Design and Optimisation of Terahertz Frequency Photoconductive Antenna Arrays on Optically Transparent Insulating Substrates

Terahertz (THz) radiation is desirable for many different applications due to the low photon energy exhibited at these wavelengths. With many elementary excitations in materials occurring in this region, the use of THz radiation in non-linear spectroscopy has become a valuable tool for material investigations. Specifically, High-field spectroscopy has been used to probe non-linear effects in solid-state materials, these require intense THz sources which are harder to produce. A promising source is the Photo-Conductive Antennae (PCA), these devices are compact and easily integrated into systems with the ability of electrical modulation and lack of phase matching requirements. In this thesis a systematic approach has been taken to investigate the effects of device geometry on the emission of THz radiation from PCA devices fabricated from LT-GaAs on optically transparent insulating substrates. Although these devices have been shown to be able to produce high-field THz electric field strengths, no investigations have been undertaken to improve device geometries and improve upon the generation efficiency. Work on the effects
of electrode designs and emitting gap sizes to support higher electrical biasing fields and optical fluences is presented. The results of the investigations led to the fabrication of a large area PCA device capable of producing ∼ 200 kV/cm, the largest THz field strengths to date from LT-GaAs PCA devices on sapphire substrates. Also presented in this work is an investigation of LT-GaAs PCAs operated at an excitation wavelength of 1030 nm. The generation characteristics are compared to that of devices operated at 800 nm. Investigations into the operation of LT-GaAs on sapphire PCA devices using high repetition rate lasers of this wavelength could prove invaluable due to the lack of photoconductive materials currently available.