Longer Laser Pulses Research Articles

Features of the Ignition of a Laser Fusion Target by a Converging Shock Wave

The compression and burning of a fusion target ignited by a focused shock wave produced at the action of a time-profiled second harmonic laser pulse of a Nd laser have been calculated and theoretically studied. The main energy features of the shock ignition scheme have been considered. The use of the second harmonic radiation corresponds to a higher energy and a longer laser pulse necessary for ignition by this method compared to the use of the third harmonic radiation. Nevertheless, the method of ignition by the focused shock wave with the second harmonic radiation makes it possible to reach the fusion target gain that is two or three times higher than that at the traditional spark ignition with the laser pulse energy higher than in the former case by a factor of 1.5. The numerical calculations have been performed with one-dimensional hydrodynamic codes.

A numerical study on the pulse duration dependence of a magnetic field generated using a laser-driven capacitor-coil target

Herein, a two-dimensional magnetohydrodynamic (MHD) simulation has been performed to understand the MHD and the generated magnetic field (B-field) behaviors of a capacitor-coil target. The results showed that a peak B-field strength was generated in the capacitor-coil target, followed by a current peak. This B-field depends on a peak current with a pulse duration of 2 ns. For a peak current of 2 ns, the average B-field in the capacitor-coil target is estimated by the center of the B-field from the conventional circular current. The time difference between the peak current and the generated B-field is up to 2 ns. Furthermore, the time difference of the capacitor-coil target corresponds to the inductance and resistance of the capacitor-coil target. Overall, the results suggests that a longer laser pulse will improve the strength of B-field generated by the capacitor-coil target.

Enhanced optical emission in laser-induced breakdown spectroscopy by combining femtosecond and nanosecond laser pulses

The role of inter-pulse delay time in laser-induced Cu plasma with femtosecond and nanosecond double-pulse irradiation had been investigated by optical emission spectroscopy. The experimental results showed that a significant enhancement effect in the atomic line emission was observed by changing the inter-pulse delay time. The spectral intensity of Cu plasma produced by a femtosecond + nanosecond double-pulse was higher than that produced by a nanosecond + femtosecond dual-pulse. That is to say, the shorter laser pulse (femtosecond) was efficient in coupling to the sample and the longer laser pulse (nanosecond) was suitable for re-exciting the produced plasma. In addition, the corresponding plasma temperature and electron density were also calculated. The plasma temperature in the case of femtosecond + nanosecond was lower than that in the case of nanosecond + femtosecond, while the electron density in the case of femtosecond + nanosecond was higher than that in the case of nanosecond + femtosecond. This study is helpful for the research on sample detection using femtosecond and nanosecond double-pulse laser-induced breakdown spectroscopy.

Is perpendicular magnetic anisotropy essential to all-optical ultrafast spin reversal in ferromagnets?

All-optical spin reversal presents a new opportunity for spin manipulations, free of a magnetic field. Most of all-optical-spin-reversal ferromagnets are found to have a perpendicular magnetic anisotropy (PMA), but it has been unknown whether PMA is necessary for spin reversal. Here we theoretically investigate magnetic thin films with either PMA or in-plane magnetic anisotropy (IMA). Our results show that spin reversal in IMA systems is possible, but only with a longer laser pulse and within a narrow laser parameter region. Spin reversal does not show a strong helicity dependence where the left- and right-circularly polarized light lead to the identical results. By contrast, the spin reversal in PMA systems is robust, provided both the spin angular momentum and laser field are strong enough while the magnetic anisotropy itself is not too strong. This explains why experimentally the majority of all-optical spin-reversal samples are found to have strong PMA and why spins in Fe nanoparticles only cant out of plane. It is the laser-induced spin–orbit torque that plays a key role in the spin reversal. Surprisingly, the same spin–orbit torque results in laser-induced spin rectification in spin-mixed configuration, a prediction that can be tested experimentally. Our results clearly point out that PMA is essential to spin reversal, though there is an opportunity for in-plane spin reversal.

The dynamics of Al/Pt reactive multilayer ignition via pulsed-laser irradiation

Reactive multilayers consisting of alternating layers of Al and Pt were irradiated by single laser pulses ranging from 100 μs to 100 ms in duration, resulting in the initiation of rapid, self-propagating reactions. The threshold intensities for ignition vary with the focused laser beam diameter, bilayer thickness, and pulse length and are affected by solid state reactions and conduction of heat away from the irradiated regions. High-speed photography was used to observe ignition dynamics during irradiation and elucidate the effects of heat transfer into a multilayer foil. For an increasing laser pulse length, the ignition process transitioned from a more uniform to a less uniform temperature profile within the laser-heated zone. A more uniform temperature profile is attributed to rapid heating rates and heat localization for shorter laser pulses, and a less uniform temperature profile is due to slower heating of reactants and conduction during irradiation by longer laser pulses. Finite element simulations of laser heating using measured threshold intensities indicate that micron-scale ignition of Al/Pt occurs at low temperatures, below the melting point of both reactants.

Impact of pulse duration on Ho:YAG laser lithotripsy: treatment aspects on the single-pulse level.

Holmium-YAG (Ho:YAG) laser lithotripsy is a multi-pulse treatment modality with stochastic effects on the fragmentation. In vitro investigation on the single-pulse-induced effects on fiber, repulsion as well as fragmentation was performed to identify potential impacts of different Ho:YAG laser pulse durations. A Ho:YAG laser system (Swiss LaserClast, EMS S.A., Nyon, Switzerland) with selectable long- or short-pulse mode was tested with regard to fiber burn back, the repulsion capacity using an underwater pendulum setup and single-pulse-induced fragmentation capacity using artificial (BEGO) stones. The laser parameters were chosen in accordance with clinical application modes (laser fiber: 365 and 200 µm; output power: 4, 6 and 10 W in different combinations of energy per pulse and repetition rate). Evaluation parameters were reduction in fiber length, pendulum deviation and topology of the crater. Using the long-pulse mode, the fiber burn back was nearly negligible, while in short-pulse mode, an increased burn back could be observed. The results of the pendulum test showed that the deviation induced by the momentum of short pulses was by factor 1.5-2 higher compared to longer pulses at identical energy per pulse settings. The ablation volumes induced by single pulses either in short-pulse or long-pulse mode did not differ significantly although different crater shapes appeared. Reduced stone repulsion and reduced laser fiber burn back with longer laser pulses may result in a more convenient handling during clinical application and thus in an improved clinical outcome of laser lithotripsy.

Study of the time-resolved emission spectra of the ejected plume generated by ultrashort laser ablation of graphite

In this paper ultrashort laser pulses with different fluences (18 J/cm2-115 J/cm2) and pulse widths (50 fs-4 ps) are employed to ablate highly oriented pyrolytic graphite in vacuum (4×10-4 Pa). By recording the time-resolved emission spectra of the ablated plume, the ultrafast time evolution of the ablation process is investigated. The Swan bands of C2 radicals, the spectral band near 416 nm which may be assigned to the electronic transition from 1Σu+ to X1Σg+ of C15 clusters, and the emission continuum ranging from 370-700 nm are observed. From the recorded time-resolved emission spectra of the ablated plume, it is seen that at larger time delays only the emission continuum is observed. The decay process of the emission continuum of the plume generated by 50 fs, 115 J/cm2 laser pulses can be divided into a fast decreasing stage (before 20 ns time delay) and a slow decreasing stage (after 20 ns time delay), indicating that the emission continuum may come from two different compositions. During the fast decreasing process, the bremsstrahlung of the ablation-generated carbon plasma contributes to the major part of the continuum; while during the slow decreasing process, the thermal radiation of carbon clusters generated at a later stage of ablation mainly contributes to the continuum. In addition, the existence time of the continuum generated by 50 fs laser pulses increases with the decrease of laser fluence, indicating that laser pulses with lower fluences can generate more carbon clusters at later stages of ablation. It is also found that for the 50 fs pulses, when the laser fluence increases at the early stage of ablation, the quantities of carbon plasma and excited C2 radicals in the plume increase significantly, but the quantity of excited C15 radicals with larger mass only increases slightly. Therefore the laser fluence has a great impact on the concentrations of different compositions in the ejected plume, implying that different material removal mechanisms exist for ablation induced by laser pulses with different laser fluences. Finally, pulse width plays an important role in the time evolution manner of the emission continuum. As the laser pulse width increases, the two-stage decay process of the emission continuum gradually changes into one-stage process, indicating that the existence time intervals of carbon plasma and carbon clusters overlap each other for longer laser pulse width. And the whole evolution process of the emission continuum induced by 4 ps laser pulses is much slower than that induced by 50 fs laser pulses. Longer laser pulse width also causes the decrease of the spectral intensity of C2 radicals, and thus higher laser intensity favors the generation of excited C2 radicals.

Impact of pulse duration on Ho:YAG laser lithotripsy: fragmentation and dusting performance.

ObjectivesIn vitro investigations of Ho:YAG laser-induced stone fragmentation were performed to identify potential impacts of different pulse durations on stone fragmentation characteristics.Materials and methodsA Ho:YAG laser system (Swiss LaserClast, EMS S.A., Nyon, Switzerland) with selectable long or short pulse mode was tested with regard to its fragmentation and laser hardware compatibility properties. The pulse duration is depending on the specific laser parameters. Fragmentation tests (hand-held, hands-free, single-pulse-induced crater) on artificial BEGO stones were performed under reproducible experimental conditions (fibre sizes: 365 and 200 µm; laser settings: 10 W through combinations of 0.5, 1, 2 J/pulse and 20, 10, 5 Hz, respectively).ResultsDifferences in fragmentation rates between the two pulse duration regimes were detected with statistical significance for defined settings. Hand-held and motivated Ho:YAG laser-assisted fragmentation of BEGO stones showed no significant difference between short pulse mode and long pulse mode, neither in fragmentation rates nor in number of fragments and fragment sizes. Similarly, the results of the hands-free fragmentation tests (with and without anti-repulsion device) showed no statistical differences between long pulse and short pulse modes.ConclusionThe study showed that fragmentation rates for long and short pulse durations at identical power settings remain at a comparable level. Longer holmium laser pulse duration reduces stone pushback. Therefore, longer laser pulses may result in better clinical outcome of laser lithotripsy and more convenient handling during clinical use without compromising fragmentation effectiveness.

Generation of nanoparticles of bronze and brass by laser ablation in liquid

Nanoparticles of brass and bronze are generated by ablation of corresponding bulk targets in liquid ethanol. The experiments were performed using three pulsed lasers with different pulse duration: ytterbium fiber laser (80ns), a Neodymium:YAG laser (10ps), and femtosecond Ti:sapphire laser (200fs). The generated nanoparticles (NPs) are characterized by UV–vis absorption spectroscopy, X-ray diffractometry, Raman scattering, and Transmission Electron Microscopy. The size of generated NPs lies in the range 10–25nm depending on the laser source. The X-ray diffractometry reveals the change of phase composition of brass NPs compared to the initial target in case of ablation with 80ns laser source, while with 10ps laser pulses this effect is less pronounced. Brass NPs generated with pico- and femtosecond laser radiation show the plasmon resonance in the vicinity of 560nm and no plasmon peak for NPs generated with longer laser pulses. Raman analysis shows the presence of Cu2O in generated NPs. The stability of generated NPs of both brass and bronze to oxidation is compared to that of Cu NPs generated in similar experimental conditions.

Polarization instability of ultrashort pulses as a source of vectorial supercontinuum

Polarization instability (PI) of ultrashort light pulses, giving rise to vectorial supercontinuum generation, is demonstrated using a subwavelength-core, highly birefringent, normally dispersive optical fiber. The evolution of ultrashort pulses in the regime of PI is shown to radically differ from polarization-instability dynamics of cw fields and longer laser pulses. As the peak power of the laser field decreases along the propagation path due to dispersion-induced pulse stretching, the Poincaré-sphere map of field dynamics is shown to evolve from the behavior typical of PI in the highly nonlinear regime toward the beating dynamics of uncoupled polarization modes, characteristic of low field intensities and cw fields.

Effect of phonon spectrum heating on the formation of laser-induced electron plasma in wide-gap insulators

The effect of lattice heating by laser pulses on the dynamics of electron plasma generation in transparent solids has been theoretically studied. Several ways of taking into account the contribution of the phonon spectrum heating to the electron avalanche dynamics, depending on the type of the effective (with respect to the field energy transfer to electrons) phonons and laser pulse duration, have been proposed. A comparative analysis of the results of Monte Carlo computation of electron gas heating in the laser pulse field, which were obtained for cold and heated lattices, has been performed. It is shown that the consideration of the effect of lattice heating on the probabilities of electron-phonon and electron-phonon-photon scattering leads to an increase in the avalanche rate, which is more pronounced at longer wavelengths of the incident radiation and under longer laser pulses. Some qualitative features of the redistribution of the energy, absorbed during a pulse, between the electron plasma and lattice are revealed, which suggest initiation of irreversible microscopic changes in the insulator. In particular, the ratio R of the energy accumulated in the electron subsystem to the excess (with respect to the initial equilibrium state) energy in the phonon subsystem has been calculated for different initial lattice temperatures. It is shown that this ratio increases with a decrease in the laser wavelength in the computation scheme with lattice heating disregarded and decreases at all pulse durations when the lattice heating is taken into account.

Porous nanoparticles of Al and Ti generated by laser ablation in liquids

Experimental results are presented on the generation of porous nanoparticles of either Al or Ti by laser ablation of solid targets in ethanol, water, and n-propanol saturated with hydrogen. The nanoparticles are characterized by high resolution transmission electron microscopy (HR TEM) and optical absorption spectroscopy. Saturation of the liquid with gaseous hydrogen leads to the formation of internal cavities in nanoparticles. In the case of short laser pulses (180fs, Ti:sapphire laser at 800nm wavelength), the nanoparticles are mostly spherical with the size of 30–50nm at concentration about 1015cm−3. The cavity occupies from 20 to 50% of the particle volume. Longer laser pulses (70ns, Nd:YAG laser at 1064nm wavelength) generate facetted nanoparticles with facetted cavities inside. The mechanism of formation of cavities is discussed on the basis of temperature-dependent solubility of hydrogen in metals.

The role of multi-return induced collision-ionization in atomic nonsequential double ionization

The nonsequential double ionization of helium by high-intensity (1.5×1015 W/cm2) few-cycle laser pulses is investigated using a three-dimensional semi-classical rescattering model, with emphasis on the longitudinal correlated momentum spectra. We find that the percentage of opposite-hemisphere emission in this case is remarkably higher than those produced by laser pulses with medium-intensity (e.g., 7×1014 W/cm2) and low-intensity (e.g., 2.5×1014 W/cm2) those procuced by the laser pulses pulses with, and also the same intensies but longer laser pulses. Meanwhile, the V-shape structure is more pronounced. We identify the underlying mechanisms with tracing back the individual trajectories, and find that the single-and the multi-return induced collision-ionization are mainly responsible for the opposite- and the same-hemisphere emission, respectively. The dependences of these two different mechanisms on peak intensity and pulse length are predicted.

Ellipsometric characterization of AlN films synthesized by Pulsed-Laser-Deposition

AlN films were synthesized on Si(100) by pulsed laser deposition at 800oC and different incident laser fluences in vacuum and in nitrogen at 0.1 Pa. Two KrF* (λ = 248 nm) excimer laser sources were used generating pulses of 7.4 and 25 ns duration, respectively. The incident laser intensity on target was kept constant in all experiments within (3–4)×108 W/cm2. The films were studied by ellipsometry (SE) in the spectral range λ = 190÷900 nm and the optical parameters, such as refractive index, high frequency dielectric constant and single oscillator energies were estimated. The analysis of the SE results revealed that the films, deposited with short laser pulses and low laser fluence (3.7 J/cm2) were characterized with refractive index values higher than 2 and an optical band gap energy value of ∼ 5 eV, suggesting that their structure was polycrystalline with cubic crystallites. With longer laser pulses and large fluence the refractive index values decreased and the values of the energetic parameters increased suggesting that the films, deposited in vacuum were polycrystalline with hexagonal phase, while those, deposited in nitrogen at 0.1 Pa were amorphous.

Angular effects of nanostructure-covered femtosecond laser induced periodic surface structures on metals

In this paper, we study nanostructure-covered laser induced periodic surface structures (NC-LIPSSs) on metals produced by femtosecond laser pulses incident at various angles. For the first time, we show that the dependence of NC-LIPSS period on the incident beam angle deviates significantly from that of regular LIPSSs studied previously using longer laser pulses. Our study shows that this deviation is due to the nanostructures on LIPSSs that can be explained by the Maxwell–Garnett theory of effective media.

A study on laser drilling of thin steel sheet in air and underwater

In laser drilling of a thin stainless steel sheet in air with Nd:YAG laser pulses of 0.5–1 ms durations it was observed that the 0.5 ms duration laser pulse was more effective in drilling a through-hole than the relatively longer laser pulses with proportionately more energy. Further, laser drilling could be readily done when the sheet was placed at the focal point of the lens and below it but not above the focal point. On the other hand, the underwater laser drilling could be done when the sheet was placed above the focal point. An attempt has been made to explain these experimental observations considering various processes involved in laser drilling in air and underwater. While the recoil pressure of the vapor and plasma played an important role in laser drilling in air; the radial gradient of recoil pressure of evaporation, the Marangoni force induced by the surface tension gradient in melt pool and the cavitation effect of bubble collapse were believed to be responsible for the material removal in underwater drilling process.

Dynamics of two-photon-induced three-photon absorption in nanosecond, picosecond, and femtosecond regimes

Based on five-level rate-equation theory, we develop the laser-pulse-duration dependence of two-photon-absorption-induced singlet and triplet excited-state absorptions (ESAs). We present analytical expressions for the effective three-photon absorption coefficients caused by both singlet and triplet ESAs under the pulsed excitation on time scales from femtoseconds to microseconds. We demonstrate that the triplet ESA is predominant with longer laser pulses (microseconds to tens of nanoseconds) and that the resultant nonlinear absorption (NLA) can be adequately interpreted by a simplified four-level model. Under the excitation of picosecond laser pulses, generally speaking, the competition between singlet and triplet ESAs is observable. In this instance, the photodynamics of the system can be understood by a five-level model. In the femtosecond regime, however, a three-level model is validated in the prediction of NLA, because the triplet ESA becomes negligible.

Systematic experiments for proof of Poisson statistics on direct-detection laser radar using Geiger mode avalanche photodiode

The single pulse detection and false-alarm probability in measuring time of flight (TOF) of laser pulse are caused by the creation of primary electrons in Geiger mode avalanche photodiode (APD). Measuring the detection and false-alarm probabilities with longer laser pulse than time bin and artificial noise source, we have experimentally proved that the creations of primary electrons in Geiger mode APD is Poisson distributed. It is also shown theoretically that there is little difference between smaller and larger laser pulse width than time bin cases on measurements of both detection and false-alarm probability.

Research and development in the optics of solid-state lasers with high peak radiation power at the S I Vavilov State Optical Institute

The goal of this article is to consider the main optical problems of creating lasers with supershort pulse widths and high peak radiation power and to give a review of research on this specialization at the State Optical Institute. Fundamental effects that limit the peak power are of the greatest interest, and methods of suppressing them are considered. One such method is to convert to a laser architecture with pulse compression, which makes it possible to sharply improve the characteristics of the radiation by comparison with lasers that use direct amplification of short pulses. This is achieved by suppressing nonlinear effects while amplifying longer laser pulses. The main nonlinear effects that limit the radiation brightness in solid-state lasers and pulse-compression methods for creating a new class of lasers are considered. The role of S. I. Vavilov State Optical Institute in research and development in powerful laser systems is considered in detail.

Acetylene-vinylidene isomerization in ultrashort intense laser fields studied by triple ion-coincidence momentum imaging

The isomerization of acetylene via hydrogen migration in intense laser fields (8 x 10(14) W/cm2) has been investigated by coincidence momentum imaging of the three-body Coulomb explosion process, C2H2 (3+)-->H+ + C+ + CH+. When ultrashort (9 fs) laser pulses are used, the angle between the momenta of C+ and H+ fragments exhibits a sharp distribution peaked at a small angle ( approximately 20 degrees ), showing that the hydrogen atom remains near the original carbon site in the acetylene configuration. On the other hand, a significantly broad distribution extending to larger momentum angles ( approximately 120 degrees ) is observed when the pulse duration is increased to 35 fs, indicating that the ultrafast isomerization to vinylidene is induced in the longer laser pulse.