Particular features of higher harmonics generation in nanocluster-containing plasmas using single- and two-color pumps

The wavelength conversion of femtosecond laser pulses in laser plasmas containing clusters of different nature and dimension (fullerenes, metal nanoparticles) is studied. Pulses of a titanium-sapphire laser are used in combination with orthogonally polarized second-harmonic pulses as radiation to be converted. Variations in the generation efficiency of higher harmonics are analyzed under conditions of phase-modulated pulses. It is shown that the optimization of components of a nonlinear optical plasma medium, of plasma excitation conditions by single- and two-color pumps, and of phase and spectral parameters of radiation to be converted leads to a considerable increase in the generation efficiency of higher harmonics.     

Spectrum and wave shape of intense laser pulse propagating through Raman active medium

Results of the analytical solution of non-linear equations describing the propagation of an ultra-short intense laser pulse through a Raman-active medium are presented. The pulse passing through the medium generates a large number of high-order harmonics if the pulse intensity is close to some thresholds. The medium is described by the density matrix for two-level systems. The laser pulse duration was much shorter than both the period of oscillation and the relaxation times of an oscillator. Using this solution, the spectrum of such a pulse was expressed in terms of integrals of the initial field. The solution allows the harmonic spectrum dependence on the pulse intensity and effects of propagation to be investigated. Estimations of the conversion efficiency of the initial pumping frequency into high-order harmonics are given. The evolution of the pulse shape as a function of the distance passed by the pulse through the media was investigated. This revised version was published online in November 2006 with corrections to the Cover Date.     

Dynamical description of transient x-ray lasers seeded with high-order harmonic radiation through Maxwell-Bloch numerical simulations

The Maxwell-Bloch code COLAX has been upgraded to use detailed hydrodynamical and collisional-radiative simulations of a soft x-ray laser plasma with traveling-wave pumping. The seeding of short pulses of high-order harmonics of the pump laser into the x-ray laser medium has been simulated. The amplification is shown to be a dynamic, two-stage process: the atomic dipoles of the lasing ions are first coherently excited by the short pulse, and subsequently generate a radiation wake which is amplified along its path through the plasma, with consequences on the experimentally recorded spectra.     

High-order harmonic generation in laser plasma: Recent achievements

Recent studies of high-order harmonic generation of laser radiation in laser-produced plasma show new attractive developments in this field. Those include generation of extended harmonics in plasma plumes, new approaches in application of two-color pump, generation of extremely broadened harmonics, further developments in harmonic generation in clusters (fullerenes, carbon nanotubes, in-situ produced nanoparticles), destructive interference of harmonics from different emitters, resonance-induced enhancement of harmonics, applications of high pulse repetition rate lasers for the enhancement of average power of generating harmonics, observation of quantum path signatures, etc. We review some of these recent developments.     

Entanglement generation between two colliding particles

Harmonics with the photon energy of up to ~150 eV were studied using extended ablation plumes as the nonlinear media. The application of the ablation pulses of different duration covering the range between a few tens of femtoseconds and a few tens of nanoseconds revealed the advanced features of the extended plasmas produced by the subnanosecond pulses for efficient harmonic generation. The examples of the quasi-phase-matching of a group of harmonics in the plateau range and the advantages of using the two-color pump and cluster-containing plasmas are presented. We analyze the spatial and coherence characteristics of the high-order harmonics produced during propagation of the 64 fs pulse through the extended plasma plume produced by 370 ps pulses. It is shown that the divergence of plasma harmonics in the plateau range is 7 times smaller than the divergence of the driving radiation used for high-order harmonic generation. The measurements of the coherence properties of the lower-order harmonics showed that the visibility of interference fringes in the far field was in the range of 0.54–0.73.     

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.     

激光驱动晶体发射低阶谐波强度随激光波长的变化规律

通过数值求解激光驱动下电子在一维周期势场中运动的薛定谔方程,研究了晶体在激光场中发射的低阶谐波强度随激光波长的变化规律,结果表明,晶体发射低阶谐波强度随激光波长的变化规律与晶体发射高次谐波第一平台区域的变化规律不同.已有的研究表明晶体发射高次谐波第一平台区域的强度会随激光波长的增加而衰减,而我们发现晶体发射低阶谐波的强度会随激光波长的增加而增加.通过对晶体发射低阶谐波的时频分析、晶体价带能量变化与激光光子能量的关系,解释了晶体发射低阶谐波强度随激光波长增加而增加的原因.     

Influence of surface waves on plasma high-order harmonic generation

The influence of surface plasma waves on high-order harmonic generation from the interaction of intense lasers with overdense plasma is analyzed. It is shown that the surface waves lead to the emission of harmonics away from the optical axis, whereas the high-order on-axis harmonics are lowered in intensity. Our simulation results indicate that surface plasma wave generation plays a crucial role in surface high-order harmonic generation experiments. Furthermore, a novel surface plasma wave generation process different from the well-known two-surface wave decay is observed in the highly relativistic regime.     

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.     

Low- and high-order nonlinear optical characterization of C60-containing media

Third- and higher-order nonlinear optical processes in fullerenes were studies to define the influence of low-order nonlinearities on the high-order harmonic generation in these media. We measured the nonlinear absorption coefficients of the C₆₀:toluene solution using the 532 and 1064 nm, 50 ps pulses. The high-order harmonic generation was studied during propagation of the 790 nm, 150 fs pulses through the plasmas produced on surfaces containing fullerene powder. These studies have shown that the low-order nonlinearities of fullerenes have no impact on the generation of harmonics in such mediums in the vacuum ultraviolet range at optimal intensity of laser radiation.     

High-order nonlinear optical processes in ablated carbon-containing materials: Recent approaches in development of the nonlinear spectroscopy using harmonic generation in the extreme ultraviolet range

The nonlinear spectroscopy using harmonic generation in the extreme ultraviolet range became a versatile tool for the analysis of the optical, structural and morphological properties of matter. The carbon-contained materials have shown the advanced properties among other studied species, which allowed both the definition of the role of structural properties on the nonlinear optical response and the analysis of the fundamental features of carbon as the attractive material for generation of coherent short-wavelength radiation. We review the studies of the high-order harmonic generation by focusing ultrashort pulses into the plasmas produced during laser ablation of various organic compounds. We discuss the role of ionic transitions of ablated carbon-containing molecules on the harmonic yield. We also show the similarities and distinctions of the harmonic and plasma spectra of organic compounds and graphite. We discuss the studies of the generation of harmonics up to the 27th order (λ = 29.9 nm) of 806 nm radiation in the boron carbide plasma and analyze the advantages and disadvantages of this target compared with the ingredients comprising B₄C (solid boron and graphite) by comparing plasma emission and harmonic spectra from three species. We also show that the coincidence of harmonic and plasma emission wavelengths in most cases does not cause the enhancement or decrease of the conversion efficiency of this harmonic.     

High-order harmonic generation in nanoparticle-containing laser-produced plasmas

Nanoparticle-containing media can be used for the efficient high-order harmonic generation (HHG) of laser radiation in the extreme ultraviolet range. We review the results of recent studies of the HHG in laser-produced plasmas containing Ag, Au, Pd, Pt, Ru, GaN, BaTiO₃, and SrTiO₃ nanoparticles. The harmonics of femtosecond radiation up to the 55th order were achieved using the nanoparticle-containing plumes, when the femtosecond radiation propagated through the preformed plasma. These results are compared with the high-order harmonics generated from the plasma produced on the surface of bulk targets at different delays between the subnanosecond heating prepulse and femtosecond pulse. We discuss a six-fold enhancement of the HHG yield, which was achieved in the case of nanoparticle-containing plumes with regard to the monoparticle-containing plasmas.     

Theoretical study of armchair single-walled carbon nanotubes in the presence of a strong laser field: high harmonic generation

The nonlinear optical response of armchair single-walled carbon nanotubes under high-intensity laser irradiation is investigated theoretically and numerically because the generation of high harmonics requires a strong laser field and come from the nonlinear motion of π electrons in carbon nanotubes. A nonperturbative approach is performed to investigate the effect of group velocity on the high harmonics spectrum by nanotubes. A set of the quantum kinetic equations is derived, which includes coupled equations for the density matrix. By solving the density matrix and the current density equations numerically, we have studied the high-order harmonic generation from metallic carbon nanotubes driven by an electromagnetic external field.     

Generation of higher harmonics of femtosecond radiation in clusters

The frequency conversion of laser radiation in cluster media is studied by the method of laser ablation of surfaces containing different nanoparticles (Cr₂O₃, In₂O₃, Ag, Sn, Au, and Cu). Using plasma that contains In₂O₃ nanoparticles as an example, it is shown that the resonant amplification of certain harmonics in the plateau-like distribution of harmonics, which is characteristic of the ablation of certain solid targets, is considerably modified in the case of targets that contain cluster formations. An increase in the conversion efficiency to harmonics in nanoparticle-containing media is discussed. In order to determine optimal characteristics of laser plasma with nanoparticles, their morphology is studied before and after laser ablation.     

Quasi-phase matching of high-order harmonic generation at high photon energies using counterpropagating pulses

We extend all-optical quasi-phase matching of high-order harmonic generation into spectral regions where conventional phase matching is not possible. The high laser intensities required to generate harmonics at energy >130 eV, coupled with the resulting high level of ionization, preclude conventional phase matching in all nonlinear media. Selective enhancement factors between 40 and 150 in the flux of harmonics at photon energies around 140 eV are demonstrated using a train of two counterpropagating pulses.     

Nonlinear excitation of the second harmonic in a semiconductor superlattice placed into a magnetic field

The nonlinear interaction of waves in a periodic structure placed into a magnetic field is considered. The structure is comprised of alternate semiconducting and insulating layers. The three-wave interaction technique is applied to studying the nonlinear processes. It is shown that nonlinear phenomena in media with translational symmetry and in homogeneous media differ. Conditions for the resonant interaction between the first and second harmonics are analyzed. It is found that the second harmonic is generated when the interacting first spatial harmonics both co-and counterpropagate, which is possible only in periodic structures. Resonance events improving the generation efficiency are discussed.     

Role of the longitudinal piston error in a tiled-grating compressor in second and high-order harmonic generation

In this work, we study the limitations for overcoming the longitudinal piston error in a femtosecond tiled-grating compressor using nonlinear measurements like second harmonic generation. In particular, we observe the influence of this error when developing high-power laser experiments such as high-order harmonic generation. The generation of nonlinear processes with femtosecond pulses compressed in tiled-grating systems is studied. Special attention is paid to the role of the longitudinal piston error which is the most difficult to overcome in the compressor alignment. A complex spatio-temporal structure is expected to appear due to that misalignment. Both second harmonic generation in nonlinear crystals and high-order harmonic generation in gases are studied and a strong dependence with piston error is found, thus leading to a sub-micron modulation in the generated signal. In particular, the high sensitivity of the high-order harmonics to the longitudinal position allows one to use this processes for the accurate alignment of the compressor to few tens of nanometers.     

Phase characteristics of high-order harmonics in the cut-off region

The behavior of the phases of high-order harmonics generated upon interaction of intense laser radiation with matter is investigated. Some specific features typical of the harmonic phases in the high-frequency part of plateau (cut-off harmonic phases) are found. First, the phase difference between neighboring harmonics is a constant value. The width of the spectral range in which this regularity occurs determines the minimum duration of the attosecond pulse obtained from these harmonics by the so-called amplitude gating technique. Second, it is shown that the phase of each harmonic in the cut-off region depends linearly on the laser intensity. The proportionality coefficient is the same for all harmonics in this region and proportional to the cube of laser wavelength. Thus, this dependence is especially important for generating high-order harmonics by lasers with a wavelength of few micrometers.     

Phase-matched high-order harmonics by interaction of Ar atoms with high-repetition-rate low-energy femtosecond laser pulses

High-order harmonics are generated by coherent interaction of an intense laser and atoms or molecules[1]. With the development of the intense ultrashort pulse laser, the research of high-order harmonic generation has reached the water-window in spectral region[2] and subfemtosecond in time domain[3]. Especially, the generation and application of subfemtosecond pulse led the study of high-order harmonic generation into a completely new world[4, 5]. It has made the study of ultrafast science fro…     

Resonance-parametric mechanism of optical rectification and high harmonic generation

The generation of zero and high-order harmonics in the spectrum of a laser pulse propagating through a medium containing quantum particles whose constant resonance transition dipole moment is nonzero is studied theoretically. The consideration is performed in the approximation of slowly varying envelopes modified for the case of the medium with the nonzero permanent dipole moment. It is shown that this modification requires consideration of antiresonance terms, in particular, the Bloch-Siegert shift in equations. The conditions are revealed for the efficient optical rectification and excitation of the second harmonic at a quasi-monochromatic signal applied to the medium.