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An alternative approach to determining the conditions of phase synchronism in the generation of harmonics in gases is proposed to take into account changes in the wave vector detuning during the propagation of an intense femtosecond laser pulse. Based on a numerical study of the third-harmonic generation by an argon medium, laws of scaling laser radiation absorption in the medium and of the phase modulation of spectral field components, which link macroparameters of interaction (pressure, intensity, and degree of medium ionization), have been identified. The possibility was investigated of increasing the third harmonic signal by increasing the gas jet pressure while keeping absorption at a low level. [ABSTRACT FROM AUTHOR]

A study is performed of the dynamics of free carriers in the conduction band of a dielectric when it is induced by femtosecond near-infrared range laser radiation. It is shown that the dispersion term in the Fokker–Planck equation that describes such dynamics must be considered. [ABSTRACT FROM AUTHOR]

The dynamics of free carriers in CaF2 induced by intense femtosecond radiation of the near- (1.24 μm) and mid- (4.4 μm) infrared ranges has been studied. It has been shown that the energy distribution of carriers formed after the passage of a laser pulse with a fluence sufficient for active impact ionization has a shape independent of the parameters (fluence, duration, and wavelength) of laser radiation and the width of the band gap of the material. The mean kinetic energy of free carriers after the action of the pulse is 0.56 of the width of the band gap. The inclusion of the dispersion term in rate carrier dynamics models makes it possible to reduce the difference of the mean kinetic energy from the statistical Fokker–Planck model by 30%. The application of rate carrier dynamics models instead of statistical models requiring large time and computational resources allows one to accelerate calculations by a factor of 20. [ABSTRACT FROM AUTHOR]

Being the second most abundant element on earth after oxygen, silicon remains the working horse for key technologies for the years. Novel photonics platform for high-speed data transfer and optical memory demands higher flexibility of the silicon modification, including on-chip and in-bulk inscription regimes. These are deepness, three-dimensionality, controllability of sizes and morphology of created modifications. Mid-IR (beyond 4 µm) ultrafast lasers provide the required control for all these parameters not only on the surface (as in the case of the lithographic techniques), but also inside the bulk of the semiconductor, paving the way to an unprecedented variety of properties that can be encoded via such an excitation. We estimated the deposited energy density as 6 kJ cm−3 inside silicon under tight focusing of mid-IR femtosecond laser radiation, which exceeds the threshold value determined by the specific heat of fusion (~ 4 kJ cm−3). In such a regime, we successfully performed single-pulse silicon microstructuring. Using third-harmonic and near-IR microscopy, and molecular dynamics, we demonstrated that there is a low-density region in the center of a micromodification, surrounded by a "ring" with higher density, that could be an evidence of its micro-void structure. The formation of created micromodification could be controlled in situ using third-harmonic generation microscopy. The numerical simulation indicates that single-shot damage becomes possible due to electrons heating in the conduction band up to 8 eV (mean thermal energy) and the subsequent generation of microplasma with an overcritical density of 8.5 × 1021 cm−3. These results promise to be the foundation of a new approach of deep three-dimensional single-shot bulk micromachining of silicon. [ABSTRACT FROM AUTHOR]

Low-order (fifth, seventh, and ninth) harmonics have been generated under the interaction of intense (I ~ 1014 W/cm2) femtosecond mid-infrared radiation of a laser system based on a Fe:ZnSe crystal (wavelength is 4.55 μm, pulse duration by the FWHM level of intensity is 160 fs, and the pulse energy is up to 3.5 mJ) with an argon jet (pressure is up to 10 bar) in the tunneling ionization regime (Keldysh parameter is ). The maximum energy efficiencies of the 5th, 7th, and 9th harmonic generation are 2 × 10–7, 6 × 10‒9, and 3 × 10–10, respectively. It has been established that nonlinear effects of propagation of generating radiation under an increase in the pressure of the gas jet begin to significantly affect the process of generation. [ABSTRACT FROM AUTHOR]

The theoretical and experimental studies of the excitation, detection, and propagation of magnons in antiferromagnetic nanostructures have been reviewed. The properties of antiferromagnetic materials such as the absence of a macroscopic magnetization, the presence of strong exchange interactions, and a complex magnetocrystalline structure make it possible to implement new types of memory and functional electronic devices. The study of possible magnon effects in antiferromagnetic materials in micro- and nanoscales requires new experimental and theoretical approaches. In this review, the recent results on the excitation of magnetic oscillations—magnons—in antiferromagnetic materials induced by the current and optical radiation are described and systematized. The main theoretical results on antiferromagnets and multilayer antiferromagnetic heterostructures are presented. Models for description of phenomena induced by the current and optical pulses in nanoheterostructures including antiferromagnets are considered. Methods for studying antiferromagnetic micro- and nanostructures by means of Brillouin scattering, as well as prospects of the application of antiferromagnetic spintronics and magnonics, are briefly discussed. [ABSTRACT FROM AUTHOR]

The change in the polarization state of the atomic response due to the appearence of the longitudinal component, which has been predicted in [A. Andreev, S. Stremoukhov, and O. Shoutova, Europhys. Lett. 120, 14003 (2017)], has been studied. The dependence of the vector properties of the atomic current induced in an atom with unit angular momentum in the ground state on the properties of a two-color laser field with orthogonally polarized components has been analyzed. It has been shown that the variation of the delay time between the frequency components of the laser field makes it possible to efficiently control the polarization of the components of the atomic response field. The effect of the mutual orientation of the electric component of the laser field and the angular momentum of the atom on the longitudinal component of the atomic current and on the polarization properties of the generated radiation has been demonstrated. [ABSTRACT FROM AUTHOR]

Within the framework of the non-perturbative light-atom interaction theory, we consider the population dynamics of the ground-state sublevels corresponding to the different values of the orbital quantum number ( l) projections and its influence on the high optical harmonics. We study the problem of high optical harmonics generation in two geometries: (1) two linearly polarized fields at frequencies ω and 2 ω for different angles of azimuthal projection difference and arbitrary orientation with respect to the angular momentum of the atom direction; (2) two elliptically polarized fields at frequencies ω and 2 ω rotating in opposite directions with variable mutual ellipticity. We show that the unequal population dynamics of the levels with different projections of the atom angular momentum (magnetic quantum number) has a significant impact on the value of the ellipticity of the generated radiation. [ABSTRACT FROM AUTHOR]

A theory of the nonlinear optical response of an atom interacting with a superposition of arbitrarily polarized fields is developed. The theory is based on the analytical solution of the boundary-value problem for an electron moving in a spherically symmetric intraatomic field and in the field of an external electromagnetic field. By means of the example of an argon atom interacting with a bichromatic field formed by the first and second harmonics of a Ti:sapphire laser, it is shown that, when an atom interacts with the field of two polarized pulses the polarization directions of which are not collinear, the response spectrum significantly depends on the laser radiation parameters-the duration and intensity of pulses, the time of delay between them, and the angle between the directions of polarization vectors. Generation of THz radiation is shown to be possible in the ionization-free regime due to intraatomic nonlinearity. [ABSTRACT FROM AUTHOR]

We develop the theory of interaction of the electromagnetic field and a single atom being in an arbitrary state and having an arbitrary direction of the angular momentum of the atomic electron with respect to the direction of the field polarization vector. It is shown that the atom response current has a tensor structure and depends on both the direction of the angular momentum of the atom, and the polarization vector of the external field. The tensor character of the response is determined by the externally induced anisotropic distribution of the probability density of spatial localization of the atomic electron. It is shown that the induced-anisotropy effects clarify the harmonic generation mechanism at play during the non-resonance interaction of laser radiation with atomic media. The developed theory is applied to the analysis of the problem about the generation of terahertz waves in a two-color laser field. It is shown that the change in the mutual orientation of wave polarization vectors leads to a significant increase in the efficiency of conversion of high-frequency fields to low-frequency ones. It is shown for the first time that the generation of terahertz waves is possible in the preionization regime, when the generation mechanism is related to atomic nonlinearity. [ABSTRACT FROM AUTHOR]

theory of the nonresonant response of a single atom in a state with arbitrary magnitude and direction of the angular momentum of an atomic electron with respect to the polarization vector of the acting electromagnetic field has been developed. It has been shown that the atomic response current has a tensor structure and depends both on the direction of the angular momentum of the atom and on the polarization vector of the external field. The tensor character of the response is due to the effects of the anisotropy of probability density distribution of the atomic electron as compared to the case of the free atom. The selection rules for the axisymmetric problem of the atom in the field have been analyzed. The manifestation of the selection rules in the angular spectra of photoelectrons has been demonstrated. The probability of the ionization of the atom has been analyzed as a function of the amplitude and duration of the pulse. It has been shown that the width of the generation spectrum is a nonlinear function of the field strength and is saturated in the region of nearly atomic fields. Methods for controlling the parameters of the atomic response spectrum have been proposed on the basis of the use of a sequence of laser pulses with various time profiles, carrier frequencies, and polarization states. It has been shown that the generation of terahertz radiation is possible in the preionization regime, where the generation mechanism is attributed to atomic nonlinearity. [ABSTRACT FROM AUTHOR]

Specific features of ionization of single atoms by laser fields of a near-atomic strength are investigated. Calculations are performed for silver atoms interacting with femtosecond laser pulses with wavelengths λ = 800 nm (Ti:Sapphire) and λ = 1.064 μm (Nd:YAG). The dependences of the probability of ionization and of the form of the photoelectron energy spectra on the field of laser pulses for various values of their duration are considered. It is shown that the behavior of the probability of ionization in the range of subatomic laser pulse fields is in good agreement with the Keldysh formula. However, when the field strength attains values close to the atomic field strength, the discrepancies in these dependences manifested in a decrease in the ionization rate (ionization stabilization effect) or in its increase (accelerated ionization) are observed. These discrepancies are associated with the dependence of the population dynamics of excited discrete energy levels of the atom on the laser pulse field amplitude. [ABSTRACT FROM AUTHOR]

Abstract  Ionization and high-harmonics generation in a single hydrogen-like atom driven by a laser pulse of near-atomic field strength is the subject of this paper. We use exact solutions of the eigenvalue problem on particle motion in the cylindrically symmetric field (CSF) as a basis for the wave-function expansion. The superposition of the spherically symmetric intra-atomic field and linearly polarized laser field has the cylindrical symmetry. Hence, the use of the free-atom eigenfunctions as a basis for the wave-function expansion requires an infinitely increasing number of spherical harmonics (i.e., free-atom eigenfunctions) when the laser field strength approaches the intra-atomic field value. The eigenfunctions of the CSF problem depend on the laser field strength; therefore, the appropriate matrix elements show how the spectral width of atomic response and angular selection rules vary with increase in the laser field strength. The introduction gives a phenomenological semiclassical illustration of the problem. [ABSTRACT FROM AUTHOR]

High harmonic generation in gases is developing rapidly as a soft X-ray femtosecond light-source for applications. This requires control over all the harmonics characteristics and in particular, spatial properties have to be kept very good. In previous literature, measurements have always included several harmonics contrary to applications, especially spectroscopic applications, which usually require a single harmonic. To fill this gap, we present here for the first time a detailed study of completely isolated harmonics. The contribution of the surrounding harmonics has been totally suppressed using interferential filtering which is available for low harmonic orders. In addition, this allows to clearly identify behaviors of standard odd orders from even orders obtained by frequency-mixing of a fundamental laser and of its second harmonic. Comparisons of the spatial intensity profiles, of the spatial coherence and of the wavefront aberration level of 5ω at 160 nm and 6ω at 135 nm have then been performed. We have established that the fundamental laser beam aberrations can cause the appearance of a non-homogenous donut-shape in the 6ω spatial intensity distribution. This undesirable effect can be easily controlled. We finally conclude that the spatial quality of an even harmonic can be as excellent as in standard generation. [ABSTRACT FROM AUTHOR]