Propagation characteristics of free-space terahertz electromagnetic pulses

Free-space terahertz electromagnetic pulses driven by femtosecond lasers have been utilized for studying the far-infrared and transient characteristics of various materials and applied to the terahertz imaging. Those experiments can be significantly improved by optimizing the terahertz pulse characteristics such as the pulse width, dispersion, frequency spectrum, etc. However, the characteristics have yet been studied theoretically because they are determined not only by the transport dynamics of photoconductivity generated carriers but also by the antenna and guiding structures. Using finite-difference time-domain (FD-TD) techniques, we characterize the dispersion properties of terahertz pulses at various spatial positions, for example, in the near- and far-field regions, and at the on- and off-axis points. We also consider the carrier transport effect on the dispersion.