arXiv subjects

Yuerui Lu

Publications and source records attributed to Yuerui Lu.

At least 37 records · Page 2Linked to original sources

arXiv2023

Graphene-Enhanced Single Ion Detectors for Deterministic Near-Surface Dopant Implantation in Diamond

Colour centre ensembles in diamond have been the subject of intensive investigation for many applications including single photon sources for quantum communication, quantum computation with optical inputs and outputs, and magnetic field sensing down to the nanoscale. Some of these applications are realised with a single centre or randomly distributed ensembles in chips, but the most demanding application for a large-scale quantum computer will require ordered arrays. By configuring an electronic-grade diamond substrate with a biased surface graphene electrode connected to charge-sensitive electronics, it is possible to demonstrate deterministic single ion implantation for ions stopping between 30 and 130~nm deep from a typical stochastic ion source. An implantation event is signalled by a charge pulse induced by the drift of electron-hole pairs from the ion implantation. The ion implantation site is localised with an AFM nanostencil or a focused ion beam. This allows the construction of ordered arrays of single atoms with associated colour centres that paves the way for the fabrication of deterministic colour center networks in a monolithic device.

cond-mat.mes-hall↗

arXiv2023

Variant Plateau Law in Atomically Thin Transition Metal Dichalcogenide Dome Networks

Since its fundamental inception from soap bubbles, Plateau law has sparked extensive research in equilibrated states. However, most studies primarily relied on liquids, foams or cellular structures, whereas its applicability has yet to be explored in nano-scale solid films. Here, we observed a variant Plateau law in networks of atomically thin domes made of solid two-dimensional (2D) transition metal dichalcogenides (TMDs). Discrete layer-dependent van der Waals (vdWs) interaction energies were experimentally and theoretically obtained for domes protruding in different TMD layers. Significant surface tension differences from layer-dependent vdWs interaction energies manifest in a variant of this fundamental law. Meanwhile, the remarkable mechanical properties, gas impermeability and interlayer vdWs interaction energy of TMD films enable domes and the networks to sustain high gas pressure and exist in a fundamentally variant nature for several years. Our findings pave the way towards exploring variant discretised states with applications in opto-electro-mechanical devices.

cond-mat.mtrl-sci↗

arXiv2022

12.6 dB squeezed light at 1550 nm from a bow-tie cavity for long-term high duty cycle operation

Squeezed states are an interesting class of quantum states that have numerous applications. This work presents the design, characterisation, and operation of a bow-tie optical parametric amplifier (OPA) for squeezed vacuum generation. We report the high duty cycle operation and long-term stability of the system that makes it suitable for post-selection based continuous-variable quantum information protocols, cluster-state quantum computing, quantum metrology, and potentially gravitational wave detectors. Over a 50 hour continuous operation, the measured squeezing levels were greater than 10 dB with a duty cycle of 96.6%. Alternatively, in a different mode of operation, the squeezer can also operate 10 dB below the quantum noise limit over a 12 hour period with no relocks, with an average squeezing of 11.9 dB. We also measured a maximum squeezing level of 12.6 dB at 1550 nm. This represents one of the best reported squeezing results at 1550 nm to date for a bow-tie cavity. We discuss the design aspects of the experiment that contribute to the overall stability, reliability, and longevity of the OPA, along with the automated locking schemes and different modes of operation.

quant-ph↗

arXiv2022

Exciton-polaron interactions in monolayer WS$_2$

Interactions between quasiparticles are of fundamental importance and ultimately determine the macroscopic properties of quantum matter. A famous example is the phenomenon of superconductivity, which arises from attractive electron-electron interactions that are mediated by phonons or even other more exotic fluctuations in the material. Here we introduce mobile exciton impurities into a two-dimensional electron gas and investigate the interactions between the resulting Fermi polaron quasiparticles. We employ multi-dimensional coherent spectroscopy on monolayer WS$_2$, which provides an ideal platform for determining the nature of polaron-polaron interactions due to the underlying trion fine structure and the valley specific optical selection rules. At low electron doping densities, we find that the dominant interactions are between polaron states that are dressed by the same Fermi sea. In the absence of bound polaron pairs (bipolarons), we show using a minimal microscopic model that these interactions originate from a phase-space filling effect, where excitons compete for the same electrons. We furthermore reveal the existence of a bipolaron bound state with remarkably large binding energy, involving excitons in different valleys cooperatively bound to the same electron. Our work lays the foundation for probing and understanding strong electron correlation effects in two-dimensional layered structures such as moiré superlattices.

cond-mat.mes-hall↗

arXiv2022

Proton-driven patterning of bulk transition metal dichalcogenides

At the few-atom-thick limit, transition metal dichalcogenides (TMDs) exhibit a host of attractive electronic optical, and structural properties. The possibility to pattern these properties has a great impact on applied and fundamental research. Here, we demonstrate spatial control over the light emission, lattice deformation, and hydrogen storage in bulk TMDs. By low-energy proton irradiation, we create uniquely favorable conditions for the production and accumulation of molecular hydrogen just one or few monolayers beneath the crystal basal plane of bulk WS2, WSe2, WTe2, MoSe2, and MoS2 samples. H2 therein produced coalesces to form bubbles, which lead to the localized swelling of one X-M-X plane prevalently. This results eventually in the creation of atomically thin domes filled with molecular hydrogen at 10 atm. The domes emit light strongly well above room temperature and can store H2 indefinitely. They can be produced with the desired density, well-ordered positions, and size tunable from the nanometer to the micrometer scale, thus providing a template for the manageable and durable mechanical and electronic structuring of two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2021

Tunable Unidirectional Nonlinear Emission from Transition-Metal-Dichalcogenide Metasurfaces

Nonlinear light sources are central to a myriad of applications, driving a quest for their miniaturisation down to the nanoscale. In this quest, nonlinear metasurfaces hold a great promise, as they enhance nonlinear effects through their resonant photonic environment and high refractive index, such as in high-index dielectric metasurfaces. However, despite the sub-diffractive operation of dielectric metasurfaces at the fundamental wave, this condition is not fulfilled for the nonlinearly generated harmonic waves, thereby all nonlinear metasurfaces to date emit multiple diffractive beams. Here, we demonstrate the enhanced single-beam second- and third-harmonic generation in a metasurface of crystalline transition-metal-dichalcogenide material, offering the highest refractive index. We show that the interplay between the resonances of the metasurface allows for tuning of the unidirectional second-harmonic radiation in forward or backward direction, not possible in any bulk nonlinear crystal. Our results open new opportunities for metasurface-based nonlinear light-sources, including nonlinear mirrors and entangled-photon generation.

physics.optics↗

arXiv2020

Two-dimensional Ga$_2$O$_3$ glass: a large scale passivation and protection material for monolayer WS$_2$

Atomically thin transition metal dichalcogenide crystals (TMDCs) have extraordinary optical properties that make them attractive for future optoelectronic applications. Integration of TMDCs into practical all-dielectric heterostructures hinges on the ability to passivate and protect them against necessary fabrication steps on large scales. Despite its limited scalability, encapsulation of TMDCs in hexagonal boron nitride (hBN) currently has no viable alternative for achieving high performance of the final device. Here, we show that the novel, ultrathin Ga$_2$O$_3$ glass is an ideal centimeter-scale coating material that enhances optical performance of the monolayers and protects them against further material deposition. In particular, Ga$_2$O$_3$ capping of commercial grade WS$_2$ monolayers outperforms hBN in both scalability and optical performance at room temperature. These properties make Ga$_2$O$_3$ highly suitable for large scale passivation and protection of monolayer TMDCs in functional heterostructures.

cond-mat.mtrl-sci↗

arXiv2020

Engineered creation of periodic giant, non-uniform strains in MoS2 monolayers

The realization of ordered strain fields in two-dimensional crystals is an intriguing perspective in many respects, including the instauration of novel transport regimes and the achievement of enhanced device performances. In this work, we demonstrate the possibility to subject micrometric regions of atomically-thin molybdenum disulphide (MoS2) to giant strains with the desired ordering. Mechanically-deformed MoS2 membranes can be obtained by proton-irradiation of bulk flakes, leading to the formation of monolayer domes containing pressurized hydrogen. By pre-patterning the flakes via deposition of polymeric masks and electron beam lithography, we show that it is possible not only to control the size and position of the domes, but also to create a mechanical constraint. Atomic force microscopy measurements reveal that this constraint alters remarkably the morphology of the domes, otherwise subject to universal scaling laws. Upon the optimization of the irradiation and patterning processes, unprecedented periodic configurations of large strain gradients -- estimated by numerical simulations -- are created, with the highest strains being close to the rupture critical values (> 10 %). The creation of such high strains is confirmed by Raman experiments. The method proposed here represents an important step towards the strain engineering of two-dimensional crystals.

physics.app-ph↗

arXiv2020

Direct Measurement of Folding Angle and Strain Vector in Atomically thin WS$_2$ using Second Harmonic Generation

Structural engineering techniques such as local strain engineering and folding provide functional control over critical optoelectronic properties of 2D materials. Accurate monitoring of local strain vector (both strain amplitude and direction) and folding angle in 2D materials is important to optimize the device performance. Conventionally, the accurate measurement of both strain amplitude and direction requires the combined usage of multiple tools, such as atomic force microscopy (AFM), electron microscopy, Raman spectroscopy, etc. Here, we demonstrated the usage of a single tool, polarization-dependent second harmonic generation (SHG) imaging, to determine the folding angle and strain vector accurately in atomically thin tungsten disulfide (WS2). We find that trilayer WS2 folds with folding angle of 600 show 9 times SHG enhancement due to vector superposition of SH wave vectors coming from the individual folding layers. Strain dependent SHG quenching and enhancement is found parallel and perpendicular respectively to the direction of the compressive strain vector. However, despite a variation in strain angle, the total SHG remains constant which allows us to determine the local strain vector accurately using photoelastic approach. We also demonstrate that band-nesting induced transition (C peak) can highly enhance SHG, which can be significantly modulated by strain. Our results would pave the way to enable novel applications of the TMDs in nonlinear optical device.

physics.optics↗

arXiv2020

Tunable Optoelectronic Properties of WS$_2$ by Local Strain Engineering and Folding

Local strain engineering is an exciting approach to tune the optoelectronic properties of materials. Two dimensional (2D) materials such as 2D transition metal dichalcogenides (TMDs) are particularly well suited for this purpose because they have high flexibility and they can withstand high deformations before rupture. Local strain engineering in 2D TMDs is achieved via strained wrinkles. Wrinkles on thick layers of TMDs are reported to show interesting photoluminescence enhancement due to bandgap modulation and funneling effect. However, the wrinkles in ultrathin TMDs have not been investigated because they can easily fall down to form folds in these ultrathin layers of TMDCs. Here, we have achieved both wrinkles and folds simultaneously in 1-3L WS2 using a new fabrication technique. A layer dependent reduction in surface potential is found for both folded layers and perfect pack layers due to the dominant interlayer screening effect. Strain dependent modulation in semi conductive junction properties is observed for strain induced wrinkles through current scanning. Thermo-ionic modelling suggests that the strained (1.6%) wrinkles can lower the Schottky barrier height (SBH) by 20%. Upon illumination, SBH reduces significantly due to photo-generated carriers. Our results present an important advance towards controlling the optoelectronic properties of atomically thin WS2 via strain engineering, with applications in optoelectronics, quantum optics and nanophotonics device fabrication.

physics.optics↗

arXiv2020

Super-transport of Excitons in Atomically Thin Organic Semiconductors at the 2D Quantum Limit

Long-range and fast transport of coherent excitons is important for development of high-speed excitonic circuits and quantum computing applications. However, most of these coherent excitons have only been observed in some low-dimensional semiconductors when coupled with cavities, as there are large inhomogeneous broadening and dephasing effects on the exciton transport in their native states of the materials. Here, by confining coherent excitons at the 2D quantum limit, we firstly observed molecular aggregation enabled super-transport of excitons in atomically thin two-dimensional (2D) organic semiconductors between coherent states, with a measured a high effective exciton diffusion coefficient of 346.9 cm2/sec at room temperature. This value is one to several orders of magnitude higher than the reported values from other organic molecular aggregates and low-dimensional inorganic materials. Without coupling to any optical cavities, the monolayer pentacene sample, a very clean 2D quantum system (1.2 nm thick) with high crystallinity (J type aggregation) and minimal interfacial states, showed superradiant emissions from the Frenkel excitons, which was experimentally confirmed by the temperature-dependent photoluminescence (PL) emission, highly enhanced radiative decay rate, significantly narrowed PL peak width and strongly directional in-plane emission. The coherence in monolayer pentacene samples was observed to be delocalized over 135 molecules, which is significantly larger than the values (a few molecules) observed from other organic thin films. In addition, the super-transport of excitons in monolayer pentacene samples showed highly anisotropic behaviour. Our results pave the way for the development of future high-speed excitonic circuits, fast OLEDs, and other opto-electronic devices.

cond-mat.mtrl-sci↗

arXiv2019

Atomic Localization of Quantum Emitters in Multilayer Hexagonal Boron Nitride

The recent discovery of single-photon emitting defects hosted by the two-dimensional wide band gap semiconductor hexagonal boron nitride (hBN) has inspired a great number of experiments. Key characteristics of these quantum emitters are their capability to operate at room temperature with a high luminosity. In spite of large theoretical and experimental research efforts, the exact nature of the emission remains unresolved. In this work we utilize layer-by-layer etching of multilayer hBN to localize the quantum emitters with atomic precision. Our results suggest the position of the emitters correlates with the fabrication method: emitters formed under plasma treatment are always in close proximity to the crystal surface, while emitters created under electron irradiation are distributed randomly throughout the entire crystal. This disparity could be traced back to the lower kinetic energy of the ions in the plasma compared to the kinetic energy of the electrons in the particle accelerator. The emitter distance to the surface also correlates with the excited state lifetime: near-surface emitters have a shorter compared to emitters deep within the crystal. Finite-difference time-domain and density functional theory simulations show that optical and electronic effects are not responsible for this difference, indicating effects such as coupling to surface defects or phonons might cause the reduced lifetime. Our results pave a way toward identification of the defect, as well as engineering the emitter properties.

cond-mat.mes-hall↗

arXiv2019

Ferroelectric driven exciton and trion modulation in monolayer MoSe2 and WSe2

In this work, we show how domain engineered lithium niobate can be used to selectively dope monolayer MoSe2 and WSe2 and demonstrate that these ferroelectric domains can significantly enhance or inhibit photoluminescence (PL) with the most dramatic modulation occurring at the heterojunction interface between two domains. A micro-PL and Raman system is used to obtain spatially resolved images of the differently doped transition metal dichalcogenides (TMDs). The domain inverted lithium niobate causes changes in the TMDs due to electrostatic doping as a result of the remnant polarization from the substrate. Moreover, the differently doped TMDs (n-type MoSe2 and p-type WSe2) exhibit opposite PL modulation. Distinct oppositely charged domains were obtained with a 9-fold PL enhancement for the same single MoSe2 sheet when adhered to the positive (P+) and negative (P-) domains. This sharp PL modulation on the ferroelectric domain results from different free electron or hole concentrations in the materials conduction band or valence band. Moreover, excitons dissociate rapidly at the interface between the P+ and P- domains due to the built-in electric field. We are able to adjust the charge on the P+ and P- domains using temperature via the pyroelectric effect and observe rapid PL quenching over a narrow temperature range illustrating the observed PL modulation is electronic in nature. This observation creates an opportunity to harness the direct bandgap TMD 2D materials as an active optical component for the lithium niobate platform using domain engineering of the lithium niobate substrate to create optically active heterostructures that could be used for photodetectors or even electrically driven optical sources on-chip.

cond-mat.mtrl-sci↗

arXiv2019

An Adaptive Soft Plasmonic Nanosheet Resonator

Current micro nanomechanical system are usually based on rigid crystalline semiconductors that normally have high quality factors but lack adaptive responses to variable frequencies, a capability ubiquitous for communications in the biological world, such as bat and whale calls. Here, we demonstrate a soft mechanical resonator based on a freestanding organic-inorganic hybrid plasmonic superlattice nanosheet, which can respond adaptively to either incident light intensity or wavelength. This is achieved because of strong plasmonic coupling in closely-packed nanocrystals which can efficiently concentrate and convert photons into heat. The heat causes the polymer matrix to expand, leading to a change in the nanomechanical properties of the plasmonic nanosheet. Notably, the adaptive frequency responses are also reversible and the responsive ranges are fine-tunable by adjusting the constituent nanocrystal building blocks. We believe that our plasmonic nanosheets may open a new route to design next-generation intelligent bio-mimicking opto-mechanical resonance systems.

physics.app-ph↗

arXiv2019

Space-compatible cavity-enhanced single-photon generation with hexagonal boron nitride

Sources of pure and indistinguishable single-photons are critical for near-future optical quantum technologies. Recently, color centers hosted by two-dimensional hexagonal boron nitride (hBN) have emerged as a promising platform for high luminosity room temperature single-photon sources. Despite the brightness of the emitters, the spectrum is rather broad and the single-photon purity is not sufficient for practical quantum information processing. Here, we report integration of such a quantum emitter hosted by hBN into a tunable optical microcavity. A small mode volume of the order of $λ^3$ allows us to Purcell enhance the fluorescence, with the observed excited state lifetime shortening. The cavity significantly narrows the spectrum and improves the single-photon purity by suppression of off-resonant noise. We explore practical applications by evaluating the performance of our single-photon source for quantum key distribution and quantum computing. The complete device is compact and implemented on a picoclass satellite platform, enabling future low-cost satellite-based long-distance quantum networks.

quant-ph↗

arXiv2018

Radiation tolerance of two-dimensional material-based devices for space applications

Characteristic for devices based on two-dimensional materials are their low size, weight and power requirements. This makes them advantageous for use in space instrumentation, including photovoltaics, batteries, electronics, sensors and light sources for long-distance quantum communication. Here, we present for the first time a comprehensive study on combined radiation effects in earth's atmosphere on various devices based on these nanomaterials. Using theoretical modeling packages, we estimate relevant radiation levels and then expose field-effect transistors, single-photon sources and monolayers as building blocks for future electronics to gamma-rays, protons and electrons. The devices show negligible change in performance after the irradiation, suggesting robust suitability for space use. Under excessive $γ$-radiation, however, monolayer WS$_2$ showed decreased defect densities, identified by an increase in photoluminescence, carrier lifetime and a change in doping ratio proportional to the photon flux. The underlying mechanism was traced back to radiation-induced defect healing, wherein dissociated oxygen passivates sulfur vacancies.

cond-mat.mtrl-sci↗

arXiv2018

Fabrication and deterministic transfer of high quality quantum emitter in hexagonal boron nitride

Color centers in solid state crystals have become a frequently used system for single photon generation, advancing the development of integrated photonic devices for quantum optics and quantum communication applications. In particular, defects hosted by two-dimensional (2D) hexagonal boron nitride (hBN) are a promising candidate for next-generation single photon sources, due to its chemical and thermal robustness and high brightness at room temperature. The 2D crystal lattice of hBN allows for a high extraction efficiency and easy integration into photonic circuits. Here we develop plasma etching techniques with subsequent high temperature annealing to reliably create defects. We show how different fabrication parameters influence the defect formation probability and the emitter brightness. A full optical characterization reveals the higher quality of the created quantum emitters, represented by a narrow spectrum, short excited state lifetime and high single photon purity. We also investigated the photostability on short and very long timescales. We utilize a wet chemically-assisted transfer process to reliably transfer the single photon sources onto arbitrary substrates, demonstrating the feasibility for the integration into scalable photonic quantum information processing networks.

quant-ph↗

arXiv2016

Molecule-Induced Conformational Change in Boron Nitride Nanosheets with Enhanced Surface Adsorption

Surface interaction is extremely important to both fundamental research and practical application. Physisorption can induce shape and structural distortion (i.e. conformational changes) in macromolecular and biomolecular adsorbates, but such phenomenon has rarely been observed on adsorbents. Here, we demonstrate theoretically and experimentally that atomically thin boron nitride (BN) nanosheets as an adsorbent experience conformational changes upon surface adsorption of molecules, increasing adsorption energy and efficiency. The study not only provides new perspectives on the strong adsorption capability of BN nanosheets and many other two-dimensional nanomaterials but also opens up possibilities for many novel applications. For example, we demonstrate that BN nanosheets with the same surface area as bulk hBN particles are more effective in purification and sensing.

cond-mat.mes-hall↗