ИСТИНА

A spark was the first source of man-made electromagnetic radiation (EMR) obtained by Hertz, later by Bose, Lebedev and others . While the initial experiments used electric breakdown in air, the later ones switched to the use of liquid, namely oil. This resulted in the increase in stability and intensity of radiation, although its physical mechanism was not studied at the time. The study of the source of THz EMR on the basis of a laser spark follows the same pattern - from air to liquid. In this papers an experiment in which a liquid, namely, water was used for the conversion of femtosecond radiation into the THz one. Being a polar liquid, water has considerable absorption in the THz frequency range and the authors of previously published works had to use very thin water films of a few hundred micrometer thickness in their experiments. The source of electromagnetic radiation in the THz band on the basis of laser spark was firstly presented earlier. Laser an experiment was carried out in which a liquid, namely, water was used for the conversion of femtosecond radiation into the THz. Water is a polar liquid which has high absorption in the THz frequency range and the authors of previously published works have to use for the experiments the very thin water films. Unlike water, considerable absorption both in THz and NIR ranges is absent in liquid nitrogen. We describes broadband generation of THz radiation first obtained in liquefied gas. In comparison with THz radiation in air plasma, THz radiation from LN reacts in a very different way to the change in the length of the pulse and the intensity of laser radiation. We demonstrated that both the ionization of the medium and its nonlinear susceptibility play a considerable role in the generation of THz radiation in LN. We assumed that the mobility of ions and electrons in liquid can play an essential role, forming quasi-static electric field by means of ambipolar diffusion mechanism. This quasi-stationary field can be an additional factor which could increase the effectiveness of transformation of optical radiation into the THz one. NLO susceptibility of the third order, which is much higher in nitrogen compared to air, enables us to carry out polarization control of THz radiation with the help of polarization parameters of femtosecond lasers.