Enhancement Of High-order Harmonics Research Articles

Amplification and ellipticity enhancement of high-order harmonics in a neonlike x-ray laser dressed by an IR field

In Khairulin et al. [Sci. Rep. 12, 6204 (2022)] we proposed a method for amplifying a train of sub-femtosecond pulses of circularly or elliptically polarized extreme ultraviolet radiation, composed by high-order harmonics of an infrared (IR) laser field, in a neonlike active medium of a plasma-based x-ray laser, additionally irradiated with a replica of a fundamental frequency laser field used to generate harmonics. Here we present the analytical theory and numerical study of an amplification process of an individual harmonic with either preservation of its polarization state or ellipticity enhancement. We also analyze in detail the spectral-temporal and polarization properties of a sub-femtosecond pulse train formed by a set of elliptically or circularly polarized harmonics during the propagation through the medium. We discuss also the possibility of an experimental implementation of the suggested technique in an active medium of an x-ray laser based on neonlike ${\mathrm{Ti}}^{12+}$ ions irradiated by an IR laser field with a wavelength of 3.2 \textmu{}m.

High-Order Harmonic Enhancement in Nanowire by Inhomogeneous Field

We theoretically investigate high-order harmonic generation from nanowire in inhomogeneous laser field. By solving the time-dependent Schrodinger equation, the simulated harmonic spectrum exhibits a double-plateau structure. The cutoff of the second plateau is extended and the intensity of the high-order harmonics in the second plateau is increased by about two to three orders of magnitude compared with that from the homogeneous laser field. Moreover, we analyze the influence of the inhomogeneous laser field polarization orientation to the nanowire. It is shown that the intensity of the high-order harmonics in the second plateau can be effectively controlled by varying the inhomogeneity parameter and the laser field polarization orientation to the nanowire. These phenomena are attributed to the time-dependent population of the electron, which has a strong relevance with the intensity of the harmonic.

Resonance-enhanced harmonics in mixed laser-produced plasmas

The enhancement of high-order harmonics of ultrashort pulses is crucial for application of coherent extreme ultraviolet (EUV) pulses in optoelectronics. Strong resonances of ionic species are useful for enhancement of the yield of single harmonic in EUV region. Another attractive concept is the quasi-phase matching (QPM) allowing the growth of conversion efficiency of the groups of harmonics in different ranges of EUV. Here we demonstrate simultaneous enhancement of 9th, 13th and 17th harmonics of 800 nm, 30 fs pulses in laser-produced plasmas (LPP) created on the surfaces of Zn, In and Sn targets. The enhancement of harmonics is attributed to the resonance transitions of singly charged particles of those materials. Strong 9th harmonic is generated in Zn plasma in a broad range of the delays between heating and probe laser pulses. We analyze strong even harmonics enhanced in Zn + In and Sn + In LPP at the conditions of two-color pump. We also show the concurrence of micro- and macroscopic processes in indium plasma by analyzing the resonance conditions for 13th harmonic and QPM for 21st and 31st harmonic using different configurations of LPP. We find the phase differences of harmonics generated in indium and zinc plasma, and show the pronounced effect of the resonances on the attosecond timing of the harmonic emission.

Effects of Laser Plasma Formation on Quasi-Phase Matching of High-Order Harmonics from Nanoparticles and Atoms.

The application of nanoparticles (NPs) and quasi-phase matching (QPM) each play an important role in the enhancement of high-order harmonics (HHG) of ultrashort laser pulses. We analyze various regimes of nanoparticle plasma formation for the creation conditions for maximal QPM-induced enhancement of the groups of harmonics in the extreme ultraviolet (XUV). Laser plasmas were formed on the surfaces of NPs- and microparticle (MPs)-contained targets using ablation by nanosecond, picosecond, and femtosecond pulses. Different conditions of laser plasma formation (extended and perforated plasma) and variable concentrations of free electrons in these three cases of laser ablation led to modifications of QPM conditions. We demonstrate novel approaches in the optimization of QPM at the conditions of laser ablation of NPs and MPs by pulses of different durations. The formation of QPM conditions using femtosecond and picosecond heating pulses during HHG in such plasmas allowed the growth of conversion efficiency of the groups of harmonics, with the enhancement factors exceeding 25× in different ranges of XUV, contrary to less efficient QPM in the case of nanosecond pulse-induced ablation.

Collisional dynamics in laser-induced plasmas: evidence for electron-impact excitation

We have experimentally investigated the collisional dynamics in femtosecond-laser-induced plasmas and presented the evidence for electron-impact excitation through enhanced high-order harmonic (HH) generation. The measurements were carried out by using an elliptically polarized pump pulse to induce the underdense plasmas and by using a time-delayed linearly polarized probe pulse to drive the HH generation from the plasmas. We found that the rise time of this enhanced HH generation was insensitive to the ellipticity degree (ED) of pump pulse but sensitive to its laser intensity (LI). With further comparison between physical scenarios and qualitative analysis, we demonstrated that the atomic excitation causing the HH enhancement should be attributed to the electron-impact excitation, i.e., the excitation from the collision between neutral atoms and electrons during the lifetime of the underdense plasma.

Enhancement of high-order harmonics in a plasma waveguide formed in clustered Ar gas.

Generation of high-order harmonics (HHs) is intensified by using a plasma waveguide created by a laser in a clustered gas jet. The formation of a plasma waveguide and the guiding of a laser beam are also demonstrated. Compared to the case without a waveguide, harmonics were strengthened up to nine times, and blue-shifted. Numerical simulation by solving the time-dependent Schrödinger equation in strong field approximation agreed well with experimental results. This result reveals that the strengthening is the result of improved phase matching and that the blue shift is a result of change in fundamental laser frequency due to self-phase modulation (SPM).

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.

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-induced enhancement of high-order harmonics during two-colour pump of multi-jet plasmas

The first demonstration of quasi-phase-matching during high-order harmonic generation (HHG) of laser radiation in multi-jet plasmas produced on the silver and chromium targets under the conditions of two-colour pump (804 and 402 nm) is presented. We analyse the enhancement of the groups of odd and even harmonics along the plateau range and compare the harmonic yields using a different number of plasma jets. The comparison of extended line plasma and multi-jet plasma shows the advanced properties of the latter formation for the efficient HHG using the two-colour pump. The 25× and 13× enhancement factors of quasi-phase-matched harmonics were obtained in the cases of the two-colour and single-colour pumps of multi-jet plasmas.

Quasi-phase-matching of laser harmonics using variable multi-jet plasmas

In this paper, we demonstrate the enhancement of high-order harmonics induced by the quasi-phase-matching (QPM) in different spectral ranges. The tuning of QPM harmonics was accomplished by changing the sizes of multiple plasma jets. The 40× and 15× enhancement factors for the QPM harmonics were achieved in the silver and manganese multiple-jet plasmas for the 35th and 55th harmonics.

Modulated phase matching and high-order harmonic enhancement mediated by the carrier-envelope phase

We report a novel quasi-phase matching technique for high-order harmonic generation in low-density gases. Numerical simulations show that in few-optical cycle pulsed Bessel beams it is possible to control the pulse envelope and phase velocities which in turn allows to control the carrier-envelope phase during propagation. The resulting oscillations in the peak intensity allow to phase-match the high-order harmonic generation process with a nearly two decade enhancement in the XUV power spectrum.

Orbital symmetry and interference effects in molecular high-order harmonic generation

We investigate harmonic generation from $\text{H}_{2}{}^{+}$ molecules driven by intense few-cycle laser pulses whose linearly polarization axis makes an arbitrary angle $\ensuremath{\chi}$ with respect to the molecular axis. The $\text{H}_{2}{}^{+}$ molecule is considered initially in various orbitals with nodal planes. It is found that a strong enhancement of high-order harmonics (HOHs) occurs when the laser polarization axis overlaps with major axes of electron distribution in the active orbital, while broad suppression of HOHs occurs when the laser polarization axis is parallel to a nodal plane of the active molecular orbital. We show that this harmonic suppression is enhanced by destructive interferences when the nodal and the laser polarization axes are parallel to the internuclear axis, which leads to a shortening of the harmonic cutoff. It follows that the orientation dependence of HOHs intensities mimics the electronic density in active orbitals through the angular dependence of ionization and recombination processes.

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.

Efficient extension and enhancement of high-order harmonics of He+ by combined laser pulses

We theoretically study the interaction of a He+ ion with fundamental laser pulses and different combined laser pulses. For low laser intensity the cut-off energy is extended; however, for higher laser intensity the cut-off of the enhanced harmonics is extended under the combination of two fundamental laser pulses whose wavelengths are 800 nm and 394 nm, respectively. We investigate the yield of ionization as a function of peak laser intensity and find that the yield of ionization is different for low and higher laser intensity regimes. The efficient extension of cut-off energy of enhanced harmonics is demonstrated by a classical analysis.

The quantitative analysis of enhancement of high-order harmonics in two-color intense laser fields

The time-dependent Schrodinger equation of the interaction of laser pulse with He+ is solved by using the asymptotic boundary condition and symplectic algorithm in fundamental laser-field and two-color laser fields. We find that the conversion efficiency of high-order harmonic generation (HHG) is higher in the two-color laser fields than in the fundamental laser field, especially for the combination of ω 0 − 19ω 0. To explain these phenomena, the ionization, the average distance, the probability of first excited sate, and the transition probability are calculated. We give the qualitative and quantitative analysis for the enhancement of conversion efficiency of HHG.

High-order harmonic generation in a narrow spectral range

It is proposed to use the two-component pumping of atoms by a strong low-frequency field and a high-frequency ultrashort pulse for the generation of high-order harmonics. For a fixed relative phase of the pumping fields, high-order harmonics are generated in a rather narrow spectral range. The interaction of atoms with the two-component field also determines the limiting efficiency of high-order harmonic generation. The effect can be reversed for the enhancement of high-order harmonics by using additional high-frequency pump radiation.