We fabricated a large-aperture photoconductive terahertz (THz) emitter array on a semi-insulating GaAs substrate. The device was composed of seven 1 cm2 photoconductive antenna units having microstructured interdigitated electrodes with 10 µm lines and spaces. By illuminating it with amplified femtosecond optical pulses, a large THz field comparable to that obtained from conventional large-aperture photoconductive antennas was obtained at a bias voltage as low as 30 V. The coherent superposition of the output of the seven units was observed.

Real-time high-speed terahertz (THz) two-dimensional imaging at a frame rate as high as 1 kHz was performed using intense half-cycle THz electromagnetic pulses. The THz source was a 3-cm-gap photoconductive antenna. The distribution of THz electric field was detected by a single optical probe pulse using a high-speed charge-coupled device camera by adopting a phase-sensitive electrooptic detection method. Using the system, we demonstrated imaging of a moving metal object by observing the transmitted beam. The time delay of optical pulses was fixed to probe the THz pulse at the time when the axial THz waveform has the steepest transient, which yields a good image quality. This research opens the window of high-speed imaging in the THz frequency regime.

Terahertz electromagnetic pulses can serve as a new and unique tool for various types of spectroscopy. We first characterized the temporal and spatial properties of THz pulses generated from a large-aperture photoconductive antena, and then used them for the study of the ultrafast dynamics of electrons in semiconductros. We studied the dynamics of electrons generated by femtosecond optical pulses with positive and negative excess energies in GaAs and InP by observing the waveform of the emitted THz radiation. Subpicosecond intraband relaxation was observed with positive excess energies. With negative excess energies, a picosecond transition from the Urbach state to free carrier states was observed. q

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