Plasmon-enabled photodetection in van-der-Waal heterostructures

Abstract

This alternative format thesis details theoretical studies into plasmon-enabled photodetection, as well as the tuning of properties within a graphene-based heterostructure. These studies outline the design of high-performance photodetectors that operate within the terahertz (THz) gap. A derivative of graphene is also described, that has thermoelectric characteristics that can be optimized via the application of a displacement field, making it an excellent candidate for use in thermocouples. The first study entails the modelling of a gated field-effect transistor (FET) with a Corbino-disk geometry, such that the drain of the device spans its circumference while the inner ring acts as its source. An AC terahertz potential difference is applied between source and gate while a static source-drain voltage is measured. After a magnetic field is applied perpendicular to the device, resonances appears at the cyclotron frequency for all plasmons with positive winding number. To enable the mixing of these resonance peaks in the DC source-drain voltage, the inner ring of the device is moved off-center. This is named the eccentric Corbino-disk geometry. The mixing of these resonant peaks in the DC photo-voltage with different winding numbers results in a strong resonance. The strength of which can be tuned by changing the eccentricity of the disk. Furthermore, it is shown that there is an optimum value of the eccentricity for which the responsivity of photodetector is maximised. Following this, in the absence of a magnetic field, the same photodetector is modelled with geometries based on variations of the Sierpinski carpet fractal. In these geometries the source and drain span the left and right edges of the device respectively. For any given fractal recursion depth, the resonant peaks in the DC photo-voltage occur at frequencies that are thrice those of the previous fractal recursion depth. These are amplified while all other existing peaks are diminished. A new peak is found with a frequency dependant on the device geometry's fractal dimension. An estimate to the functional form of this resonant frequency is given. The last study investigates the thermoelectric properties of twisted bilayer graphene (TBG) and twisted mono-bilayer graphene (TMBG) over a range of displacement fields with twist angles close to the magic-angle of TBG. The displacement field is found to provide no benefit to the power factor of TBG, however, for TMBG, large maxima in the power factor are found for select displacement fields. Additionally, these maxima are found to be resistant to minor shifts in the twist angle. Analysis of the band structure for these values of the displacement field suggest the power factor is amplified due to valley convergence.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Alessandro Principi (Supervisor) & Vladimir Falko (Supervisor)