Terahertz Ultrashort Pulse Behavior: Near-Field and Far-Field Propagation

An investigation of beam propagation properties for terahertz ultrashort pulses emitted from a photoconductive switch antenna is presented. The propagation of terahertz pulsed beams is studied based on the diffraction theory of electromagnetic fields. It is shown that under the assumption that the pulse's spectrum radiates from a common initiating beam waist, the terahertz temporal pulse shape can be predicted elsewhere without the need for paraxial or far-field approximations. An algorithm is given that enables simple calculation of a terahertz pulse and its spatial distribution, based on simplifications of previous works. A numerical simulation model is presented that predicts spatiotemporal behavior and shows excellent agreement with experimental results. The presented algorithm provides a practical tool for many relevant applications. It also assists in designing and understanding practical pulsed terahertz laboratory setups based on photoconductive switches and can be generalized to predict the behavior of ultrashort pulses in other spectral regions, such as the visible spectrum.