Table of contents

The general concepts for generation and amplification of the X-pulses in optical parametric amplifiers under the plane-wave and localised (Bessel beam, or more generally, X-pulse) pump are reviewed. It is shown numerically and experimentally that X-pulse phase-matching gives rise to spontaneous emergence of the localised light structures in the regime of the parametric frequency down-conversion. The parametric amplification technique of localised waves is extended to the chirped X-pulse optical parametric amplification concept, which allows one to achieve few optical cycle, high-peak power localised wave packets for laser—matter interactions.

The domains of existence and peculiarities of exact analytic solutions of the problem of quasi-synchronous interaction of four plane collinear monochromatic waves — modes in a quadratically nonlinear medium during cascade frequency conversion are analysed. It is shown that the unusual types of multicomponent cnoidal and solitary soliton-like waves (of periodic and aperiodic energy-exchange regimes) are realised. Two of the four components of the latter are proportional to the real and imaginary parts of the well-known Lorentzian dependence, which is commonly used to describe the dispersion of contributions from resonance transitions to the complex permittivity in the case of homogeneous line broadening.

The effect of the real part of the Raman suscep-tibility on the Stokes spectrum excited by Gaussian and Bessel beams is studied theoretically and experimentally. This part of the susceptibility is shown to be responsible for the increase in the overlap integral of the Stokes beam and the pump beam and, as a result, for the increase in the Raman amplification in the region of frequencies higher than the frequency of the exact Raman resonance. This, in particular, is observed in the spectral shift of the axial component of Raman generation in the case of Bessel pump and Stokes radiation in the case of the Gaussian pump to the high-frequency region. It is shown that the shift caused by the Bessel pump significantly exceeds the shift caused by the Gaussian pump. The conical component in the case of the Bessel pump is generated at the frequency of the exact Raman resonance.

We report an experimental study of the first cw solid-state Raman laser operating simultaneously at the frequencies of the first and second Stokes components. Simultaneous generation is ensured by a cavity with an enhanced finesse at both Stokes frequencies. The threshold pump powers for the first (3.4 W) and second (3.67 W) Stokes components suggest that the second Stokes generation follows a cascade mechanism. We demonstrate for the first time Raman conversion with intensity stability exceeding the pump radiation stability and show that this approach may find application in Raman spectroscopy.

Sum-frequency generation from the surface of an isotropic gyrotropic medium by two normally incident homogeneously elliptically polarised Gaussian beams is studied theoretically. Analytic expressions, completely describing the transverse spatial distribution of the intensity and polarisation in the cross section of the reflected beam at the sum frequency, were derived taking into account both the local and nonlocal contributions of the quadratic nonlinearity of the medium thicknesses and the nonlinear contribution of its surface. It is shown that a special selection of the parameters of the fundamental waves allows one to determine components of the surface susceptibility tensor.

We demonstrate a new method of generation of frequency-tunable megawatt femtosecond pulses in the infrared based on multisoliton supercontinuum generation in a large-mode-area photonic-crystal fibre (PCF) followed by the temporal compression of the PCF output. Photonic-crystal fibres with a core area of about 710 μm² are employed to convert microjoule femtosecond laser pulses with a spectrum centreed at 1.39 μm into pulses with a central wavelength of 1520 nm, a pulse width of 210 fs, and a peak power of about 1 MW.

Four-photon laser scattering spectra of bidistilled water and aqueous solutions of biopolymers (proteins and DNA), carbon nanotubes and hydrogen peroxide have been measured in the range ±10 cm^-1. The spectra show rotational resonances of H₂O₂, ortho-H₂O and para-H₂O molecules. The resonance contribution of the H₂O rotational spectrum to the four-photon scattering signal in the solutions of the biopolymers and hydrophobic nanoparticles is an order of magnitude larger in comparison with water, which points to free rotation of the water molecules near the surface of such particles. This effect is due to the formation of water depletion layers near hydrophobic nanoparticles, as predicted in earlier theoretical studies.

Tunable optical parametric generators and amplifiers (OPA), proposed and developed by Akhmanov and his colleagues, have become the working horses in exploration of dynamical processes in physics, chemistry, and biology. In this paper, we demonstrate the possibility of using ultrafast polarisation-sensitive two-colour spectroscopy, performed with a set of two OPAs, to study charge photogeneration and transport in conjugated polymers and their donor-acceptor blends.

Formulas are derived for evaluating the diffusion coefficient and size of gas molecules from transient coherent anti-Stokes Raman scattering measurements. Numerical estimates are presented for hydrogen.

Supercontinuum emission observed upon filamentation of transform-limited collimated femtosecond laser pulses in a transparent condensed medium (fused KU-1 quartz) is studied experimentally and numerically. The splitting of diverging conical supercontinuum emission into discrete rings was observed with increasing the pulse energy.

Various methods for controlling the onset of filamentation of a high-power laser pulse in extended vertical atmospheric paths are analysed. It is shown that the increase in the structure constant of the atmospheric turbulence on average leads to the earlier formation of 'hot' spots when the initial pulse power exceeds the critical self-focusing power by an order of magnitude and more. It is found in the numerical experiments that the use of broad focused beams is preferable for achieving the minimal standard deviation of a distance for the filament onset.

The propagation of ultrashort 10-μm laser pulses of power exceeding the critical self-focusing power in xenon and air is numerically simulated. It is shown that the pulse duration in certain regimes in xenon can be decreased by 3–4 times simultaneously with the increase in the pulse power by 2–3 times. It is found that the average energy of electrons in a filament upon filamentation of 10-μm laser pulses in air can exceed 200 eV. The features of the third harmonic and terahertz radiation generation upon filamentation are discussed.

We report a study of hot electron generation via the interaction of femtosecond laser pulses of subrelativistic intensity (10¹⁵ to 2×10¹⁷ W cm^-2), having different linear polarisations and nanosecond-scale contrasts, with the surface of 'transparent' (quartz glass) and 'absorbing' (silicon) targets. As the incident pulse intensity increases from 10¹⁵ to 10¹⁷ W cm^-2, the difference in hard X-ray yield and average hot electron energy between s- and p-polarised beams rapidly decreases. This effect can be understood in terms of relativistic electron acceleration mechanisms.

It is shown for the first time that the use of autoionisation states for phase matching leads to the efficient selection of a single harmonic generated in a plateau region in plasma. The selected harmonic frequency can be tuned by changing the relative concentration of plasma components and tuning the fundamental radiation frequency. It is shown that the contrast of the selected harmonic can exceed 10⁴.

The self-organisation of the surface-relief nanostructures in solids under the action of energy and particle fluxes is interpreted as the instability of defect-deformation (DD) gratings produced by quasi-static Lamb and Rayleigh waves and defect-concentration waves. The allowance for the nonlocality in the defects—lattice atom interaction with a simultaneous account for both (normal and longitudinal) defect-induced forces bending the surface layer leads to the appearance of two maxima in the dependence of the instability growth rate of DD waves on the wave number. Three-wave interactions of quasi-static coupled DD waves (second harmonic generation and wave vector mixing) are considered for the first time, which are similar to three-wave interactions in nonlinear optics and acoustics and lead to the enrichment of the spectrum of surface-relief harmonics. Computer processing of experimental data on laser-induced generation of micro- and nanostructures of the surface relief reveals the presence of effects responsible for the second harmonic generation and wave vector mixing.

We describe a new type of spatially periodic structure (lattice models): a polaritonic crystal formed by a two-dimensional lattice of trapped two-level atoms interacting with the electromagnetic field in a cavity (or in a one-dimensional array of tunnelling-coupled microcavities), which allows polaritons to be fully localised. Using a one-dimensional polaritonic crystal as an example, we analyse conditions for quantum degeneracy of a lower-polariton gas and those for quantum optical information recording and storage.