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International audience; The extreme intensity provided by ultrafast laser pulses has enabled a number of scientific and technological advances in a number of areas in photonics and material science. The laser-generated plasma can be particularly used to create Extreme UV sources, to synthetize new materials phases or, if it is dense and hot enough, it can relax into Warm Dense Matter, which is an intermediate state in-between hot plasmas and solid-state. It is still particularly challenging to study and model and is of primary importance in research on inertial confinement fusion or on the core of several astrophysics objects.Up to here, dense plasmas were generated at the surface of materials, where the expansion rapidly cools down the plasma. It would be particularly attractive to generate such warm dense matter and hot plasmas within the bulk of solid dielectrics, where the infrared laser can create a localized plasma in 3D. However, because of the beam defocusing due to the plasma generated by the onset of the pulse, only underdense plasmas could be generated inside solids- apart in the case of extreme focusing where the applicability is very limited.We demonstrate with a combination of experimental and Particle-In-Cell simulations that ultrafast Bessel beams can generate over-dense plasmas in the bulk of transparent solids. We show that collisionless resonance absorption is a primary mechanism of the absorption inside the plasma. It yields an extreme mass energy density of several MJ/kg and we estimate from our measurements that the temperature can reach 10 eV.

International audience; Many contaminants of agricultural origin are released into rural environments, particularly at the soil surface. Their fate has been extensively investigated in repacked soils, but only few studies have addressed their transport in structurally preserved natural soils. Much remains unknown about their fate and transfer within and between This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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International audience; PRACE-funded PULSAR-PIC project is part of the ERC-funded project PULSAR which aims at developing new

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International audience; Transparent dielectrics such as glass are becoming increasingly used in nanotechnologies. However, most of the intended use require deep drilling, which is extremely demanding for lithography. In this framework, ultrafast laser pulses provide the benefit of being able to process materials in 3 dimensions from the bulk itself. A single laser pulse can be used to generate an extremely high aspect ratio nanochannel. The process relies on generating a high density plasma along a cylinder with nanometric diameter, whose relaxation generates in turn a microexplosion opening a void even inside the hardest materials.With the objective of generating nanometric channels inside transparent dielectrics with high efficiency, a key enabling tool is to deposit energy within glass on the smallest possible diameters. We report several successful approaches to confine the interaction. For this, we used combination of pulse sequence, high angle focussing. We have demonstrated higher efficiency for nanochannel drilling with high aspect ratio and reduced heat affected zone. This allowed us to generate nanochannels in glass with diameters close to 100 nm with a single laser pulse. The physics of the interaction will be discussed. Applications to stealth dicing of glass will be demonstrated. For the application to nanomachining of thick glass, we designed a new Bessel beam shaper which allowed us to cleave glass with up to 1 cm thickness after a single pass illumination.

International audience; Direct laser writing is a very useful technique to process dielectrics for many applications, like micro-fluidics, photo-electronics or glass cutting for consumer electronics. Material processing requires accurate control on energy deposition profiles inside the transparent material, which strongly depends on irradiation conditions. Gauss-Bessel beams have been successfully used for ultra-high aspect ratio processing because of the enhanced stability of these beams in comparison with the unstable and complex filamentation regime of Gaussian beams. In this work, we temporally split the input 100 fs laser pulse in two equal pulses with a variable delay. We compare single and double pulse illumination as a function of delay on the drilling of high aspect ratio nanochannels in borosilicate glass. Our main result is that, with equal energy, splitting the pulse allows increasing the channel width and enables channel drilling for conditions when only index modification was created. The results strongly depend on the sub-nanosecond delay (ie at delays below the standard burst mode of lasers). We report measurements of energy absorption and interpret our results in terms of confinement of the energy deposition. Our results raise novel fundamental questions on laser-matter interaction and we anticipate a wide impact on technological applications to laser processing of transparent materials.

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International audience; We review a selection of recent results where applications of concepts that harness the “shape of light” in spatial or temporal domain have been applied widely to yield significant advances in areas such as computer-vision sensing with digital holography, spatial shaping of complex laser beams and photonic neural networks.Computer vision is a powerful contact-less measurement tools successfully applied in numerous domains of application, where depth of field and working distances are constrained by the imaging magnification chosen. The use of pseudo-periodic patterns on the target of interest overcomes these usual computer-vision limitations leading to sub-pixel resolutions and making the absolute measurement range independent of the field-of-observation of the imaging system [1]. The approach was also validated using digital holography as imaging method with a tremendous enlargement of the allowed working distance range [2], and seems very well suited to diverse application needs in the micro-robotic and biomedical domains.Principles of digital holography can be used also for spatial shaping of complex laser beams such as Bessel, Airy or arbitrary beams using liquid crystal spatial phase modulators (SLM). Applications in micro&nano-machining by non-diffracting ultrashort laser pulses in various materials have been proposed during recent years [3]. This will also open new perspectives for applications of complex beams for applications in microscopy, optical coherence tomography or ultrafast physics.Photonic systems have revolutionized the hardware implementation of Recurrent Neural Networks and Reservoir Computing, in particular [4]. The fundamental principles of ReservoirComputing strongly facilitate a realization in such complex analog systems. Especially delay systems, which potentially provide large numbers of degrees of freedom even in simple architectures, can efficiently be exploited for information processing. We also demonstrated learning in large-scale neural networks with numerous nonlinear nodes in an architecture using SLM [5]. This last scheme is fully parallel and the passive weights maximize energy efficiency and bandwidth.In high-tech areas such as micro-robotics and photonics, measurement requirements are increasing in terms of high resolution and their controls are based on multi-scale and complex parameters. Increasingly real-time processing remains a big challenge for future applications, where next generation of systems will need to implement new hardware architectures, maybe based on photonic neural networks.

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International audience; We report high aspect ratio nanochannel fabrication in glass using single-shot femtosecond Bessel beams of sub-3 μJ pulse energies at 800 nm. We obtain near-parallel nanochannels with diameters in the range 200–800 nm, and aspect ratios that can exceed 100. An array of 230 nm diameter channels with 1.6 μm pitch illustrates the reproducibility of this approach and the potential for writing periodic structures. We also report proof-of-principle machining of a through-channel of 400 nm diameter in a 43 μm thick membrane. These results represent a significant advance of femtosecond laser ablation technology into the nanometric regime.

International audience; Bessel beams are invariant solutions to the Helmoltz equation that can also propagate, with finite pulse energy at high intensity, in a quasi-invariant regime in transparent dielectrics. Homogeneous energy is deposited along a line focus by infrared ultrashort pulses. If the cone angle is sufficiently high, the laser-deposited energy density is enough to open nanochannels in glasses or sapphire with a single laser pulse. This has found applications in the field of glass cutting via the technique of "stealth dicing". Here we address two important challenges in this field. First, high quality Bessel beams are essential for controlled energy deposition. Second, the maximal angle used up to here for channel drilling was 26° for 800 nm laser central wavelength. This enabled the formation of channels with diameters down to typically 300 nm in glass and sapphire. It is questionable if higher cone angles could also produce channels with potentially smaller diameters. Here, we generate high quality Bessel-Gauss beams with a setup based on reflective, off-axis axicons. The Bessel zone exceeds 100 µm for cone angles up to 35 degrees. This corresponds to central spot diameter down to 0.5 µm FWHM. We qualified these beams with a 100 fs laser source centered at 800 nm wavelength. We report nanochannel drilling down to typically 100 nm over at least 30 µm length in glass. Our approach opens novel perspectives for high quality Bessel beam generation but also for the highly confined laser-matter interaction for high precision processing of transparent dielectrics.

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International audience; Ultrafast lasers have revolutionized materials science because of the high peak power they can generate even in the bulk of transparent materials. They allow for extremely localized nonlinear ionization. Besides, the processing of glass and sapphire is of recent particular importance for applications in consumer electronics (displays, touch-screens), microelectronics, substrates for LED growth, etc. However, conventional ablative techniques are slow and generate a large amount of debris. In contrast, stealth dicing enables material separation with minimal debris [1]. It consists of creating an internal damage defining a plane, which leads to a fracture under mechanical bending or stretching. This eventually leads to material separation at high illumination speed.In this context, we show that ultrafast Bessel beams are very advantageous. We have demonstrated that they allow controlling quasi-uniform energy deposition within transparent dielectrics. This is highly differs from the filamentation process created by Gaussian beams which undergo nonlinear distortions [2]. In single shot, Bessel beam generate formation of high aspect ratio nanochannels appearing either as a through-channel, or as a terminated channel opened on only one of the sample' surfaces [3] or as voids fully enclosed within the material's bulk [4]. The last case shows that the pressure created after picosecond or femtosecond pulse illumination is enough to compress the material around the void. We have shown the high influence of the pulse duration on void morphology.Creating channels side by side allows for inducing a weakened plane for the stealth dicing process. We report novel results where we engineer the transverse intensity profile to control the tensile stress distribution within the material and allow for a precisely defined cleaving plane. We show that the high precision of the fracture plane with regard to the laser-produced void is a particularly important parameter for the strength of the material after the separation. Finally, we report a new and different approach for crystal cleaving where the cleaving plane is created by controlled crack formation [5].

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International audience; Filamentation of Bessel and Bessel-like beams has enabled a number of applications in the field of transparent materials processing. Spatial phase is an important parameter to control four-wave-mixing and the cross section of plasma tracks.

International audience; Stealth dicing is a laser separation method for transparent materials, which generates virtually no debris. A single-pass laser illumination generates a weakened plane in the material’s bulk. Then, a weak bending yields cleaving: a fracture is guided along the weakened plane, which separates the material in two parts. The weakened plane can easily and quickly be written using ultrafast laser pulses that allow for a precise energy deposition in the material bulk thanks to the confinement of the highly non-linear interaction [1]. Stealth dicing has been reported using filamentation, offering energy deposition along extended depth [2]. Non-diffractive beams such as Bessel beams provide an even better energy deposition control. Nanochannels series with few microns pitch have indeed shown to be great candidates for stealth dicing applications [3].However cylindrically-symmetric channels do not induce a well-confined stress distribution when the workpiece is bended (fig 1.a). In this case, cleaving does not follow precisely the laser machined plane as one can observe from fig 1.d. To solve this problem, we have developed a novel elliptical nondiffracting Bessel beams that create channels featuring elliptical cross-section.The beam has been developed from spatial filtering of a Bessel beam (fig 1.c) and preserves the non-diffracting properties [4]. Using such beam for single-shot ablation allows for generating nanochannels that exhibit both &gt200 aspect-ratio and 2:1 elliptical cross-section (fig 1.f). The elliptical cross-section oriented along the processing axis enhances and confines the stress at ellipse vertices (fig 1.b). Here we report a sub-micron stealth dicing precision (fig 1.e) along the whole sample 20 mm length and 150 μm depth. In addition, we demonstrate with 3 line-bending measurements that elliptical nanochannels strongly improve cut glass cleavability and resistance in comparison with cylindrically-symmetric laser illumination [5].

International audience; The controlled laser energy deposition in dielectrics is a key technological issue for applications thatrequire drilling or structuring glass materials in the bulk. High angle Bessel beams delivered by a fem-tosecond laser have been used for extremely high aspect ratio nanochannels drilling and microstructuringin glass with a single laser shot [1,2]. Several types of filamentary damages were obtained with highangle Bessel beams and were reported to depend on laser pulse duration and energy [2]. I will addressthe question of optimal laser parameters for nonlinear absorption of laser energy in the bulk of dielectricmaterials, along the focal line of a Bessel beam.A semi-analytical model was developed for modeling the generation of a plasma column with fem-tosecond and picosecond laser pulses focused by a conical lens in dielectrics. The associated nonlinearabsorption of energy was calculated so as to highlight the influence of the pulse duration, laser pulseenergy and cone angle, on laser energy absorption. From a map of the density of nonlinear absorption oflaser energy in BK7 as a function of pulse duration and peak fluence, a specific pulse duration is shownto correspond to the highest nonlinear absorption at given pulse energy and cone angle. This durationcorresponds to a transition between two propagation regimes of the laser pulse featured by uniform ornon-uniform laser energy deposition in the Bessel zone. A uniform energy deposition is obtained in theabsence of pulse temporal dynamics.I will show that transmission measurements in BK7 are qualitatively well reproduced by the results ofthe semi-analytical model, which is also in agreement with numerical simulations of the nonlinear pulsepropagation

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International audience; Ultrafast lasers used for micro and nano machining now offer the reliability and performance for massproduction. In the particular field of transparent materials processing, a key capability of ultrashort pulses is that theycan drill and modify matter from inside the material itself. We demonstrate that using appropriate beam shaping, it ispossible to produce voids or nano-channels using a single pulse in even the hardest materials, and this has recentlyled to major advances in the field of stealth dicing, which is a non-ablative technique used to cleave and separatetransparent materials at extremely high processing speeds. We report novel recent developments where Bessel beamhave been used to create cracks and cleave sapphire, but also where symmetry has been broken to enhancecleaveability of laser-processed glass.

International audience; Ultrafast lasers have been a revolution in a number of areas of science and technology. In materials processing,the ultrafast pulse duration brought the ability to deposit energy into the free-electron gas before it couples to thelattice and therefore generate extremely steep thermodynamic gradients. A second benefit is the ultra-high peakpower which allows for ionizing dielectrics, even from the bulk material. The processing of glass and sapphire isof particular importance for several technological applications to mass fabrication (consumer electronics,microelectronics, etc). In this field, nondiffracting Bessel beams have brought the ability of controlling energydeposition by a single laser pulse on a long propagation distance inside dielectrics. In particular, this has allowedavoiding nonlinear instabilities generated by the filamentation regime of Gaussian beams yielding inhomogeneousdamage in materials [1]. Here, we report further advances in this field where additional benefit can be found in other beam shapes andpulse sequence. For applications to the so-called "stealth-dicing" technology, where material is cleaved after laserillumination, we have developed elliptical Bessel beams. We have demonstrated these beams remain alsopropagation-invariant and allow for generating nanochannels with elliptical cross section (Fig. 1(a)) [2]. We havedemonstrated those allow for cleaving glass (Fig 1(b)) with enhanced reliability and with sub-μm accuracy [3].In addition, we will discuss recent novel results where the illumination has been performed by double pulses withequal power. We show that, depending on the initial pulse energy, double femtosecond pulses enhance the abilityof channel drilling in glasses. In particular, channels with larger diameters are produced [4]. Energy absorptionmeasurements will be discussed.

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International audience; Ultrafast laser pulses create microexplosions under extreme focusing in transparent materials. The pressure created by few nano to micro joules of light is enough to generate new materials phases in the compressed zone around a laser-created void in sapphire or silicon [1,2]. Reaching extreme pressures is of fundamental interest not only for the formation of new material phases, but also for the study of the Warm Dense Matter, reproducing the state of the cores of planets with table top experiments.In this context, we have recently shown that Bessel beams create, in single shot, high aspect ratio nanovoids in the bulk of sapphire crystal. The Bessel beam possesses a central intense core surrounded by several lobes of lower intensity. The focal line can be arbitrarily long while having a constant focus diameter. In our case, the focal diameter is 0.66 µm FWHM, extending over ~30 µm. The voids were generated by short, 140 fs, and long, 3 ps, laser pulses at 2 µJ, 800 nm. [3]The analysis of the post-mortem samples shows that the pressures reached are in the range of tens of Megabars on a volume that is 2 orders of magnitude higher than what was reached with extremely focused Gaussian beams. Yet the light-matter interaction entangles propagation and permittivity change due to the swift conversion of the initial transparent medium into a dense plasma state, the Bessel beam is a quasi-2D system where Kerr nonlinearity is almost negligible for high cone angle focusing as in our case. We report on a new interaction mode provided by the Bessel beam focusing: numerical simulations, both from dynamical nonlinear Schrödinger equation for pulse propagation and static Maxwell models provide insights in the interplay between the incident laser field and the gradient of permittivity. Significant field-enhancement is observed which explains the extreme absorption and therefore extreme thermodynamic conditions reported experimentally.The new interaction provided by the Bessel beam is a novel approach to reach extreme conditions on macroscopic volumes. We believe this will open novel routes for applications and fundamental physics based on laser-matter interaction.

International audience; Bessel beams are a class of non-diffracting solution to the propagation equation. They exhibit a constant size of their main lobe over propagation distances that are much longer than the Rayleigh range of Gaussian beams with identical lobe size [1]. Such beam have been widely used for, femto-laser ablation [2], microscopy and optical coherence tomography [3-5].

International audience; Sapphire is a transparent crystalline dielectric of high hardness with many important applications, specifically to the next-generation touchscreens and to the LED growth, as substrates. However, sapphire cutting by ablative techniques is rather slow therefore fast material separation techniques are needed. Material separation by “stealth dicing” has been recently developed, it is based on material cleaving along a plane weakened by multiple ultrafast laser illuminations. This allows usually generating taper-free cutting and avoids material loss. However, the illuminated plane needs small spacing between the shot to shot (typically a few μm) and long damages inside the bulk. This requires lasers with both high repetition rate and high energy to allow high speed cutting and high aspect ratio damages. Here we report a novel approach based on controlled crack formation in crystalline materials. We demonstrate the possibility to generate a crack in a single direction with a femtosecond pulse along one crystalline axis. The cracks can extend over more than 30 μm laterally and can be joined to form a plane of cleaving which requires much less laser pulses. We use Bessel beams to create longitudinally uniform damage [1] and we demonstrate the application to sapphire wafer cleaving.

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International audience; Shaping complex light fields such as nondiffracting beams, provide important novel routes to control laser materials processing. Nondiffracting beams are produced from an interference between waves with an angle kept constant along the propagation direction. These beams are of outmost importance for laser materials processing because they offer invariant light-matter interaction conditions. We have used and developed several families of beams generated with phase and amplitude shaping and we will review their impact for laser surface processing and high aspect ratio laser processing in the bulk of transparent materials. Bessel beams and higher order Bessel beams allow for high aspect ratio channel drilling, elongated void creation in the bulk of transparent media, or tubular damage creation. We will also discuss the impact of accelerating beam shaping, ie beams with a curved main intensity lobe, to dice materials with a curved edge. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No 682032-PULSAR)

International audience; We investigate high-speed glass cleaving with ultrafast laser beams with engineered transverse intensity profile. We achieve accuracy of ~ 1 µm at 25 mm/s and drastically enhance cleavability compared to standard Bessel beams.

International audience; The material processing technique of “stealth” nanomachining is based on translating a longitudinally extended beam such as a Bessel beam into a transparent sample to generate extended nanochannels, which leads to subsequent internal stress that facilitates high quality cleaving. In this letter, we compare the quality of such cleaving in glass samples obtained using Bessel beams with both circularly symmetric and elliptical transverse profiles. We find that the use of an elliptical Bessel beam generates elliptical nanochannels, which greatly improves the cleavage quality and cuts material strength by aligning the centre of the cleavage plane with the centre of the machined channels. These results are interpreted using numerical simulations that show how elliptical nanochannels enhance the intensity and localization of the tensile stress distribution in glass under bending when compared to channels with circular cross-sections.

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National audience; Graphene is an important material for next-generation technologies, but the development of applications in the field of photonics and optoelectronics require micro and nanostructuring of the graphene films [1]. Femtosecond laser ablation is a promising technique because it can process large surfaces at extremely high speed and is easily reconfigurable. Previous studies of fs-laser ablation of graphene in accumulative thermal regime (burning) have shown high accuracy (down to

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International audience; Femtosecond laser processing is a versatile and reproducible technology with a large potential for mass micro-nano fabrication. However, the control of energy deposition within transparent materials is a challenge because of the nonlinear character of the propagation of laser light into the medium to ablate. We have recently developed several strategies to overcome this problem, based on the use of solutions that are propagation-invariant, even in the nonlinear regime, such as Bessel beams, accelerating beams and Bessel vortices. These beams allow for novel technological applications: curved edge profiling, high-aspect ratio drilling in single shot and extreme material compression.Interestingly, the long range of laser-plasma interaction within these beams also provide novel insights in the laser-plasma interaction within the bulk of materials. We have developed an experimental strategy to enable non-ambiguous and direct comparison with nonlinear propagation models. We uncover reasons for extreme confinement of the laser-matter interaction.

National audience; Réalisé pour l’année de la lumière, un montage d’holographie numérique intégrant un SLM, est dédié à la pédagogie en Cursus Master en Ingénierie physique.

National audience; Le contrôle de la propagation non-linéaire d’impulsions femtoseconde dans les diélectriques par mise en forme spatiale du faisceau ouvre de nouvelles perspectives scientifiques et technologiques.