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We compare supercontinuum generation in $$\hbox {CaF}_2$$ CaF 2 crystal under tight and loose focusing of 150 fs, 515 nm second harmonic pulses from an amplified Yb:KGW laser at a repetition rate of 10 kHz. It is demonstrated that supercontinuum generation geometry applying loose focusing ($$\hbox {NA}=0.004$$ NA = 0.004 ) of the pump beam into a long (25 mm) $$\hbox {CaF}_2$$ CaF 2 sample is advantageous in terms of supercontinuum spectral extent and durability of damage-free operation of the nonlinear material as compared to a commonly used supercontinuum generation setup which employs tight focusing ($$\hbox {NA}=0.012$$ NA = 0.012 ) into a short (5 mm) sample and to setup which uses tight focusing into a long (25 mm) sample. More specifically, loose focusing into a long sample showed remarkably longer (20 min) damage-free operation of the nonlinear material, which was not translated with respect of the pump beam, while in tight focusing condition the sample is damaged just within 2 min of operation, leading to a complete extinction of the supercontinuum spectrum. The evolution of optical degradation of the nonlinear material in time and its impact to supercontinuum spectrum is studied in terms of filament-induced luminescence due to self-trapped exciton emission and light scattering at the pump wavelength indicating the onset of optical damage. Our findings are supported by the numerical simulations which compare relevant parameters related to filament propagation in tight and loose focusing conditions.

We have experimentally studied supercontinuum generation in an undoped LiSAF crystal using ultraviolet, visible and near infrared pump pulses provided by fundamental and second harmonics of amplified femtosecond Ti:sapphire and Yb:KGW lasers. We have found that the optical degradation of the crystal, manifested by gradual narrowing of the supercontinuum spectrum, starts much faster using infrared pump pulses. This is attributed to the role of impact ionization, which increases with increasing the pump wavelength. The most reliable operation is achieved with the shortest pump wavelength of 400 nm (the second harmonic of a Ti:sapphire laser), where LiSAF produces a stable, almost 1.3 octave-spanning supercontinuum spectrum with a short-wavelength cut-off at 252 nm and shows no apparent optical degradation for one hour of continuous operation at 500 Hz repetition rate without crystal translation.

In this work, a burst mode laser is used for micromachining of 20 &micro, m&ndash, 250 &micro, m thick Invar (Fe64/Ni36) foils. Holes were drilled by firing multiple pulses transversely onto the sample without moving the beam (percussion drilling). The utilized laser system generates a burst of a controllable number of pulses (at 1030 nm) with tunable pulse-to-pulse time spacing ranging from 200 ps to 16 ns. The sub-pulses within the burst have equal amplitudes and a constant duration of 300 fs that do not change regardless of the spacing in time between them. In such a way, the laser generates GHz to MHz repetition rate pulse bursts with a burst repetition rate ranging from 100 kHz to a single shot. Drilling of the material is compared with the non-burst mode of kHz repetition rate. In addition, we analyze the drilling speed and the resulting dependence of the quality of the holes on the number of pulses per burst as well as the average laser power to find the optimal micromachining parameters for percussion drilling. We demonstrate that the micromachining throughput can be of an order of magnitude higher when using the burst mode as compared to the best results of the conventional kHz case, however, excess thermal damage was also evident in some cases.

Results of experimental and numerical investigation of supercontinuum generation in polarization maintaining photonic crystal fiber using a burst of two orthogonal polarization femtosecond pump pulses are presented. The initial pump pulse source was a mode-locked Yb:KGW laser generating 52 nJ energy 90 fs duration pulses at 1033 nm with 76 MHz repetition rate. Analysis of cross-correlation frequency-resolved optical gating traces and numerical simulation results reveal that there are slight changes in supercontinuum generation when temporal delay between pump pulses in the burst is changed. Weak dependence of supercontinuum characteristics on temporal distance between the pump pulses is related to supercontinuum generation dynamics which is analyzed in detail.

Femtosecond lasers are widely applied in scientific and industrial fields. Recent trends in the laser market show decreasing prices for femtosecond units, which will ultimately lead to the opening of new markets that were inaccessible in the past due to the high costs of such systems. To this end, new techniques that enable micromachining of materials with increased efficiency are interesting. In this article, we demonstrate a technique that may be used for cutting and drilling various materials. By placing a layer of water on top of the samples and loosely focusing laser light on the surface, it was found that the micromachining throughput is increased by up to 10-fold as compared with micromachining without the water layer (conventional focusing in air), however, the main reasons for the increase in fabrication efficiency have not been fully understood until now. By modelling the propagation of the femtosecond pulses by means of the nonlinear modified Schrodinger equation through the water layer, we show that the increased throughput is attributed to the changing of the Gaussian intensity profile. In addition, we confirm these findings by numerically modelling the ablated crater formation.

Bessel beams are perfect candidates for laser microprocessing of transparent materials due to generation of high aspect ratio micro voids [1], and ability to control micro crack formation enables to efficiently cut them [2, 3]. The crack propagation direction is controlled by inducing asymmetry to beam profile [3], and allows ultra-fast glass cutting over non-straight lines. The laser-matter interaction of various nondiffracting beams is heavily investigated and their utilisation for laser microprocessing is soon to be expected. Vector beams that are more exotic than scalar counterparts and can have additional beneficial properties for micro-manufacturing of materials. For instance, vector Bessel beam has a doughnut shaped intensity profile with complex spatial distribution of polarisation and can be de-composed to two transverse polarisation intensity components [4].

We experimentally study filamentation and supercontinuum generation in bulk silicon crystal using femtosecond mid-infrared pulses with carrier wavelengths in the range of 3.25–4.7 μm, in the presence of three-, four-, and five-photon absorption. Spectral measurements show a fairly stable blueshifted cutoff in the 2.5–2.7 μm range and gradual increase of the long-wave extent with increasing wavelength of the incident pulses, eventually yielding an octave-spanning supercontinuum, covering the wavelength range from 2.5 to 5.8 μm with the input pulses at 4.7 μm. The recorded spatiotemporal intensity distributions of a single filament revealed pulse splitting after the nonlinear focus, in line with the pulse-splitting-based filamentation scenario inherent to normally dispersive dielectric nonlinear media.

International audience; We report on the optical transmission of near-infrared (1.8 μm) femtosecond laser pulses during multiple pulses laser ablation of fused silica surface. In addition to the previously reported ciliary white light phenomenon (Phys. Rev. Lett. 110, 097601 (2013)), two new optical transmission phenomena were observed for relatively low energy laser pulses. We systemically examined the damage morphologies of the ablation site as a function of laser pulse energy and laser shot number. Laser induced periodic surface structures (LIPSS) with different periods and orientations were observed for different pulse energy regimes. Comparison of the damage morphologies and the optical transmission reveals that the two new optical transmission phenomena origin from the refraction of the incident near-infrared pulses on the LIPSS covered damage surface followed by nonlinear propagation of the beamlets and even filamentation if the laser energy was high enough. We found that the interference of the neighboring white light emissions gives rise to radially orientated optical fringes. Based on this observation, we proposed another interpretation for the ciliary white light phenomenon.

We study supercontinuum generation with femtosecond mid-infrared laser pulses in NaCl and KBr crystals when pumped at the vicinity of their zero group velocity dispersion points. Almost three octave-spanning SC spectra (covering the 0.7–5.4 μm and 0.85–5.4 μm ranges in 5 mm-thick NaCl and KBr samples, respectively) were produced by filamentation of 70 fs, 3.6 μm input pulses in continuously translated samples. In the static setup, we show that rapid formation of persistent color centers (within a few seconds at 500 Hz repetition rate) inside these materials results in a marked decrease (by 40%) of the overall energy transmittance and in significant narrowing of SC spectra. We also demonstrate that the effect of color centers on the SC spectrum could be adequately simulated numerically using a simple phenomenological model which considers color centers as impurities with a certain energy bandgap.

We have experimentally investigated supercontinuum (SC) generation and the evolution of optical damage in sapphire and YAG crystals with 180 fs, 1035 nm pulses from an amplified Yb:KGW laser with variable repetition rates, both in tight and loose focusing conditions. In this Letter, we demonstrate that the extinction of the SC spectrum always correlates with an occurrence of conical third harmonic generation, which readily serves as an indication of the onset of in-bulk optical damage. Damage-related structural changes of the nonlinear material are also justified by an increased intensity and large red shift of crystal luminescence spectrum corresponding to the F center emission. The SC spectrum in sapphire starts shrinking on the time scale between seconds and minutes by varying the focusing condition from tight to loose at the laser repetition rate of 200 kHz, whereas the YAG crystal produces stable performance for several hours at least.

In this work, we present efficient generation of a high-quality vector Bessel beam using an S-wave plate (radial/azimuth polarization converter) together with an ordinary glass axicon. We examine laser-induced modifications in glass with different pulse durations. We achieve material cracking and observe dominant crack propagation directions caused by the generated beam’s intensity asymmetry. By translating the beam, we demonstrate potential application of vector Bessel beams and their transverse polarization components for microprocessing of transparent materials using ultra-short pulses.

We demonstrate that polycrystalline strontium barium niobate (SBN) serves as an excellent nonlinear material for supercontinuum (SC) generation in the near- and mid-infrared, as pumped by femtosecond pulses in the regions of its normal, zero, and anomalous group velocity dispersion. We also show that broadband, octave-spanning planar second-harmonic emission generated via random quasi phase matching usefully serves to precisely monitor focusing/refocusing cycles of a filament and to determine position of the nonlinear focus in particular, which was used to estimate the nonlinear index of refraction of the material applying Marburger’s law. The measured n 2 values are remarkably large: 44 ± 7 × 10 − 16 c m 2 / W at 1.2 µ m , 81 ± 23 × 10 − 16 c m 2 / W at 2.0 µ m , and 100 ± 15 × 10 − 16 c m 2 / W at 2.4 µ m , and thus imply very low energy (below 100 nJ) and power (below 1 MW) thresholds for filamentation and SC generation in the infrared spectral range.

U.S. Air Force Office of Scientific Research under MURI [FA9550-16-1-0013]; Laserlab-Europe Project [HIJ-FSU002344]; Friedrich-Schiller-Universitat Jena; University of Arizona

We investigate the nonlinear absorption of laser energy in the bulk of transparent dielectrics for femtosecond and picosecond laser pulses focused by a conical lens. We highlight the influence of the pulse duration, laser pulse energy, and cone angle on laser energy absorption in transparent dielectrics. We provide a semi-analytical model allowing the calculation of maps for the density of nonlinear absorption of energy in BK7 and in SiO2 as a function of the pulse duration and peak fluence in the focal region. The comparison of the density of nonlinear absorption of energy with the available energy density determines optimal pulse durations and Bessel beam cone angles compatible with uniform laser energy deposition in the Bessel zone. The results reproduce quantitatively the transmission measurements for experiments in BK7 with picosecond pulses and suggest that the loss of propagation invariance and uniform laser energy deposition is responsible for a previously reported transition between different types of damage morphology in SiO2 [M. K. Bhuyan et al., Appl. Phys. Lett. 104, 021107 (2014)].

International audience; Nonlinear propagation of intense femtosecond laser pulses in bulk transparent media leads to a specific propagation regime, termed femtosecond filamentation, which in turn produces dramatic spectral broadening, or super-broadening, termed supercontinuum generation. Femtosecond supercontinuum generation in transparent solids represents a compact, efficient and alignment-insensitive technique for generation of coherent broadband radiation at various parts of the optical spectrum, which finds numerous applications in diverse fields of modern ultrafast science. During recent years, this research field has reached a high level of maturity, both in understanding of the underlying physics and in achievement of exciting practical results. In this paper we overview the state-of-the-art femto-second supercontinuum generation in various transparent solid-state media, ranging from wide-bandgap dielectrics to semiconductor materials and in various parts of the optical spectrum, from the ultraviolet to the mid-infrared spectral range. A particular emphasis is given to the most recent experimental developments: multioctave supercon-tinuum generation with pumping in the mid-infrared spectral range, spectral control, power and energy scaling of broadband radiation and the development of simple, flexible and robust pulse compression techniques, which deliver few optical cycle pulses and which could be readily implemented in a variety of modern ultrafast laser systems.

The propagation of intense ultrashort light pulses in transparent dielectric media gives rise to one of the most spectacular nonlinear optical phenomenon — supercontinuum generation (SC). The nature of this process is fairly well understood and involves intricate coupling between temporal and spatial effects leading to filamentation and an explosive growth of the pulse spectrum [1]. In recent years the rapidly growing field of applications has seen a rising demand for achieving broadband radiation with desired temporal and spectral properties. However, the spectral extent of the SC is essentially determined by the input wavelength and the linear or nonlinear properties of the medium such as material dispersion, bandgap and the nonlinear refractive index [2], all of which remain fixed in an isotropic medium for a given input wavelength. On the other hand, the use of birefringent nonlinear media, which possess both, quadratic and cubic nonlinearities, may provide the desired degree of control. The second-order cascading, which results from the phase-mismatched second harmonic (SH) generation [3] produces a cascaded-quadratic Kerr-like nonlinearity, ncasc 2 ∝ −d2 eff /Δk, which contributes to the nonlinear index of refraction. By setting an appropriate phase mismatch, Δk = k 2ω − 2k ω , the interplay between the cascaded and cubic nonlinearities may be exploited to either enhance or suppress the nonlinear effects [4, 5] and to access qualitatively different regimes of nonlinear propagation.

International audience; We experimentally investigate filamentation and supercontinuum generation in a birefringent medium (BBO crystal), in the self-focusing regime where intrinsic cubic nonlinearity is either enhanced or reduced by the second-order cascading due to phase-mismatched second harmonic generation. We demonstrate that the supercontinuum spectral extent is efficiently controlled by varying the phase mismatch parameter. In the range of negative phase mismatch, we achieve full control of the blue-shifted spectral broadening, which is very robust and independent on the input pulse energy. In the range of positive phase mismatch, both the blue-shifted and the red-shifted spectral broadenings are controlled simultaneously, however showing a certain dependence on the input pulse energy. The results are interpreted in terms of complex interplay between the self-phase-matched second harmonic generation, which is a process inherent to narrow ultrashort pulsed laser beams and concurrent self-steepening processes which arise from cubic and cascaded-quadratic nonlinearities.

We present a detailed experimental and numerical study of supercontinuum generation in sapphire crystal at the vicinity of its zero group velocity dispersion point (1.3 μm) when pumped by relatively long (210 fs) femtosecond pulses. We uncover very different evolutions of the spectral broadening versus the input pulse energy when the incident beam is focused either onto the input face or inside the 4-mm-thick sapphire sample. In particular, when the input beam was focused inside the crystal, we captured a surprising variation of the spectral width as a function of the input pulse energy, demonstrating supercontinuum generation, its suppression, and eventually, its recovery. The experimental findings were nicely reproduced by the numerical simulations, revealing a specific supercontinuum generation scenario, which relies on strong reshaping of the wave packet due to defocusing and absorption of free electron plasma, subsequent replenishment of the pulse on the propagation axis, and its splitting. We also demonstrate that this particular supercontinuum generation scenario takes place regardless of the external focusing geometry.

Acoustic signals generated by filamentation of ultrashort TW laser pulses in water are characterized experimentally. Measurements reveal a strong influence of input pulse duration on the shape and intensity of the acoustic wave. Numerical simulations of the laser pulse nonlinear propagation and the subsequent water hydrodynamics and acoustic wave generation show that the strong acoustic emission is related to the mechanism of superfilamention in water. The elongated shape of the plasma volume where energy is deposited drives the far-field profile of the acoustic signal, which takes the form of a radially directed pressure wave with a single oscillation and a very broad spectrum., 9 pages, 12 figures

We investigate the generation of high aspect ratio microstructures across 0.7 mm thick glass by means of single shot Bessel beam laser direct writing. We study the effect on the photoinscription of the cone angle, as well as of the energy and duration of the ultrashort laser pulse. The aim of the study is to optimize the parameters for the writing of a regular microstructure due to index modification along the whole sample thickness. By using a spectrally resolved single pulse transmission diagnostics at the output surface of the glass, we correlate the single shot material modification with observations of the absorption in different portions of the retrieved spectra, and with the absence or presence of spectral modulation. Numerical simulations of the evolution of the Bessel pulse intensity and of the energy deposition inside the sample help us interpret the experimental results that suggest to use picosecond pulses for an efficient and more regular energy deposition. Picosecond pulses take advantage of nonlinear plasma absorption and avoid temporal dynamics effects which can compromise the stationarity of the Bessel beam propagation.

International audience; We present a series of measurements, which characterize filamentation dynamics of intense ultrashort laser pulses in the space–time domain, as captured by means of three-dimensional imaging technique in sapphire and fused silica, in the wavelength range of 1.45–2.25 μm, accessing the regimes of weak, moderate and strong anomalous group velocity dispersion (GVD). In the regime of weak anomalous GVD (at 1.45 μm), pulse splitting into two sub-pulses producing a pair of light bullets with spectrally shifted carrier frequencies in both nonlinear media is observed. In contrast, in the regimes of moderate (at 1.8 μm) and strong (at 2.25 μm) anomalous GVD we observe notably different transient dynamics, which however lead to the formation of a single self-compressed quasistationary light bullet with an universal spatiotemporal shape comprised of an extended ring-shaped periphery and a localized intense core that carries the self-compressed pulse.

The propagation of near-infrared ultra-short laser pulses in the regime of anomalous dispersion of transparent solids is associated with a host of self-induced effects including a significant spectral broadening extending from the ultraviolet into the infrared region, pulse self-compression down to few-cycle pulse durations, free and driven third harmonic generation, conical emission and the formation of stable filaments over several \(\mathop{\mathrm{cm}}\nolimits\) showing the emergence of conical light bullets. We review measurements performed in different experimental conditions and results of numerical simulations of unidirectional propagation models showing that the interpretation of all these phenomena proceeds from the formation of non-spreading conical light bullets during filamentation.

Acoustic signals generated by filamentation of TW laser pulses in water are characterized experimentally and numerically revealing a strong influence of the input pulse duration on the shape of the acoustic signal.

International audience; Part of the chain in petawatt laser systems may involve extreme focusing conditions for which nonparaxial and vectorial effects have high impact on the propagation of radiation. We investigate the possibility of using propagation equations to simulate numerically the focal spot under these conditions. We derive a unidirectional propagation equation for the Hertz vector, describing linear and nonlinear propagation under situations where nonparaxial diffraction and vectorial effects become significant. By comparing our simulations to the results of vector diffraction integrals in the case of linear tight-focusing by a parabolic mirror, we establish a practical criterion for the critical f -number below which initializing a propagation equation with a parabolic input phase becomes inaccurate. We propose a method to find suitable input conditions for propagation equations beyond this limit. Extreme focusing conditions are shown to be modeled accurately by means of numerical simulations of the unidirectional Hertz-vector propagation equation initialized with suitable input conditions.

International audience; We investigate experimentally and numerically the nonlinear propagation of intense Bessel–Gauss vortices in transparent solids. We show that nonlinear Bessel–Gauss vortices preserve all properties of nonlinear Bessel–Gauss beams while their helicity provides an additional control parameter for single-shot precision micro structuring of transparent solids. For sufficiently large cone angle, a stable hollow tube of intense light is formed, generating a plasma channel whose radius and density are increasing with helicity and cone angle, respectively. We assess the potential of intense Bessel vortices for applications based on the generation of hollow plasma channels.

International audience; We investigate the early stage of propagation of Bessel–Gauss vortex beams where a transition regime shows a progressive lateral expansion of the main intensity ring before reaching a diffraction-free regime. The eikonal equation is used to characterize the beam structure. The beam is featured by a family of hyperboloids with variable waists, generating a tapered tubular caustic. Our analytical results are in excellent agreement with numerical and experimental results. We show the transition regime can be well eliminated by using hollow input beams.

We report on filamentation and supercontinuum generation in polycrystalline ZnSe by intense femtosecond laser pulses whose center wavelength is tunable in the 1.5–2.4 μm range. By varying the input pulse energy, single and multiple filamentation regimes were accessed, as verified by the measurements of near field intensity distributions. Along with supercontinuum generation, efficient simultaneous generation of broadband second, third, and fourth harmonics is observed. We uncover that extremely efficient harmonics generation stems from randomly quasi phase matched three wave mixing processes, which are facilitated by filamentation of the input beam, as verified by the spectral and energy conversion dynamics, and by polarization measurements.

The interaction between a large number of laser filaments brought together using weak external focusing leads to the emergence of few filamentary structures reminiscent of standard filaments, but carrying a higher intensity. The resulting plasma is measured to be one order of magnitude denser than for short-scale filaments. This new propagation regime is dubbed superfilamentation. Numerical simulations of a nonlinear envelope equation provide good agreement with experiments., Comment: 5 pages, 4 figures

International audience; Filamentation in the anomalous dispersion regime is experimentally shown to give rise to an extreme blueshifted continuum peak in the visible region even when the filament is formed by near-infrared pulses. Measurements and numerical simulations allow us to identify this peak as an axial component of the conical emission. Its features are characterized and quantitatively reproduced by the effective three-wave mixing model.

International audience; We report on generation of ultrabroadband, more than 4 octave spanning supercontinuum in thin CaF2 crystal, as pumped by intense mid-infrared laser pulses with central wavelength of 2.4 μm. The supercontinuum spectrum covers wavelength range from the ultraviolet to the mid-infrared and its short wavelength side is strongly enhanced by cascaded generation of third, fifth and seventh harmonics. Our results capture the transition from Kerr-dominated to plasma-dominated filamentation regime and uncover that in the latter the spectral superbroadening originates from dramatic plasma-induced compression of the driving pulse, which in turn induces broadening of the harmonics spectra due to cross-phase modulation effects. The experimental measurements are backed up by the numerical simulations based on a nonparaxial unidirectional propagation equation for the electric field of the pulse, which accounts for the cubic nonlinearity-induced effects, and which reproduce the experimental data in great detail.

International audience; We study the propagation of intense, high repetition rate laser pulses of picosecond duration at 1.03 µm central wavelength through air. Evidence of filamentation is obtained from measurements of the beam profile as a function of distance, from photoemission imaging and from spatially resolved sonometric recordings. Good agreement is found with numerical simulations. Simulations reveal an important self shortening of the pulse duration, suggesting that laser pulses with few optical cycles could be obtained via double filamentation. An important lowering of the voltage required to induce guided electric discharges between charged electrodes is measured at high laser pulse repetition rate.-repetition-rate picosecond pump laser based on a Yb:YAG disk amplifier for optical parametric amplification

We analyze numerically and experimentally the effect of the input pulse chirp on the nonlinear energy deposition from $5\ \mu$J fs-pulses at $800$ nm to water. Numerical results are also shown for pulses at $400$ nm, where linear losses are minimized, and for different focusing geometries. Input chirp is found to have a big impact on the deposited energy and on the plasma distribution around focus, thus providing a simple and effective mechanism to tune the electron density and energy deposition. We identify three relevant ways in which plasma features may be tuned., Comment: 9 pages, 7 figures