V.A. Makarov

Ultrabroad Terahertz Spectrum Generation from an Air-Based Filament Plasma.

We have solved the long-standing problem of the mechanism of terahertz (THz) generation by a two-color filament in air and found that both neutrals and plasma contribute to the radiation. We reveal that the contribution from neutrals by four-wave mixing is much weaker and higher in frequency than the distinctive plasma lower-frequency contribution. The former is in the forward direction while the latter is in a cone and reveals an abrupt down-shift to the plasma frequency. Ring-shaped spatial distributions of the THz radiation are shown to be of universal nature and they occur in both collimated and focusing propagation geometries. Experimental measurements of the frequency-angular spectrum generated by 130-fs laser pulses agree with numerical simulations based on a unidirectional pulse propagation model.

Arrest of self-focusing collapse in femtosecond air filaments: higher order Kerr or plasma defocusing?

Experimentally measured conical emission rings on the blue side of the filament supercontinuum of a 800 nm 50 fs pulse in air are reproduced in simulations with plasma and the third-order Kerr as the nonlinear terms. This agreement indicates plasma as the dominant mechanism arresting the self-focusing collapse. The higher order Kerr terms with the recently measured coefficients stop the collapse at a lower intensity than the plasma does and lead to the spherical angle-wavelength spectrum without blueshifted rings.

Transformation of terahertz spectra emitted from dual-frequency femtosecond pulse interaction in gases

Experimental and theoretical study of the mechanisms affecting the spectral form of the pulsed THz emission generated due to the interaction between a bichromatic laser field and a gaseous medium is presented. The influence of a photoionization process and Raman rotational molecular transitions on the THz spectrum is discussed. Typical spectral modifications of THz emission are tracked down and linked to the duration of the two-color laser pump pulse.

Theory of Nonlinear Optical Activity in Isotropic Media and Liquid Crystals

The phenomenological theory of nonlinear optical activity and nonlinear circular dichroism is developed. Exact analytical solutions are found in the planewave approximation which make it possible to analyse conditions for observation of contributions of the aforementioned effects.

Polarization rotation due to femtosecond filamentation in an atomic gas

The linear-to-elliptical transformation of a 400 nm femtosecond-probe pulse in the birefringent filament in argon of an 800 nm linearly polarized femtosecond-pump pulse is studied numerically and experimentally. The rotation of the probe elliptical polarization is the largest in the high-intensity filament core. With propagation, the rotated radiation diffracts outward by the pump-produced plasma. The transmission of the analyzer crossing the probes polarization is maximum at the pump-probe angle of 45 degrees and gives equal values for each pair of angles symmetrically situated at both sides of the maximum.

Filamentation of arbitrary polarized femtosecond laser pulses in case of high-order Kerr effect

We developed a model of femtosecond filamentation which includes high-order Kerr effect and an arbitrary polarization of a laser pulse. We show that a circularly polarized pulse has maximum filament intensity. Also, we show that, independently of the initial pulse polarization, the value of a maximum filament intensity tends to the maximum intensity of either linearly or circularly polarized pulse.

Emission of terahertz pulses from vanadium dioxide films undergoing metal-insulator phase transition

This paper describes research on the optics of functional materials, which can change their dielectric properties according to their function. Vanadium dioxide is a good example of such a material where the insulator-to-metal phase transition offers the possibility to control dielectric properties and to use them as a triggering element for photonic applications in the wide spectral range from optical to terahertz frequencies. We observed emission of terahertz (THz) radiation from VO2 films in insulating and conductive phase states under femtosecond pulse irradiation. We found that the efficiency of THz emission increases up to 30 times after the insulator-to-metal phase transition. This process occurs in thin films while it is fundamentally forbidden in the bulk material, and polarization analysis of the emitted radiation reveals the crucial importance of interface contributions. The properties of the THz radiation emitted by VO2 are determined by displacement photocurrents at the VO2–air and VO2–substrate interfaces induced by the incident laser light. In each phase state the contributions of the two boundaries are different. Properties of the effective dielectric susceptibility χ(2) tensor for the insulating phase were defined. In demonstrating the conversion of optical into THz radiation in VO2 films, we found that fundamental symmetry restrictions are not applicable to problems of nonlinear optics of thin films.

Spectroscopy of Nonlinear Gyrotropic Medium and Surface Diagnostics Based on Polarization Effects Due to Self-action of Light

Abstract Amplitude and polarization characteristics of the light reflected and passed through crystals are found. Four mechanisms of polarization-ellipse nonlinear optical rotation and deformation are studied. A new scheme of nonlinear polarization spectroscopy of uniaxial and cubic crystals is put forward.

Specific features of the self-action of elliptically polarized light pulses and the formation of vector solitons in an isotropic medium with the anomalous frequency dispersion and the spatial dispersion of cubic nonlinearity

Numerical methods are used to study the effect of local and nonlocal nonlinear optical susceptibilities on the parameters of a soliton whose degree of ellipticity depends on time and that is formed at a distance of several dispersion lengths in a medium with the anomalous frequency dispersion. The rotation angle of the major axis of the polarization ellipse does not depend on time and linearly increases with an increasing propagation coordinate. If the tensor components of the local nonlinear susceptibility have opposite signs, the incident elliptically polarized pulse can propagate in the regime that involves the pulse splitting into components for which the absolute values of the electric-field degrees of ellipticity are close to unity. In this case, the electric-field vector at the center of the pulse rotates in the opposite direction with respect to the rotation at the edges.

Chirped elliptically polarized waves in an isotropic gyrotropic nonlinear medium: approximate solution to the propagation problem

An approximate solution to the nonintegrable propagation problem for chirped elliptically polarized waves in an isotropic gyrotropic medium with local and non-local parts of the cubic nonlinear optical response and second order frequency dispersion has been obtained. Conditions for excitation of waves with periodic changes in the polarization state (chirped elliptically polarized cnoidal waves) and implementation of aperiodic propagation regimes similar to polarization ‘chaos’ have been determined.