Optical emission from defects in hexagonal boron nitride: deterministic patterning, emission enhancement and optically detected magnetic resonance

Abstract

The core focus of this PhD thesis is defects in the 2D material hexagonal Boron Nitride (hBN). Defects in the planar honeycomb crystal structure have energy states within the large hBN band gap. These embedded defect energy levels allow the defects to fluoresce at visible wavelengths and act as single photon emitters (SPE). Single photon emitters are important for quantum information processes (QIP) like quantum encryption. Some hBN defects have energy states with optically addressable spin states and these allow the defect to be operated as a spin qubit. Spin qubits have the potential as a building block in the construction of quantum computers and for quantum sensing applications.

In this work native defects hosted in dropcast hBN nanoflakes are shown to be a promising source of room temperature single photons. This motivated an investigation into scalable deterministic patterning methods including focused ion beam (FIB) milling, reactive ion etching (RIE) and ion implantation so as to pattern optically active SPE defects into larger area hBN flakes and films. We initially show a collection of important null results that highlight issues in the reproduction of published methods for patterning SPE hBN defects. Instead it is shown that a process of 10 keV carbon ion implantation is able to deterministically pattern ensembles of negatively charged boron vacancy defects that emit broadly, with a centre of emission ranging between 800-820 nm. Additionally a process of 2keV Carbon or Nitrogen implantation yielded promising signs of deterministic patterning of a sharper emitter with emission centred around 635nm.

An ensemble of negatively charged boron vacancy defects are shown to have spin (S=1) addressable energy states which allowed the defects to be operated as a spin qubit ensemble. Using the technique of optically detected magnetic resonance (ODMR) the VB− defect is further studied with the aid of a hBN/coplanar waveguide (CPW) device capable of pumping the spins with an oscillating magnetic field at microwave frequencies. Here we show coherent control of the ground state spins by measuring Rabi oscillations with a high power to field conversion ratio, ΩRabi/√P = 134MHz/√W , which is comparable to state of the art devices with NV-centres in diamond. The high power to field conversion is a consequence of the strong in-plane B-field at the surface of the CPW and the proximity of the hBN layer. The strong CPW magnetic pump field, when used in combination with a time resolved photo-luminescence (TRPL) ODMR method, led to the first published measurement of the zero field splitting of the VB− excited state. Zeeman splitting of both the ground and excited state spins demonstrates an ability to tune the spin resonances with an external static magnetic field. With this we were able to demonstrate the possibility that the hBN/CPW device could be operated as a magnetic field sensor in the field of magnetometry.

Finally this work explores fluorescence enhancement of VB− defects present in existing hBN/CPW devices through the design and construction of plasmonic nanocavities assembled by depositing nanoantennas on top of existing hBN/CPW devices. The design process was aided by finite difference time domain (FDTD) simulations (Lumerical) which showed promising relative factor of enhancement of 103 for the radiative emission and quantum efficiency. In practice the fabricated devices did not yield the hoped for enhancement of the VB− defects. However, we were able to identify the sample criteria need to achieve a measurable enhancement.