High-order harmonic generation by nonlinear reflection of an intense high-contrast laser pulse on a plasma

We demonstrate the use of a plasma mirror to obtain 60-fs 10-TW laser pulses with a temporal contrast of 10(8) on a nanosecond time scale and 10(6) on a picosecond time scale, and we use these high-contrast pulses to generate high harmonics by nonlinear reflection on a plasma with a steep electronic density gradient. Well-collimated harmonics up to 20th order are observed for a laser intensity of approximately equal to 3 x 10(17) W/cm2, whereas no harmonics are obtained without the plasma mirror.     

Probing hot and dense laser-induced plasmas with ultrafast XUV pulses

In this Letter, we demonstrate the instantaneous creation of a hot solid-density plasma generated by focusing an intense femtosecond, high temporal contrast laser on an ultrathin foil (100 nm) in the 10(18) W/cm2 intensity range. The use of high-order harmonics generated in a gas jet, providing a probe beam of sufficiently short wavelengths to penetrate such a medium, enables the study of the dynamics of this plasma on the 100 fs time scale. The comparison of the transmission of two successive harmonics permits us to determine the electronic density and the temperature with accuracies better than 15%, never achieved up to this date in the regime of laser pulses at relativistic intensity.     

强激光和超薄等离子体薄膜相互作用产生高次谐波

通过一维粒子（particle in cell,简称PIC）模拟，研制了超薄等离子体薄膜在两列旋转方向相反的圆偏振激光脉冲作用下的高次谐波辐射。在10^21W/cm^2强度的激光作用下，可以获得200次以上的高次谐波。当激光脉冲宽度较短时，由于等离子体薄膜被压缩所导致的多普勒频移对高次谐波产生了显著的频率调制。而这种多普勒频移可以通过对抽运光脉冲引入频率啁啾来补偿。     

Transition to the relativistic regime in high order harmonic generation

Experiments and computer simulations on the generation of high order harmonics from steep plasma gradients using intense femtosecond laser pulses are presented. Qualitative changes in the harmonic emission take place when the intensities are increased above 10(19) W/cm2 and/or the plasma scale length is varied. Good agreement between experimental and calculated spectra makes it possible to clearly distinguish between nonrelativistic and relativistic mechanisms of harmonic generation.     

Interference and spectral broadening of higher harmonics of laser radiation in plasma torches containing ions,nanoparticles, and fullerenes

We analyze the interference between two processes of higher harmonic generation (HHG) in plasma containing mixtures of different materials (silver and gold nanoparticles, as well as graphite and boron). We find that, for mixtures and individual ingredients, the limiting orders of generated harmonics of laser radiation approximately coincide with one another. At the same time, for plasma torches formed by the ablation of mixtures of materials, the HHG efficiency is considerably reduced compared to the case of the frequency transformation of laser radiation in individual ingredients of these mixtures as a result of destructive interference in the former case. We demonstrate a considerable spectral broadening of harmonics generated in laser plasma with pulses passed through filaments formed in air. In this case, the HHG efficiency increases fourfold (from 3 × 10⁻⁶ to 1.2 × 10⁻⁵) compared to the case of radiation free of phase and frequency modulation. The generation of harmonics is also observed upon the passage of 120-fs laser pulses through plasma containing fullerenes. In this case, the limiting value of generated harmonics achieves the 33rd order. The efficiency of harmonics in fullerene plasma considerably exceeds a similar process in silver plasma.     

Stable generation of high-order harmonics of femtosecond laser radiation from laser produced plasma plumes at 1 kHz pulse repetition rate

We present a method for the creation of stable weakly ionized plasmas from laser ablation of solid targets using a 1 kHz pulse repetition rate laser, which can be used for stable high-order harmonic generation from plasma plumes. The plasma plumes were generated from cylindrical rotating targets. Without target rotation the intensity of harmonics in the 40-80 nm range drops by more than one order of magnitude during less than 10(3) shots, while, with rotation of the target at typically 30 revolutions per minute, stable emission of high-order harmonics from aluminum plasma plumes with variation of less than 10% was maintained for >10(6) laser shots.     

Coherent focusing of high harmonics: a new way towards the extreme intensities

We demonstrate analytically and numerically that focusing of high harmonics produced by the reflection of a few-femtosecond laser pulse from a concave plasma surface opens a new way to unprecedentally high intensities. The key features allowing the boosting of the focal intensity are the harmonics coherency and the small exponent of the power-law decay of the harmonics spectrum. Using similarity theory and direct particle-in-cell simulations, we find that the intensity at the focus scales as I(CHF) alpha a(3)(0)I(0), where a(0) and I(0) alpha a(2)(0) are the dimensionless relativistic amplitude and the intensity of the incident laser pulse. The scaling suggests that due to the coherent harmonic focusing (CHF), the Schwinger intensity limit can be reached using lasers with I(0) approximately 10(22) W/cm(2). The pulse duration at the focus scales as tau(CHF) alpha 1/a(2)(0) and reaches the subattosecond range.     

Coherent wake emission of high-order harmonics from overdense plasmas

We present a new mechanism for high-order harmonic generation by reflection of a laser beam from an overdense plasma, efficient even at moderate laser intensities (down to Igamma2 approximately 4x10(15) W cm-2 microm2). In this mechanism, a transient phase matching between the electromagnetic field and plasma oscillations within a density gradient leads to the emission of harmonics up to the plasma frequency. These plasma oscillations are periodically excited in the wake of attosecond electron bunches which sweep across the density gradient. This process leads to a train of unevenly spaced chirped attosecond pulses and, hence, to broadened and chirped harmonics. This last effect is confirmed experimentally.     

Dynamics of the critical surface in high-intensity laser-solid interactions: modulation of the XUV harmonic spectra

The generation of harmonics from the interaction of an intense (I>or=10(18) W cm(-2)) laser with a solid surface is investigated. Modulation of the harmonic emission spectrum with a periodicity of 2 to 4 harmonics is observed at higher laser intensities. A similar modulation is predicted by a particle-in-cell simulation. The modulation is shown to be caused by the higher modes of oscillation of the critical surface during the interaction. As a result, the dynamics of the critical surface can be inferred from the shape of the harmonic spectrum.     

Laser radiation frequency conversion in carbon- and cluster-containing plasma plumes under conditions of single and two-color pumping by pulses with a 10-Hz repetition rate

This work reviews a series of investigations of different plasma plumes using single- and two-color laser systems that emit femtosecond pulses with a 10-Hz repetition rate. Results of investigation of the resonant enhancement of harmonics in tin plasma with the use of two types of pumps are analyzed, and it is shown that the tuning of the wavelengths of harmonics to ion-resonance levels plays an important role in increasing the conversion efficiency to high-order harmonics of the radiation to be converted. Investigations of different carbon-containing plasma media (carbon nanotubes, graphite, carbon aerogel, etc.) exhibit attractive properties of the nonlinear medium of this type for efficient generation of high-order harmonics. The results of the first experiments on the use of nanoparticles produced directly in the course of laser ablation of metals for increasing the efficiency of harmonics generated in this cluster-containing medium are analyzed. It is shown that new approaches realized in these investigations give hope that the nonlinear optical response of plasma media in the far-ultraviolet range can be further increased.     

Double plasma mirror for ultrahigh temporal contrast ultraintense laser pulses

We present and characterize a very efficient optical device that employs the plasma mirror technique to increase the contrast of high-power laser systems. Contrast improvements higher than 10(4) with 50% transmission are shown to be routinely achieved on a typical 10 TW laser system when the pulse is reflected on two consecutive plasma mirrors. Used at the end of the laser system, this double plasma mirror preserves the spatial profile of the initial beam, is unaffected by shot-to-shot fluctuations, and is suitable for most high peak power laser systems. We use the generation of high-order harmonics as an effective test for the contrast improvement produced by the double plasma mirrors.     

High harmonic generation in a laser plasma

The results of investigation of high harmonics of radiation of a Ti:sapphire laser propagating through a laser plasma generated on the surface of different targets are presented. For most of the targets, the intensity distribution of the high harmonics generated is found to form a plateaulike pattern similar to that observed in the case of gas jets. The generation of high harmonics (up to the 65th harmonic, λ = 12.24 nm) is caused by the interaction of femtosecond laser radiation with ions. The conversion efficiency in the plateau region varies from 10^?7 to 8 × 10^?5 depending on the target. The main restriction on the conversion efficiency and the peak intensity of the harmonics generated is caused by the self-defocusing of the femtosecond radiation due to free charge carriers formed as a result of tunnel ionization.     

Second Harmonic Generation of Self‐Focused Cosh‐Gaussian Laser Beam in Thermal Quantum Plasma by Excitation of an Electron Plasma Wave

This paper presents a scheme for second harmonic generation (SHG) of an intense Cosh‐Gaussian (ChG) laser beam in thermal quantum plasmas. Moment theory approach in W.K.B approximation has been adopted in deriving the differential equation governing the propagation characteristics of the laser beam with distance of propagation. The effect of relativistic increase in electron mass on propagation dynamics of laser beam has been incorporated. Due to relativistic nonlinearity in the dielectric properties of the plasma, the laser beam gets self‐focused and produces density gradients in the transverse direction. The generated density gradients excite electron plasma wave (EPW) at pump frequency that interacts with the incident laser beam to produce its second harmonics. Numerical simulations have been carried out to investigate the effects of laser parameters on selffocusing of the laser beam and hence on the conversion efficiency of its second harmonics. Simulation results predict that within a specific range of decentered parameter the ChG laser beams show smaller divergence as they propagate and, thus, lead to enhanced conversion efficiency of second harmonics. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Harmonic emission from the rear side of thin overdense foils irradiated with intense ultrashort laser pulses

The harmonic emission from thin solid carbon and aluminum foils, irradiated by 150 fs long frequency-doubled Ti:sapphire laser pulses at lambda=395 nm and peak intensities of a few 10(18) W/cm(2), has been studied. In addition to the harmonics emitted from the front side in the specular direction, we observe harmonics up to the 10th order, including the fundamental from the rear side in the direction of the incident beam, while the foil is still strongly overdense. The experimental observations are well reproduced by particle-in-cell simulations. They reveal that strong coupling between the laser-irradiated side and the rear side occurs via the nonlocal electron current driven by the laser light.     

Generation of higher harmonics of laser radiation in plasma formed by pulses with a 1-kHz repetition rate

Studies of higher-order nonlinearities in plasma plumes by the method of higher-harmonic generation (HHG) upon laser ablation of different materials by pulses with a repetition rate of 1 kHz are reviewed. It is shown that an increase in the pulse repetition rate compared to the previous works on HHG in plasma using lasers with a pulse repetition rate of 10 KHz considerably increases the average power of converted radiation in the far-ultraviolet (FUV) region. The review considers the results of works on the interference of quantum trajectories of accelerated electrons, two-color pumping, HHG in graphite plasma, HHG in gases compared to HHG in plasma plumes, resonant enhancement of harmonics, and stabilization of the output parameters of radiation converted in plasma. It is shown that the new methods of optimization of the HHG process considerably increase the potential of the use of coherent FUV radiation owing to the improvement of some characteristics of harmonics in plasma generated by pulses with a high repetition rate.     

Generation of strong coherent extreme ultraviolet radiation from the laser plasma produced on the surface of solid targets

We demonstrate the generation of high harmonics (up to the 65th order, λ=12.24 nm) of a Ti:sapphire laser radiation after the propagation of femtosecond laser pulses through the low-excited plasma produced by a picosecond prepulse radiation on the surface of different targets. High-order harmonics generated from the surface plasma of most targets showed a plateau pattern. It is assumed that the harmonic generation in these conditions occurs due to the interaction of the femtosecond pulses with the ions. The conversion efficiencies at the plateau region were varied between 1×10^-7 to 8×10^-6, depending on the target. The main contribution to the limitation of harmonic generation efficiency and cutoff energy was attributed to the self-defocusing of main pulse. A considerable restriction of the 27th harmonic generation was observed at different focusing conditions in the case of chromium plasma. Our observation of the resonance-induced enhancement of a single harmonic (λ=61.2 nm) at a plateau region with the efficiency of 8×10^-5 in the case of In plasma can offer some expectation that analogous processes can be realized in other plasma samples in the shorter wavelength range where the highest harmonics were achieved. PACS 42.65.Ky; 52.35.Mw; 52.38.-r     

Generation of lower and higher harmonics in different plasma media with small <Emphasis Type="Italic">Z</Emphasis>

The generation of lower (third) and higher harmonics of femtosecond laser radiation in plasmas produced by laser ablation of different targets with a small atomic number Z (B, Be, Li) has been investigated. The high (10⁻³) efficiency of third-harmonic generation was observed in plasma produced on the boron surface. Efficient third-harmonic generation was also observed in beryllium plasma using femtosecond pulses of Ti:sapphire laser radiation (λ = 790 nm) and its second harmonic (395 nm). We could tune the higher harmonics generation spectrum by tuning the crystal converter when using 395-nm radiation to be converted. It is shown that, in plasmas formed on targets with small Z, the conversion efficiency and limiting generated harmonic order depend on the delay between the ablation pulse and the pulse to be converted.     

基于半解析自洽理论研究相对论激光脉冲驱动下阿秒X射线源的产生

发展了一种描述相对论激光脉冲与稠密等离子体相互作用产生阿秒X射线源的半解析自洽理论.该理论模型不仅可以获得等离子体界面的振荡轨迹、振荡面电场和磁场等物理参数,而且能够精确计算出激光脉冲驱动下阿秒X射线源的频谱,结果与粒子模拟程序一致.理论计算结果表明阿秒X射线源的辐射特性与等离子体界面随时演化过程相关,在周期量级激光场驱动下等离子体界面振荡振幅呈现中心不对称,通过改变激光场的载波包络相位实现对等离子体界面振荡的控制,获得准单阿秒X射线源.     

Diagnostics of Tin Plasma Produced by Visible and IR Nanosecond Laser Ablation

We present the optical emission spectroscopic studies of the Tin (Sn) plasma, produced by the fundamental (1064 nm) and second (532 nm) harmonics of a Q switched Nd: YAG pulsed laser having pulse duration of 5 ns and 10 Hz repetition rate which is capable of delivering 400 mJ at 1064 nm, and 200 mJ at 532 nm using Laser Induced Breakdown Spectroscopy (LIBS). The laser beam was focused on target material by placing it in air at atmospheric pressure. The experimentally observed line profiles of four neutral tin (Sn I) lines at 231.72, 248.34, 257.15 and 266.12 nm were used to extract the electron temperature (T_e) using the Boltzmann plot method and determined its value 6360 and 5970 K respectively for fundamental and second harmonics of the laser. Whereas, the electron number density (N_e) has been determined from the Stark broadening profile of neutral tin (Sn I) line at 286.33 nm and determined its value 5.85 x 10¹⁶ and 6.80 x 10¹⁶cm^–3 for fundamental and second harmonics of the laser respectively. Both plasma parameters (T_e and N_e) have also been calculated by varying distance from the target surface along the line of propagation of plasma plume and also by varying the laser irradiance. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Extreme ultraviolet interferometry measurements with high-order harmonics

We demonstrate that high-order harmonics generated by short, intense laser pulses in gases provide an interesting radiation source for extreme ultraviolet interferometry, since they are tunable, coherent, of short pulse duration, and simple to manipulate. Harmonics from the 9th to the 15th are used to measure the thickness of an aluminum layer. The 11th harmonic is used to determine the spatial distribution of the electron density of a plasma produced by a 300-ps laser. Electronic densities higher than 2-10(20)electrons/cm>(3) are measured.