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The dependence of the polymerization and optical damage thresholds in multi-photon polymerization (MPP) lithography was studied using a broadly-tunable laser system with group delay dispersion (GDD) control. The order of non-linearity and the light–matter interaction mechanisms were investigated using the resolution bridges method for non-photosensitized SZ2080TM and photosensitized SZ2080TM + IRG369 prepolymers. Energy deposition, voxel dimension growth, and the size of the dynamic fabrication window (DFW) were measured in the 700–1300 nm wavelength range at three different pulse durations measured at the sample – 100, 200 and 300 fs. Polymerization was observed at all wavelengths and pulse durations without significant differences in the achieved minimal spatial dimension ( < 300 ${< } 300$ nm). This was achieved despite the broad range of excitation wavelengths used which spanned two- and three-photon absorption bands, and the differences in the absorption spectra of the prepolymers. The lateral and longitudinal voxel growth dynamics revealed an abrupt change in the power dependence of polymerization and a significant variation of the DFW – from 1 at 1250 nm to 29 at 700 nm. This result can be interpreted as a consequence of a change in the instantaneous refractive index and a lowering of the polymerization but not the damage threshold. The optimization of energy delivery to the material by a wavelength-tunable laser source with pulse duration control was experimentally validated. These findings are uncovering the complexity of polymerization mechanisms and are useful in further development of MPP technology.

Polymerized voxels by focusing ultrashort pulses varying wavelength over three-octaves wide range for femtosecond laser direct write 3D nanoprinting are studied. A possibility to use any color of spectrum from 500-nm-to-1200-nm with a controlled pulse width of 100-fs reveals a delicate interplay of photo-physical mechanisms more than just two-photon absorption inducing localized photo-polymerization. None of the applied exposure conditions ensured significantly higher spatial definition than 200-nm in width using SZ2080TM photosensitized and pure materials, yet it allowed tuning the voxel aspect ratio and greatly influenced dynamic fabrication window determined by the resolution bridges method. An effective order of absorption, i.e., the n ef -photon process of absorption, and exposure per pulse (energy, fluence, intensity) was assessed for ultra-short pulses varying wavelengths as it has the paramount influence to the established multi-photon polymerization (MPP) process. Here an evolution of the polymerised volume via different energy delivery mechanisms: one-/two-/three-photon absorption, avalanche ionization, and thermal diffusion leading to controlled photo-polymerization are revealed. The results can be used to tailor polymerized volume for increasing the 3D nano-printing performance. A non-trivial energy deposition by X-photon absorption was observed with an onset of a strong lateral size increase at the higher pulse energy at longer wavelengths and can be understood as due to reaching epsilon-near-zero conditions. Findings are valuable for developing MPP technology to reduce the footprint size and increase its efficiency. Understanding mechanisms and appearance of λ -tunable commercial lasers are benefiting broad applications in advanced optical additive manufacturing areas of micro-optics, nano-photonic devices, meta-materials, and integrated-chips, and tissue engineering.

Polymerized voxels by focusing ultrashort pulses varying wavelength over three-octaves wide range for femtosecond laser direct write 3D nanoprinting are studied. A possibility to use any color of spectrum from 500-nm-to-1200-nm with a controlled pulse width of 100-fs reveals delicate interplay of photo-physical mechanisms more than just two-photon absorption inducing localized photo-polymerization. None of the applied exposure conditions ensured significantly higher spatial definition than 200-nm in width using SZ2080TM photosensitized and pure materials, yet it allowed tuning the voxel aspect ratio and greatly influenced dynamic fabrication window determined by the resolution bridges method. An effective order of absorption, i.e., the nef-photon process of absorption, and exposure per pulse (energy, fluence, intensity) was assessed for ultra-short pulses varying wavelengths as it has the paramount influence to the established multi-photon polymerization (MPP) process. Here an evolution of the polymerised volume via different energy delivery mechanisms: one-/two-/three-photon absorption, avalanche ionization, thermal diffusion leading to controlled photo-polymerization are revealed. The results can be used to tailor polymerized volume for increasing the 3D nano-printing performance. A non-trivial energy deposition by X-photon absorption observed with an onset of a strong lateral size increase at the higher pulse energy at longer wavelengths and can be understood as due to reaching epsilon-near-zero conditions. Findings are valuable for the developing MPP technology to reduce the footprint size and increase its efficiency. Understanding mechanisms and appearance of λ-tunable commercial lasers are benefiting broad applications in advanced optical additive manufacturing areas of micro-optics, nano-photonic devices, meta-materials, and integrated-chips, and tissue engineering.

High repetition rate femtosecond filaments in transparent solids produce conical third harmonic generation due to filament-induced material reorganization in the form of periodic volume nanogratings. Here we report on conical third harmonic generation that accompanies supercontinuum generation in fused silica using broadly tunable femtosecond pulses. The measurement of third harmonic cone angles with driving wavelengths in the 1–3- μ m range fully supports the noncollinear phase-matching scenario that involves a reciprocal lattice vector of the filament-inscribed nanograting. The nanograting provides an octave-spanning phase-matching bandwidth, as attested by the measurements of the angle-resolved spectra of broadband conical third harmonic emission.

Here, we present thorough research into high-order vector Bessel-Gauss beams from a theoretical perspective, with experimental realization and application to transparent-material laser micromachining. Efficient, stable, high-quality, and power-vector Bessel-Gauss beam generation using two geometric-phase-based optical elements, a high-order $S$-wave plate and an axicon in combination with simple optical elements, such as lenses, wave plates, and polarizers, is demonstrated. Ultrashort pulse-vector Bessel-Gauss beams are used to induce modifications in the glass, creating three-dimensional tubular structures of type I and type II modification in D263T glass. Furthermore, samples containing tubular modifications are tested for etch selectivity in the femtosecond-laser-induced chemical-etching method with KOH solution by etching throughout channels.

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.

Direct laser ablation is one of the most widespread laser microtechnology used in the industry. Many different types of lasers at a broad range of processing parameters have been tested on various surfaces with the primary goal to achieve highly predictable and regularly patterned geometry. Such a task remains a big challenge for the large band-gap materials, where the best results can be expected in a linear laser absorption regime. Therefore excimer UV lasers and high harmonics of nanosecond solid-state laser are successfully applied for surface patterning tasks [1] , [2] . However, the recent advancement of industrial-grade femtosecond laser systems with high harmonics modules expand the range of usable laser sources for this application. It is well known that shorter pulse processing, due to characteristic pulse timescales smaller than the processing material’s atomic vibrations timescale, has reduced heat-affected zones (HAZ), which causes better ablation precision compared to the longer pulses. This leads to a lower surface roughness of an ablated region, and, most importantly, as the absorption remains linear, the roughness and ablation depth can be directly controlled. The choice of patterning algorithm is also important. i.e., Tanvir et al. introduced a model to fine-tune processing parameters to reproduce a particular surface microstructure [3] , while Žemaitis et al. produced an ablation model which takes into account the decrease in ablation threshold and saturation of the ablation depth with the increasing number of pulses per spot [4] . These models help increase the processing area, however, they still struggle to correctly predict ablation depth and surface roughnesses required for the fabrication of diffractive optical elements, microfluidic chips, hydrophobic surfaces, and other devices

During the last decade the zeroth order Bessel-Gauss laser beam has found many uses in the transparent material processing. The high aspect ratio channels can be created that slice through various thin transparent materials and increase the efficiency of cutting. However, the generation of high-quality Bessel-Gauss beam remains a challenge due to imperfection of glass axicon manufacturing, i.e. rounded tip, not smooth surface etc. These imperfections generate intensity modulation along propagation axis or even modify transversal central core intensity distribution, that results in worsening of micro-machining quality. The diffractive optical element (DOE) is a great alternative that do not suffer from previously mentioned problems. In this study we show the possibility of generating high quality Bessel-type beams with geometric phase optical elements (GPOEs) (manufactured by Workshop of Photonics). These elements act as precise flat DOEs that have very high diffraction efficiency (>90%), high optical damage threshold and can be freely customized for specific needs. Therefore, with the use of high-power laser they can be applied to process transparent materials. In this work, controllable phase shifts are implemented in axicon phase masks to create unique and fanciful Bessel-type beams as well as asymmetric core beams for thin glass modification/cutting application. Using numerical simulations and experimental data we compare performance of GPOEs and demonstrate thin glass processing using powerful laser with reshaped intensity distribution by GPOE.

A well-known and already having many material processing applications zero-order Bessel beam makes a great base for improvements to have even broader applicability. In our work, we analyze vector Bessel beams (VBB), which can be generated with high efficiency and quality via the use of Geometric Phase Optical Elements. The beam transverse polarization distribution enables to change intensity distribution easily, i.e. a polarizer in front of the beam will generate invariant over the propagation multi-peak ring- the shaped structure which could be very beneficial to modify material at multiple sights with a single laser shot. We analyze higher-order VBB generated modifications in thin glass and analyze the applicability for etching of large diameter holes.

We report on conical third harmonic generation by filamentation of high repetition rate femtosecond laser pulses in transparent dielectrics which results from in-bulk optical damage of the material in the form of quasiperiodic nanograting.

We report on observations of conical third harmonic emission that emerges during supercontinuum generation produced by self-focusing and filamentation of high (20-200 kHz) repetition rate 180 fs, 1035 nm pulses from an amplified Yb:KGW laser in various nonlinear crystals and glasses: YAG, sapphire, YLF, LiF, CaF2, MgF2, LiSAF, fused silica and BK-7 glass. We show that conical third harmonic generation is a phase-matched four-wave mixing process, where noncollinear phase matching is achieved by means of reciprocal lattice vector, inversely proportional to the period of nanograting, which is inscribed by femtosecond filament in the volume of nonlinear material. The existence of a particular period required to phase match conical third harmonic generation was indirectly verified by investigations of periodicity features of high and low spatial frequency laser-induced periodic surface structures, in which matter is reorganized in a similar fashion.

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.

Biocompatible polymers are used for many different purposes (catheters, artificial heart components, dentistry products, etc.). An important field for biocompatible polymers is the production of vision implants known as intraocular lenses or custom-shape contact lenses. Typically, curved surfaces are manufactured by mechanical means such as milling, turning or lathe cutting. The 2.5 D objects/surfaces can also be manufactured by means of laser micromachining, however, due to the nature of light&ndash, matter interaction, it is difficult to produce a surface finish with surface roughness values lower than ~1 &micro, m Ra. Therefore, laser micromachining alone can&rsquo, t produce the final parts with optical-grade quality. Laser machined surfaces may be polished via mechanical methods, however, the process may take up to several days, which makes the production of implants economically challenging. The aim of this study is the investigation of the polishing capabilities of rough (~1 &micro, m Ra) hydrophilic acrylic surfaces using bursts of femtosecond laser pulses. By changing different laser parameters, it was possible to find a regime where the surface roughness can be minimized to 18 nm Ra, while the polishing of the entire part takes a matter of seconds. The produced surface demonstrates a transparent appearance and the process shows great promise towards commercial fabrication of low surface roughness custom-shape optics.

Nondiffracting beams are known for their long line of focus, which has various applications in laser materials processing. Zeroth order Bessel beam is usually generated with an axicon and has a distinct circular spatial spectra. The nature of higher order Bessel beams, elliptical and parabolic nondiffracting beams is also conical and their spatial spectra have their own azimuthal modulation. We study numerically and verify experimentally generation of vortical Bessel beams, their superpositions along with elliptical and parabolical beams using an axicon. Laser induced modifications in glasses for various durations and beam powers using generated pulsed beams are analyzed.

Beam profile engineering, where a desired optical intensity distribution is generated by phase shifting and/or amplitude changing elements, is a promising approach in various laser-related applications. For example, vector geometrical phase elements enable various flat special optical elements such as top-hat converters. We present a study on engineering efficient top-hat converters inscribed in the glass by femtosecond laser pulses. We start with an amplitude encoded top hat converter and demonstrate how its efficiency can be further increased by introduction of phase masks and by the polarization of the incident beam. Experimental verification of the concept is also presented.

Zeroth order Bessel beams are widely used in laser micromachining of transparent materials. The small diameter of central core and elongated focus enables to generate high aspect ratio voids. The simplest way to generate this beam is to induce a conical shape phase with an axicon. However, the quality of the axicon tip is very crucial to generate smooth Bessel beams since it is known that a blunt axicon tip induces large intensity modulation in propagation direction. Alternative Bessel beam generation method is to use a Diffractive Optical Elements (DOEs) that do not suffer from previously mentioned problem. In this work we demonstrate generation of a zeroth order Bessel beam with Geometric Phase Optical Elements (GPOEs) (manufactured by Workshop of Photonics) acting as a diffractive beam shaping element. Having absolute control of induced beam phase, we have modified mask phase so that half of it had additional phase shift or spatial transposition resulting in creation of fanciful induced beam phase patterns. With the use of laser beam propagation numerical modeling we show that these new phase masks can create various beam transverse intensity patterns such as asymmetrical central core, generation of multiple peaks or even large rings that are highly demanded for various laser micromachining applications. We have chosen couple of most perspective beam shapes and manufactured GPOEs to generate them. The experimentally generated beams were compared to numerical simulations. As the GPOEs are able to work with high power pulses we have also investigated induced transparent material modifications.

Invariant Bessel–Gauss beams have attracted great interest for transparent material microprocessing applications. The small central peak and elongated focal region of the zeroth-order Bessel–Gauss beam provide advantages in laser micromachining. However, many transparent material processing applications need asymmetric beams to induce directional cracking. Therefore, in this work, we investigate analytically, numerically, and experimentally a beam that was generated by spatially displaced axicon halves, which induces noncylindrical symmetry in the beam phase profile. Analytical solutions were found and verified experimentally. High power and quality beams were generated by writing a needed phase alteration of the beam with a geometric phase element. These beams were able to demonstrate applicability to micromachine processing of transparent material.

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 report on supercontinuum generation in silicon carbide (6H-SiC), pumped by tunable femtosecond pulses in the wavelength range of 1.3 - 2.4 μm, which cover the regions of normal, zero, and anomalous group velocity dispersion of the material. More than an octave spanning infrared supercontinuum spectra were measured, demonstrating almost constant blue cut-offs around 0.9 μm. Low energy thresholds for supercontinuum generation (depending on the pump wavelengths, but generally below 200 nJ) were demonstrated due to large nonlinear refractive index of the material (n2 = 97 ± 19 × 10−16 cm2/W), which was experimentally evaluated by measurement of the nonlinear transmission at 2.0 μm.

Development of pulsed lasers for space-based science missions entail many additional challenges compared to terrestrial experiments. For systems requiring short pulses ≪1 ns with energies >100 mJ and fast repetition rates >10 kHz there are currently few if no laser architectures capable of operating with high electrical efficiency >20% and have good system stability. The emergence of a mulit-channel fiber-based Coherent-Amplifying-Network or CAN laser potentially enables such capability for space based missions. Here in this article we present an analysis of two such missions scaling up in pulse energy from ≈100 mJ for a supercontinuum LIDAR application utilising atmospheric filamentation to the higher energy demands needed for space debris remediation requiring ≈10 J pulses. This scalability of the CAN laser provides pathways for development of the core science and technology where many new novel space applications can be made possible.

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.