Development of the next generation of THz devices: detectors to modulators

The development of devices operating in the terahertz (THz) frequency is of paramount importance for the progress of THz science and technology. The THz spectral region is particularly appealing for many applications; it is widely known that drugs or explosives present spectral fingerprints in the THz, while, conversely, plastic material are transparent to this radiation, which uniquely finds application in security scanning. Moreover, diverse chemical agents, or molecules are absorbing in the mid/far infrared; common pollutants, such as nitrogen dioxide, or greenhouse gases, such as carbon monoxide, dioxide, or methanol, present ro-vibrational and rotational transitions in this spectral region, thus lending itself to environmental monitoring.
The development of efficient and fast THz detectors and switches/modulators is critical to a number of these applications. A number of detection mechanisms utilising two-dimensional electron gases (2DEGs) in semiconductor devices have been reported. Our group has recently developed a novel gate biasing device architecture that makes the 2DEG non-uniform such that THz photo-response is achieved not by exploiting conventional electron heating but from photon-assisted tunnelling. This detection mechanism allows for the exploitation of the inherent ultra fast response of this process. More complex detector architectures will be explored that not only aim to optimise the antenna design to improve THz detection sensitivity, but also the development of detector arrays for THz imaging applications.
THz applications utilising a new class of quantum materials known as Topological Insulators (TI) has great potential. One exciting property of topological materials is dissipationless surface transport, through topological surface states (TSS). Theoretically the photo-response can be extended to the THz spectral regime, with a potential SNR that would largely exceed those of conventional photo-detecting approaches. An interesting device architecture would be a TI based THz photodetector/modulator. This would enable THz illumination to actively control the transition between topological and non-topological phases, altering the transport characteristics of the device. This would enable THz photons to act as a 'switch' to turn the topological surface state (TSS) on/off optically.