arXiv subjects

Lesley Spencer

Publications and source records attributed to Lesley Spencer.

8 recordsLinked to original sources

arXiv2026

Intermediate Excited State Relaxation Dynamics of Boron Vacancy Spin Defects in Hexagonal Boron Nitride

Optically addressable spin defects in hexagonal boron nitride offer promising potential for 2D quantum sensing, though excited-state dynamics remain poorly understood. In particular, the non-radiative relaxation paths from the excited triplet states to the ground state, specially those involving a shelving intermediate state (IS), remain largely hypothetical, and the rate constants have yet to be directly measured. In this work, we investigate the relaxation dynamics of the IS in the optical pumping cycle in a broad temperature range. We measure a 24.0(3) ns relaxation time from IS to the ground state at room temperature, which approximately doubles at low temperatures. Simulations reveal how spin populations and ground-state polarization evolve with varying excitation rate. Accordingly, we optimize optically detected magnetic resonance pulse sequences to account for the effects of IS relaxation. This considerably enhances spin manipulation efficiency, allowing substantial optimization of the quantum sensor's sensitivity based on boron vacancies.

quant-ph↗

arXiv2025

Quantifying Spin Defect Density in hBN via Raman and Photoluminescence Analysis

Negatively charged boron vacancies ($\mathrm{V_B^-}$) in hexagonal boron nitride (hBN) are emerging as promising solid-state spin qubits due to their optical accessibility, structural simplicity, and compatibility with photonic platforms. However, quantifying the density of such defects in thin hBN flakes has remained elusive, limiting progress in device integration and reproducibility. Here, we present an all-optical method to quantify $\mathrm{V_B^-}$ defect density in hBN by correlating Raman and photoluminescence (PL) signatures with irradiation fluence. We identify two defect-induced Raman modes, D1 and D2, and assign them to vibrational modes of $\mathrm{V_B^-}$ using polarization-resolved Raman measurements and density functional theory (DFT) calculations. By adapting a numerical model originally developed for graphene, we establish an empirical relationship linking Raman (D1, $E_\mathrm{2g}$) and PL intensities to absolute defect densities. This method is universally applicable across various irradiation types and uniquely suited for thin flakes, where conventional techniques fail. Our approach enables accurate, direct, and non-destructive quantification of spin defect densities down to $10^{15}$ defects/ cm${}^3$, offering a powerful tool for optimizing and benchmarking hBN for quantum optical applications.

quant-ph↗

arXiv2025

A Van der Waals Moiré Bilayer Photonic Crystal Cavity

Enhancing light-matter interactions with photonic structures is critical in classical and quantum nanophotonics. Recently, Moiré twisted bilayer optical materials have been proposed as a promising means towards a tunable and controllable platform for nanophotonic devices, with proof of principle realisations in the near infrared spectral range. However, the realisation of Moiré photonic crystal (PhC) cavities has been challenging, due to a lack of advanced nanofabrication techniques and availability of standalone transparent membranes. Here, we leverage the properties of the van der Waals material hexagonal Boron Nitride to realize Moiré bilayer PhC cavities. We design and fabricate a range of devices with controllable twist angles, with flatband modes in the visible spectral range (~ 450 nm). Optical characterization confirms the presence of spatially periodic cavity modes originating from the engineered dispersion relation (flatband). Our findings present a major step towards harnessing a two-dimensional van der Waals material for the next-generation of on chip, twisted nanophotonic systems.

physics.optics↗

arXiv2024

Double Etch Method for the Fabrication of Nanophotonic Devices from Van der Waals Materials

The integration of van der Waals (vdW) materials into photonic devices has laid out a foundation for many new quantum and optoelectronic applications. Despite tremendous progress in the nanofabrication of photonic building blocks from vdW crystals, there are still limitations, specifically with large-area devices and masking. Here, we focus on hexagonal boron nitride (hBN) as a vdW material and present a double etch method that overcomes problems associated with methods that employ metallic films and resist-based films for masking. Efficacy of the developed protocol is demonstrated by designing and fabricating a set of functional photonic components including waveguides, ring resonators and photonic crystal cavities. The functionality of the fabricated structures is demonstrated through optical characterization over several key spectral ranges. These include the near-infrared and blue ranges, where the hBN boron vacancy (VB-) spin defects and the coherent B center quantum emitters emit, respectively. The double etch method enables fabrication of high-quality factor optical cavities and constitutes a promising pathway toward on-chip integration of vdW materials.

physics.optics↗

arXiv2024

Fibre-integrated van der Waals quantum sensor with an optimal cavity interface

Integrating quantum materials with fibre optics adds advanced functionalities to a variety of applications, and introduces fibre-based quantum devices such as remote sensors capable of probing multiple physical parameters. However, achieving optimal integration between quantum materials and fibres is challenging, particularly due to difficulties in fabrication of quantum elements with suitable dimensions and an efficient photonic interface to a commercial optical fibre. Here we demonstrate a new modality for a fibre-integrated van der Waals quantum sensor. We design and fabricate a hole-based circular Bragg grating cavity from hexagonal boron nitride (hBN), engineer optically active spin defects within the cavity, and integrate the cavity with an optical fibre using a deterministic pattern transfer technique. The fibre-integrated hBN cavity enables efficient excitation and collection of optical signals from spin defects in hBN, thereby enabling all-fibre integrated quantum sensors. Moreover, we demonstrate remote sensing of a ferromagnetic material and of arbitrary magnetic fields. All in all, the hybrid fibre-based quantum sensing platform may pave the way to a new generation of robust, remote, multi-functional quantum sensors.

quant-ph↗

arXiv2023

Monolithic Integration of Single Quantum Emitters in hBN Bullseye Cavities

The ability of hexagonal boron nitride to host quantum emitters in the form of deep-level color centers makes it an important material for quantum photonic applications. This work utilizes a monolithic circular Bragg grating device to enhance the collection of single photons with 436 nm wavelength emitted from quantum emitters in hexagonal boron nitride. We observe a 6- fold increase in collected intensity for a single photon emitter coupled to a device compared to an uncoupled emitter, and show exceptional spectral stability at cryogenic temperature. The devices were fabricated using a number of etching methods, beyond standard fluorine-based reactive ion etching, and the quantum emitters were created using a site-specific electron beam irradiation technique. Our work demonstrates the potential of monolithically-integrated systems for deterministically-placed quantum emitters using a variety of fabrication options.

physics.app-ph↗

arXiv2023

Monolithic Platform for Integrated Quantum Photonics with Hexagonal Boron Nitride

Integrated quantum photonics (IQP) provides a path to practical, scalable quantum computation, communications and information processing. Realization of an IQP platform requires integration of quantum emitters with high quality photonic circuits. However, the range of materials for monolithic platforms is limited by the simultaneous need for a high-quality single photon source, high optical performance and availability of scalable nanofabrication techniques. Here we demonstrate the fabrication of IQP components from the recently emerged quantum material hexagonal boron nitride (hBN), including tapered waveguides, microdisks, and 1D and 2D photonic crystal cavities. Resonators with quality factors greater than 4000 are achieved, and we engineer proof-of-principle complex, free-standing IQP circuitry fabricated from single crystal hBN. Our results show the potential of hBN for scalable integrated quantum technologies.

physics.optics↗

arXiv2021

Coupling spin defects in hexagonal boron nitride to monolithic bullseye cavities

Color centers in hexagonal boron nitride (hBN) are becoming an increasingly important building block for quantum photonic applications. Herein, we demonstrate the efficient coupling of recently discovered spin defects in hBN to purposely designed bullseye cavities. We show that the all monolithic hBN cavity system exhibits an order of magnitude enhancement in the emission of the coupled boron vacancy spin defects. In addition, by comparative finite difference time domain modelling, we shed light on the emission dipole orientation, which has not been experimentally demonstrated at this point. Beyond that, the coupled spin system exhibits an enhanced contrast in optically detected magnetic resonance readout and improved signal to noise ratio. Thus, our experimental results supported by simulations, constitute a first step towards integration of hBN spin defects with photonic resonators for a scalable spin photon interface.

physics.optics↗