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Ultrafast imaging is essential in physics and chemistry to investigate the femtosecond dynamics of nonuniform samples or of phenomena with strong spatial variations. It relies on observing the phenomena induced by an ultrashort laser pump pulse using an ultrashort probe pulse at a later time. Recent years have seen the emergence of very successful ultrafast imaging techniques of single non-reproducible events with extremely high frame rate, based on wavelength or spatial frequency encoding. However, further progress in ultrafast imaging towards high spatial resolution is hampered by the lack of characterization of weak probe beams. For pump–probe experiments realized within solids or liquids, because of the difference in group velocities between pump and probe, the determination of the absolute pump–probe delay depends on the sample position. In addition, pulse-front tilt is a widespread issue, unacceptable for ultrafast imaging, but which is conventionally very difficult to evaluate for the low-intensity probe pulses. Here we show that a pump-induced micro-grating generated from the electronic Kerr effect provides a detailed in-situ characterization of a weak probe pulse. It allows solving the two issues of absolute pump–probe delay determination and pulse-front tilt detection. Our approach is valid whatever the transparent medium with non-negligible Kerr index, whatever the probe pulse polarization and wavelength. Because it is nondestructive and fast to perform, this in-situ probe diagnostic can be repeated to calibrate experimental conditions, particularly in the case where complex wavelength, spatial frequency or polarization encoding is used. We anticipate that this technique will enable previously inaccessible spatiotemporal imaging in a number of fields of ultrafast science at the micro- and nanoscale., The article reports a technique to characterize, inside a sample, ultrafast probe pulses with weak intensity: zero delay, optimal compression, pulse front tilt. The measurement is non-destructive, valid for all wavelengths and polarizations.

Second-harmonic emission at a frequency that is twice the laser frequency is an important diagnostic for nonlinear laser-plasma interaction. It is forbidden for centrosymmetric materials such as the bulk of sapphire. The symmetry, however, can be broken by dielectric discontinuities as a result of plasma generation inside a solid dielectric. In the present work, we explore the basic characteristics of experimentally observed second-harmonic emission during focusing a femtosecond Bessel beam inside sapphire. We employ three-dimensional particle-in-cell simulations and the Helmholtz wave equation for theoretical investigations. We analyze how the efficiency of second-harmonic generation and its polarization depend on the plasma parameters. We find that the second-harmonic is generated either due to the coalescence of two surface electromagnetic waves or nonlinear interaction between the transverse electromagnetic wave and the longitudinal electron plasma wave driven by linear mode conversion. Experimental results agree with the theoretical predictions and confirm the existence of over-critical plasma inside the sapphire that is essential for the resonance of plasma waves or excitation of surface plasmons., Comment: 10 pages, 4 figures, published in Physics of Plasmas

The creation of high energy density ($\gtrsim10^6$ joules per cm$^3$) over-critical plasmas in a large volume has essential applications in the study of warm dense matter, being present in the hot cores of stars and planets. It was recently shown that femtosecond Bessel beams enable creating over-critical plasmas inside sapphire with sub-wavelength radius and several tens of micrometers in length. Here, the dependence of field structure and absorption mechanism on the plasma density transverse profile are investigated by performing self-consistent Particle-In-Cell (PIC) simulations. Two { limiting} cases are considered: one is a homogeneous step-like profile, that can sustain plasmon formation, the second is an inhomogeneous Gaussian profile, where resonance absorption occurs. Comparing experimental absorption measures to analytical predictions allows determining the plasma parameters used in PIC simulations. The PIC simulation results are in good agreement with experimental diagnostics of total absorption, near-field fluence distribution, and far-field radiation pattern. We show that in each case an ambipolar field forms at the plasma surface due to the expansion of the hot electrons and that electron sound waves propagate into the over-critical region., Comment: 13 pages, 10 figures, published in Physics of Plasmas

International audience; Ultrafast imaging requires accurate synchronization of pump and probe pulses, as well as the removal of anypulse front tilt of the probe pulse. Indeed, any pulse front tilt is highly detrimental to the accuracy of the imagingtechnique. In addition, the ”zero delay” between pump and probe is only true for a specific propagation distance.This is because along the path of pump and probe beams, the dispersion strongly modifies the retardance betweenthe pulses. As an example, for pump and probe beams in a collinear configuration, the propagation through 100 µmthick glass slide is enough to induce a shift by more than 37 fs between 800 nm and 400 nm pulses because of thedifference in terms of refractive index. However, it is extremely difficult to characterize weak probe pulses.

International audience; Second-harmonic (SH) radiation spectrum is an important diagnostic in nonlinear laser-plasma interactions.

Ultrafast pump-probe imaging requires both an accurate synchronization of probe pulses with the pump and that the probe pulses are free from spatio-temporal distortions. However, characterizing weak probes inside transparent solids reveals to be particularly difficult. We report a new in-situ diagnostic for ultrashort probes using a micrometric-sized Kerr-based transient grating induced in the sample by a shaped pump pulse. Our configuration allows us to synchronize pump and probe pulses in-situ, to measure the ultrashort probe pulse duration, and to remove pulse front tilt of the weak probe. Our approach is valid for any probe wavelength and polarization.

International audience; High absorption is experimentally measured during the propagation of femtosecond Bessel pulses in the bulk of sapphire. Using self-consistent particle-in-cell simulations, we have explained this process by resonance absorption in the pulse generated nanoscale plasma rod.

Ultrafast laser pulses spatially shaped as Bessel beams in dielectrics create high aspect ratio plasma channels whose relaxation can lead to the formation of nanochannels. We report a strong enhancement of the nanochannel drilling efficiency with illumination by double pulses separated by a delay between 10 and 500 ps. This enables the formation of nanochannels with diameters down to 100 nm. Experimental absorption measurements demonstrate that the increase of drilling efficiency is due to an increase of the confinement of the energy deposition. Nanochannel formation corresponds to a drastic change in absorption of the second pulse, demonstrating the occurrence of a phase change produced by the first pulse. This creates a highly absorbing, long-living state. Our measurements show that it is compatible with the semi-metallization of warm dense glass which takes place within a timescale of

International audience; The main advantage of ultrafast laser-based processing techniques is the capability of structuring the material in three dimensions, even inside the bulk of transparent materials. It allows for a very precise control and versatility of the manufacturing process. The formation of high aspect ratio voids in glass with diameters in the hundreds of nanometres range using only a single ultrafast laser pulse is particularly useful for many applications. Drilling of high aspect ratio nanochannels has been particularly successful when using zeroth-order Bessel beams [1, 2]. However, depending on the material and illumination geometry, limitations arise on the maximal and minimal nanochannel diameter (Fig. 1a). Here, we demonstrate that double pulses drastically improve the efficiency of the drilling (Fig. 1b) and absorption measurement allow for deciphering the mechanism.

Pump-probe experiments are essential tools to investigate ultrafast dynamics of laser-matter interaction. We are particularly interested in the dynamics of transparent dielectrics under high numerical aperture focusing. Two main challenges arise for the weak probe pulse. First, we need a precise knowledge of the probe delay with respect to the pump pulse. Second, dispersion compensation of the ultrashort probe pulse generally requires a prism compressor, which can generate angular dispersion, and therefore incorrect interpretation of the pump­probe measurements.

We developed a new numerical method to build a multidimensional hydrodynamic code solving the complex two-fluid plasma model equations. The major advantage of this method is its strong simplicity of implementation but still preserving accuracy and robustness. This approach can be very helpful in the field of laser-plasma interaction, high-intensity optics and plasmonics.

We report on the development of an ultrafast beam shaper capable of generating Bessel beams of high cone angle that maintain a high-intensity hot spot with subwavelength diameter over a propagation distance in excess of 8~mm. This generates a high-intensity focal region with extremely high aspect ratio exceeding 10~000:1. The absence of intermediate focusing in the shaper allows for shaping very high energies, up to Joule levels. We demonstrate proof of principle application of the Bessel beam shaper for stealth dicing of thick glass, up to 1~cm. We expect this high energy Bessel beam shaper will have applications in several areas of high intensity laser physics., Comment: 3 figures

Ultrafast Bessel beams demonstrate a significant capacity of structuring transparent materials with a high degree of accuracy and exceptional aspect ratio. The ability to localize energy on the nanometer scale (bypassing the 100-nm milestone) makes them ideal tools for advanced laser nanoscale processing on surfaces and in the bulk. This allows to generate and combine micron and nano-sized features into hybrid structures that show novel functionalities. Their high aspect ratio and the accurate location can equally drive an efficient material modification and processing strategy on large dimensions. We review, here, the main concepts of generating and using Bessel non-diffractive beams and their remarkable features, discuss general characteristics of their interaction with matter in ablation and material modification regimes, and advocate their use for obtaining hybrid micro and nanoscale structures in two and three dimensions (2D and 3D) performing complex functions. High-throughput applications are indicated. The example list ranges from surface nanostructuring and laser cutting to ultrafast laser welding and the fabrication of 3D photonic systems embedded in the volume.

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.

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 develop a novel concept for ultra-high speed cleaving of crystalline materials with femtosecond lasers. Using Bessel beams in single shot, fracture planes can be induced nearly all along the Bessel zone in sapphire. For the first time, we show that only for a pulse duration below 650 fs, a single fracture can be induced in sapphire, while above this duration, cracks appear in all crystallographic orientations. We determine the influential parameters which are polarization direction, crystallographic axes and scanning direction. This is applied to cleave sapphire with a spacing as high as 25 μm between laser impacts.

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.

Ultrafast lasers have revolutionized material processing, opening a wealth of new applications in many areas of science. A recent technology that allows the cleaving of transparent materials via non-ablative processes is based on focusing and translating a high-intensity laser beam within a material to induce a well-defined internal stress plane. This then enables material separation without debris generation. Here, we use a non-diffracting beam engineered to have a transverse elliptical spatial profile to generate high aspect ratio elliptical channels in glass of dimension 350 nm x 710 nm, and subsequent cleaved surface uniformity at the sub-micron level., Comment: 5 pages, 4 figures

International audience; We have developed a strategy to image the nonlinear propagation of high-intensityfemtosecond pulses while they propagate in fused silica. We have studied the propagation ofBessel beams in the regime where they allow for drilling high-aspect ratio nanochannels in singleshot. Experimental results provide useful insights for the modelling.

Femtosecond pulses provide an extreme degree of confinement of light matter-interactions in high-bandgap materials because of the nonlinear nature of ionization. It was recognized very early on that a highly focused single pulse of only nanojoule energy could generate spherical voids in fused silica and sapphire crystal as the nanometric scale plasma generated has energy sufficient to compress the material around it and to generate new material phases. But the volumes of the nanometric void and of the compressed material are extremely small. Here we use single femtosecond pulses shaped into high-angle Bessel beams at microjoule energy, allowing for the creation of very high 100:1 aspect ratio voids in sapphire crystal, which is one of the hardest materials, twice as dense as glass. The void volume is 2 orders of magnitude higher than those created with Gaussian beams. Femtosecond and picosecond illumination regimes yield qualitatively different damage morphologies. These results open novel perspectives for laser processing and new materials synthesis by laser-induced compression.

In the context of laser cutting of transparent materials, we investigate glass cleaving with Bessel beams and report that a modification of the beam with 3 main lobes drastically enhances cleavability and reduces defects.

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.

International audience; Ultrafast lasers in association to beam shaping have shown to be excellent candidates for transparent material processing. Non-diffractive solutions such as Bessel beams allows for precise energy deposition since they are robust to undesired non-linear effects and as they do not distort along the propagation. This offers important opportunities in laser-assisted cleaving, i.e. mechanical medium separation after single-pass laser illumination. Here we break the Bessel beam cylindrical symmetry using a novel anisotropic and non-diffractive solutions to investigate both lateral intensity contributions on material response and induced processing effect for non-cylindrical defects. Using such beam shape, we report a strong cleavability enhancement as well as an improvement of the final robustness of the separated glass in comparison with Bessel beams. We demonstrate cleaving for laser-writing speed as high as 25mm/s with ~1μm accuracy over the whole 20mm sample length.