Abstract

In LPD-based THz emitters where a metal masks a semiconductor and an ultrafast laser is focused in the interface of metal and semiconductor [1]. Barnes et al. proposed a mechanism for the emitters described in [1] that combined carrier diffusion and suppresion of radiation of the THz dipole under the metal. The PD dipole formed under the metal mask is suppressed due to a π phase shift on reflection off the metal [2, 3]. We demonstrate a double-metal emitter, which uses repeatable strips of two partially overlapping metals with different skin depths in the THz range to reduce the dipole suppression on one side of the repeatable element [4]. Using two metals with different work functions also allows the manipulation of a net surface current created by the differing Schottky barriers [4]. These two effects combine to create a net THz emission. The chosen metal pairings for the emitters were gold and lead (Au/Pb), gold and aluminium (Au/Al) and copper and chrome (Cu/Cr). 7 µm wide metal strips were fabricated using photolithography and were evaporated onto a semi-insulating Gallium Arsenide (SI-GaAs) substrate over a period of 15 µm. A second Au/Pb emitter with a layer of SiO₂ between the semiconductor and metal was fabricated to analyse the effect on the resultant THz emission by insulating against the Schottky effects. The emitters were mounted in a Helium flow cryostat situated in a typical THz-TDS setup. A Menlo Systems photoconductive antenna was used as a detector. An 80 MHz, 100 fs Ti:sapphire laser centered at 780 nm and modulated with an optical chopper illuminated both the emitter and detector, with 100 mW and 6 mW respectively. The full laser spot size of 1.4 mm was used to illuminate the emitters as neither a focussing element for the pump beam or a Si-lens for the produced THz were required.

© 2015 IEEE