Photoexcitation of biased semiconductor photoconductive antennas by femtosecond pulses is the most common and
convenient technique for generating strong terahertz (THz) pulses. In this paper, we use the three-dimensional (3D)
finite-difference-time-domain (FDTD) to analyze electric field distribution of THz pulses in the near-field from a
photoconductive antenna. The simulation is based on solving Maxwell’s equations and the carrier rate equations
simultaneously on realistic dipole antenna structures. The 3D FDTD simulation gives detailed features of THz electric field distribution in and out of the antenna. It is found that the difference of near-field distribution between the substrate and free space is considerably large. The fields of the alternating-current dipole exhibit an unsymmetrical distribution and a large deviation from those calculated using the simple Hertzian dipole theory. The magnitude of THz field in and out of the substrate attenuates rapidly while it holds the line in the gap center. The high-frequency components of THz radiation emission come only from the dipole antenna, while the low-frequency components are from both the center electrodes and coplanar stripline waveguide. This work can be used to optimize the design of antenna geometry and raise the radiation field power.