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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.
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Polaritons in polar biaxial crystals with extreme anisotropy offer a promising route to manipulate nanoscale light-matter interactions. The dynamic modulation of their dispersion is of great significance for future integrated nano-optics but remains challenging. Here, we report tunable topological transitions in biaxial crystals enabled by interface engineering. We theoretically demonstrate such tailored polaritons at the interface of heterostructures between graphene and α-phase molybdenum trioxide (α-MoO3). The interlayer coupling can be modulated by both the stack of graphene and α-MoO3 and the magnitude of the Fermi level in graphene enabling a dynamic topological transition. More interestingly, we found that the wavefront transition occurs at a constant Fermi level when the thickness of α-MoO3 is tuned. Furthermore, we also experimentally verify the hybrid polaritons in the graphene/α-MoO3 heterostructure with different thicknesses of α-MoO3. The interface engineering offers new insights into optical topological transitions, which may shed new light on programmable polaritonics, energy transfer, and neuromorphic photonics.
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Lead iodide (PbI2) is a van der Waals layered semiconductor with a direct bandgap in its bulk form and a hexagonal layered crystalline structure. The recently developed PbI2 nanosheets have shown great promise for high-performance optoelectronic devices, including nanolasers and photodetectors. However, despite being widely used as a precursor for perovskite materials, the optical properties of PbI2 nanomaterials remain largely unexplored. Here, we determine the nonlinear optical properties of PbI2 nanosheets by utilising nonlinear microscopy as a non-invasive optical technique. We demonstrate the nonlinearity enhancement dependent on excitonic resonances, crystalline orientation, thickness, and influence of the substrate. Our results allow for estimating the second- and third-order nonlinear susceptibilities of the nanosheets, opening new opportunities for the use of PbI2 nanosheets as nonlinear and quantum light sources.
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Recently, two rich and exciting research fields, layered two-dimensional (2D) materials and metamaterials, have started overlapping. Metamaterials are artificial, engineered materials with broad metaphotonic prospects such as negative refraction, perfect lensing, subwavelength imaging, and cloaking. The possibility of achieving metaphotonic properties using metamaterials based on layered 2D materials has been extensively exploited. Because they are highly tunable and adjustable with the ease of micro- and nanofabrication, 2D materials exhibit diverse optical properties such as natural negative refraction, natural anisotropic behavior, and even hyperbolic dispersion. A combination of 2D materials with conventional metamaterials promises a variety of prospective applications. In this review, we illustrate how the concept of metamaterials and their associated metaphotonic capabilities are naturally born in 2D materials. The multifunctionality of 2D materials may enable the manufacture of novel optical devices that work in a broad frequency range, from visible to terahertz, with particularly low loss, high speed, gated tunability, and miniaturized sizes. This new area of research links the fields of photonics, optoelectronics, and plasmonics with that of metamaterials and may provide insights to future innovations for 2D-material-inspired metaphotonic devices.
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Surface phonon polaritons (SPhPs) in polar dielectrics offer new opportunities for infrared nanophotonics. However, bulk SPhPs inherently propagate isotropically with limited photon confinement, and how to collectively realize ultralarge confinement, in-plane hyperbolicity, and unidirectional propagation remains elusive. Here, we report an approach to solve the aforementioned issues of bulk SPhPs in one go by constructing a heterostructural interface between biaxial van der Waals material (e.g., α-MoO3) and bulk polar dielectric (e.g., SiC, AlN, and GaN). Because of anisotropy-oriented mode couplings, the hybridized SPhPs with a large confinement factor (>100) show in-plane hyperbolicity that has been switched to the orthogonal direction as compared to that in natural α-MoO3. More interestingly, this proof of concept allows steerable and unidirectional polariton excitation by suspending α-MoO3 on patterned SiC air cavities. Our finding exemplifies a generalizable framework to manipulate the flow of nanolight in many other hybrid systems consisting of anisotropic materials and polar dielectrics.
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Owing to its good air stability and high refractive index, two-dimensional (2D) noble metal dichalcogenide shows intriguing potential for versatile flat optics applications. However, light field manipulation at the atomic scale is conventionally considered unattainable because the small thickness and intrinsic losses of 2D materials completely suppress both resonances and phase accumulation effects. Here, we demonstrate that losses of structured atomically thick PtSe2 films integrated on top of a uniform substrate can be utilized to create the spots of critical coupling, enabling singular phase behaviors with a remarkable π phase jump. This finding enables the experimental demonstration of atomically thick binary meta-optics that allows an angle-robust and high unit thickness diffraction efficiency of 0.96%/nm in visible frequencies (given its thickness of merely 4.3 nm). Our results unlock the potential of a new class of 2D flat optics for light field manipulation at an atomic thickness.
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In the growing list of 2D semiconductors as potential successors to silicon in future devices, metal-halide perovskites have recently joined the family. Unlike other conversional 2D covalent semiconductors such as graphene, transition metal dichalcogenides, black phosphorus, etc., 2D perovskites are ionic materials, affording many distinct properties of their own, including high photoluminescence quantum efficiency, balanced large exciton binding energy and oscillator strength, and long carrier diffusion length. These unique properties make 2D perovskites potential candidates for optoelectronic and photonic devices such as solar cells, light-emitting diodes, photodetectors, nanolasers, waveguides, modulators, and so on, which represent a relatively new but exciting and rapidly expanding area of research. In this Review, the recent advances in emerging 2D metal-halide perovskites and their applications in the fields of optoelectronics and photonics are summarized and insights into the future direction of these fields are offered.
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A new approach for efficiently recovering the wasted light energy in conventional flexible organic light-emitting diodes (FOLEDs) is developed by implementing disordered micro-meander structures (DMMs) via laser speckle holography technology. Compared to conventional flat device architecture, the structured FOLEDs with DMMs result in substantial improvement of the device efficiency and superior angular color stability. The resulting current efficiency (CE) and external quantum efficiency (EQE) are 1.31 and 1.39 times that of a common flat structure, respectively. Moreover, the proposed DMMs micro-structure simultaneously offers the unique characteristics of angular color stability with a wide viewing angle, which is usually considered as the criteria of the high-quality lighting applications. We hope that the demonstrated method could provide an alternative way for the development of high efficiency flexible OLEDs.
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Two novel D-A bipolar blue phosphorescent host materials based on phenothiazine-5,5-dioxide: 3-(9H-carbazol-9-yl)-10-ethyl-10H-phenothiazine-5,5-dioxide (CEPDO) and 10-butyl-3-(9H-carbazol-9-yl)-10H-phenothiazine-5,5-dioxide (CBPDO) were synthesized and characterized. The photophysical, electrochemical and thermal properties were systematically investigated. CEPDO and CBPDO not only have a high triplet energy but also show a bipolar behavior. Moreover, their fluorescence emission peaks are in the blue fluorescence region at 408 nm and the fluorescence quantum efficiency (Φ) of CEPDO and CBPDO were 62.5% and 59.7%, respectively. Both CEPDO and CBPDO showed very high thermal stability with decomposition temperatures (Td) of 409 and 396 °C as well as suitable HOMO and LUMO energy levels. This preferable performance suggests that CEPDO and CBPDO are alternative bipolar host materials for the PhOLEDs.
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It is challenging in realizing high-performance transparent organic light-emitting diodes (OLEDs) with symmetrical light emission to both sides. Herein, an efficient transparent OLED with highly balanced white emission to both sides is demonstrated by integrating quasi-periodic nanostructures into the organic emitter and the metal-dielectric composite top electrode, which can simultaneously suppressing waveguide and surface plasmonic loss. The power efficiency and external quantum efficiency are raised to 83.5 lm W-1 and 38.8%, respectively, along with a bi-directional luminance ratio of 1.26. The proposed scheme provides a facile route for extending application scope of transparent OLEDs for future transparent displays and lightings.
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Organo-lead halide perovskites (OHPs) have recently emerged as a new class of exceptional optoelectronic materials, which may find use in many applications, including solar cells, light emitting diodes, and photodetectors. More complex applications, such as lasers and electro-optic modulators, require the use of monocrystalline perovskite materials to reach their ultimate performance levels. Conventional methods for forming single crystals of OHPs like methylammonium lead bromide (MAPbBr3 ) afford limited control over the product morphology, rendering the assembly of defined microcavity nanostructures difficult. We overcame this by synthesizing for the first time (MA)[PbBr3 ]â DMF (1), and demonstrating its facile transformation into monocrystalline MAPbBr3 microplatelets. The MAPbBr3 microplatelets were tailored into waveguide based photonic devices, of which an ultra-low propagation loss of 0.04â dB µm-1 for a propagation distance of 100â µm was demonstrated. An efficient active electro-optical modulator (AEOM) consisting of a MAPbBr3 non-linear arc waveguide was demonstrated, exhibiting a 98.4 % PL intensity modulation with an external voltage of 45â V. This novel synthetic approach, as well as the demonstration of effective waveguiding, will pave the way for developing a wide range of photonic devices based on organo-lead halide perovskites.
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Flexible organic light-emitting diodes (OLEDs) are emerging as a leading technology for rollable and foldable display applications. For the development of high-performance flexible OLEDs on plastic substrate, we report a transparent nanocomposite electrode with superior mechanical, electrical, and optical properties, which is realized by integrating the nanoimprinted quasi-random photonic structures into the ultrathin metal/dielectric stack to collectively optimize the electrical conduction and light outcoupling capabilities. The resulting flexible OLEDs with green emission yield the enhanced device efficiency, reaching the maximum external quantum efficiency of 43.7% and luminous efficiency of 154.9 cd/A, respectively.
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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.
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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.
Assuntos
Fônons , Fótons , Transferência de Energia , Movimento (Física) , FísicaRESUMO
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.
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Negative reflection occurs when light is reflected toward the same side of the normal to the boundary from which it is incident. This exotic optical phenomenon is not only yet to be visualized in real space but also remains unexplored, both at the nanoscale and in natural media. Here, we directly visualize nanoscale-confined polaritons negatively reflecting on subwavelength mirrors fabricated in a low-loss van der Waals crystal. Our near-field nanoimaging results unveil an unconventional and broad tunability of both the polaritonic wavelength and direction of propagation upon negative reflection. On the basis of these findings, we introduce a device in nano-optics: a hyperbolic nanoresonator, in which hyperbolic polaritons with different momenta reflect back to a common point source, enhancing the intensity. These results pave way to realize nanophotonics in low-loss natural media, providing an efficient route to control nanolight, a key for future on-chip optical nanotechnologies.
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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.
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The diffusion length of photogenerated carriers is a crucial parameter in semiconductors for optoelectronic applications. However, it is a challenging task to determine the diffusion length in layered nanoplatelets due to their anisotropic diffusion of photogenerated carriers and nanometer-thin thickness. Here, we demonstrate a novel method to determine the in-plane diffusion length of photogenerated carriers in layered nanoplatelets using local time-resolved photoluminescence. Also, the in-plane carrier diffusion length of 1.82 µm is obtained for an exfoliated (BA)2PbI4 (BA = CH3(CH2)3NH3) perovskite nanoplatelet. This method is particularly useful for weak luminescent materials and the materials that are easily damaged by long-term laser beam because of the high detection sensitivity. This technique is extendable to other layered materials and therefore plays a valuable role in the development and optimization of two-dimensional (2D) and three-dimensional (3D) semiconductor materials and devices for photovoltaic and photonic applications.
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Ultrathin flat lenses based on metasurfaces or metamaterials have shown great promise in recent years as essential components in nano-optical system, with capability of abrupt changes of light wavefronts. However, such structural designs require complex nanopatterns and a time-consuming nanofabrication process. In this regard, flat lenses are developed based on 2D perovskite nanosheets, using a cost-effective mask-free femtosecond direct laser writing system. The optical properties of the 2D perovskite are rationally adjusted through facile composition engineering as well as thickness-dependent quantum-size confinement. A diffraction theory model is derived to understand the focusing mechanism of the 2D perovskite nanosheets flat lenses. The as-fabricated lenses exploit the tunable material property variations to effectively manipulate not only the amplitude but also the phase of the incident light to focus into a 3D focal spot with a sub-wavelength resolution in the range of 0.5-0.9λ. The results pave the way toward low-cost and large-scale high-resolution imaging applications in the future.
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Highly confined and low-loss polaritons are known to propagate isotropically over graphene and hexagonal boron nitride in the plane, leaving limited degrees of freedom in manipulating light at the nanoscale. The emerging family of biaxial van der Waals materials, such as α-MoO3 and V2O5, support exotic polariton propagation, as their auxiliary optical axis is in the plane. Here, exploiting this strong in-plane anisotropy, we report edge-tailored hyperbolic polaritons in patterned α-MoO3 nanocavities via real-space nanoimaging. We find that the angle between the edge orientation and the crystallographic direction significantly affects the optical response, and can serve as a key tuning parameter in tailoring the polaritonic patterns. By shaping α-MoO3 nanocavities with different geometries, we observe edge-oriented and steerable hyperbolic polaritons as well as forbidden zones where the polaritons detour. The lifetime and figure of merit of the hyperbolic polaritons can be regulated by the edge aspect ratio of nanocavity.