector design and the use of well adapted nozzle materials to avoid debris formation by plasma erosion. After injection, the xenon is recycled and highly purified to reach a low cost round- the-clock operation. This source provides both high conversion efficiency and low debris flux. These are necessary conditions for its industrial application in the future EUV microlithography. The conception of the so-called ELSA (EUV Lithography Source Apparatus) prototype allows in principal 2 years full operation on the French lithography test bench BEL (Banc d'essai pour la lithographie) that has been developed during PREUVE. In parallel, the EXULITE consortium that is coordinated by Alcatel Vacuum Technology France (AVTF) has started its activities in the frame of the European MEDEA+ initiative on EUV source development. In collaboration with Thales and the CEA, AVTF develops a prototype power source for EUV lithography production tools by the end of 2004. A low cost and modular high power laser system architecture has been chosen and is developed by Thales and the CEA to pump the laser plasma- produced EUV source.

As a mass limited target the water droplet laser plasma source has been shown to have many attractive features as a continuous, almost debris-free source for extreme ultraviolet (EUV) and X-ray applications. Through a dual experimental and theoretical study, we analyze the interaction physics between the laser light and the target. The hydrodynamic laser plasma simulation code, Medusa103 is used to model the electron density distribution for comparison to electron density distributions obtained through Abel inversion of plasma interferograms. In addition, flat field EUV spectra are compared to synthetic spectra calculated with the atomic physics code RATION.

This paper reviews the basic phenomena and technologies associated with design of repetitively pulsed CO₂ lasers operating at power levels of approximately 100 kW and pulse repetition rates of approximately 100 Hz. Such systems are of potential interest for ablative propulsion investigations and as potential drivers for higher power systems for launching of small payloads to space.

A modified electron beam controlled pulsed CO2 laser is used as a multi spectral multi purpose test bed in order to generate high power fundamental and first overtone laser transitions in CO. The revisited concept includes an all solid state power supply which provides a highly reproducible operation at pulse repetition frequencies of up to 100 Hz. The active gas mixture is recirculated in a closed loop and kept at near room temperature using conventional water cooling. Discrimination of the CO fundamental band is obtained by using specially coated dielectric mirrors and introducing additional intracavity diaphragms. Unprecedented laser pulse energies of 25 J are reported in the overtone transitions covering a spectral range between 2 micrometers and 3.5 micrometers . Further scaling of pulse energies is expected in the near future using larger diameter resonator mirrors.

This work presents an improved disk laser concept, where a diode- pumped disk is hydrostatically clamped to a rigid substrate and continuously cooled by a microchannel heat exchanger. Effective reduction of thermo-optical distortions makes this laser suitable for continuous operation at ultrahigh-average power.

Revision of theoretical model of CO laser kinetics was performed including new data on intra- and inter-molecular vibration exchange processes and vibration relaxation in highly excited CO molecules, spontaneous radiation Einstein coefficients for fundumental and overtone bands of CO molecules and cross sections for cascade excitation and de-excitation of CO molecules by electron impact.