This PDF file contains the front matter associated with SPIE Proceedings Volume 10908, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

The industrial use of ultrashort laser pulses has made considerable progress in recent years. The reasons for this lie in the availability of high average powers at pulse repetition rates in the several 100 kHz range. The advantages of using ultrashort laser pulses in terms of processing precision can thus be fully exploited. However, high laser intensities on the workpiece can also lead to the generation of unwanted X-rays. Even if the emitted X-ray dose per pulse is low, the accumulated X-ray dose can become significant for high-repetition-rate laser systems so that X-ray exposure safety limits must be considered. The X-ray emission during ultrashort pulse laser processing was investigated for a pulse duration of 925 fs at 1030 nm wavelength and 400 kHz repetition rate. Industrially relevant materials such as steel, aluminum and glass were treated. Tungsten served as reference. X-ray spectra were recorded, and X-ray dose measurements were performed for laser treatment in air. For laser intensities > 2 × 10¹³ W/cm² , X-ray doses exceeding the regulatory exposure limits for members of the public were found. Suitable X-ray protection strategies are proposed.

We theoretically study the interaction of three dimensional topological Weyl semimetals with an ultrafast circularly polarized optical pulse. We use the lower-energy approximation of the full Hamiltonian of the system in the reciprocal space near the Weyl points. We present the results for TaAs, which has two pairs of Weyl points. The ultrafast pulse causes a finite electron conduction band population both during and after the pulse. We show that the electron dynamics for such materials is coherent and highly irreversible, i.e., the residual conduction band population is comparable to the maximum conduction band population during the pulse. For a pulse propagating in the z direction, the large population of electrons is located near the Weyl points and along the separatrix which is defined as a set of the initial points for which electron trajectories in the reciprocal space pass precisely through the (k_x, k_y) = (0, 0) point for different values of k_z. For small k_z, the system behaves similar to graphene and, the interband dipole matrix elements are highly localized near (k_x, k_y) = (0, 0) point and the conduction band population has sharp maximum along the separatrix. However, for large k_z, the system behaves as a gapped graphene with delocalized interband dipole matrix and the transfer of electrons between the valence band and the conduction bands are not confined within a narrow region. We also show that the optical pulse causes electrical current and net charge transfer through the system during the pulse.

We experimentally demonstrated formation of electrically conductive structures by modifying native polydimethylsiloxane (PDMS) with femtosecond laser. By irradiating femtosecond laser pulses to native PDMS, black structures with electrical conductivity were formed. Analyses using X-ray diffraction (XRD) show that the formed structures were composed of β-silicon carbide (β-SiC). Our technique enables the spatially selective formation of β-SiC on the surface of PDMS, leading to open a novel route to develop a simple method to fabricate flexible or stretchable MEMS devices with SiC microstructures.

Ultrafast laser micromachining has been extensively researched for its “clean, cold” cutting potential in fields from microelectronics to dentistry. It is clear that the mechanism of laser ablation with pulses shorter than about 500 fs differs significantly different from the light-to-heat dominated processes with longer pulsed (ns, ps) and CW laser machining. However, the details of the femtosecond laser ablation mechanism remain incompletely understood. The ablation threshold (J/cm^2) is widely used for characterizing laser machining efficiency. Unfortunately, it is not entirely clear what the ablation threshold means in the ultrashort pulse regime. For example, our diameter regression measurements of the ablation thresholds of several materials using 800 nm, 120 fs laser pulses reveal multiple distinct ablation regimes, each characterized by a different effective beam waist. Evidence of similar behavior can be found in the literature, however it is often unremarked upon. In this paper, we present thorough characterization of the ultrafast laser ablation for a diverse collection of materials (undoped silicon, sapphire, stainless steel and cortical bone). For example, for undoped silicon we find three ablation regimes each characterized by a different ablation threshold and apparent beam waist: (1) 1.56 J/cm^2, 11.8 µm; (2) 1.21 J/cm^2, 51.9 µm; and (3) 0.85 J/cm^2, 159.9 µm. We show the presence of up to three different ablation regimes that vary depending on the type of material. Using computational modeling, we address the mechanistic underpinnings of these observations, particularly the dependence upon pulse energy and spatial beam shape.

It is generally admitted that femtosecond laser pulses applied to fused silica in a non-ablative regime at repetition rates excluding cumulative effects leads to structural modifications of mainly two types: the formation of nanogratings and a reduction of volume leading to a localized densification. Here, we report recent observations that show that the taxonomy of material modifications induced by the laser is significantly richer than previously known. In particular, we observe evidences of complex polymorphic phases, including possible Si nano-cristallites, eventually embedded in an amorphous shell. In addition, exploring the cumulative regime further, we have observed that nanogratings can also be found in this regime characterized by a melting of the glass phase. This surprising results point out towards complex self-organization phenomena taking place also in the cumulative regime. Both observations, a richer taxonomy of phase transformations in the non-cumulative regime and the occurrence of nanogratings in the cumulative regime, beyond challenging our understanding of laser-induced phase transformation, have interesting applications potential that will be outlined in this presentation.

The annual revenue of dental implants is estimated on 33 billion USD in 2019 and the efforts to keep the teeth functionality and aesthetics is continuously growing over the years. However, loosening of dental implants induced by infection is still a critical and common problem worldwide. In this scenario, the development of new implant manufacturing strategies is of utmost importance. Every surface exposed in the oral cavity, both the tooth and the implant surface, are covered by a layer of salivary proteins, the so-called pellicle. The initial formation of a pellicle is followed by the attachment of bacterial cells onto it. Well-developed biofilms on dental implant surfaces become the main source of pathogenic microbes causing Peri-Implantitis, which is one of the main causes of dental implant failure. The surface topography and chemical composition of an implant are key factors in controlling surface wettability, which directly affects the formation of the biological films. In this sense, ultrafast laser surface nanotexturing rises as a promising advanced technology for controlling implant surface biological properties. Laser-processing parameters such as laser wavelength λ, fluence F and number of pulses N are essential for surface texturing. Thus, this paper presents promising results on the influence of different laser induced periodic surface structures (LIPSS) on the composition of the pellicle and the biofilm formation on biomedical grade 5 Ti-6Al-4V dental abutments. Moreover, a biofilm reactor was built and adapted to assess the effect of the LIPSS on the biofilm formation.

Low-NA focusing systems can offer longer working distances than the high-NA ones, thereby enabling fabrication of 3D structures of great heights/thicknesses. However, degradation of the longitudinal resolution occurs at low NAs as a consequence of diffraction of the light waves. Here, we report on high-resolution laser printing of three dimensional (3D) structures of heights up to a few centimeters with a sub-10 μm longitudinal resolution by incorporating a simultaneous spatiotemporal focusing (SSTF) scheme into the femtosecond laser direct writing. Remarkably, the SSTF can ensure generation of symmetric focal spots (i.e., spherical focal spots) of comparable sizes along both the horizontal and vertical directions. The resolutions are tunable in real time by dynamically varying the power of the writing laser beam. The ability to simultaneous achieving the large heights and high longitudinal resolutions in femtosecond laser 3D micromachining is of great use for applications ranging from microfluidics to infrared and THz photonics.

Microfluidic lasers are very attractive sources for integrated lab-on-chip systems, medicine, spectroscopy, and many more. Although there have been already variety of impressive results on microfluidic dye lasers, the complexity of fabrication of such systems is still not fully tackled. We propose a new concept for microfluidic channel fabrication that allows to fabricate any shape of micro-channel with a single bulk material and laser processing. We use a femtosecond laser machining for locally modifying the material, followed by wet chemical etching with a bath of 2.5 % HF solution. The final step is the closure of the channel that is done with laser reflow by scanning a CO2 laser over the elongated channel in which the dye is passing through. As a result, we can fabricate channels with various diameters and cross-section shapes ranging from ~10 µm to hundreds of microns and this, over any arbitrary length. Based on this, we setup a dye laser experimental setup. We use Rhodamine 6G as a dye with dimethyl sulfoxide (DMSO) as solvent, with 0.15 g/l of concentration. Since DMSO has slightly higher refractive index than fused silica (1.478 at 570 nm), the microfluidic channel will be able to guide light. For forming a cavity we use gold coated fused silica pieces with cylindrical shape. For pumping, a commercially available special green laser (Necsel) at 532 nm is used, with a maximum power of 2.5 W. With a help of a collimating aspherical and focusing cylindrical lenses we match the pump beam size to the micro-channel size at the focus. Here, we report the working principle of this optofluidic laser, its preliminary performances as well as the various fabrication steps and in particular, the laser-based closing mechanism for the fluidic cavity. We will also introduce a laser cavity in which the dye is constantly refreshed.

By internal modification of transparent glasses and crystals with ultrafast laser radiation and subsequent wet-chemical etching (here named SLE = selective laser-induced etching), very precise 3D structures have been produced, especially in quartz glass (fused silica), for more than a decade. By the combination of a high precision three-axis system to move the glass sample and a fast 3D beam steering system to move the laser focus, the SLE process is now suitable to produce more complex structures in a shorter time [1]. We have programmed a printer driver for commercial CAD software and the flexible machine software enabling automated production of complex 3D glass parts with the LightFab 3D Printer. Some examples of 3D precision glass parts e.g. for lab-on-a-chip applications (cell-sorting microfluidics), electronics (glass via and connectors), semiconductor (quartz chucks), optics and precision mechanics are presented. The SLE process is very scalable for high throughput since a faster writing speed results in higher selectivity and thus larger precision of the resulting structures. Thus SLE is a process which is suitable for mass production of 3D structures in glasses. Some examples of rapidly produced structures using our high speed beam deflection modules are demonstrated, which are the basis of our special machines enabling mass-production. Since the LightFab 3D Printer includes an ultrafast laser with programmable pulse duration and variable repetition rates also other in-volume processes than SLE like 2-photon-polymerization, direct writing of waveguides in glasses, internal glass welding can be done in the same machine. As example of process chains the combination of SLE with glass welding by ultrafast laser radiation will be shown. [1] J. Gottmann, M. Hermans, N. Repiev, J. Ortmann (2017) Selective Laser-Induced Etching of 3D Precision Quartz Glass Components for Microfluidic Applications—Up-Scaling of Complexity and Speed, Micromachines 8, pp. 110-120

Several approaches exist to induce the internal modifications in fused silica by femtosecond laser irradiation depending on the dose: direct writing of refractive index change (type I modification), birefringence control by nanogratings for geometric phase elements and polarisation sensitive imaging (type II modification) and new phenomena arising from double pulse utilisation. In this presentation, we focus on two recent topics: enhancement of nanograting formation using a double pulse processing and fabrication of high-efficient diffractive optical elements (DOE) by I type modification in fused silica. Most of the studies show that the orientation of the LIPSS is perpendicular to the first pulse polarisation. However, the intra-volume modifications with the induced nanogratings have the depth dimension where the double-pulse fabrication can provide more sophisticated morphology depending on the temporal delay and energy relation between two pulses. The nanogratings induced using the double-pulse irradiation with perpendicular polarisations demonstrates the grid-like structure at ~ 10 ps temporal delay, while the 45 degrees tilted gratings appear without delay between pulses. Variation of the nanograting period was observed in the case of parallel polarisation. Those new phenomena can be widely used for writing the two-dimensional diffraction gratings or the information coding applications and requires more deep investigations. Most of the effects are observed at focusing with high numerical aperture objectives, which working range is limited by spherical aberration below 1 mm depth. However, that makes possible to restrict the radial size of modification below the diffraction limit and extend the longitudinal length of modification up to 50 µm during the single scan. Therefore such approach is capable of recording more compact volume diffractive optical elements with the total diffraction efficiency > 90%. By varying the refractive index within ~1x10-4 to 1x10-3, it is possible to get up to π/24 phase retardance resolution attractive to design the phase change optical elements for the low-loss Top-hat beam shaping and multi-beam splitters.

Photonic time-stretch has established world’s fastest real-time spectrometers and cameras with applications in biological cell screening, tomography, microfluidics, velocimetry and vibrometry. In time-stretch imaging, the target’s spatial information is encoded in the spectrum of the broadband laser pulses, which is stretched in time and then detected by a single-pixel detector and digitized by a real-time ADC, and processed by a CPU or a dedicated FPGA or GPU. In time-of-flight LiDAR measurement, the maximum detectable distance scales with the temporal duration of the chirped illumination source. The bearing angle is proportional to the bandwidth of the source. In order to have a large detection angle and depth, a large chirp-bandwidth product is required. Various methods have been proposed to generate a chirped output to realize time-stretch, including single mode fibers, dispersion compensating fibers, chirped Bragg grating, and chromo-modal dispersion (CMD). But none of those methods provide the chirped source with a large time-bandwidth product. Moreover, the chirp profile and the operating wavelength can be changed with minimum freedom in those methods. In this study, we demonstrate the discrete time-stretch method that can generate the giant time-bandwidth product with arbitrary nonlinear chirp at operating wavelength from the visible to the infrared. A chirped pulse train with chirp time-bandwidth product at the order of 106 is easily feasible, rendering time-of-flight imaging of long-ranging distance and large bearing angle possible. We show its application in spectral-temporal LIDAR with the foveated vision at MHz refresh rate.

Low-intensity light pulses with energies of ~10fJ and pulse lengths of ~10ps will likely play key roles in next-generation optical telecommunications. While techniques exist that can measure weak pulses, some require a time-synchronized reference pulse, and another requires a high-temperature, aperiodically poled LiNiO₃ waveguide, which can be difficult to work with and is not readily available. Others involve difficult alignment procedures, complex apparatuses, and expensive electronics (>$100,000), and still others measure only the coherent artifact. Thus, it has not been possible to reliably and practically measure the complete intensity and phase of ultraweak ps pulses. To solve this problem, we are developing a self-referenced technique based on the simple method, GRENOUILLE. As in standard GRENOUILLE devices, we also use the natural phase-matching angular dispersion of a thick second-harmonic generation (SHG) crystal to spectrally resolve the SH light. To improve the sensitivity, however, we exchange the crossed-beam line-focus geometry and single-shot functionality, for a point-focus collinear-beam geometry and scan the delay, while using and even thicker SHG crystal. This generates an “interferometric FROG” trace from which a conventional SHG FROG trace can be extracted. It is considerably more sensitive than current GRENOUILEs due to the point focus (which yields much higher SHG efficiency than the usual line focus) and FROGs due to its thick crystal.

The fundamental limits set by diffraction in optics have motivated the investigation of ‘diffraction-free’ beams. Examples of monochromatic diffraction-free beams include those whose profiles conform to Bessel, Mathieu, Weber, and Airy functions. For pulsed beams (wave packets or optical bullets), different functional forms of the field in space and time have been found to be propagation-invariant. Here we describe experiments on the synthesis and characterization of a unique class of pulsed optical beams called ‘space-time’ (ST) wave packets that are diffraction-free and dispersion-free in free space. The most salient features of such wave packets that determines their characteristics is the tight spatio-temporal spectral correlations underlying their construction. We identify 10 unique classes of ST wave packets according to the magnitude and sign of the group velocity, and whether the ST wave packet is ‘baseband’ or ‘sideband’; that is, whether low spatial frequencies are allowed in the wave-packet construction or are forbidden. Such wave packets can be propagation invariant even for extended distances. Furthermore, sculpting the spatio-temporal spectrum of the wave packet allows control over its propagation characteristics in optical materials, namely over the group velocity and group velocity dispersion, whether the material itself is dispersive or non-dispersive.

A picosecond laser emitting alternatively two wavelengths separated by 1 nm around 780 nm is demonstrated. This source is designed for advanced Raman spectroscopic measurements for which two spectra at two different pump wavelengths are acquired. The difference between the two measurements permits to discriminate the Raman contribution from the noise (SERDS). For such purpose, we developed a mode-lock fiber laser delivering a broad optical spectrum around 1560 nm which provides, through filtering with Fiber Bragg Gratings, the two required wavelengths operation. Second Harmonic Generation at 780 nm is then performed with a fibered coupled bulk PPLN.

We report on the selective modal control of tailored femtosecond-written long period fiber gratings (LPFG). It is shown that the excitation of higher order cladding modes is possible with strong selectivity and high precision. The coupling behavior of several gratings dependent on the modified core cross section is determined theoretically and confirmed by experiments. Additional mode field measurements proof our concept. The presented tool could pave the way for a completely new branch of fiber integrated devices such as highly efficient transmission gratings or mode converters.

Multichannel filter elements based on fiber Bragg gratings in multicore fibers (MCF) feature excellent potential for applications in astrophotonics. However, due to slight core-to-core variations of the refractive index (RI), the resonance wavelength of the inscribed FBGs differs. For an optimized functionality, the filter properties, especially the resonance wavelengths, should be equal across all cores. Therefore, we present a post-treatment procedure by ultrashort pulsed laser irradiation in order to tune the resonance wavelength of an individual FBG. To shift the resonance wavelength, a constant RI change in addition to the RI modulation is induced. Precisely tuned wavelength shifts up to 255 pm are demonstrated. Finally, we demonstrate precise tuning of the FBG resonance wavelength in a single core inside an MCF.

Not only are waveguides fundamental as a light carrier, yet they are also key elements for countless optical components such as couplers, modulators and oscillators to name a few. Modulating waveguides is usually performed using electro-optics or acousto-optics principles involving, among others, specific crystals such as Lithium-Niobate or glass thermal poling to introduce second order non-linearity. In this work, we investigate a waveguide phase-modulation based on optomechanics and in particular photoelasticity. Specifically, a fused silica suspended 3D waveguide suitable for a broad visible and near-infrared spectrum and able to carry a large single mode is implemented in the form of a double-clamped suspended beam. This optomechanical device oscillates up to kHz frequencies thanks to the use of dielectrophoresis excitation, resulting from a varying non-linear electric field. The suspended waveguide seats in a V-shape groove providing the electrostatic field. The full device is manufactured out of a single piece of silica through femtosecond laser exposure combined with chemical etching. In addition, a CO2-laser polishing step is added to achieve high surface quality and prevent scattering losses. The dynamic response of this optomechanical device can be further tuned - using the same femtosecond laser - to shift from a non-linear hardening frequency response to a linear one or to a softening mode.

Direct laser writing in glasses using femtosecond lasers has been extensively studied during the last two decades. It provides a robust and efficient way to directly inscribe 3D photonic structures in the interior of bulk glasses. More specifically, the inscription of 3D embedded waveguides has been well established. However, direct laser writing of waveguides neighboring the glass surface was only demonstrated in few works. This is owed to ablation that arises when approaching the glass surface. This shortcoming was recently overcome using additional processing and/or reinforced glasses. In silver containing glasses, direct laser writing induces the creation of silver clusters around the interaction voxel inducing a Type A refractive index change. It allows the inscription of optical waveguides using very low pulse energy, differing from standard type I waveguides which are inscribed with higher energy pulses. The low energy regime allows for the inscription of waveguides in close proximity to the surface while inhibiting surface ablation. In this work, we demonstrate direct laser writing of near-surface waveguides in a non-reinforced bulk glass sample without resorting to any additional processes, as well as a demonstration of a high-sensitivity refractive index sensor based on such near-surface waveguides. This work highlights the novelty and benefits of type A waveguides inscribed in silver containing glasses and their high potential for sensing applications compatible with laser manufacturing approaches. Finally, our results could be easily transposed to silver containing ribbon shaped glass fibers, thus paving the way towards fiber based sensing applications.

Lately, there has been a strong incentive for the development of photonics devices which operate farther in the infrared wavelength range. In this communication, we present a study of the photosensitivity to femtosecond pulses of Mid- Infrared transmitting materials, which are promising candidates to be used as substrates for photo-induced devices, namely, Germanium-Sulfide glass (Ge-S), Barium Gallo-Germanate glass (BGG), and sapphire. We report the formation of single mode waveguides operating at a wavelength of 2.85 μm, in all three materials. In addition, the inscription of a low-loss depressed clad waveguide in sapphire is demonstrated.

Novel integrated photonics components are highly demanded to develop photonic integrated circuits, or sensing applications with lab-on-chip or lab-on-fiber approaches. Femtosecond laser inscription has demonstrated to be a very attractive approach being highly versatile as well as compatible with laser manufacturing technological transfer. In this framework, some of the innovations to come will result from optimized laser-based processing in prepared materials, namely in specialty glasses with a designed photosensitivity enhancement under laser irradiation. In this framework, we recently demonstrated a new type of waveguides. Here, the positive refractive index modification, labelled type A, is sustained by the laser-induced photochemistry of silver-containing phosphate glasses, with a non-trivial distribution that corresponds to the creation of fluorescent silver clusters. Some of the inner features of the refractive index structures and associated waveguides show mesoscale dimensions of a few hundreds of nanometers, which is of interest for produce sub-wavelength modifications. In this paper, we will present recent results concerning the exploitation of such new waveguides, namely with the investigation of curvature losses and the creation of directional couplers. Additionally, periodically modulated waveguides are considered to address laser-fabricated waveguide Bragg grating functionality. Such development is in progress, especially targeting reflectivity in the visible range.

Intense near-fields of surface plasmon polaritons (SPPs) excited with femtosecond (fs) laser pulses can sculpt nanometer-size structures on various kinds of solid materials through ablation. To control the formation, we need to understand the characteristic properties of the SPPs such as plasmon wavelength, damping, and spatial modes. Recently we succeeded to measure surface plasmon resonance curves of Si gratings with the intense p-polarized 100-fs laser pulses and observe the nanoablation on the surface. The experimental results and calculation for model targets have shown that SPPs with low internal damping can be excited with the fs pulse at higher fluence. This indicates that the propagation length of SPPs on Si can be controlled by the laser fluence of fs laser pulses.

Ultrafast laser micromachining that utilises pulses on a femtosecond timescale is a rapidly growing area of research with applications in a wide variety of fields, from microelectronics to microsurgery. Femtosecond pulses are often praised for their ability to perform precise cutting of materials through a ‘cold-cutting’ mechanism which avoids mechanical and thermal collateral damage to the surrounding area. However, the high precision and clean ablation features associated with ultrafast laser micromachining can be counteracted through the intense plasma in air that is generated at high pulse energies. The highly reflective plasma generated above the sample surface can result in a distorted beam profile at the target machining plane, producing machined features with reduced edge quality and accuracy. In addition, the highly reflective plasma results in underutilised portions of the incident pulse energy, therefore decreasing machining efficiency. We present the ablation threshold data and trends for a variety of materials including undoped silicon, stainless steel and sapphire laser machined under vacuum and other ambient conditions. Ablation thresholds were determined using the diameter regression technique with 130 fs, 800 nm laser pulses at a repetition rate of 500 Hz. Ablation features are analysed extensively to observe the impact of the ambient conditions on the resulting feature quality.

Ultrashort pulse laser induced processes in the nanotechnology at interfaces are presented. A special focus is placed on femtosecond far field investigations of defect generation in solids [1-4] and on electrochemical in-situ techniques in graphene nanosheet synthesis [5]. Further, deterministic nanostructuring of solids and hot electron electrochemistry is discussed [6]. Apertureless scanning near-field nanolithography with a femtosecond Yb-doped fiber laser oscillator allows non-thermal electromagnetic energy transfer [7]. [1] W. Kautek and O. Armbruster, Springer Series in Materials Science 191 (2014) 43-66. [2] O. Armbruster, A. Naghilou, M. Kitzler, W. Kautek, J. Phys. Chem. C 119 (2015) 22992−22998. [3] O. Armbruster, A. Naghilou, M. Kitzler, W. Kautek, Appl. Surf. Sci. 396 (2017) 1736–1740. [4] A. Naghilou, O. Armbruster, W. Kautek, Appl. Surf. Sci. 418 (2017) 487-490. [5] M. Pfaffeneder-Kmen, I. Falcon Casas, A. Naghilou, G. Trettenhahn, W. Kautek, Electrochim. Acta 255 (2017) 160-167. [6] O. Armbruster, H. Pöhl, W. Kautek, (2018), in publication. [7] I. Falcón Casas, W. Kautek, Nanomaterials 8 (2018) 536

In simultaneous spatial and temporal focusing (SSTF) a wide bandwidth pulse with transverse spatial chirp is focused, resulting in a pulse that is temporally compressed only near the focal plane. The pulse also has a pulse front tilt angle that depends on the amount of initial transverse chirp. Using an improved design of an asymmetric pulse compressor, we can easily vary the amount of output spatial chirp and thus the pulse front tilt at the focus. We direct this beam into a vacuum chamber and focus it onto an argon gas jet to achieve high harmonic generation (HHG). Since the harmonics are created with a tilted pulse, they emerge from the focus with an amount of angular chirp based on the input spatial chirp and harmonic number. We angularly disperse the harmonics in the direction perpendicular to the spatial chirp with a curved reflective grating, which focuses in the spectral direction onto an x-ray CCD camera. We observe that each of the harmonics possesses angular spatial chirp. To our knowledge, this is the first experimental verification of our earlier published theory of spatially chirped high harmonics. These harmonics are in a sense the Fourier complement to harmonics produced with the Lighthouse Effect. In that case, the attosecond pulse train is angularly dispersed while here each harmonic has angular spectral dispersion. This technique could be used for hyperspectral XUV spectroscopy and, when the beam is refocused, would allow for temporal focusing of the attosecond pulse train.

Since the first observation of High-order Harmonic Generation (HHG) in gas twenty years ago, the combination of intense research together with technological developments, has led to impressive progress in the field of extreme ultraviolet spectroscopy and attosecond science. Beam lines based on HHG extend over several meters and are based on instrumentation that requires careful alignment and even active stabilization systems. Therefore, a miniaturization of HHG beams will reduce the cost of these light sources and pave the way to their application in numerous new fields. Femtosecond laser micromachining followed by chemical etching (FLICE) has already demonstrated its high potential in the fabrication of fused silica lab-on-a-chip devices; it can directly produce microfluidic networks in a 3D geometry directly buried in the glass substrate. Until now, they have been extensively used for the manipulation of fluids but they are perfectly suitable for the manipulation of gas as well. In this work, we will demonstrate HHG in a gas filled microchannel network fabricated by the FLICE technique. The device structure will be based on hollow waveguides: several inlets will deliver the gas into a central hollow waveguide where the ultrafast laser will be coupled and HHG will take place. The high versatility of the FLICE technique will allow us to fabricate devices with modulated gas concentration and waveguide profile to achieve quasi-phase-matching conditions. Moreover, we will also demonstrate an integrated filtering chip that will allow to geometrically separate the main laser radiation from the XUV generated beam.

The spectral range spanning from ~100 eV to 1 keV is highly attractive for a large number scientific applications including the study of ultrafast chemical reaction in the liquid phase, the study of ultrafast demagnetization at the L-edges of 3d transition metals composing magnetic materials or, more simply, nano-imaging and micro-tomography of deep structures such as semiconductor components. The brilliance of table-top coherent soft X-rays sources does not compete yet with large-scale synchrotron beam lines: the conversion efficiency of High order Harmonic Generation (HHG) [1], i.e. the physical process used to produce photons up to 1 keV from a near- or middle- infrared femtosecond laser, is low and the photon flux in the X-UV is actually clamped by the availability of powerful enough driving lasers. The advent of picosecond Ytterbium solid-state lasers delivering average powers in the kW range is about to change this statement. When combined with nonlinear conversion devices such as optical parametric chirped-pulse amplifiers (OPCPA), these industrial lasers can be turned into powerful tunable sources with favorable properties for HHG up to soft-x-rays [2] such as mid-infrared wavelength, few-cycle pulse duration, high peak intensity, high energy and high-repetition. Additionally, few-cycle pulses reduce the number of attosecond bursts up to, ideally, a single isolated attosecond pulse. In that case, Carrier Envelope Phase (CEP) stability and control is paramount but also ensures a shot-to-shot reproducibility of the driving electric field as well as the HHG yield and spectra. In this talk we present the experimental results acquired during the commissioning at ELI-ALPS (Szeged, Hungary) of a supercontinuum-seeded optical parametric chirped-pulse amplifier (OPCPA) generating 4-cycle pulses at ~3.2 µm with a pulse energy >150 µJ at 100 kHz (15 W average power), a Strehl ratio >0.8 and a shot-to-shot energy stability of 0.7% over 8h. This system was optimized for long-term energy and CEP stability and exhibits a CEP noise of 65 mrad RMS over 8h. To date, this is the best recorded non-averaged CEP stability for an amplified system, independently of the wavelength, pulse duration or repetition rate. This OPCPA also delivers the highest reported peak power (~3.8 GW) at 100 kHz within the 2-4 µm wavelength range without post-compression Last, we present our development strategy toward the extension of these high-flux OPCPA sources toward the mid- and far-infrared as well innovative ideas to adapt these sources to multi-dimensional spectroscopy. References: [1] M. Lewenstein et al, "Theory of high-harmonic generation by low-frequency laser fields." Phys. Rev. A 49, 2117-2132 (1994). [2] T. Popmintchev et al, ”Bright coherent ultrahigh harmonics in the keV X-ray regime from mid-infrared femtosecond lasers”, Science 336, 1287-1291 (2012).

Ultrafast lasers contribute essentially to the development of micro/nanotechnologies, being able to structure materials with utmost precision. Present advances in photonics include the development of optical devices based on laser-induced refractive index engineering. Ultrafast laser photoinscription can confine energy in micro-domains of arbitrary geometries, modifying the material refractive index and laying down the concept of 3D design for efficient optical functions. Here nanoscale precision can deliver high levels of performance. Therefore bypassing the diffraction limit is key for a new range of applications in optics requiring optical access at the nanoscale. We discuss the capability of Gauss and Bessel-Gauss pulses with engineered dispersion to localize light on subwavelength scales. We show how sculpting beams in space and time can bring advantages for controlling the interaction between light and matter and for achieving extreme confinement of energy. We discuss physical mechanisms of photoinscription by following the dynamics of excitation over the entire evolution cycle, serving as guidelines for control. We explore the influence of pulse temporal and spatial design in achieving index structures on 100 nm scales, either in direct focusing or in self-organization schemes in fused silica. Non-diffractive beam excitation takes advantage of this localization and achieve unprecedented high-aspect-ratio structuring. Subsequently we present photonic systems where hybrid micro/nanoscale features can develop advanced optical functionalities. We will show their capability to transport, manipulate and access electrical fields, either for Bragg sensing or for reconstruction spectral information. Finally we indicate a range of applications, from telecom to astrophotonics.

A W-type co-axial chalcogenide optical fiber structure is designed and numerically analysed for the broadband and highly coherent supercontinuum sources in the mid-IR region. The structural parameters of the designed W-type optical fiber are optimized to obtain small absolute group velocity dispersion in broad spectral range in the mid-IR region. The proposed W-type fiber structure possesses a flat dispersion profile with the flatness of the dispersion of ±2.45 ps/nm/km in the spectral range of 4.9 – 12.6 μm. The broadband and coherent mid-IR supercontinuum spectrum extending from 2.28 μm to 15.52 μm at -40 dB level is obtained using a 4 cm long chalcogenide W-type fiber pumped by 200 fs laser pulse of peak power of 10 kW at 7 μm. The average coherence property of the supercontinuum spectrum is almost unity in the full spectral range for the chalcogenide W-type fiber. Such broad and highly coherent mid-IR supercontinuum spectrum is very important because most of the biological tissue possesses their molecular fingerprints within this spectral range. Therefore, this region of electromagnetic spectrum is extremely useful to determine a tissue spectral map which provides very important information concerning the existence of the critical diseases such as cancer. The W-type chalcogenide fiber structure reported in this paper is a promising candidate for the development of the coherent broadband mid-IR supercontinuum sources which have potential applications in early cancer diagnostic, food quality control, gas sensing, and imaging.