Powerful Femtosecond Laser Pulses Research Articles

Dynamics of a Discharge Initiated by a Powerful Femtosecond Laser Pulse in Atmospheric Pressure Air in Pre-Breakdown Electrical Fields

Numerical modeling of the dynamics of a discharge initiated by a high-power femtosecond laser pulse in air at atmospheric pressure in pre-breakdown fields was carried out. Calculations were conducted within the framework of a 1D-axisymmetric model that describes the evolution of the radial profiles of the main parameters of the discharge under study. The model includes a system of reaction that determine gas heating and a detailed description of the kinetic processes in a given discharge, as well as a system of gas-dynamic equations to describe the expansions of the heated channel. The results of calculations of the breakdown time of the discharge gap are conсistent with the measurement data over the entire studied range of electric field strengths, E = 9–17 kV/cm. It is shown that one of the key factors determining the evolution of the parameters of a given discharge is the rate of gas heating.

Dynamics of an Electrical Discharge Initiated by a Powerful Femtosecond Laser Pulse in Atmospheric Pressure Air in Pre-Breakdown Fields

Dynamics of an Electrical Discharge Initiated by a Powerful Femtosecond Laser Pulse in Atmospheric Pressure Air in Pre-Breakdown Fields

Fluorescence and lasing of neutral nitrogen molecules inside femtosecond laser filaments in air: mechanism and applications.

High power femtosecond laser pulses launched in air undergo nonlinear filamentary propagation, featuring a bright and thin plasma channel in air with its length much longer than the Rayleigh length of the laser beam. During this nonlinear propagation process, the laser pulses experience rich and complex spatial and temporal transformations. With its applications ranging from supercontinuum generation, laser pulse compression, remote sensing to triggering of lightning, the underlying physical mechanism of filamentation has been intensively studied. In this review, we will focus on the fluorescence and cavity-free lasing effect of the plasma filaments in air. The different mechanisms underlying the fluorescence of the excited neutral nitrogen molecules will be throughly examined and it is concluded that the electron collision excitation is the dominant channel for the formation of the excited nitrogen molecules. The recently discovered "air lasing" effect, a cavity-free bidirectional lasing emission emitted by the filaments, will be introduced and its main properties will be emphasized. The applications of the fluorescence and lasing effect of the neutral nitrogen molecules will be introduced, with two examples on spectroscopy and detection of electric field. Finally, we discuss the quenching effect of the lasing effect in atmosphere and the mechanisms responsible will be analyzed. An outlook for the achievement of backward lasing in air will be briefly presented.

10 PW peak power femtosecond laser pulses at ELI-NP

Abstract We report on the generation and delivery of 10.2 PW peak power laser pulses, using the High Power Laser System at the Extreme Laser Infrastructure – Nuclear Physics facility. In this work we demonstrate for the first time, to the best of our knowledge, the compression and propagation of full energy, full aperture, laser pulses that reach a power level of more than 10 PW.

Temporal contrast enhancement and compression of output pulses of ultra-high power lasers.

The peak power and temporal intensity contrast of powerful femtosecond laser pulses were enhanced simultaneously by broadening the pulse spectrum in transparent dielectrics due to self-phase modulation and subsequent reflection from chirping mirrors with a symmetrical dip in the reflection coefficient in the center of the broadened spectrum. This dip provides almost zero reflection of the pulse pedestal, only slightly distorting the pulse itself.

Theoretical and experimental progress of high-order harmonics and attosecond pulse generation based on solid overdense plasma

<p indent="0mm">Since the laser was invented, harmonic generation and wavelength conversion have been important research topics in nonlinear optics regime. In 1987, McPherson A et al. used sub-picosecond KrF laser <sc>(248 nm</sc> wavelength) to interact with noble gas to obtain high-order harmonics for the first time. In 2001, the first single attosecond <sc>(10^–18 s)</sc> pulse with a pulse width of 650 as was measured experimentally by Krausz et al. by using the broadband high-order harmonic generation. Since then, high-order harmonics and attosecond pulses have been widely used in atomic and molecular physics, materials science and other fields. However, due to the low efficiency of attosecond pulses generated by the gas media, the energy of attosecond pulses is limited, which limits the detection methods of attosecond time dynamics research (currently mainly IR+XUV pumping/detection) and its applications in many fields. How to obtain high-brightness, high-energy attosecond pulses has always been the pursuit in this field since the generation of the first attosecond pulse. With the development of femtosecond laser technology, the output peak power of the femtosecond laser system has been rapidly increased in the past two decades. At present, the laser system can output the pulse with a peak power of up to 10 PW (1 PW<sc>=10¹⁵ W).</sc> Its intensity can be much higher than <sc>10¹⁸ W/cm²</sc> and its interaction with the overdense plasma can efficiently generate high-order harmonics at the level of hundreds of eV or even keV, which brings new opportunities for the development and application of high-order harmonics and attosecond pulses. Generally speaking, the interaction of high peak power femtosecond laser pulses with solid-density plasmas has unique advantages in high-luminance, high-energy high-order harmonics and attosecond pulse generation. In the ELI-ALPS plan, solid plasma high-order harmonics and attosecond pulse generation are very important parts, especially to promote their high brightness and their high photon energy. In terms of the attosecond pulse energy, the goal of their first phase is that the attosecond pulse energy reaches the μJ level, which will be advanced to the mJ level in the next phase, and the photon energy of the attosecond pulse will reach a few keV. This review gives a brief introduction to its physical mechanisms and their current experimental research progress. Three mechanisms (coherent wake emission (CWE), relativistic oscillating mirror (ROM), coherent synchrotron emission (CSE)) will be discussed. They are working under different laser intensity. CWE can emit high efficiency harmonics at the laser intensity below <sc>10¹⁸ W/cm².</sc> But when the peak laser intensity is very high, the research of the CSE mechanism shows the generation of high-luminance high-order harmonics under super-relativistic light intensity, which can be used to produce extremely high field-strength pulse of attoseconds, even tens of zeptoseconds. If the laser parameters and the plasma condition can be well controlled, the laser pulse may even generate higher harmonic radiation with field strength much higher than the pump laser field strength, which may be a possible way to study the 39th question (“What is the most powerful laser researchers can build? ”) among the 125 most challenging scientific questions published by <italic>Science</italic> on its 125th anniversary.

Experimental Study of Nanosecond Laser-Generated Plasma Channels

Generation of plasma-channels by interaction of gas targets with nanosecond laser beams was investigated experimentally. Such laser-generated plasma channels are very promising for subsequent guiding of high peak power femtosecond laser pulses, over several tens of centimeters, as required in laser wake field electron-acceleration (LWFA). The experimental setup was based on the use of a cylindrical lens (100 mm of focal length) with the aim of proposing a technical solution easy to be integrated into a compact experimental setup for acceleration of multi-GeV electron beams using high peak-power laser systems. A pilot experiment, showing production of asymmetric plasma channels over a length of several millimeters in N and Ar targets with initial neutral-gas atomic density around 5 × 1019 cm−3, is reported. Plasma effective threshold formation was estimated, along with future optimization of the optical setup for a symmetrization of such plasma channel. Scalability of this concept to several tens of centimeters is preliminarily discussed, along with the corresponding critical requirements for an optimal LWFA scheme.

Structural dynamics of free molecules and condensed matter

Advances in the development of pulsed lasers provided a further breakthrough in the study of the structural dynamics of nuclei and electrons. As a result of this progress, the use of powerful femtosecond laser pulses, both for exciting a sample and for generating ultrashort (down to femto- and even subfemtosecond) photoelectron bunches synchronized with optical pulses for sensing matter, made it possible to observe the coherent dynamics of nuclei and electrons in samples at the required spatiotemporal scales. The possibility of direct observations of reaction processes is a major breakthrough in chemical physics. The many-particle potential is so complex that the degree of the interaction nonadiabaticity cannot be determined with an accuracy sufficient for predicting reaction paths. How can this information and a new look at the reaction dynamics be used in the future? This question arises in connection with the development of a new conceptual foundation of natural sciences incorporating the convergence of experimental and theoretical tools in studies of systems of any complexity with atomic resolution. In this approach, an ‘atomic–molecular’ movie is obtained by using mutually complementary information extracted from simultaneous studies of ultrafast electron (or X-ray) diffraction, spectroscopy, and the quantum dynamic theory of matter.

Full path single-shot imaging of femtosecond pulse collapse in air turbulence.

In a single shot, we measure the full propagation path, including the evolution to pulse collapse, of a high power femtosecond laser pulse propagating in air. Our technique enables examination of the effect of parameters that fluctuate on a shot-to-shot basis, such as pulse energy, pulse duration, and air turbulence-induced refractive index perturbations. We find that even in lab air over relatively short propagation distances, turbulence plays a significant role in determining the location of pulse collapse.

Time-resolved luminescence Z-scan of CsI using power femtosecond laser pulses

Time-resolved Z-scan technique gives the opportunity to obtain consistent data for dependence of the yield and decay kinetics from excitation density. The excitation by 4th harmonics of femtosecond Ti:Sapphire laser (6.2 eV) allows to measure with wide dynamical range of excitation density (from 1017 to 1022 excitations per cubic centimeter) CsI luminescence. In the density range 1018-1019 cm−3 the yield of 310 nm band (fast intrinsic luminescence) increases linearly with energy of the laser pulse, and yield of 430 nm band decreases inversely. In the same density range the subnanosecond decay of 310 nm band becomes slower and in microsecond kinetics of 430 nm band increases the contribution of nanosecond components.

Enhanced self-phase modulation effect: an effective method of generating high average and peak power femtosecond laser pulses

This work experimentally explores the important application of the enhanced self-phase modulation (SPM) effect for the generation of high average power and high peak power femtosecond laser pulses in an all-fiber mode-locked laser oscillator pumped by a single-mode laser diode. Depending on the utilization of a section of nonlinear spectral broadening fiber with a 4.2 µm mode field diameter (MFD) and the optimization of the polarization-maintaining (PM) fiber length in a polarization-based all-fiber Lyot filter, the oscillator delivers 350 mW of average power at a 46.2 MHz repetition rate, resulting in 7.5 nJ pulse energy. The laser produces SPM-broadened spectrum and the spectrum features symmetrical multi-peak distribution. The resulting compressed pulse duration and average power are 107 fs and 271 mW, respectively, which corresponds to 54.8 kW of peak power. Additionally, comparative experiments have also been performed by replacing the nonlinear spectral broadening fiber with equal-length standard fiber with 6.2 µm MFD and shortening the length of the 45° aligned PM fiber of the all-fiber polarization-based Lyot filter, respectively. The presented experimental scheme demonstrates a notable improvement in terms of average and peak power compared to conventional experimental methods. Simultaneously, the experimental scheme also shows that the enhanced SPM effect in a mode-locked fiber laser oscillator has important application value for minimizing pulse duration and lowering the mode-locking threshold.

Modification of Graphene on a Copper Grid during Femtosecond Laser Irradiation: Electron Diffraction and Raman Spectroscopy Studies

The modification of a graphene sheet, which was fabricated by chemical vapor deposition on a close-meshed copper grid, during powerful femtosecond laser irradiation is studied. The process of long-term irradiation of a sample in vacuum is found to be accompanied by (1) breaking carbon bonds, which is indicated by a decrease in the intensities of the reflections corresponding to electron-beam diffraction by graphene and (2) the formation of new products, which is indicated by the appearance of new diffraction rings and a change in the Raman spectrum of the sample. The compositions of the forming chemical compounds are analyzed. Copper oxides Cu2O and CuO are shown to form predominantly on the sample surface as a result of the chemical reactions induced by powerful femtosecond laser pulses.

Avalanche parametric conversion and white spectrum generation from infrared femtosecond pulses in glasses.

The observation of discrete lines in the white spectrum at the initial stage of filamentation of powerful femtosecond laser pulses, propagating in silica glasses, as well as the filamentation without plasma channels observed in the experiments in air, pushed us to look for other nonlinear mechanisms for describing these effects. In this paper, we present a new parametric conversion mechanism for asymmetric spectrum broadening of femtosecond laser pulses towards higher frequencies in isotropic media. This mechanism includes cascade generation with THz spectral shift for solids and GHz shift for gases. The process works simultaneously with the four-photon parametric wave mixing. The theoretical model proposed agrees well with the experimental data.

Generation of multi-wavelength femtosecond laser pulse based on nonlinear propagation of high peak power ultrashort laser pulse in single-mode fiber and spectral selectivity technology

Highly-integrated high-reliability widely-tunable femtosecond laser sources have important application values in various research and application fields, such as ultrafast spectroscopy, quantum optics, remote sensing and bio-imaging. In multi-photon excited fluorescence microscopy, femtosecond laser sources with moderate pulse energy and wide wavelength tunable range can not only meet the application requirements of the different tissue structures for the peak power and excitation wavelength, but also improve the nonlinear fluorescence efficiency and imaging resolution of the sample, and thus enhancing the penetration depth. Considering the extensive application prospect and important scientific research significance of the widely tunable femtosecond laser, in this paper we conduct an experimental research of the high repetition rate multi-wavelength femtosecond laser generation in compact sized and low-cost configuration based on the nonlinear propagation scheme of the high peak power femtosecond laser pulses in single-mode fiber. In experiment, we first construct a highly-integrated reliable all-polarization-maintaining fiber femtosecond laser amplifier, which mainly consists of an environmentally stable all-polarization-maintaining fiber mode-locked laser oscillator, single-mode fiber stretcher, a single-mode power pre-amplifier, a dual-cladding Yb-fiber amplifier, and transmission grating-pair compressor. Self-starting mode-locked operation is assured with a semiconductor saturable absorber mirror, and intra-cavity dispersion compensation is realized by a chirped fiber Bragg grating in the mode-locked oscillator. The mode-locked oscillator, which delivers laser pulses with center wavelength peaked at 1035 nm, is robust operation as temperature changes from 10℃ to 40℃ and the measured power fluctuation is less than 1% RMS over 168 hours at 23℃. The amplified high repetition rate laser pulses are compressed in a double-pass 1000 lines/mm transmission grating-pair compressor. After compression, laser pulses with 5.83 W average power and 264 fs pulse duration at 34 MHz repetition rate can be obtained. Simultaneously, we also study the dependence of the compressed pulse duration on the amplified output power. Employing a home-made high reliable compact sized all-polarization-maintaining fiber femtosecond laser as a pump source and low-cost single-mode fiber as a nonlinear medium, the generation technology of the widely tunable femtosecond laser in only fiber format is also studied based on the self-phase modulation nonlinear spectral broadening mechanism. Simultaneously, in order to reduce the effect of the dispersion on the spectral broadening as much as possible, an 80-mm-long fiber is used in experiment. The used single-mode spectral broadening fiber has a 6-m-diameter core and 20 fs2/mm dispersion coefficient. By coupling the femtosecond pump laser pulses into the 6-m-diameter fiber core, the output spectrum presents a significant nonlinear broadening. The coupled pump power can be continuously adjusted by a combination of a half-wave plate and a Glan laser polarizer. After bandpass filtering the leftmost and rightmost spectral lobes in self-phase modulation and self-steeping induced broadened spectrum with bandpass filters centered at 980, 1000, 1050, 1070 and 1100 nm, the laser pulses with 203, 195, 196, 187, and 194 fs pulse duration can be obtained at the corresponding center wavelengths. The experimental scheme presented in this paper, which is based on the nonlinear spectral broadening of the highreliability femtosecond laser pulse in single-mode fiber and the spectral selectivity technology, provides a new research approach to the realization of the highly-compacted reliable widely-tunable femtosecond laser sources and has important research significance.

New method of a “point-like” neutron source creation based on sharp focusing of high-current deuteron beam onto deuterium-saturated target for neutron tomography

A possibility of a compact powerful point-like neutron source creation is discussed. Neutron yield of the source based on deuterium-deuterium (D-D) reaction is estimated at the level of 1011 s−1 (1013 s−1 for deuterium-tritium reaction). The fusion takes place due to bombardment of deuterium- (or tritium) loaded target by high-current focused deuterium ion beam with energy of 100 keV. The ion beam is formed by means of high-current quasi-gasdynamic ion source of a new generation based on an electron cyclotron resonance (ECR) discharge in an open magnetic trap sustained by powerful microwave radiation. The prospects of proposed generator for neutron tomography are discussed. Suggested method is compared to the point-like neutron sources based on a spark produced by powerful femtosecond laser pulses.

Modification of narrow ablating capillaries under the influence of multiple femtosecond laser pulses

Powerful femtosecond laser pulses that propagate through narrow ablating capillaries cause modification of capillary walls, which is studied experimentally and theoretically. At low intensities, the laser-induced periodic surface structures and porous coating composed of sub-micron particles appear on the walls. At higher intensities, the surface is covered by deposited droplets of the size up to 10 μm. In both cases, the ablated material forms a solid plug that completely blocks the capillary after several hundreds or thousands of pulses. The suggested theoretical model indicates that the plug formation is a universal effect. It must take place in any narrow tube subject to ablation under the action of short laser pulses.

Control of the configuration of multiple femtosecond filaments in air by adaptive wavefront manipulation.

We demonstrate the ability to position single and multiple filaments arbitrarily within the energy reservoir of a high power femtosecond laser pulse. A deformable mirror controlled by a genetic algorithm finds the optimal phase profile for producing filaments at user-defined locations within the energy reservoir to within a quarter of the nominal filament size, on average. This proof-of-principle experiment demonstrates a potential technique for fast control of the configuration of the filaments.

Enhanced relativistic laser–plasma coupling utilizing laser-induced micromodified target

The interaction of slighly relativistic femtosecond laser radiation with microstructured Si targets was studied. The microstructuring was performed by nanosecond pulse laser ablation with additional chemical etching of the target material. An analysis was made of the optical damage under the action of femtosecond radiation near the ablation threshold. It was experimentally demonstrated that the hot electron temperature increases appreciably in the laser-driven plasma (from ~370 to almost 500 keV) as well as radiation yield in the MeV range at the interaction of a high power femtosecond laser pulse with a microstructured surface in comparison with a flat surface. Numerical simulations using 3D3V PIC code Mandor revealed that the charged particle energy growth is caused by the stochastic motion of electrons in the complex field formed by the laser field and the quasistatic field at the sharp tips of micromodifications.

Transient magnetized plasma as an optical element for high power laser pulses

Underdense plasma produced in gas jets by low intensity laser prepulses in the presence of a static magnetic field, $B\ensuremath{\sim}0.3\text{ }\text{ }\mathrm{T}$, is shown experimentally to become an optical element allowing steering of tightly focused high power femtosecond laser pulses within several degrees along with essential enhancement of pulse's focusability. Strong laser prepulses form a density ramp perpendicularly to magnetic field direction and, owing to the light refraction, main laser pulses propagate along the magnetic field even if it is tilted from the laser axis. Electrons generated in the laser pulse wake are well collimated and follow in the direction of the magnetic field; their characteristics are measured to be not sensitive to the tilt of magnetic field up to angles $\ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{}$.

Nonequilibrium plasma channel in gaseous media formed by powerful UV laser as a waveguide for transportation and amplification of short microwave pulses

The evolution of strongly nonequilibrium plasma in a channel created in gaseous media by powerful UV femtosecond laser pulse is studied. It is demonstrated that such a plasma channel can be used as a waveguide for both transportation and amplification of the microwave radiation. The specific features of such a plasma waveguide are studied on the basis of the self-consistent solution of the kinetic Boltzmann equation for the electron energy distribution function in different spatial points of the gas media and the wave equation in paraxial approximation for the microwave radiation guided and amplified in the channel. The results of modeling for rare gases (xenon) and air are compared. The amplification factor in xenon plasma in dependence on channel radius, intensity and frequency of the input RF radiation is analyzed.