ИСТИНА

We studied the interaction of an intense THz pulse with the doped Si exploiting the new regime when the THz field strength is far above the breakdown threshold by the DC electric field. Firstly, we consider self-induced transparency of a medium under the action of terahertz radiation (or bleaching) that was previously observed in semiconductors irradiated with a THz field of relatively low strength and increase in transmission was of a few percent only. In this work we reported that transmission by energy of the 700 fs THz pulse through n-doped Si sample increases almost two orders of magnitude to∼ 8% with field strength up to 5–7 MV/cm and then gradually decreases nearly twofold at higher electric fields of 10–20 MV/cm [1,2]. Secondly, we study an extreme few cycle terahertz (THz) pulses action at the p-doped Si wafer in transmission mode. The THz pump –weak infrared probe measurements revealed complex and unusual temporal dependence of the second order optical nonlinearity induced by the THz pulse. This was attributed to: (i) direct delayed transverse acoustic phonons’ excitation at the maximal THz field strength of 2-6 MV/cm; (ii) anharmonicity of this process at higher fields leading to chaotic phonons’ behaviour and (iii) modulation at the optical field frequency of the THz field induced impact ionization rate due to modulation of the free carriers’ drift kinetic energy by the probe field at the THz field strength above 10 MV/cm. Hence our study opens up new intriguing area of research, where an extreme ultrashort THz pulse changes medium’s drastically.