Focal spot imaging of terahertz subpicosecond pulse by THz-field-induced optical second harmonic generation

Optical imaging of spatial distribution of intensity in the focal spot of the terahertz pulse was developed. The technique is based on the THz electric field-induced second harmonic generation in the nonlinear centrosymmetric crystal $(\rm SrTiO_3)$. The measurements of the spatial distribution of focal spot of terahertz radiation with the electric field strength over $1$ MV/cm were conducted.