Realisation of an efficient Terahertz source using Quantum dot devices

Abstract

The development of a compact, tunable, room-temperature operating THz source remains one of the key unsolved tasks in the scientific community to unlock the numerous advantages and applications of THz radiation in spectroscopy, communication, sensing, and imaging among others. Pulsed terahertz systems requires femtosecond optical pumping from mainly bulky lasers, while earlier approaches of continuous-wave(CW) THz generation involved using pumped gas lasers which were very bulky and barely tunable. However, more recent sources of CW THz mostly use Quantum Cascade lasers (QCL) and Photoconductive antennas (PCA); The QCL suffers from cryogenic operating conditions and production complexity that keeps it out of commercial reach while conventional Photoconductive antennas are mainly limited by low optical-terahertz conversion efficiency. The use of quantum dots (QD) in PCA substrate material for THz generation has been implemented by research in this thesis to achieve both pulsed and continuous wave terahertz radiation, to provide access to optical pumping from compact semiconductor lasers and more importantly to reduce the carrier lifetime in PCAs to enable more efficient and optimised photoconductive antenna for THz generation. This research has demonstrated a tunable continuous-wave Quantum Dot external cavity laser emitting at two frequencies as an optical pump for continuous wave terahertz generation. The external cavity QD Laser has been characterised with tunability of 152nm and a tuning range from 1143nm-1295nm that lies within the THz difference frequency for the generation of THz radiation from PCAs. This research work also presents the enhancement of THz PCA’s power output with Quantum dots at pump powers and operating conditions that are analogous to that of semiconductor lasers for a compact THz system. The generation of pulsed THz radiation from the designed quantum dot photoconductive antennas (PCAs) pumped at 800nm and 700nm and optical pump power of 1mW-10mW with an applied bias voltage of 2V-20V has been recorded and presented in chapter 4. This PhD project investigates the output and characteristics of the generated THz from the QD PCAs alongside a comparison with a commercial antenna from Teravil with low-temperature grown GaAs substrate. The QD PCAs outputs significantly higher THz power than the commercial PCA at low pump powers that are representative of semiconductor lasers. This provides a significant step towards the realisation of an efficient compact THz system.