High-intensity THz radiation pulses from a scalable photoconductive device

Photoconductive emitters are an attractive way for impulsive generation of THz radiation. There are two main categories, namely large-aperture emitters and interdigitated electrodes coupled to antennas. Large-aperture emitters have the advantage of a high active volume, while interdigitated structures provide high electric fields for efficient acceleration of photogenerated carriers. We present a large-aperture emitter consisting of a interdigitated metal-semiconductor-metal (MSM) structure, which combines both advantages. A second metallization layer, which is electrically insulated from the first one, blocks the optical excitation in every second period of the MSM structure, resulting in an unidirectional acceleration of carriers in the device. Focussing fs optical pulses with an average power of 100 mW from a Ti:sapphire oscillator on the emitter lead to THz field amplitudes of up to 85 V/cm (U_bias = 65 V). Excitation with unfocussed radiation from a 1 kHz repetition rate Ti:sapphire amplifier system (average power 10 mW) provided THz field amplitudes of 6 kV/cm (U_bias = 23 V). In case of the excitation with the Ti:sapphire amplifier system a pronounced nonlinear behavior of the THz field amplitude with respect to both the excitation density and the bias electric field was observed.