THz Generation from Photoconductive Antennas

An electromagnetic (EM) pulse is generated upon acceleration of charge. THz pulses are generated by biasing a semiconductor. Upon photoexcitation, electrons are excited into the conduction band where they are accelerated by the applied bias field.

The emitted pulse is proportional to the time derivative of the change in current. Since charge is accelerated only during or shortly after the laser pulse, the near-field shape of the resultant electromagnetic pulse roughly traces the envelope of the laser pulse. This results in pulses in the THz region of the electromagnetic spectrum. The onset of photocurrent can be no faster than the rising edge of the laser pulse, and is typically slower due to screening effects, etc.

Very large bias field are required to produce THz pulse with impressive signal-to-noise ratios. In order to produce large bias fields, a transmission line is lithographically deposited on the insulating semiconductor of choice. The lines are typically 10 - 30 mm apart.

The emitted field, radiates in a typical point dipole radiation pattern and thus a focusing element is needed to collimate the radiation.