Gallium sulphide (GaS) is an emerging monochalcogenide material that has recently attracted interest in optical technologies due to its tunable bandgap in the near-UV region. In this work, we employ in-situ, in-operando X-ray diffraction to investigate local atomic modifications in GaS induced by 400- nm femtosecond laser pulses. We identify the energy threshold at which irreversible structural changes occur and observe a laser-induced elongation of the unit cell along the c-axis. This elongation is expected to enhance the anisotropy of the material physical properties. Ab initio calculations further reveal that the experimentally observed ≈ 10% elongation along the c-axis leads to a transition from a direct to an indirect bandgap, accompanied by a bandgap increase of approximately 0.45 eV. Additional ab-initio optical simulations show that this structural transformation results in a nearly constant in-plane refractive index contrast of ∆n ≈ 0.1 across a wide spectral range, from the visible to the near-infrared, with negligible optical losses, which could be of interest for reconfigurable photonics applications.

Laser-based Kα X-ray sources offer compact and cost-effective alternatives to large-scale facilities like synchrotrons, providing high photon flux and ultrashort pulses ideal for advanced imaging. However, challenges such as flux variability, target refresh limitations and acquisition constraints hinder their use in tomography. This study presents tomographic strategies leveraging a priori knowledge of the sample and advanced denoising techniques, tested on simulated data.

Using lasers to achieve controlled crystallographic phase changes in silicon with high spatial precision promises new manufacturing solutions in semiconductor technologies, including silicon photonics. Recent demonstrations of improved amorphization thicknesses position ultrafast lasers as an optimum tool to meet current challenges. Here, the literature on silicon transformations is reviewed and complemented with new experimental data. This includes amorphization and ablation response as a function of pulse duration (tau = 13.9 to 134 fs at lambda = 800 nm) and laser wavelength (lambda = 258 to 4000 nm with tau = 200 fs pulses). For pulse duration-dependent studies on Si(111), the amorphization fluence threshold decreases with shorter durations, emphasizing the significance of non-linear absorption in the range of considered conditions. For wavelength-dependent studies, the amorphization threshold increases sharply from lambda = 258 to 1030 nm, followed by near-constant behavior up to lambda = 3000 nm. Conversely, the ablation threshold fluence increases in these specified ranges. Differences in the obtained amorphization thicknesses on Si(111) and Si(100) are also discussed, identifying an anomalously large fluence range for amorphization at lambda = 258 nm. Finally, the question of the lateral resolution, shown as independent of the interaction nonlinearity is addressed. This study investigates the limits of silicon amorphization using ultrashort laser pulses. It explores fluence ranges for ablation and amorphization, optimizing amorphization thickness, and aspects related to spatial resolution control. Thoroughly examined pulse duration (from 13.9 to 134 fs) and laser wavelength (from 258 to 4000 nm) effects reveal fluence thresholds and maximum amorphization depths. Results can inform theoretical models and advance femtosecond laser strategies in silicon photonics and semiconductor technologies. image

All-dielectric nanophotonic devices are usually fabricated by engraving arrays of nanoholes at the surface of high-index materials, to engineer dedicated optical functions. However, their direct 3D integration in the volume of a material is challenging, inaccessible to current planar nanolithography methods. Here is introduced an ultrafast laser-machining method that opens the possibility to realize scalable arrays of hollow nanochannels directly inside the bulk of silica glass within a single-step, maskless, and digital approach. Using a custom-shaped micro-Bessel beam and by tuning laser pulse durations from femtoseconds to picosecond to boost processing versatility, dense assemblies of nanochannels with adjustable lengths (up to 30 mu m), and submicron lattice periodicity (down to 0.7 mu m) are achieved. As a proof-of-principle demonstration, a gradient-index metaphotonic structure is realized and its performance is experimentally characterized, demonstrating its relevancy for imprinting phase functions with magnitude up to pi/2$\pi /2$ in the short-wave infrared spectral range. Results show that the unique flexibility and scalability provided by individual control of each channel opens a new realistic alternative approach for 3D fabrication of monolithic integrated nanophotonic devices inside a wide range of low-index standard optical materials. Laser direct machining of nanophotonic structures composed of arrays of hollow nanochannels embedded in glass is demonstrated. Flexible adjustment of channels dimensions and spacings opens new possibilities for versatile effective-index engineering. A gradient-index metaprism is fabricated and tested, as a demonstrator of custom phase functions accessible by metamaterials laser-engraved inside standard low-index dielectric materials and operating in the infrared spectral-range. image

Periodic arrangements of micro- and nano-holes with controllable period, diameter and depth at the surface of materials are of high interest for a large range of applications. Here we present a laser-based approach using micro-Bessel beams with adjustable length to machine arrays of holes with subwavelength diameters and depths reaching several micrometers at the surface of fused silica. Suitability and limitations of the technique are investigated, including the challenge of avoiding crosstalk effects. The performance level shows the potential of the direct-laser-processing method towards the realization of integrated devices, as a flexible and cost-effective alternative technique to current multistep nanofabrication methods.

Periodic nanohole arrangements constitute an important building block of advanced photonic devices. Aside from standard nanofabrication tools, a direct laser‐based approach is introduced here, that enables single‐step and point‐by‐point machining of arrays of holes with subwavelength diameters and depths reaching several micrometers at the surface of fused silica. The method relies on a simple optical arrangement including an axicon combined with an amplitude mask to shape the laser intensity in appropriately truncated micro‐Bessel beams of adjustable length. The suitability and limitations of the technique are investigated to fabricate arrays of cylindrical nanoholes with tunable depths. In particular, the challenge of avoiding crosstalk effects during the laser‐writing process of high‐density arrays is explored. The achievability of square arrays of nanoholes at the surface of fused silica, with diameters down to 200 nm and variable depths from 3 to 20 µm at a spatial density defined by a pitch of 1.5 µm is demonstrated. The performance level shows the potential of the direct‐laser‐processing method towards the realization of integrated devices, offering a highly flexible and cost‐effective alternative technique to current nanofabrication methods.

We demonstrate laser-fabrication of length-controlled channels in the bulk of dielectrics with a spatial periodicity down to 0.7 µm and moderate aspect ratio (1:10), by single-shot ablation using different (fs/ps) pulse durations. We take advantage of beam shaping technique using an axicon and annular aperture to generate a Bessel beam with an extra control of the so-called “non-diffracting” length. The dimensions and pitch attained are suitable to envision the writing of NIR nanophotonic components. As a proof-of-principle demonstration, we fabricate patterns whose arrangements mimic photonic-crystal devices like waveguide, Y-coupler, and structures with square and triangular lattices as their basic units.

Realization of photonic components made of micro and nanoscale features has paved the path towards creating photonic integrated circuits, which find application in data communication, biomedical field [1] as well as lab-on-a-chip devices [2]. The building blocks of such structures are regularly ordered arrays of nanoholes or nanochannels with a submicrometric spatial periodicity. There remain several challenges to deliver these structures featured with large lengths, controlled shape, high throughput, and larger flexibility with classical procedures like photo/electron lithography, focused ion beam and nanoimprint techniques. Moreover, these nanofabrication tools are restricted to surface processing. Therefore, albeit the spatial resolution is obviously not comparable, direct femtosecond laser ablation could find a relevant alternative to facilitate high designability in making complex structures, with unique in-volume processing abilities.

Arrays of nanoholes or nanochannels constitute the building block of integrated devices that open attractive applications like 2D photonic crystals, 2D metamaterials or nanostructured surfaces. Here we present a laser-based technique that enables to generate short-length micro-Bessel beams (irrespective of their core diameter) that we further use to machine depth-controlled holes with a cylindrical depth profile. We illustrate the potential of this method by fabricating square arrays of subwavelength-diameter holes with several-micrometers depth by direct laser ablation at the surface of fused silica.

We study the thermally induced aberrations in the compressor of a high average power ultrashort laser. We demonstrate a significant reduction of these distortions by selecting a substrate of diffraction gratings with adequate thermomechanical properties.

Over the past several years, non-ablative femtosecond laser exposure with spatially overlapping (i.e., spatially cumulative) pulses has emerged as a key process in three-dimensional writing of patterns in bulk of dielectric substrates. When temporally non-cumulative and combined with post-processing steps, this process defines a novel manufacturing technique in fused silica, finding uses in a broad number of applications, including—but not limited to—micromechanics, integrated optics, microelectronics, microfluidics, information storage, and combinations of these fields for novel integrated sensing applications. For fused silica, evidence has shown that there is a pulse-length duration threshold around 200 fs, marking the boundary between two radically different characteristic material modification regimes, each leading to a specific application. Pulse widths below 200 fs lead to localized densification, enabling the direct-write of optical waveguides, while pulse widths above this value produce self-organized nanostructures causing a localized volume expansion and enhanced etching susceptibility to various chemicals. Here, we focus our attention on the regime below 200 fs, using low repetition rates and temporally non-cumulative pulses. In particular, we use very short pulses, i.e., in the range of 30 fs—a regime as yet unexplored from the viewpoint of spatially cumulative modifications. Our goal is to understand how structural modifications obtained by overlapping pulses evolve with varying pulse overlap, and how shorter pulse duration may correlate with higher material densification. This knowledge is particularly important for the next generation of photonics devices, where increasing the level of laser-induced densification is a key factor for high-density photonic integration.

Over the past decades, the rapid progress in generating laser pulses with high peak power enabling peak intensities in excess of 10 18 W/cm 2 has opened up a new field of physics. Such laser pulses are used for various applications, such as high energy physics or the production of highly energetic particles and photons. In parallel to the increase of peak power, constant effort has been undertaken to increase the repetition rate of such systems. Indeed, it allows to improve the average power of the laser-induced secondary sources enabling systematic studies and parameters scans backed up by statistical data, leading to deeper and more consistent analysis.

Ultra-short breakdown in the bulk of transparent materials has been intensively investigated in the last years, especially in dielectrics [1]. Whereas three-dimensional (3D) femtosecond laser micromachining in dielectrics is highly advanced, it remains extremely challenging in narrow bandgap materials such as silicon (Si). Recent numerical and experimental investigations show that only an underdense microplasma can be generated inside the bulk of crystalline Si by two-photon absorption [2]. The energy deposition inside the material is drastically limited by significant losses in the prefocal region and strong plasma effects. We provide in this study an experimental evidence of this optical limitation by 3D-imaging of the beam propagation with 60-fs, 1300-nm pulses focused 1 mm under the Si-surface. Even for high Numerical Aperture (NA) (up to 0.65), we observe a strict clamping of the delivered energy far below the level needed for material breakdown. For comparison, the horizontal line in Fig.1.a is the measured fluence threshold for surface breakdown, which can be taken as the minimal target for breakdown in the bulk.

An important challenge in the field of three-dimensional ultrafast laser processing is to achieve permanent modifications in the bulk of silicon and narrow-gap materials. Recent attempts by increasing the energy of infrared ultrashort pulses have simply failed. Here, we establish that it is because focusing with a maximum numerical aperture of about 1.5 with conventional schemes does not allow overcoming strong nonlinear and plasma effects in the pre-focal region. We circumvent this limitation by exploiting solid-immersion focusing, in analogy to techniques applied in advanced microscopy and lithography. By creating the conditions for an interaction with an extreme numerical aperture near 3 in a perfect spherical sample, repeatable femtosecond optical breakdown and controllable refractive index modifications are achieved inside silicon. This opens the door to the direct writing of three-dimensional monolithic devices for silicon photonics. It also provides perspectives for new strong-field physics and warm-dense-matter plasma experiments. Ultrafast laser processing is a versatile three-dimensional photonic structuring method but it has been limited to wide band gap materials like glasses. Here, Chanal et al. demonstrate direct refractive-index modification in the bulk of silicon by extreme localization of the energy deposition.

Carrier kinetics in the density range of N=1017−1020 cm−3 is investigated inside the bulk of crystalline silicon. Most conventional experimental techniques used to study carrier mobility are indirect and lack sensitivity because of charging effects and recombination on the surface. An all optical technique is used to overcome these obstacles. By focusing 1.3-μm femtosecond laser pulses in the volume, we inject an initial free-carrier population by two-photon absorption. Then, we use pump-and-probe infrared microscopy as a tool to obtain simultaneous measurements of the carrier diffusion and recombination dynamics in a microscale region deep inside the material. The rate equation model is used to simulate our experimental results. We report a constant ambipolar diffusion coefficient Da of 2.5 cm2 s−1 and an effective carrier lifetime τeff of 2.5 ns at room temperature. A discussion on our findings at these high-injection levels is presented.

We report on a functional experimental design for Bessel beam generation capable of handling high-energy ultrashort pulses (up to 1.2 mJ per pulse of 50 fs duration). This allows us to deliver intensities exceeding the breakdown threshold for air or any dielectric along controlled micro-filaments with lengths exceeding 4 mm. It represents an unprecedented upscaling in comparison to recent femtosecond Bessel beam micromachining experiments. We produce void microchannels through glass substrates to demonstrate that aspect ratios exceeding 1200∶1 can be achieved by using single high-intensity pulses. This demonstration must lead to new methodologies for deep-drilling and high-speed cutting applications.

Ultrafast carrier dynamics in arrays of single crystal and relatively uniform-diameter Ge nanowires (NWs) are investigated by transient absorption measurements and effective medium simulations. We present the first quantitative analysis of a Ge NW-air metamaterial, translating the photon response of the assemblies to carrier dynamics. Three time regimes of the ultrafast recombination process are identified: Auger recombination dominant (0-5 ps), "fast" surface trapping and recombination dominant (5-20 ps), and a mix of "fast" recombination and "slow" surface trapping (20-200 ps). The rates of surface recombination and their dependences on pump fluence are determined, highlighting the different interactions of electrons and holes with Ge NW surface and interface states. Structural and excitation conditions can be engineered to extend the photogenerated electron and hole lifetimes. Small wire diameters and low pump powers enhance the electron lifetime because charging of defect states in the surface oxide layer produces a potential barrier for electrons to be trapped at Ge/GeOx interface. This phenomenon simultaneously causes an enhancement of hole lifetime for relatively large wire diameters and large pump powers.

Dielectrics as single layers and broadband high-reflective stacks were deposited by electron beam deposition processes compatible with 1-meter class optics. After being physically and optically characterized, samples were irradiated with several ultrafast lasers (KYW:Yb 500fs, Ti:Sa 40fs and Ti:Sa 11fs) with single and multi-pulses. The setups of the test platforms, laser-induced damage threshold investigations of intrinsic materials, dielectric multilayers and hybrid metal/dielectric multilayers and electric field intensity distributions are described.

Two-photon ionization by focused femtosecond laser pulses initiates the development of micrometer-scale plasmas in the bulk of silicon. Using pump-and-probe transmission microscopy with infrared light, we investigate the space-time characteristics of these plasmas for laser intensities up to 1012 W/cm2. The measurements reveal a self-limitation of the excitation at a maximum free-carrier density of ≅1019 cm−3, which is more than one order of magnitude below the threshold for permanent modification. The plasmas remain unchanged in the ∼100 ps timescale revealing slow carrier kinetics. The results underline the limits in local control of silicon dielectric permittivity, which are inherent to the use of single near-infrared ultrashort Gaussian pulses.