All-optical metasurface modulators for THz radiation

The studentship is part of the UK’s Centre of Doctoral Training in Metamaterials (XM2) based in the Departments of Physics and Engineering on the Streatham Campus in Exeter. Its aim is to undertake world-leading research, while training scientists and engineers with the relevant research skills and knowledge, and professional attributes for industry and academia.

The ability to control and manipulate THz radiation has been a long-term goal in electromagnetic research. For this spectral band, all-optical modulation is a convenient mechanism to utilize: here, one relies on the change in conductivity induced in a high mobility semiconductor when excited by optical light. This process is very fast, with pico to micro-second switching times, depending on the semiconductor. However, it can also be rather inefficient, often requiring very intense CW light sources or femotosecond pulses.

To increase the effectiveness of all-optical modulators, one can employ the resonant enhancement of antennas or cavities [see Applied Spectroscopy Reviews 50, 707 (2015) and references there-in]. For example, optically induced modulation of THz radiation has been demonstrated, where patterned photoexcitation of a semiconductor using a continuous wave laser source allowed tuning of localised surface plasmons resonances [Optic. Express 37, 1391 (2012)]. Alternatively, metallic resonators can be used on a semiconductor surface, as in refs. [Nat. Comm. 3, 1151 (2012)] and [PRB 75, 235305 (2007)], to create a “metasurface” photo-modulator. This form of modulators is particularly promising, as they are predicted to enhance the modulation efficiency by up to two
orders of magnitude in very low loss environments [PRB 75, 235305 (2007)].

The goal of this PhD project is to design, characterise and optimise meta-surface modulators for specific THz frequency bands, and working with Q-Eye personnel, to trial as part of the multiplexing system of Q-Eye’s high performance sensor. We will begin by considering designs similar to those in [Nat. Comm. 3, 1151 (2012)], featuring resonant antenna arrays on a semiconductor surface. We will also consider matching resonant etalon effects, and even doubly resonant surfaces on both the front and rear surfaces [APL 104, 103508 (2014)]. Dimensions and structures will need to be optimised to provide narrow resonances (aiming for <10% bandwidth), while exhibiting relatively low angular dependence and low emissivity. Simultaneously, the student will consider the best semiconducting materials for the modulator itself, including high mobility 2-DEG materials, assessing both the modulation magnitude and switching times. Strained and unstrained Si and Ge, of great interest to Q-Eye, and whose staff have extensive experience of these materials, will be investigated. Fast and efficient modulators are crucial to many imaging and signal processing applications, and the optimised designs from the project will be incorporated into THz imaging modalities.