We study, both experimentally and theoretically, the second-harmonic (SH) generation (SHG) of optical radiation assisted by intense terahertz electromagnetic pulses from the surface of high-resistivity Si(111). The study was performed within the framework of the analysis of the superposition of different SH sources, including the surface-dipole, bulk-quadruple, and electric-field-induced (EFI) bulk-dipole (arising from internal built-in and external terahertz fields) contributions. Azimuthal rotation anisotropy of SHG for an arbitrary mutual polarization of the terahertz, fundamental, and SH optical fields was calculated and experimentally verified. The strongest impact of the terahertz field on SHG was observed in the case where the polarizations of the terahertz and SH fields coincide. In these polarization configurations, the terahertz field added mainly an isotropic term to the SH azimuthal dependences. It has been demonstrated that the application of the terahertz field can be helpful for distinguishing the SHG mechanism. For the sample under study, the contribution to the SH energy from the built-in field-induced source was found to be >4 times greater than the contribution from the surface and quadrupole nonlinear sources. The dominance of the EFISHG allowed us to find the ratio between the components of a silicon third-order nonlinear tensor ${\chi }_{}^{\left(3\right)}(2\omega ;\omega ,\omega ,0)$: ${\chi }_{1122}={\chi }_{1212}\approx 0.51{\chi }_{1111}$ and ${\chi }_{1212}\approx {\chi }_{1221}$ for the fundamental optical wavelength 790 nm.

We propose and demonstrate experimentally a nonlinear optical technique that allows for detection and characterization of invisible (or low-contrast) microscale objects buried in the bulk of materials transparent in the optical and terahertz frequency ranges. The technique is based on the effect of terahertz-field-induced second harmonic generation and uses collinearly propagating femtosecond optical and picosecond terahertz pulses to probe a sample. Due to a difference between the optical and terahertz velocities, the pulses can be overlapped in different regions of the sample by varying the time delay between them. Overlapping in the bulk of the material does not produce optical second harmonic emission, whereas overlapping at the microobject does produce the emission. The technique was verified experimentally for two plates of fused quartz glued by a thin (15–35 μm thick) layer of optical adhesive. The presence of the adhesive was detected, and its third-order nonlinear susceptibility was measured.

Spontaneous optical emission of graphene irradiated by intense single-cycle terahertz pulses was studied experimentally and explained theoretically. We found that the emitted photons are mainly polarized normally to the electric field of the terahertz pulse, which proves that the terahertz field not only heats the electrons, but also creates a strongly nonequilibrium momentum distribution. The comparison of the measured optical spectrum and polarization anisotropy with the results of numerical modeling allowed us to estimate the momentum isotropization time for electrons in graphene to be $\sim 25$ fs and roughly reconstruct the time evolution of the distribution function in $k$ space.

Third-order nonlinear susceptibilities ${\chi }^{\left(3\right)}\left(-2\omega ;\omega ,\omega ,0\right)$ and ${\chi }^{\left(3\right)}\left(-\omega ;\omega ,-\omega ,\omega \right)$ responsible for electric-field-induced second harmonic generation and the optical Kerr effect were measured and directly compared for tellurite and chalcogenide glasses. The nonlinear coefficients were found by measuring the second harmonic radiation from samples under the action of an external field of terahertz pulses and by the classical z-scan technique, respectively. The influence of ambient air and helium gas on second harmonic generation was analyzed. It was demonstrated that both susceptibilities ${\chi }^{\left(3\right)}\left(-2\omega ;\omega ,\omega ,0\right)$ and ${\chi }^{\left(3\right)}\left(-\omega ;\omega ,-\omega ,\omega \right)$ have close absolute values, which shows the dominant role of nonresonant electronic nonlinearity in the studied glasses.

The problem of creating optical parametric amplifiers of femtosecond pulses up to the multiterawatt level was fiist discussed in [l, 21. In these papers, as well as in subsequent studies [3-91, only nonlinear elements made from KDP crystals, whose aperture can be as great as 30cm, were considered as nonlinear elements for final cascades of optical parametric amplifiers (OPAs). The most promising pump source is the second harmonic of a Nd:glass laser with a wavelength of 3L3=527nm. At this pumping, the KDP crystal has two serious disadvantages: it absorbs greatly at wavelengths higher than 1200nm and has maximal gain bandwidth (1000-1200~m~[l~]) at the wavelength of signal 1050-1 1 OOnm, i.e., at degenerated (or nearly degenerated) phasematching. In this wavelength range, however, it is difficult to create light sources with pulse duration of less than 50fs.

Experiments with interacting high-velocity flows in a laser plasma can help answer fundamental questions in plasma physics and improve understanding of the mechanisms behind some astrophysical phenomena, such as the formation of collisionless shock waves, deceleration of accretion flows, and evolution of solar and stellar flares. This work presents the first direct experimental observations of stagnation and redirection of counterstreaming flows (jets) of laser plasma induced by intense laser pulses with intensity I ∼ 2 × 1018 W/cm2. Hybrid particle-in-cell–fluid modeling, which takes into account the kinetic effects of ion motion and the evolution of the pressure tensor for electrons, demonstrates the compression of counterdirected toroidal self-generated magnetic fields embedded in counterstreaming plasma flows. The enhancement of the toroidal magnetic field in the interaction region results in plasma flow stagnation and redirection of the jets across the line of their initial propagation.