Fs Laser Pulses Research Articles

The influence of x-ray pulse length on space-charge effects in optical pump/x-ray probe photoemission

Pump-probe photoelectron spectroscopy (PES) is a versatile tool to investigate the dynamics of transient states of excited matter. Vacuum space-charge effects can mask these dynamics and complicate the interpretation of electron spectra. Here we report on space-charge effects in Au 4f photoemission from a polycrystalline gold surface, excited with moderately intense 90 ps (FWHM) soft x-ray probe pulses, under the influence of the Coulomb forces exerted by a pump electron cloud, which was produced by intense 40 fs laser pulses. The experimentally observed kinetic energy shift and spectral broadening of the Au 4f lines, measured with highly-efficient time-of-flight spectroscopy, are in good agreement with simulations utilizing a mean-field model of the electrostatic pump electron potential. This confirms that the line broadening is predominantly caused by variations in the take-off time of the probe electrons without appreciable influence of local scattering events. Our findings might be of general interest for pump-probe PES with picosecond-pulse-length sources.

Femtosecond Laser Double Pulses Nanofabrication on Silicon

The temporal shaping femtosecond (fs) laser is realized by dividing a fs laser pulse into two identical sub-pulses. The double-pulse fs laser can be used to effectively control the initial free electron state such as electron temperature, capacity and density, etc. to improve the surface morphology quality. In this experiment, silicon was fabricated using the double-pulse fs laser. It was found that the average diameter of ablated micro-or nanoholes decreases with the increasing pulse delay (up to 1.5 ps) under the same laser fluence and the phenomenon was explained quantitatively by the plasma model. Furthermore, nanoholes were achieved using the double-pulse fs laser, which cannot be obtained by single pulse laser, and the processing size can be reduced to below 200 nm.

Direct patterning of nitrogen-doped chemical vapor deposited graphene-based microstructures for charge carrier measurements employing femtosecond laser ablation

Chemical vapor deposited nitrogen-doped graphene, transferred onto a SiO2/Si substrate, was selectively patterned by femtosecond laser ablation for the formation of the topology dedicated to charge carrier measurements. Ultrashort 1030 nm wavelength Yb:KGW fs-laser pulses of 22 μJ energy,14 mJ cm−2 fluence, 96% pulse overlap, and a scanning speed of 100 mm s−1, were found to be the optimum regime for the high throughput microstructure ablation in graphene, without surface damage of the substrate in the employed fs-laser micromachining workstation. Optical scanning electron and atomic force microscopy, as well as Raman spectroscopy, were applied to clarify the intensive fs-laser light irradiation effects on graphene and the substrate, and to also verify the quality of the graphene removal. Measurements of magnetotransport properties of the fs-laser ablated nitrogen-doped graphene microstructure in the Hall configuration enabled the determination of the type, as well as concentration of charge carriers in a wide range of temperatures.

Grating-like nanostructures formed by the focused fs laser pulse in the volume of transparent dielectric.

The evolution of optical discharge induced in the volume of transparent dielectric by a focused fs laser pulse is studied under condition of ionization instability that results in deep small-scale periodic modulation of breakdown plasma parameters in the direction of laser polarization. Laser pulse intensity range is found where the instability evolving from very small seed perturbations leads to formation of the contrast subwavelength periodic structure containing narrow zones with overcritical plasma density and enhanced energy deposition. The latter allows considering this structure as underlying the nanograting formation observed experimentally in the fused silica irradiated by series of repeated fs pulses.

Ge-Sb-Te Chalcogenide Thin Films Deposited by Nanosecond, Picosecond, and Femtosecond Laser Ablation.

Ge-Sb-Te thin films were obtained by ns-, ps-, and fs-pulsed laser deposition (PLD) in various experimental conditions. The thickness of the samples was influenced by the Nd-YAG laser wavelength, fluence, target-to-substrate distance, and deposition time. The topography and chemical analysis results showed that the films deposited by ns-PLD revealed droplets on the surface together with a decreased Te concentration and Sb over-stoichiometry. Thin films with improved surface roughness and chemical compositions close to nominal values were deposited by ps- and fs-PLD. The X-ray diffraction and Raman spectroscopy results showed that the samples obtained with ns pulses were partially crystallized while the lower fluences used in ps- and fs-PLD led to amorphous depositions. The optical parameters of the ns-PLD samples were correlated to their structural properties.

Laser nanojoining of copper nanowires

Single nanowire becomes attractive as a basic component of various nanodevices. Nanowire joining is one of the crucial operations in micro-nano manufacturing. Compared with other joining methods, such as electrical current heating and mechanical pressure, laser illumination can yield metallic contacts through local precision nanosoldering. In this paper, the authors report the joining of one single copper nanowire (CuNW) to a silver film by a laser illumination method. They first cut the silver film on a flexible substrate to form a gap of about 1–4 μm by a 1030 nm femtosecond (fs) pulsed laser. Then, they coat CuNWs by dripping nanowire solution onto the gap, bridge the gap, and then join single CuNW with the fs pulsed laser. By comparison with scanning electron microscope images after and before laser illumination, it is found that the local silver film melts, then wraps partial CuNW, and finally crystallizes. So, the joining is through a part of CuNW embedded in a silver film. In addition, the authors further analyze the joining situation by directly measuring the resistance variation between after joining (Ra) and before joining (Rb). It is demonstrated that the resistance ratio of Ra to Rb decreases from 0.835 to 0.172 with increasing laser power from 10 to 40 mW for 3 s of the laser illumination time. The resistance ratio of Ra to Rb decreases from 0.936 to 0.368 with an increment of the laser illumination time from 1 to 4 s at a laser power of 30 mW. Finally, the authors find that the welded single CuNW enables sensing of glucose at a concentration of 10−5M.

Method for an accurate measurement of nonlinear refractive index in the case of high-repetition-rate femtosecond laser pulses

In this report, we propose a new method for measuring the nonlinear refractive index in liquid and glass samples using femtosecond (fs)-laser pulses with high pulse repetition rates. This method is based on solving the heat equation to derive an expression for the normalized transmittance of closed-aperture Z-scan measurements as a function of the accumulative thermal lens induced by a high-repetition-rate fs laser pulse. To validate this method, the nonlinear refractive indices of acetone as a liquid and soda-lime as a glass material have been measured using a 100 fs pulse duration, 80 MHz repetition rate, and 800 nm wavelength. The extracted nonlinear refractive index is in a good agreement with the reported one using 1 kHz repetition rate fs laser pulses.

Microcavity-enhanced Kerr nonlinearity in a vertical-external-cavity surface-emitting laser.

Self-mode-locking has become an emerging path to the generation of ultrashort pulses with vertical-external-cavity surface-emitting lasers. In our work, a strong Kerr nonlinearity that is so far assumed to give rise to mode-locked operation is evidenced and a strong nonlinearity enhancement by the microcavity is revealed. We present wavelength-dependent measurements of the nonlinear absorption and nonlinear refractive index change in a gain chip using the Z-scan technique. We report negative nonlinear refraction up to 5x10-12cm2/W in magnitude in the (InGa)As/Ga(AsP) material system close to the laser design wavelength, which can lead to Kerr lensing. We show that by changing the angle of incidence of the probe beam with respect to the gain chip, the Kerr nonlinearity can be wavelength-tuned, shifting with the microcavity resonance. Such findings may ultimately lead to novel concepts with regard to tailored self-mode-locking behavior achievable by peculiar Kerr-lens chip designs for cost-effective, robust and compact fs-pulsed semiconductor lasers.

Experimental study of fast electron generation in intense short duration laser solid interaction at grazing incidence

We report a study on the generation of relativistic fast electrons during the interaction of an ultrashort (45 fs) laser pulse with a variety of targets (Al, Cu, Mo, Si, polyvinyl chloride, and Perspex), at a nearly grazing angle of incidence (∼70°) at an intensity of ∼1.2 × 1018 W/cm2. The fast electron emission direction was found to be dependent on the laser pulse amplified spontaneous emission (ASE) contrast, its extent, and laser pulse duration. For a 45 fs laser pulse and an ASE extent of 1 ns, the electron emission direction changes from a nearly target surface (∼7° ± 2°, divergence: ∼30°) to a specular direction (∼20° ± 2°, divergence: ∼20°) when the contrast was decreased from ∼10−7 to ∼10−6. It further shifts toward target normal with an increase in the ASE duration from 1 ns to 5 ns. For an ASE contrast of ∼10−7 and ∼10−6 with a 1 ns duration, preplasma density scale lengths of ∼30 μm and ∼45 μm were estimated, respectively, from the HELIOS hydrodynamic code. For a longer ASE duration in the range of 1–5 ns, the density scale length was in the range of ∼45–140 μm. Further, for a 45 fs pulse and a 1 ns ASE level, the electron beam charge was found to be higher in the case of Mo (∼52 pC) and Si (∼122 pC) targets compared to the other targets (∼23–34 pC). A further increase in the charge of the electron beam of ∼1.45× (∼75 pC) and ∼2.7× (∼141 pC) was also observed with Mo for a longer pulse duration of ∼170 fs (ASE 1 ns) and a larger ASE duration of 5 ns at (45 fs pulse), respectively. The electron beam properties were found to be independent of laser polarization (p, s, and circular) except a slightly higher charge (∼1.5×) was observed in the case of p-polarized laser light. The fast electron spectrum showed a maximum energy of 1 MeV with electron temperatures of ∼390 keV and ∼320 keV for the target surface and specular direction, respectively. Electron acceleration and their emission are explained in terms of applicable mechanisms in preformed plasma on the target surface considering the role of surface fields which is also established by the 2D particle in cell simulation.

Ultrafast dynamics of magnetic vortices and pulse collapse in a laser-under dense plasma interaction

The energy of an intense ultrashort laser pulse interacting with high density (still under dense) plasma is typically transformed into electron heating and in excitation of nonlinear coherent structures such as magnetic vortices, solitons, or post-solitons. Using 33 TW 30 fs laser pulses and a high-density nitrogen gas jet, we experimentally investigated magnetic vortices in the laser pulse collapse region where electrons are efficiently accelerated and heated. Those vortices, which are associated with rapidly decaying magnetic fields, are found to be immobile and persist for several picoseconds. A collisionless plasma was formed due to the quasi-static field ionization of the gas associated with the hot and fast electron currents. The evolution dynamics of such nonlinear plasma phenomena have been monitored by using a 30-fs probe laser beam through employing the polarimetric and shadowgraphic techniques. Our experimental results are also supported by particle-in-cell simulations.

Femtosecond laser generation of microbumps and nanojets on single and bilayer Cu/Ag thin films.

The formation mechanisms of microbumps and nanojets on films composed of single and double Cu/Ag layers deposited on a glass substrate and irradiated by a single 60 fs laser pulse are investigated experimentally and in atomistic simulations. The composition of the laser-modified bilayers is probed with the energy dispersive X-ray spectroscopy and used as a marker for processes responsible for the modification of the film morphology. For the bilayer with the top Ag layer facing the laser, the increase in fluence is found to result in a sequential appearance of a Ag microbump, the exposure of the Cu underlayer by removal of the Ag layer, a Cu microbump, and a frozen nanojet. The Cu on Ag bilayer exhibits a partial spallation of the top Cu film, followed by the generation of surface structures that mainly consist of Ag at higher fluences. The experimental observations are explained with atomistic simulations, which reveal that the stronger electron-phonon coupling of Cu results in the confinement of the deposited laser energy in the top Cu layer in the Cu on Ag case and channelling of the energy from the top Ag layer to the underlying Cu layer in the Ag on Cu case. This difference in the energy (re)distribution directly translates into differences in the morphology of the laser-modified bilayers. In all systems, the generation of microbumps and nanojets occurs in the molten state. It is driven by the dynamic relaxation of the laser-induced stresses and, at higher fluences, the release of vapor at the interface with the substrate. The resistance of the colder periphery of the laser spot to the ejection of spalled layers as well as the rapid solidification of the transient molten structures are largely defining the final shapes of the surface structures.

Effect of the electronic kinetics on graphitization of diamond irradiated with swift heavy ions and fs-laser pulses

Diamond preliminary damaged with neutrons was irradiated with swift heavy ions (SHI, 1030 MeV 209Bi) decelerated in the electronic stopping regime as well as with fs-laser pulses. The initial excess electronic energy densities appearing in the nanometric vicinity of the SHI trajectories and within the absorbing layers in laser spots were comparable (∼1024 eV cm−3).Graphitization of diamond in the central parts of the lased spots was observed above the threshold fluence of 15–30 J/cm−2. It was also found that the lower threshold fluence is required for initiating graphitization as well as destruction of the pre-damaged crystal by laser pulses in comparison to that for undamaged diamond. This indicates a noticeable effect of an existing defect ensemble on the kinetics of diamond transformations in laser spots.However, X-ray diffraction, atomic-force microscopy, and electron microscopy detected no graphitic domains within the SHI-irradiated pre-damaged crystal.The research demonstrated that the density of the initial excess electronic energy cannot be treated as the sole parameter governing subsequent structure transformations in diamond. Large differences between the spatial as well as temporal scales finally results in different pathways of the relaxation kinetics of this excess energy in laser spots and SHI tracks in diamond.

Ultrafast dynamics of single-pulse femtosecond laser-induced periodic ripples on the surface of a gold film

The ultrafast dynamics of periodic ripples are studied during their formation on the surface of a gold film with a prefabricated nanogroove. These transient ripples are induced by a single 800-nm, 50-fs laser pulse and are observed by a collinear pump-probe imaging method. When the laser polarization is parallel to the nanogroove, transient ripples begin appearing after an elapsed time of 25--80 ps, and become clear and regular at 400--600 ps. The ripple period increases from 685 to 770 nm as the laser fluence $F$ increases from 0.73 to $3.42\phantom{\rule{0.16em}{0ex}}\mathrm{J}/\mathrm{c}{\mathrm{m}}^{2}$. The evolution of temperature and lattice temperature are theoretically studied using the two-temperature model. When the laser fluence $F$ is above $0.73\phantom{\rule{0.16em}{0ex}}\mathrm{J}/\mathrm{c}{\mathrm{m}}^{2}$, the electron temperature rises to several ${10}^{4}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and the collision frequency rises above ${10}^{16}/\mathrm{s}$, which further causes the localization of hot electrons. Moreover, the $d$-band electrons can be excited through two-photon absorption and become free electrons. Using the dielectric constant of the excited states, which includes the effects of hot-electron localization and $d$-band transitions, the period predicted by the surface-plasmon-polarization (SPP) model accords well with experimental results. Both theory and experiment give support to SPP excitations playing a prominent role in the formation of periodic ripples induced by femtosecond laser pulses.

Intensified proton and carbon ion flux from femtosecond laser driven plasma source

Ion acceleration from aluminium foils irradiated with a 30 fs laser pulse of ∼1020 W/cm2 intensity at an incidence angle of 45° was investigated. Laser intensity contrast enhancement by a factor of 100 resulted in a nearly 7 and 30 times increase in proton and carbon ion flux, respectively, while their maximum energy remains almost unchanged. More than 1013 protons and 1014 carbon C4+ ions per MeV bandwidth per steradian solid angle were measured. Simulations, being in a good agreement with the experimental findings, have revealed that the difference in proton emission between the low and high contrast cases is a narrower angular distribution of protons at high laser pulse contrast. In the low contrast scenario, the plasma density gradient increases the hot electron divergence, leading to the reduction of particle flux in a fixed solid angle. The analytical model verifies the concept of the theoretical limit of particle flux. These results open up the possibility for further optimization of the laser driven bright source of energetic particles.

Strong third-order optical nonlinearities of Ag nanoparticles synthesized by laser ablation of bulk silver in water and air

We demonstrate the correlation between strong nonlinear optical response of silver nanoparticles (Ag NPs) out of plasmon resonance and efficient third harmonic generation in the plasmas containing Ag NPs. The dynamics of nonlinear optical response of the 8 nm and 50 nm Ag NPs prepared by laser ablation of bulk silver in deionized water using 6 ns, 200 ps, and 60 fs laser pulses is systematically analyzed. Their optical limiting properties are studied at the wavelengths of 800 and 355 nm, using femtosecond and nanosecond laser pulses. Nonlinear absorption coefficient of 8 nm Ag NPs at the wavelength of 1064 nm was measured to be as high as 3 × 10−5 cm W−1. Nonlinear refraction shows the change of sign with variation of the wavelength and duration of probe laser pulses. The theoretical calculation of silver ablation shows the formation of a few nanometer sized particles. We also analyze third harmonic generation in the laser-produced plasmas containing Ag NPs and attribute the enhancement of this process to the influence of small silver clusters. The conversion efficiency of 800 nm towards the third harmonic was measured to be 4 × 10−3, which was a few times larger compared with similar process in air.

Giant impact of self-photothermal on light-induced ultrafast insulator-to-metal transition in VO2 nanofilms at terahertz frequency.

Ultrafast detection and switching of light are key processes in high-speed optoelectronic devices. However, the performances of VO2-based optoelectronics are strongly degraded by photothermal. The mechanism of the latter is still unclear. Here, by using femtosecond-laser (fs-laser) driven kinetic terahertz wave absorption, we quantitatively separate slow photothermal response and ultrafast photodoping response (e.g. light-induced insulator-to-metal transition) from second- to picosecond-timescales, and discover the competing interplay between them. With self-photothermal (mainly determined by fs-laser pulse repetition rate and pump fluence), the ultrafast transition time was degraded by 190% from 50 ps to 95 ps, the ultrafast transition threshold was decreased to 82% from 11mJ/cm2 to 9mJ/cm2, while the amplitudes of the two photoresponse are competing. Percolation theory, along with the macroscopic conductivity response, is used to explain the competing interplay. Our findings are relevant for designing and optimizing VO2-based ultrafast optoelectronic devices.

Femtosecond laser pulse generation with self-similar amplification of picosecond laser pulses.

Compressing picosecond laser pulses to the femtosecond level is an attractive shortcut for obtaining femtosecond laser pulses. However, dechirped pulses generated by nonlinear compression with self-phase modulation (SPM) show obvious pedestals, which are induced by nonlinear chirp accumulation in spectral broadening process and cannot be easily suppressed. Here, we report systematic numerical simulations and experimental studies on self-similar amplification of picosecond pulses in a short gain fiber for obtaining ~100-fs laser pulses with nearly transform-limited (TL) temporal quality. It is demonstrated that self-similar amplification with picosecond seed pulses is only sensitive to pulse duration and pulse energy. Based on this optimization guideline, we built a compact self-similar amplification fiber system with a picosecond fiber laser as the seed source. This system outputs 66-fs pulses with 6.1-W average power at a repetition rate of 30 MHz. Due to the linear chirp produced in self-similar evolution process, compressed pulses show nearly TL temporal quality. It promises an efficient way of obtaining high-quality femtosecond laser pulses from a picosecond laser source.

Defect effects on spatiotemporal evolution of photoinduced martensitic transition in MnNiSn

Martensitic transition and reverse transition in ferromagnetic shape memory alloy MnNiSn contain a variety of structural dynamic features accompanied directly by atomic motions and micro-domain alterations. To investigate the effects of crystalline defects on the dynamical structural phase transitions, we use ultrafast transmission electron microscopy (UTEM) to directly image the rapid structural phase transition in MnNiSn initiated by femtosecond laser pulses. Via high spatiotemporal resolution images, we reveal the pinning effect by the grain boundary on the phonon-driven martensitic transition after fs-laser pulse excitations, and the structural oscillation is also observed as driven by coherent acoustic phonons that start at the sites of the grain boundary and propagate with the speed of sound. These results elucidate the roles of crystallographic defects in the dynamical processes of martensitic transition and highlight the unprecedented capability of UTEM for direct imaging lattice motions with nanometer spatial and picosecond temporal resolutions.

Femtosecond Laser-Induced Periodic Surface Structures on Fused Silica: The Impact of the Initial Substrate Temperature.

The formation and properties of laser-induced periodic surface structures (LIPSS) were investigated upon fs-laser irradiation of fused silica at different initial substrate temperatures, TS. For substrate heating between room temperature, TRT, and TS = 1200 °C, a continuous wave CO2 laser was used as the radiation source. The surface structures generated in the air environment at normal incidence with five successive fs-laser pulses (pulse duration, τ = 300 fs, laser wavelength, λ = 1025 nm, repetition frequency, frep = 1 kHz) were characterized by using optical microscopy, scanning electron microscopy, and 2D-Fourier transform analysis. The threshold fluence of fused silica was systematically investigated as a function of TS. It was shown that the threshold fluence for the formation of low-spatial frequency LIPSS (LSFL) decreases with increasing TS. The results reveal that the initial spatial period observed at TRT is notably increased by increasing TS, finally leading to the formation of supra-wavelength LIPSS. The findings are discussed in the framework of the electromagnetic interference theory, supplemented with an analysis based on thermo-convective instability occurring in the laser-induced molten layer. Our findings provide qualitative insights into the formation mechanisms of LIPSS, which allow improvements of the control of nanostructure formation to be made for corresponding applications of dielectric materials in the future.

Femtosecond laser pulse assisted photoporation for drug delivery in Chronic myelogenous leukemia cells

Chronic myelogenous leukemia (CML) is a myeloproliferative disorder occurs in the pluripotent hematopoietic stem cell. Currently, first-generation tyrosine kinase inhibitor (TKI) imatinib is the mainstay for the treatment of CML. Second generation TKI's like ponatinib, dasatinib, nilotinib, and bafetinib were treated against resistant CML. However, several CML patients develop resistance towards all existing inhibitors. Curcumin (Curcuma longa) a plant-derived natural compound is an effective bioactive component against various cancers including CML. Many studies have shown that curcumin induces time- and dose-dependent apoptosis in CML cells by regulating various downstream molecular regulators. Despite curcumin's selective cytotoxicity towards cancer cells, it has very poor bioavailability both in in-vitro and in-vivo conditions. In this present study, we have used femtosecond laser (fs-laser) pulses to ablate the cell membrane and standardized the conditions required for creating a cell membrane pores with less lethality. Following fs-laser pulse irradiation, K562 cells were incubated along with curcumin 30 μM for 0 h, 6 h,12 h and 24 h. Interestingly irradiated cells have shown higher sensitivity towards curcumin than non-irradiated cells. Immunoblotting studies showed higher induction levels of cleaved caspase 3 and 9 in irradiated population than non-irradiated. In summary, the results prove that irradiation by fs-laser pulses enhanced the bioavailability of curcumin and shows caspase-mediated cell death in irradiated CML cells than other populations.