Indium phosphide based photodiodes for continuous wave terahertz generation

This thesis is concerned with the generation of Continuous Wave Terahertz signals from photomixing sources based on the InP material system. Continuous Wave systems are expected to dominate future Terahertz technology where agile, high resolution systems are required. Photomixing offers a low cost and compact solution for broadband Terahertz systems. Driven by the established Telecommunications industry, the InP material system has numerous devices to offer operating at 1550 nm. In this thesis the Travelling-Wave Uni- Travelling Carrier Photodiode (TW-UTC-PD) is proposed as a novel photomixing element that combines the advantages of other previously demonstrated devices. The theory of ultra-fast photodetectors is studied thoroughly and theoretical considerations towards advanced InP-based photodetectors are given. A figure of merit for the Photonic-to-Terahertz conversion efficiency is defined here. In order to maximise this efficiency different input optical waveguides and output planar antennas are studied. A semi-analytical model that predicts the frequency response and the efficiency is developed as an essential tool for the design of these devices. TW-UTC-PDs integrating a mode converting optical waveguide and two types of output configurations were fabricated. Devices with a Coplanar Waveguide output were tested at DC and in the millimetre-wave range. DC responsivities of up to 0.53 A/W at 1550 nm were achieved together with a 3-dB bandwidth of 105 GHz. The same devices demonstrated an output power of 1 mW at 200 GHz. Antenna integrated devices achieved 40 μW at 510 GHz, 5 μW at 1.02 THz and 0.5 μW at 1.53 THz at a photocurrent of 13 mA when pumped with less than 40 mW of optical power. Packaged devices showed broadband response up to 1 THz. The achievements described in this thesis are anticipated to play an important role towards Continuous Wave photomixing Terahertz systems based on InP-based components. In combination with recent demonstrations of detection of millimetre-wave signals with the same device, efficient and compact spectroscopic and communication systems based on the TW-UTC-PD that are also capable of a high degree of photonic integration should be feasible.