A generalized kinetic model of atomic level populations in an optically dense plasma excited by laser pulses of arbitrary duration is formulated and studied. This model is based on a nonstationary expression for the probability of excitation of an atomic transition and takes into account the effects of laser pulse penetration into an optically dense medium. A universal formula for the excitation probability as a function of time and propagation length is derived and applied to the case of a Lorentzian spectral profile of an atomic transition excited by a laser pulse with a Gaussian envelope. The features of nonstationary excitation probabilities are presented for different optical depths of the plasma, laser pulse durations, and carrier frequencies. The formulas derived here will be useful for the description of atomic populations excited by laser pulses under realistic conditions of dense plasmas.

Scattering of ultrashort laser pulses (USP) in optically dense plasma is presented, accounting for the peculiarities of ultrashort electromagnetic interaction and absorption of radiation in plasmas. The results are presented in terms of a universal dimensionless total scattering coefficient (TSC). A universal closed expression for TSC is derived and analyzed. Numerical calculations of TSC are carried out for the special case of USP resonance scattering on H-like ions in hot plasmas. Fine structure splitting of the resonant transition and Doppler broadening are taken into account. The dependences of the TSC on USP parameters and the optical thickness of plasma layers are derived. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license.

Abstract A simple efficient approach describing the interaction of various quantum targets with short electromagnetic (EM) pulses firstly proposed in [1] is reviewed. This approach is based on the expression for the photoprocess probability in terms of the cross section and the Fourier transform of the EM pulse. Our method makes it possible to take into account the internal dynamics of the target and the influence of the pulse parameters in a simple way. The proposed approach is used both for the probability for the entire pulse duration, and for its dependence on the current time. In addition, the developed method is applied to generalize the simulation of the population kinetics of a quantum system to take into account the dependence of the photoprocess rate on time. It is also shown that our expression for the probability of excitation of a micro-target corresponds to the formula describing the energy loss of a non-monochromatic field in a dispersive medium and can be generalized on other EM processes.

This study is devoted to generalization of the traditional approach for the description of photon scattering on free electrons in the case of ultrashort laser pulses. In the framework of the second order of quantum mechanical perturbation theory with the use of the Klein–Nishina formula, we derived an expression for the total scattering probability during the whole time of the pulse action that is applicable in the relativistic limit. The redshift of scattered pulse spectra under an increase in the scattering angle in the relativistic case was studied. The trends of the total scattering probability on the duration of ultrashort laser pulses were categorized.

This study examines the excitation of transverse optical phonons in a semiconductor sample under the action of electromagnetic pulses of arbitrary duration, in view of their passage through the interface and absorption in matter. A universal formula for the total absorption coefficient through the components of the complex refractive index of the medium is derived. The dependences of phonon excitation efficiency on the sample thickness and excitation pulse parameters are examined using the example of a GaAs sample and pulses with Gaussian envelope. The specific features of the considered process are established for the carrier frequencies of the pulse inside and outside the photonic forbidden zone.

The dependence of energy transferred to a damped harmonic oscillator by a pulse of periodic force on pulse duration, carrier frequency, and shape of the pulse envelope is investugated. The cases of pulses with an exponential and Gaussian shape of the envelope are analyzed. Analytical expressions describing the process are obtained. It is established that the dependence of energy transferred to the oscillator on pulse duration in the case of an exponential pulse can have an inflection point, while extrema appear in the discussed dependence at sufficiently large offset of the pulse carrier frequency from the oscillator natural frequency in the case of a Gaussian pulse. It is demonstrated that the number of quanta and the amplitude of the average value of the coordinate operator upon excitation of a quantum oscillator are determined by the same dependences on the driving-pulse parameters as in the case of the classical oscillator.

It is shown that a consistent microscopic quantum approach for describing the interaction of ultrashort electromagnetic pulses with matter, developed earlier by the author in the paper [1], leads to the well-known macroscopic formula for the dissipation of the energy of a non-monochromatic field in a medium. This formula is used to study the features of energy transfer from various types of ultrashort pulses both with and without carrier frequency to a solid target.

To explain the dynamics of the population of a quantum system excited by an ultrashort laser pulse, the generalized kinetic model is proposed and tested. It is shown that this model is accurate within the constraints of the applicability of the perturbation theory for any values of the laser pulse's duration and carrier frequency. By comparing the outcomes of our model to the two conventional methods and by precisely resolving the Bloch equations, we can assess the effectiveness of them. It is demonstrated that the usual techniques considerably overestimate the population values of the higher state of the excited quantum system outside the boundaries of their applicability. When using the Bloch equations is problematic or impossible, the suggested model can be utilized to characterize the population kinetics in the general situation.

A brief review of the classical and quantum description of the interaction of electromagnetic radiation with matter based on the model of a harmonic oscillator is presented. This review includes the generalized Bohr correspondence principle, the excitation of a quantum oscillator by electromagnetic pulses including saturation effect, the harmonic limit of the Bloch equations, and a phenomenological account of the damping of the quantum oscillator. In all cases, at the mathematical level, the relationship between the classical and quantum descriptions of the electromagnetic interaction is established and the conditions for such compliance are identified.

The excitation of a quantum oscillator at transitions between stationary states by wavelet pulses with zero and nonzero area as a function of their duration and amplitude is studied. The excitation from the ground state is examined in detail, together with the specific features of this process during excitation from excited states. Analytical expressions for the main characteristics of excitation probability under weak and strong perturbation of quantum oscillator were obtained within limits of strict consideration. Particularly, it was found that, on a qualitative and quantitative level, there is no difference between excitation by pulses with zero and non-zero area.

The time dependence (on the pulse duration and on current time) of resonant photoprocesses induced by electromagnetic pulses of various durations (including ultrashort and quasi-monochromatic pulses) is analyzed using perturbation theory. Pulses with the Gaussian and exponential envelopes, as well as the Lorentzian and Gaussian spectral profiles of a photoprocess cross section are considered. Simple analytic expressions are obtained for the probability in the long-time limit. The time dependence for a given pulse duration is investigated analytically in the monochromatic and ultrashort limits and numerically for intermediate values of parameters. Specific features of the temporal dynamics, which are common for resonant photoprocesses and depend on the pulse shape and the spectral cross section profile are established.

It is well known that the harmonic oscillator model can be employed for weakly excited mechanical systems. When excitation grows, anharmonicity begins to be noticeable and more sophisticated models should be used.

We investigate the excitation of a quantum harmonic oscillator by pulses with different envelopes in terms of the excitation probability during the action of a pulse. The majority of attention is given to the dependence of the probability on the pulse duration and the pulse carrier frequency for three envelopes: exponential, double exponential, and rectangular. The choice of these envelopes makes it possible to cover various features of the excitation of a quantum oscillator by an external pulse. In particular, the presence of weak and strong excitation modes is established, for each of which the dependences of the process probability on pulse parameters are studied.

The saturation effect during the excitation of a two-level system by laser pulses is investigated in the framework of two approaches: excitation probability based on Karplus–Schwinger spectral profile and exact solution of Bloch equations. Simple analytical expression for the excitation probability by exponential pulse is derived. The excitation spectra obtained using this expression were compared with the result of solving the Bloch equations for various values of the pulse duration and the Rabi frequency, which describes the strength of the electromagnetic interaction. It is shown that in the case of long pulses, there is a satisfactory correspondence between the two approaches, but in short-pulse limit and strong saturation, the probability description based on Karplus–Schwinger spectral profile and perturbation theory does not provide satisfactory results.

The effect of plasma Coulomb microfied dynamics on spectral line shapes is under consideration. The analytical solution of the problem is unachievable with famous Chandrasekhar–Von-Neumann results up to the present time. The alternative methods are connected with modeling of a real ion Coulomb field dynamics by approximate models. One of the most accurate theories of ions dynamics effect on line shapes in plasmas is the Frequency Fluctuation Model (FFM) tested by the comparison with plasma microfield numerical simulations. The goal of the present paper is to make a detailed comparison of the FFM results with analytical ones for the linear and quadratic Stark effects in different limiting cases. The main problem is connected with perturbation additions laws known to be vector for small particle velocities (static line shapes) and scalar for large velocities (the impact limit). The general solutions for line shapes known in the frame of scalar perturbation additions are used to test the FFM procedure. The difference between “scalar” and “vector” models is demonstrated both for linear and quadratic Stark effects. It is shown that correct transition from static to impact limits for linear Stark-effect needs in account of the dependence of electric field jumping frequency in FFM on the field strengths. However, the constant jumping frequency is quite satisfactory for description of the quadratic Stark-effect. The detailed numerical comparison for spectral line shapes in the frame of both scalar and vector perturbation additions with and without jumping frequency field dependence for the linear and quadratic Stark effects is presented.

The excitation of a linear quantum oscillator (LQO) upon a collision with a charged particle moving in a rectilinear trajectory is analyzed. The probability and the cross section of the process are calculated beyond the framework of perturbation theory for different charges of the incident particle, including multiply charged ions. The excitation between the LQO stationary states, as well as total excitation from the ground state, are considered. The characteristic features of the process depending on the problem parameters are analyzed.