Profile of a photoconductive THz Emitter excited by an amplified laser system

We present a large photoconductive THz emitter[1] consisting of two interdigitated metallization layers on a semi-insulating GaAs substrate.
The photoexcited carriers are unidirectional accelerated by a bias voltage providing an electric field. This leads to a subsequent emission of THz radiation. The second metallization inhibits the optical excitation in every second period of the electrode structure in order to prevent destructive interference. We analyse the spatial profile of such an emitter excited with unfocused fs optical pulses from an Ti:sapphire amplifier with an average power of 60mW at a 1kHz repetition rate. The resulting THz beam has a bandwidth from 0.1 THz to 4 THz and a field amplitude of up to 6kV/cm (Ubias = 30V ). The focussed THz spot was mapped out and analyzed with respect to the frequency. A strong increase of the beam diameter with decreasing frequency was found. Saturation behavior was observed by changing the excitation density.
[1] A. Dreyhaupt, S.Winnerl, T.Dekorsy, and M.Helm, Appl. Phys 86,
121114 (2005)