On focusing of broadband terahertz pulses

The particular features of the focusing of a broadband THz pulse with a Gaussian transverse amplitude distribution by a lens are investigated theoretically. The expressions for the spatiotemporal field distribution on the beam axis and in the focal plane of the lens are obtained in the quasi-optical approximation. It is shown that, for the focusing to be efficient, it is necessary to use lenses with a focal distance that is much shorter than the characteristic diffraction length at the frequency corresponding to the spectral maximum of the THz pulse.