This conference presentation was prepared for High-Power Laser Ablation VIII, 2024.

Alexander (Sasha) Rubenchik lived in Petropavlovsk-Kamchatsky and Novosibirsk, Russia before moving to Livermore in 1992. He received a doctorate degree in physics from Novosibirsk State Univ. in Russia and dedicated his career to laser-matter interactions, non-linear optics and plasma physics, working for the Lawrence Livermore National Lab. for 30 years. Dr. Rubenchik authored 350 publications and held over 50 patents. He was awarded four R&D 100 awards. He traveled all over the world and shared his knowledge with the scientists of many countries.

Femtosecond lasers are a powerful tool for high-precision material processing and functionalization. In my lab, the laser processing led to a range of technological developments, including the so-called black and colored metals, superhydrophillic and superhydrophobic surfaces. In this talk, I will discuss our recent developments in femtosecond laser micro- and nano-scale surface patterning, including the formation dynamics, the drastically altered surface functionalities, and the various applications.

The mechanism of epitaxy loss in laser powder bed fusion additive manufacturing is studied through single crystal laser scan experiments. Results suggests that contrary to currently accepted mechanism of stray grain nucleation, epitaxy loss occurs through gradual accumulation of crystallographic misorientation from a combination of plastic deformation and morphological changes in solidification dendrites. Highly disparate misorientation distribution was observed inside and outside the melt pool. Although alignment with cell/colony boundaries was often observed inside the melt pool, misorientation frequently develops independent of the solidification features, indicating combined effect of solidification and residual stress-induced plasticity. On the other hand, a gradual decaying misorientation develops approximately 100 micrometers outside the melt pool, which indicates residual stress as the dominant mechanism. Interaction of multiple melt pools causes a superposition of misorientation fields, yielding random occurrences of high misorientation boundaries that lead to formation of new grains.

Single-crystal alumina (sapphire) and transparent polycrystalline alumina are compelling candidates for laser processing in optical applications. In this study, single-shot laser irradiations (~10^15 W/cm2) on sapphire and polycrystalline alumina are investigated. A laser in the femtosecond regime (1030 nm, 490 fs) is used to examine the mechanisms of laser-induced damage on sapphire and polycrystalline alumina. The damage morphologies are characterized using a Scanning Electron Microscope (SEM), Atomic Force Microscopy (AFM), and optical profilometer. When irradiated with a single-shot ultrafast laser pulse, sapphire and polycrystalline alumina show dissimilar damage mechanisms, attributed mainly to the difference in the microstructure. In addition, a quantitative analysis of crater diameter, depth, and volume is conducted. The laser-induced damage thresholds of the materials are determined. The quantitative analysis provides insight into the scaling relationship between the laser parameters and damage morphologies for sapphire and polycrystal alumina.

This conference presentation was prepared for High-Power Laser Ablation VIII, 2024.

Laser and plasma-based techniques have been demonstrated to be effective for obtaining surfaces with functional properties i.e. adherent or repellent to different types of cells. Two Photon Polymerization-Direct Writing (2PP-DW) allowed obtaining complex hierarchical structures: a first level consisted of micrometric mushroom-like constructs, and in the second level, the mushrooms' hats were "decorated" with micro- and nanostructures in the shape of elliptical ripple-like patterns. Their wettability was found to strongly depends on the morphology aspect ratio and dimensions. Also, Matrix Assisted Pulsed Laser Evaporation of Polyvinyl formal (Formvar) - PVF allowed obtaining functional coatings with tunable adherence properties (hydrophobic/hydrophilic), depending on the experimental deposition parameters. The irradiation of ns-laser processed Polyethylene terephthalate (PET) with ripples using a cold atmospheric pressure radio frequency plasma source shown controlled changes of the surface functionality.

This presentation summarizes recent work at the Laser Thermal Laboratory on the laser chemical processing of two-dimensional (2D) layered materials and the laser-aided atomic laser etching (ALEt) of semiconductors. Spatially selective laser doping of transition metal dichalcogenides (TMDCs), reversible writing of dopant patterns in graphene and fabrication of functional devices have been accomplished. Digital self-limited etching of semiconductors has been demonstrated.

The mechanisms of picosecond laser fragmentation of gold nanoparticles in water are investigated in a closely integrated atomistic simulations and time-resolved X-ray probing. The results of this joint computational and experimental study reveals a sequence of nonequilibrium processes triggered by the laser irradiation, from heating, melting, and resolidification of nanoparticles proceeding under conditions of strong superheating and undercooling, to evaporation of Au atoms followed by condensation into atomic clusters and small satellite nanoparticles, and to the regime of rapid (explosive) phase decomposition of superheated nanoparticles into small liquid droplets and vapor phase atoms. The transition to the phase explosion fragmentation regime is signified by prominent changes in the small-angle X-ray scattering profiles measured in experiments and calculated in simulations. A good match between the experimental and computational diffraction profiles gives credence to the physical picture of the cascade of thermal fragmentation regimes revealed in the simulations.

Nanoparticles (NPs) applications in energy, medicine, additive manufacturing, or catalysis require the development of synthesis techniques offering solvent and material versatility, morphology, size control, and high purity, together with industrial-scale productivities. Pulsed laser ablation in liquids (PLAL) comes close to meet these requirements; however, NP size control and further productivity increase remain a challenge. The spatial and temporal modification of the laser beam appear as an ideal approach to modify the cavitation bubble dynamics and influence NP size distribution, and to increase productivity by reducing cavitation bubble pulse shielding. Here, a (9 ± 1) wt% reduction of the characteristic NP bimodality is shown by a double pulse configuration with inter-pulse delay of 600 ps. Furthermore, synchronous double pulse PLAL with controlled inter-pulse distance is shown to modify bubble merging dynamics, resulting approximately in a factor 3 NP size increase. Finally, multi-beam PLAL is proposed as an alternative to increase inter-pulse distance and reduce cavitation bubble pulse shielding, showing a factor 4 maximum productivity increase compared to standard PLAL.

Ablation plume analysis showed that the most important problems of PLD, i.e. complex deviations of the film composition are clearly related to the properties of the ablation plume. The variation of the deposition conditions combined with plume analysis suggests also certain approaches how to overcome these problems, e.g. by varying background gases and pressures, but also when more drastic approaches must be used, e.g. an enrichment of certain elements in the target.

Pulsed laser interactions provide unique, highly nonequilibrium conditions for synthesis and processing of new materials, enabling access to metastable phases and nanostructures that can be explored by pulsed laser ablation (PLA), pulsed laser deposition (PLD), and laser processing. Current demands for quantum materials require understanding of the structure and properties at the atomic-level to reveal defects and their correlated properties. Atomically-thin 2D layered materials (such as graphene, MoS2 and other transition metal dichalcogenides (TMDs)) and their heterostructures – formed by stacking layers in different orientations – form a tunable palette of materials that are synthesizable, computationally tractable, and can be atomistically characterized. Here, we describe progress in both the implementation and automation of real-time in situ diagnostics during PLD and laser processing to reveal the synthesis pathways and metastable states of atomically-thin 2D materials as they grow, and advance efforts towards their autonomous synthesis.

Functional oxides thin films are a class of materials which are embedded in a wide range of devices. Depending on the device/type of application, simple or complex oxide are used. We report on the properties of the WO3 and of several lead-free perovskite materials such as titanate-based (Ba(Ti0.8 Zr0.2)O3-x(Ba0.7 Ca0.3)TiO3 -BCTZ) and ferrite-based (BiFeO3-BFO; Y-doped BiFeO3- BYFO; LaFeO3- LFO) thin films growth by pulsed laser deposition (PLD). Applications in energy storage/generation, photonics, sensors, biology are discussed, depending on their properties as crystallinity, morphology, thickness.

The use of ultrashort laser pulses for volume nanostructuring of dielectric materials shows tremendous potential for 3D photonics. Key aspect in the optical design is the capability to confine energy to the smallest scales, bypassing the diffraction limit. The nonlinearity of the light-matter interaction, harvesting far and nearfield effects as well as material transients is at the base of refractive index engineering on the nanoscales, enabling to develop new embedded optical functions. It will be shown how extreme scales, down to a tenth of the wavelength, can be obtained. Using quantitative optical imaging techniques, a time dynamic perspective will be given over the excitation mechanisms and the relaxation pathways, reconstructing thermomechanical trajectories. Demonstrating nanoscale features, I will pinpoint the potential for the fabrication of complex hybrid micro-nano optical systems, capable of transporting, manipulating and reconstructing optical signals. Applications in telecom and astrophotonics for sensing and imaging will be indicated.

Pulsed-laser deposition scheme (PLD) is one of feasible coating methods and we applied this method to hydroxyapatite coating on medical implants. Our previous study showed that ablated droplets disturbed to coat dense, high purity, and high crystallinity hydroxyapatite layer at low annealing temperature. Therefore, we applied “eclipse-type” PLD scheme, which could capture ablated droplets by an obstacle ball and only tiny particles as atoms, ions, and molecules were deposited on a substrate. Raman spectrum from the coating layer showed coating layer changed to crystal layer over 360 ℃, and as increase the annealing temperature crystallinity became better. The adhesion strength of the coating layer was also measured by a scratched test. As increasing of the annealing temperature, the adhesion also became strength stronger. We concluded this high-quality coating layer is suitable for medical implants coating.

After the discovery of graphene prepared by peeling graphite off using scotch tape, many methods are proposed, such as thermal decomposition of silicon carbide (SiC), and chemical vapor deposition (CVD) method. The CVD method seems like to be most popular among those methods. However, the CVD requires metal catalyst (Cu, Ni), and films deposited on such metals are required to transfer onto insulating substrates for device applications. Another method we have previously reported employs pencils and paper. Paper sheet drown using a lead pencil was irradiated by both femtosecond laser and laser cutter (consumer product, a few hundred dollars), and graphitic materials remain on the paper sheet [1]. In this presentation, another method using pulsed laser deposition (PLD) in carbon dioxide[2] will be proposed, and we estimated an adsorption energy between clusters and substrate surface for choice of substrates. A molecular dynamics was used to estimate the adsorption energy[3]. [1] Kaneko et. al. Jpn. J. Appl. Phys. 55 (2016) 01AE24. [2] Kaneko et. al. ACS Omega 2 (2017) 1523. [3] Kaneko et. al. Sci. Rep. 12(2022)15809.

Nanohybrid surface layers consisting of semiconductor transmission metal oxide nanoparticles and carbon based nanomaterials were prepared by matrix assisted pulsed laser evaporation (MAPLE). The photocatalytic decomposition efficiency of the nanocomposite layers was studied against microorganisms, yeast, bacteria, and virus cells constituents under UV, visible, and solar light irradiation. Transition metal oxide semiconductor materials are widely investigated photocatalysts, being non-toxic, eco-friendly, and cost effective. However, their use in practical applications is constrained by their relatively wide band gap, limiting the absorption range to the UV spectrum of the solar radiation, and high recombination rate of photo-induced electron-hole pairs. Our purpose in this study was to overcome these inconveniencies. The enhanced photocatalytic efficiency of the nanohybrid layers as compared to the reference single component layers was attributed to the synergistic effects between the constituent nanomaterials.

This conference presentation was prepared for High-Power Laser Ablation VIII, 2024.

We present recent results on fibers for directed energy applications. These fibers use new designs for high transverse mode instability thresholds obtained by increasing the higher-order mode losses. This approach allows for simultaneously increasing the MFD of the fundamental mode while also maintaining high TMI thresholds, even for higher-absorption fibres with increased heat load. Using such an approach, 5 kW of signal power at 26 GHz linewidth was obtained from a 7 m long amplifier using a gain fiber with 20 µm MFD and 400 µm cladding. For high-power, high-energy pulse lasers, we have used this design approach to demonstrate 30/400 fibers with 25 µm MFD that amplified millijoule, nanosecond pulses to killowatt average powers.

The Modern Directed Energy HEL is nearly entirely based on the combined fiber laser array. These lasers are compact, rugged, low SWAP with high beam performance. Currently at 2-3kW of near diffraction limited output, they are imminently scalable from 10kW CUAS lasers to 1MW strategic lasers. OEI has been utilizing its decades long expertise in fiber glass processing to create lasers that have even higher levels of performance while greatly reducing the structural overhead currently required for laser and beam control. In this talk OEI will survey the current state of the art in DE Laser Fiber laser glass processing in areas including cleaving, tapering, etching and splicing, and thermal management and mode stripping well as future developments in combiners, mode adapters, and -65dB kW tap couplers. System enhancements such as monolithic fiber laser control, advanced output structures, as well as fiber array passive alignment, and novel structures such as the Counter Pumped Tapered End Cap (CPTEC) and the Fly’s Eye Fiber Laser Array (FEFLA) will also be discussed.

Among possible approaches to fusion energy, we regard the Proton Fast Ignition (PFI) as the most credible. PFI as an alternate route to ignition was triggered by the discovery of ultra-bright beams of protons produced by ultra-intense lasers. Protons are advantageous to other ion species and electrons. Because of their highest ionic charge-to-mass ratio, they are accelerated most efficiently up to the highest energies. They can penetrate deep into a target to reach the high-density region, where the hot spot is to be formed. And they exhibit a characteristic maximum energy deposition at the end of their range, desirable to heat a localized volume. Thus, Focused Energy Inc. has chosen PFI for the primary pathway to fusion energy.

The advancement of laser technology has greatly increased the intensity of focused light over the past seventy years. This has allowed for the study of ultrafast events and has opened up new fields of science and various applications in society. From understanding atomic processes to manipulating particle beams, lasers have had a significant impact in scientific research. However, many laser technologies are now reaching their limits after decades of progress. During this talk, we will present recent results leveraging attosecond-level frequency comb stabilty, such as heterodyne and homodyne detection techniques for quantum sensing applications. We will also review and address the challenges on emerging technologies from first principles quantum electrodynamics to materialize both large-scale facility and compact hard X-ray sources exhibiting attosecond pulse durations to probe electronic dynamics with atomic specificity simultaneously.

Acceleration of particles in photonic nanostructures fabricated using semiconductor manufacturing techniques and driven by ultrafast solid state lasers is a new and promising approach to developing future generations of compact particle accelerators. Substantial progress has been made in this area in recent years, fueled by a growing international collaboration of universities, national laboratories, and companies. Performance of these micro-accelerator devices is ultimately limited by laser-induced material breakdown limits, which can be substantially higher for optically driven dielectrics than for radio-frequency metallic cavities traditionally used in modern particle accelerators, allowing for 1 to 2 order of magnitude increase in achievable accelerating fields. The lasers required for this approach are commercially available with moderate (micro-Joule class) pulse energies and repetition rates in the MHz regime. We summarize recent experimental progress and potential near-term applications and offshoot technologies.

The University of Dayton Research Institute (UDRI) in collaboration with the Air Force Research Laboratory (AFRL) investigated the use of femtosecond laser processing methods for the athermal processing of Polymer Matrix Composites (PMC) and advanced coating systems using lasers up to 100W in both 1030nm and 15W in 343nm wavelengths. Results of work including Composite Laser Ablation Surface Preparation (CLASP), laser peel-ply scoring, coating manipulation/removal, will be presented. Practical implementation progress, including robotic integration and handheld implementation of these technologies, will also be reviewed. Finally, the broader impacts of advanced/femtosecond laser technologies for manufacturing across the DoD will also be considered.

we present a numerical and experimental study on the characterization of the temporal ablation pressure due to a laser with and without a confinement layer on aluminum target. Based on velocimetry measurements, we have built a large experimental data of laser configurations varying the pulse duration, the energy and the wavelength. Then we are able to reproduce the results with a single numerical model using the lagrangian 1D ESTHER hydrocode in both regimes. Finally we use the code to propose analytical laws for temporal ablation pressure in a large interaction domain for aluminum targets.

Spallation caused by shock waves in optical components such as those used in the Laser MegaJoule facility during laser operation leads to material fracture during a Laser-Induced damage event. One solution may be to use a viscoelastic thin film on these components to mitigate spallation, but it must have excellent optical, mechanical, and resistance to laser damage properties. Among the viscoelastic materials investigated were Nafion and polydimethylsiloxane-based Ormosil. These materials, as thin films deposited on a fused silica substrate, were studied under nanosecond pulsed lasers at 1064 and 532 nm with different diagnostics in situ and post-mortem. In particular, the effect of the films on spallation was studied using the laser shock technique. Preliminary results showed that these thin films have interesting properties that could help to reduce mechanical damage to optical components.

Three-dimensional microprinting via two-photon absorption is the additive manufacturing technique of choice for complex micro-optical systems. Since post-processing of printed micro-optics is not possible in most cases, deviations between design and printed samples affect the intended function and therefore need to be minimized. This is a difficult task since important material properties such as shrinkage and refractive index depend on the cross-linking density and thus on the process parameters. We present first results towards a detailed prediction of 3D printed structures based on a modeling approach combined with machine learning to adjust the corresponding process parameters.

We present a scalable method for producing all-glass meta-surfaces with high mechanical stability and robust resistance to laser-induced damage. The process is based on dewetting a thin metal film on a glass substrate, followed by dry etching and metal mask removal. Laser scan is used to pattern the spatial pattern of the end-result metasurface. We will present process advances which enable formation of lenses, antireflection surfaces, and birefringence elements.

We present the development of x-ray sources based on laser-wakefield acceleration of electrons with high-intensity lasers. They are developed for applications including non-destructive imaging, medical imaging, or high energy density science at large-scale laser facilities.

Laser-plasma acceleration (LPA) sources exhibit extraordinary beam properties that create unique application opportunities but also pose challenges. LPA can generate energetic ions or electrons. The resulting bunches feature a high number of particles concentrated in temporally short bunches, making them capable of delivering ultra-high dose rates (UHDR), e.g., for radiobiological research. However, LPA sources suffer from unfavorable beam properties such as shot-to-shot fluctuation, high divergence, and a broad energy distribution. The presentation focuses on results obtained at the Draco PW laser facility. Draco PW drives an LPA proton source that was stabilized by tuning the laser’s spectral phase. Pulsed high-field magnets efficiently tailor the beam to meet the demands of application experiments, here, the world’s first radiobiological animal studies with laser-accelerated particle beams (zebrafish embryos and tumors in a mouse model). The role of LPA particle sources in the context of UHDR radiobiology and FLASH radiotherapy – possibly the next breakthrough in the fight against cancer – will be discussed.

We report advances in the development of a robust, laser-driven X-ray source. An ultrafast, high-average-power laser driver (1 kHz, 25 mJ, 1.5 ps) allows us to reach sufficient X-ray photon flux to demonstrate the source performance in a realistic imaging application by realizing a complete tomographic sequence in under-15min exposure time, edging toward clinical imaging relevance. This kind of compact, high-brilliance, hard X-ray source holds promise to unlock the development of practical phase-contrast medical imaging.

Shocks produced by laser plasma can generate strong (GPa range) and short (ns range) stresses in materials. By playing on their propagations, it can sollicitate structural assembly interfaces for example. This issue is important since it concerns the detection of weakbond. Since several years, we have been interested in the development of this technique by addressing all technical and scientific issues. This presentation will share the latest scientific and technical advances concluded by demonstrations on representative parts. It will also discuss futures issues that remain to be developed to reach maturity for the industrialization of the process.

Laser ablation propulsion and hypervelocity impacts are developing areas of research. This work aims to understand the contribution of different laser and target parameters to the momentum transfer during laser ablation through ballistic pendulum experiments. The data presented are the results of three experimental campaigns using different pulse durations, wavelengths and energies. The momentum was calculated from Photonic Doppler Velocimetry (PDV) and pendulum deflection measurements, while the contribution of the ejecta was estimated by camera imaging. The experimental results were complemented by 1D simulations of the momentum and ejecta contribution using the ESTHER code.

Rare earth doped polycrystalline ceramics are promising new gain medium for high power lasers because of their outstanding thermal and mechanical properties. One major difficulty is that polycrystals suffer more from scattering compared with single crystals and glass. Experiments have shown high optical transparency can be obtained with finer grains and grain orientation alignment. Therefore, it is of crucial importance to understand the loss mechanism behind both qualitatively and quantitatively. Here we present a first principles birefringent scattering model which predicts the scattering loss from realistic microstructure. The model can be used to interpret experimental results as well as make quantitative predictions with varying grain morphology and texture parameters.

Electron-phonon coupling plays a central role in describing the energy relaxation dynamics of solids excited by ultrafast laser pulses. It depends on electronic and phononic properties in different ways for different metals. In many calculations of the electron-phonon coupling parameter, the phonons are assigned a secondary role. In this work, we study the influence of the maximum phonon energy on the electron-phonon coupling parameter within the framework of the Debye model. We find a large increase of the coupling parameter with the Debye energy for all considered metals.

Wide-bandgap materials such as silicon dioxide (fused silica, α-quartz) can undergo strong excitation when exposed to high-power ultrashort laser pulses. This leads to a high transient electron density in the conduction band, causing distortion in the bands and resulting in a significant bandgap renormalization. Additionally, there is a spatial redistribution of the excited charges, leading to weakening of silica bonds and subsequent reorganization of the crystal structure, further contributing to the change in the bandgap. Through the use of Density Functional Theory, Time-Dependent Density Functional Theory, and GW approximation, the evolution of the bandgap is studied at different levels of excitation, revealing changes of up to several electronvolts on ultrashort timescales.

By the application of a multi-physics model describing the nonlinear propagation of a femtosecond, near-infrared laser pulse in bulk sapphire, we show that even under extreme focusing conditions, the ionization is rigidly clamped at about one tenths of the electron density in the upper valence band. The earlier estimates of approximately 10 TPa pressure that could be attainable through the internal excitation of transparent dielectrics by tightly focused ultrafast laser beams is shown to be off by two orders of magnitude. We discuss potential routes towards overcoming the clamping limit and present experimental results on the generation of internal voids in bulk sapphire by ultrafast, simultaneously spatially and temporally focused (SSTF) laser beams.

Several distinct mechanisms of femtosecond laser ablation of thin Ag films from a silica substrate are established in large-scale atomistic simulations and are mapped to the space of film thickness and absorbed fluence. For a fixed film thickness, the increase in fluence results in sequential transitions from melting with no ejection of the film, to film splitting or spallation, to an explosive decomposition of the top part of the film and generation of a residual layer in the lower part of the ablation plume, and to a complete phase decomposition of the film into small droplets and vapor. To facilitate the experimental validation of the computational predictions, the variation of the scattering and reflectivity of the ablation plume is calculated from atomic configurations predicted in the simulations and related to the results of pump-probe optical imaging of the ablation plume.

A liquid-assisted, laser machining process is described for high-aspect ratio micro-holes (<200-um diameter), which are produced in a variety of glass substrates with precise geometry control. Such features are difficult or impossible to fabricate using alternative approaches.

High energy ultrashort pulses can generate filaments during propagation, plasma channels which balance self-focusing to allow laser light propagation at a clamped diameter and intensities exceeding 10^13 W/cm^2 for many times the Rayleigh length. These filaments are attractive options for laser ablation at a distance but are limited in the total energy that can be delivered to a single point on target. In this work, temporally structuring femtosecond pulses into gigahertz bursts of spatially overlapped filaments lead to an increase in ablation. Material-interaction effects are compared between these nanosecond duration bursts of femtosecond pulses and traditional nanosecond pulses of similar total energy and duration in both aluminum and silicon targets.

Since a wide variety of microscopic living forms in contact with medical implant in human body are known to be sensitive and reactive to the surface topography, it is of active interest to optimize the implant surface of for desired integration. Ultrafast laser is a powerful tool for modifying the surface of medical implants, at the micro- /nano-scale, for either improving or limiting living tissues adhesion ability. Laser processing and living microorganisms’ response to laser texturing are discussed. Bacteria reduction and human cells adhesion at implant surfaces are reviewed in connection with ultrafast laser induced surface micro- / nanostructures.

Laser wakefield accelerators can be an alternative to huge linear accelerators and cyclotrons. Electron bunches with 150-200 MeV energies are needed for Very High Energy Electron radiotherapy. Injection of electrons and their acceleration take place when the focused laser beam interacts with a gas plasma target. We utilise a combined laser micromachining technology with short-pulse and ultra-short-pulse lasers to manufacture complex gas nozzles in fused silica. TW-class lasers are able to accelerate electrons to high energies in a very short distance. A stable operation with electron energy around 3 MeV was demonstrated at a 1 kHz repetition rate. Flexibility in 3D carving within fused silica with lasers allows tailoring plasma targets to particular beams of ultra-high intensity lasers and achieving high energy of accelerated electrons with low energy spread and divergence. Electron energy above 100 MeV could be achieved using new kHz-class OPCPA lasers operating at pulse energy >50 mJ.

Plasmonic cell fusion is a recently developed technique for artificial induction of cell fusion. It uses high-intensity femtosecond laser pulses for resonant irradiation of specific cells conjugated by gold nanospheres. Various experiments were conducted for demonstrating the potential of this technique for addressing several key clinical challenges in medicine, including stimulating a selective immune response and treating muscle injuries.

Non-linear ionization is the physical excitation mechanism by which an ultrashort laser locally transforms a dielectric from an initial insulating to a highly conductive state with considerable changes on its local electrical and optical properties. When controlled, this local excitation of the material yields well-calibrated macroscopic post-mortem transformation on which every optimized process of laser micromachining builds on. The work presented here is part of this perspective to get control of such laser-induced transformation, aiming to propose an accurate experimental evaluation of laser energy deposition at the surface of a dielectric material exposed to ultrashort pulses, and in particular of its produced free-electron – hole plasma density. For that objective, we conceive appropriate single-shot energy balance experiments with specifically designed dielectric targets consisting of wedged and plane-parallel samples of different thickness (below and above the Rayleigh of the focused laser) in order to observe the free-electron hole plasma produced at the dielectric front surface and also to act on the ratio between photo-ionization and total beam losses. Experiments consists in measuring the incident, reflected and transmitted energy on a wide range of incident fluence (<< Fth to ~ Fth, where Fth indicates the laser-induced ablation threshold) by means of calibrated and identical photodiodes in order to establish a precise energy balance of the interaction. We choose fused silica material because of its popularity for micromachining and photonics applications and we also use ultrashort femtosecond pulses ( 15 - 100 fs, 800 nm) to vary the photo-ionization rate and to well decorrelate ionization process from hydrodynamics and any energy transfer and recombination processes. The information retrieved from these experiments and further confronted to Drude-Lorentz theoretical framework helps us understanding ionization processes (photo-ionization, impact ionization) and their respective importance for dielectric macroscopic transformation in the ablation regime.

Solar cells based on III-V semiconductors are the world's most efficient, but their use is currently limited to applications such as space power because of their high cost. Post-growth device processing, which includes metallizing the solar cells and isolating the active areas, makes up a significant portion of fabrication cost. Mesa isolation is typically done by masking active areas with polymer-based photolithography and chemically etching the surrounding epitaxial layers to electrically isolate the solar cells from, for example, flaws in the epitaxy on the edge of the wafer, or from each other. In this work, we use a femtosecond laser micromachining system to mesa-isolate GaAs and GaInP solar cells, two common subcell materials in multijunction III-V devices, as an alternative to photolithography and etching. We demonstrate GaAs solar cells mesa isolated with laser ablation with a performance similar to that of a baseline, but a significant degradation of performance for GaInP solar cells. We will discuss the damage we observe from laser ablation of these materials, including an Auger electron spectroscopy analysis of the ablated regions.