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We have explored the possibility of forming H⋅ and H 2 from H 2 S, CH 4 , H 2 O and SiH 4 in a series of ab initio calculations. The key finding is that H 2 S can give rise to direct photolytic ultrafast formation of H 2 This finding is in agreement with the available experimental data and is a result of a directed pathway that leads toward H 2 formation at easily accessible wavelengths. It arises at the (conical) intersection between the two lowest-lying electronic states. This intersection is a result of the valence orbital interaction of the two hydrogen atoms. This interaction is becoming more favorable as the S-H bonds simultaneously becomes longer. The H-S σ-σ∗ bonding interaction cannot compete with the bond formation that takes place between the hydrogens as two bonds stretch simultaneously. For oxygen this interaction it predominant. The reason is that sulfur has a higher principal quantum number and the interaction with the 1s of hydrogen therefore is less favorable. The excited states that are involved in the H 2 formation from H 2 S could be populated by, for example, a two-photon 400 nm excitation. The other hydrides have a more entangled potential energy landscape close to the Franck-Condon region in the direction of the reaction coordinate that leads to H 2 loss and as a result the direct loss of a hydrogen atom is favored. A two-photon excitation could be induced by femtosecond laser pulses that at the same time is proposed as a future platform for sensing the H 2 S molecules with the backward transient absorption scheme. The implication would be that both sensing and photoinduced H 2 formation could be in place at the same time which justifies the use of expensive femtosecond light sources getting "two for the price of one". • H 2 S can form H 2 when exposed to two 400 nm photons. • Interaction with short light pulses is predicted in induce non-statistical dynamics in H 2 S and H 2 O. • The findings could be a platform for sensing the H 2 S molecules with the backwards transient absorption scheme. [ABSTRACT FROM AUTHOR]

Ni-Mn-Ga-based magnetic shape memory (MSM) alloy microactuators have a high potential in miniaturized devices for which conventional mechanisms or materials are no longer feasible. However, manufacturing single-crystal-based Ni-Mn-Ga micro actuators is challenging due to their high brittleness, which precludes conventional machining methods. The present work introduces and develops a novel manufacturing method – femtosecond pulse width laser (FPWL) ablation micromachining – for the manufacture of a functional actuators from oriented Ni-Mn-Ga single crystals. Notably, using a green laser (wavelength 515 nm) permits micromachining in the martensitic phase due to the low heating effect of the process. Optimized FPWL parameters are here used to machine a functional actuator from a five-layered modulated (10 M) martensite Ni-Mn-Ga single crystal. The actuator develops a fully reversible and repeatable magnetic-field-induced strain of 6.5% without any post-processing. [Display omitted] [ABSTRACT FROM AUTHOR]

The possibility of hydrogen production by the action of laser pulses (1064 nm, 14 ns, 10 Hz, 0.5–6 J/cm2) on a suspension containing 0.03 wt % aluminum (100 nm) in water is shown. It was found that aluminum nanoparticles absorb the laser energy and heat up. As a result of heating, the continuity of the oxide film is disrupted, and metallic aluminum reacts with water. It is shown that there is a complete transformation of aluminum into products (gibbsite, boehmite, bayerite). The maximum hydrogen yield V m = 8.4 mL does not depend on the energy density of the laser radiation. The time to reach V m decreases according to the hyperbolic law with increasing laser energy density, reaching 3.5 min at an energy density of 6 J/cm2. [Display omitted] • The time of complete transformation of Al depends on the laser energy density. • Laser radiation is absorbed directly by aluminum nanoparticles. • Rapid heating of aluminum nanoparticles leads to the destruction of their oxide film. • It is not required to heat the entire volume of the suspension. • There is a complete transformation of metallic aluminum into reaction products. [ABSTRACT FROM AUTHOR]

We investigated the removal of spray paint from heritage granite surfaces using a nanosecond pulse laser and compared the results, in terms of the effectiveness of treatment and preservation of the stone surface from potential damage, with results obtained using a femtosecond pulse laser. We show that, with the nanosecond laser, the ablation threshold of the stone was measured at 0.5 J·cm-2, but it was only possible to remove certain types of spray paints, such as blue and green paints, at a higher fluence of 0.9 J·cm-2. To remove other types of paints, such as red and yellow, it was necessary to increase even further the laser fluence to 1.5 J·cm-2, well above the ablation threshold of the stone. In this case, damage was induced in the minerals, such as melting of biotite, and a general roughening of the other minerals' surfaces was observed. Despite nanosecond pulse lasers being more widespread for cleaning purposes, we demonstrate that femtosecond pulse lasers allowed better effectiveness in removing various colours of paint, without leaving residues, while keeping the laser energy below the damage threshold of the underlying stone, allowing complete preservation of the substrate, and avoiding the melting of the most sensitive mineral grains (particularly biotite). We determined the ablation efficiency of spray paint using the femtosecond pulse laser and found a maximum ablation regime centered around 6.2 J·cm-2, at 8.36 mm3·(min·W)-1. This study highlights the benefits of using a laser with femtosecond pulses instead of longer pulse durations (nanosecond range) for the preservation and maintenance of heritage stone. [ABSTRACT FROM AUTHOR]

Ytterbium ferrites are being used in many promising applications, such as visible-light photocatalysis, solar cells, magnetooptic devices, electro-magnetic equipment, etc., due to their fantastic ferroelectric and ferromagnetic properties. However, despite their good magnetic and radiopaque features, the use of ytterbium ferrites as multiplatform contrast agents in magnetic resonance imaging (MRI) and X-ray computed tomography (CT) is still under-developed. This is mainly due to difficulties in obtaining stable and biocompatible aqueous colloidal dispersions of ytterbium ferrite nanoparticles. In order to overcome this limitation, this work explores an eco-friendly method to directly synthesize such dispersions by liquid-assisted pulsed laser ablation of ytterbium ferrite massive targets. First, orthorhombic bulk YbFeO 3 targets were obtained by a reaction-sintering method. Then, colloidal dispersions of nanoparticles were produced directly in both distilled water and ethanol by irradiating the bulk YbFeO 3 targets with high-power infrared nanosecond lasers pulses. A battery of techniques has been used to characterize the as synthesized YbFeO 3 targets and colloidal dispersions of YbFe nanoparticles to determine their composition, structure, magnetic properties, X-ray attenuation potentials, and colloidal properties. Moreover, the biocompatibility of the systems was also analysed by MTT cell viability assay. Results indicated that the use of distilled water as ablation medium yields colloidal dispersions consisted mainly of paramagnetic ytterbium ferrite nanoparticles. Contrarily, the use of ethanol as solvent leads to colloidal dispersions of polycrystalline nanoparticles with both ferromagnetic and paramagnetic behaviour, due to the coexistence, in each nanoparticle, of ytterbium ferrite, ytterbium oxide, and iron oxide crystalline phases. Both colloidal dispersions exhibit also high biocompatibility and suitable X-ray attenuation properties. Moreover, they show bio-safe hydrodynamic sizes (lower than 200 nm) with acceptable overall hydrodynamic polydispersity index values (under 0.4), being stable in water for several weeks. These results pave the way for the future evaluation of Yb–Fe based nanoparticles as multiplatform contrast agents in multimodal MRI and CT imaging. [Display omitted] [ABSTRACT FROM AUTHOR]

In this work, coherent phonons in glassy carbon using transient reflection measurements with the pump-probe spectroscopy are investigated. Detection of coherent oscillation of optical phonons in a polished glassy carbon sample is enabled by wavelength-selected measurements for reflected probe pulse. We demonstrate that the G and D mode oscillations occur at 46.8 and 38.2 THz frequency, respectively, and have approximately the same lifetime of 0.17 ps. Furthermore, we find that the D mode peak is anisotropic and spreads towards lower energies. [Display omitted] • Femtosecond transient reflection signal showed a coherent oscillation of optical phonons in a polished glassy carbon. • Wavelength selective detection with an optical bandpass filter enhanced the phonon oscillation. • Both G and D mode oscillations had approximately the same lifetime of 0.17 ps. [ABSTRACT FROM AUTHOR]

Red-colored high-pressure, high-temperature synthesized (HPHT) diamond was exposed in its bulk in a number of patterns to millions of 0.3-ps, 515-nm laser pulses focused by a 0.25-NA micro-objective. The resulting arrays of bright micrometer-scale spots produced at variable laser exposure and pulse energy was characterized by optical transmission microspectroscopy, room-temperature dynamic and stationary 3D confocal photoluminescence (PL) microspectroscopy, FT-IR microspectroscopy. These bleached spots exhibit 1) reduced IR-absorbance of C- and A-centers at the pertaining absorbance of interstitials, 2) stronger reduced visible-range absorbance of C centers, comparing to that of NV0 and NV− centers, 3) considerably depth-wise reduced PL intensity of NV0 and NV− centers in favor of the emerging blue-green (480–550 nm) PL band of indistinguishable higher-aggregated H3, H4 centers. The photo-activated aggregation process of nitrogen centers exhibits second-power dependence on laser energy and is apparently related to direct agitation of low-aggregated centers, rather than to photo-generation of interstitial-vacancy pairs. [Display omitted] [ABSTRACT FROM AUTHOR]

Simultaneous nitric oxide (NO) laser-induced fluorescence (LIF), atomic oxygen (O) two-photon LIF (TPLIF), and molecular oxygen (O 2) LIF were demonstrated using a single broadband femtosecond (fs) laser in methane- and hydrogen-fueled flames. An amplified Ti:Sapphire laser with ∼80-fs pulse duration was used to pump an optical parametric amplifier (OPA) to generate 226.1-nm pulses having 2.3 nm (∼450 cm−1) bandwidth to excite all three species simultaneously. The specific excitation transitions are NO A-X (0,0) system, O 3 p 3 P ← ← 2 p 3 P two-photon transition, and O 2 B 3 Σ u − − X 3 Σ g − Schumann-Runge system. Detailed emission characterization was performed using a 1D imaging spectrometer to identify suitable emission bands for interference-free detection of each species. A simultaneous detection system consisting of a visible and a UV camera provided the optimal detection of all three species. Imaging studies revealed high concentrations of NO, O, and O 2 at the edge of the flame cone of Bunsen flames because NO and O are produced at the hot flame front. O 2 LIF signal was present only at the flame front because of the strong temperature dependence of the O 2 Schumann-Runge system. Direct imaging of O TPLIF enabled single-shot as well as high-fidelity shot-averaged line images in stable laminar flames. Equivalence ratio scans in the CH 4 /air Hencken flames showed good agreements for O and O 2 with Cantera equilibrium predictions performed using GRI-Mech 3.0. Although the NO measurements deviated from the equilibrium calculations on the fuel-rich side, it agreed well with previously reported NO LIF measurements in a similar flame. This discrepancy is likely due to Prompt NO, which was not considered in the simulation. NO LIF measurements as a function of height-above-the-burner in the H 2 /air Hencken flames agreed well with calculated NO mole fractions using the UNICORN flame code. The present study lays the foundation for single-laser imaging of three critical flame species in NO formation pathways in combustion systems. [ABSTRACT FROM AUTHOR]

Halite deposits have long been utilized for interrogating past climate conditions. Microthermometry on halite fluid inclusions has been used to determine ancient water temperatures. One notable obstacle in performing microthermometric measurements, however, is the lack of a vapor bubble in the single-phase liquid inclusions at room temperature. (Pseudo-) isochoric cooling of the inclusions to high negative pressures, far below the homogenization temperature, has commonly been needed to provoke spontaneous vapor bubble nucleation in the liquid. High internal tensile stress in soft host minerals like halite, however, may induce plastic deformation of the inclusion walls, resulting in a wide scatter of measured homogenization temperatures. Nucleation-assisted (NA) microthermometry, in contrast, employs single ultra-short laser pulses provided by a femtosecond laser to stimulate vapor bubble nucleation in metastable liquid inclusions slightly below the expected homogenization temperature. This technique allows for repeated vapor bubble nucleation in selected fluid inclusions without affecting the volumetric properties of the inclusions, and yields highly precise and accurate homogenization temperatures. In this study, we apply, for the first time, NA microthermometry to fluid inclusions in halite and we evaluate the precision and accuracy of this thermometer utilizing (i) synthetic halite crystals precipitated under controlled laboratory conditions, (ii) modern natural halite that precipitated in the 1980s in the Dead Sea, and (iii) Late Pleistocene halite samples from a sediment core from Death Valley, CA. Our results demonstrate an unprecedented accuracy and precision of the method that provides a new opportunity to reconstruct reliable quantitative temperature records from evaporite archives. • Fluid inclusions trapped in halite host crystals are valuable paleoclimate archives. • Primary halite inclusions act as physical thermometers at time of entrapment. • Nucleation-assisted microthermometry is precise and accurate in halite inclusions. • Formation temperatures are measured for Lab-grown, Dead Sea, & Death Valley halites. [ABSTRACT FROM AUTHOR]

• The laser can be controlled intelligently. • The algorithm can derive the simulation model of the laser. • The algorithm can improve the efficiency and accuracy of the intelligent laser. • The intelligent laser can realize the shortest pulse search. Ultrafast fiber lasers based on nonlinear polarization rotation can generate femtosecond pulses with different pulse durations and high peak powers, which are powerful tools for engineering applications and scientific research. However, achieving a precise and repeatable polarization state for generating the ultrashort pulses with the shortest pulse duration remains a significant challenge. In this paper, we extend the use of recurrent neural networks and adaptive optimization algorithms, specifically designed to optimize repetitive processes in optical systems, to facilitate intelligent search and control aimed at achieving the minimum pulse duration within a mode-locked fiber laser cavity. Our multi-algorithm-based intelligent system can fully simulate and optimize the processes involved in hands-on experiments. Our intelligent system identified a mode-locked fiber laser with the shortest pulse duration of 465 fs, which was experimentally verified. The proposed intelligent algorithm not only identifies the shortest pulse but also holds significant potential for selecting related laser characteristic parameters. We believe this work opens up a novel avenue for exploration and optimization in mode-locked lasers and the intelligent laser can find practical applications in engineering and scientific research. [ABSTRACT FROM AUTHOR]

Investigation of the metastable dissociations clarifies the characteristics of ionic states. Tandem mass spectrometry using an ion trap mass spectrometer, a time-of-flight mass spectrometer (TOF-MS), and the combination of those spectrometers were used for the analysis of metastable dissociation. However, investigation of the metastable dissociations of multiply charged ions is not straightforward because collision-induced charge transfer reactions to lose charges should be avoided. TOF-MS equipped with a reflectron (refTOF-MS) maintained under high vacuum condition is one of the suitable instruments. However, another difficulty arises due to the working principle of refTOF-MS: The product ions whose m / z is greater than that of multiply charged precursor ions can pass through a reflectron under the experimental conditions for detecting the product ions of a singly charged precursor ion. In this study, we report the ionization of decafluorobiphenyl (DFB) by femtosecond laser pulses and the determination of the product ions emerged from doubly charged precursor ions using a refTOF-MS. Detection of ions behind a reflectron by varying the retarding potential of a reflectron enables us to identify the m / z of product ions in two ways: measurement of the threshold retarding potential reflecting product ion; comparing the relative flight time of the product ions obtained by experiments and ion trajectory simulations. We have reported three and one metastable dissociation channels of DFB+ and DFB2+, respectively, by a conventional product ion analysis, that is the selection of a precursor ion and its product ions using an ion gate followed by the reflection and separation of them by a reflectron. In this study, we further identified the products that passed through a reflectron: charge transfer product of C2+; the product ion of C 4 F 2 2+ which is a secondary product of DFB ion; the product ion of DFB2+. The detection of the product ions that passed through a reflectron expanded the measurable m / z range of product ions emerged from doubly charged precursor ions. [Display omitted] • Detection of product ion whose m / z was greater than that of the precursor ion. • Complementary detection of the product ions that were passed through and reflected by a reflectron. • Precision of ion trajectory simulation was confirmed by observing charge transfer reactions of Xe. [ABSTRACT FROM AUTHOR]

Laser-induced breakdown spectroscopy (LIBS) is a powerful method for non-invasive elemental composition monitoring which can be applied for processing graded metal alloys, provided that challenges with reliable and rapid spectral data processing are overcome to enable composition analysis in real-time. In this study, we describe an integrated experimental-computational methodology for analyzing LIBS spectra data obtained from graded composition metal alloy surfaces with a mean rate of composition analysis up to 15.7 Hz. The accuracy and speed of this method was assessed via analysis of a graded Al Cu film prepared by magnetron sputtering representing a gradient alloy processed by additive manufacturing. Using a pulsed femtosecond laser and gated sCMOS detector, a series of LIBS measurements were taken across the composition gradient of an Al Cu sputtered alloy. Energy Dispersive Spectroscopy (EDS) was used both to calibrate composition measurements and quantify the error of the LIBS-based rapid analysis method demonstrated in this paper, which yielded a mean percent error relative to the EDS data in the composition measurement along the gradient as low as 1.4 % when optimized for the cumulative number of probe laser pulses on a single measurement spot. The femtosecond probe pulse method described in this paper allows for the alloy composition to be accurately measured in hundredths of a second, potentially allowing for real-time composition monitoring in gradient alloy processing, including thin film sputtering, additive manufacturing, and other rapid processing technologies for graded metal alloys. • Rapid quantitative spectroscopic analysis method for composition measurement of graded alloys • Integrated computational-experimental approach for real time, in situ LIBS analysis • Accuracy validation of composition measurements using graded, sputtered Al Cu thin film • Optimization of number of cumulative laser pulses for reduced error in composition data [ABSTRACT FROM AUTHOR]

We investigate the random lasing properties of polycrystalline MAPbBr 3 grown in a hollow fiber. Coupling of the random lasing on the inner wall of the hollow fiber into the annular cylinder and guided by the inner surface of hollow cavity enable directional output of the random lasing emission. The polycrystalline MAPbBr 3 was grown by the capillary effect, when one end of the hollow fiber was dipped into the precursor solution. A two-photon pumping scheme was employed, where femtosecond pulses centered at about 800 nm was focused onto the MAPbBr 3 polycrystals through the side wall of the hollow fiber. Random lasing was achieved due to the strong optical scattering by the interfaces within the polycrystals on the inner wall of the hollow fiber. The random lasing output was re-collimated into a roughly parallel beam with a circular transverse mode, which favors far-field applications of random lasers. Two-photon pumping enables larger penetration depth and larger excitation volume than single-photon pumping, so that strong lasing spot can also be observed in the center area of the output laser beam. • Growth of polycrystalline layer of MAPbBr 3 on the inner wall of a hollow fiber with optimized precursor solution for the fabrication of a random laser device. • Recollimated laser beam was achieved for the random lasing emission inside a hollow fiber, where spatial confinement and direction guiding by the hollow fiber are the responsible mechanisms. • Random lasing beam with a small divergence and regular transverse mode is very important for the practical far-field applications. • Two-photon pumping scheme was employed for such a random laser device, where large thickness of the active medium allows large penetration depth of the pump laser. [ABSTRACT FROM AUTHOR]

Femtosecond laser ablation associates with a series of multiphysics phenomena (electron thermalization, electron thermal conduction, electron-phonon coupled energy transport, melting, vaporization and phase explosion) and the instant variation of material transient properties, which impacts the laser energy absorption and the subsequent material removal. In this work, a multiphysics model with material transient properties is developed to study the femtosecond laser ablation of Ti6Al4V. A two-dimensional axisymmetric simulation is performed. The predicted ablation depth and ablation diameter with material transient properties show good agreements with the experimental data. The necessity of material transient properties for femtosecond laser ablation is demonstrated. Besides analyzing the roles of material transient properties on femtosecond laser ablation, the ablation thresholds of Ti6Al4V are probed for different pulse durations. In addition, the impact of pulse duration on femtosecond laser heat affected zone is studied. This work contributes to understand roles of material transient properties in femtosecond laser ablation. • A multiphysics model of femtosecond laser ablation is developed. • Material transient properties in femtosecond laser ablation are included. • Impacts of material transient properties are quantitatively studied. • Optical penetration depth is found as the dominant transient material property. • Heat affected zones at different laser pulse durations are compared. [ABSTRACT FROM AUTHOR]

• Laser-induced oxidative or laser ablative LIPSS can be formed on the thin films by Gaussian pulses or femtosecond laser pulse trains, depending on laser fluence, scanning speed, and pulse duration. • The range of laser fluence thresholds for oxidative nanostructure formation by femtosecond laser train is significantly wider compared to Gaussian pulses. • Simulation results reveal that the periodic energy distribution persists for an extended duration when using pulse trains, creating more favorable conditions for the formation of periodic oxidative nanostructures. Laser-induced periodic surface structure is a universal phenomenon observed when solid materials are irradiated with pulsed lasers. Typically, these structures are generated using Gaussian pulses. Here, we investigate the formation of periodic nanostructures on titanium nitride thin films using two different laser sources: Gaussian pulses and femtosecond pulse trains generated through a Fabry-Perot cavity. Our findings indicate that the formation of self-organized nanostructures on the thin films can occur through either laser-induced oxidation or laser ablation processes, depending on laser fluence, scanning speed, and pulse duration. Interestingly, when utilizing femtosecond pulse trains, we observe that the range of laser fluence thresholds for oxidative nanostructure formation is significantly wider compared to Gaussian pulses. To comprehend the underlying mechanisms driving these observations, we employ a two-temperature model to calculate the ultrafast dynamics of hot electrons when exposed to laser irradiation. Our results reveal that the initial sub-pulse generates a substantial population of hot electrons, exciting surface plasmons and resulting in the creation of a periodic energy distribution. Before these hot electrons can dissipate, subsequent sub-pulses reach the thin film, re-exciting a large number of hot electrons. Consequently, the periodic energy distribution persists for an extended duration, creating more favorable conditions for the formation of periodic oxidative nanostructures when using pulse trains. These findings contribute to a comprehensive understanding of the mechanisms involved in the formation of periodic nanostructures under femtosecond laser irradiation. [ABSTRACT FROM AUTHOR]

• The written CFBG achieves an average reflectivity of 98.2%, and demonstrates an ultra-flat reflection spectrum. • The high group velocity dispersion (GVD) of 60.7 ps2 signifies its potential in fiber chirped pulse amplification systems. • The fs-laser+PM writing method shows advantages of high efficiency and low energy consumption. Chirped fiber Bragg gratings (CFBGs) have been extensively used in applications such as ultrafast lasers, fiber sensors, and fiber communications. This work presents a comprehensive investigation of CFBG written using a 257 nm femtosecond laser and a linear chirped phase mask. The CFBG, written on a hydrogen-loaded polarization-maintaining fiber, exhibited a center wavelength of 1031.7 nm and a bandwidth of 11.97 nm. The CFBG achieved an average reflectivity of 98.2 % within its bandwidth, demonstrating an ultrahigh flatness of the reflection spectrum with a low spectral reflectivity variance of 0.00003 (σ2). The high group velocity dispersion (GVD) of 60.7 ps2 signifies its potential in fiber chirped pulse amplification systems and dispersion compensation system. Meanwhile, the fs-laser + PM writing method shows advantages of high efficiency and low energy consumption (as low as 2 nJ), owing to the high photosensitivity of the fiber to the UV band, which holds significant promise in fiber grating inscription. [ABSTRACT FROM AUTHOR]

• The morphology and structural transformation of 6H-SiC under irradiation of single pulse with varying energies was investigated. Three distinct modification regimes were identified based on pulse energy. • At low pulse energies just above the modification threshold, the sample surface undergoes moderate modification, no larger craters and amorphous silicon are produced. At even higher energies, nonlinear absorption and ionization occur under ultrafast laser irradiation, Si-C crystal bonds are broken and melting, resolidification and the generation of amorphous and crystalline silicon phase appeared. • Tunneling ionization and avalanche ionization are responsible for the generation of free electrons in SiC sample and melting/resolidification happens when a large portion of energy is transferred from electrons to the lattice. Silicon carbide (SiC) is promising in semiconductor devices operating in extreme environments owing to its excellent properties including wide forbidden band, high thermal conductivity, thermal stability, chemical inertness, and high saturation rate. However, its Mohs hardness of 9 poses challenges for conventional machining techniques. Femtosecond laser processing offers an effective alternative for SiC processing, yet the underlying mechanism of interaction between laser pulse and SiC remains unclear. Herein we studied the mechanism of laser interacted with SiC, thoroughly investigated the structural transformation and morphology change of single crystal 6H-SiC induced by single ultrafast laser pulse irradiation at different pulse energies. The morphology changes of single pulse induced crater, i.e., diameter and depth, are characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The structural transformation and changes in the chemical composition are analyzed using micro-Raman spectroscopy. Moreover, the evolution of lattice arrangement is revealed by high-resolution transmission electron microscopy (TEM). At low pulse energies just above the modification threshold, the sample surface undergoes moderate modification, no larger craters and amorphous silicon are produced. At even higher energies, nonlinear absorption and ionization occur under ultrafast laser irradiation, the energy of the hot free electrons is transferred to the cold crystal lattice through electron–phonon scattering, causing a sharp increase in the lattice temperature. Si-C crystal bonds are broken and melting, resolidification and the generation of amorphous and crystalline silicon phase appeared. [ABSTRACT FROM AUTHOR]

Femtosecond laser pulses were spatially modulated with fast reconfiguration of computer-generated holograms (CGHs) using a ferroelectric liquid-crystal-on-silicon spatial light modulator (FLCoS-SLM). Reconfiguration was executed at a frame rate of more than 1 kHz while changing the CGH every frame. The CGHs were designed as binary phases because the FLCoS-SLM can represent only binary values. The optimization method was modified by the weighted iterative Fourier transform algorithm (IFTA) to make the binary phase. The fundamental performance of the CGH reconstruction and femtosecond laser processing with 6.3 kHz pulse-to-pulse spatial light modulation was demonstrated. The reconfiguration speed will drastically change the applicability to science and industrial fields and open a door for new CGH applications. • Femtosecond pulses were modulated with CGH reconfiguration of 6.3kHz using FLCoS-SLM. • To our knowledge, this is the highest frame rate for laser processing using LCoS-SLM. • To make the binary phase mask, the weighted IFTA was modified. • The fast reconfiguration speed will be useful for next-generation laser processing. [ABSTRACT FROM AUTHOR]

[Display omitted] • Irradiation of crystalline Si and Ge with single 400 nm, 100 fs laser pulses. • fs-resolved optical microscopy over a wide temporal window, up to 15 ns. • Unravel dynamics of free-electron plasma formation, non-thermal melting, ablation. • At high fluences, anomalous transient high-reflectivity state. • Indicative of recoil pressure-induced liquid–liquid phase transition. 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 high-reflectivity 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. [ABSTRACT FROM AUTHOR]

This paper presents the study and implementation of a modified Z-scan technique using a chopper to change the thermal load of the sample, while keeping the input peak irradiance constant. It was verified numerically and experimentally that by changing the frequency of the chopper and keeping the power of the laser beam constant, it is possible to vary the thermal load on the sample. This technique is illustrated under the study of the nonlinear optical (NLO) properties of nucleated Au–Pt metallic nanoparticles (NPs) by ion implantation. The nonlinear optical response was studied using the Z-scan. technique using a Ti:Sapphire laser with femtosecond pulses at a wavelength of 800 nm and a repetition rate of 76 MHz. By scanning the sample using the Z-scan technique it was It is possible to determine the refractive and absorbing contributions to the nonlinear response, in this case as a consequence of a high pulse repetition rate, cumulative pulse-to-pulse thermal effects occurred. Because of this, a modification of the Z-scan technique allowed to separate the resolution of the electronic and thermal contributions on the sample. • We present a technique that allows the determination of the electronic and thermal contributions to the third-order nonlinear optical response of a material, through the use of a variable frequency optical chopper in the z-scan technique. • We present a complete calculation of the heating produced in a sample by a femtosecond pulse train of finite duration. • The technique is illustrated by a study of the nonlinear response of a dual system of metallic nanoparticles (NPs) consisting of Au and Pt NP distributions embedded at different depths in a glass substrate, which was produced by ion implantation, using a femtosecond laser oscillator. • The technique allowed us to extract the thermal and electronic contributions to the nonlinearity of the sample studied, together with some thermal parameters. [ABSTRACT FROM AUTHOR]

Compared to conventional oven-based annealing in ambient air of a silicon surface layer nanotextured and hyperdoped in CS 2 fluid by femtosecond laser (black nano-silicon), low-intensity nanosecond laser annealing facilitated its delicate recrystallization, preserving quasi-regular nanoscale topography and high n-doping concentration by sulfur donor impurities. The retained and recrystallized topography of the laser-annealed nano-silicon with a few-percent doping level exhibits broadband optical absorption in the visible−near-infrared (vis−NIR) range, rendering this optical material promising for photovoltaic and IR-photonic applications. • Nanosecond laser annealing of a silicon, nanotextured and hyperdoped in CS 2 fluid by femtosecond laser carried out. • Two-wavelength scanning Raman microspectroscopy indicated almost complete recrystallization of the hyperdoped nanotexture. • The laser-annealed nano-silicon exhibited broadband (mid-IR-vis) reduced reflectance and transmittance. [ABSTRACT FROM AUTHOR]

• Building an all-fiber watt-level femtosecond laser pumping source using chirped pulse amplification and fiber compression. • Bridging high numerical aperture fibers for low-loss fusion splicing. • Utilizing Ge-doped fiber as a nonlinear medium to realize a watt-level high-flatness supercontinuum source. • The output supercontinuum pulse is a noise-like pulse with a tip pulse duration of only 62 fs. • The whole supercontinuum laser system is characterized by compactness and high stability. The high-power ultra-flat supercontinuum (SC) sources play an important role in various fields. In this letter, we experimentally demonstrated an all-fiber, high-power, ultra-flat SC source in GeO 2 highly nonlinear fiber (HNLF) pumped by a femtosecond fiber laser system. The system is mode-locked by a semiconductor saturable absorber mirror with an all-polarization-maintaining (PM) structure. Based on the technology of chirped pulse amplification, a maximum power of 3.12 W was realized with a repetition rate of 80 MHz at 1.56 μm, and the pulse duration was compressed to 124 fs in a segment of PM single-mode fiber. Furthermore, by optimizing the length of the HNLF, a wideband SC spectrum of nearly 700 nm was obtained at10 dB with a maximum power of 1.82 W, and the flatness of the spectrum was better than 5 dB in the range of 1600–2200 nm. We observed that the pulse evolved into a noise-like pulse with a narrowest pulse duration of 62 fs. Thanks to the all-PM structure of the fiber laser, the long-term stability of SC power was manifested with a relative fluctuation as low as 0.53 % over 6 h. [ABSTRACT FROM AUTHOR]

We report a 790–1000 nm tunable femtosecond laser source by second harmonic generation (SHG) of a 1580–2000 nm soliton self-frequency shift (SSFS) fiber laser. The SSFS fiber laser with an all polarization-maintaining fiber configuration consists of passively mode-locked figure-9 Er-doped fiber oscillator, two-stage fiber amplifiers and a nonlinear fiber, delivering femtosecond Raman solitons in a tunable range of 1580 to 2016 nm. The tunable Raman solitons are further injected into a specifically-designed chirped periodically-poled lithium niobite (CPPLN) crystal for efficient SHG, and therefore femtosecond pulses with wide wavelength tuning from 790 to 1000 nm are obtained. The shortest pulse duration is < 100 fs at 890 nm, and the average power is 7.55 mW with a repetition rate of 44.9 MHz. This is, to the best of our knowledge, the first demonstration of a < 1000 nm broadband tunable fiber-based femtosecond source almost covering the whole short-wavelength near-infrared spectrum, which may have important applications in the nonlinear microscopy, biomedical imaging, and material processing. [ABSTRACT FROM AUTHOR]

Due to its extremely short timescale, the non-equilibrium melting of metals is exceptionally difficult to probe experimentally. The knowledge of melting mechanisms is thus based mainly on the results of theoretical predictions. This work reports on the investigation of ultrafast melting of thin polycrystalline Pd films studied by optical laser pump – X-ray free-electron laser probe experiments and molecular-dynamics simulations. By acquiring X-ray diffraction snapshots with sub-picosecond resolution, we capture the sample's atomic structure during its transition from the crystalline to the liquid state. Bridging the timescales of experiments and simulations allows us to formulate a realistic microscopic picture of the crystal-liquid transition. According to the experimental data, the melting process gradually accelerates with the increasing density of deposited energy. The molecular dynamics simulations reveal that the transition mechanism progressively varies from heterogeneous, initiated inside the material at structurally disordered grain boundaries, to homogenous, proceeding catastrophically in the crystal volume on a picosecond timescale comparable to that of electron-phonon coupling. We demonstrate that the existing models of strongly non-equilibrium melting, developed for systems with relatively weak electron-phonon coupling, remain valid even for ultrafast heating rates achieved in femtosecond laser-excited Pd. Furthermore, we highlight the role of pre-existing and transiently generated crystal defects in the transition to the liquid state. [Display omitted] [ABSTRACT FROM AUTHOR]

This work demonstrates for the first time the formation of nanogratings inside Barium Gallo-Germanate (BGG) glasses using femtosecond laser direct writing. These heavy oxide glasses, with a characterized transparency window up to 6 μ m, along with the polarization sensitive birefringent nanostructures induced by femtosecond laser, offer an excellent platform for the development of mid-Infrared photonic applications. The fabricated glasses, containing both rare earth elements (Gd, Y) and transition metals (Zn, Ta), are directly compared to amorphous GeO 2 and commercial alumino-germanate (Corning-9754) glasses commonly used in this field. The nanogratings processing window in each glass was investigated by varying laser parameters (pulse duration, energy, repetition rate), and electron microscopy imaging directly evidenced the porous nanostructures. Among the results, a strong form birefringence in 8% ZnO containing BGG glass, e.g., up to −0.029 ± 0.001, was reported. Finally, birefringent embedded Fresnel GRIN plates were directly written in a BGG glass, to demonstrate the functionalization potential of these materials. [ABSTRACT FROM AUTHOR]

Isopenicillin N synthase (IPNS) catalyzes formation of the β-lactam and thiazolidine rings of isopenicillin N from its linear tripeptide L-δ-(α-aminoadipoyl)-L-cysteinyl-D-valine (ACV) substrate in an iron- and dioxygen (O2)-dependent four-electron oxidation without precedent in current synthetic chemistry. Recent X-ray free-electron laser studies including time-resolved serial femtosecond crystallography show that binding of O2 to the IPNS--Fe(II)--ACV complex induces unexpected conformational changes in α-helices on the surface of IPNS, in particular in α3 and α10. However, how substrate binding leads to conformational changes away from the active site is unknown. Here, using detailed 19F NMR and electron paramagnetic resonance experiments with labeled IPNS variants, we investigated motions in α3 and α10 induced by binding of ferrous iron, ACV, and the O2 analog nitric oxide, using the less mobile α6 for comparison. 19F NMR studies were carried out on singly and doubly labeled α3, α6, and α10 variants at different temperatures. In addition, double electron--electron resonance electron paramagnetic resonance analysis was carried out on doubly spin-labeled variants. The combined spectroscopic and crystallographic results reveal that substantial conformational changes in regions of IPNS including α3 and α10 are induced by binding of ACV and nitric oxide. Since IPNS is a member of the structural superfamily of 2-oxoglutarate-dependent oxygenases and related enzymes, related conformational changes may be of general importance in nonheme oxygenase catalysis. [ABSTRACT FROM AUTHOR]

Silicon carbide (SiC) is an attractive semiconductor material for devices that operate under extreme conditions. However, its high hardness and chemical stability hinder micromachining. Femtosecond laser has been proposed extensively as an effective micromachining tool for SiC. However, the fundamental mechanisms of laser-material interaction during femtosecond laser irradiation process remain elusive. This paper presents a comprehensive study of the structural transformation and residual stress induced by irradiating a 4H–SiC target with a single-pulse femtosecond laser. The energy dependence of the structural characteristics at the spot center and the spatial distribution across the laser spot were determined using optical microscopy and micro-Raman spectroscopy. The effect of the laser fluence on the residual stress was discussed. The obtained results showed that no structural changes occurred at low energy, whereas bond breaking occurred among crystalline SiC (c-SiC) at high energy. A central disk with no structural transformation and no residual stress was formed for the fluence of 22.2 J/cm2, owing to phase explosion-induced spluttering. Meanwhile, an inhomogeneous distribution of the structural transformation was generated from the spot centre to the edge. Based on this, threshold fluences for modification and structural transformation were proposed and calculated. The fundamental mechanisms for different laser-fluence regimes are discussed, and some suggestions for improving the surface quality are put forward. This study provides deep insights into the laser-material interaction mechanisms and is beneficial for optimising the utilisation of femtosecond laser for 4H–SiC micromachining. • Structural characteristics and residual stress at the spot centre depend on laser energy. • Spatial distribution of structural transformation from the spot centre to the edge is inhomogeneous. • Threshold fluences for modification and structural transformation are proposed and calculated. • The fundamental mechanisms of laser-material interaction for different laser-fluence regimes are analysed. • The residual stress in central disk region vanishes for the laser fluence of 22.2 J/cm2, owing to explosive boiling. [ABSTRACT FROM AUTHOR]

Nitrogen doping and amino-group functionalization through chemical modification can engender enhance electron donation. Using a homemade femtosecond titanium:sapphire laser optical system operated at a low energy and short photoexcitation time [power: 200.0 nJ pixel−1; scanning frequency: 150 scans (∼0.80–1.33 s); excitation wavelength: 870, 910, or 970 nm], this study conducted these processes on the large π-conjugated system of graphene quantum dot (GQD)-based materials functioning as electron donors; this improved the efficiency of charge transfer to the prepared amino-N-GQDs, resulting in enhanced two-photon absorption, excitation-wavelength-independent photoluminescence (EWI-PL), radiative efficiency, excitation absolute cross section, and excitation from the near-infrared (NIR)-I to the NIR-II region. The lifetime decreased and quantum yield (QY) increased. The sorted amino-N-GQDs exhibited two-photon EWI-PL emissions between the ultraviolet and NIR-I regions, leading to the generation of reactive oxygen species (ROS), which functioned as two-photon photosensitizers for photodynamic therapy (PDT). Increasing the mean lateral size of the particles increased the N-functionality-dependent photochemical and electrochemical activities, which improved the PL QY and thus enhanced the efficiency of two-photon PDT. Additionally, a polystyrene sulfonate (PSS) coating was applied (forming sorted amino-N-GQD-PSS); the sulfur atoms introduced by the PSS induced the radiative recombination of localized electron–hole pairs and then improved the sorted amino-N-GQD surfaces by strengthening the quantum confinement of their emissive energy. Thus, compared with the sorted amino-N-GQDs that were not coated, the PSS-coated materials had superior two-photon properties. No ROS were detected. Accordingly, these materials are suitable for use as two-photon contrast probes for analyte tracking and localization. [Display omitted] [ABSTRACT FROM AUTHOR]

Custom shaped magnetic flux guiding channels have been fabricated on superconducting Nb thin films by laser nanopatterning of their surface. Preferential pathways are defined by suitable combination of imprinted anisotropic pinning domains through laser-induced periodic surface structures (LIPSS). Generated by the selective energy deposition of femtosecond UV laser pulses, quasi-parallel ripple structures are formed under optimized irradiation conditions. On average, each domain is formed by grooves with a lateral period of 260–270 nm and a depth about 80 nm. By combination of scanning and transmission electron microscopy, magneto-optical imaging, and conductive atomic force microscopy techniques, we conclude that the boundaries of the LIPSS-covered domains play a prominent role in the magnetic flux diversion process within the film. This is confirmed by dedicated modeling of the flux dynamics, combined with the inversion of the magneto-optical signal. The created metasurfaces enable control of the flux penetration process at the microscale. [Display omitted] • Laser-induced surface domains of periodic nanostructures are created on Nb thin films • Controlled boundaries between domains allow to manipulate the magnetic flux dynamics • MOI reveals magnetic flux dynamics features as: diversion, steering, and parceling • Micromagnetic modeling discloses the relation between structure and magnetic behavior [ABSTRACT FROM AUTHOR]

• We demonstrate a novel line-by-line (LbL) WBGs written by femtosecond laser in bulk glass for the first time. The Bragg resonance reaches 25 dB in a length of only 2 mm, which is the strongest to our knowledge for a second-order WBG inscribed in bulk glass. • Due to the relatively small pulse energy of the femtosecond laser used for inscribing, the LbL grating has relatively low insertion loss of 0.3 dB. • We show the different-order and phase-shifted LbL gratings in this work, demonstrating the universality and flexibility of this LbL method for fabricating gratings. • This LbL WBG has a strong polarization dependence, the birefringence is 5.74 × 10−4. A novel line-by-line (LbL) waveguide Bragg grating (WBG) written by femtosecond laser in bulk glass is demonstrated for the first time. The Bragg resonance reaches 25 dB in a length of only 2 mm, which is the strongest to our knowledge for a second-order WBG inscribed in bulk glass. Due to the use of relatively small pulse energy, the insertion loss caused by the 2 mm LbL grating is only 0.3 dB. Different-order WBGs and phase-shifted WBGs have been realized to prove the applicability of the LbL method. This method demonstrates the potential for fabricating compact and low-loss strong grating devices. [ABSTRACT FROM AUTHOR]

• Demonstration of low-noise, continuous-wave mode-locking of a Yb:KLuW waveguide laser operating at GHz repetition rates. • Femtosecond pulse generation employing single-walled carbon nanotubes and semiconductor saturable absorbers. • Experimental and theoretical investigation of timing noise characteristics of waveguide lasers. We demonstrate low-noise, continuous-wave mode-locking of a diode-pumped Yb:KLuW waveguide laser operating at fundamental repetition rates in the GHz range. Utilizing a femtosecond-laser-inscribed Yb:KLuW channel waveguide laser with a tunable cavity length of a few centimeters, we successfully generate femtosecond pulses near 1030 nm, employing either single-walled carbon nanotubes or semiconductor saturable absorbers. Although timing noise is one of the most critical factors in the implementation of high-repetition-rate laser sources for various applications, investigations into timing noise characteristics in waveguide lasers remain limited. This study fills this gap by presenting timing jitter measurements for GHz mode-locked waveguide lasers with diverse cavity parameters. To quantitatively analyze noise spectral characteristics, including relaxation oscillation processes induced by intensity noise features, we introduce a theoretical modeling approach. The present investigation directly enables the provision of strategies for further suppression of timing jitter and designing high-performance, low-noise mode-locked lasers. [ABSTRACT FROM AUTHOR]

A Two-photon 3D printing Fabry-Perot (FP) sensor for simultaneous measurement of temperature and non-contact pressure is proposed and verified by experiments. A fiber Bragg grating (FBG) was written into a single mode fiber (SMF) core near the microprobe by femtosecond laser micromachining. FP demonstrates sensitivity to both pressure and temperature, while FBG is only responsive to temperature, so FBG can be used to compensate for temperature crosstalk. The cavity length of the FP microprobe is 45 μm, the outer diameter is 100 μm, the wavelength of the FP microprobe is near 1602 nm, and the pressure sensitivity of the FP microprobe can reach 6.6649 nm/MPa in the range of 0 –1 MPa, remaining stable at 6.4941 nm/MPa under repetitive experiments. In the range of 5 –35 °C, the temperature sensitivity of FP microcavity is 0.105 nm/°C, and the temperature sensitivity of FBG is 9.5 pm/°C. The sensitivity matrix method can realize the two-parameter accurate measurement of both non-contact pressure and temperature. [ABSTRACT FROM AUTHOR]

• Homogeneous LSFL (H-LSFL) covered with HSFL can be generated on 4H-SiC. • Separated line segments are produced in a single scanning path by a vector beam. • The formation of the HSFL on H-LSFL is enhanced by a beam expander. Femtosecond lasers are widely recognized for generating laser-induced periodic surface structures (LIPSS) on various materials. Moreover, the homogeneous low spatial frequency LIPSS (H-LSFL) can be achieved by employing linear scanning with polarized femtosecond laser pulses and precise control of parameters, such as fluence and scanning speed. Unlike traditional LSFLs, these H-LSFLs are characterized by uniform structures. This study demonstrates the high-precision fabrication of hybrid periodic nanostructures, specifically H-LSFL with vertically aligned HSFL (high-spatial frequency LIPSS) on 4H-SiC using azimuthally polarized femtosecond laser beams. The experimental results revealed that the rapid fabrication of two parallel line patterns featuring hybrid periodic nanostructures can be achieved in a single scanning pass. Furthermore, a beam expander was employed to reduce the laser's focal spot size, increasing the distribution of HSFL concerning H-LSFL as the distance between line segments decreased. [ABSTRACT FROM AUTHOR]

The demand for precise and high-quality machining of wide-bandgap materials, such as glasses and crystals, is of considerable significance in science and industry. Among these materials, sapphire stands out as an appealing choice due to its exceptional mechanical and optical attributes, high thermal conductivity and stability, low electrical conductivity, and resilience against harsh chemicals. Despite its hardness, sapphire is brittle, making it prone to cracking during conventional machining attempts. Recently, alternative non-contact approaches, like laser ablation, have emerged as potential solutions to improve the machining quality. However research of laser processing of wide-bandgap materials, especially utilizing the high harmonics of femtosecond solid-state laser systems, remains incomplete. Our study focuses on investigating the nonthermal laser ablation of c-cut sapphire crystals using femtosecond (300 fs) deep UV (206 nm) laser pulses and comparing the results with traditional IR femtosecond ablation. The publication encompasses a comprehensive depiction of the ablation process and a review of the various achieved morphologies with accompanying scanning electron microscope images. Our findings indicate that efficient ablation with surface roughness below 100 nm can be achieved through a single-step process within specific laser processing parameter ranges. The ablation process of sapphire encompasses a strong incubation effect, hence the pulses need to be tightly overlapped. Additionally, we provide a detailed description of methodology used to extract surface roughness which was utilized in all the presented research and offers a practical framework for characterizing ablation results obtained from diverse laser systems. • Deep ultraviolet femtosecond pulses minimize thermal effects in sapphire ablation. • Ultraviolet wavelengths surpass infrared for achieving low surface roughness. • The ablation process exhibits a significant incubation effect. • Efficient ablation regime produces deep structures with low roughness. • Ablation originates from impurities and breakup mechanism is photomechanical. [ABSTRACT FROM AUTHOR]

• Ablation threshold of graphite for different number of pulses. • Surface morphology of created laser spots on graphite electrode. • Low spatial frequency laser induced periodic surface structures. • High spatial frequency laser induced periodic surface structures. • Formation of Nanoparticles and Filaments by laser ablation. This study investigates the effects of femtosecond laser irradiation on graphite, a highly sought-after material known for its exceptional chemical stability and ability to withstand extreme temperatures. The ablation threshold and surface morphology of graphite were determined using a tightly focused laser beam with varying pulse numbers and laser fluences. The results showed that the laser beam produced diverse morphological structures within different regions of a single laser spot, and the ablation threshold and surface morphology were significantly influenced by the number of pulses and the energy density of the incident laser. The study found that the ablation threshold of graphite was 107, 90, 76, and 56 ± 0.5 mJ/cm2 for 5, 50, 200, and 500 laser pulses, respectively. The resulting surface structures ranged from quasi-periodic to periodic, with varying periodicities, and the ablation debris exhibited morphologies from fine particles to long filaments. The crystal structure of the specimen was analyzed via X-ray diffraction and micro-Raman spectroscopy, with compositional analysis performed using energy-dispersive X-ray spectroscopy (EDX). Notably, a significant broadening of the Raman spectrum peaks with increasing pulse energy suggested the formation of a more disordered structure. These findings demonstrate that femtosecond laser ablation can precisely manipulate the surface morphology and nanoparticle production, presenting potential applications in fields such as nanoparticle generation and engineering surface properties like adhesion, biocompatibility, spectral reflectivity, wear resistance, and wettability. [ABSTRACT FROM AUTHOR]

• Efficiency of THz generation by the plasma of femtosecond optical filaments in air depends on the filament spatial structure; • THz yield from laser filaments is 1.5-fold increased by a thin turbulent layer of heated air placed in the femtosecond optical beam; • The effect is theoretically explained on the base of incoherent superposition of multiple linear dipole sources; Remote generation of electromagnetic waves in the terahertz (THz) by the volume of air medium gains increasing scientific interest due to the demand for this spectral range in solving various practical problems including those belonging to the nonlinear atmospheric optics. During self-focusing and filamentation of high-power femtosecond laser pulses in air, the source of THz radiation is the plasma of laser filaments. One of the main challenges of THz generation by a laser filament is increasing the value of optical energy conversion to the long-wavelength band. In this paper, we present the results of our studies on DC-biased THz generation during single-color filamentation in air of focused femtosecond Ti:Sapphire-laser pulses (800 nm, pulse energy up to 40 mJ). The distinctive feature of our study is that a femtosecond optical pulse propagates through a spatially-localized heated air layer containing randomly inhomogeneous refractive index perturbations, which mimics strong air turbulence. For the first time to our knowledge, we show, that the turbulent air layer formed at the beginning of the optical path allows increasing the yield of THz radiation up to 1.5 times due to the formation of multiple chaotically-arranged filaments resulting from random perturbations of the optical beam energy profile that reduces the subsequent THz (re)absorption in the filament plasma. [ABSTRACT FROM AUTHOR]

We characterize the polarization state of few-femtosecond laser pulses by implementing the dispersion scan technique in combination with the tomographic ultrafast retrieval of transverse light E-fields procedure (D-TURTLE). We demonstrate, both numerically and experimentally, that D-TURTLE unambiguously identifies the pulse ellipticity, its handedness and which polarization component travels ahead. Given the robustness of this technique, and its self-referenced and in-line nature, D-TURTLE can be extremely useful to characterize polarized few-cycle laser pulses. • Technique to measure polarization state of few-cycle laser pulses. • The inline self-referenced d-scan technique is combined with TURTLE (D-TURTLE). • Simulations and experiments demonstrate the robustness and generality of D-TURTLE. • D-TURTLE unambiguously identifies the ellipticity and handedness of polarized pulses. • D-TURTLE even finds the relative temporal position of components separated in time. [ABSTRACT FROM AUTHOR]

A broadband two-photon fluorescence (TPF) imaging system, based on a femtosecond oscillator, has been constructed. This system enables TPF imaging of rabbit mammary gland tissue samples by detecting TPF signals generated from the tissue upon excitation by broadband femtosecond pulses. Specifically, TPF imaging utilizes a detection window of 565–615 nm to capture TPF signals from the fluorescent adenine dinucleotide (FAD) present in lactating cells within breast tissue. Moreover, the 3D reconstruction algorithm has been optimized to visualize the 3D structure of the rabbit mammary gland acinar cavity.The optimized three-dimensional reconstruction algorithm can reduce ambiguous signal regions, making the reconstructed three-dimensional structure more reliable and accurate. This is of significant importance for studying the microscopic structure and cell distribution of breast tissue, contributing to a deeper understanding of the tissue's physiological status. [Display omitted] [ABSTRACT FROM AUTHOR]

Millimeter-scale temperature and strain gradient measurements are crucial for understanding the mechanical properties of buoyancy materials. In this paper, we successfully fabricated ultrashort and high-density fiber Bragg grating (FBG) array based on femtosecond laser point-by-point writing technology. By optimizing the energy and number of femtosecond laser pulse, and introducing secondary tilt apodization technology, submillimeter-spatial-resolution FBG arrays were prepared. To the best of our knowledge, this is the first application of femtosecond FBG array for high spatial resolution temperature and strain gradient in situ monitoring of buoyancy materials. FBGs exhibit excellent anti-chirp performance and achieve submillimeter spatial resolution curing monitoring under temperature and strain monitoring gradients of 2.6 °C/mm and 231 με/mm, respectively. • Tilt apodization technique to increase the side mode suppression ratio and the reflectivity of fs-FBG. • First application of fs-FBG arrays for temperature and strain gradients monitoring during the curing process of buoyancy materials. • Submillimeter-scale temperature and strain gradient measurement. [ABSTRACT FROM AUTHOR]

The Modulation Transfer Function (MTF) offers an objective measure to assess the imaging performance of streak cameras. This study aims to enhance the diagnostic precision of streak camera used in laser-driven inertial confinement fusion research. We introduce a novel method for evaluating the MTF of a recently developed large imaging area streak tube. This method integrates the electron optical MTF theory and the production of a reticle plate-type photocathode. Using ultraviolet and femtosecond lasers, we established two calibration platforms for the streak camera in both static and dynamic modes. We measured the radius of the electrons reaching fluorescent screen and obtained MTFs. Our findings reveal that the ultimate spatial resolutions at off-axis distances of 1.2, 4.7, 8.2, and 11.7mm are 28.5, 27.2, 26.4, 24.9 lp/mm (static) and 25.0, 22.8, 23.0, 21.2 lp/mm (dynamic). Notably, we successfully charted the ultimate spatial resolution distributions across the streak camera's full imaging area in both modes. • A novel method for calibrating the MTF of a large imaging area streak tube is reported. • The radius of the electrons reaching fluorescent screen and the static and dynamic MTFs are measured. • The ultimate spatial resolution distributions across the streak camera's full imaging area in both image modes are obtained. [ABSTRACT FROM AUTHOR]

• Laser ignition of a lean methane/air mixture by dual-color femtosecond pulses is investigated. • A strong wavelength effect in fs laser ignition (fs-LI) is found. • The role of the ionization enhancement in the wavelength effect is clarified. • The mechanism of fs-LI is ascribed to the non-resonant photochemical ignition. Laser ignition (LI) is promising for green combustion of lean-fuel mixtures with controllable ignition timing and location. It was recently discovered that despite the inferior energy deposition and low thermal temperature in femtosecond (fs) laser-induced plasma, fs laser pulses can achieve a robust ignition of lean-fuel mixture through forming a "line" kernel by filamentation. Here, to clarify fs-LI mechanism, we investigated a dual-color (DC: 800 nm at 1.5 mJ and 400 nm at 0.43 mJ, ∼50 fs) fs-LI of a lean methane/air mixture with an equivalence ratio of φ = 0.87. An optical emission spectroscopy study was conducted to probe the N 2 + and OH emissions and characterize the ignition success rate. It was demonstrated that fs-LI can be achieved at a lower minimum ignition energy (MIE) (<0.46 mJ) by the DC scheme than that (>0.7 mJ) by a single-color (SC: 800 nm at 2.0 and 2.4 mJ, ∼50 fs) scheme, indicating a strong wavelength effect on the successful ignition. A pump-probe measurement was carried out to reveal the effect of the ionization enhancement on the successful ignition. It was found that only when the two-color fs pulses are temporally overlapped, the OH yield is strongly enhanced and the MIE is decreased. By comparing the variation trend of the fluorescence intensity of OH with that of the direct ionization product N 2 +, we ascribed fs-LI to a non-resonant photochemical ignition mechanism, in which the enhancement in the multiphoton/tunnel ionization of the lean-fuel mixture by the high-energy 400-nm photon can increase the yields of the reactive radicals through various dissociation and chain reaction pathways, and thus result in the successful ignition at the micro-joule level. This work unravels the essential role of the non-resonant photochemical ignition mechanism in fs-LI, and provides a promising route for the ignition of lean-fuel engines by compact ultrashort-pulsed lasers in the filamentation regime. [ABSTRACT FROM AUTHOR]

[Display omitted] • The possibility of generating hydrophobic surfaces on Ni-Mn-Ga-based alloys is studied for the first time. • Micro- and nanostructures generated with varied laser parameters are characterized. • A novel multi-stage process to generate hydrophobic surfaces has been introduced. • The potential for generating metasurfaces with controllable hydrophobicity using the shape memory function is explored. • These findings are crucial for developing multifunctional devices in fields like microfluidics for drop manipulation. The generation of hydrophobic surfaces through laser ablation has garnered considerable attention, particularly for its prospective diverse applications across various industries. This study explores the possibility of generating controllable hydrophobic metasurfaces on Ni-Mn-Ga-based magnetic shape memory (MSM) alloys using femtosecond pulse width laser (FPWL). While hydrophobic surfaces have been achieved on different materials through a variety of different techniques, the research marks the first systematic attempt to tailor hydrophobicity on the surface of Ni-Mn-Ga-based alloys. By characterizing surfaces treated with different laser parameters, the distinct morphologies and hydrophobic properties corresponding to each surface were identified. This newfound control over surface properties with specific machining parameters opens possibilities for applications in microfluidic devices. Additionally, the potential of utilizing the magnetic-field-induced strain (MFIS) exhibited by Ni-Mn-Ga single crystals to alter surface hydrophobicity was explored. Metasurfaces mimicking the dimensional changes in elongation induced by MFIS demonstrated higher static contact angles (SCAs) for water droplets compared to the original surfaces. This approach presents a promising avenue for creating multifunctional microdevices with controllable hydrophobicity using Ni-Mn-Ga-based alloys. Our findings not only offer insights into tailoring of hydrophobic/hydrophilic properties on Ni-Mn-Ga-based MSM alloys but also provide a novel methodology for fabricating functional metasurfaces on other metals. [ABSTRACT FROM AUTHOR]

Silicon nanoparticles with Mie resonances are among the most prospective building blocks for state-of-the-art all-dielectric metasurfaces. Here we focus on linear and nonlinear optical responses of silicon nanoparticles printed by laser-induced transfer from silicon-on-insulator wafer. Second-harmonic generation and broadband multiphoton-absorption-induced luminescence are studied as a function of pump wavelength. The key role of magnetic quadrupole Mie resonances in the nonlinear optical response from silicon nanoparticles is revealed. We also show the influence of Si nanoparticle shape and structure modification, realized by additional femtosecond laser irradiation, on their linear and nonlinear optical properties. [Display omitted] • The key role of magnetic quadrupole Mie resonances in the nonlinear optical response from silicon nanoparticles is revealed. • Both SHG efficiency and broadband luminescence are measured for individual Si NPs as a function of the optical pump frequency. • An ability of nonlinear optical response control via additional femtosecond laser irradiation post-treatment is demonstrated. [ABSTRACT FROM AUTHOR]

• Generation of laser tracks utilizing varying pulse energies and scanning velocities. • Examination of the surface morphology of laser tracks formed on soda lime glass. • Production of laser-induced periodic surface structures with low spatial frequency. • Comprehensive investigation of laser tracks through top-plane and cross-sectional analysis using FESEM. • In-depth study of nanoparticle morphology through TEM examination. Glass, due to its high transparency and chemical stability, has found extensive use in electrical, electronic, and optoelectronic devices. This study presents a novel approach to modify the morphological features of laser-generated tracks and produced nanoparticles in soda-lime glass using the accuracy of micro processing by femtosecond laser pulses. The investigation has meticulously examined how variations in the incident laser pulse energy and scanning velocity dictate the formation of distinct morphological characteristics on the glass surface. FESEM has revealed three distinct morphological formations, including particulate, self-ordered periodic surface structures, and a combination of the two. Additionally, TEM analysis has shown the generation of both spherical and irregular-shaped nanoparticles with diameters ranging from 10 nm to less than 2 μm, and an amorphous structure confirmed by SAED patterns. The periodicity of the LIPSS was found to be 550±25 nm, and they were oriented perpendicular to the laser polarization vector. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Low-power non-coherent light-triggered two-photon absorption process with long-lived transition state: consecutive photo-induced electron transfer and triplet triplet annihilation upconversion. • Supramolecular approaches to extending the lifetime of radical anion or triplet state of dye molecules. • Applications in photoredox catalysis, 3D printing, cancer treatment and bioimaging. • The challenges and future directions are proposed. Materials with multi-photon absorption (MPA) feature, are highly desirable for applications such as deep-seated tumor treatment, high spatiotemporal resolution bioimaging, sophisticated micro-nano fabrication, optical data storage, frequency-upconverting laser, and optical limiting. The classical two-photon absorption (TPA) process relies on an extremely short-lived virtue state, leading to the requirement of an ultrahigh power density of femtosecond pulsed laser. To break this application barrier, the key solution is to extending the lifetime of the transition state in TPA. Recently, the operation of TPA with low-power non-coherent excitation (LPNC-TPA) was achieved by leveraging the mono-reduced species (such as radical anion) and the triplet excited state of dye molecules as the transition state with a relatively long lifetime. In this review, the mechanism of these LPNC-TPA processes will first be introduced, followed by the approaches to extend the lifetime of the mono-reduced species and the triplet state. Then, considering its ability to tune the aggregation mode of dye molecules, the metal–organic framework (MOF) will be emphasized as an efficient tool to operate efficient LPNC-TPA in the solid state. The merits and features of LPNC-TPA materials will be revealed through their emerging applications in photoredox catalysis, photopolymerization, 3D printing, in vivo cancer treatment, bioimaging, and biosensing. Finally, the future directions and challenges of LPNC-TPA are proposed, along with the possible solutions. [ABSTRACT FROM AUTHOR]

Nanoparticles are the key in nanotechnology and nano science, and are widely implemented in various domains, including biochemistry, biophysics, and nanoscience. Laser processing is the ideal method for manufacturing colloidal alloy nanoparticles with high-purity surface in a liquid by laser pulses, which is a sustainable and convenient production process. Molecular dynamics proves to be a powerful tool to provide a numerical insight for the laser interaction with material, and helps understand the mechanisms better. This paper discusses the application of molecular dynamics simulations in the characteristics and behaviors of laser processing metal nanoparticles. The cluster-based two-temperature model molecular dynamics emerges as an efficacious method for scrutinizing the microscopic dynamics of gold nanoparticles. The two-temperature model integrated with molecular dynamics model effectively emulates energy transfer and relaxation processes that underpin the ultimate nanoparticle size and morphology. Additionally, this paper delves into the interaction of laser with nanoparticles suspended in liquid and the influence of metal film thickness on nanoparticle size and distribution. Furthermore, the paper explains the impact of the liquid medium and laser energy on nanoparticle morphology, size, and properties. In summary, this review offers a comprehensive synopsis of recent advancements in molecular dynamics simulations of laser-processed metal nanoparticles. • The development of MD study in preparing nanoparticles by LAL, LFL and LML methods is reviewed. • The development of the two-temperature method combined with the MD method. • Discussed the performance comparison of LAL, LFL, and LML. • The future direction of nanoparticles development is predicted. [ABSTRACT FROM AUTHOR]