Terahertz (THz) radiation from the interaction of ultrashort laser pulses with gases is studied both theoretically and experimentally.
We theoretically study the THz generation based on transient ionization current model and give the relation
between the final THz field and the initial transient ionization current. Recent experimental results on optimization of THz
radiation in laser air interaction are also shown. We find by use of a simple aperture to change the laser field distribution,
the terahertz wave amplitudes can be enhanced by more than eight times than those of aperture-free cases. We use two
dimensional particle-in-cell codes to simulate the experiments and give possible explanations.