Petr Solyankin

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.

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.

Terahertz emission during interaction of ultrashort laser pulses with gas cluster beam

We present the results of experimental study of terahertz (THz) generation in gas cluster beam excited by intense femtosecond laser pulses. Cluster beam was produced by partial condensation of pure Ar and mixtures CF2Cl2+He, Ar+He during their expansion through a conical nozzle into vacuum. There were used two excitation schemes in our experiments: single color and two color (fundamental frequency mixed with its second harmonic). We have studied how THz signal scales with various control parameters such as laser pulse duration, gas backing pressure and laser pulse energy. Simultaneously we measured intensity of X-Ray emission which originates from laser-cluster interaction. We found that in a single color scheme energy of THz pulses from Ar cluster beam strongly decreases in the region of minimum laser pulse duration while X-Ray power is maximal under these conditions. Both in single- and two color excitation regimes THz signal demonstrated growth without saturation with increasing of optical pulse energy up to its peak value of 25 mJ.

Polarization Of THz radiation generated during two-color filamentation of arbitrarily polarized laser pulses

We examined experimentally and theoretically polarization of THz radiation generated during dual-color (800nm+400nm) co-propagation of high-peak-power femtosecond laser pulses in gases. We reveal that THz radiation polarization is predominantly defined by the generation of the nonlinear photocurrent in the self - induced laser plasma and remains relatively stable with respect to the change of the initial polarization angle between the 800 nm and 400 nm light fields.

Spatio-spectral characteristics of THz radiation from two-color femtosecond filament

Spatio-spectral distribution of THz radiation generated by two-color femtosecond laser breakdown in air is investigated theoretically. The theoretical model is based on the fast oscillating light field propagation and self-consistent free electron generation process. We find that the THz emission spectrum has both the low-frequency component related to the transient photocurrent with the maximum spectral intensity at ~1 THz, and the high-frequency component at ~10 THz related to the nonlinear response of bound electrons.