A. A. Korolev

Characteristic features of generation of few-cycle pulses in infrared and terahertz spectral ranges upon interaction of two femtosecond pulses of different frequencies in air

The results of a theoretical analysis of the generation of broadband radiation in the infrared and terahertz spectral ranges upon the excitation of plasma in air by two femtosecond pulses at the fundamental and second-harmonic frequencies of a Ti-sapphire laser are presented. It is found that the appearance of long-wavelength radiation in a strong field of pulses of different frequencies can be described in terms of strongly anharmonic oscillations of optical electrons, whereby electrons are pulled far away from their atoms; these oscillations are accompanied by cascade transitions of electrons from their ground state to a bound excited state, followed by a transition to the continuum. It is shown that the generated infrared and terahertz radiation appears in the form of pulses containing a few oscillations of the light field. The efficiency of terahertz generation varies periodically with an increase in the interaction length of the femtosecond pulses of different frequencies.

Dynamics of the strong field of a few-cycle optical pulse in a dielectric medium

The dependence of the conditions for the dominance of different physical factors in the self-action of few-cycle optical pulses in dielectrics on the intensity, duration, and spectrum of radiation has been theoretically analyzed. It is shown that the larger the pulse width and the central wavelength, the stronger the effect of plasma nonlinearity. For example, for a quartz glass in the field of pulses with a duration of 10 fs and a central wavelength of 780 nm, this nonlinearity mechanism is dominant at intensities exceeding 3 × 1013 W cm−2.

Strong-field dynamics of a light pulse composed of a small number of vibrations when plasma is excited in a dielectric medium

The paper discusses the spectral-broadening processes of femtosecond pulses in the high-frequency region when optical breakdown occurs in a gaseous medium. It is shown that the spectrum of the generated high-frequency radiation contains several isolated maxima whose positions are determined by the properties of the medium and by the thickness of the layer of gas. An illustration is given of the role of taking into account the dispersion of the nonlinear refractive index in the high-frequency region of the optical range of the spectrum when optical breakdown is being modelled. The results of the modelling are compared with experimental data from an investigation of the optical breakdown of a layer of sulfur hexafluoride.

Few-cycle strong light field dynamics in dielectric media

A nonlinear wave equation suitable for describing a propagation of a light pulse containing few oscillations of a strong electric field in isotropic dielectric media is deduced. It describes non-resonant dispersion of linear refractive index and non-inertial third-order non-linearity as well as inertia of dielectric non-linearity of electron nature, including parts caused by energy state population dynamics and free electron motion. The dependence of the conditions for the dominance of different physical factors in the self-action of few-cycle optical pulses in dielectrics on the intensity, duration, and spectrum of radiation has been theoretically analyzed. It is shown that the larger the pulse width and the central wavelength, the stronger the effect of plasma nonlinearity. For example, for a quartz glass in the field of pulses with a duration of 10 fs and a central wavelength of 780 nm, this nonlinearity mechanism is dominant at intensities exceeding 3 - 1013 W/cm2.

Displacement of the spectrum of a light pulse composed of a small number of oscillations toward shorter wavelengths in a dielectric with plasma nonlinearity

The principal regularities are determined of the self-action of high-intensity ultrashort laser pulses, caused by ionization of a dielectric medium as the pulses propagate in it. It is shown that the plasma nonlinearity generated in dielectrics in the strong fields of pulses composed of a small number of oscillations causes profiling of the emission spectrum and shifts its maximum toward shorter wavelengths.