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Title: Optical and mechanical studies of semiconductor resonators
Author: Maguire, Joseph
ISNI:       0000 0004 8509 1895
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2019
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This thesis concerns the investigation of micromechanical resonators formed either from group III-V semiconductors, or two-dimensional (2D) transition metal dichalcogenides (TMDs). The work is motivated by furthering the understanding of nonlinear resonator dynamics at the micron-scale, and, separately, the possibility of coupling the mechanics to an embedded quantum emitter. The latter is of particular interest for sensing applications using micromechanical resonators. First, gallium arsenide (GaAs) nanowires (NWs) are grown with cross-sectional dimensions of varied elongation, and the effects of the elongation on the resonator dynamics are studied. The single-mode dynamics of the NWs are found to agree with predictions made using Euler-Bernoulli (EB) beam theory. The NWs are then driven into the large amplitude regime of motion, and the nonlinear response is used to estimate the cubic Duffing nonlinearity. The nonlinear response of NWs gives rise to coupled mode dynamics. In the coupled mode regime, a quadratic dependence between the change in the fundamental (and second order) mode frequencies on the drive amplitude of the coupled mode is observed. Depending on the NW elongation, and which flexural modes are driven, a reversal in the direction of the frequency change is observed. This response is explained using the coupled, nonlinear Duffing equations of motion. Strain coupling between the mechanical motion of a GaAs cantilever and the emission properties of an embedded indium arsenide (InAs) quantum dot (QD) is then investigated. The cantilever is driven at the fundamental resonance frequency, and the effect of the cantilever motion on the QD emission energy is evaluated. The QD emission energy is modulated at the cantilever’s resonance frequency via the deformation potential, and is used to estimate the QD-cantilever optomechanical coupling rate. Computational modelling is used to predict the strain fields within the cantilever, and therefore estimate the optomechanical coupling rate. This is found to be in good agreement with predictions made from the experimental observations. This research is working towards the realisation of strain-induced sensing applications using micromechanical resonators formed from III-V semiconductors. Next, GaAs cantilevers, similar to those studied for the previous strain tuning application, are integrated with a one-dimensional (1D) photonic crystal cavity (PhCC), and a 1D perturbing PhC structure. The PhCC acts as an on-chip spectral filter or cavity for enhancement of the QD emission. In this system, displacement of the cantilever results in an out-of-plane separation between the PhCC and the perturbing PhC structure, which can be used to tune the PhCC mode resonance indirectly. Here, indirect tuning of the PhCC resonance is attempted through electrostatic actuation of the cantilever. Computational modelling is carried out to predict the optical response of the PhCC in response to the out-of-plane separation of the perturbing PhC structure, and the technological challenges involved with fabricating the structures are outlined. This research has applications in on-chip integrated quantum optical circuits. Finally, monolayer tungsten diselenide (WSe2) integrated within an optically and electrically active van der Waals heterostructure is studied, with specific focus given to the emission properties of embedded single defect emitters (SDEs). Electrical tuning of the SDEs is demonstrated, which has promising applications for quantum information processing (QIP). Observations of SDEs in monolayer TMDs motivated the study of the mechanical properties of suspended molybdenum diselenide (MoSe2) monolayer resonators, which could be used as mechanical strain sensors. The resonators are electrostatically driven by applying a bias to the suspended structures with time varying (AC) and constant voltage (DC) components. The initial tension within the monolayer is tuned by controlling DC bias, which in turn allows for tuning of the resonator’s resonance frequency. Then, the monolayers are driven into the large amplitude regime of motion (similar to previous demonstrations using GaAs NWs and cantilevers) and nonlinear motion is observed. These observations contribute to the fundamental understanding of the dynamical properties of TMD monolayer resonators.
Supervisor: Wilson, Luke Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available