This thesis describes the development and experimental investigation of a series of III-V semiconductor nano-photonic circuit elements. A recently emerged approach known as photonic topological insulators is used for realization of these photonic components. In this approach, geometrical ideas are utilised to engineer photonic channels to control the flow of light at the nano-scale. Harnessing topological concepts from mathematics and electronic condensed matter physics, these channels are deterministically engineered to offer favourable features such as robustness against imperfections and disorder in the system, as well as control over the direction of transport for propagation of light down to the single photon regime.
Motivated by the extremely appealing features of these topologically protected photonic channels, as well as the possibility of realization of fundamentally new concepts and applications, some of the most important optical elements such as straight and bent waveguides, closed optical ring resonators and integrated single/multi channel waveguide-coupled-ring-resonator photonic crystals are developed and investigated experimentally.
Chapter one provides the background and motivation for integrated photonic circuits as an excellent potential platform for quantum computation, commutation, sensing, imaging and metrology, followed by a detailed introduction to electronic and photonic topological insulators. Relevant topological concepts, band structure engineering and key advantageous features for photonic circuits are discussed in detail.
In chapter two, steps involved in development of the nano-photonic topological circuits are discussed, including the iterative process of conceiving ideas, semi-automated nano-photonic design and 3D electromagnetic wave simulations. This is followed by a discussion of the nano-fabrication optimisation and quantum optical measurements of device operation and performance improvement of a series of topological nano-photonic structures. An introduction to the general characteristics of III-V semiconductor quantum emitters, which were integrated in these devices as embedded single photon emitters, is also covered in this chapter.
Chapters 3-5 of this thesis are presented in the thesis publication format, in which each chapter is either a published or drafted (unpublished) paper.
In chapter three, the design, simulation and the first experimental demonstration of a hexagonal topological photonic crystal ring resonator with chip-integrated quantum dots is presented. This closed optical circuit is made utilizing a Spin-Hall topological interface, and can serve as a key component for quantum optical filters and gates. The formation of ring resonator modes in this hexagonal optical circuit, as well as the helicity of the modes and topological protection for in-plane propagation of light against defects and sharp bends is demonstrated using finite-difference time-domain simulations. Then, formation, as well as control of the lateral confinement of the modes is experimentally demonstrated via photo-luminescence spectroscopy, utilising GaAs quantum dots as an internal light source in the ring resonator.
In chapter four, a design is proposed in order to further improve and move beyond the optical properties of the Spin-Hall resonator. An upgraded ring resonator is proposed utilizing a Valley-Hall topological interface. Significant improvement of the quality factor of modes is demonstrated both in simulations and experiment. To demonstrate the helicity of the modes, first, single photon routing and chiral emission from an embedded QD into a topological guided mode is presented in a straight waveguide geometry. Then, by evanescently coupling the topological ring resonator modes to a topological bus waveguide, chiral coupling of a QD to a resonator mode is presented. Significantly, direct comparison of a trial vs non-trivial topological modes is demonstrated for the first time in the same nano-photonic device.
Chapter five discusses the demonstration of on-chip optical filtering using a novel, integrated, multi-channel topological optical circuit. Device performance flexibility in terms of optical filtering as well as generation and control of the direction of propagation of single photons in this topological photonic add drop filter is demonstrated experimentally for the first time.
A summary of results and conclusion of the findings in these thesis, as well as future directions and challenges for these topological photonic circuits is provided in chapter six.
