arXiv subjects

Milos Toth

Publications and source records attributed to Milos Toth.

At least 55 records · Page 3Linked to original sources

arXiv2021

Purcell enhancement of a cavity-coupled emitter in hexagonal boron nitride

Integration of solid state quantum emitters into nanophotonic circuits is a critical step towards fully on-chip quantum photonic based technologies. Among potential materials platforms, quantum emitters in hexagonal boron nitride have emerged over the last years as viable candidate. While the fundamental physical properties have been intensively studied over the last years, only few works have focused on the emitter integration into photonic resonators. Yet, for a potential quantum photonic material platform, the integration with nanophotonic cavities is an important cornerstone, as it enables the deliberate tuning of the spontaneous emission and the improved readout of distinct transitions for that quantum emitter. In this work, we demonstrate the resonant tuning of an integrated monolithic hBN quantum emitter in a photonic crystal cavity through gas condensation at cryogenic temperature. We resonantly coupled the zero phonon line of the emitter to a cavity mode and demonstrate emission enhancement and lifetime reduction, with an estimation for the Purcell factor of ~ 15.

physics.optics↗

arXiv2021

Coupling Spin Defects in a Layered Material to Nanoscale Plasmonic Cavities

Spin defects in hexagonal boron nitride, and specifically the negatively charged boron vacancy (VB) centres, are emerging candidates for quantum sensing. However, the VB defects suffer from low quantum efficiency and as a result exhibit weak photoluminescence. In this work, we demonstrate a scalable approach to dramatically enhance the VB- emission by coupling to a plasmonic gap cavity. The plasmonic cavity is composed of a flat gold surface and a silver cube, with few-layer hBN flakes positioned in between. Employing these plasmonic cavities, we extracted two orders of magnitude in photoluminescence enhancement associated with a corresponding 2 fold enhancement in optically detected magnetic resonance contrast. The work will be pivotal to progress in quantum sensing employing 2D materials, and realisation of nanophotonic devices with spin defects in hexagonal boron nitride.

physics.optics↗

arXiv2021

Integration of hBN quantum emitters in monolithically fabricated waveguides

Hexagonal boron nitride (hBN) is gaining interest for potential applications in integrated quantum nanophotonics. Yet, to establish hBN as an integrated photonic platform several cornerstones must be established, including the integration and coupling of quantum emitters to photonic waveguides. Supported by simulations, we study the approach of monolithic integration, which is expected to have coupling efficiencies that are 4 times higher than those of a conventional hybrid stacking strategy. We then demonstrate the fabrication of such devices from hBN and showcase the successful integration of hBN single photon emitters with a monolithic waveguide. We demonstrate coupling of single photons from the quantum emitters to the waveguide modes and on-chip detection. Our results build a general framework for monolithically integrated hBN single photon emitter and will facilitate future works towards on-chip integrated quantum photonics with hBN.

physics.app-ph↗

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↗

arXiv2021

Phonon dephasing and spectral diffusion of quantum emitters in hexagonal Boron Nitride

Quantum emitters in hexagonal boron nitride (hBN) are emerging as bright and robust sources of single photons for applications in quantum optics. In this work we present detailed studies on the limiting factors to achieve Fourier Transform limited spectral lines. Specifically, we study phonon dephasing and spectral diffusion of quantum emitters in hBN via resonant excitation spectroscopy at cryogenic temperatures. We show that the linewidths of hBN quantum emitters are phonon broadened, even at 5K, with typical values of the order of one GHz. While spectral diffusion dominates at increasing pump powers, it can be minimized by working well below saturation excitation power. Our results are important for future utilization of quantum emitters in hBN for quantum interference experiments.

quant-ph↗

arXiv2021

Scalable and Deterministic Fabrication of Quantum Emitter Arrays from Hexagonal Boron Nitride

We demonstrate the fabrication of large-scale arrays of single photon emitters (SPEs) in hexagonal boron nitride (hBN). Bottom-up growth of hBN onto nanoscale arrays of dielectric pillars yields corresponding arrays of hBN emitters at the pillar sites. Statistical analysis shows that the pillar diameter is critical for isolating single defects, and diameters of ~250 nm produce a near-unity yield of a single emitter at each pillar site. Our results constitute a promising route towards spatially-controlled generation of hBN SPEs and provide an effective and efficient method to create large scale SPE arrays. The results pave the way to scalability and high throughput fabrication of SPEs for advanced quantum photonic applications.

physics.app-ph↗

arXiv2021

Recoil Implantation Using Gas-Phase Precursor Molecules

Ion implantation underpins a vast range of devices and technologies that require precise control over the physical, chemical, electronic, magnetic and optical properties of materials. A variant termed recoil implantation - in which a precursor is deposited onto a substrate as a thin film and implanted via momentum transfer from incident energetic ions - has a number of compelling advantages, particularly when performed using an inert ion nano-beam [Fröch et al., Nat Commun 11, 5039 (2020)]. However, a major drawback of this approach is that the implant species are limited to the constituents of solid thin films. Here we overcome this limitation by demonstrating recoil implantation using gas-phase precursors. Specifically, we fabricate nitrogen-vacancy (NV) color centers in diamond using an Ar ion beam and the nitrogen-containing precursor gases N2, NH3 and NF3. Our work expands the applicability of recoil implantation to most of the periodic table, and to applications in which thin film deposition or removal is impractical.

physics.app-ph↗

arXiv2021

Bottom-Up Synthesis of Hexagonal Boron Nitride Nanoparticles with Intensity-Stabilized Quantum Emitters

Fluorescent nanoparticles are widely utilized in a large range of nanoscale imaging and sensing applications. While ultra-small nanoparticles (size <10 nm) are highly desirable, at this size range their photostability can be compromised due to effects such as intensity fluctuation and spectral diffusion caused by interaction with surface states. In this letter, we demonstrate a facile, bottom-up technique for the fabrication of sub-10-nm hBN nanoparticles hosting photostable bright emitters via a catalyst-free hydrothermal reaction between boric acid and melamine. We also implement a simple stabilization protocol that significantly reduces intensity fluctuation by ~85% and narrows the emission linewidth by ~14% by employing a common sol-gel silica coating process. Our study advances a promising strategy for the scalable, bottom-up synthesis of high-quality quantum emitters in hBN nanoparticles.

physics.optics↗

arXiv2021

Fabrication of photonic resonators in bulk 4H-SiC

The design and engineering of photonic architectures, suitable to enhance, collect and guide light on chip is needed for applications in quantum photonics and quantum optomechanics. In this work we apply a Faraday cage based oblique angle etch method to fabricate various functional photonic devices from 4H Silicon Carbide - a material that has attracted attention in recent years, due to its potential in optomechanics, nonlinear optics and quantum information. We detail the processing conditions and thoroughly address the geometrical and optical characteristics of the fabricated devices. Employing photoluminescence measurements we demonstrate high quality factors for suspended microring resonators of up to 3500 in the visible range. Such devices will be applicable in the future to augment the properties of SiC in integrated on chip quantum photonics.

physics.optics↗

arXiv2020

Versatile Direct Writing of Dopants in a Solid State Host Through Recoil Implantation

Modifying material properties at the nanoscale is crucially important for devices in nanoelectronics, nanophotonics and quantum information. Optically active defects in wide band gap materials, for instance, are vital constituents for the realisation of quantum technologies. Yet, the introduction of atomic defects through direct ion implantation remains a fundamental challenge. Herein, we establish a universal method for material doping by exploiting one of the most fundamental principles of physics - momentum transfer. As a proof of concept, we direct-write arrays of emitters into diamond via momentum transfer from a Xe+ focused ion beam (FIB) to thin films of the group IV dopants pre-deposited onto a diamond surface. We conclusively show that the technique, which we term knock-on doping, can yield ultra-shallow dopant profiles localized to the top 5 nm of the target surface, and use it to achieve sub-50 nm lateral resolution. The knock-on doping method is cost-effective, yet very versatile, powerful and universally suitable for applications such as electronic and magnetic doping of atomically thin materials and engineering of near-surface states of semiconductor devices.

physics.app-ph↗

arXiv2020

Grain Dependent Growth of Bright Quantum Emitters in Hexagonal Boron Nitride

Point defects in hexagonal boron nitride have emerged as a promising quantum light source due to their bright and photostable room temperature emission. In this work, we study the incorporation of quantum emitters during chemical vapor deposition growth on a nickel substrate. Combining a range of characterization techniques, we demonstrate that the incorporation of quantum emitters is limited to (001) oriented nickel grains. Such emitters display improved emission properties in terms of brightness and stability. We further utilize these emitters and integrate them with a compact optical antenna enhancing light collection from the sources. The hybrid device yields average saturation count rates of ~2.9 x106 cps and an average photon purity of ~90%. Our results advance the controlled generation of spatially distributed quantum emitters in hBN and demonstrate a key step towards on-chip devices with maximum collection efficiency.

physics.app-ph↗

arXiv2020

Identifying Carbon as the Source of Visible Single Photon Emission from Hexagonal Boron Nitride

Single photon emitters (SPEs) in hexagonal boron nitride (hBN) have garnered significant attention over the last few years due to their superior optical properties. However, despite the vast range of experimental results and theoretical calculations, the defect structure responsible for the observed emission has remained elusive. Here, by controlling the incorporation of impurities into hBN and by comparing various synthesis methods, we provide direct evidence that the visible SPEs are carbon related. Room temperature optically detected magnetic resonance (ODMR) is demonstrated on ensembles of these defects. We also perform ion implantation experiments and confirm that only carbon implantation creates SPEs in the visible spectral range. Computational analysis of hundreds of potential carbon-based defect transitions suggest that the emission results from the negatively charged VBCN- defect, which experiences long-range out-of-plane deformations and is environmentally sensitive. Our results resolve a long-standing debate about the origin of single emitters at the visible range in hBN and will be key to deterministic engineering of these defects for quantum photonic devices.

physics.app-ph↗

arXiv2020

Optical Thermometry with Quantum Emitters in Hexagonal Boron Nitride

Nanoscale optical thermometry is a promising non-contact route for measuring local temperature with both high sensitivity and spatial resolution. In this work, we present a deterministic optical thermometry technique based on quantum emitters in nanoscale hexagonal boron-nitride. We show that these nanothermometers exhibit better performance than that of homologous, all-optical nanothermometers both in sensitivity and range of working temperature. We demonstrate their effectiveness as nanothermometers by monitoring the local temperature at specific locations in a variety of custom-built micro-circuits. This work opens new avenues for nanoscale temperature measurements and heat flow studies in miniaturized, integrated devices.

physics.app-ph↗

arXiv2020

Quantum Random Number Generation using a Solid-State Single-Photon Source

Quantum random number generation (QRNG) harnesses the intrinsic randomness of quantum mechanical phenomena. Demonstrations of such processes have, however, been limited to probabilistic sources, for instance, spontaneous parametric down-conversion or faint lasers, which cannot be triggered deterministically. Here, we demonstrate QRNG with a quantum emitter in hexagonal boron nitride; an emerging solid-state quantum source that can generate single photons on demand and operates at room temperature. We achieve true random number generation through the measurement of single photons exiting one of four integrated photonic waveguides, and subsequently, verify the randomness of the sequences in accordance with the National Institute of Standards and Technology benchmark suite. Our results open a new avenue to the fabrication of on-chip deterministic random number generators and other solid-state-based quantum-optical devices.

physics.optics↗

arXiv2020

Photonic nanobeam cavities with nano-pockets for efficient integration of fluorescent nanoparticles

Integrating fluorescent nanoparticles with high-Q, small mode volume cavities is indispensable for nanophotonics and quantum technologies. To date, nanoparticles have largely been coupled to evanescent fields of cavity modes, which limits the strength of the interaction. Here, we developed both a cavity design and a fabrication method that enable efficient coupling between a fluorescent nanoparticle and a cavity optical mode. The design consists of a fishbone-shaped, one-dimensional photonic crystal cavity with a nano-pocket located at the electric field maximum of the fundamental optical mode. Furthermore, the presence of a nanoparticle inside the pocket reduces the mode volume substantially and induces subwavelength light confinement. Our approach opens exciting pathways to achieve strong light confinement around fluorescent nanoparticles for applications in energy, sensing, lasing and quantum technologies.

physics.app-ph↗

arXiv2020

Strain Engineering of Quantum Emitters in Hexagonal Boron Nitride

Quantum emitters in hexagonal boron nitride (hBN) are promising building blocks for the realization of integrated quantum photonic systems. However, their spectral inhomogeneity currently limits their potential applications. Here, we apply tensile strain to quantum emitters embedded in few-layer hBN films and realize both red and blue spectral shifts with tuning magnitudes up to 65 meV, a record for any two-dimensional quantum source. We demonstrate reversible tuning of the emission and related photophysical properties. We also observe rotation of the optical dipole in response to strain, suggesting the presence of a second excited state. We derive a theoretical model to describe strain-based tuning in hBN, and the rotation of the optical dipole. Our work demonstrates the immense potential for strain tuning of quantum emitters in layered materials to enable their employment in scalable quantum photonic networks.

cond-mat.mtrl-sci↗

arXiv2019

Coupling hBN quantum emitters to 1D photonic crystal cavities

Quantum photonics technologies require a scalable approach for integration of non-classical light sources with photonic resonators to achieve strong light confinement and enhancement of quantum light emission. Point defects from hexagonal Boron Nitride (hBN) are amongst the front runners for single photon sources due to their ultra bright emission, however, coupling of hBN defects to photonic crystal cavities has so far remained elusive. Here we demonstrate on-chip integration of hBN quantum emitters with photonic crystal cavities from silicon nitride (Si3N4) and achieve experimentally measured Q-factor of 3,300 for hBN/Si3N4 hybrid cavities. We observed 9-fold photoluminescence enhancement of a hBN single photon emission at room temperature. Our work paves the way towards hybrid integrated quantum photonics with hBN, and outlines an excellent path for further development of cavity quantum electrodynamic experiments and on-chip integration of 2D materials.

physics.app-ph↗

arXiv2019

Low-temperature electron-phonon interaction of quantum emitters in hexagonal Boron Nitride

Quantum emitters based on atomic defects in layered hexagonal Boron Nitride (hBN) have emerged as promising solid state 'artificial atoms' with atom-like photophysical and quantum optoelectronic properties. Similar to other atom-like emitters, defect-phonon coupling in hBN governs the characteristic single-photon emission and provides an opportunity to investigate the atomic and electronic structure of emitters as well as the coupling of their spin- and charge-dependent electronic states to phonons. Here, we investigate these questions using photoluminescence excitation (PLE) experiments at T=4K on single photon emitters in multilayer hBN grown by chemical vapor deposition. By scanning up to 250 meV from the zero phonon line (ZPL), we can precisely measure the emitter's coupling efficiency to different phonon modes. Our results show that excitation mediated by the absorption of one in-plane optical phonon increases the emitter absorption probability ten-fold compared to that mediated by acoustic or out-of-plane optical phonons. We compare these measurements against theoretical predictions by first-principles density-functional theory of four defect candidates, for which we calculate prevalent charge states and their spin-dependent coupling to bulk and local phonon modes. Our work illuminates the phonon-coupled dynamics in hBN quantum emitters at cryogenic temperature, with implications more generally for mesoscopic quantum emitter systems in 2D materials and represents possible applications in solid-state quantum technologies.

cond-mat.mes-hall↗