High-intensity terahertz radiation from a microstructured large-area photoconductor

Abstract

We present a planar large-area photoconducting emitter for impulsive generation of terahertz ͑THz͒ radiation. The device consists of an interdigitated electrode metal-semiconductor-metal ͑MSM͒ structure which is masked by a second metallization layer isolated from the MSM electrodes. The second layer blocks optical excitation in every second period of the MSM finger structure. Hence charge carriers are excited only in those periods of the MSM structure which exhibit a unidirectional electric field. Constructive interference of the THz emission from accelerated carriers leads to THz electric field amplitudes up to 85 V / cm when excited with fs optical pulses from a Ti:sapphire oscillator with an average power of 100 mW at a bias voltage of 65 V applied to the MSM structure. The proposed device structure has a large potential for large-area high-power THz emitters. 6 Several attempts have been made to increase the THz emission efficiency by employing photoconductors with laterally structured emitter electrodes. 7,8 The driving forces for further developments of impulsive THz emitters are applications which require a large bandwidth and/or high THz electric field amplitudes. The performance of THz emitters based on photoconductors ͑PC͒ is limited by several conditions. In order to achieve a high bandwidth the electric field applied to the PC should be high in the 100 kV/ cm range, i.e., close to the breakdown voltage of the PC. On the other hand the optically excited area should be large to prevent local heating of the PC with the exciting laser pulses which would have a detrimental effect on the carrier mobility and thus decrease the bandwidth. Therefore the solution of using pin-diodes has advantages over lateral electrical contacts to the PC, since in a pin-diode high electric fields exist in the whole area of the diode. Yet the acceleration of carriers in a pin-diode is perpendicular to the surface which results in a bad outcoupling efficiency of the dipole radiation with the main intensity being emitted parallel to the surface. Hence the acceleration of carriers in the plane of the PC would be favorable. However, large lateral electric fields applied over large areas, i.e., large electrode spacing, require pulsed high voltage sources in the kV range which one tries to avoid because of the electronic interference with sensitive electronic equipment in the laboratory. 9 Another limitation of PC emitters with lateral carrier excitation is the emission of the radiation via coupling to a microstructured dipole antenna, which typically exhibits a narrower bandwidth than the intrinsic carrier acceleration in the PC.Here we investigate the THz radiation from photoconductive MSM structures modified in a way to achieve unidirectional carrier acceleration on a large active area for high excitation powers. The emission is based on the intrinsic acceleration of carriers in the PC and hence does not require a narrow-band antenna. High electric fields of the order of 100 kV/ cm are achieved by convenient dc voltages of abo...

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Photo-Dember terahertz emitter excited with an Er:fiber laser

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