Nanophotonic Integration of hBN Quantum Emitters

Abstract

The central motivation for this doctoral research project is the study and nanophotonic integration of hexagonal Boron Nitride (hBN) quantum emitters. We aim to develop methods for integrating quantum emitters on chip, seeking to support real world applications, such as room temperature single photon sources (SPSs) for quantum communication, or ultra-sensitive quantum magnetic field sensors. By integrating emitters optically and electronically on chip, we negate the need for bulky, manually aligned free space optical components and instead, enable fibre coupling and packaging.

Initially, we focus on single photon emitters in hBN nanoflakes. We develop fabrication techniques based on the correlation of emitter photoluminescence (PL) with high resolution images of nanoflakes, to allow localisation of flakes hosting emitters and electron beam lithography alignment to fabricate nanophotonic structures. We design and model circular Bragg gratings (CBGs) to control the far field intensity pattern of emitters, to enhance light outcoupling. We show successfully fabricated structures, broadband reflection spectroscopy results, and photoluminescence spectra of integrated emitters. We highlight the extreme challenges of fabricating nanophotonic structures without destroying emitters, and highlight the need to use emitter-compatible processes and materials. More work is required to increase the yield of successful devices and to improve performance further, particularly to embed emitters in structures for best performance, and to etch near emitters without compromising performance.

Next, we turn our attention to spin defects. The integration of ensembles of negatively charged Boron vacancy (VB− ) defects for quantum sensing requires both electrical and optical integrated devices. We develop a doped-silicon permittivity model for microwave Co-Planar Waveguide (CPW) simulations, and numerically model in-plane magnetic field distributions for a slot waveguide design, which allows vertical optical access to a buried hBN defect ensemble, as well as the standard CPW designs. We model CPWs on silicon substrates with a range of doping levels to evaluate performance of devices based on standard CMOS (Complementary Metal-Oxide-Semiconductor)- compatible substrates. This is important to allow simultaneous optical integration with silicon nitride waveguides. We go on to model coupling of defect PL to a silicon nitride slab waveguide and perform numerical simulations to verify compatibility of the nitride platform with gold CPWs. We overcome challenges in transferring hBN flakes to rough substrates resulting from the three-dimensional (vertically stacked) integration of a CPW and optical waveguide and give PL and optically detected magnetic resonance results for an isotopically purified, ion implanted hBN flake on a gold CPW.

Finally, we apply an inverse nanophotonic design tool (adjoint topology optimisation) to design novel, doubly resonant, small footprint, light-extractors based on monolithic hBN. We optimise the far field mode overlap between a dipole emitter and a Gaussian target mode for efficient fibre or free space light collection. We target the wavelength of strong absorption in the V− defect, at 480 nm, as well as simultaneously, the central wavelength of the PL fluorescence at 800 nm.

hBN emitters are singly well suited to near-term applications of quantum emitters. They provide bright, room temperature single photon and spin active emitters in a robust, chemically inert, two-dimensional platform. In overcoming some of the integration challenges still associated with these emitters we bring them closer to useful applications.