Laser Pulse Regime Research Articles

Spatio-temporal ablation dynamics and plasma chemistry of aluminum induced by temporally modulated ytterbium fiber laser

In this work, we studied single-pulse ablation dynamics of a temporally modulated continuous wave laser–material interaction with Al using in situ multimodal time-resolved diagnostics that describe in detail the associated physical and chemical processes. Time-resolved scattering, emission imaging, and optical emission spectroscopy unveiled a sequence of events spread out across three distinct phases: (i) early phase ablation process, associated with particle generation and liquid Al column formation (<20 μs), (ii) secondary detonation when sufficient ejected material is accumulated over the surface (20–50 μs), and (iii) molten liquid Al pool oscillation on the surface, followed by large droplet ejection from the liquid pool (100–500 μs). Atomic Al and AlO were observed with optical emission spectroscopy at different ratios during the entire lifetime of the event, verifying the formation of oxidized Al vapor upon its interaction with air. Morphological and compositional characterization confirmed surface oxidation and material re-solidification in the form of protrusions produced during the irradiation process. This work provides insights into the complex physical and chemical mechanisms of single-pulse ablation in the sub-millisecond laser pulse regime, which are critically important for parameter optimization in a variety of laser processing, microfabrication, and deposition applications.

Generation of intense coherent electromagnetic radiation during the interaction of a multi-terawatt laser pulse with a nanowire target* *Reported at the 4th International Conference on Ultrafast Optical Science (Russia, Moscow, FIAN, 28 September – 2 October 2020).

Generation of coherent radiation in the IR and terahertz ranges during the propagation of a multi-terawatt laser pulse along a nanowire target is investigated. In the process of interaction, dense electron bunches are displaced from the target and accelerated in the laser field, generating intense electromagnetic radiation. Three regimes of interaction can be realised depending on the duration and shape of the laser pulse. In the first regime, when the laser pulse is long enough (tens and hundreds of femtoseconds), electrons are only partially forced out of the target. The characteristics of the low-frequency part of the spectrum of the generated radiation are determined in this case by the duration of the laser pulse, as well as by its amplitude and target parameters (geometric dimensions and concentration of electrons). In the second regime, the laser pulse has a large amplitude and a steep rising edge (the amplitude of the first half-wave is of the order of the maximum pulse amplitude); as a result, most of the electrons are displaced from the target already at the initial moment of interaction. In this regime, unipolar and bipolar pulses with a duration of tens of laser field periods can be formed. Changing the target length makes it possible to control the period of field oscillations and their number in the generated radiation. In the intermediate regime of short laser pulses with an insufficiently steep rising edge, oscillations of the formed electron bunches can occur in the macroscopic Coulomb attraction field of the charged target, which gives rise to radiation with a frequency several times lower than that of laser radiation. In this case, the pulses of the generated radiation contain a few cycles of the field with decreasing amplitude and increasing frequency. Using numerical simulation in three regimes of interaction, the characteristics of IR and terahertz radiation are found, in particular, the pulse shapes, ranges of generated frequencies, amplitudes and angular distributions of radiation are determined. It is shown that the amplitude of the generated pulse can reach subrelativistic and relativistic values (the field strength is more than 1 TV m−1 at a frequency ten times lower than the laser radiation frequency), and the energy conversion efficiency can be of the order of one percent.

Parametric study of ultra-intense laser interaction with uniform and nano-porous near-critical plasmas

Responses of the uniform near-critical plasma (UNCP) and nano-porous near-critical plasma (NPNCP) upon interaction with a short-intense laser have been scrutinized using two-dimensional (2D) particle-in-cell simulations. Maximum proton energy variation by the deposition of uniform and nano-porous layers in front of a solid target for a wide range of laser intensities (normalized amplitude a0 = 5–25) and average densities of the front layer ne = 0.3 − 3nc (where nc is the critical density) has been parametrically studied. It is found that the proton maximum energy for the front layers with sub-10 µm thicknesses is independent of the target porosity and density. However, in the relatively thick targets, the nano-porous structure decreases the laser energy absorption and, subsequently, the maximum proton energy compared to the uniform one. The results indicate that by employing UNCPs instead of NPNCPs, at the moderate laser intensity, the maximum proton energy reveals a 23% enhancement. This increment could be explained by rapid self-focusing of the laser pulse and dominant direct laser electron acceleration regime on the well-formed plasma channel in the UNCP layer. However, in the case of NPNCPs, the laser scattering from the plasma structure makes it less intense and more disordered, which influences the efficient laser energy coupling to the electrons.

Effect of nanosecond and femtosecond pulse laser on the formation of WS2 nanostructures and field emission characteristics

The pulse laser ablation is a simple and direct method used for the generation of nanostructures in liquid environment. In this paper, formation of tungsten sulfide [Formula: see text] nanostructures has been carried out using nanosecond (ns) and femtosecond (fs) laser ablation with ablation time 120 min. In addition, the field emission studies were performed of the prepared sample. The characterization techniques such as the transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV-visible spectroscopy have been used to confirm the surface morphology as well as structural details. It is observed that the laser pulse regime plays a vital role on the size reduction of [Formula: see text] nanostructure. The X-ray spectra reveal the formation of [Formula: see text] nanoparticles for ns laser ablation, while [Formula: see text] nanosheets for fs laser ablation. The TEM images indicate that the transformation of [Formula: see text] microsheet into nanosheet of the size about 15–200 nm for the sample treated with ns laser and fs laser ablation. Also, the results clearly shows the improvement in the field emission characteristics for laser ablated [Formula: see text] nanosheets. The enhanced field emission characteristics is attributed to the formation and separation of few layer [Formula: see text] nanosheets. The [Formula: see text] nanostructures may have great potential for the applications in optoelectronic devices.

The Influence of Laser Alloying of Ti13Nb13Zr on Surface Topography and Properties

Abstract The laser alloying is a continually developing surface treatment because of its significant and specific structuration of a surface. In particular, it is applied for Ti alloys, being now the most essential biomaterials’ group for load-bearing implants. The present research was performed on the Ti13Nb13Zr alloy subject to laser modification in order to determine the treatment effects on surface topography and its some mechanical properties like nanohardness, Young’s modulus, roughness. A pulse laser Nd:YAG was applied at three different laser pulse regimes: either 700 W, 1000 W or 1000 W treatment followed by 700 W modification at a pulse duration of 1 ms. The surface topography and morphology were examined using light microscopy and scanning electron microscopy with spectroscope of X-ray energy dispersion. The mechanical properties were determined by nanoindentation tests and surface roughness with a use of profilograph. The wettability was tested with a goniometer. The obtained results demonstrate complex behavior of the material surface: decrease in penetration distance and increase in hardness after first laser treatment, maintenance of this trend when machining using a higher laser pulse power, followed by an increase in penetration and decrease in hardness after additional laser treatment at lower power input, due to which a surface with fewer defects is obtained. The change in Young’s modulus follows the change in other mechanical properties, but not a change in roughness. Therefore, the observed hardening with the increase of the laser pulse power and then a small softening with the use of additional treatment with lower power can be attributed to some processes of remelting, diffusion and crystallization, sensitive to the previous surface state and heat energy flux. Despite that, the laser treatment always caused a significant hardening of the surface layer.

Sensitivity improvement by optically-absorbent plastics of electro-optical probes for high-intensity electromagnetic-fields generated by laser-matter interaction

The characterization of intense radiofrequency-microwave electromagnetic pulses\\break (EMPs) generated by the interaction of powerful lasers with matter is a very hot topic of research, for both inertial-confinement-fusion and laser-plasma acceleration facilities spread around the world. These fields are often detrimental for the operation and even the survival of active diagnostics placed around the interaction region, but they might be also used as a plasma diagnostics or for application to studies on materials and devices in these extreme-field conditions. The reliable measurement of these intense fields by classical conductive probes is a well-known problem and for this reason fully electro-optical (EO) probes have been recently developed and proved to be a very interesting solution even at petawatt laser-pulse regimes. Indeed, in some experiments performed with the nanosecond ABC laser we found that part of the diffused main laser-pulse may occasionally be coupled with the optical path of the probing CW laser within the crystal of the electro-optical probe, and lead to important reductions of the detector sensitivity. To resolve this problem we investigated here, by two experimental campaigns, the use of thick dielectric shields for the crystal, to be highly absorbing for the main laser wavelength and in general for visible radiation, and at the same time highly transparent to the wide radiofrequency-microwave range of the EMP fields to be measured. As result of this study we found that Ertalon 6XAU is an excellent material for these purposes, and thus we carefully designed and realized a suitable shielding for the EO probes made of this material.

Effect of covalent linkage between hexagonal boron nitride and porphyrins on the optical nonlinearities

Nonlinear optical nanostructured materials are gaining increased interest as optical limiters for various applications. In the present case, two soluble porphyrins (TPP) covalently functionalized hexagonal boron nitride (h-BN) nanohybrids (BN-TPP 1 and BN-TPP 2) as optical limiter were prepared for the first time, and characterized by various spectroscopic techniques. The covalent attachment of porphyrins onto the surfaces of h-BN improves the ease of processing of these BN-porphyrins composite materials. The results of Z-scan measurements at 532 nm in the nanosecond laser pulse regime indicated that the BN-TPP nanohybrids exhibited markedly improved nonlinear absorption and optical limiting performances compared to a control sample consisting of a covalently unattached physical blend of h-BN and porphyrins, and the porphyrin covalently functionalized graphene nanohybrids, as well as to either h-BN or TPP alone. The enhanced optical nonlinearities can be ascribed to the synergetic effects originating from charge transfer between the two counterparts. The effects of porphyrins on the structure, thermal, linear and nonlinear optical responses of the h-BN were investigated in detail. The efficient nonlinear optical performances make the as-prepared nanohybrids excellent candidates for optical limiting applications.

Influence of laser pulse regime on the structure and optical properties of TiO2 nanolayers

TiO2 films were deposited onto glass substrates by pulsed laser deposition (PLD) in high vacuum under monopulse and multipulse excitation. After deposition, a thermal treatment on air atmosphere was done to promote crystallization. Films were studied by x-ray diffraction, energy dispersive spectroscopy, scanning electron, atomic force microscopy, UV–vis spectroscopy, and ellipsometry. After air annealing, films gain a different amount of oxygen: TiO1.9 in multipulse regime versus TiO1.7 in the monopulse one. Splashing is observed in both regimes although in the multipulse mode greater particles are found, that derived in a less compact film after annealing which could be the cause of the better oxygen diffusion. The optical band gap of the film prepared with monopulse excitation is 3.09 eV. This value increased to 3.34 eV with annealing, corresponding to that of anatase. The film made with multipulses has an Eg = 3.12 eV which was invariant upon annealing. The difference in the properties of the films grown in the different regimes was attributed to the re-excitation of the plasma during the ablation process in the multipulse ablation that leads to an increased splashing density and thereafter a less compact film and the presence of off-stoichiometry inclusions within the film bulk.

Efficient tailored nonlinear optical responses by nanoassemblies: Focus on spindle β-FeOOH nanorods

Assemblies of β-iron oxyhydroxide (β-FeOOH) nanostructures were synthesized by a facile, mild, and non-toxic ultrasonication method. Quasi-rectangular, chrysanthemum-like, and silk-like architectures, respectively, were obtained by introducing a urea surfactant, synthesized Fe3O4 template, and natural kapok seed template. This versatile and efficient synthetic method can be extended to the fabrication of a series of novel, multifunctional assembled architectures. Hierarchical structures with identical morphologies were constructed from porous β-FeOOH nanorods with high surface areas. The effects of the assembly mode on the nonlinear optical (NLO) properties of the β-FeOOH nanorod assemblies were explored using the open-aperture Z-scan technique. Individual β-FeOOH nanorods and their analogous nanoassemblies exhibited excellent NLO performances upon excitation at 532 and 1064 nm in the nanosecond laser pulse regime, both in liquid and sol–gel solid-state matrices. These nanomaterials are promising candidates for broadband optical limiters; however, their abilities to attenuate intense and potentially dangerous laser beams differ. Our results confirm that nanoassembly formation can be used to effectively tailor the NLO activities of β-FeOOH nanorods, and the possible mechanisms for these behaviors are discussed. The observed NLO effects originate from nonlinear scattering and free-carrier absorption at 532 and 1064 nm, respectively. This study provides new insights into the assembly of one-dimensional building blocks, which will be useful in the rational design and preparation of hierarchical materials and enable tailoring and improvement of the NLO properties of materials.

Unexpected Broadband Optical Limiting Properties of Antimonene Quantum Dots

Antimonene has been found to possess excellent nonlinear refractive and saturable absorption properties recently. To further investigate its optical limiting properties, we prepared uniform antimonene quantum dots (AMQDs) via a facile ionic liquid-assisted exfoliation method. Thereafter the nonlinear absorption of the atomically thin AMQDs was studied using the open aperture Z-scan technique in the nanosecond laser pulse regime. Z-scan measurements indicate that the prepared AMQDs exhibited superior optical limiting properties at 532 nm and 1064 nm excitation, implying the antimonene hold potential for developing broadband nonlinear optical devices.

Extracting spectroscopic molecular parameters from short pulse photo-electron angular distributions.

Using a quantum wave packet simulation including the nuclear and electronic degrees of freedom, we investigate the femtosecond and picosecond energy- and angle-resolved photoelectron spectra of the E(Σg+1) electronic state of Li2. We find that the angular distributions of the emitted photoelectrons depend strongly on the pulse duration in the regime of ultrashort laser pulses. This effect is illustrated by the extraction of a time-dependent asymmetry parameter whose variation with pulse duration can be explained by an incoherent average over different ion rotational quantum numbers. We then derive for the variation of the asymmetry parameter a simple analytical formula, which can be used to extract the asymptotic CW asymmetry parameters of individual transitions from measurements performed with ultra-short pulses.

Effect of Microsecond Pulse Laser Modification on Electromagnetic Properties of Grain Oriented Silicon Steel

A microsecond pulsed laser beam was used to local magnetic domain modification of electrical grain oriented silicon steel. It was carried out using three different laser pulse regimes: a single pulse laser regime, a multipulse laser regime and a multipulse laser regime with modulation of laser pulses. The laser processing variables were pulse energy and and number of pulses. The samples were tested for nanohardness and coercivity before and after laser treatment. Light optical microscopy, scanning electron microscopy and magnetic force microscopy were used to observe the cross-sectional profile, surface of the samples, and magnetic domain visualization, respectively. The local laser treatment of grain oriented silicon steel surface has been studied in terms of its influence on the magnetic domains and coercivity. It was found that laser-modified samples showed coercivity improvement in comparison to the non-treated samples. The most significant improvement in coercivity was obtained in the modulated multipulse regime and negligible improvement in the single pulse laser regime. Three main effects responsible for the observed improvement were identified, namely: magnetic domain refinement, influence of number of laser pulses and shape of laser HAZ profile. The present work highlights on differences in the magnetic domain structure, microstructure of the laser modified material and basic electromagnetic and mechanical properties. In present study, the pulse laser surface processing was presented as a useful energy efficient alternative to other techniques e.g. mechanical scribing, electrical discharge scribing, plasma jet scribing, etc. The refined magnetic domains in electrosteels are responsible for the observed low coercivity, which indicates perspective application of the investigated laser modified steels in the power transformer cores with lower core losses.

Study on the influence of laser pulse duration in the long nanosecond regime on the laser induced plasma spectroscopy

By using a high power pulsed fiber laser, this study reports the experimental investigation of the laser-induced plasma characteristics for the laser pulse duration range extended from 40ns to 200ns. The experiments were performed with keeping the laser fluence constant at 64J/cm2. The measurements show that, for the early phase of plasma formation, the spectral line intensities and the continuum emissions as well as the plasma characteristics decay to a certain extent with the increase of the pulse duration. On the other hand, as the plasma evolves in post laser pulse regime, the electron density and the degree of ionization increase slightly for the longer pulses, while the plume temperature is more or less independent from the pulse duration. Furthermore, the ablation characteristics, such as the ablation rate, coincide with the results of plasma characteristics for the different pulse durations. Eventually, with keeping the laser fluence constant at 64J/cm2, the analytical performance of Laser-Induced Plasma Spectroscopy (LIPS) for the corresponding pulse duration range is examined by using a temporal gating and non-gating analyses. The measurements show that, in the case of gating analysis, all pulse durations yield almost the same range of limits of detections LODs. On the other hand, for non-gating analysis, the longer pulse durations provide lower LODs (better) than the shorter ones by orders of magnitude. Moreover, the calculated absolute limit of detection (LODAbs) for the longest pulse duration (i.e. 200ns) is lower by approximately factor 2 than that of the shortest one (i.e. 40ns).

Enhanced photoemission from laser-excited plasmonic nano-objects in periodic arrays

The process of photoelectron emission from gold surfaces covered with nano-objects that are organized in the form of a periodic array is addressed in the short laser pulse regime ( fs) at moderate intensities W cm−2 and for various laser wavelengths. The emission spectrum from a gold single crystal measured under the same conditions is used for reference. The comparison of the photo-emission yield and the energy of the ejected electrons with their counterparts from the (more simple) reference system shows that the periodic conditions imposed on the target surface drastically enhance both quantities. In addition to the standard mechanism of Coulomb explosion, a second mechanism comes into play, driven by surface plasmon excitation. This can be clearly demonstrated by varying the laser wavelength. This interpretation of the experimental data is supported by predictions from model calculations that account both for the primary quantum electron emission and for the subsequent surface-plasmon-driven acceleration in the vacuum. Despite the fact that the incident laser intensity is as low as W cm−2, such a structured target permits generating electrons with energies as high as 300 eV. Experiments with two incident laser beams of different wavelengths with an adjustable delay, have also been carried out. The results show that there exist various channels for the decay of the photo-emission signal, depending on the target type. These observations are shedding light on the various relaxation mechanisms that take place on different timescales.

Efficient energy absorption of intense ps-laser pulse into nanowire target

The interaction between ultra-intense laser light and vertically aligned carbon nanotubes is investigated to demonstrate efficient laser-energy absorption in the ps laser-pulse regime. Results indicate a clear enhancement of the energy conversion from laser to energetic electrons and a simultaneously small plasma expansion on the surface of the target. A two-dimensional plasma particle calculation exhibits a high absorption through laser propagation deep into the nanotube array, even for a dense array whose structure is much smaller than the laser wavelength. The propagation leads to the radial expansion of plasma perpendicular to the nanotubes rather than to the front side. These features may contribute to fast ignition in inertial confinement fusion and laser particle acceleration, both of which require high current and small surface plasma simultaneously.

Porous microstructures induced by picosecond laser scanning irradiation on stainless steel surface

A study of porous surfaces having micropores significantly smaller than laser spot on the stainless steel 304L sample surface induced by a picosecond regenerative amplified laser, operating at 1064nm, is presented. Variations in the interaction regime of picosecond laser pulses with stainless steel surfaces at peak irradiation fluences(Fpk=0.378–4.496J/cm2) with scanning speeds(v=125–1000μm/s) and scan line spacings(s=0–50μm) have been observed and thoroughly investigated. It is observed that interactions within these parameters allows for the generation of well-defined structured surfaces. To investigate the formation mechanism of sub-focus micropores, the influence of key processing parameters has been analyzed using a pre-designed laser pulse scanning layout. Appearances of sub-focus ripples and micropores with the variation of laser peak fluence, scanning speed and scan line spacing have been observed. The dependencies of surface structures on these interaction parameters have been preliminarily verified. With the help of the experimental results obtained, interaction parameters for fabrication of large area homogeneous porous structures with the feature sizes in the range of 3–15μm are determined.

Functional multilayered capacitor pixels printed by picosecond laser-induced forward transfer using a smart beam shaping technique

The printing of multilayered pixels using single picosecond laser pulses is investigated by using an improved laser-induced forward transfer (LIFT) technique. Micrometric low-k dielectric capacitors of hybrid organic polymer (parylene-C) and silver thin films have been transferred on laser-printed pads of annealed silver nanoparticles (AgNP) paste. This printing technique allows to design and print directly on virtually any substrate, in order to make the capacitors directly interface with other components, such as transducers or sensing devices, and which are further used for improving or complementing the characteristics of these devices. This study focuses on the use of the optimized LIFT technique using a double mask setup, i.e. smart beam shaping (SBS) in the picosecond laser pulse regime. The purpose is to optimize the energy repartition of the Gaussian beam on the thick multilayer donor by using a beam profile with increased intensity at the edges and low intensity in the center. This allows to keep the incoming irradiation fluence on the donor layer as low as possible in the centre area, and thus preventing the multilayer pixels from damage induced by laser irradiation. Electrical investigations of the SBS-LIFT printed micro-capacitors demonstrate that they are fully functional and stable over time, with capacities in the pF range.

Broadband optical limiting response of a graphene-PbS nanohybrid.

Graphene-based materials have shown promising nonlinear optical properties in the visible range. To extend their nonlinear optical response to the near infrared (NIR) region, we prepared a new nanohybrid consisting of uniform PbS quantum dots (QDs) attached on the reduced graphene oxide, named rGO-PbS, via a facile, low-cost, and phosphine-free method. The rGO-PbS nanohybrid exhibited superior optical limiting properties to either graphene oxide or PbS QDs upon both 532 nm and 1064 nm excitation in the nanosecond laser pulse regime, which is attributed to the synergetic effects stemming from charge transfer between the two components. Meanwhile, the thin films containing the rGO-PbS nanohybrid dispersed in polymethylmethacrylate (PMMA) also showed excellent optical limiting properties with high transparency, implying the potential applications of this hybrid material in broadband nonlinear optical devices.

Optical limiting of high-repetition-rate laser pulses by carbon nanofibers suspended in polydimethylsiloxane

The optical limiting (OL) behavior of carbon nanofibers (CNFs) in polydimethylsiloxane (PDMS) was studied and compared with that of CNFs in water, and polyhedral multi-shell fullerene-like nanostructures (PMFNs) also in water. It was shown that when switching from single-shot to pulse-periodic regime of laser pulses (10 Hz), the CNF in PDMS suspension retains its OL characteristics, while in the aqueous suspensions, considerable degradation of OL characteristics is observed. It was also observed that a powerful laser pulse causes the CNF in PDMS suspension to become opaque for at least three seconds, while such a pulse brings out a bleaching effect in aqueous PMFN and CNF suspensions. The processes of OL degradation in aqueous suspensions, bleaching and darkening of the studied materials are discussed herein.

Formation of corrugated and porous steel surfaces by femtosecond laser irradiation

Novel interaction regime of femtosecond laser pulses with steel surface at high peak irradiation fluences (0.82–10J/cm2) has been observed and thoroughly investigated. This interaction regime allows the generation of well defined microrough surfaces. To investigate the mechanisms involved in the formation of sub-focus microscale cavities and protrusions, the influence of key structuring parameters has been analyzed using a multi-pulse/multi-point layout. Threshold-like appearance of large amplitude sub-focus corrugations and micropores with the variation of laser fluence, hatch spacing, and radiation exposure has been observed. The dependencies of these threshold values on ambient gas pressure and focal spot size have been verified. The established experimental background has been applied for the fabrication of ordered rough-porous patterns with the feature sizes in the range of 1–25μm.