Abstracts

The terahertz (THz) region of the electromagnetic spectrum remains relatively underdeveloped and unexploited. This is due to the lack of compact and high-power sources which are able to output radiation between 100 GHz and 10 THz. In this thesis two separate methodsof generating THz radiation are considered; the quantum cascade laser (QCL) and the photoconductive (PC) emitter. THz-QCLs generate radiation through the intersubband transition of electrons within quantum wells. Electrons are then transported into the adjacent quantum well, where they are recycled. Alternatively, PC emitters generate THz radiation through the excitation, acceleration and recombination of electron-hole pairs in semiconducting material,leading to the emission of short pulse THz radiation.In this work, the gain dynamics of two different designs of THz-QCL are investigated using well-established time domain spectroscopy (TDS) techniques. The gain recovery time of a bound-to-continuum (BTC) device, a property known to be responsible for the preventionof mode-locking in the laser, is recovered through use of THz-pump-THz-probe TDS. This was performed multiple times, both with and without anti-reflective coating applied to the QCL facets. This coating prevents the QCL from lasing. Furthermore, this approach was thenused to investigate the carrier dynamics of a hybrid active region device, which yielded some interesting preliminary results.To facilitate these measurements, a new form of quartz-mounted PC device was designed and fabricated. This entails removing the LT-GaAs active layer from its SI-GaAs growth substrate and bonding it directly to a layer of z-cut quartz. In emission, this device has been shown to produce significantly higher THz output fields, when compared to the previously used PC devices. In addition to this, the use of an optically transparent substrate provides the ability to perform back-side illumination, which is shown to improve the output characteristicsof the device. Furthermore, when used in detection, the newly fabricated devices have shown significantly increased sensitivity, when compared to other methods of detection.