Topological polaritons and photonic magic angles in twisted α-MoO3 bilayers.

Twisted two-dimensional bilayer materials exhibit many exotic electronic phenomena. Manipulating the 'twist angle' between the two layers enables fine control of the electronic band structure, resulting in magic-angle flat-band superconductivity1,2, the formation of moiré excitons3-8 and interlayer magnetism9. However, there are limited demonstrations of such concepts for photons. Here we show how analogous principles, combined with extreme anisotropy, enable control and manipulation of the photonic dispersion of phonon polaritons in van der Waals bilayers. We experimentally observe tunable topological transitions from open (hyperbolic) to closed (elliptical) dispersion contours in bilayers of α-phase molybdenum trioxide (α-MoO3), arising when the rotation between the layers is at a photonic magic twist angle. These transitions are induced by polariton hybridization and are controlled by a topological quantity. At the transitions the bilayer dispersion flattens, exhibiting low-loss tunable polariton canalization and diffractionless propagation with a resolution of less than λ0/40, where λ0 is the free-space wavelength. Our findings extend twistronics10 and moiré physics to nanophotonics and polaritonics, with potential applications in nanoimaging, nanoscale light propagation, energy transfer and quantum physics.

Spin-orbital angular momentum degeneracy breaking in nanoplasmonic metachain.

The spin and orbital angular momentum (namely SAM and OAM) mode division provides a promising solution to surmount exhausted available degrees of freedom in conventional optical communications. Nevertheless, SAM and OAM are often subjected to the degeneracy of total angular momentum (AM) because they both have integer variables of quantum eigenstates, which inevitably brings about the shortcomings specific to limited signal channels and multiplexing cross talk. Herein, we present a nanoplasmonic metachain that can discriminatively couple any input SAM and OAM components to an extrinsic orbital AM, corresponding to the chirality and topological charge of incident light. Importantly, the unambiguous measurement has a prominent advantage of detecting the arbitrary AM component rather than the total AM. The miniature metadevice offers the possibility of harnessing AM division on chip or in fiber and holds great promise to delve the spin-orbit interactions for topological photonics and quantum cryptography.

Interfacial engineering of halide perovskites and two-dimensional materials.

Recently, halide perovskites (HPs) and layered two-dimensional (2D) materials have received significant attention from industry and academia alike. HPs are emerging materials that have exciting photoelectric properties, such as a high absorption coefficient, rapid carrier mobility and high photoluminescence quantum yields, making them excellent candidates for various optoelectronic applications. 2D materials possess confined carrier mobility in 2D planes and are widely employed in nanostructures to achieve interfacial modification. HP/2D material interfaces could potentially reveal unprecedented interfacial properties, including light absorbance with desired spectral overlap, tunable carrier dynamics and modified stability, which may lead to several practical applications. In this review, we attempt to provide a comprehensive perspective on the development of interfacial engineering of HP/2D material interfaces. Specifically, we highlight the recent progress in HP/2D material interfaces considering their architectures, electronic energetics tuning and interfacial properties, discuss the potential applications of these interfaces and analyze the challenges and future research directions of interfacial engineering of HP/2D material interfaces. This review links the fields of HPs and 2D materials through interfacial engineering to provide insights into future innovations and their great potential applications in optoelectronic devices.

A Magnetic Field SPR Sensor Based on Temperature Self-Reference.

In this paper, a novel D-shaped photonic crystal fiber sensor for simultaneous measurements of magnetic field and temperature is proposed and characterized. Based on the surface plasmon resonance theory, the D-shaped flat surface coated with a gold layer is in direct contact with magnetic fluid to detect magnetic field, and one of the relatively small air holes near the fiber core is filled with polydimethylsiloxane (PDMS) to sense temperature. The realization of measuring the magnetic field and temperature separately through two channels depends on the fact that the magnetic field only changes the refractive index of the magnetic fluid, but has no effect on the refractive index of PDMS. The refractive index of the magnetic fluid and PDMS can be affected by temperature at the same time. The sensor designed in this work can separate the variations of the magnetic field and temperature simultaneously, therefore solving the cross-sensitivity problem to further improve the magnetic field sensitivity. When the thickness of the gold film is 50 nm and the radius of the filling hole is 0.52 µm, the magnetic field sensitivity and the temperature sensitivity of magnetic field sensor based on temperature self-reference can reach 0.14274 nm/Oe and -0.229 nm/°C, respectively.

Broadband High-Efficiency Chiral Splitters and Holograms from Dielectric Nanoarc Metasurfaces.

Simultaneous broadband and high efficiency merits of designer metasurfaces are currently attracting widespread attention in the field of nanophotonics. However, contemporary metasurfaces rarely achieve both advantages simultaneously. For the category of transmissive metadevices, plasmonic or conventional dielectric metasurfaces are viable for either broadband operation with relatively low efficiency or high efficiency at only a selection of wavelengths. To overcome this limitation, dielectric nanoarcs are proposed as a means to accomplish two advantages. Continuous nanoarcs support different electromagnetic resonant modes at localized areas for generating phase retardation. Meanwhile, the geometric nature of nanoarc curvature endows the nanoarcs with full phase coverage of 0-2π due to the Pancharatnam-Berry phase principle. Experimentally incorporated with the chiral-detour phase principle, a few compelling functionalities are demonstrated, such as chiral beamsplitting, broadband holography, and helicity-selective holography. The continuous nanoarc metasurfaces prevail over plasmonic or dielectric discretized building block strategies and the findings lead to novel designs of spin-controllable metadevices.

Resonance-free ultraviolet metaoptics via photon nanosieves.

Ultraviolet (UV) light with high-energy photons is widely used in various areas such as nano-lithography, biology, and photoemission spectroscopy. The flexible control over its amplitude and phase is a longstanding problem due to the strong absorption from most materials. Here, we propose a nano-aperture platform to control the amplitude and phase of UV light and experimentally demonstrate amplitude- and phase-type holograms at a wavelength of 355 nm. In principle, nano-apertures etched on a metal film can be filled in vacuum, so that the material issue about optical absorption is not involved in this configuration, allowing us to manipulate UV light through the geometry of nano-apertures even when plasmonic resonances are absent. A binary-amplitude nanosieve is used to reconstruct three holographic images at different cut-planes by tuning the constructive interference elaborately. Meanwhile, rectangular nano-apertures are employed to demonstrate UV holograms with geometric phase that is controlled by the orientation of the nano-apertures. This platform could be extended to other UV regions.

Metamaterial Lensing Devices.

In recent years, the development of metamaterials and metasurfaces has drawn great attention, enabling many important practical applications. Focusing and lensing components are of extreme importance because of their significant potential practical applications in biological imaging, display, and nanolithography fabrication. Metafocusing devices using ultrathin structures (also known as metasurfaces) with superlensing performance are key building blocks for developing integrated optical components with ultrasmall dimensions. In this article, we review the metamaterial superlensing devices working in transmission mode from the perfect lens to two-dimensional metasurfaces and present their working principles. Then we summarize important practical applications of metasurfaces, such as plasmonic lithography, holography, and imaging. Different typical designs and their focusing performance are also discussed in detail.

Metadevices with Potential Practical Applications.

Metamaterials are "new materials" with different superior physical properties, which have generated great interest and become popular in scientific research. Various designs and functional devices using metamaterials have formed a new academic world. The application concept of metamaterial is based on designing diverse physical structures that can break through the limitations of traditional optical materials and composites to achieve extraordinary material functions. Therefore, metadevices have been widely studied by the academic community recently. Using the properties of metamaterials, many functional metadevices have been well investigated and further optimized. In this article, different metamaterial structures with varying functions are reviewed, and their working mechanisms and applications are summarized, which are near-field energy transfer devices, metamaterial mirrors, metamaterial biosensors, and quantum-cascade detectors. The development of metamaterials indicates that new materials will become an important breakthrough point and building blocks for new research domains, and therefore they will trigger more practical and wide applications in the future.

Maskless fabrication of slanted annular aperture arrays.

We report maskless fabrication of high-aspect-ratio slanted annular aperture arrays (SAAAs) in gold films using focused ion beam lithography. By tilting the substrate, SAAAs with the desired tilting angle can be fabricated. Our experimental results demonstrate accurate control over aperture size, obliqueness, and reproducibility. We also show that the resulted plasmonic resonances of SAAAs can be effectively tuned via obliqueness control. This versatile approach may enable fabrication of more complicated plasmonic nanostructures. The demonstrated gold SAAAs could also find many potential applications in plasmon-assisted sensing and surface enhanced spectroscopy.

Optically isotropic, electrically tunable liquid crystal droplet arrays formed by photopolymerization-induced phase separation.

Phase separation has been an interesting and important topic in liquid crystal (LC)-polymer composites. We investigated the photopolymerization-induced phase separation in an LC-polymer composite through a maskless lithographic system based on an amplitude-modulated spatial light modulator. By optimizing both exposure conditions and materials, we achieved a two-dimensional (2D) liquid crystal droplet array (LCDA) in the LC-polymer composite. Further investigations revealed that such 2D LCDAs, working as microlens arrays, demonstrated polarization-independent, electrically tunable focusing properties under a certain voltage. With advantages in cost-effectiveness, fast fabrication, and polarization-independent, electrically tunable focusing, such phase-separated microlens arrays in the LC-polymer composite could find many potential optical applications.

All-optical, polarization-insensitive light tuning properties in silver nanorod arrays covered with photoresponsive liquid crystals.

Active plasmonics has been an interesting and important topic recently. Here we demonstrate the all-optical, polarization-insensitive tunable manipulation of a hybrid system that integrates a silver nanorod array with photoresponsive liquid crystals. The large-area plasmonic nanorod arrays are fabricated by laser interference lithography and ion milling. By covering a layer of photoresponsive liquid crystals, tunable control of plasmon resonance is achieved under an external light pump. The silver nanorod array also enables the homeotropic alignment of the liquid crystals, which makes the all-optical tuning behavior polarization-insensitive. With its advantages of cost-effective fabrication, easy integration, all-optical control, and polarization-insensitivity, the hybrid system could be valuable in many nanophotonic applications.

Incident-angle dependent color tuning from a single plasmonic chip.

We report on a broad color tuning effect covering the visible range from a single plasmonic chip. By simply tilting the orientation of the designed plasmonic chip within a certain range, the photon-plasmon coupling interactions between the incident light and the plasmonic nanostructures on the chip can be finely tuned, resulting in an angle-dependent continuous color filtering effect. The physical mechanism of the device is investigated through the full-wave calculations, which provide important guidance for the design and optimization of the proposed devices. The broad color tuning from the demonstrated single chip will potentially benefit visualization and display technologies, and is particularly useful for the construction of reflection-based spatial light modulators.

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.

From Volumetric to Planar Multiplexing: Phase-Coded Metasurfaces without the Bragg Effect.

Metasurfaces consisting of planar subwavelength structures with minimal thickness are appealing to emerging technologies such as integrated optics and photonic chips for their small footprint and compatibility with sophisticated planar nanofabrication techniques. However, reduced dimensionality due to the 2D nature of a metasurface poses challenges to the adaptation of a few useful methods that have found great success with conventional optics in 3D space. For instance, Bragg diffraction is the foundation of the well-established technique of phase-coded multiplexing in volume holography. It relies on interference among the scattered waves from multiple layers across the thickness of a sample. In this work, despite losing the dimension in thickness, a metasurface is devised to experimentally demonstrate phase-coded multiplexing by replacing free-space light with a surface wave in its output. The in-plane interference along the propagation of the surface wave resembles the Bragg diffraction, thus enabling phase-coded multiplexing in the 2D design. An example of code-based all-optical routing is also achieved by using a multiplexed metasurface, which can find applications in photonic data processing and communications.

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.

Plasmon-Enhanced Fluorescence Emission of an Electric Dipole Modulated by a Nanoscale Silver Hemisphere.

The spontaneous emission of a fluorophore is altered by the surrounding electromagnetic field. Therefore, the radiation of the fluorophore can be engineered by inter-coupling with the nanoscale plasmons. This work proposes a nanoscale hemisphere structure that enhances the electric field and further modulates its effects on fluorophores by adjusting the radius of the hemisphere. A full-wave simulation is carried out using the finite element method, and the radiation characteristics of the nanoscale hemisphere are studied in detail. Compared with free space, the structure has generated significant enhancement exceeding 30. Through curve fitting, the relationship between the enhanced peak wavelength and the radius of the hemisphere is obtained.

A High Refractive Index Plasmonic Micro-Channel Sensor Based on Photonic Crystal Fiber.

A new concave shaped high refractive index plasmonic sensor with a micro-channel is proposed in this work, which comprises an analyte channel in the core hole. The sensor is elaborately designed to reduce the interference effect from the metal coating. Furthermore, the impact of the proposed structure on the sensitivity is also investigated by engineering the geometric parameters using the finite element method. We select gold as the plasmonic material in this theoretical study because it is widely used to fabricate plasmonic and metamaterial devices due to its chemical stability and compatibility. According to wavelength interrogation technique, simulations results show that this sensor can obtain maximal wavelength sensitivity of 10,050 nm/refractive index unit. In view of the excellent indicators of this device, it has important development potential in chemical and biological research fields.

Unidirectionally excited phonon polaritons in high-symmetry orthorhombic crystals.

Advanced control over the excitation of ultraconfined polaritons-hybrid light and matter waves-empowers unique opportunities for many nanophotonic functionalities, e.g., on-chip circuits, quantum information processing, and controlling thermal radiation. Recent work has shown that highly asymmetric polaritons are directly governed by asymmetries in crystal structures. Here, we experimentally demonstrate extremely asymmetric and unidirectional phonon polariton (PhP) excitation via directly patterning high-symmetry orthorhombic van der Waals (vdW) crystal α-MoO3. This phenomenon results from symmetry breaking of momentum matching in polaritonic diffraction in vdW materials. We show that the propagation of PhPs can be versatile and robustly tailored via structural engineering, while PhPs in low-symmetry (e.g., monoclinic and triclinic) crystals are largely restricted by their naturally occurring permittivities. Our work synergizes grating diffraction phenomena with the extreme anisotropy of high-symmetry vdW materials, enabling unexpected control of infrared polaritons along different pathways and opening opportunities for applications ranging from on-chip photonics to directional heat dissipation.

Super-Resolution Imaging with Graphene.

Super-resolution optical imaging is a consistent research hotspot for promoting studies in nanotechnology and biotechnology due to its capability of overcoming the diffraction limit, which is an intrinsic obstacle in pursuing higher resolution for conventional microscopy techniques. In the past few decades, a great number of techniques in this research domain have been theoretically proposed and experimentally demonstrated. Graphene, a special two-dimensional material, has become the most meritorious candidate and attracted incredible attention in high-resolution imaging domain due to its distinctive properties. In this article, the working principle of graphene-assisted imaging devices is summarized, and recent advances of super-resolution optical imaging based on graphene are reviewed for both near-field and far-field applications.

Efficient and Tunable Reflection of Phonon Polaritons at Built-In Intercalation Interfaces.

Phonon polaritons-light coupled to lattice vibrations-in polar van der Waals crystals offer unprecedented opportunities for controlling light at the nanoscale due to their anisotropic and ultralow-loss propagation. While their analog plasmon polaritons-light coupled to electron oscillations-have long been studied and exhibit interesting reflections at geometrical edges and electronic boundaries, whether phonon polaritons can be reflected by such barriers has been elusive. Here, the effective and tunable reflection of phonon polaritons at embedded interfaces formed in hydrogen-intercalated α-MoO3 flakes is elaborated upon. Without breaking geometrical continuity, such intercalation interfaces can reflect phonon polaritons with low losses, yielding the distinct phase changes of -0.8π and -0.3π associated with polariton propagation, high efficiency of 50%, and potential electrical tunability. The results point to a new approach to construct on-demand polariton reflectors, phase modulators, and retarders, which may be transplanted into building future polaritonic circuits using van der Waals crystals.