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Yuerui Lu

Publications and source records attributed to Yuerui Lu.

At least 19 recordsLinked to original sources

Ultrafast tuning of the focusing efficiency of a nonlinear atomically thin lens

Control over light focusing is paramount for optical and imaging systems. However, its implementation in active and integrated nanophotonic devices is hindered by the bulky and static nature of standard lenses. Here, we show that Fresnel Zone Plate Lenses (FZPL) based on a monolayer Transitional Metal Dichalcogenide are a viable solution to the challenges of miniaturization, integration, and active control of the focusing efficiency, featuring both ultrafast and large modulation depth. Using a monolayer $WSe_2$ FZPL, we demonstrate all-optical ultrafast ($\sim$ ps) modulation of the focusing efficiency close to $\sim$30\%, that we achieve by exploiting the nonlinear exciton-resonant enhancement and modulation of second harmonic generation.

physics.optics

Real-Space Imaging of Guided Exciton Polaritons in Free-standing Monolayer WSe2

Monolayers of transition metal dichalcogenides (TMDCs), known for their strong excitonic states with high binding energies in the visible spectrum at room temperature, offer great potential for polariton-driven devices. While polariton guided modes in bulk TMDCs have been reported the real space experimental observation of 2D exciton-polariton guided modes in a monolayer remains challenging due to various mode cut-off conditions that arise as the TMDC layer becomes thinner, including cut-off frequency, mode confinement and boundary conditions. Here using scanning near-field optical microscopy (s-SNOM), we directly visualized the real-space propagation of these guided modes for the first time in an angstrom-thick, suspended monolayer of WSe2. Through numerical simulations we have also validated that the guided mode can only exist in a monolayer WSe2 when symmetric cladding conditions are closely applied. By tuning the excitation laser energy and analysing the guided mode distribution, we observed a pronounced back-bending dispersion around the A exciton, indicating strong light-matter interactions, and confirmed the existence of the fundamental TE0 exciton polariton (EP) propagation mode. The unique dispersion characteristics of these modes were further validated through theoretical modelling of the mode in free-standing monolayer WSe2. Our findings provide crucial experimental evidence of guided mode EPs in atomically thin TMDCs, opening new possibilities for nanoscale photonic applications.

physics.optics

In-Line Fiber-Integrated Photon-Pair Generation from van der Waals Crystals

Miniaturized quantum light sources that operate directly in optical fibers are an attractive platform for optical quantum technologies. However, most miniaturized spontaneous parametric down- conversion (SPDC) sources still rely on objective-lens-based free-space pumping and collection, which limits compactness, robustness, and direct compatibility with fiber-based systems. Here we demonstrate a lens-free in-line SPDC photon-pair source by integrating a van der Waals NbOI2 flake directly onto the end facet of an optical fiber. In this configuration, the generated photon- pairs are efficiently collected into optical fibers, eliminating the need for bulk free-space collection optics. Despite the limited numerical aperture of the single-mode fiber, efficient photon-pair collection with high purity, characterized by a coincidence-to-accidental ratio of up to ~4600, is achieved in an ultracompact configuration. These results establish van der Waals ferroelectric materials as a promising platform for fiber-integrated quantum light sources and provide a pathway toward compact, alignment-free quantum photonic devices.

quant-ph

Air-stable bright entangled photon-pair source from graphene-encapsulated van der Waals ferroelectric NbOI2

Van der Waals (vdW) ferroelectrics are emerging nonlinear photonic materials that combine large second-order susceptibility \c{hi}(2) with heterostructure compatibility, offering an attractive route toward miniaturized spontaneous parametric down-conversion (SPDC) sources. However, vdW SPDC sources operating under continuous irradiation in air remain limited in low brightness and poor operational stability, as oxygen and moisture exposure, together with pump-induced heating, lead to material degradation and permanent damage. Here we demonstrate an air-stable, bright SPDC source based on ferroelectric NbOI2 enabled by graphene encapsulation. Graphene provides robust environmental protection and can effectively supress pump induced degradation by enhancing heat dissipation. We report a record photon-pair generation absolute rate of 258 Hz and a normalized brightness of 19,900 Hz/(mW.mm). Leveraging this stabilized platform, we further generate polarization entangled photon pairs with 94% fidelity with respect to the maximally entangled Bell states from graphene-encapsulated 90{\deg} twisted bilayer NbOI2. Our results establish a practical and air-stable vdW ferroelectric SPDC platform that overcomes key limitations of existing vdW quantum light sources and provides a viable pathway toward scalable, integrated entangled photon sources for on chip quantum photonics.

physics.app-ph

Polarization Engineering of Second-Harmonic Generation in 3R-MoS$_2$ Waveguides

Chip-scale nonlinear optics enables strong light-matter interactions within compact devices, serving as a fundamental platform for multifunctional integrated photonics from classical optical signal processing to quantum information technologies. Transition metal dichalcogenide (TMDC) waveguides have recently emerged as a highly promising platform owing to their giant material nonlinearity and extended interaction lengths. To date, however, research has predominantly focused on conversion efficiency, leaving the mechanisms governing the polarization state of nonlinear signal largely unexplored. Here, we establish a comprehensive framework for engineering the polarization of second-harmonic generation (SHG) in 3R-MoS$_2$ waveguides. By synergizing polarization-resolved measurements with theoretical modeling, we reveal that the SHG polarization is determined by guided-mode interactions constrained by waveguide geometry and crystal symmetry, and further reshaped during propagation. We demonstrate that thickness-dependent guided-mode confinement and in-plane crystal symmetry provide robust, static control over SHG polarization, while propagation length offers a dynamic tuning knob for continuously tailoring the nonlinear output. Our findings provide a deterministic approach for on-chip polarization engineering, opening opportunities for reconfigurable nonlinear light sources and quantum photonic circuits.

physics.optics

Light matter interaction in van der Waals heterostructures with Mie voids

Recently introduced concept of Mie voids allows to enhance the field localization inside air cavities embedded in high-index materials. Mie voids provide an alternative approach to conventional dielectric resonators that confine optical fields within bulk high-index materials. Building on this concept, here we present a hybrid photonic platform that integrates monolayer WS2 with Mie void resonators patterned in a high-index Bi2Te3 substrate. By carefully aligning the dipolar void resonance with the excitonic transition of WS2, we achieve substantially enhanced photoluminescence and second-harmonic generation. Far-field imaging of the harmonic fields reveals spatially resolved hotspots that directly map localized resonant modes, with their positions tunable by cavity geometry and pump wavelength. This approach enables real-space control of nonlinear emission at the single-resonator level, offering a robust and reconfigurable platform for next-generation nonlinear photonics and surface-enhanced optical sensing.

physics.optics

Counter-propagating Entangled Photon Pairs from a Monolayer

Non-phase-matched spontaneous parametric down-conversion (SPDC) in atomically thin materials provides new degrees of freedom and enhanced quantum information capacity compared to conventional phase-matched sources. These systems emerged as promising platforms for quantum computing, communication, and imaging, with the potential to support higher-order nonlinear processes. However, direct observation of photon-pair emission from a monolayer has remained experimentally challenging. In this work, we theoretically modeled SPDC emission across the full angular space from a monolayer GaSe film and experimentally validated the model through measurements of both co- and counter-propagating photon pairs. We demonstrated two-photon quantum correlations in the telecom C-band from the thinnest SPDC source reported to date. The spatially symmetric, broadband emission predicted by theory was confirmed experimentally. Furthermore, we observed high-fidelity Bell states in the counter-propagating configuration, marking the first realization of polarization-entangled photon pairs from a monolayer. Our results revealed the emission characteristics of SPDC in the deeply subwavelength, non-phase-matched regime, and introduced atomically thin, counterpropagating SPDC as a scalable and integrable platform for programmable quantum state generation, extendable via moir\'e superlattice engineering.

quant-ph

Enhanced photon-pair generation from a van der Waals metasurface

Quantum photon pairs play a pivotal role in many quantum applications. Metasurfaces, two-dimensional arrays of nanostructures, have been studied intensively to enhance and control pair generation via spontaneous parametric downconversion (SPDC). Van der Waals (VdW) layered materials have emerged as promising candidates for nonlinear materials in quantum light sources, owing to their high nonlinear susceptibility and compatibility with on-chip integration. In this work, we present the first demonstration of SPDC from a metasurface composed of the VdW material 3R-MoS2. The nanoresonators support quasi-bound states in the continuum (qBIC) with a quality factor of up to 120, enhancing light-matter interactions. This design achieves a 20-fold increase in SPDC rate compared to an unstructured film and significantly higher brightness, resulting in enhanced quantum photon-pair generation. This work establishes a new approach for utilizing van der Waals metasurfaces in the generation of quantum photon pairs, opening avenues for advanced quantum applications.

physics.optics

Dynamic Control of Nonlinear Emission by Exciton-Photon Coupling in WS2 Metasurfaces

Transition metal dichalcogenides (TMDCs) have demonstrated significant potential as versatile quantum materials for light absorption and emission. Their unique properties are primarily governed by exciton-photon interactions, which can be substantially enhanced through coupling with resonant photonic structures. For example, nonlinear light emission, such as second harmonic generation (SHG) is doubly enhanced when the incident wave is resonant simultaneously with the excitonic and photonic resonance. However, the excitonic absorption of incident waves can significantly dump the SHG emission. Here, we propose and demonstrate a tunable enhancement of SHG by leveraging virtual coupling effects between quasi-bound states in the continuum (qBIC) optical resonances and tunable excitons in arrays of high-index WS2 crescent metaatoms. These crescent metaatoms excites a pure magnetic type qBIC resonance, enabling dynamic control and enhancement of nonlinear optical processes in visible spectrum. Our findings demonstrate that an array of WS2 crescent metaatoms, exhibiting qBIC resonance at half the exciton energy, enhances SHG efficiency by more than 98-fold compared to monolayer WS2 (1L-WS2) and four orders of magnitude relative to unpatterned WS2 film. This substantial SHG enhancement is tunable as a function of temperature and polarization angle of incident light, allowing us to obtain control of the virtual coupling and SHG efficiency in the visible spectrum (600-650 nm). Our work opens new avenues toward next-generation reconfigurable meta-optics devices.

physics.optics

Giant Second Harmonic Generation from 3R-MoS$_2$ Metasurfaces

Metasurfaces have long served as a cornerstone technique to enhance nonlinear processes, enabling frequency conversion, efficient light manipulation and integrated photonic devices. However, traditional bulk materials often suffer from high absorption losses, hindering the second harmonic generation (SHG) efficiency. Here, we develop a novel approach exploiting quasi-bound state in continuum (qBIC) to achieve giant SHG efficiency in metasurfaces utilizing 3R-MoS$_2$, with high index, superior damage threshold and inherent nonlinearity. The high refractive index of 3R-MoS$_2$, facilitates the high-quality factor (Q) metasurfaces, leading to reduced radiation leakage and localized light confinement within qBIC resonances, with which a remarkable 2000-fold enhancement in SHG intensity has been experimentally demonstrated. Additionally, the twist angle between the lattice orientation and the metasurface unit geometry exhibits a 120$^\circ$ periodicity in its influence on SHG behaviour. By strategically designing to realize the qBIC and exciton dual resonances and optimized twist angle (30$^\circ$), SHG conversion efficiency was boosted to ~1%, which is around 2 orders of magnitude higher than those of the best metasurfaces on traditional bulk materials. This approach enables potential applications in various areas of nonlinear optics, including frequency conversion, light manipulation, integrated photonics, and quantum communications.

physics.app-ph

Optical Visualization of Carrier Surfing in 2D Monolayers Driven by Surface Acoustic Waves

Charge carrier transport is pivotal in advancing nanoelectronics. Despite progress in exciton transport within ultra-thin semiconductors, the intertwined transport of free carriers and excitons presents challenges. Surface Acoustic Waves (SAWs) offer a compelling solution, enabling remote, real-time control of excitonic states at room temperature via surfing carriers in 2D materials, a relatively unexplored domain. SAWs create a versatile platform for tailoring excitonic states from microwave to optical frequencies. This study first demonstrates a simple route to visualize directional light transport and carriers drift driven by non-perfect Rayleigh-SAWs. We observed a maximum drift velocity of ~16.4 um/s for ionized carriers with SAW, significantly surpassing their natural movement in monolayers, though free electrons drift remains in the order of ~103 m/s. Enhanced exciton emission was achieved through standing SAWs, generating periodic oscillations. By combining traveling and standing wave portions, controllable, on-demand single-chip emission is feasible. Our findings open avenues for light manipulation, photonic circuits, and on-chip communications technologies.

physics.optics

Quasi-Phase-Matching Enabled by van der Waals Stacking

Quasi-phase matching (QPM) is a technique extensively utilized in nonlinear optics for enhancing the efficiency and stability of frequency conversion processes. However, the conventional QPM relies on periodically poled ferroelectric crystals, which are limited in availability. The 3R phase of molybdenum disulfide (3R-MoS2), a transition metal dichalcogenide (TMDc) with the broken inversion symmetry, stands out as a promising candidate for QPM, enabling efficient nonlinear process. Here, we experimentally demonstrate the QPM at nanoscale, utilizing van der Waals stacking of 3R-MoS2 layers with specific orientation to realize second harmonic generation (SHG) enhancement beyond the non QPM limit. We have also demonstrated enhanced spontaneous parametric down-conversion (SPDC) via QPM of 3R-MoS2 homo-structure, enabling more efficient generation of entangled photon pairs. The tunable capacity of 3R-MoS2 van der Waals stacking provides a platform for tuning phase-matching condition. This technique opens interesting possibilities for potential applications in nonlinear process and quantum technology.

physics.optics

Long-Propagating Ghost Phonon Polaritons Enabled by Selective Mode Excitation

The precise control of phonon polaritons(PhPs) is essential for advancements in nanophotonic applications like on-chip optical communication and quantum information processing. Ghost hyperbolic phonon polaritons (g-HPs), which have been recently discovered, feature in-plane hyperbolic dispersion and oblique wavefronts, enabling long-range propagation. Despite their potential, controlling the directionality and selective excitation of g-HPs remains challenging. Our research demonstrates that modifying the shape of the launching micro/nano antenna can achieve this control. Using an asymmetric triangular gold antenna on a calcite crystal surface, we achieve highly directional g-HP excitation by selectively targeting specific polariton modes. Additionally, the mode of g-HPs can be adjusted by changing the excitation wavelength or rotating the antenna. Remarkably, our near-field imaging experiments show g-HP propagation over distances exceeding 35 micrometers, more than twice the length reported in previous studies. This work merges g-HP theory with structural engineering, enhancing the control over g-HPs and paving the way for innovative applications in mid-IR optoelectronics.

physics.optics

Nonlinear chiral metasurfaces based on structured van der Waals materials

Nonlinear chiral photonics explores nonlinear response of chiral structures, and it offers a pathway to novel optical functionalities not accessible through linear or achiral systems. Here we present the first application of nanostructured van der Waals materials to nonlinear chiral photonics. We demonstrate the three orders of magnitude enhancement of the third-harmonic generation from hBN metasurfaces driven by quasi-bound states in the continuum and accompanied by strong nonlinear circular dichroism at the resonances. This novel platform for chiral metaphotonics can be employed for achieving large circular dichroism combined with high-efficiency harmonic generation in a broad frequency range.

physics.optics

Dipolar many-body complexes and their interactions in stacked 2D heterobilayers

Highly customizable interfaces created by van der Waals stacked 2D materials provide an extremely flexible opportunity for engineering and effectively controlling material properties. The atomic-thin nature and strong scalability of transition metal dichalcogenides (TMDs), the star family of two-dimensional semiconducting materials, allow for the modulation of their inherent optical and electrical characteristics by utilizing various environmental stimuli. In such a material system, the stacking mechanism with spatial separation in the structure enables recent observations of dipolar many-body complexes with the interplay of multi-particles, leading to some exotic and novel excitonic phenomena and enabling the closer study of high-correlated quantum physics. The presence of powerful dipole-dipole interactions among long-lived interlayer excitons can cause the system to enter unique classical and quantum phases with multiparticle correlations, such as dipolar liquids, dipolar crystals and superfluids. The strong binding energy of interlayer excitons in TMD-based hetero-bilayers especially enhances the critical temperature of these exotic phenomena. Here, we provide a concise summary of the recent frontier research progress on dipolar complexes and many-body effects in TMD double layers, encompassing fundamental theory and properties modulation. We reveal the significance and current challenges of this research field and present the potential developing directions of the hetero-bilayers in quantum physics and quantum devices by adding new levels of external control or integration.

cond-mat.mtrl-sci

Twisted van der Waals Quantum Materials: Fundamentals, Tunability and Applications

Twisted vdW quantum materials have emerged as a rapidly developing field of 2D semiconductors. These materials establish a new central research area and provide a promising platform for studying quantum phenomena and investigating the engineering of novel optoelectronic properties such as single-photon emission, non-linear optical response, magnon physics, and topological superconductivity. These captivating electronic and optical properties result from, and can be tailored by, the interlayer coupling using moiré patterns formed by vertically stacking atomic layers with controlled angle misorientation or lattice mismatch. Their outstanding properties and the high degree of tunability position them as compelling building blocks for both compact quantum-enabled devices and classical optoelectronics. This article offers a comprehensive review of recent advancements in the understanding and manipulation of twisted van der Waals structures and presents a survey of the state-of-the-art research on moiré superlattices, encompassing interdisciplinary interests. It delves into fundamental theories, synthesis and fabrication, and visualization techniques, and the wide range of novel physical phenomena exhibited by these structures, with a focus on their potential for practical device integration in applications ranging from quantum information to biosensors, and including classical optoelectronics such as modulators, light emitting diodes (LEDs), lasers, and photodetectors. It highlights the unique ability of moiré superlattices to connect multiple disciplines, covering chemistry, electronics, optics, photonics, magnetism, topological and quantum physics. This comprehensive review provides a valuable resource for researchers interested in moiré superlattices, shedding light on their fundamental characteristics and their potential for transformative applications in various fields.

cond-mat.mtrl-sci

A Tunable Transition Metal Dichalcogenide Entangled Photon-Pair Source

Entangled photon-pair sources are at the core of quantum applications like quantum key distribution, sensing, and imaging. Operation in space-limited and adverse environments such as in satellite-based and mobile communication requires robust entanglement sources with minimal size and weight requirements. Here, we meet this challenge by realizing a cubic micrometer scale entangled photon-pair source in a 3R-stacked transition metal dichalcogenide crystal. Its crystal symmetry enables the generation of polarization-entangled Bell states without additional components and provides tunability by simple control of the pump polarization. Remarkably, generation rate and state tuning are decoupled, leading to equal generation efficiency and no loss of entanglement. Combining transition metal dichalcogenides with monolithic cavities and integrated photonic circuitry or using quasi-phasematching opens the gate towards ultrasmall and scalable quantum devices.

quant-ph

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