This PDF file contains the front matter associated with SPIE Proceedings Volume 7005, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

In recent years new generations of precision lasers have been demonstrated and are increasingly available on an industrial level. For example high beam quality and diffraction limited Fiber lasers, Slab lasers, Disk lasers and still Rod lasers are used very successfully. This paper focuses on - ns and μs drilling of shaped holes by helical drilling¹ - drilling of extreme aspect ratios in dielectrics/glass by ns-slab lasers² - nm-size periodic structuring of polymers by interferometric approaches - ablation by ns- and ps-pulses for metal moulds - generation of waveguide structures in glass by fs-pulses.³ On the laboratory scale a next generation of diffraction limited short pulse lasers is at the horizon.⁴ In particular, ps-lasers at multi-hundred watts of average power with repetition rates of several MHz,² fs-lasers at 400W² average power and green, frequency doubled lasers at 200W are under construction. At the short end of pulses, attosecond lasers have been demonstrated and themselves shall open a new domain of interaction of light and matter⁵.

Femtosecond time-resolved small and wide-angle x-ray diffuse scattering techniques are applied to investigate the ultrafast nucleation processes that occur during the ablation process in semiconducting materials. Following intense optical excitation, a transient liquid state of high compressibility characterized by large-amplitude density fluctuations is observed and the build-up of these fluctuations is measured in real-time. Small-angle scattering measurements reveal the first steps in the nucleation of nanoscale voids below the surface of the semiconductor and support MD simulations of the ablation process.

An impulse measurement device and analysis package was conceived, designed, constructed, tested, and demonstrated to be capable of: measuring nanoNewton-seconds to milliNewton-seconds of impulse due to laser-ablation; being transported as carry-on baggage; set-up and tear-down times of less than an hour; target exchange times of less than two minutes (targets can be ablated at multiple positions for thousands of shots); measurements in air and in vacuum; error of just a few percent; repeatability over a wide range of potential systematic error sources; and time between measurements, including ring-down and analysis, of less than 30 seconds. The instrument consists of a cantilever (i.e. leaf spring), whose time-dependent displacement/oscillation is measured and analyzed to determine the impulse imparted by a laser pulse to a target. These shapes are readily/commercially available, and any target material can be used, provided it can be fashioned in the form of a cantilever, or as a coating/film/tape, suitable for mounting on a cantilever of known geometry. The instrument was calibrated both statically and dynamically, and measurements were performed on brass, steel, and Aluminum, using laser pulses of ~7ns, ~500ps, and ~500fs. The results agree well with those published in the literature, with surface effects, atmosphere, and pre-/post-pulses demonstrating interesting effects and indicating areas for further study. In addition to exploring space-propulsion applications, measurements were performed to explore the strong beneficial effects of depositing lines of energy ahead of supersonic and hypersonic vehicles. This deposition creates a low-density channel, through which a vehicle can travel with dramatically reduced drag. Temperature and pressure are both also reduced on the front surfaces of the vehicle, while density and pressure are increased at the vehicle base. When applied off-center, this technique can be used to control the vehicle, employing the entire body as the control surface and eliminating the need for actuators. Numerical results for drag-reduction, temperature-reduction, and control forces are indicated here.

The physical mechanisms and processes underlying the erosion of a surface induced by cluster bombardment or short-pulse laser irradiation are highlighted. When the average energy delivered per atom in the vicinity of the surface becomes comparable to the cohesive energy of the solid, sputtering from a so-called spike may result. Such a spike leads to abundant sputtering (surface erosion) and crater formation. Direct atomization in the region of highest energy deposition, as well as melt flow and gas flow contribute to the erosion. The materials phenomena occurring after ultra-fast laser irradiation of a metal in the ps- or fs-regime are reviewed. With increasing laser fluence, the film melts, voids are formed, the film tears (spallation), and finally fragments to form a multitude of clusters. These processes are universal in the sense that they occur in widely differing materials such as metals or van-der-Waals bonded materials. We investigate a Lennard-Jones solid as well as four different metals (Al, Cu, Ti, W), which vary widely in their cohesive energy, melting temperature, bulk modulus, and crystal structure. When the energy transfer starting the process is scaled to the cohesive energy of the material, the thresholds of these processes adopt similar values. A comparison of the similarities and differences of the mechanisms underlying surface erosion under cluster ion impact and ultrafast laser irradiation will be drawn.

We model an interaction of femtosecond laser pulses (800 nm, 100 fs, 10E¹²-10E¹⁴ W/cm²) with metal targets (Al, Au, Cu and Ni). A detailed analysis of laser-induced phase transitions, melting wave propagation and material decomposition is performed using a thermodynamically complete two-temperature equation of state with stable and metastable phases. Material evaporation from the surface of the target and fast melting wave propagation into the bulk are observed. On rarefaction the liquid phase becomes metastable and its lifetime is estimated using the theory of homogeneous nucleation. Mechanical fragmentation of the target material at high strain rates is also possible as a result of void growth and confluence. In our simulation several ablation mechanisms are observed but the major output of the material is found to originate from the metastable liquid state. It can be decomposed either into a liquid-gas mixture in the vicinity of the critical point, or into droplets at high strain rates and negative pressure. The simulation results correlate with available experiments.

Ultrafast intense femtosecond laser pulses spontaneously undergo critical collapse in air and condensed media above some critical power. In normally dispersive media, such pulses can spontaneously generate dynamical X-waves where distinct X-features appear in the spectrally-resolved far-field. These nonlinear self-trapped pulses resemble linear Bessel beams - the latter exhibit extended line rather than point foci and are robust to strong perturbations. Nonlinear X-waves can be directly generated by using an axicon lens and have the potential to generate highly nonlinear, extended interaction zones relative to pulses with Gaussian profiles. Potential applications of these pulsed sources to controlling and extending white light supercontinuum and plasma channel generation will be discussed. X-wave generation in normally dispersive media is associated witha cascade of pulse splittings where individual split pulses have been identified with different arms of the spectrally observed X-feature. This allows for novel pump-probe experiments where a seed pulse can selectively generate Raman Stokes shifted waves by scattering off of different arms of the X-feature. We will discuss a 3-wave interaction picture that allows for a transparent physical interpretation of these complex spatio-temporal events.

The aim of this paper is to give a critical review of the influence of constant effective-mass approximation on the results provided by current theoretical models of laser-induced ionization of the solids transparent at input laser wavelength. Making transparent qualitative analysis of the models, we come to conclusion that assuming electrons and holes to have constant effective mass, one eliminates specific photo-ionization effects taking place at high intensity. The assumption of constant effective mass of conduction-band electrons is also shown to provide wrong description of the avalanche ionization almost at any laser wavelength and intensity. The presented analysis allows to give simple quantitative criteria determining the range of laser and material parameters for which the effective mass can be assumed constant.

Based on experiments and a theoretical analysis, we raise questions on two fundamental mechanisms of femtosecond laser desorption/ablation of solids, namely Coulomb explosion (CE) and plasma etching. The effects of laser-induced ionization and surface charging are analyzed which can be responsible for ultrafast ions observed in time-of-flight mass-spectra under ultrashort laser irradiation of solids. The importance of surface charging in formation of velocity distributions of desorbed/ablated species has been revealed for conditions when the CE mechanism is inhibited. The influence of ambient plasma formation on the dynamics of heating of metallic targets by femtosecond laser pulses is studied based on 2D modeling of laser-induced target heating and dynamics of the ambient plasma. The calculations show an intriguing picture of the laser-induced ambient gas motion. We propose a model of laser-induced breakdown of an ambient gas in a region in front of the irradiated target and analyze plasma-chemical processes which can affect laser processing of surfaces in the presence of air or highly reactive media.

Photolithography is well established in the fabrication of microfluidic networks; however, it is difficult to fabricate designs which require multiple depths. Furthermore, the cost/time to produce photolithographic masks is problematic, particularly when prototyping. Here we describe fabrication of microfluidic branching networks with multi-depth structures, ranging from 10s to 100s of microns, using a femtosecond fiber laser with 10 W average power, followed by chemical etching in a 10:1 solution of 49% HF and 69% HNO₃. While this technique was originally developed using a nanosecond laser, this unique femtosecond laser enables greater processing precision and faster overall processing speed.

This paper describes the status of the stadium-sized National Ignition Facility (NIF), the world's largest laser system and first operational multi-megajoule laser. The 192-beam NIF, located at Lawrence Livermore National Laboratory (LLNL), is 96% complete and scheduled for completion in March 2009. The NIF laser will produce nanosecond laser pulses with energies up to approximately 4 MJ in the infrared (laser wavelength = 1.053-μm) and 2MJ in the ultraviolet (laser wavelength = 0.35-μ m). With these energies NIF will access conditions of pressure and temperature not previously available on earth, allowing it to conduct experiments in support of the nation's national security, energy, and fundamental science goals. First ignition experiments at NIF are scheduled for FY2010. This paper will provide an overview of the NIF laser and the ignition, energy, and fundamental science activities at NIF.

We investigate the fundamental mechanisms of resonant-infrared laser ablation of polymers using polystyrene as a model material. Time-resolved plume shadowgraphy coupled with laser-induced temperature-rise calculations indicate that spinodal decomposition of a superheated surface layer is the primary mechanism for the initial stages of material removal. The majority of the ablated material is then released by way of recoil-induced ejection of liquid which proceeds for some tens of microseconds following a ~μs laser pulse excitation. The recoil-induced ejection of liquid material as the dominant ablation mechanism helps to explain previous observations of laser deposition of intact polymeric material.

Laser-Induced Breakdown Spectroscopy (LIBS) has been used since 40 years on typical samples such as metals, alloys, rocks. Detection of organic hazards or analysis of biological compounds under atmospheric pressure with LIBS represents a new challenge. For this purpose, we need better understandings of the physico-chemical properties of the plasma in atmosphere and their influences on the LIBS signal. As a model sample of organic materials, Nylon 6-6 has been studied under nanosecond ablation at different wavelengths (1064 nm and 266 nm) and energies (from 1 to 5 mJ) in order to observe the influence of these parameters. Shadowgraph technique is used to image the plasma at its early stage of expansion (0 to 40 ns). Time-resolved LIBS signal is recorded for longer times (50 ns to 5 μs). In the infrared regime, the expansion of the plume is faster along the laser axis, perpendicular to the sample surface. On the contrary, for UV ablation, the expansion of the plume is quite isotropic. We can also observe different regimes of expansion due to Laser-Supported Detonation Waves (LSDW) above 3 mJ in the UV regime. In particular, these observations provide us ideas to understand the kinetics of the CN emission in the LIBS signal. In the IR regime, a formation of CN due to carbon present in the sample and nitrogen in the air via the reaction 2C + N₂ → 2CN can be observed. In the UV regime, the direct ablation of CN bonds is clearly seen but other effects like screening and recombination due to LSDW have also been observed.

Femtosecond laser micromachining of grooves in the SiO₂ coated crystal silicon is investigated using 300 fs laser pulses at a center wavelength of 1030 nm. A novel chirped pulse amplified femtosecond Yb:KGW laser source (Pharos, Light Conversion, Lithuania) with high pulse repetition rate of 1- 350 kHz and high average power up to 8 W is employed. The ablation depth of grooves as a function of pulse repetition rate, number of passes over the same groove, and the light polarization relative to the cutting direction is investigated. Different scanning algorithms as well as influence of the focal plane height relative to the sample are investigated.

Ultrafast dynamic ellipsometry, a technique that probes a sample with chirped laser pulses at two angles and with two orthogonal polarizations, was used to measure the effective refractive index across the ablation region of a Si(111) wafer exposed to a 100 fs ablation pulse. The resulting refractive index data show a significant increase in the extinction coefficient, indicative of the melting of silicon.

In this paper, we are focused on the understanding the underlying physical mechanisms of femtosecond laser interactions with metallic and multi-layer optical materials. The results of the numerical modeling provide an estimation of damage and/or ablation threshold for different laser parameters (pulse duration, fluence, angle of incidence, polarization) and target material properties (metal, dielectric, or multilayer with variable metal layer thickness). These results are compared with the experimental measurements of the thresholds obtained by using different techniques. In particular, dielectric ionization and ablation mechanisms are analyzed based on the experimental findings.

Transparent solids may absorb energy from a laser beam of sufficient high intensity. Several models are under consideration to describe the evolution of the free-electron density. Some of these models keep track of the energy distribution of the electrons. In this work we compare different models and give rules to estimate which one is applicable. We present the inclusion of a term in the multiple rate equation approach, recently introduced, describing fast recombination processes to exciton states. Moreover, we present experimental results with temporally asymmetric femtosecond laser pulses, impinging on a surface of fused silica. We found different thresholds for surface material modification with respect to an asymetric pulse and its time reversed counterpart. This difference is due to a different time-and-intensity dependence of the main ionization processes, which can be controlled with help of femtosecond shaped laser pulses.

Modifications of bulk aluminum irradiated well above ablation threshold (F < 300 J^.cm^-2) have been investigated in situ by means of shadowgraphy and transient quantitative phase microscopy (TQPm) using ultrafast laser radiation (t_p=80 fs, λ=800 nm). This novel pump-probe technique enables quantitative time-resolved measurements of object's properties, e.g. dimensions of melt droplets and layer thickness or transient refractive index changes. A series of time-resolved phase images of vaporized material and/or melt, which are induced by n=1..8 pulses on an aluminum target, are obtained using TQPm. Dynamics and characteristics of melting, dependence of the ablated material volume on process parameters and thereby induced structural modifications have been studied. An increase of material ejection rate is observed at delay time of approximately τ=300 ns and τ>800 ns after the incident pulse. Transient refractive index modifications have been investigated in technical glass (Schott D263) by means of TQPm. By using high-repetition rate ultra-short pulsed laser radiation (t_p=400 fs, λ=1045 nm, f_rep=1 MHz) focused by a microscope objective (w₀ ≈ 4 μm) heat accumulation and thereby glass melting as well as welding is enabled. Transient optical phase variation has been measured up to τ=2.1 μs after the incident pulse and can be attributed to the generation of free charge carriers and compression forces inside glass.

Non-thrombogenic blood contacting surfaces and appropriate blood flow characteristics are essential for clinical application. State-of-the-art coatings are based on heparin and struggle with the problem of bleeding. Thus, there is increasing demand for developing new coating materials for improved human body acceptance. Materials deposited by vacuum coating techniques would be an excellent alternative if the coating temperatures can be kept low due to the applied substrate materials of low temperature resistance (mostly polymers). Under these circumstances, adequate film structure and high adhesion can be reached by the Pulsed Laser Deposition at room temperature (RT-PLD), which was developed to an industrial-scaled process at Laser Center Leoben. This process was applied to deposit Ti, TiN, TiCN and diamond-like carbon (DLC) on polyurethane, titanium and silicon substrates to study the biological interactions to blood cells and the kinetic mechanism of eukaryote cell attachment. Besides high biological acceptance, distinct differences for the critical delamination shear stress were found for the coatings, indicating higher adhesion at higher carbon contents.

Ablation with nanoscale spatial resolution needs special tools to overcome conventional diffraction limit. A few methods have been successfully applied for this purpose. These include: surface nanostructuring by laser illuminated tip; Near-field Scanning Optical Microscopy (NSOM) nano-patterning; Surface nano-processing based on optical resonances and near-field effects with transparent particles as well as the field enhancement by plasmonic nanoparticles. All these methods permit localized laser ablation on the scale beyond 100 nm. In this paper we report our recent work related to field enhancement by laser illuminated tip, near-field laser ablation with transparent particles and field enhancement by plasmonic effects.

The main objective of this study is to explain the experimental observations. To simulate material ablation, plume formation and its evolution, we developed a combined molecular dynamics (MD) and direct simulation Monte Carlo (DSMC) computational study of laser ablation plume evolution. The first process of the material ablation is described by the MD method. The expansion of the ejected plume is modelled by the DSMC method. To better understand the formation and the evolution of nanoparticles present in the plume, we first used separate MD simulations to analyse the evolution of a cluster in the presence of background gas with different properties (density, temperature). In particular, we examine evaporation and growth reactions of a cluster with different size and initial temperature. As a result of MD calculations, we determinate the influence of the background gas parameters on the nanoparticles. The reactions rates such as evaporation/condensation, which are obtained by MD simulations, are directly transferred to the DSMC part of our combined model. Finally, several calculations performed by using MD-DSMC model demonstrate both plume dynamics and longer-time cluster evolution. Calculations results are compared with experimental findings.

Laser-induced phase transitions in a-Ge/Si heterostructures (amorphous Ge films on crystalline Si substrate) have been studied by optical diagnostics and numerical simulation methods. The samples were irradiated by (i) a ruby laser with pulse duration 80 ns (FWHM) and wavelength 694 nm and (ii) an ArF excimer laser (10 ns and 193 nm). Time resolved reflectivity measurements showed the discrepancy in dynamics of reflectivity of probing beam for different regimes of laser irradiation. This discrepancy can be explained by differing kinetics of solid-liquid phase transitions in Ge films: (i) intermediate crystallization or (ii) simultaneous solidification of molten Ge layer from the surface and from the substrate.

Ice made of ultrapure water or water doped with 1 % polymer (polyethylene glycol, "PEG") was irradiated by laser light with fluences between 2 and 80 J/cm² in the ultraviolet (UV) regime at 355 nm and in the infrared (IR) regime at 1064 nm in vacuum. In the UV regime there is a threshold for plasma formation at 3.5 J/cm², whereas the threshold is at 8.5 J/cm² in the IR regime. The ions from the plasma plume were studied by a Langmuir probe. The ion yield was much higher for UV laser irradiation than for IR laser irradiation. The peak of the time-of-flight spectra comprises ions of velocity from 60 to 110 km/s. Generally, the ion yield was slightly larger for ice samples doped with PEG than for pure ones. The threshold behavior was much more pronounced in the IR regime than in the UV regime. These results indicate that the behavior of the plasma current can be understood in terms of ionization breakdown at the ice surface.

Pulsed Laser Deposition (PLD) is an ideal technique to be used for combinatorial approaches. By simply changing the deposition targets one can obtain alternating layers with different periodicities both vertically and laterally, along the substrate surface. By changing the laser impact area location and the number of pulses on each target used for ablation, one can grow films with a continuous variation of the chemical composition, which will be a function of the location on the substrate. To illustrate the advantages and versatility of this Combinatorial PLD (C-PLD) technique, several examples of films used in applications where more than one property should be tailored or optimized are presented. Investigations of thermo-chemical stability, chemical bonding and crystalline structure of thin films of mixtures of HfO₂ and Al₂O₃ that are used as high-k dielectric layers in advanced C-MOS transistors is the first example, followed by a study of structural, mechanical, optical and electrical properties of mixtures of indium tin oxide and doped or pure zinc oxide that are used as transparent and conductive layers. The third example is from the deposition of multilayers of ZrC and ZrN with variable thicknesses to obtain hard coatings.

The laser-driven in-tube accelerator in which the propellant is supplied from laser-ablated gas from the tube wall was developed. Proof-of concept demonstrations of vertical launch were successfully done. The device had a 25mm X 25mm square cross-section; two opposing walls were made of polyacetal and acted as the propellant, the other two acrylic window with guide grooves to the projectile. The upper end of the launch tube was connected to a vacuum chamber of an inner volume of 0.8 m², in which the initial pressure was set to lower than 20 Pa. With plugging the bottom end of the launch tube, a momentum coupling coefficient exceeding 2.5 mN/W was obtained. Even with the bottom end connected to the same vacuum chamber through a different duct, the projectile was vertical launched successfully, obtaining 0.14 mN/W.

Photonic Laser Thruster (PLT) is an innovative photon thruster that amplifies photon thrust by orders of magnitude by exploiting an active resonant optical cavity formed between two mirrors on paired spacecraft. PLT is predicted to be able to provide the thrust to power ratio (T/P) approaching that of conventional thrusters, such as laser ablation thrusters and electrical thrusters. Yet, PLT has the highest I_sp of 3x10⁷ sec, which is orders of magnitude larger than that of other conventional thrusters. We have demonstrated the photon thrust amplification in PLT for the first time. The T/P obtained with an OC mirror with R= 0.99967±0.00002 was 20±1 μN/W, and the maximum photon thrust obtained was 35 μN, resulting in an apparent photon thrust amplification factor of 2,990±150. Scaling-up of PLT is promising, and PLT is predicted to enable wide ranges of space endeavors. Low thrust (T<N) PLTs may enable nanometer precision spacecraft formation for forming ultralarge space telescopes and radars, and provide economically viable solution to Fractionated Spacecraft Architecture, the System F-6. Medium thrust (N<T<kN) PLTs may enable precision propellantless orbit changing and docking. High thrust (T>kN) PLTs may enable propelling spacecraft at speeds orders of magnitude greater than that by conventional thrusters.

Basic characteristics of the laser-based engine will be compared with theoretical predictions and important stages of further technology implementation (low frequency resonance). Relying on a wide cooperation of different branches of science and industry organizations it is very possible to use the accumulated potential for launching of nano - vehicles during the upcoming years.

The application of energetic polymers has resulted in an increased thrust in micro laser plasma thrusters compared to standard polymers. In this study we tested a novel concept for micro laser plasma thrusters, i.e. the application of liquid polymeric fuels, by using polymer solutions of the energetic materials with different viscosity. Shadowgraphy experiments suggest that for higher viscosity solutions ablation without splashing is possible, indicating that liquids are applicable as fuels in laser plasma thrusters. First thrust measurements on a viscous polymer solution confirmed this by yielding a specific impulse similar to a solid material.

Laser cleaning research at Macquarie University has concentrated on a "hard science" approach to research and development of processes for optics and microphotonics applications (removing contaminants from optical materials). In more recent times we have carried out pulsed laser cleaning and processing studies for conservation of articles from Indigenous Australian cultural heritage. This paper introduces our research on laser cleaning of optical materials using UV pulsed lasers, and related surface modification and laser processing outcomes. It then introduces and reports some results from the laser conservation studies along with discussion of how activity in these two contrasting areas have informed our approach to the laser conservation studies, and vice versa.

We present two laser driven shock wave loading techniques utilizing long pulse lasers, laser-launched flyer plate and confined laser ablation, and their applications to shock physics. The full width at half maximum of the drive laser pulse ranges from 100 ns to 10 μs, and its energy, from 10 J to 1000 J. The drive pulse is smoothed with a holographic optical element to achieve spatial homogeneity in loading. We characterize the flyer plate during flight and dynamically loaded target with temporally and spatially resolved diagnostics. The long duration and high energy of the drive pulse allow for shockless acceleration of thick flyer plates with 8 mm diameter and 0.1-2 mm thickness. With transient imaging displacement interferometry and line-imaging velocimetry, we demonstrate that the planarity (bow and tilt) of the loading is within 2-7 mrad (with an average of 4±1 mrad), similar to that in conventional techniques including gas gun loading. Plasma heating of target is negligible in particular when a plasma shield is adopted. For flyer plate loading, supported shock waves can be achieved. Temporal shaping of the drive pulse in confined laser ablation enables flexible loading, e.g., quasi-isentropic, Taylor-wave, and off-Hugoniot loading. These dynamic loading techniques using long pulse lasers (0.1-10 μs) along with short pulse lasers (1-10 ns) can be an accurate, versatile and efficient complement to conventional shock wave loading for investigating such dynamic responses of materials as Hugoniot elastic limit, plasticity, spall, shock roughness, equation of state, phase transition, and metallurgical characteristics of shock-recovered samples, in a wide range of strain rates and pressures at meso- and macroscopic scales.

Multicomponent noble metal nanoparticles were synthesized using a novel laser-assisted dry processing approach. Combining attributes from conventional pulsed laser deposition and matrix-assisted pulsed laser evaporation, nanoparticles of Au-Ag and Au-Ag-Pd were deposited on Si substrates and electron microscopy grids from metal precursors. The mean diameter of Au-Ag particles was approximately 2.8 nm, while that of Au-Ag-Pd particles was approximately 2.2 nm. Significant compositional non-uniformity was observed in deposited particles and is attributed to the inhomogeneity of the target solutions and the decomposition behavior of the selected material systems.

Coarse-grained molecular dynamics simulations are performed to investigate the origins of the surface features observed in films deposited by the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique. The simulations of MAPLE are performed for polymer concentrations up to 6 wt.% and a broad range of laser fluences. The polymer molecules are found to be ejected only in the ablation regime and are always incorporated into polymer-matrix clusters/droplets generated in the process of the explosive disintegration of the overheated matrix. The entanglement of the polymer molecules facilitates the formation of intricate elongated viscous droplets that can be related to the complex morphologies observed in polymer films deposited by MAPLE. The effect of dynamic molecular redistribution in the ejected matrix-polymer droplets, leading to the generation of transient "molecular balloons" in which polymer-rich surface layers enclose the volatile matrix material, has been identified as the mechanism responsible for the formation of characteristic wrinkled polymer structures observed experimentally in films deposited by MAPLE.

Ultrafast lasers emerged as promising tools to process refractive index changes in band-gap materials, resulting in waveguiding functions. Positive refractive index changes were often reported in fused silica matrices. However, in glasses characterized by slow electronic relaxation and high thermal expansion, the refractive index change is usually negative, detrimental for waveguide writing. This relates to the formation of hot regions, where, due to thermal expansion, material is quenched in low-density phases. We discuss control mechanisms related to spatio-temporal heat-source design which may be tuned by temporally shaped laser radiation. Programmable temporal tailoring of pulse envelopes triggers transitions from thermal expansion to directional inelastic flow. Consequently, material compaction leads to a positive refractive index change and guiding structures may thus be created. From an application perspective, the structuring quality degrades with the focusing depth due to wavefront distortions generated at the air-dielectric interface inducing spatial energy dispersion. Spatial beam tailoring corrects beam propagation distortion, improving the structuring accuracy. The corrective process is becoming important when laser energy has to be transported without losses at arbitrary depths, with the purpose of triggering mechanisms of positive index change.

ZnO is a wide and direct band-gap material (3.37 eV at room temperature) making this compound very suitable for UV photodetector applications as well as for UV and blue light emitting devices. As an electronic conductor, ZnO may be used as transparent and conducting electrodes for flat panel displays and solar cells. ZnO doped with various atoms can also lead to new or enhanced functional properties. For example, doping with Al, Ga, In, Si or H allows decreasing its resistivity to below 10^-4 Ω.cm, while keeping the high optical transparency. Rare-earth doped ZnO thin films have been studied for optics and optoelectronics such as visible or infrared emitting devices, planar optical waveguide amplifiers. Ferromagnetic semiconductors can be obtained by doping ZnO with transition metal atoms (Mn, Co, Ni...) that could be used as spin injectors in spintronics. These new and exciting properties of pure and doped ZnO request the use of thin films or multilayer structures. ZnO thin film growth by pulsed-laser deposition (PLD) with or without any dopants or alloyed atoms has been intensively studied. In this paper, we will review the aspects of ZnO thin films grown by PLD, in order to prepare dense, stoichiometric and crystalline epitaxied ZnO layers or to form nanocrystalline films. Then, the optical and electrical properties will be discussed with a special emphasis on the growth conditions in relation to the physical properties for applications in p-n junctions, light emission devices, spintronics and bandgap tuning.

We investigate post-annealing effects using an epi-GaN substrates for ZnO thin film growth by pulsed laser deposition (PLD). The growth of ZnO nanorods on a Si(100) substrate through a two-step process, annealing and off-axis PLD, without a metal catalyst is demonstrated as well. The as-grown films were annealed for one hour under atmospheric pressure air. ZnO morphologies after annealing were measured and the post-annealed ZnO films grown at T_g = 700^oC had very smooth surfaces and the rms roughness was about 0.5 nm. Finally, ZnO post-annealed buffer layer was inserted between ZnO epi-layer and GaN/sapphire substrates. It was evident by AFM that growth temperature of 700^oC helps the films grow in a step-flow growth mode. It was confirmed by cathode luminescence (CL) spectrum that the ZnO film grown at 700^oC had very low visible luminescence, resulting in a decrease of the deep level defects. In the case of ZnO nanorods, controlling growth parameters during deposition enabled the adjustment of the dimensions of nanorods. The diameters of the grown nanorods ranged from 50 to 700 nm and the lengths are from 2 to 10 μm. The CL spectra were used to evaluate the states of defects within the ZnO nanorods. According to the CL results, the thinnest nanorod arrays were found to have fewer defects, while more defects were introduced as nanorods became thicker.

After an introductory overview of different types of laser propulsion schemes a review of the corresponding laser requirements will be given ranging from low power lasers sources to inertial confinement laser facilities. A subsequent status of work reveals the impasse in which the laser propulsion community is currently engaged. Revisiting the basic relations leads to new avenues in ablative and direct laser propulsion for ground based and space based applications. Hereby, special attention is devoted to the space qualification of available laser sources using a microgravity installation. A new approach to debris removal project is discussed with respect to the use Satellite Laser Ranging (SRL) facilities. Finally, a novel aspect for direct photon propulsion in space is derived by introducing the concept of a "long" optical resonator. Potential and limits of this concept are presented opening the possibility of photon propulsion over 100 km path under realistic experimental conditions.

A variety of laser applications in space, past, present, future and far future are reviewed together with the contributions of some of the scientists and engineers involved, especially those that happen to have South African connections. Historically, two of the earliest laser applications in space, were atmospheric LIDAR and lunar ranging. These applications involved atmospheric physicists, several astronauts and many of the staff recruited into the Soviet and North American lunar exploration programmes. There is a strong interest in South Africa in both LIDAR and lunar ranging. Shortly after the birth of the laser (and even just prior) theoretical work on photonic propulsion and space propulsion by laser ablation was initiated by Georgii Marx, Arthur Kantrowitz and Eugen Saenger. Present or near future experimental programs are developing in the following fields: laser ablation propulsion, possibly coupled with rail gun or gas gun propulsion; interplanetary laser transmission; laser altimetry; gravity wave detection by space based Michelson interferometry; the de-orbiting of space debris by high power lasers; atom laser interferometry in space. Far future applications of laser-photonic space-propulsion were also pioneered by Carl Sagan and Robert Forward. They envisaged means of putting Saenger's ideas into practice. Forward also invented a laser based method for manufacturing solid antimatter or SANTIM, well before the ongoing experiments at CERN with anti-hydrogen production and laser-trapping. SANTIM would be an ideal propellant for interstellar missions if it could be manufactured in sufficient quantities. It would be equally useful as a power source for the transmission of information over light year distances. We briefly mention military lasers. Last but not least, we address naturally occurring lasers in space and pose the question: "did the Big Bang lase?"

From theoretical considerations it is well known that pulsed CO₂ lasers with beam peak powers of 50 MW and a pulse length of 20 μs should be able to launch small satellites. To overcome limitations from ultra high power densities in a single laser source, a new concept proposes a beam source which consists of several individual laser systems. Short laser pulses emitted by 16 Q-switched CO₂ laser sources with more than 50 MW power, as of coaxial electrode geometry with excellent beam power to volume ratio, will be combined on a common optical beam path to form a longer single pulse as required. Coaxial lasers have already been built successfully, although without Q-switching. As a main component of the above concept a new optical beam switching element - a "plasma mirror" - which can withstand ultra high power densities that must serve as a Q switch and as a beam path switch is proposed. From the literature it is well known that very dense plasmas are able to reflect an incoming laser beam totally if the plasma frequency, depending on the electron density, equals the laser radiation frequency. As a first step for the development of such a device the absorptivity and reflectivity of iron argon plasmas for CO₂ laser beams has been studied theoretically and experimentally by the authors with the result, that for plasma electron densities of 10¹⁷ cm^-3 nearly 100% are absorbed due to "inverse bremsstrahlung", but that the plasma frequency and thus reflectivity can not be reached, since the electron density is too small in plasmas as contained in electrical arcs.

The AEgis Technologies Group and RTI International are developing microsensors for High Energy Laser (HEL) diagnostic applications. The conformal sensor array will measure the irradiance profile of an incident laser beam, and concomitant rise in surface temperature of the target. The open mesh architecture allows 90% of the beam to impact the surface. A critical part of this program is developing a protective coating that ensures sensor survivability at high irradiance levels for operational lifetimes on the order of 10 seconds. The protective coating must transmit a measurable amount of light to the irradiance sensor. We have conducted experiments to evaluate candidate heat shield materials. In the first round of experiments, a 10kW CO₂ laser was used to irradiate pure materials, including metals and carbon foils. Although many of the metal foils were perforated by the laser, no significant amount of material was ablated away. In fact, most of the test samples gained mass, presumably due to oxidation. Analysis of high speed video shows that once the metal melted, surface tension caused the molten metal to coalesce into droplets around the rim of the hole. The second and third rounds of testing, conducted with a 3kW, 1.07μm fiber laser, included samples of highly reflective metals and ceramics, standard plasma-sprayed coatings, and multilayer stacks. We have also measured the performance of temperature sensors and irradiance sensors fabricated from nanoparticle solutions deposited by advanced printing technology and have completed a preliminary investigation of high temperature adhesives.

The paper discusses optimization of gain and output power and scaling of a pulser-sustainer discharge excited oxygen-iodine laser. For this, NO addition to the laser mixture and iodine vapor dissociation in an auxiliary high-voltage, nanosecond pulse duration, repetitively pulsed discharge ("side" discharge) are used. Iodine dissociation fraction generated in the side discharge and measured in the M=3 laser cavity is up to 50%. The experiments showed that additional iodine dissociation generated in the side discharge only moderately increased laser gain, by 10-15%. Parametric gain optimization by varying main discharge pressure, O₂ and NO fractions in the flow, I2 flow rate, pulsed discharge frequency, and sustainer discharge power, with the side discharge in operation produced gain up to 0.08 %/cm. Two parameters that critically affect gain are the energy loading per molecule in the discharge and the NO flow rate controlling the O atom concentration in the flow. Operation at the main discharge pressure of 60 torr resulted in significantly higher gain than at 100 torr, 0.080 %/cm vs. 0.043 %/cm, due to high discharge energy loading per molecule at the lower pressure. Laser output power measured at the gain optimized conditions is 1.4 W. Experiments with a scaled-up laser with a large volume pulser-sustainer discharge (10 cm x 10 cm x 2 cm vs. 5 cm x 5 cm x 2 cm) and longer gain path (10 cm vs. 5 cm) demonstrated stable discharge operation at discharge powers up to at least 2.9 kW. Singlet delta yield and gain measurements in the scaled-up laser are underway.

The electric oxygen iodine laser (EOIL) offers a vastly more practical, implementable, and safer alternative to its predecessor, the chemical oxygen iodine laser (COIL), particularly for airborne or other mobile military applications. Despite its promise and after 25 years effort, numerous laboratories around the world have not succeeded in providing the known basic physical requirements needed to electrically convert O₂ into O₂(a¹Δ) with the fractional yields and efficiencies needed to make a practical laser. Hence, as of this date, the world record power generated from an EOIL device is only 6.5 watts. In this paper, a 30% conversion from O₂ into O₂(a¹Δ) operating at substantial oxygen mass flow rates (0.090 moles O₂/sec at 50 torr) and 40% electrical efficiency is reported. The O₂(a¹Δ) flow stream being produced carries 2400 watts. Gain measurements are currently in progress, to be followed shortly by power extraction. Current conditions imply that initial power extraction could push beyond 1 KW. Efforts to date have failed to generate substantial laser power because critical criteria have not been met. In order to achieve good O₂(a¹Δ) fractional yield, it is normally mandatory to impart on the order of 100 KJ/mole O₂ while efficiently removing the waste heat energy from the generator so that less than a few hundred degrees Kelvin rise occurs due to gas heating. The generator must be excited by an electric field on the order of 10 Td. This is far below glow potential; hence, a fully externally sustained plasma generation technique is required. Ionization is supplied by means of applying short (tens of nanosecond) pulses to the O₂(a¹Δ) generator at 50,000 PPS, which are on the order of ten times breakdown potential. This enables a quasi-steady adjustable DC current to flow through the generator, being conducted by application of a DC, 10 to 14 Td pump E-field. This field is independently tunable. The result is that up to 180 KJ/mole O₂ gets imparted to the gas by means of the 6 KW sub-breakdown pump field, while another 2700 watts is applied to the controlled avalanche field. The generator consists of 24 each, 1 cm diameter tubes that are submerged in rapidly circulating cold fluorinert. Heat is efficiently removed so that that the gas temperature, initially 273°K, raises only by 125°K, as evidenced by spectrographic analysis of the fine structure of O₂(b¹Σ) at lower pressure. Since all necessary conditions have been met, a 30% conversion rate of O₂ to O₂(a¹Δ) has been achieved. Fortuitously, neither excited O atom production nor O₂(b¹Σ) production is visible in the spectra of the higher pressure, best yield runs. Essentially all other spectral lines are dwarfed in comparison the O₂(a¹Δ) line. Energy normally partitioned to O₂(b¹Σ) and apparently O atoms now feeds into O₂(a¹Δ) directly, enabling electrical efficiency to exceed 40%. As a continuation of this work, an I₂ disassociating mixing section - then subsequently a 20 cm transverse M = 2.5 laser channel - has been coupled to the O₂(a¹Δ) generator. The effects of titrating NO, NO₂, etc. to scavenge O atoms and O₃ atoms is under current investigation. Laser power extraction will commence after having optimized all parameters to achieve maximum gain. Power extraction has been delayed due to substantial mechanical equipment failure; however, the apparatus has now been fully restored. Also, several modes of potential discharge instabilities peculiar to high O₂(a¹Δ) concentrations have been discovered. These phenomenon and their means of prevention will be discussed.

Influence of nitrogen oxides NO and NO₂ on specific input energy (SIE) and time behavior of singlet delta oxygen (SDO) luminescence excited by pulsed e-beam sustained discharge in oxygen was experimentally and theoretically studied. NO and NO₂ addition into oxygen results in small increase and decrease of SIE, respectively, the latter being connected with large energy of electron affinity to NO₂. The addition of 0.1-0.3% nitrogen oxides was experimentally and theoretically demonstrated to result in notable enhancement of SDO lifetime, which is related to a decrease of atomic oxygen concentration in afterglow. There was experimentally demonstrated that for getting high SDO concentration at gas pressure 30-60 Torr for the time interval less than ~0.5 s one needs to add not less than 0.2% nitrogen oxides into oxygen. Temperature dependence of relaxation constant for SDO quenching by unexcited oxygen was estimated by using experimental data on time behavior of SDO luminescence.

We report a novel experience gathered in the development of powerful optical sources of the vacuum ultraviolet (VUV) radiation, which are based on high-current multichannel surface discharges of submicrosecond duration. The peak intensity of the VUV radiation produced by the designed large-area (~0.1 m²) optical sources reaches 130 kW/cm², whereas the intrinsic efficiency of the discharge emission within the spectral range of 120-200 nm attains 3.2 %. Application of such sources to pump a gaseous active medium of the multipass XeF(C-A) amplifier allows us obtaining the total gain factors exceeding 10² for the blue-green ultrashort optical pulses of 150 fs duration. The results presented in the report show a considerable potential of the developed laser technology for the femtosecond pulse amplification up to petawatt peak powers.

Experimental investigations of radio-frequency discharges in O₂/He/NO mixtures in the pressure range of 1-100 Torr and power range of 0.1-2.5 kW have indicated that O₂(a¹Δ) production is a strong function of geometry, pressure and diluent ratio. The goal of these investigations was maximization of both the yield and flow rate (power flux) of O₂(a¹Δ) in order to produce favorable conditions for application to an electric oxygen-iodine laser (EOIL). As pressure is increased, yield performance is dominated by the influence of geometry and diluent ratio. Numerous measurements of O₂(a¹Δ), oxygen atoms, and discharge excited states are made in order to describe the discharge performance dependence on various parameters.

The results of experimental investigations of a volume avalanche discharge initiated by an e-beam (VADIEB) and surface layer of Cu and AlBe foils modifications at the plasma action of VADIEB are given. The volume discharge in the air of atmosphere pressure formed in the gap with the cathode having small curvature radius and with high voltage pulses of nanosecond duration and positive and negative polarity. A supershort avalanche electron beam (SAEB) with formation conditions in gases under atmospheric pressure have been investigated. It is proved that the surface layer is cleared of carbon at foil treatment, and atoms of oxygen penetrate into a foil. It is show that the cleaning depth depends on polarity of voltage pulses. At positive polarity of a copper foil electrode the cleaning is observed at the depth over 50 nm, and atoms of oxygen penetrate at the depth up to 25 nm. Plasma of the superpower volume discharge of nanosecond duration with a specific excitation power of hundreds of MW/cm³, and SAEB, and the discharge plasma radiation of various spectral ranges (including UV, VUV and X-ray) has the influence on the anode. The supershort avalanche electronic beam is generated only at negative polarity of a voltage pulse on an electrode with a small radius of curvature. SAEB influence on modifications of the copper foil surface is registered. VADIEB is easily realized in various gases and at various pressures, and, at gas pressure decrease the density of the beam current in helium can achieve 2 kA/cm². It allows predicting an opportunity of VADIEB application for metal surface modifications in various technological processes, and for surface dielectric modifications at the certain design of the anode.

An optically pumped overtone HBr laser is investigated experimentally and theoretically. The frequency tuning and stabilization of the Nd:YAG pump laser is described. Results of HBr laser emission are presented. The simulation shows promising features of both pulsed and cw pumped systems concerning efficiency, frequency tuning and heat dissipation.

Recent studies of fundamental issues of target material format and laser radiation parameters have revealed the attractiveness of LPP EUV source technology based on Sn target and multi-kW CO₂ laser driver. In recent work we have reported 8kW of average power at 100kHz repetition frequency and 20ns pulse duration produced by our MOPA CO₂ laser driver built on a chain of Fast Axial Flow (FAF) amplifiers. However, the oscillator power is insufficient to saturate the input stages and significant amount of available laser energy (>80%) is untapped. In this paper we report a step towards an improvement of laser driver power and efficiency. For the first time, to our knowledge, the performance of a novel multi-pass pre-amplifier based on RF-excited slab waveguide CO₂ laser technology has been numerically modeled. The calculations show the feasibility of this approach. We carried out amplification experiments to validate the numerical model. In our experiments we have obtained power gain of 10 at 13-pass configuration from a slab of 60x600mm2 geometry at 20ns pulse length and 100kHz repetition frequency at diffraction-limited output and no self-oscillation. The experiment has validated the numerical model, which will be used at this stage to design and optimize a pre-amplifier for our current FAF amplifier chain. Furthermore, these results enable us to design and optimize next generation of LPP laser driver based entirely on compact slab-waveguide amplifiers.

A model that solves simultaneously both the electron and atomic kinetics was used to generate synthetic X-ray spectra to characterize high intensity ultrashort-laser-driven target experiments. A particle-in-cell simulation was used to model the laser interaction for both cluster and foil targets and provided the initial electron energy distribution function (EEDF) for a Boltzmann solver. Previously reported success in the spectroscopic characterization of an irradiated Ar cluster target has motivated the authors to apply this technique in a feasibility study to assess the possibility of recording time resolved spectra of a 10 micron Ti foil target irradiated by a 500 fs, I= 1.0 × 10¹⁸W/cm² short-pulse laser. Though this model suggests that both Ar cluster and Ti foil plasmas are held in a highly non-equilibrium state for both the EEDF and the ion level populations for several picoseconds, the spectral line features of the foil experiment was shown to evolve too quickly to be seen by current ultrafast time resolved spectrometers.

A theoretical model is developed for the interaction of intense femtosecond laser pulses with solid targets on the basis of the two-temperature equation of state for an irradiated substance. It allows the description of the dynamics of the plasma formation and expansion. Comparison of available experimental data on the amplitude and phase of the complex reflection coefficient of aluminum with the simulation results provides new information on the transport coefficients and absorption capacity of the strongly coupled Al plasma over a wide range of temperatures and pressures.

Recently, using a femtosecond laser surface structuring technique we turned highly reflective metals to highly absorptive, creating the so-called "black metals". In this study, we made an even more significant advancement. Here, we demonstrate that our femtosecond laser structuring technique not only allows us to create black metals but also gray and even color metals. We show that our technique essentially provides a controllable modification of optical properties of metals from the UV to THz spectral range via surface structuring on the nano-, micro-, and submillimeter-scales.

Laser induced breakdown of single-layer, ion-beam sputtered Ti_xSi_1-xO₂ composite films was studied using single and multiple pulses from a femtosecond Ti:sapphire laser. The bandgap of this coating material can be gradually adjusted with the composition parameter x. A scaling law with respect to the bandgap energy and pulse duration dependence of the single-pulse damage threshold that was observed previously for pure oxide films was found to apply to the composite films as well. The dependence of the damage threshold as a function of pulse number F(N) was similar to the behavior observed for pure oxide films. It was possible to explain the dependence as a function of pulse number using a theoretical model based on the formation and accumulation of defects. The shape of F(N) can be used to estimate the role of shallow traps and deep traps on the multiple-pulse breakdown behavior.

The motion of both Lennard-Jones solids and metals induced by ultrashort laser irradiation near the ablation threshold is investigated by molecular dynamics simulation. The universality of the ablation threshold fluence with respect to the cohesion energy of solids irradiated by femtosecond laser pulses is demonstrated for Lennard-Jones solid and metals simulated by many-body EAM potentials.

Recently we became interested in applying previous work with liquid fueled laser powered minithrusters for spacecraft orientation to the conceptual design of a multi-newton thruster based on the same principles. Solid-fuel configurations (such as the fuel tapes used in the Photonic Associates microthruster) are not amenable to the range of mass delivery rates (g/s to g/s) necessary for such an engine. We will discuss problems for this design which have been solved, including identifying a practical method of delivering liquid fuel to the laser focus, avoiding splashing of liquid fuels under pulsed laser illumination, and avoiding optics clouding due to ablation backstreaming on optical surfaces from the laser-fuel interaction region. We have already shown that I_sp = 680 seconds can be achieved by a viscous liquid fuel based on glycidyl azide polymer and an IR-dye laser absorber. The final problem is mass: we will discuss a notional engine design which fits within a 10-kg "dry mass" budget. This engine, 80kg mass with fuel, is designed to fit within a 180-kg spacecraft, and use 3kW of prime power to deliver a Δv of 17.5 km/s to the spacecraft in sixteen months. Its specific impulse will be adjustable over the range 200sp<3,600 seconds and maximum thrust will be 6N, based on performance which has been demonstrated in the laboratory. Such an engine can put small satellites through demanding maneuvers in short times, while generating the optimum specific impulse for each mission segment. We see no reason why I_sp = 10,000 seconds cannot be achieved with liquid fuels.

The fluence dependence of the laser ablation of selected polymers was studied within the range from 1-150 J/cm². A TEA CO₂ laser operating at 10.6 μm with 300 ns main pulse length and up to 20 J pulse energy was used to ablate prepared polymer samples with single pulses of laser energy. Measurements of parameters such as the ablated mass per spot area (Δm_a), momentum coupling coefficient (C_m), specific impulse (I_sp), and internal efficiency (η_i) will be plotted as functions of fluence. Critical threshold effects observed throughout the experiments will be described in detail.

Problems of laser-plasma thruster development for space applications are analyzed. Results of laboratory research concerning the choice of solid-state laser, operating mode and target material are considered. Characteristics of micro-thruster with diode pumping are discussed.

The concept of power-scalable, high beam-quality diode pumped alkali lasers was introduced in 2003 [Krupke, US Patent No. 6,643,311; Opt. Letters, 28, 2336 (2003)]. Since then several laboratory DPAL devices have been reported on, confirming many of the spectroscopic, kinetic, and laser characteristics projected from literature data. This talk will present an overview of the DPAL concept, summarize key relevant properties of the cesium, rubidium, and potassium alkali vapor gain media so-far examined, outline power scaling considerations, and highlight results of published DPAL laboratory experiments.

Optically pumped alkali vapor lasers have been constructed using excitation of the D₂ line (²P_3/2-²S_1/2) followed by lasing on the D₁ line (²P_1/2-²S_1/2). Collisional relaxation is used to transfer population from ²P_3/2 to the ²P_1/2 level. The collision partner used for this step must have a large cross section for inducing transfer between the ²P_J levels, combined with a very small cross section for electronic quenching of the form M(²P_J)+Q→M(²S_1/2)+Q (where M is an alkali metal and Q is the energy transfer agent). Ethane has proved to be an effective energy transfer agent for optically pumped Rb and Cs lasers. However, modeling of data for the Rb/C₂H₆ laser with the literature value for the quenching rate constant was unsuccessful. We have reexamined the quenching of Rb(²P_J) by C₂H₆ using time-resolved fluorescence techniques. Radiation trapping was significant under the conditions of our measurements, and an analysis of the interplay between the kinetics of trapping and quenching was carried out. It was found that quenching of Rb(²P_J) by C₂H₆ was very inefficient. The upper bound established for the quenching cross section was two orders of magnitude lower than that indicated by the previous determination.

Resonance transition rubidium laser (5²P_1/2→5²S_1/2) is demonstrated with a hydrocarbon-free buffer gas. Prior demonstrations of alkali resonance transition lasers have used ethane as either the buffer gas or a buffer gas component to promote rapid fine-structure mixing. However, our experience suggests that the alkali vapor reacts with the ethane producing carbon as one of the reaction products. This degrades long term laser reliability. Our recent experimental results with a "clean" helium-only buffer gas system pumped by a Ti:sapphire laser demonstrate all the advantages of the original alkali laser system, but without the reliability issues associated with the use of ethane. We further report a demonstration of a rubidium laser using a buffer gas consisting of pure ³He. Using isotopically enriched ³He gas yields enhanced mixing of the Rb fine-structure levels. This enables efficient lasing at reduced He buffer gas pressure, improved thermal management in high average power Rb lasers and enhanced power scaling potential of such systems.

The Laser and Optics Research Center of the US Air Force Academy started a research program on optically pumped alkali lasers in 2004. We demonstrated the first diode pumped alkali (cesium) vapor laser, the first optically pumped potassium laser, the most efficient (slope efficiency higher than 80%) cesium laser, and diode pumped Rb and Cs lasers with highest output powers (17 W and 48 W respectively). We have developed an efficient Cs amplifier with a small signal amplification factor of 145 and tunable single mode Cs laser for scientific applications. In this paper we present a review of our main results and recent achievements in high power alkali laser development, discuss some problems existing in this field and ways to solve them.

Diode pumped alkali lasers have developed rapidly since their first demonstration. These lasers offer a path to convert highly efficient, but relatively low brightness, laser diodes into a single high power, high brightness beam. General Atomics has been engaged in the development of DPALs with scalable architectures. We have examined different species and pump characteristics. We show that high absorption can be achieved even when the pump source bandwidth is several times the absorption bandwidth. In addition, we present experimental results for both potassium and rubidium systems pumped with a 0.2 nm bandwidth alexandrite laser. These data show slope efficiencies of 67% and 72% respectively.

The absolute absorption and stimulated emission cross-sections, including the effects of hyperfine splitting and pressure broadening at low to moderate pressures are computed and compared with experimental results. The comparison is excellent and requires no fit parameters. An analysis of the degree to which the lineshape can be approximated by a single Lorentzian profile is provided as a function of background pressure.

This paper discusses the possibility of producing high concentrations of O₂(a¹Δ_g) states at pressures up to atmospheric in rare-gas/oxygen/NO mixtures by using micro-plasmas. Micro-plasmas refer to electric discharges created in very small geometries which have been proven able to operate in DC mode at high pressure and high power loading without undergoing any glow to arc transition. The so-called Micro Cathode Sustained Discharge (MCSD), which is a three-electrode configuration using a Micro Hollow Cathode Discharge (MHCD) as a plasma cathode, can be operated as a non-self-sustained discharge with low values of the reduced electric field and of the gas temperature. As a result, these MCSDs can efficiently generate large amounts of singlet delta oxygen. In Ar/O₂/NO mixtures, at an oxygen partial pressure of 10 mbar, high values of O₂(a¹Δ_g) number density (1.5 10¹⁶ cm^-3) and of the production yield (6.7 %) can be simultaneously obtained. For lower O₂ partial pressure, yields higher than 10 % have been measured. In He/O₂/NO mixtures, O₂(a¹Δ_g) number densities around 10¹⁶ cm^-3 were achieved at atmospheric pressure for flow rates in the range 5-30 ln/mn, which could give rise to new applications.

In this paper we describe a quasi-two level analytic model for end pumped Alkali metal vapor lasers. The model is developed by considering the steady state rate equations for the number densities of the, ²S_1/2, ²P_3/2, and ²P_1/2, energy states for the three level laser system. The approximation is then made that the relaxation between the two upper levels, ²P_3/2 and ²P_1/2, caused by collisions with additive ethane is much faster, in fact infinitely fast, by comparison with any other process in the system including stimulated emission. With this assumption the ratio of the number densities for the upper two levels, ²P_3/2 and ²P_1/2, is given by its statistical equilibrium value and the mathematical description becomes that of a quasi-two level system from which an analytic solution can be extracted. The analytic model description gives expressions for the threshold pump power and the slope efficiency including intra-cavity losses. Applications of the model and comparisons with the steady state three level model developed by Beach et al. will be presented.

A CW diode-pumped alkali laser (DPAL) based on the D1 rubidium resonance transition has been investigated. The pump sources for these experiments are two 780 nm fiber-coupled diode modules, incorporating volume holographic gratings for wavelength control. Total pump power is up to 64 W. Rb laser output at the 794.8 nm fundamental wavelength is up to 7.8 W. Intracavity second harmonic generation in BIBO generates up to 250 mW at 397.4 nm.

Surface cleaning is a key step in many industrial processes and especially in laser surface treatments. During laser cleaning of metallic alloys using pulsed lasers, surface modification can be induced due to transient thermal effect. In ambient atmospheric conditions, an oxidation of the cleaned surface can be detected. The aim of this work was to characterize this transient oxidation that can occur below the laser energy domain leading to any phase change (melting, ablation) of the cleaned substrate. A Q-switched Nd:YAG laser (1.06 μm) with 10 ns pulse duration was used for this study. X-ray photoelectron spectroscopy and secondary ion mass spectroscopy were used for surface analysis of irradiated samples. Thermal oxidation took place on the aluminium-magnesium alloy (5000 series) during the irradiation in air (fluence range 0.6-1.4 Jcm^-2). It has been demonstrated that this 10 ns laser thermal oxidation and the steady state thermal oxidation have the same mechanism. When the laser fluence reached 1 J cm -2 , the oxide formed by the thermal oxidation became in a large extent crystalline and its outer part was entirely covered by a continuous magnesium oxide layer.

The present study deals with calibration-free analysis of materials by Laser-Induced Breakdown Spectroscopy (LIBS). A numerical code computes the spectral radiance emitted from a plasma in local thermal equilibrium. The numerical code includes the calculation of the plasma composition by solving the system of Saha-equations, mass conservation and neutrality equation. The chemical reactions within the plasma plume are considered, especially in case of organic materials ablation. The line intensities are then computed. In the present study, the lines are considered to be optically thin. The modeling of the chemical composition of an organic material and a steel sample is presented. Comparing the experimental spectra to the computed ones, it was possible to measure the elemental concentrations of the steel with good accuracy without any requirement of preliminary calibration.

The laser annealing of Ge/Si heterostructures with Ge quantum dots (QD's) embedded on the depth of 0.15 and 0.3 μm has been studied. The samples were irradiated by 80-nanosecond ruby laser pulses. Irradiation energy density was close to the melting threshold of Si surface. The nanocluster structure was analyzed by Raman spectroscopy. Changes in composition of QD's were observed for both types of samples. The decrease in dispersion of nanocluster sizes after laser irradiation was obtained for samples with QD's embedded on 0.3 μm depth. The numerical simulations on the basis of Stefan problem showed that the maximum temperatures on the depth of QD's bedding differ by ~ 100 K. This difference is likely to lead to different effects of laser annealing of heterostructures with QD's.

Theoretical consideration of the ablation of laser heated metal target based on two-temperature hydrodynamic calculation is performed for aluminum and gold targets. Concurrent with the hydrodynamic calculation the molecular dynamics simulation of the ablation was carried out in the case of aluminum. The initial state of matter for the molecular dynamics is taken as a final state of hydrodynamic calculation. Molecular dynamics simulation is extended to cover late stages of the evolution of two-phase foam placed between the crater and spalled cupola. Theoretical results are in a good agreement with the experimental data obtained by the microinterferometer diagnostics of the femtosecond laser ablation both for aluminum and gold.

Actively mode-locked electron-beam-sustained-discharge CO-laser producing ~10 ns (FWHM) pulses repetition rate 10 MHz for both single-line and multiline mode of operation was experimentally studied. The total laser pulse duration was ~0.5 ms for both mode-locked and free running laser conditions. The specific output energy in multiline CO-laser mode of operation was up to 20 J l^-1 Amagat^-1 and the laser efficiency up to 3.5%. The active mode-locking was achieved for single-line CO-laser mode of operation in the spectral range 5.2-5.3 micron. The radiation can be used for laser ablation, for pumping an optical parametric amplifier in optical stochastic cooling of relativistic heavy ions, and for studying vibrational and rotational relaxation of NO molecules.

The concept of high energy density (HED) radiation driven momentum coupling (momentum transfer), C_M, to a targets in a vacuum is analytically developed and applied via successive plasma, ablative, and hydrodynamic interfaces undergoing both weak and strong shocks. C_M are derived from equations of state (EOS) variables and serve as figures of merit to determine energy efficiency conversion into target momentum. Generally, C_M are proportional to the inverse of the interaction speed and related variables for each interaction regime. This approach provides a formalism allowing computation of hitherto intractable HED radiation and mechanical momentum coupling interactions encountered in astrophysics, planetary physics, inertial confinement fusion, near-Earth object hazard mitigation, and HED explosives modeling. C_M is generally not scale invariant as are the hydrodynamic Euler equations. This analytic procedure supports interpretation of experiments using EOS response of material targets to HED interactions on the meso - and macro-scales to describe C_M.

Effects related to the use of high repetition rate lasers in ablation of metals (aluminum, copper, stainless steel) and silicon were investigated. The multi-pulse irradiation with the laser beam significantly lowered the ablation threshold and led to a relative increase in the ablation rate at the higher repetition rate. The reason of alteration could be accumulation of structural defects on the metal surface formed by irradiation with a laser of the sub-threshold fluence. The mean volumetric ablation rate in laser milling experiments was a non-linear function of the pulse energy. Plasma shielding was the main limiting factor in processing efficiency of metals with the high power picosecond lasers. Increasing the repetition rate keeping the pulse energy below the plasma formation threshold is a way to increase the efficiency of material removal with nanosecond lasers. Thermal management of the specimen could be a problem at high repetition rates because of the laser energy wasted in the bulk. The reduction in the ablation threshold by irradiation with a series of laser pulses might be useful in application of the high- repetition-rate lasers with the low pulse energy.

Sandia National Laboratories NLS (1064 nm) and Z-Beamlet (527 nm) pulsed lasers @ ~ 100 GW/cm² and 10 TW/cm² were used to attain pressures at 20 - 525 GPa on a variety of metallic and mineral targets. A simple, inexpensive and innovative electro-optical real-time methodology monitored rear surface mechanical deformation and associated particle and shock wave velocities that differ considerably between metals and non-metals. A reference calibration metal (Aluminum) and a reference non-metal (graphite) were used to demonstrate the validity of this methodology. Normative equations of state and momentum coupling coefficients were obtained for dunite, carbonaceous meteorites, graphite, iron and nickel. These experimental results on inhomogeneous materials can be applied to a variety of high energy density interactions involving stellar and planetary material formation, dynamic interactions, geophysical models, space propulsion systems, orbital debris, materials processing, near-earth space (lunar and asteroid) resource recovery, and near-earth object mitigation models.

One of the many challenges faced by laser propulsion is the long-term performance of the propellant. The chemical changes that can take place at the propellant surface during ablation can greatly modify the in-flight performance characteristics. For stable regimes for propulsion, such chemical action should be minimized. A TEA (Transverse Electrical discharge in gas at Atmospheric pressure) CO₂ laser of 10.6 μm wavelength, 300 ns pulse length, and up to 20 J pulse energy was used to ablate several types of polymer targets with a range of observable chemical changes at the surface following ablation. After 10 subsequent shots, the target samples were measured using Attenuated Total Reflectance Fourier Transform Infrared (ATR FTIR) spectroscopy then compared to unablated samples of the same polymer. An analysis of the results was made with an emphasis on laser propulsion applications, with a comparison of the propulsion performance of the targets, specifically regarding the ablated mass per spot area (Δm_a). Chemical reaction pathways for combustion and vaporization are discussed on the basis of the differences observed in the FTIR spectra, and the consequences for using such materials as laser propulsion propellants are explored.

A CO₂ laser of 300 ns pulse length, operating at 10.6 μm wavelength and from 1-4 J pulse energy was used to ablate carbon-doped Delrin^® (polyoxymethylene, or POM) targets in a set of conical aluminum minithrusters at standard temperature and pressure. Nozzles with lengths ranging from 0.5 - 5 cm were used (corresponding to expansion ratios of about 4 to 16), as well as a bare sample with no nozzle. A piezoelectric force sensor was used to record the imparted impulse for fluences in the range of 1-100 J/cm² for each thruster. The effect of increasing the expansion ratio on the impulse generation for single pulse laser propulsion experiments will be described. The study will also clarify the effect of confining air from an ambient atmosphere in augmenting impulse generation.

Momentum coupling coefficients of TEA CO₂ laser pulses for a parabolic aluminum shell were investigated. Momentum coupling coefficients were measured with a pendulum in a chamber, the energy of the incident laser pulse was varied from 8.3J to 50.9J, and the gas pressure in the chamber was changed from 100 kPa to 20 kPa in our experiments. Experimental data were analyzed thoroughly. It was found that the coupling coefficients under the air pressure of 100kPa decreased very slowly from 242 N/MW to 170 N/MW for the incident energy from 50.9J to 15.1J but decreased sharply for the energy between 15.1 J to 13.8 J. And it was different for the air pressure below 100 kPa. Indoor free flight of our parabolic shell was also analyzed, coupling coefficients and some other parameters were deduced from the experimental data.

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

Emission characteristics of a nanosecond discharge in nitrogen, inert gases and its halogenides without preionization of the gap from an auxiliary source have been investigated. A volume discharge, initiated by an avalanche electron beam (VDIAEB) was realized at pressures up to 12 atm. It has been shown that at VDIAEB excitation no less than 90% energy in the 120-850 nm range is emitted by Xe, Kr, Ar dimers. Xenon spectra in the range 120-850 nm and time-amplitude characteristics have been recorded and analyzed for various excitation regimes. In xenon at pressure of 1.2 atm, the energy of spontaneous radiation in the full solid angle was ~ 45 mJ/cm³, and the FWHM of a radiation pulse was ~ 110 ns. The spontaneous radiation power rise in xenon was observed at pressures up to 12 atm. Pulsed power densities of radiation of inert gases halogenides excited by VDIAEB was ~ 4.5 kW/cm² at efficiency up to 5.5 %.

We are developing a laser produced plasma light source for high volume manufacturing (HVM) EUV lithography. The light source is based on a high power, high repetition rate CO₂ laser (10.6μm) system, a tin (Sn) target and a magnetic ion guiding for Sn treatment. We evaluated the characteristics of Sn debris generated by a CO₂ laser produced plasma. Experiments were performed with bulk Sn-plate targets and Mo/Si multilayer mirror samples were used for debris analysis. We observed very thin and uniform Sn layers of nano/sub-nano size debris particles. The layer deposition rate at 120mm from the plasma is, without magnetic field, about 30nm per million shots. The fast Sn ion flux was measured with Faraday cups and the signal decreased by more than 3 orders of magnitude on application of a magnetic field of 1T. The Sn deposition on the Mo/Si multilayer mirror decreased in small magnetic field space by a factor of 5. In a large magnetic field space, the effectiveness of the magnetic guiding of Sn ions is examined by monitoring the fast Sn ions. The ion flux from a Sn plasma was confined along the magnetic axis with a maximum magnetic field of 2T.

We perform a comparison study of periodic structures on the surfaces of three different noble metals, Cu, Ag, and Au, following femtosecond laser radiation. Under identical experimental conditions, laser-induced surface patterns show distinctly different level of morphological clearness on the three different metals. Simply calculations based on metal melting fail to explain the pattern differences. We show that our observation result from the competition of two ultrafast processes, electron-phonon energy coupling and hot electron diffusion, following femtosecond laser heating of metals.

Hybrid space propulsion has been a feature of most space missions. Only the very early rocket propulsion experiments like the V2, employed a single form of propulsion. By the late fifties multi-staging was routine and the Space Shuttle employs three different kinds of fuel and rocket engines. During the development of chemical rockets, other forms of propulsion were being slowly tested, both theoretically and, relatively slowly, in practice. Rail and gas guns, ion engines, "slingshot" gravity assist, nuclear and solar power, tethers, solar sails have all seen some real applications. Yet the earliest type of non-chemical space propulsion to be thought of has never been attempted in space: laser and photon propulsion. The ideas of Eugen Saenger, Georgii Marx, Arthur Kantrowitz, Leik Myrabo, Claude Phipps and Robert Forward remain Earth-bound. In this paper we summarize the various forms of nonchemical propulsion and their results. We point out that missions beyond Saturn would benefit from a change of attitude to laser-propulsion as well as consideration of hybrid "polypropulsion" - which is to say using all the rocket "tools" available rather than possibly not the most appropriate. We conclude with three practical examples, two for the next decades and one for the next century; disposal of nuclear waste in space; a grand tour of the Jovian and Saturnian moons - with Huygens or Lunoxod type, landers; and eventually mankind's greatest space dream: robotic exploration of neighbouring planetary systems.

We report femtosecond laser cutting of ultrathin ferroelectric sheets. This process enables one to do rapid patterning of microns-thick films of complex oxides such as LiNbO3, which are obtained via ion-beam exfoliation from standard wafers. Cutting these fragile samples is extremely difficult using standard methods but can be done effectively with ultrafast lasers. To achieve fast writing speed, we employ a high-repetition-rate amplified Ti:sapphire laser system with a pulse peak power of ~100MW. Optimization of the depth and quality of cut were determined as a function of laser pulse energy, crystallographic axes, optical polarization, and pre- and post-ablation chemical treatments.