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Title: Optical gain and loss in nanophotonics
Author: Nguyen, Ngoc B.
ISNI:       0000 0004 7655 5264
Awarding Body: University of London
Current Institution: Imperial College London
Date of Award: 2018
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The topic of gain/loss and dispersion is fundamental to the construction of various nanophotonic devices. Through a unique branch of physics called Parity Time Symmetry (PTS), the interplay between gain and loss opens many opportunities to make gain/loss and dispersion necessary parts of the operation of PTS optical devices. The spatial refractive index profile of an optical PTS system, n(x,y,z) requires R{n(x,y,z)} to be spatially symmetric and I{n(x,y,z)} to be spatially antisymmetric. Such a system exhibits exceptional point behaviours which give rise to unusual physical phenomena including non-reciprocal propagation of light—the operational principle of optical diodes. Most solid state devices investigated in this thesis are based on GaAs to supply gain. To apply the concept of PTS to practical optical platforms where high loss/gain exists, it is essential to understand the mechanism behind the high gain in GaAs and associated gain-induced dispersion. To assess the feasibility of generating high gain and the extent of dispersion effects in nanooptics, we demonstrate experimentally a practical GaAs plasmonic laser based on the hybrid gap plasmon waveguide. The analysis of the modal gain and losses in these nanolasers suggests that even though lasing actions can be obtained in this high loss environment, the severe effects of dispersion pose potential issues in practical PTS systems. In PTS coupled waveguides, the detrimental influence of gain-induced dispersion is counteracted by working with non-identical waveguides and bias pumping of the optical waveguides. The coupled mode theory provides the necessary formulation for an in-depth analysis of such a highly dispersive system. The useful application of dispersion in optical PTS is investigated in a Silicon-based active PTS grating. The results of these works improve our understanding of the gain/loss and dispersion mechanism in PTS optical devices and lay the foundation for an efficient design of PTS optical diodes.
Supervisor: Maier, Stefan A. ; Hong, Minghui ; Oulton, Rupert F. Sponsor: Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral