Interface nano-optics with van der Waals polaritons

Regioselective super-assembly of Prussian blue analogue

The construction of high-asymmetrical structures demonstrates significant potential in improving the functionality and distinctness of nanomaterials, but remains a considerable challenge. Herein, we develop a one-pot method to fabricate regioselective super-assembly of Prussian blue analogue (PBA) -- a PBA anisotropic structure (PBA-AS) decorated with epitaxial modules--using a step-by-step epitaxial growth on a rapidly self-assembled cubic substrate guided by thiocyanuric acid (TCA) molecules. The epitaxial growth units manifest as diverse geometric shapes, which are predominantly concentrated on the {100}, {111}, or {100}+{111} crystal plane of the cubic substrate. The crystal plane and morphology of epitaxial module can be regulated by changing the TCA concentration and reaction temperature, enabling a high level of controllability over specific assembly sites and structures. To illustrate the advantage of the asymmetrical structure, phosphated PBA-AS demonstrates improved performance in the oxygen evolution reaction compared to simple phosphated PBA nanocube. This method offers valuable insights for designing asymmetrical nanomaterials with intricate architectures and versatile functionalities.

Enhanced water decontamination via photogenerated electron delocalization of π → π* and D-π-A synergistically

Mixed-dimensional van der Waals heterojunctions (MD-vdWhs), known for exceptional electron transfer and charge separation capabilities, remain underexplored in photocatalysis. In this study, we leveraged the synergistic effect of intermolecular π → π* and D-π-A dual channels to fabricate novel MD-vdWhs. Owing to the synergistic effect, it exhibits superior electron transfer and delocalization ability, thereby enhancing its photocatalytic performance. The Optimal photocatalyst can degrade 98.78 % of 20 mg/L tetracycline (TC) within 15 min. Additionally, we introduced a novel proof strategy for investigating the photoelectron transfer path, creatively demonstrating the synergistic dual channels effect, which can be attributed to the carbonyl density and light-excitation degree. Notably, even under low-power light sources, it achieved complete inactivation of Escherichia coli within just 7 mins, far surpassing current cutting-edge research. This theoretical framework holds promise for broader applications within related studies.

Highly Confined Hybridized Polaritons in Scalable van der Waals Heterostructure Resonators

The optimization of nanoscale optical devices and structures will enable the exquisite control of planar optical fields. Polariton manipulation is the primary strategy in play. In two-dimensional heterostructures, the ability to excite mixed optical modes offers an additional control in device design. Phonon polaritons in hexagonal boron nitride have been a common system explored for the control of near-infrared radiation. Their hybridization with graphene plasmons makes these mixed phonon polariton modes in hexagonal boron nitride more appealing in terms of enabling active control of electrodynamic properties with a reduction of propagation losses. Optical resonators can be added to confine these hybridized plasmon-phonon polaritons deeply into the subwavelength regime, with these structures featuring high quality factors. Here, we show a scalable approach for the design and fabrication of heterostructure nanodisc resonators patterned in chemical vapor deposition-grown monolayer graphene and h-BN sheets. Real-space mid-infrared nanoimaging reveals the nature of hybridized polaritons in the heterostructures. We simulate and experimentally demonstrate localized hybridized polariton modes in heterostructure nanodisc resonators and demonstrate that those nanodiscs can collectively couple to the waveguide. High quality factors for the nanodiscs are measured with nanoscale Fourier transform infrared spectroscopy. Our results offer practical strategies to realize scalable nanophotonic devices utilizing low-loss hybridized polaritons for applications such as on-chip optical components.

Monolithically Structured van der Waals Materials for Volume-Polariton Refraction and Focusing

Polaritons, hybrid light and matter waves, offer a platform for subwavelength on-chip light manipulation. Recent works on planar refraction and focusing of polaritons all rely on heterogeneous components with different refractive indices. A fundamental question, thus, arises whether it is possible to configure two-dimensional monolithic polariton lenses based on a single medium. Here, we design and fabricate a type of monolithic polariton lens by directly sculpting an individual hyperbolic van der Waals crystal. The in-plane polariton focusing through sculptured step-terraces is triggered by geometry-induced symmetry breaking of momentum matching in polariton refractions. We show that the monolithic polariton lenses can be robustly tuned by the rise of van der Waals terraces and their curvatures, achieving a subwavelength focusing resolution down to 10% of the free-space light wavelength. Fusing with transformation optics, monolithic polariton lenses with gradient effective refractive indices, such as Luneburg lenses and Maxwell's fisheye lenses, are expected by sculpting polaritonic structures with gradually varied depths. Our results bear potential in planar subwavelength lenses, integrated optical circuits, and photonic chips.

Hyperbolic Shear Metasurfaces

Polar dielectrics with low crystal symmetry and sharp phonon resonances can support hyperbolic shear polaritons, which are highly confined surface modes with frequency-dependent optical axes and asymmetric dissipation features. So far, these modes have been observed only in bulk natural materials at midinfrared frequencies, with properties limited by available crystal geometries and phonon resonance strength. Here, we introduce hyperbolic shear metasurfaces, which are ultrathin engineered surfaces supporting hyperbolic surface modes with symmetry-tailored axial dispersion and loss redistribution that can maximally enhance light-matter interactions. By engineering effective shear phenomena in these engineered surfaces, we demonstrate geometry-controlled, ultraconfined, low-loss hyperbolic surface waves with broadband Purcell enhancements applicable across a broad range of the electromagnetic spectrum.

Spectroscopic and Interferometric Sum-Frequency Imaging of Strongly Coupled Phonon Polaritons in SiC Metasurfaces

Phonon polaritons enable waveguiding and localization of infrared light with extreme confinement and low losses. The spatial propagation and spectral resonances of such polaritons are usually probed with complementary techniques such as near-field optical microscopy and far-field reflection spectroscopy. Here, infrared-visible sum-frequency spectro-microscopy is introduced as a tool for spectroscopic imaging of phonon polaritons. The technique simultaneously provides sub-wavelength spatial resolution and highly-resolved spectral resonance information. This is implemented by resonantly exciting polaritons using a tunable infrared laser and wide-field microscopic detection of the upconverted light. The technique is employed to image hybridization and strong coupling of localized and propagating surface phonon polaritons in a metasurface of SiC micropillars. Spectro-microscopy allows to measure the polariton dispersion simultaneously in momentum space by angle-dependent resonance imaging, and in real space by polariton interferometry. Notably, it is possible to directly image how strong coupling affects the spatial localization of polaritons, inaccessible with conventional spectroscopic techniques. The formation of edge states is observed at excitation frequencies where strong coupling prevents polariton propagation into the metasurface. The technique is applicable to the wide range of polaritonic materials with broken inversion symmetry and can be used as a fast and non-perturbative tool to image polariton hybridization and propagation.

Highly confined epsilon-near-zero and surface phonon polaritons in SrTiO3 membranes

Recent theoretical studies have suggested that transition metal perovskite oxide membranes can enable surface phonon polaritons in the infrared range with low loss and much stronger subwavelength confinement than bulk crystals. Such modes, however, have not been experimentally observed so far. Here, using a combination of far-field Fourier-transform infrared (FTIR) spectroscopy and near-field synchrotron infrared nanospectroscopy (SINS) imaging, we study the phonon polaritons in a 100 nm thick freestanding crystalline membrane of SrTiO3 transferred on metallic and dielectric substrates. We observe a symmetric-antisymmetric mode splitting giving rise to epsilon-near-zero and Berreman modes as well as highly confined (by a factor of 10) propagating phonon polaritons, both of which result from the deep-subwavelength thickness of the membranes. Theoretical modeling based on the analytical finite-dipole model and numerical finite-difference methods fully corroborate the experimental results. Our work reveals the potential of oxide membranes as a promising platform for infrared photonics and polaritonics.

Focusing Micromechanical Polaritons in Topologically Nontrivial Hyperbolic Metasurfaces

Vertically stacked multiple atomically thin layers have recently widened the landscape of rich optical structures thanks to these quantum metamaterials or van der Waals (vdW) materials, featuring hyperbolic polaritons with unprecedented avenues for light. Despite their far-reaching implications, most of their properties rest entirely on a trivial band topological origin. Here, a 2D approach is adopted toward a micromechanical vdW analogue that, as a result of engineered chiral and mirror symmetries, provides topologically resilient hyperbolic radiation of mechanical vibrations in the ultrasonic regime. By applying laser vibrometry of the micrometer-sized metasurface, we are able to exhibit the exotic fingerprints of robust hyperbolic radiation spanning several frequencies, which beyond their physical relevance, may enable ultrasonic technologies.

Steering and cloaking of hyperbolic polaritons at deep-subwavelength scales

Polaritons are well-established carriers of light, electrical signals, and even heat at the nanoscale in the setting of on-chip devices. However, the goal of achieving practical polaritonic manipulation over small distances deeply below the light diffraction limit remains elusive. Here, we implement nanoscale polaritonic in-plane steering and cloaking in a low-loss atomically layered van der Waals (vdW) insulator, α-MoO3, comprising building blocks of customizable stacked and assembled structures. Each block contributes specific characteristics that allow us to steer polaritons along the desired trajectories. Our results introduce a natural materials-based approach for the comprehensive manipulation of nanoscale optical fields, advancing research in the vdW polaritonics domain and on-chip nanophotonic circuits.

Graphene-integrated microring cavity for electronically controlled molecular fingerprinting

Microring cavities supporting whispering-gallery modes (WGMs) have an exceptionally high quality factor (Q) and a small mode volume, greatly improving the interaction between light and matter, which has attracted great attention in various microscale/nanoscale photonic devices and potential applications. Recently, two-dimensional van der Waals (vdW) materials such as graphene have emerged as a potential platform for next-generation biosensing by enabling the confinement of light fields at the nanoscale. Here, we propose what we believe to be a novel approach to achieve molecular fingerprint retrieval by integrating graphene into a microring cavity and conducting numerical simulations using the finite-difference time-domain (FDTD) method. The hybrid cavity exhibits high-quality WGMs with a high Q factor of up to 800. Moreover, the resonant wavelength can be electronically controlled through modulation of graphene's Fermi level, enabling coverage of the entire free spectral range at infrared frequencies. By depositing a thin layer of biomolecular material (e.g., CBP) onto the surface of our hybrid cavity, we are able to accurately read out the absorption spectrum at multiple spectral points, thereby achieving broadband fingerprint retrieval for the targeted biomolecule. Our results pave the way for highly sensitive, chip-integrated, miniaturized, and electrically modulated infrared spectroscopy biosensing.

All-optical subcycle microscopy on atomic length scales

Bringing optical microscopy to the shortest possible length and time scales has been a long-sought goal, connecting nanoscopic elementary dynamics with the macroscopic functionalities of condensed matter. Super-resolution microscopy has circumvented the far-field diffraction limit by harnessing optical nonlinearities1. By exploiting linear interaction with tip-confined evanescent light fields2, near-field microscopy3,4 has reached even higher resolution, prompting a vibrant research field by exploring the nanocosm in motion5-19. Yet the finite radius of the nanometre-sized tip apex has prevented access to atomic resolution20. Here we leverage extreme atomic nonlinearities within tip-confined evanescent fields to push all-optical microscopy to picometric spatial and femtosecond temporal resolution. On these scales, we discover an unprecedented and efficient non-classical near-field response, in phase with the vector potential of light and strictly confined to atomic dimensions. This ultrafast signal is characterized by an optical phase delay of approximately π/2 and facilitates direct monitoring of tunnelling dynamics. We showcase the power of our optical concept by imaging nanometre-sized defects hidden to atomic force microscopy and by subcycle sampling of current transients on a semiconducting van der Waals material. Our results facilitate access to quantum light-matter interaction and electronic dynamics at ultimately short spatio-temporal scales in both conductive and insulating quantum materials.

Nonlocal Acoustic Moiré Hyperbolic Metasurfaces

The discovery of the topological transition in twisted bilayer (tBL) materials has attracted considerable attention in nano-optics. In the analogue of acoustics, however, no such topological transition has been found due to the inherent nondirectional scalar property of acoustic pressure. In this work, by using a theory-based nonlocal anisotropic design, the in-plane acoustic pressure is transformed into a spatially distributed vector field using twisted multilayer metasurfaces. So-called "acoustic magic angle"-related acoustic phenomena occur, such as nonlocal polariton hybridization and the topological Lifshitz transition. The dispersion becomes flat at the acoustic magic angle, enabling polarized excitations to propagate in a single direction. Moreover, the acoustic topological transition (from hyperbolic to elliptic dispersion) is experimentally observed for the first time as the twist angle continuously changes. This unique characteristic facilitates low-loss tunable polariton hybridization at the subwavelength scale. A twisted trilayer acoustic metasurface is also experimentally demonstrated, and more possibilities for manipulating acoustic waves are found. These discoveries not only enrich the concepts of moiré physics and topological acoustics but also provide a complete framework of theory and methodologies for explaining the phenomena that are observed.

Compensating losses in polariton propagation with synthesized complex frequency excitation

Surface plasmon polaritons and phonon polaritons offer a means of surpassing the diffraction limit of conventional optics and facilitate efficient energy storage, local field enhancement and highsensitivity sensing, benefiting from their subwavelength confinement of light. Unfortunately, losses severely limit the propagation decay length, thus restricting the practical use of polaritons. While optimizing the fabrication technique can help circumvent the scattering loss of imperfect structures, the intrinsic absorption channel leading to heat production cannot be eliminated. Here, we utilize synthetic optical excitation of complex frequency with virtual gain, synthesized by combining the measurements made at multiple real frequencies, to compensate losses in the propagations of phonon polaritons with dramatically enhanced propagation distance. The concept of synthetic complex frequency excitation represents a viable solution to the loss problem for various applications including photonic circuits, waveguiding and plasmonic/phononic structured illumination microscopy.

Tunable anisotropic van der Waals films of 2M-WS2 for plasmon canalization

In-plane anisotropic van der Waals materials have emerged as a natural platform for anisotropic polaritons. Extreme anisotropic polaritons with in-situ broadband tunability are of great significance for on-chip photonics, yet their application remains challenging. In this work, we experimentally characterize through Fourier transform infrared spectroscopy measurements a van der Waals plasmonic material, 2M-WS2, capable of supporting intrinsic room-temperature in-plane anisotropic plasmons in the far and mid-infrared regimes. In contrast to the recently revealed natural hyperbolic plasmons in other anisotropic materials, 2M-WS2 supports canalized plasmons with flat isofrequency contours in the frequency range of ~ 3000-5000 cm-1. Furthermore, the anisotropic plasmons and the corresponding isofrequency contours can be reversibly tuned via in-situ ion-intercalation. The tunable anisotropic and canalization plasmons may open up further application perspectives in the field of uniaxial plasmonics, such as serving as active components in directional sensing, radiation manipulation, and polarization-dependent optical modulators.

Spatiotemporal beating and vortices of van der Waals hyperbolic polaritons

In conventional thin materials, the diffraction limit of light constrains the number of waveguide modes that can exist at a given frequency. However, layered van der Waals (vdW) materials, such as hexagonal boron nitride (hBN), can surpass this limitation due to their dielectric anisotropy, exhibiting positive permittivity along one optic axis and negativity along the other. This enables the propagation of hyperbolic rays within the material bulk and an unlimited number of subdiffractional modes characterized by hyperbolic dispersion. By employing time-domain near-field interferometry to analyze ultrafast hyperbolic ray pulses in thin hBN, we showed that their zigzag reflection trajectories bound within the hBN layer create an illusion of backward-moving and leaping behavior of pulse fringes. These rays result from the coherent beating of hyperbolic waveguide modes but could be mistakenly interpreted as negative group velocities and backward energy flow. Moreover, the zigzag reflections produce nanoscale (60 nm) and ultrafast (40 fs) spatiotemporal optical vortices along the trajectory, presenting opportunities to chiral spatiotemporal control of light-matter interactions. Supported by experimental evidence, our simulations highlight the potential of hyperbolic ray reflections for molecular vibrational absorption nanospectroscopy. The results pave the way for miniaturized, on-chip optical spectrometers, and ultrafast optical manipulation.

Twisted photonic Weyl meta-crystals and aperiodic Fermi arc scattering

As a milestone in the exploration of topological physics, Fermi arcs bridging Weyl points have been extensively studied. Weyl points, as are Fermi arcs, are believed to be only stable when preserving translation symmetry. However, no experimental observation of aperiodic Fermi arcs has been reported so far. Here, we continuously twist a bi-block Weyl meta-crystal and experimentally observe the twisted Fermi arc reconstruction. Although both the Weyl meta-crystals individually preserve translational symmetry, continuous twisting operation leads to the aperiodic hybridization and scattering of Fermi arcs on the interface, which is found to be determined by the singular total reflection around Weyl points. Our work unveils the aperiodic scattering of Fermi arcs and opens the door to continuously manipulating Fermi arcs.

Unconventional Breathing Currents Far beyond the Quantum Tunneling Distances in Large-Gapped Nanoplasmonic Systems

Localized surface plasmon resonance (LSPR) in plasmonic nanoparticles propels the field of plasmo-electronics, holding promise for transformative optoelectronic devices through efficient light-to-current conversion. Plasmonic excitations strongly influence the charge distribution within nanoparticles, giving rise to electromagnetic fields that can significantly impact the macroscopic charge flows within the nanoparticle housing material. In this study, we present evidence of ultralow, unconventional breathing currents resulting from dynamic irradiance interactions between widely separated nanoparticles, extending far beyond conventional electron (quantum) tunneling distances. We develop an electric analogue model and derive an empirical expression to elucidate the generation of these unconventional breathing currents in cascaded nanoplasmonic systems under irradiance modulation. This technique and theoretical model have significant potential for applications requiring a deeper understanding of current dynamics, particularly on large nanostructured surfaces relevant to photocatalysis, energy harvesting, sensing, imaging, and the development of future photonic devices.

Optical Coupling in Atomic Waveguide for Vertically Integrated Photonics

Integrated 2-dimensional (2D) photonic devices such as monolayer waveguide has generated exceptional interest because of their ultimate thinness. In particular, they potentially permit stereo photonic architecture through bond-free van der Waals integration. However, little is known about the coupling and controlling of the single-atom guided wave to its photonic environment, which governs the design and application of integrated system. Here, we report the optical coupling of atomically guided waves to other photonic modes. We directly probe the mode beating between evanescent waves in a monolayer 2D waveguide and a silicon photonic waveguide, which constitutes a vertically integrated interferometer. The mode-coupling measures the dispersion relation of the guided wave inside the atomic waveguide and unveils it strongly modifies matter's electronic states, manifesting by the formation of a propagating polariton. We also demonstrated light modulating and spectral detecting in this compact nonplanar interferometer. These findings provide a generalizable and versatile platform toward monolithic 3-dimensional integrated photonics.

Twisted moiré conductive thermal metasurface

Extensive investigations on the moiré magic angle in twisted bilayer graphene have unlocked the emerging field-twistronics. Recently, its optics analogue, namely opto-twistronics, further expands the potential universal applicability of twistronics. However, since heat diffusion neither possesses the dispersion like photons nor carries the band structure as electrons, the real magic angle in electrons or photons is ill-defined for heat diffusion, making it elusive to understand or design any thermal analogue of magic angle. Here, we introduce and experimentally validate the twisted thermotics in a twisted diffusion system by judiciously tailoring thermal coupling, in which twisting an analog thermal magic angle would result in the function switching from cloaking to concentration. Our work provides insights for the tunable heat diffusion control, and opens up an unexpected branch for twistronics -- twisted thermotics, paving the way towards field manipulation in twisted configurations including but not limited to fluids.

Wandering principal optical axes in van der Waals triclinic materials

Nature is abundant in material platforms with anisotropic permittivities arising from symmetry reduction that feature a variety of extraordinary optical effects. Principal optical axes are essential characteristics for these effects that define light-matter interaction. Their orientation - an orthogonal Cartesian basis that diagonalizes the permittivity tensor, is often assumed stationary. Here, we show that the low-symmetry triclinic crystalline structure of van der Waals rhenium disulfide and rhenium diselenide is characterized by wandering principal optical axes in the space-wavelength domain with above π/2 degree of rotation for in-plane components. In turn, this leads to wavelength-switchable propagation directions of their waveguide modes. The physical origin of wandering principal optical axes is explained using a multi-exciton phenomenological model and ab initio calculations. We envision that the wandering principal optical axes of the investigated low-symmetry triclinic van der Waals crystals offer a platform for unexplored anisotropic phenomena and nanophotonic applications.

Optically addressable spin defects coupled to bound states in the continuum metasurfaces

Van der Waals (vdW) materials, including hexagonal boron nitride (hBN), are layered crystalline solids with appealing properties for investigating light-matter interactions at the nanoscale. hBN has emerged as a versatile building block for nanophotonic structures, and the recent identification of native optically addressable spin defects has opened up exciting possibilities in quantum technologies. However, these defects exhibit relatively low quantum efficiencies and a broad emission spectrum, limiting potential applications. Optical metasurfaces present a novel approach to boost light emission efficiency, offering remarkable control over light-matter coupling at the sub-wavelength regime. Here, we propose and realise a monolithic scalable integration between intrinsic spin defects in hBN metasurfaces and high quality (Q) factor resonances, exceeding 102, leveraging quasi-bound states in the continuum (qBICs). Coupling between defect ensembles and qBIC resonances delivers a 25-fold increase in photoluminescence intensity, accompanied by spectral narrowing to below 4 nm linewidth and increased narrowband spin-readout efficiency. Our findings demonstrate a new class of metasurfaces for spin-defect-based technologies and pave the way towards vdW-based nanophotonic devices with enhanced efficiency and sensitivity for quantum applications in imaging, sensing, and light emission.

Polariton Microfluidics for Nonreciprocal Dragging and Reconfigurable Shaping of Polaritons

Dielectric environment engineering is an efficient and general approach to manipulating polaritons. Liquids serving as the surrounding media of polaritons have been used to shift polariton dispersions and tailor polariton wavefronts. However, those liquid-based methods have so far been limited to their static states, not fully unleashing the promise offered by the mobility of liquids. Here, we propose a microfluidic strategy for polariton manipulation by merging polaritonics with microfluidics. The diffusion of fluids causes gradient refractive indices over microchannels, which breaks the symmetry of polariton dispersions and realizes the microfluidic analogue to nonreciprocal polariton dragging. Based on polariton microfluidics, we also designed a set of on-chip polaritonic elements to actively shape polaritons, including planar lenses, off-axis lenses, Janus lenses, bends, and splitters. Our strategy expands the toolkit for the manipulation of polaritons at the subwavelength scale and possesses potential in the fields of polariton biochemistry and molecular sensing.

Fabrication of angstrom-scale two-dimensional channels for mass transport

Fluidic channels at atomic scales regulate cellular trafficking and molecular filtration across membranes, and thus play crucial roles in the functioning of living systems. However, constructing synthetic channels experimentally at these scales has been a significant challenge due to the limitations in nanofabrication techniques and the surface roughness of the commonly used materials. Angstrom (Å)-scale slit-like channels overcome such challenges as these are made with precise control over their dimensions and can be used to study the fluidic properties of gases, ions and water at unprecedented scales. Here we provide a detailed fabrication method of the two-dimensional Å-scale channel devices that can be assembled to contain a desired number of channels, a single channel or up to hundreds of channels, made with atomic-scale precision using layered crystals. The procedure includes the fabrication of the substrate, flake, spacer layer, flake transfers, van der Waals assembly and postprocessing. We further explain how to perform molecular transport measurements with the Å-channels to directly probe the intriguing and anomalous phenomena that help shed light on the physics governing ultra-confined transport. The procedure requires a total of 1-2 weeks for the fabrication of the two-dimensional channel device and is suitable for users with prior experience in clean room working environments and nanofabrication.

Ultrabroadband Terahertz Near-Field Nanospectroscopy with a HgCdTe Detector

While near-field infrared nanospectroscopy provides a powerful tool for nanoscale material characterization, broadband nanospectroscopy of elementary material excitations in the single-digit terahertz (THz) range remains relatively unexplored. Here, we study liquid-Helium-cooled photoconductive Hg1-XCdXTe (MCT) for use as a fast detector in near-field nanospectroscopy. Compared to the common T = 77 K operation, liquid-Helium cooling reduces the MCT detection threshold to ∼22 meV, improves the noise performance, and yields a response bandwidth exceeding 10 MHz. These improved detector properties have a profound impact on the near-field technique, enabling unprecedented broadband nanospectroscopy across a range of 5 to >50 THz (175 to >1750 cm-1, or <6 to 57 μm), i.e., covering what is commonly known as the "THz gap". Our approach has been implemented as a user program at the National Synchrotron Light Source II, Upton, USA, where we showcase ultrabroadband synchrotron nanospectroscopy of phonons in ZnSe (∼7.8 THz) and BaF2 (∼6.7 THz), as well as hyperbolic phonon polaritons in GeS (6-8 THz).

Hyperbolic Polaritonic Rulers Based on van der Waals α-MoO3 Waveguides and Resonators

Low-dimensional, strongly anisotropic nanomaterials can support hyperbolic phonon polaritons, which feature strong light-matter interactions that can enhance their capabilities in sensing and metrology tasks. In this work, we report hyperbolic polaritonic rulers, based on microscale α-phase molybdenum trioxide (α-MoO3) waveguides and resonators suspended over an ultraflat gold substrate, which exhibit near-field polaritonic characteristics that are exceptionally sensitive to device geometry. Using scanning near-field optical microscopy, we show that these systems support strongly confined image polariton modes that exhibit ideal antisymmetric gap polariton dispersion, which is highly sensitive to air gap dimensions and can be described and predicted using a simple analytic model. Dielectric constants used for modeling are accurately extracted using near-field optical measurements of α-MoO3 waveguides in contact with the gold substrate. We also find that for nanoscale resonators supporting in-plane Fabry-Perot modes, the mode order strongly depends on the air gap dimension in a manner that enables a simple readout of the gap dimension with nanometer precision.

Visualization of Confined Electrons at Grain Boundaries in a Monolayer Charge-Density-Wave Metal

1D grain boundaries in transition metal dichalcogenides (TMDs) are ideal for investigating the collective electron behavior in confined systems. However, clear identification of atomic structures at the grain boundaries, as well as precise characterization of the electronic ground states, have largely been elusive. Here, direct evidence for the confined electronic states and the charge density modulations at mirror twin boundaries (MTBs) of monolayer NbSe2 , a representative charge-density-wave (CDW) metal, is provided. The scanning tunneling microscopy (STM) measurements, accompanied by the first-principles calculations, reveal that there are two types of MTBs in monolayer NbSe2 , both of which exhibit band bending effect and 1D boundary states. Moreover, the intrinsic CDW signatures of monolayer NbSe2 are dramatically suppressed as approaching an isolated MTB but can be either enhanced or suppressed in the MTB-constituted confined wedges. Such a phenomenon can be well explained by the MTB-CDW interference interactions. The results reveal the underlying physics of the confined electrons at MTBs of CDW metals, paving the way for the grain boundary engineering of the functionality.

Photonic Bound States in the Continuum in Nanostructures

Bound states in the continuum (BIC) have garnered considerable attention recently for their unique capacity to confine electromagnetic waves within an open or non-Hermitian system. Utilizing a variety of light confinement mechanisms, nanostructures can achieve ultra-high quality factors and intense field localization with BIC, offering advantages such as long-living resonance modes, adaptable light control, and enhanced light-matter interactions, paving the way for innovative developments in photonics. This review outlines novel functionality and performance enhancements by synergizing optical BIC with diverse nanostructures, delivering an in-depth analysis of BIC designs in gratings, photonic crystals, waveguides, and metasurfaces. Additionally, we showcase the latest advancements of BIC in 2D material platforms and suggest potential trajectories for future research.

Near- and Far-Field Observation of Phonon Polaritons in Wafer-Scale Multilayer Hexagonal Boron Nitride Prepared by Chemical Vapor Deposition

Polaritons in layered materials (LMs) are a promising platform to manipulate and control light at the nanometer scale. Thus, the observation of polaritons in wafer-scale LMs is critically important for the development of industrially relevant nanophotonics and optoelectronics applications. In this work, phonon polaritons (PhPs) in wafer-scale multilayer hexagonal boron nitride (hBN) grown by chemical vapor deposition are reported. By infrared nanoimaging, the PhPs are visualized, and PhP lifetimes of ≈0.6 ps are measured, comparable to that of micromechanically exfoliated multilayer hBN. Further, PhP nanoresonators are demonstrated. Their quality factors of ≈50 are about 0.7 times that of state-of-the-art devices based on exfoliated hBN. These results can enable PhP-based surface-enhanced infrared spectroscopy (e.g., for gas sensing) and infrared photodetector applications.

Nonlinear Plasmonics in Nanostructured Phosphorene

Phosphorene has emerged as an atomically thin platform for optoelectronics and nanophotonics due to its excellent optical properties and the possibility of actively tuning light-matter interactions through electrical doping. While phosphorene is a two-dimensional semiconductor, plasmon resonances characterized by pronounced anisotropy and strong optical confinement are anticipated to emerge in highly doped samples. Here we show that the localized plasmons supported by phosphorene nanoribbons (PNRs) exhibit high tunability in relation to both edge termination and doping charge polarity and can trigger an intense nonlinear optical response at moderate doping levels. Our explorations are based on a second-principles theoretical framework, employing maximally localized Wannier functions constructed from ab initio electronic structure calculations, which we introduce here to describe the linear and nonlinear optical response of PNRs on mesoscopic length scales. Atomistic simulations reveal the high tunability of plasmons in doped PNRs at near-infrared frequencies, which can facilitate the synergy between the electronic band structure and plasmonic field confinement to drive efficient high-harmonic generation. Our findings establish nanostructured phosphorene as a versatile atomically thin material candidate for nonlinear plasmonics.

Visible to mid-infrared giant in-plane optical anisotropy in ternary van der Waals crystals

Birefringence is at the heart of photonic applications. Layered van der Waals materials inherently support considerable out-of-plane birefringence. However, funnelling light into their small nanoscale area parallel to its out-of-plane optical axis remains challenging. Thus far, the lack of large in-plane birefringence has been a major roadblock hindering their applications. Here, we introduce the presence of broadband, low-loss, giant birefringence in a biaxial van der Waals materials Ta2NiS5, spanning an ultrawide-band from visible to mid-infrared wavelengths of 0.3-16 μm. The in-plane birefringence Δn ≈ 2 and 0.5 in the visible and mid-infrared ranges is one of the highest among van der Waals materials known to date. Meanwhile, the real-space propagating waveguide modes in Ta2NiS5 show strong in-plane anisotropy with a long propagation length (>20 μm) in the mid-infrared range. Our work may promote next-generation broadband and ultracompact integrated photonics based on van der Waals materials.

Editorial for the Special Issue on Micro/Nano-Structure Based Optoelectronics and Photonics Devices

In the ever-evolving fields of optoelectronics and photonics, the introduction of carefully designed micro-/nanostructures enables personalized customization of the electrical and optical properties of optoelectronic and photonic devices [...].

Functionalizing nanophotonic structures with 2D van der Waals materials

The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered a wide spectrum of optical and optoelectronic applications. Photonic structures of conventional materials typically lack efficient reconfigurability or multifunctionality. Atomically thin 2D materials can thus generate new functionality and reconfigurability for a well-established library of photonic structures such as integrated waveguides, optical fibers, photonic crystals, and metasurfaces, to name a few. Meanwhile, the interaction between light and van der Waals materials can be drastically enhanced as well by leveraging micro-cavities or resonators with high optical confinement. The unique van der Waals surfaces of the 2D materials enable handiness in transfer and mixing with various prefabricated photonic templates with high degrees of freedom, functionalizing as the optical gain, modulation, sensing, or plasmonic media for diverse applications. Here, we review recent advances in synergizing 2D materials to nanophotonic structures for prototyping novel functionality or performance enhancements. Challenges in scalable 2D materials preparations and transfer, as well as emerging opportunities in integrating van der Waals building blocks beyond 2D materials are also discussed.

Charged biexciton polaritons sustaining strong nonlinearity in 2D semiconductor-based nanocavities

Controlling the interaction between light and matter at micro- and nano-scale can provide new opportunities for modern optics and optoelectronics. An archetypical example is polariton, a half-light-half-matter quasi particle inheriting simultaneously the robust coherence of light and the strong interaction of matter, which plays an important role in many exotic phenomena. Here, we open up a new kind of cooperative coupling between plasmon and different excitonic complexes in WS2-silver nanocavities, namely plasmon-exciton-trion-charged biexciton four coupling states. Thanks to the large Bohr radius of up to 5 nm, the charged biexciton polariton exhibits strong saturation nonlinearity, ~30 times higher than the neutral exciton polariton. Transient absorption dynamics further reveal the ultrafast many-body interaction nature, with a timescale of <100 fs. The demonstration of biexciton polariton here combines high nonlinearity, simple processing and strong scalability, permitting access for future energy-efficient optical switching and information processing.

Tunable heterostructural prism for planar polaritonic switch

The study of phonon polaritons in van der Waals materials at the nanoscale has gained significant attention in recent years due to its potential applications in nanophotonics. The unique properties of these materials, such as their ability to support sub-diffraction imaging, sensing, and hyperlenses, have made them a promising avenue for the development of new techniques in the field. Despite these advancements, there still exists a challenge in achieving dynamically reversible manipulation of phonon polaritons in these materials due to their insulating properties. In this study, we present experimental results on the reversible manipulation of anisotropic phonon polaritons in α-MoO3 on top of a VO2 film, a phase-change material known for its dramatic changes in dielectric properties between its insulating and metallic states. Our findings demonstrate that the engineered VO2 film enables a switch in the propagation of polaritons in the mid-infrared region by modifying the dielectric properties of the film through temperature changes. Our results represent a promising approach to effectively control the flow of light energy at the nanoscale and offer the potential for the design and fabrication of integrated, flat sub-diffraction polaritonic devices. This study adds to the growing body of work in the field of nanophotonics and highlights the importance of considering phase-change materials for the development of new techniques in this field.

Ultrafast anisotropic dynamics of hyperbolic nanolight pulse propagation

Polariton pulses-transient light-matter hybrid excitations-traveling through anisotropic media can lead to unusual optical phenomena in space and time. However, studying these pulses presents challenges with their anisotropic, ultrafast, and nanoscale field variations. Here, we demonstrate the creation, observation, and control of polariton pulses, with in-plane hyperbolic dispersion, on anisotropic crystal surfaces by using a time-resolved nanoimaging technique and our developed high-dimensional data processing. We capture and analyze movies of distinctive pulse spatiotemporal dynamics, including curved ultraslow energy flow trajectories, anisotropic dissipation, and dynamical misalignment between phase and group velocities. Our approach enables analysis of polariton pulses in the wave vector time domain, demonstrating a time-domain polaritonic topological transition from lenticular to hyperbolic dispersion contours and the ability to study the polariton-induced time-varying optical forces. Our findings promise to facilitate the study of diverse space-time phenomena at extreme scales and drive advances in ultrafast nanoimaging.

Optical nanoimaging of highly-confined phonon polaritons in atomically-thin nanoribbons of α-MoO3

Phonon polaritons (PhPs), collective modes hybridizing photons with lattice vibrations in polar insulators, enable nanoscale control of light. In recent years, the exploration of in-plane anisotropic PhPs has yielded new levels of confinement and directional manipulation of nano-light. However, the investigation of in-plane anisotropic PhPs at the atomic layer limit is still elusive. Here, we report the optical nanoimaging of highly-confined phonon polaritons in atomically-thin nanoribbons of α-MoO3 (5 atomic layers). We show that narrow α-MoO3 nanoribbons as thin as a few atomic layers can support anisotropic PhPs modes with a high confinement ratio (∼133 times smaller wavelength than that of light). The anisotropic PhPs interference fringe patterns in atomic layers are tunable depending on the PhP wavelength via changing the illumination frequency. Moreover, spatial control over the PhPs interference patterns is also achieved by varying the nanostructures' shape or nanoribbon width of atomically-thin α-MoO3. Our work may serve as an empirical reference point for other anisotropic PhPs that approach the thickness limit and pave the way for applications such as atomically integrated nano-photonics and sensing.

Wafer-scale δ waveguides for integrated two-dimensional photonics

The efficient, large-scale generation and control of photonic modes guided by van der Waals materials remains as a challenge despite their potential for on-chip photonic circuitry. We report three-atom-thick waveguides-δ waveguides-based on wafer-scale molybdenum disulfide (MoS2) monolayers that can guide visible and near-infrared light over millimeter-scale distances with low loss and an efficient in-coupling. The extreme thinness provides a light-trapping mechanism analogous to a δ-potential well in quantum mechanics and enables the guided waves that are essentially a plane wave freely propagating along the in-plane, but confined along the out-of-plane, direction of the waveguide. We further demonstrate key functionalities essential for two-dimensional photonics, including refraction, focusing, grating, interconnection, and intensity modulation, by integrating thin-film optical components with δ waveguides using microfabricated dielectric, metal, or patterned MoS2.

Tunable optical topological transitions of plasmon polaritons in WTe2 van der Waals films

Naturally existing in-plane hyperbolic polaritons and the associated optical topological transitions, which avoid the nano-structuring to achieve hyperbolicity, can outperform their counterparts in artificial metasurfaces. Such plasmon polaritons are rare, but experimentally revealed recently in WTe2 van der Waals thin films. Different from phonon polaritons, hyperbolic plasmon polaritons originate from the interplay of free carrier Drude response and interband transitions, which promise good intrinsic tunability. However, tunable in-plane hyperbolic plasmon polariton and its optical topological transition of the isofrequency contours to the elliptic topology in a natural material have not been realized. Here we demonstrate the tuning of the optical topological transition through Mo doping and temperature. The optical topological transition energy is tuned over a wide range, with frequencies ranging from 429 cm-1 (23.3 microns) for pure WTe2 to 270 cm-1 (37.0 microns) at the 50% Mo-doping level at 10 K. Moreover, the temperature-induced blueshift of the optical topological transition energy is also revealed, enabling active and reversible tuning. Surprisingly, the localized surface plasmon resonance in skew ribbons shows unusual polarization dependence, accurately manifesting its topology, which renders a reliable means to track the topology with far-field techniques. Our results open an avenue for reconfigurable photonic devices capable of plasmon polariton steering, such as canaling, focusing, and routing, and pave the way for low-symmetry plasmonic nanophotonics based on anisotropic natural materials.

Flatband polaritonic router in twisted bilayer van der Waals materials

In recent years, van der Waals (vdW) polaritons excited by the hybrid of matter and photons have shown great promise for applications in nanoimaging, biosensing, and on-chip light guiding. In particular, polaritons with a flatband dispersion allow for mode canalization and diffractionless propagation, which showcase advantages for on-chip technologies requiring long-range transportation of optical information. Here, we propose a flatband polaritonic router based on twisted α-MoO₃ bilayers, which allows for on-chip routing of highly confined and low-loss phonon polaritons (PhPs) along multichannel propagating paths under different circular polarized dipole excitations. Our work combines flatband physics and optical spin- orbit coupling, with potential applications in nanoscale light propagation, on-chip optical switching, and communication.

Machine Learning for Optical Scanning Probe Nanoscopy

The ability to perform nanometer-scale optical imaging and spectroscopy is key to deciphering the low-energy effects in quantum materials, as well as vibrational fingerprints in planetary and extraterrestrial particles, catalytic substances, and aqueous biological samples. These tasks can be accomplished by the scattering-type scanning near-field optical microscopy (s-SNOM) technique that has recently spread to many research fields and enabled notable discoveries. Herein, it is shown that the s-SNOM, together with scanning probe research in general, can benefit in many ways from artificial-intelligence (AI) and machine-learning (ML) algorithms. Augmented with AI- and ML-enhanced data acquisition and analysis, scanning probe optical nanoscopy is poised to become more efficient, accurate, and intelligent.

Experimental probe of twist angle-dependent band structure of on-chip optical bilayer photonic crystal

Recently, twisted bilayer photonic materials have been extensively used for creating and studying photonic tunability through interlayer couplings. While twisted bilayer photonic materials have been experimentally demonstrated in microwave regimes, a robust platform for experimentally measuring optical frequencies has been elusive. Here, we demonstrate the first on-chip optical twisted bilayer photonic crystal with twist angle-tunable dispersion and great simulation-experiment agreement. Our results reveal a highly tunable band structure of twisted bilayer photonic crystals due to moiré scattering. This work opens the door to realizing unconventional twisted bilayer properties and novel applications in optical frequency regimes.

Hyperbolic polaritonic crystals with configurable low-symmetry Bloch modes

Photonic crystals (PhCs) are a kind of artificial structures that can mold the flow of light at will. Polaritonic crystals (PoCs) made from polaritonic media offer a promising route to controlling nano-light at the subwavelength scale. Conventional bulk PhCs and recent van der Waals PoCs mainly show highly symmetric excitation of Bloch modes that closely rely on lattice orders. Here, we experimentally demonstrate a type of hyperbolic PoCs with configurable and low-symmetry deep-subwavelength Bloch modes that are robust against lattice rearrangement in certain directions. This is achieved by periodically perforating a natural crystal α-MoO₃ that hosts in-plane hyperbolic phonon polaritons. The mode excitation and symmetry are controlled by the momentum matching between reciprocal lattice vectors and hyperbolic dispersions. We show that the Bloch modes and Bragg resonances of hyperbolic PoCs can be tuned through lattice scales and orientations while exhibiting robust properties immune to lattice rearrangement in the hyperbolic forbidden directions. Our findings provide insights into the physics of hyperbolic PoCs and expand the categories of PhCs, with potential applications in waveguiding, energy transfer, biosensing and quantum nano-optics.

Multiple and spectrally robust photonic magic angles in reconfigurable α-MoO3 trilayers

The emergence of a topological transition of the polaritonic dispersion in twisted bilayers of anisotropic van der Waals materials at a given twist angle-the photonic magic angle-results in the diffractionless propagation of polaritons with deep-subwavelength resolution. This type of propagation, generally referred to as canalization, holds promise for the control of light at the nanoscale. However, the existence of a single photonic magic angle hinders such control since the canalization direction in twisted bilayers is unique and fixed for each incident frequency. Here we overcome this limitation by demonstrating multiple spectrally robust photonic magic angles in reconfigurable twisted α-phase molybdenum trioxide (α-MoO₃) trilayers. We show that canalization of polaritons can be programmed at will along any desired in-plane direction in a single device with broad spectral ranges. These findings open the door for nanophotonics applications where on-demand control is crucial, such as thermal management, nanoimaging or entanglement of quantum emitters.

Enhanced efficiency of launching hyperbolic phonon polaritons in stacked α-MoO3 flakes

In this work, we reported a systemic study on the enhanced efficiency of launching hyperbolic phonon polaritons (PhPs) in stacked α-phase molybdenum trioxide (α-MoO₃) flakes. By using the infrared photo-induced force microscopy (PiFM), real-space near-field images (PiFM images) of mechanically exfoliated α-MoO₃ thin flakes were recorded within three different Reststrahlen bands (RBs). As referred with PiFM fringes of the single flake, PiFM fringes of the stacked α-MoO₃ sample within the RB 2 and RB 3 are greatly improved with the enhancement factor (EF) up to 170%. By performing numerical simulations, it reveals that the general improvement in near-field PiFM fringes arises from the existence of a nanoscale thin dielectric spacer in the middle part between two stacked α-MoO₃ flakes. The nanogap acts as a nanoresonator for prompting the near-field coupling of hyperbolic PhPs supported by each flake in the stacked sample, contributing to the increase of polaritonic fields, and verifying the experimental observations Our findings could offer fundamental physical investigations into the effective excitation of PhPs and will be helpful for developing functional nanophotonic devices and circuits.

Launching and Manipulation of Higher-Order In-Plane Hyperbolic Phonon Polaritons in Low-Dimensional Heterostructures

Hyperbolic phonon polaritons (HPhPs) are stimulated by coupling infrared (IR) photons with the polar lattice vibrations. Such HPhPs offer low-loss, highly confined light propagation at subwavelength scales with out-of-plane or in-plane hyperbolic wavefronts. For HPhPs, while a hyperbolic dispersion implies multiple propagating modes with a distribution of wavevectors at a given frequency, so far it has been challenging to experimentally launch and probe the higher-order modes that offer stronger wavelength compression, especially for in-plane HPhPs. In this work, the experimental observation of higher-order in-plane HPhP modes stimulated on a 3C-SiC nanowire (NW)/α-MoO₃ heterostructure is reported where leveraging both the low-dimensionality and low-loss nature of the polar NWs, higher-order HPhPs modes within 2D α-MoO₃ crystal are launched by the 1D 3C-SiC NW. The launching mechanism is further studied and the requirements for efficiently launching of such higher-order modes are determined. In addition, by altering the geometric orientation between the 3C-SiC NW and α-MoO₃ crystal, the manipulation of higher-order HPhP dispersions as a method of tuning is demonstrated. This work illustrates an extremely anisotropic low dimensional heterostructure platform to confine and configure electromagnetic waves at the deep-subwavelength scales for a range of IR applications including sensing, nano-imaging, and on-chip photonics.

Topological Metamaterials

The topological properties of an object, associated with an integer called the topological invariant, are global features that cannot change continuously but only through abrupt variations, hence granting them intrinsic robustness. Engineered metamaterials (MMs) can be tailored to support highly nontrivial topological properties of their band structure, relative to their electronic, electromagnetic, acoustic and mechanical response, representing one of the major breakthroughs in physics over the past decade. Here, we review the foundations and the latest advances of topological photonic and phononic MMs, whose nontrivial wave interactions have become of great interest to a broad range of science disciplines, such as classical and quantum chemistry. We first introduce the basic concepts, including the notion of topological charge and geometric phase. We then discuss the topology of natural electronic materials, before reviewing their photonic/phononic topological MM analogues, including 2D topological MMs with and without time-reversal symmetry, Floquet topological insulators, 3D, higher-order, non-Hermitian and nonlinear topological MMs. We also discuss the topological aspects of scattering anomalies, chemical reactions and polaritons. This work aims at connecting the recent advances of topological concepts throughout a broad range of scientific areas and it highlights opportunities offered by topological MMs for the chemistry community and beyond.

Observation of directional leaky polaritons at anisotropic crystal interfaces

Extreme anisotropy in some polaritonic materials enables light propagation with a hyperbolic dispersion, leading to enhanced light-matter interactions and directional transport. However, these features are typically associated with large momenta that make them sensitive to loss and poorly accessible from far-field, being bound to the material interface or volume-confined in thin films. Here, we demonstrate a new form of directional polaritons, leaky in nature and featuring lenticular dispersion contours that are neither elliptical nor hyperbolic. We show that these interface modes are strongly hybridized with propagating bulk states, sustaining directional, long-range, sub-diffractive propagation at the interface. We observe these features using polariton spectroscopy, far-field probing and near-field imaging, revealing their peculiar dispersion, and - despite their leaky nature - long modal lifetime. Our leaky polaritons (LPs) nontrivially merge sub-diffractive polaritonics with diffractive photonics onto a unified platform, unveiling opportunities that stem from the interplay of extreme anisotropic responses and radiation leakage.

Real-space observation of ultraconfined in-plane anisotropic acoustic terahertz plasmon polaritons

Thin layers of in-plane anisotropic materials can support ultraconfined polaritons, whose wavelengths depend on the propagation direction. Such polaritons hold potential for the exploration of fundamental material properties and the development of novel nanophotonic devices. However, the real-space observation of ultraconfined in-plane anisotropic plasmon polaritons (PPs)-which exist in much broader spectral ranges than phonon polaritons-has been elusive. Here we apply terahertz nanoscopy to image in-plane anisotropic low-energy PPs in monoclinic Ag₂Te platelets. The hybridization of the PPs with their mirror image-by placing the platelets above a Au layer-increases the direction-dependent relative polariton propagation length and the directional polariton confinement. This allows for verifying a linear dispersion and elliptical isofrequency contour in momentum space, revealing in-plane anisotropic acoustic terahertz PPs. Our work shows high-symmetry (elliptical) polaritons on low-symmetry (monoclinic) crystals and demonstrates the use of terahertz PPs for local measurements of anisotropic charge carrier masses and damping.

Mid-infrared analogue polaritonic reversed Cherenkov radiation in natural anisotropic crystals

Cherenkov radiation (CR) excited by fast charges can serve as on-chip light sources with a nanoscale footprint and broad frequency range. The reversed CR, which usually occurs in media with the negative refractive index or negative group-velocity dispersion, is highly desired because it can effectively separate the radiated light from fast charges thanks to the obtuse radiation angle. However, reversed CR at the mid-infrared remains challenging due to the significant loss of conventional artificial structures. Here we observe mid-infrared analogue polaritonic reversed CR in a natural van der Waals (vdW) material (i.e., α-MoO₃), whose hyperbolic phonon polaritons exhibit negative group velocity. Further, the real-space image results of analogue polaritonic reversed CR indicate that the radiation distributions and angles are closely related to the in-plane isofrequency contours of α-MoO₃, which can be further tuned in the heterostructures based on α-MoO₃. This work demonstrates that natural vdW heterostructures can be used as a promising platform of reversed CR to design on-chip mid-infrared nano-light sources.