Here we report on our latest results in creating highpower terahertz pulses by means of optical rectification technique and their application in various fields. We investigated terahertz generation in organic crystals DSTMS, DAST and OH1 directly pumped by a Cr: forsterite laser at central wavelength of 1.25 μm. This pump laser technology provides a laser-to-THz energy conversion efficiency higher than 3%. Phase-matching is demonstrated over a broad 0.1-8 THz frequency range. In our simple

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... , we achieved hundred μJ pulses in tight focus resulting in electric and magnetic field larger than 10 MV/cm and 3 Tesla [1]. We've applied a novel method for measuring the THz spatial distribution of in the focal plane of a parabolic mirror based on generation of the second-harmonic radiation in the optical region of spectrum in centrosymmetric crystals under action of a powerful terahertz pulse with subsequent image transfer to iCCD camera ( Fig. 1 ). Fig. 1. THz intensity distribution for OH1 crystal Application of different techniques of spatial profile transformation using interference of two chirped pulses in Mach Zehnder-type interferometer [2] as well as acoustooptical dispersion delay line [3] made it possible to generate THz pulses with a tunable center frequency in the range 0.5-2.5 THz (Fig. 2) . Fig. 2. Spectra of THz radiation under pumping of the OH1 crystal with laser pulses of different temporal shape and the delay between two chirped pulses (1) 0.5, (2) 1 and (3) 1.5 ps; (4) is the spectrum in the case of pumping with a single transform-limited pulse Special attention is devoted to THz electric fieldinduced second harmonic generation in inorganic ferroelectric and centrosymmetric antiferromagnet as well. Second Harmonic Generation induced by the electric field of a strong nearly single-cycle terahertz pulse with the pick amplitude of 300 kV/cm is studied in a classical inorganic ferroelectric thin film of BaSrTiO3. The dependences of the SHG intensity on the polarization of the incoming light is revealed and interpreted in terms of electric polarization induced in the plane of the film. As the THz pulse pumps the medium in the range of phononic excitations, the induced polarization is explained as a change of the ferrolectric order parameter. It is estimated that under action of the THz pulse the latter acquires an in-plane component up to 3% of the net polarization. Fig. 3 . Experimental geometry and polarization diagrams of the SHG intensity for various experimental geometries. (a) Experimental geometry. The axes of the chosen laboratory frame X L , Y L , Z L correspond to [100], [010] and [001] crystallographic directions, respectively. ϕ -the angle between the electric field of near-infrared probe and the X L -axis. ψ -the angle between the electric field of the THz pump pulse and the X L -axis; (b) dependence of the SHG signal on ϕ without any THz pump; (c) the same, when the THz field is applied parallel with respect to the X L -axis. The polarization of the SHG signal was set either in the P out or S out -state; (d) dependence of the SHG signal on ψ when the probe polarization was set to the P-state (ϕ = 0). The polarization of the SHG signal was set either in the P out or S outstate. Dots correspond to experimental data and lines are fits. Values for S-state multiplied by 3, 2, and 50 for (b-d) respectively Hence the data show that the THz electric field clearly affects the process of the second harmonic generation. To reveal ultrafast dynamics of these electric field induced changes, we performed pump-probe measurements of the SHG signal. In particular, the signal was measured as a function of the delay between the THzpump and near-infrared probe pulses. Fig. 4a shows timedomain trace of the electric field of the THz pulse obtained with the help of electro-optical sampling. The