Intense Laser Pulses Research Articles

Superradiance in Quantum Vacuum

A new process associated with the nonlinear optical properties of the electromagnetic quantum vacuum is described. It corresponds to the superradiant emission of photons, resulting from the interaction of an intense laser pulse with frequency ω0 with a counter-propagating high-harmonic signal with a spectrum of frequencies nω1, for n integer, in the absence of matter. Under certain conditions, photon emission from vacuum will be enhanced by the square of the number of intense spikes associated with the high-harmonic pulse. This occurs when the field created by the successive spikes is coherently emitted, as in typical superradiant processes involving atoms. Subradiant conditions, where the nonlinearity of quantum vacuum is entirely suppressed, can equally be defined.

Optical near-field imaging and nanostructuring by means of laser ablation

In this review we consider the development of optical near-field imaging and nanostructuring by means of laser ablation since its early stages around the turn of the century. The interaction of short, intense laser pulses with nanoparticles on a surface leads to laterally tightly confined, strongly enhanced electromagnetic fields below and around the nano-objects, which can easily give rise to nanoablation. This effect can be exploited for structuring substrate surfaces on a length scale well below the diffraction limit, one to two orders smaller than the incident laser wavelength. We report on structure formation by the optical near field of both dielectric and metallic nano-objects, the latter allowing even stronger and more localized enhancement of the electromagnetic field due to the excitation of plasmon modes. Structuring with this method enables one to nanopattern large areas in a one-step parallel process with just one laser pulse irradiation, and in the course of time various improvements have been added to this technique, so that also more complex and even arbitrary structures can be produced by means of nanoablation. The near-field patterns generated on the surface can be read out with high resolution techniques like scanning electron microscopy and atomic force microscopy and provide thus a valuable tool—in conjunction with numerical calculations like finite difference time domain (FDTD) simulations—for a deeper understanding of the optical and plasmonic properties of nanostructures and their applications.

Light-front approach to relativistic electrodynamics

We illustrate how our recent light-front approach simplifies relativistic electrody-namics with an electromagnetic (EM) field F μν that is the sum of a (even very intense) plane travelling wave and a static part ; it adopts the light-like coordinate ξ = ct – z instead of time t as an independent variable. This can be applied to several cases of extreme acceleration, both in vacuum and in a cold diluted pasma hit by a very short and intense laser pulse (slingshot effect, plasma wave-breaking and lasera wake-field cceleration, etc.)

Численное исследование динамики нагрева золота ультракороткими дихроматическими импульсами лазерного излучения

The interaction of femtosecond laser pulses of moderate intensity with metals by the example of gold is studied. The main attention is paid to dichromatic regimes of laser irradiation corresponding to a new trend in the field of laser pro-cessing of materials. Based on the two-temperature approximation, a new model is created which involves a dichro-matic source of the laser radiation energy and also takes into account the influence of the electron temperature on the reflection coefficient of the irradiated material and thermophysical properties of its electronic subsystem. For numerical implementation of the model, an implicit divergent finite-difference scheme is created with an improved method for describing the melting process in comparison with previous works. The model calculations have shown that the absorption of the laser energy by gold depends on the sequence of two pulses at different wavelengths and the delay time between them, and for correct description of the absorbed energy, it is necessary to take into account the change of the reflection coefficient of the surface during irradiation. The developed model may be of interest for optimizing the processing of materials with ultra-short dichromatic laser pulses.

Линейный дихроизм и двулучепреломление зондирующего излучения в спектроскопии накачка-зондирование в многоатомных молекулах

Probe beam dichroism and birefringency occurring in the excited states of polyatomic molecules under excitation with two femtosecond laser pulses have been theoreticaly studied as a function of delay between the pulses. General expressions describing the change of intensity and polarization of the probe laser pulse passed through the solution of arbitrary polyatomic molecules at any initital polarization of the laser pulses have been derived using the spherical tensor approach. The expressions take into account the coherence in excited molecule vibrational states and decay of these states due to vibrational relaxation, rotational diffusion, and radiative transitions. The expressions describe the effects of probe beam linear dichroism and birefringency occurring in molecular excited states. As shown, under certain conditions both effects can be observed simultaneously. It has been concluded that within the geometry of almost collinear propagation of the pump and probe laser pulses through the molecular sample the contributions from linear dichroism and birefringency can be completely separated from each other by an appropriate choice of the polarization analyzer placed in front of the photodetector. The obtained expressions were applied for description of the signals that can be recorded experimentally by means of the polarization-modulation scheme developed in the recent author's publication (Gorbunova et al,Phys. Chem. Chem. Phys. 2020, Vol. 22, 18155−18168). The theory predicts that the modulated signals of dichroism and birefringency appear in quadrature with respect to the double frequency reference modulation signal.

Femtosecond X-ray Lasers at λ=32.8 and 44.4 nm in a Plasma Formed by Optical Field Ionization in a Krypton Cluster Jet

Due to high efficiency, X-ray lasers based on transitions of Ni-like krypton (Kr⁸⁺) are being actively studied. The main focus is on an X-ray laser based on the conventional 3d_5/24d_5/2 [J=0] – 3d_3/24p_1/2 [J=1] transition at λ=32.8 nm. Gaseous krypton targets or krypton cluster jets are used in various experiments. X-ray lasers at 32.8 nm in a plasma formed by optical field ionization in a krypton cluster jet are widely used for research of nanoobjects. In this article, the possibility of creating an efficient X-ray laser in Ni-like krypton based on a transition with optical self-pumping 3d_3/24f_5/2 [J=1] – 3d_3/24d_5/2 [J=1] at λ=44.4 nm is predicted for the first time. The plasma filament is excited upon interaction of a jet of krypton clusters with an intense pump laser pulse. Optimal conditions to achieve the duration t_las ≤300 fs of the X-ray laser radiation are determined. The optimal electron density is in a rather narrow interval in the range n_e ~ 10²¹ - 2×10²¹ cm^-3. The optimal electron temperature is several keV. It is likely that this explains the fact that no X-ray laser has been observed on this transition in Kr⁸⁺ so far. The conversion factor per pulse is found to be ~5×10^-5. For an X-ray laser operating on the conventional transition 3d_5/24d_5/2 [J=0] – 3d_3/24p_1/2 [J=1] at λ=32.8 nm, t_las ≤ 300 fs can also be achieved; however, the conversion factor for this transition is times ~5 smaller than that for the former transition.

Controlling the plasma electron number density of copper metal using NIR picosecond laser-induced plasma spectroscopy

In this paper, we investigate a new method to control the plasma electron number density of copper metal using a near-infrared (NIR) picosecond Nd:YAG laser-induced plasma spectroscopy (LIPS) technique. The applied laser parameters are as follows; laser pulse energy and intensity varied from 29.2 to 59.4 mJ ± 3% and from 6.01×1010 to 12.35×1010 W/cm2 ± 5%, respectively, for a single pulse at 170 ps pulse duration, and beam diameter about 0.5 ± 0.1 mm. By considering the Stark broadening of a specific spectral line, electron density can be calculated using a neutral copper line at 521.8 nm, assuming the local thermodynamic equilibrium (LTE) condition. The observed electron density values were 1.09×1016, 2.24×1016, 3.60×1016, and 4.75×1016 cm–3 for the laser pulse energies 29.2, 41, 52.4, and 59.4 mJ, respectively. The plasma electron density values are increased with the increase in laser pulse energy. Such findings were interpreted due to an increase in the mass ablation rates with laser pulse energy. The obtained results explore the ability to control the plasma electron density by controlling the picosecond pulse energy. These results can contribute to the development of plasma technologies and their applications in many fields.

Scaling of laser-driven electron and proton acceleration as a function of laser pulse duration, energy, and intensity in the multi-picosecond regime

A scaling study of short-pulse laser-driven proton and electron acceleration was conducted as a function of pulse duration, laser energy, and laser intensity in the multi-picosecond (ps) regime (∼0.8 ps–20 ps). Maximum proton energies significantly greater than established scaling laws were observed, consistent with observations at other multi-ps laser facilities. In addition, maximum proton energies and electron temperatures in this regime were found to be strongly dependent on the laser pulse duration and preplasma conditions. A modified proton scaling model is presented that is able to better represent the accelerated proton characteristics in this multi-ps regime.

Dynamic evolution of ionization and recombination processes and ion front acceleration in the presence of nonthermal and trapped electrons

The dynamic evolution of ionization, three-body and radiative recombination processes in high intensity laser ion acceleration mechanisms, has been studied. For that, the expansion of a collisional thin plasma slab in vacuum is modeled using mixture hydrodynamic fluids equations for ions and neutral atoms, in the presence of fast nonthermal and slow trapped electrons, obeying a Cairns-Gurevich distribution. In addition, the characteristics of ion front acceleration and ion gained energy profiles are obtained, for three types of accelerated ions (H +, C + and Al +). It is proved that, ionization and recombination processes are responsible for the energy transfer between plasma particles. These processes are also strongly influenced by the impact of electron nonthermal phenomena, generated by the interaction of an intense laser pulse with the target. On the other hand, parametric studies have proved that ion energy profiles, maximum electric fields and ion energies at the ion front acceleration are also significantly affected by these phenomena. This study is useful in applications involving the creation of energetic ion beams, such as protontherapy.

Simulation of Plasma Density Gradient Generation by Wakefield and the Effect of Probe Pulse Time-Delay on the Photon Acceleration

Laser Wakefield is produced by ultra-high intensity laser pulse interacting with underdense plasma with special conditions for the laser wavelength and plasma density. In this mechanism, nonlinear forces appear due to the very high amplitudes of the electromagnetic wave and these forces evacuate plasma electrons from the path of the laser pulse leading to very high electron plasma density gradients. Due to the electrostatic forces which result from these density perturbations, the electrons move very fast in oscillatory manner to restore neutrality creating a wake of electron density perturbations behind the laser pulse. Detailed investigation has been dealt with the time-delay between the driver laser pulse and the probe pulse which can affect the production of high plasma gradients needed for photon acceleration process.

Broadband THz Sources from Gases to Liquids

Matters are generally classified within four states: solid, liquid, gas, and plasma. Three of the four states of matter (solid, gas, and plasma) have been used for THz wave generation with short laser pulse excitation for decades, including the recent vigorous development of THz photonics in gases (air plasma). However, the demonstration of THz generation from liquids was conspicuously absent. It is well known that water, the most common liquid, is a strong absorber in the far infrared range. Therefore, liquid water has historically been sworn off as a source for THz radiation. Recently, broadband THz wave generation from a flowing liquid target has been experimentally demonstrated through laser-induced microplasma. The liquid target as the THz source presents unique properties. Specifically, liquids have the comparable material density to that of solids, meaning that laser pulses over a certain area will interact with three orders more molecules than an equivalent cross-section of gases. In contrast with solid targets, the fluidity of liquid allows every laser pulse to interact with a fresh area on the target, meaning that material damage or degradation is not an issue with the high-repetition rate intense laser pulses. These make liquids very promising candidates for the investigation of high-energy-density plasma, as well as the possibility of being the next generation of THz sources.

Mechanisms of picosecond laser-induced damage from interaction with model contamination particles on a high reflector

The interactions of microparticles of different materials located on the surface of a multilayer dielectric mirror with intense 1053-nm laser pulses of varying fluence and duration (10 and 0.6 ps) are investigated. The particles caused localized intensification of the electric field, which becomes the dominant mechanism for the onset of damage and secondary contamination of the mirror at fluences far below the pristine (without particles) laser-induced-damage threshold of the mirror. Several interaction mechanisms leading to material modification and damage are identified, including the localized field intensification by multibeam interference and particle-induced microlensing, plasma-induced scalding, and secondary contamination via nanoparticle generation and particle melting. The resulting morphologies were observed to be vulnerable to damage growth and additional damage initiation when irradiated by subsequent pulses.

Laser-induced Coulomb-explosion imaging of the CS2 dimer: The effect of non-Coulombic interactions

Carbon disulfide dimers ${({\mathrm{CS}}_{2})}_{2}$ are doubly ionized by an intense 40-fs laser pulse leading to fragmentation into a pair of ${{\mathrm{CS}}_{2}}^{+}$ cations. Unlike previous studies on atomic dimers, their kinetic energy, gained by mutual electrostatic repulsion, deviates strongly from the value determined by applying the Coulomb interaction approximation to the ${{({\mathrm{CS}}_{2})}_{2}}^{2+}$ ion. An ab initio calculation shows that inclusion of non-Coulombic interactions in ${{({\mathrm{CS}}_{2})}_{2}}^{2+}$ is crucial to account for the observed kinetic energy. These interactions result in 16 ${{({\mathrm{CS}}_{2})}_{2}}^{2+}$ electronic microstates, which obstructs accurate determination of intermolecular bond distances in ${({\mathrm{CS}}_{2})}_{2}$ from the fragments' kinetic energy. The situation will be similar for most other weakly bonded atomic and molecular complexes.

Nonlinear propagation of an intense Laguerre–Gaussian laser pulse in a plasma channel* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61665006 and 61865011) and the Natural Science Foundation of Jiangxi Province of China (Grant Nos. 20171ACB21018, 20161BAB212041, and 20162BCB23012).

The nonlinear propagation of an intense Laguerre–Gaussian (LG) laser pulse in a parabolic preformed plasma channel is analyzed by means of the variational method. The evolution equation of the spot size is derived including the effects of relativistic self-focusing, preformed channel focusing, and ponderomotive self-channeling. The parametric conditions of the LG laser pulse and plasma channel for propagating with constant spot size, periodically focusing and defocusing oscillation, catastrophic focusing, and solitary waves are obtained. Compared with the laser pulse with fundamental Gaussian (FG) mode, it is found that the effect of vacuum diffraction is reduced by half and the effects of relativistic and wakefield focusing are decreased by a quarter due to the hollow transverse intensity profile of the LG laser pulse, while the effect of channel focusing is the same order of magnitude with that of the FG laser pulse. Thus, the matched condition for the intense LG laser pulse with constant spot size is released obviously, while the parameters of the laser and plasma for the existence of solitary waves nearly coincide with those of the FG laser pulse.

Polarization Dependent Excitation and High Harmonic Generation from Intense Mid-IR Laser Pulses in ZnO.

The generation of high order harmonics from femtosecond mid-IR laser pulses in ZnO has shown great potential to reveal new insight into the ultrafast electron dynamics on a few femtosecond timescale. In this work we report on the experimental investigation of photoluminescence and high-order harmonic generation (HHG) in a ZnO single crystal and polycrystalline thin film irradiated with intense femtosecond mid-IR laser pulses. The ellipticity dependence of the HHG process is experimentally studied up to the 17th harmonic order for various driving laser wavelengths in the spectral range 3–4 µm. Interband Zener tunneling is found to exhibit a significant excitation efficiency drop for circularly polarized strong-field pump pulses. For higher harmonics with energies larger than the bandgap, the measured ellipticity dependence can be quantitatively described by numerical simulations based on the density matrix equations. The ellipticity dependence of the below and above ZnO band gap harmonics as a function of the laser wavelength provides an efficient method for distinguishing the dominant HHG mechanism for different harmonic orders.

Evaluation of clinical response and efficiency of laser treatment for removing or significantly improving in facial nevi in north Indian population

Introduction: Melanocytic nevus is commonly occurring condition in Indian subjects. We investigate the clinical response and efficiency of laser for removing facial melanocytic nevi. Materials and Methods: The present study comprised 98 patients in which 52 are females and 46 are males with age 12 to 57 years. These patients with different Nevus were treated with different laser for a period of 38 months. Patients had skin types 4 and 5 and 6 based on fitzpatrick skin type. The results were documented and clinical photographs were taken before and after completion of treatments. Results: good-to-excellent results were presented by 96% patients. All subjects completed the study and the follow-up visits. Treatment was well tolerated overall by all participants, with no major adverse events. Out of 98 patients 20 patients received one or two session of treatment in which 10 were received very satisfactory result, 6 patients received good result and 3 received fair result. Out of 98 patients 38 patients had 95% clearance and 24 had 90% clearance of nevi with treatment sessions ranging from 3-8. Conclusion: we recommend that in cases of nevus in Indian patients, Q-switched Nd: YAG, Er: YAG, Intensive Pulsed Laser (IPL) and Pulsed Dye Laser (PDL) are easy-to perform, safe and effective treatment. Good results were taken after laser treatment on nevi, on an average 96% of the patients received good-to-excellent results. Keywords: Q-switched Nd: YAG laser, Er: YAG, Pulsed dye laser, Intensive pulsed laser facial nevus, Melanocytic nevi

Hybrid stimulated Raman scattering-two-plasmon decay instability and 3/2 harmonic in steep-gradient femtosecond plasmas.

We numerically study interaction of a very intense (I∼10^{17} to 5×10^{19}W/cm^{2}) femtosecond obliquely incident p-polarized laser pulse with a steep-gradient (L∼λ) plasma, i.e., within the conditions typical for modern experiments. It is shown that the hybrid stimulated Raman scattering-two-plasmon decay instability develops near the quarter-critical density surface and plays the dominant role for the plasma waves' excitation and energy absorption. The plasmons are excited as two wave packets confined near this surface with very wide ≈ω_{0}/c spatial spectra along its normal. Hence, phase-matching conditions for the 3/2 harmonic generation are fulfilled immediately and include the mechanism coming from the high harmonics of plasma waves. This mechanism has been proved experimentally by observing an additional 3/2 harmonic beam.

Sign reversal in magnetic field amplification by relativistic laser-driven microtube implosions

We demonstrate and explain the surprising phenomenon of sign reversal in magnetic field amplification by the laser-driven implosion of a structured target. Relativistically intense laser pulses incident on the outer surface of a microtube target consisting of a thin opaque shell surrounding a μm-scale cylindrical void drive an initial ion implosion and later explosion capable of generating and subsequently amplifying strong magnetic fields. While the magnetic field generation is enhanced and spatially smoothed by the application of a kilotesla-level seed field, the sign of the generated field does not always follow the sign of the seed field. One unexpected consequence of the amplification process is a reversal in the sign of the amplified magnetic field when, for example, the target outer cross section is changed from square to circular. Using 2D particle-in-cell simulations, we demonstrate that sign reversal is linked to the stability of the surface magnetic field of opposite sign from the seed, which arises at the target inner surface during laser irradiation. The stability of the surface magnetic field and, consequently, the sign of the final amplified field depend sensitively on the target, laser, and seed magnetic field conditions, which could be leveraged to make laser-driven microtube implosions an attractive platform for the study of magnetic fields in high energy density plasma in regimes where sign reversal either is or is not desired.

Collisionless shock acceleration in the corona of an inertial confinement fusion pellet with possible application to ion fast ignition.

Two-dimensional particle-in-cell simulations are used to explore collisionless shock acceleration in the corona plasma surrounding the compressed core of an inertial confinement fusion pellet. We show that an intense laser pulse interacting with the long scale-length plasma corona is able to launch a collisionless shock around the critical density. The nonlinear wave travels up-ramp through the plasma reflecting and accelerating the background ions. Our results suggest that protons with characteristics suitable for ion fast ignition may be achieved in this way.This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 2)’.

Characterization of microcraters fabricated on the silicon surface by single and multi-pulse laser ablation at various laser intensities

Microcraters on the silicon surface were fabricated with the help of a nanosecond pulsed laser. The features of microcraters fabricated with single laser pulse and pulse trains of various laser intensities were analyzed. In case of single pulse, the diameter and depth of micro-crater rise with the laser fluence and achieve saturation at higher laser intensities. For the range of fluence 62–122 J/cm2, the mass of ablated material was in the range of 0.27 × 10−9–2.69 × 10−9 g/pulse. Numerically simulated temperature trends were compared with the measured crater depths. The influence of laser intensity on the target temperature and onset time to melt/boil were also investigated. In case of pulse trains, the reduction in material removal efficiency is attributed to the plume shielding and accumulation of ablated material on the crater walls. We demonstrated that the laser intensity and number of laser pulses can be used to control the morphology of a microcrater.