In this work, we describe a methodology to highly improve the quality of micro-structured silicon obtained by femtosecond-laser irradiation in ambient air atmosphere. Optimum femtosecond laser irradiation conditions have been selected to obtain a high aspect ratio spike morphology at the surface of silicon. Due to the aggressive fs-laser process, the crystalline structure of the material presents extended defects, which can be removed by rapid thermal annealing. Moreover, thermal annealing triggers diffusion of oxygen to the surface, which was incorporated from the air atmosphere into the bulk upon repetitive fs-laser irradiation. This oxygen binds with silicon atoms within the amorphous surface layer, giving rise to SiOx, obtaining irregular decorations at the surface of the spike structures. These decorations can be removed by a buffered hydrofluoric acid solution, which etches the silicon oxide but not the crystalline silicon. Using this strategy we have obtained a high crystalline quality with: high absorption of around 95 % in the wavelength region from 200 to 1100 nm, minimized defectrelated absorption from 1100 to 2500 nm; a crystalline lattice free of defects and oxygen; and a high aspect-ratio micro-spike morphology formed by c-Si free of surface SiOx nano-decorations that can be useful to fabricate optoelectronic devices.

We have analyzed the formation and coherent propagation of laser-induced periodic surface structures (LIPSS) upon fs-laser irradiation of fluorine-doped tin oxide (FTO) films. The aim is the generation of large electrical anisotropies in macroscopic areas for applications including laser-written transparent heaters, sensors, or substrates for electrically controllable wettability, among others. The films have been processed with high-repetition-rate (hundreds of kHz) fs-laser pulses at 1030 nm using different laser (fluence and pulse duration) and scanning parameters (speed and line overlap). Optimal LIPSS formation and coherent propagation are conditioned by the dynamic evolution of the F content in the laser-processed regions that can be controlled via processing parameters. However, the homogeneity in the spatial distribution of F in the microscale in the pristine samples can generate important issues that are carefully considered and discussed in the present work. Still, the formation of optically and electrically highly anisotropic surfaces for laser scanning speeds well above 1.0 m/s has been demonstrated, reaching electrical resistivity anisotropy factors of >10(3). Consequently, the potential development of large-area applications using LIPSS-structured FTO films seems feasible, as we have demonstrated by fabricating an efficient laser-written electrothermal transparent device as a proof-of-concept.

Transparent Conductive Oxides (TCOs) are materials with relatively high conductivity at room temperature and low optical absorptance in the visible (VIS), which makes them particularly suitable for applications in photovoltaics (energy harvesting, light detection …), information technologies (OLEDs, displays …) and transparent heaters, among others [1]. Laser processing provides a very suitable tool to modify their composition, morphology or structure to tailor their properties for specific applications [2]. The feasibility of producing strongly anisotropic surfaces by fs-laser irradiation in indium-tin-oxide (ITO) films has been demonstrated [3], [4]. Electrical and optical anisotropies were triggered by the formation of Laser Induced Periodic Surface Structures (LIPSS) [5] characterized by alternating ablated and non-ablated stripes (ridges and valleys). In addition, under some processing conditions, the compositional changes induced in the material ridges triggered a red-shift of the ITO's transparency window, which seems extremely attractive for IR-photodetectors and photovoltaics [6].

Several transition metal oxides exhibit electrical properties that can be tuned through their crystalline structure, defect structure and composition. A critical parameter to control when modifying the conductivity of an oxide of this kind is the oxygen content, which can be generally assessed during the oxide synthesis [1]. The feasibility of modifying their electrical properties, together to their large bandgap and structure, make transition metal oxides candidates of interest in the development of energy applications (i.e. Li-ion batteries) and photocatalytic reactors.

The advent of high-power ultrafast lasers in the infrared and mid-infrared regimes is enabling the exploration of new excitation regimes in laser processing. In particular, for semiconductor materials and their transparency in the infrared domain of the spectrum, they facilitate the study of highly nonlinear regimes and the impact for novel modification possibilities, in a similar way to what has previously been achieved in dielectric materials using near-infrared ultrafast lasers.

Material processing with femtosecond lasers has attracted enormous attention because of its potential for technology and industrial applications. In parallel, time-resolved x-ray diffraction has been successfully used to study ultrafast structural distortion dynamics in semiconductor thin films or surface layers. However, real-world processing applications mostly are concerned with bulk materials, which prevents the use of x-ray surface based techniques. For processing applications, a fast and depth-sensitive probe is needed. To address this, we present a novel technique based on ultrafast x-ray dynamical diffraction (UDD) capable of imaging transient strain distributions inside bulk crystals upon laser excitation. This pump-probe technique provides a complete picture of thetemporal evolution of ultrafast distorted lattice depth profiles. We demonstrate the potential of UDD by studying a thin Si single crystal upon single pulse femtosecond optical excitation. Our study reveals that below the melting threshold strong lattice distortions not only longitudinal, but also transversal to the propagation of the strain wave appear on picosecond time scales along the single crystal. The observation of this transversal deformation after laser excitation contradicts previous work that were not able to observed it, what could be related to the high sensitivity of dynamical diffraction with respect to the lattice distortions. The speed of propagation of this ultrafast transversal strain deformation is observed to be slower to the longitudinal sound speed for Si as described in the bibliography.

The synthesis of large area micro- and nano-structured ZnO surfaces has been successfully achieved through a two-step process. It involves the irradiation of Zn metal sheets with femtosecond laser pulses (350 fs at 1030 nm) at high repetition rates (100-500 kHz), and fast scanning speeds (cm/s). Subsequently, the irradiated sheets are thermally treated in an Argon flux at 380 degrees C, a temperature significantly lower than that typically required for growing micro- and nanostructures in ZnO. Fs-laser irradiation promotes the initial development of topography and the localized oxidation of the metal. This enables the further growth of micro- and nanostructures at preferential sites with good crystalline quality and luminescent properties. Analysis of the material at different processing steps shows that the initial laser-induced oxidation is crucial in defining ZnO growth mechanisms upon thermal treatment, and determining the final properties. We have tested the potential use of these structures as reusable photocalyst. The ease of catalyst recovery in photocatalysis experiments and the degree of degradation achieved may be considered as key performance indicators. Photocatalytic activity tests performed with a Rhodamine B solution showed degradation values up to 43 % over 90 min. The morphology of the samples remains unaltered after photocatalysis experiments.

In this work, we demonstrate that ultrafast laser processing (1030 nm, 290 fs) of silicon in ambient air strongly improves the material's performance in terms of absorption, both in the visible and near infrared spectral range, which paves the way for further studies on increasing the sub-bandgap absorption after texturing, suggesting the material developed as a sustainable substitute for black silicon processed in greenhouse gases atmospheres. Our approach is based on the fabrication of spike-like morphologies in ambient air and the subsequent annealing of the material by pulsed laser melting or rapid thermal annealing to recover its crystalline phase. In particular, the influence of three main processing parameters (fluence, pulse number and repetition rate) on the properties of the spike-like structures has been investigated, each of them revealing the possibility of a direct control on the size, shape and period of the spikes, and achieving a total tuning range of the period from 4 mu m to 14 mu m for a single laser wavelength. Macroscopic areas have been fabricated using short processing times, yielding absorption values A > 94% over the UV-VIS-NIR spectral range (250 nm - 1100 nm) without hyperdoping, and A >= 20% for longer wavelengths up to 2500 nm, while preserving the electrical performance of pristine silicon.

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

Femtosecond laser processing of semiconductors has evolved into a mature, high -precision fabrication technique, enabling a wide range of applications. While initially most studies have employed pulses at near infrared wavelengths, the interest in using UV laser pulses is constantly increasing due to the different excitation conditions as a consequence of the much shorter optical penetration depth, leading to an improved resolution. In this context, fundamental studies on the temporal dynamics of phase transformations triggered by such pulses are necessary in order to comprehend and eventually control the complex phase transformation pathways. Here, we report a detailed time -resolved study on the phase transformation dynamics of crystalline silicon and germanium upon irradiation with single 400 nm, 100 fs laser pulses in the moderate and high excitation regime. To this end, we have employed fs-resolved optical microscopy with a probe wavelength of 800 nm to study the reflectivity evolution of the irradiated surface over a temporal window ranging from 100 fs up to 20 ns. At moderate excitation fluence, the data reveals the entire sequence of laser -induced processes, starting from the generation of a free -electron plasma, non -thermal melting, ablation onset and expansion of a semi -transparent ablation layer with sharp interfaces. At excitation with peak fluences more than 30 times the ablation threshold, an anomalous transient highreflectivity state is observed, which might be indicative of a recoil pressure -induced liquid -liquid phase transition. Moreover, 70 nm-thick amorphous surface layers are formed in both materials after irradiation at moderate fluences. Overall, our results provide relevant information on both, transformation dynamics and final state of both materials for fs-pulse excitation in the near -UV wavelength range.

High repetition rate femtosecond lasers are commonly used for fabricating laser-induced periodic surface structures (LIPSS) over large areas at high processing speeds. Industrially relevant metals, like steel, experience thermal modifications at repetition rates beyond several hundred kilohertz. In this work, we fabricate low spatial frequency LIPSS (LSFL) on steel, varying pulse repetition rates from 10 kHz to 2 MHz. The study characterizes laser-structured areas and redeposited debris using SEM and μ-Raman spectroscopy. A simple heat dissipation model identifies repetition rate ranges associated with thermal modifications. Morphological changes and debris impact functional wetting behavior, offering insights for optimizing parameters in high repetition rate femtosecond laser materials processing.

In this study, we explore the morphology and orientation of molybdenum diselenide, a Van der Waals 2D material, through isothermal closed space vapor deposition on both pristine and laser-structured substrates. Laser structuring is conducted on dielectric (sapphire), semiconductor (silicon), and conductive (titanium nitride) substrates using ultrashort laser pulses, resulting in smooth topographic changes such as laser-induced periodic surface structures (LIPSS) or selective ablation. Scanning electron microscopy (SEM) reveals the pivotal role of surface structuring in the growth of out-of-plane MoSe2 nanosheets. This effect is particularly pronounced on monocrystalline substrates like sapphire and silicon, exhibiting in-plane growth on pristine substrates. Additionally, Raman spectroscopy confirms the vertical orientation of flakes on structured substrates and highlights the presence of active edge sites by demonstrating an increased abundance of deposited material. Overall, our findings emphasize the controllability of directing the growth of MoSe2 flakes through appropriate pre-treatment of the substrate, with potential applications in various fields, including Surface-Enhanced Raman Scattering (SERS). Furthermore, the scalability, reproducibility, and applicability to any substrate make ultrashort laser structuration a promising general strategy for orienting 2D materials.

The success of ultrafast laser processing strategies to fabricate reproducible nanostructures below the diffraction limit was possible in part by understanding the complex material transformation pathways triggered by laser pulses. A continuously growing scientific community has focused on the development of experimental techniques, mostly based on probing matter by light, to unravel the underlying interaction mechanisms. Often supported by theoretical modelling, these techniques have greatly contributed to today's understanding of laser-matter interaction and enabled the identification of processing conditions that would have hardly been identified with empirical parameter scans. In this chapter, we review these light-based probing techniques, which interrogate in-situ the transient state of matter throughout its transformation from the initial state to the final state. Each technique is introduced and discussed in terms of its suitability for measuring certain transient changes induced, its spatial and temporal resolution, and is accompanied by results that illustrate its potential. Finally, a few examples of the applicability of some of these techniques in an industrial environment are given.

The design and fabrication of a integrated symmetric directional coupler dependent o the pumping power and operating at a 1534 nm wavelength is reported. The twin-core waveguide was inscribed into Er3+/Yb3+ co-doped phosphate glass by a femtosecond laser direct writing technique. By optical pumping, the coupling ratio can be modulated due to the changes induced in the refractive index of the material. The experimental results demonstrated that the coupling ratio can be tuned continuously from 100/0 to 50/50 by increasing the pump's power from 0 to 350 mW. The developed twin-core coupler has promising applications for on-chip all-optical signal processing and communication systems.

Bragg gratings inscribed in active waveguides combine very efficient reflective properties with the amplifying capability of rare-earths, which may lead to large amplification and lasing performance. However, the response of these photonic structures highly depends on the grating parameters and working conditions, so modeling their behavior and dependences becomes fundamental. In this work, a numerical method has been implemented to simulate the optical power propagation along an Er/Yb-codoped integrated waveguide Bragg grating as a function of its most relevant operational parameters. The results obtained show the optimal conditions to maximize its performance as a highly amplifying reflector, but also its capability as a monolithic laser. In addition, the modeling results adequately match experimental values measured in fs-laser written structures in Er/Yb-codoped phosphate glass, supporting the accuracy of the numerical method developed and its usefulness for further optimizing these promising photonic structures.

This work reports on the production of fs-laser written waveguides based on the cross migration of Ba and/or La against alkali in modified phosphate glass, and the presence of Ba-La ion migration competition effects.

Reflectivity loss due to dust deposition over primary mirrors is a key issue for ground-based astronomical observatories. Suspended atmospheric dust falls over the optical surface affecting its performance and optical quality. Several in-situ and off-site techniques have been developed for eliminating dust particulates from optical surfaces, but all of them present important drawbacks like being time-consuming, expensive, contaminant or even involving dangerous operations in off-site approaches. Alternatively, a method based on laser-matter interaction and SENER’s experience in ground telescopes, has been tested and developed. Laser cleaning is a discipline that covers a large range of applications, from semiconductor physical decontamination to the removal of oxides from metallic surfaces. The wide catalogue of laser systems currently available, allows us to investigate and design the best laser source for astronomical mirror maintenance. This has allowed us to develop a system that performs a proper cleaning for astronomical mirrors, which is currently bridging the gap between the laboratory demonstrator and the telescope application. In this communication we present the development of a laser cleaning system for the removal of dust particulates and contaminants attached to telescope mirror surfaces. Several configurations have been tested in the laboratory, under different conditions, and cleaning was performed over aluminized Zerodur® flat samples, that were naturally contaminated in Atacama’s desert and Canary Islands, aiming at the definition, manufacturing, and testing of a demonstrator prototype. Results show a large improvement of reflectivity after cleaning and contribute to the design of a custom efficient laser cleaning system adapted to different observatories.

Plasmonic metasurfaces based on the extraordinary optical transmission (EOT) effect can be designed to efficiently transmit specific spectral bands from the visible to the far-infrared regimes, offering numerous applications in important technological fields such as compact multispectral imaging, biological and chemical sensing, or color displays. However, due to their subwavelength nature, EOT metasurfaces are nowadays fabricated with nano- and micro-lithographic techniques, requiring many processing steps and carrying out in expensive cleanroom environments. In this work, we propose and experimentally demonstrate a novel, single-step process for the rapid fabrication of high-performance mid- and long-wave infrared EOT metasurfaces employing ultrafast direct laser writing. Microhole arrays composing extraordinary transmission metasurfaces were fabricated over an area of 4 mm2 in timescales of units of minutes, employing single pulse ablation of 40 nm thick Au films on dielectric substrates mounted on a high-precision motorized stage. We show how by carefully characterizing the influence of only three key experimental parameters on the processed micro-morphologies (namely, laser pulse energy, scan velocity, and beam shaping slit), we can have on-demand control of the optical characteristics of the extraordinary transmission effect in terms of transmission wavelength, quality factor, and polarization sensitivity of the resonances. To illustrate this concept, a set of EOT metasurfaces having different performances and operating in different spectral regimes has been successfully designed, fabricated, and tested. Comparison between transmittance measurements and numerical simulations has revealed that all the fabricated devices behave as expected, thus demonstrating the high performance, flexibility, and reliability of the proposed fabrication method. We believe that our findings provide the pillars for mass production of EOT metasurfaces with on-demand optical properties and create new research trends toward single-step laser fabrication of metasurfaces with alternative geometries and/or functionalities.

Monitoring the laser-induced melting and solidification dynamics of Ge upon laser irradiation is an enormous challenge due to the short penetration depth of its liquid phase. In this work, real-time pump-probe experiments in combination with finite element calculations have been employed to investigate the melting and solidification dynamics of germanium upon ns and fs laser pulse irradiation (λ = 800 nm). Excellent agreement between experiments and simulations allowed us to indirectly determine additional time- and depth-dependent information about the transformation dynamics of germanium, including the thickness evolution of the molten layer, as well as its melting and solidification velocities for the two pulse durations for different fluences. Our results reveal considerable differences in the maximum thickness of the molten Ge superficial layers at sub-ablative fluences for ns and fs pulses, respectively. Maximum melt-in velocities of 39 m s−1 were obtained for ns pulses at high fluences, compared to non-thermal melting of a thin layer within 300 fs for fs pulses already at moderate fluences. Maximum solidification velocities were found to be 16 m s−1 for ns pulses, and up to 55 m s−1 for fs pulses. Weak signs of amorphization were observed for fs excitation, suggesting that the lower limit of solidification velocities for a complete amorphization is above 55 m s−1. In addition, we show high precision measurements of the melt-in velocities over the first 20 nm by means of fs microscopy with sub-ps temporal resolution. Here, differences of the melt-in process of several orders of magnitude were observed, ranging from virtually instantaneous melting within less than 2 ps even for a moderate peak fluence up to 200 ps for fluences close to the melting threshold.