arXiv subjects

Zai-Quan Xu

Publications and source records attributed to Zai-Quan Xu.

15 recordsLinked to original sources

arXiv2021

Electrical Control of Quantum Emitters in a Van der Waals Heterostructure

Controlling and manipulating individual quantum systems in solids underpins the growing interest in development of scalable quantum technologies. Recently, hexagonal boron nitride has garnered significant attention in quantum photonic applications due to its ability to host optically stable quantum emitters. However, the large band gap of hBN and the lack of efficient doping inhibits electrical triggering and limits opportunities to study electrical control of emitters. Here, we show an approach to electrically modulate quantum emitters in n hBN graphene van der Waals heterostructure. We show that quantum emitters in hBN can be reversibly activated and modulated by applying a bias across the device. Notably, a significant number of quantum emitters are intrinsically dark, and become optically active at non-zero voltages. To explain the results, we provide a heuristic electrostatic model of this unique behaviour. Finally, employing these devices we demonstrate a nearly coherent source with linewidths of 160 MHz. Our results enhance the potential of hBN for tuneable solid state quantum emitters for the growing field of quantum information science.

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

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↗

arXiv2020

Upconversion nonlinear structured illumination microscopy

Video-rate super-resolution imaging through biological tissue can visualize and track biomolecule interplays and transportations inside cellular organisms. Structured illumination microscopy allows for wide-field super resolution observation of biological samples but is limited by the strong absorption and scattering of light by biological tissues, which degrades its imaging resolution. Here we report a photon upconversion scheme using lanthanide-doped nanoparticles for wide-field super-resolution imaging through the biological transparent window, featured by near-infrared and low-irradiance nonlinear structured illumination. We demonstrate that the 976 nm excitation and 800 nm up-converted emission can mitigate the aberration. We found that the nonlinear response of upconversion emissions from single nanoparticles can effectively generate the required high spatial frequency components in Fourier domain. These strategies lead to a new modality in microscopy with a resolution of 130 nm, 1/7th of the excitation wavelength, and a frame rate of 1 fps.

physics.optics↗

arXiv2019

Near-field energy transfer between a luminescent 2D material and color centers in diamond

Energy transfer between fluorescent probes lies at the heart of many applications ranging from bio-sensing and -imaging to enhanced photo-detection and light harvesting. In this work, we study Förster resonance energy transfer (FRET) between shallow defects in diamond --- nitrogen-vacancy (NV) centers --- and atomically-thin, two-dimensional materials --- tungsten diselenide (WSe$_2$). By means of fluorescence lifetime imaging, we demonstrate the occurrence of FRET in the WSe$_2$/NV system. Further, we show that in the coupled system, NV centers provide an additional excitation pathway for WSe$_2$ photoluminescence. Our results constitute the first step towards the realization of hybrid quantum systems involving single-crystal diamond and two-dimensional materials that may lead to new strategies for studying and controlling spin transfer phenomena and spin valley physics.

quant-ph↗

arXiv2019

Charge transition levels of quantum emitters in hexagonal boron nitride

Quantum emitters in layered materials are promising candidates for applications in nanophotonics. Here we present a technique based on charge transfer to graphene for measuring the charge transition levels ($\rm E_t$) of fluorescent defects in a wide bandgap 2D material, and apply it to quantum emitters in hexagonal boron nitride (hBN). Our results will aid in identifying the atomic structures of quantum emitters in hBN, as well as practical applications since $\rm E_t$ determines defect charge states and plays a key role in photodynamics.

cond-mat.mtrl-sci↗

arXiv2018

Enhanced Emission from WSe2 Monolayers Coupled to Circular Bragg Gratings

Two-dimensional transition-metal dichalcogenides (TMDC) are of great interest for on-chip nanophotonics due to their unique optoelectronic properties. Here, we propose and realize coupling of tungsten diselenide (WSe2) monolayers to circular Bragg grating structures to achieve enhanced emission. The interaction between WSe2 and the resonant mode of the structure results in Purcell-enhanced emission, while the symmetric geometrical structure improves the directionality of the out-coupling stream of emitted photons. Furthermore, this hybrid structure produces a record high contrast of the spin valley readout (> 40%) revealed by the polarization resolved photoluminescence (PL) measurements. Our results are promising for on-chip integration of TMDC monolayers with optical resonators for nanophotonic circuits.

physics.optics↗

arXiv2018

Bottom up engineering of near-identical quantum emitters in atomically thin materials

Quantum technologies require robust and photostable single photon emitters (SPEs) that can be reliably engineered. Hexagonal boron nitride (hBN) has recently emerged as a promising candidate host to bright and optically stable SPEs operating at room temperature. However, the emission wavelength of the fluorescent defects in hBN has, to date, been shown to be uncontrolled. The emitters usually display a large spread of zero phonon line (ZPL) energies spanning over a broad spectral range (hundreds of nanometers), which hinders the potential development of hBN-based devices and applications. We demonstrate bottom-up, chemical vapor deposition growth of large-area, few layer hBN that hosts large quantities of SPEs: 100 per 10x10 μm2. Remarkably, more than 85 percent of the emitters have a ZPL at (580{\pm}10)nm, a distribution which is over an order of magnitude narrower than previously reported. Exploiting the high density and uniformity of the emitters, we demonstrate electrical modulation and tuning of the ZPL emission wavelength by up to 15 nm. Our results constitute a definite advancement towards the practical deployment of hBN single photon emitters in scalable quantum photonic and hybrid optoelectronic devices based on 2D materials.

physics.app-ph↗

arXiv2017

Single Photon Emission from Plasma Treated 2D Hexagonal Boron Nitride

Artificial atomic systems in solids are becoming increasingly important building blocks in quantum information processing and scalable quantum nanophotonic networks. Yet, synthesis of color centers that act as single photon emitters which are suitable for on-chip applications is still beyond reach. Here, we report a number of plasma and thermal annealing methods for the fabrication of emitters in tape-exfoliated hexagonal boron nitride (hBN) crystals. A two-step process comprised of Ar plasma etching and subsequent annealing in Ar is highly robust, and yields a seven-fold increase in the concentration of emitters in hBN. The initial plasma etching step generates emitters that suffer from blinking and bleaching, whereas the two-step process yields emitters that are photostable at room temperature and have an emission energy distribution that is red-shifted relative to that of pristine hBN. An analysis of emitters fabricated by a range of plasma and annealing treatments, combined with a theoretical investigation of point defects in hBN indicates that single photon emitters characterized by a high degree of photostability and emission wavelengths greater than ~700 nm are associated with defect complexes that contain oxygen. This is further confirmed by generating the emitters by annealing hBN in an oxidative atmosphere. Our findings advance present understanding of the structure of quantum emitter in hBN and enhance the nanofabrication toolkit that is needed to realize integrated quantum nanophotonics based on 2D materials.

cond-mat.mtrl-sci↗

arXiv2017

Nanoassembly of quantum emitters in hexagonal boron nitride and gold nanospheres

Assembly of quantum nanophotonic systems with plasmonic resonators are important for fundamental studies of single photon sources as well as for on-chip information processing. In this work, we demonstrate controllable nanoassembly of gold nanospheres with ultra-bright quantum emitters in 2D layered hexagonal boron nitride (hBN). We utilize an atomic force microscope (AFM) tip to precisely position gold nanospheres to close proximity of the quantum emitters and observe the resulting emission enhancement and fluorescence lifetime reduction. A fluorescence enhancement of over 300% is achieved experimentally for quantum emitters in hBN, with a radiative quantum efficiency of up to 40% and a saturated count rate in excess of 5 million counts/s. Our results are promising for future employment of quantum emitters in hBN for integrated nanophotonic devices and plasmonic based nanosensors.

physics.optics↗

arXiv2017

Super-resolution imaging of quantum emitters in layered materials

Layered van der Waals materials are emerging as compelling two-dimensional (2D) platforms for studies of nanophotonics, polaritonics, valleytronics and spintronics, and have the potential to transform applications in sensing, imaging and quantum information processing. Amongst these, hexagonal boron nitride (hBN) is unique in that it hosts ultra-bright, room temperature single photon emitters (SPEs). However, an outstanding challenge is to locate SPEs in hBN with high precision, a task which requires breaking the optical diffraction limit. Here, we report the imaging of SPEs in layered hBN with a spatial resolution of 63 nm using ground state depletion (GSD) nanoscopy. Furthermore, we show that SPEs in hBN possess nonlinear photophysical properties which can be used to realize a new variant of GSD that employs a coincident pair of doughnut-shaped lasers to reduce the laser power that is needed to achieve a given resolution target. Our findings expand the current understanding of the photophysics of quantum emitters in layered hBN and demonstrate the potential for advanced nanophotonic and bio-imaging applications which require localization of individual emitters with super-resolution accuracy.

physics.optics↗

arXiv2017

Resonant Excitation of Quantum Emitters in Hexagonal Boron Nitride

Quantum emitters in layered hexagonal boron nitride (hBN) have recently attracted a great attention as promising single photon sources. In this work, we demonstrate resonant excitation of a single defect center in hBN, one of the most important prerequisites for employment of optical sources in quantum information application. We observe spectral linewidths of hBN emitter narrower than 1 GHz while the emitter experiences spectral diffusion. Temporal photoluminescence measurements reveals an average spectral diffusion time of around 100 ms. On-resonance photon antibunching measurement is also realized. Our results shed light on the potential use of quantum emitters from hBN in nanophotonics and quantum information.

cond-mat.mtrl-sci↗

arXiv2017

Nanodiamonds with photostable, sub-gigahertz linewidths quantum emitters

Single photon emitters with narrow linewidths are highly sought after for applications in quantum information processing and quantum communications. In this letter, we report on a bright, highly polarized near infrared single photon emitter embedded in diamond nanocrystals with a narrow, sub GHz optical linewidths at 10K. The observed zero phonon line at ~ 780 nm is optically stable under low power resonant excitation and blue shifts as the excitation power increases. Our results highlight the prospect for using new near infrared color centers in nanodiamonds for quantum applications.

cond-mat.mes-hall↗

arXiv2017

Atomic Engineering of Single Photon Sources in 2D Boron Nitride Zai-Quan

Artificial atomic systems in solids such as single photon emitters are becoming increasingly important building blocks in quantum information processing and scalable quantum nanophotonic networks. Here, we report on a controllable way to engineer emitters in two-dimensional (2D) hexagonal boron nitride (hBN) crystals using plasma processing. The method is robust, and yields a 7-fold increase in the density of emitters in hBN, which is promising for their deployment in practical devices. While as-fabricated emitters suffer from blinking and bleaching, a subsequent annealing step yields photo-stable emitters. The presented process is the first step towards controllable placement of quantum emitters in hBN for integrated on-chip quantum nanophotonics based on 2D materials.

cond-mat.mtrl-sci↗

arXiv2016

High Performance Atomically Thin Flat Lenses

We experimentally demonstrate ultrathin flat lenses with a thickness of 7 Å, which corresponds to the fundamental physical limit of the thickness of the material, is fabricated in a large area, monolayer, CVD-prepared tungsten chalcogenides single crystals using the low-cost flexible laser writing method. The lenses apply the ultra-high refractive index to introduce abrupt amplitude modulation of the incident light to achieve three-dimensional (3D) focusing diffraction-limited resolution (0.5λ) and a focusing efficiency as high as 31%. An analytical physical model based diffraction theory is derived to simulate the focusing process, which shows excellent agreement with the experimental results.

physics.optics↗