Resonance processes during harmonic generation in plasmas using mid-infrared radiation

A few recently introduced approaches of the high-order harmonic generation in laser-produced plasmas are reviewed. We show how the tuning of odd and even high-order harmonics of ultrashort pulses along the strong resonance of laser-produced indium plasma using optical parametric amplifier of white-light continuum radiation (1250?1400 nm) allows observation of different harmonics enhanced in the vicinity of the transition of In II ions possessing high oscillator strength. We discuss various peculiarities and discuss the theoretical model of the phenomenon of tunable harmonics enhancement in the region of 62 nm using indium plasma. With the theoretical analysis, we present the approach allowing reproduce the experimental observations and characterize the dynamics of the resonant harmonic emissions. We also discuss the resonance enhancement of harmonics using mid-infrared radiation in the tin, chromium, and antimony plasmas and show the calculations of this process.     

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.     

Role of resonances in the high-order harmonic enhancement in diatomic molecules

We present the study of high-order harmonic generation from the diatomic molecules produced during the laser plasma formation at the surfaces of semiconductors (InSb, InGaP, InP, GaAs). The discrepancy between the calculated molecular spectra and the observed resonance-enhanced harmonics in semiconductor molecules was attributed to the much greater influence of the strong electric field of laser radiation on the dynamical modification of the molecular spectra. The Stark shift of the energy levels of diatomic molecules was considered the main reason of the decrease of single harmonic efficiency compared to the monoatomic plasmas (In, As, Sb), where the enhancement of single harmonic was attributed to the closeness of the harmonic wavelength and the ionic transitions possessing strong oscillator strength.     

Organic compound-contained plasmas as the media for frequency conversion of ultrashort pulses

We demonstrate the high-order harmonic generation in the plasmas produced on the surfaces of organic targets. The role of resonances and two-color pump-induced enhancement of harmonics is analyzed. We show that the harmonic spectra from organic molecules-contained plasmas demonstrate the similarities and distinctions with the one from graphite ablation.     

Experimental study of conversion from atomic high—order harmonics to x—ray emissions

There are two physical phenomena in a strong laser intensity. One is the high-order harmonic emission; the other is x-ray emission from optical-field ionized plasmas. The experiment of conversion from high-order harmonics to x-ray emissions was given with a 105fs Ti:sapphire laser by adjusting laser intensities. The ingredient in plasma was investigated by the numerical simulations.Our experimental results suggested that the free electrons have detrimental effects on harmonic generation but are favourable for x-ray emission from optical-field ionized plasmas. If we want to obtain more intense harmonic signals as a coherent light source in the soft x-ray region, we must avoid the production of free electrons in plasmas. At the same time, if we want to observe x-rays for the development of high-repetition-rate table-top soft x-ray lasers, we should strip all atoms in the plasmas to a necessary ionized stage by the optical-field-ionization in the field of a high-intensity laser pulse.     

Enhancement of high-order harmonics generated in laser-produced plasma using ionic resonances and nanoparticles

We review recently emerged methods of high-order harmonic enhancement in the extended laserproduced plasmas using ionic resonances and nanoparticles and show the difference of this approach with regard to previously used methods. Particularly, we analyze the harmonic generation in graphene and a few other clustered media using 800-nm-class lasers. We also discuss the application of parametric waves for frequency conversion towards the extreme ultraviolet range and show the resonance enhancement of harmonics in laser-produced zinc plasma using tunable mid-infrared pulses. The reviewed studies demonstrated the attractiveness of the joint application of the methods of harmonic enhancement using ionic resonances and nanoparticles.     

Controlling the macro- and micro-processes influenсing harmonic generation in laser-produced plasmas

We review the studies of the spatial conditions for the quasi-phase-matching in the multi-jet laserproduced silver plasma. These studies of the off-axis and on-axis spatial components of harmonics allowed the demonstration of significant enhancement of a group of harmonics in the latter case. We analyze the role of plasma emission, together with phase-mismatch, that deteriorate the conditions of high-order harmonic generation due to significant phase distortion between the interacting waves. We also discuss the resonanceinduced enhancement of single harmonic and the quasi-phase-matching-induced enhancement of the group of harmonics during propagation of the tunable mid-infrared femtosecond pulses through the perforated laser-produced indium plasma. Those studies have shown that the enhancement of harmonics using the macro-process of quasi-phase-matching is comparable with the one using micro-process of resonantly enhanced harmonic. We conclude that joint implementation of the two methods of the increase of harmonic yield could be a useful tool for generation of strong short-wavelength radiation in different spectral regions.     

Coherent and incoherent XUV emission in helium and neon,laser-driven plasmas

The authors present results of measurements of high-order harmonic generation and XUV spontaneous emission in helium and neon plasmas excited by a short pulse laser at intensities between 10¹⁴ and 5×10¹⁷ W/cm². They compare the observed behavior of the harmonics with recent single atom calculations in helium. A wavelength dependence to the efficiency of harmonic generation that has not been previously reported is observed. Line emission from excited state transitions in Ne⁷⁺ in a short pulse laser-driven plasma is also observed. In particular, strong emission and a rapid recombination rate for the 9.8-nm (3d-2p) transition that is a possible candidate for a recombination-pumped X-ray laser is shown     

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.     

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.     

Advantages of plasma harmonic generation using high pulse repetition rate lasers: Review of recent studies

The studies of coherence properties of the harmonics generating in laser-produced plasmas, the analysis of the optical nonlinearities of deoxyribonucleic acid components, the resonance enhancement of harmonic in the cases of excitation of indium plasma by multi- and few-cycle pulses, and the application of nanoparticle-based emitters of harmonics using high-pulse repetition rate lasers are reviewed. The analysis of various aspects of plasma harmonic generation at the conditions of optimal excitation of the ablated targets irradiating by 1 kHz lasers is presented. The growth of plasma harmonic conversion efficiency, single harmonic emission, nonlinear spectroscopy of complex organic components, as well as high coherency of harmonic radiation show the advantages of using plasma harmonic technique for optimization of the sources of coherent extreme ultraviolet radiation and for the material science studies. These studies allowed a significant growth of the average power of harmonics compared with the case of 10 Hz lasers.     

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.     

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.     

Methods for Modifications of Harmonic Spectra from Laser-Produced Plasmas

We review the studies of the modifications of harmonic spectra from laser-produced plasmas. The substantial spectral broadening and blue shift of the harmonics generating in the laser plasma during transformation of the spectrum of the laser radiation formed the filaments upon passage of femtosecond pulses through the argon gas is analyzed. We also discuss the studies of harmonic spectra variations that occur as a result of the passage of the laser pulses through the carbon- and metal-containing plasmas at the variable density of ablated species and intensity of driving pulse. To show other type of modulation of harmonic emission spectra, the plasmas containing the small molecules of silver produced during ablation of the bulk silver and silver nanoparticles at the conditions suitable for efficient harmonic generation of the ultrashort pulses propagating through the laser-produced plasmas were analyzed. The time-of-flight mass-spectroscopy studies of plasmas confirmed the presence of these species in plasmas.     

Application of organic compounds for high-order harmonic generation of ultrashort pulses

The studies of the high-order nonlinear optical properties of a few organic compounds (polyvinyl alcohol, polyethylene, sugar, coffee, and leaf) are reported. Harmonic generation in the laser-produced plasmas containing the molecules and large particles of above materials is demonstrated. These studies showed that the harmonic distributions and harmonic cutoffs from organic compound plasmas were similar to those from the graphite ablation. The characteristic feature of observed harmonic spectra was the presence of bluesided lobes near the lower-order harmonics.     

High-order harmonic enhancement using the quasi-phase-matching in a laser plasma

The quasi-phase-matching of the high-order harmonics generating in the multiple plasma jets produced on the surfaces of perforated Ag, V, and Mn targets is demonstrated. The 20× and 30× enhancement factors of quasi-phase-matched harmonics of 804 nm radiation in the range of the 35th harmonic in the cases of silver and vanadium plasmas were achieved.     

Quasi-phase-matching of harmonic waves in plasmas: Calculations,new schemes,and applications

We review recent studies of the coherent phase matched conversion of ultrashort pulses in the modulated plasmas. Particularly, we discuss the influence of ablated and tunneled electrons on the quasiphase- matched high-order harmonic generation in laser-produced plasma and application of mid-infrared pulses for the quasi-phase-matching of harmonics in silver plasma. These studies demonstrate the perspectives of frequency conversion in properly modulated laser-produced plasmas.     

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.     

Enhanced harmonic generation from expanding clusters

We report controlled enhancement of optical third harmonic generation (THG) from hydrodynamically expanding clusters of approximately 6x10(5) noble-gas atoms several hundred femtoseconds following ionization and heating by ultrashort pump pulses. This resonant enhancement is more pronounced for orthogonal than for parallel pump-probe polarizations, a consequence of faster cluster expansion along the pump polarization. Simulations show that the nonlinear susceptibility chi(3) of the individual clusters and the coherence length of the clustered plasma medium are optimized nearly simultaneously as the clusters expand, and both contribute to the observed THG enhancement. This dual enhancement mechanism may be scalable to relativistic probe intensity and to generation of high-order harmonics in the soft-x-ray regime.     

Fullerenes: The attractive medium for harmonic generation

We review the results of systematic experimental studies of high-order harmonic generation (HHG) in C₆₀-rich laser-produced plasma under different plasma conditions and laser parameters. The morphology of fullerene clusters before and after ablation is analyzed to define the optimal conditions of excitation of C₆₀-containing targets. The enhancement of HHG efficiency in C₆₀-rich plasmas by using the two-color pump technique is discussed. The conversion efficiency for the odd and even harmonics in the vicinity of surface plasmon resonance of the C₆₀-containing plasma (40–70 nm) was estimated to be in the range of 10⁻⁴.