ИСТИНА

The bright THz source is essential for exploring nonlinear THz field-matter interaction and spectroscopy. Recent advances in high peak-power pulsed THz sources with intensity greater than 1 MV/cm provided unique opportunities to study transient dynamics of non-perturbative states of matter and to investigate strong light-matter interactions. The records of pulsed THz field strength are in the range of 10 to 10 MV/cm from laser-induced gas plasma, gas cluster jets and from the liquids with one- and two-color laser excitation, or up to about 80 MV/cm from organic crystals with the optical rectification. Extremely intense sub-cycle pulses pulse at THz/far-IR region will enable strong field-matter interaction and investigation of a wide range of scientific phenomena. The this work we study also the generation mechanisms of terahertz (THz) radiation from tightly focused femtosecond laser pulses in a gas medium. We measured the angular radiation pattern under different focusing conditions and observed that with the deepening of focus, the angular radiation pattern changes and optical-to-THz conversion efficiency increases. The analysis of the observed phenomena led to the assumption that the dipole radiation prevails in most cases despite the existing conception regarding the dominating role of the quadrupole mechanism of radiation. Based on these assumptions, the transient photocurrent theory of the phenomenon presented in this paper was developed by us and used for the numerical fit of the experimental data. Conventional methods used to obtain the highly synchronized pulses necessary for the pump-probe measurements rely on an optical parametric oscillator (OPO) or optical parametric amplifier (OPA), which generates synchronized signal and idler pulses at different wavelengths. In the paper we will discuss the generation of two synchronized ultrafast radiations, THz and high harmonic radiation (XUV to soft X-ray), using an intense laser pulse in visible or near-infrared ranges with selected media such as gases and nano-structured metals. We studied THz and X-ray radiation from gas clusters while irradiating them with high-intensity femtosecond optical pulses. Clusters were produced by partial condensation of the pure Ar gas and the mixtures in the process of their expansion through a conical nozzle into evacuated chamber. Simultaneous measurements of THz and X-ray radiation properties were carried out with various durations and total energies of optical pulses, in single-color and two-color schemes of clusters jet irradiation. In the two-color scheme, optical pulse bears both the fundamental and second harmonics of Ti-Sapphire laser, we observed a significant increase of THz radiation field without any change of X-ray radiation properties. We observed a non-monotonic dependency of THz radiation power upon the pulse duration at given total energy of the optical pulse. To interpret this effect we developed a theoretical model of cluster ionization which self-consistently predicts level of ionization and electron temperature in the clusters.