Candidate: Rubayet Al Maruf
Title: Integrated Waveguide Interfaces for Quantum Optics Applications
Date: August 31, 2020
Time: 10:00 AM
Place: REMOTE ATTENDANCE
Supervisor(s): Bajcsy, Michal
This dissertation explores the integration and interfacing of a variety of photonics devices with optical waveguides, aiming to minimize the optical losses and physical footprint of such systems. The specific investigations include interfacing solid-core and hollow-core optical fibres, development of a monolithic single-photon source based on a quantum dot embedded in a semiconductor nanowire aligned with a single-mode fibre, a proposal for a waveguide-integrated power limiter that can protect detectors in a quantum key distribution (QKD) network, and integrating superconducting-nanowire single photon detectors (SNSPD) with laser-written waveguides.
Hollow-core photonic-crystal fibres (HCPCFs), which can allow simultaneous tight confinement of both photons and atoms in their hollow core, offer a platform for enhancing interactions between light and atomic ensembles. However, interfacing HCPCFs with conventional solid core (SC) fibres presents a unique set of challenges including significant losses in the joint region due to partial melting and deformation of the photonic crystal during the conventional arc-fusion splicing process. We address this issue with a lithographically defined, vacuum-compatible on-chip structure acting as a mechanical splicer that allows efficient injection of light from a SC fibre to a HCPCF and vice versa, with maximum observed efficiency light injection from a single- mode SC fibre into a HCPCF of 93%.
Single photon sources (SPS) are often required in photonic quantum information applications. Among various candidates, quantum dots (QD) embedded in tapered semiconductor nanowires have demonstrated excellent progress so far to realize a SPS. However, the overall collection efficiency of the emitted photon into a single mode fibre remains as one of the major challenges, mainly due to loss in the intermediate optical components. We design a complete scheme for high efficiency light coupling from such quantum dot to a single mode fibre. We optimize the geometry of the tapered nanowire to achieve a low numerical aperture Gaussian output mode from the wire tip and use a lithographically defined structure to align the SMF collecting the emitted photons. A graded-index multimode fibre acting as a lens is spliced on the SMF end for high efficiency coupling of light from the nanowire into the fibre.
Extremely sensitive photon devices are often used in quantum optics application for detection and manipulation of signals with very low power. To prevent malfunction or even damage to these sensitive devices by excess incident power, we propose an optical power limiter based on a pair of cavities integrated with optical waveguides. Such power limiter can be used to protect sensitive electro-optical devices and prevent certain types of attacks on QKD systems.
Lastly, a 2D photonic lattice formed by laser-written waveguides in bulk glass can be used as an analogue quantum simulator. We explore the integration of SNSPDs with laser-written waveguides with the goal of making this platform fully on-chip.