Abstract Optical metamaterials have presented an innovative method of manipulating light. Hyperbolic metamaterials have an extremely high anisotropy with a hyperbolic dispersion relation. They are able to support high- k modes and exhibit a high density of states which produce distinctive properties that have been exploited in various applications, such as super-resolution imaging, negative refraction, and enhanced emission control. Here, state-of-the-art hyperbolic metamaterials are reviewed, starting from the fundamental principles to applications of artificially structured hyperbolic media to suggest ways to fuse natural two-dimensional hyperbolic materials. The review concludes by indicating the current challenges and our vision for future applications of hyperbolic metamaterials. 

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Hyperbolic metamaterials: fusing artificial structures to natural 2D materials

Polaritons are hybrid excitations of matter and photons. In recent years, polaritons in van der Waals nanomaterials—known as van der Waals polaritons—have shown great promise to guide the flow of light at the nanoscale over spectral regions ranging from the visible to the terahertz. A vibrant research field based on manipulating strong light–matter interactions in the form of polaritons, supported by these atomically thin van der Waals nanomaterials, is emerging for advanced nanophotonic and opto-electronic applications. Here we provide an overview of the state of the art of exploiting interface optics—such as refractive optics, meta-optics and moire engineering—for the control of van der Waals polaritons. This enhanced control over van der Waals polaritons at the nanoscale has not only unveiled many new phenomena, but has also inspired valuable applications—including new avenues for nano-imaging, sensing, on-chip optical circuitry, and potentially many others in the years to come. This Review discusses the state of the art of interface optics—including refractive optics, meta-optics and moire engineering—for the control of van der Waals polaritons.

Interface nano-optics with van der Waals polaritons.

Wireless power transfer—the transmission of electromagnetic energy without physical connectors such as wires or waveguides—typically exploits electromagnetic field control methods that were first proposed decades ago and requires some essential parameters (such as efficiency) to be sacrificed in favour of others (such as stability). In recent years, novel approaches to electromagnetic field manipulation have been developed that can be used to create advanced forms of wireless power transfer. Here we review the development of novel physical effects and materials for wireless power transfer. We explore techniques based on coherent perfect absorption, parity–time symmetry and exceptional points, and on-site power generation. We also explore the use of metamaterials and metasurfaces in wireless power transfer, and the use of acoustic power transfer. Finally, we highlight potential routes for the further development of wireless power transfer technology. This Review examines the development of novel physical effects and materials for wireless power transfer, considering techniques based on coherent perfect absorption, parity–time symmetry and exceptional points, and on-site power generation, as well as the use of metamaterials and metasurfaces, and acoustic power transfer.

Wireless power transfer based on novel physical concepts

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials, such as MoS2, WS2, MoSe2, and WSe2, have received extensive attention in the past decade due to their extraordinary electronic, optical and thermal properties. They evolve from indirect bandgap semiconductors to direct bandgap semiconductors while their layer number is reduced from a few layers to a monolayer limit. Consequently, there is strong photoluminescence in a monolayer (1L) TMDC due to the large quantum yield. Moreover, such monolayer semiconductors have two other exciting properties: large binding energy of excitons and valley polarization. These properties make them become ideal materials for various electronic, photonic and optoelectronic devices. However, their performance is limited by the relatively weak light-matter interactions due to their atomically thin form factor. Resonant nanophotonic structures provide a viable way to address this issue and enhance light-matter interactions in 2D TMDCs. Here, we provide an overview of this research area, showcasing relevant applications, including exotic light emission, absorption and scattering features. We start by overviewing the concept of excitons in 1L-TMDC and the fundamental theory of cavity-enhanced emission, followed by a discussion on the recent progress of enhanced light emission, strong coupling and valleytronics. The atomically thin nature of 1L-TMDC enables a broad range of ways to tune its electric and optical properties. Thus, we continue by reviewing advances in TMDC-based tunable photonic devices. Next, we survey the recent progress in enhanced light absorption over narrow and broad bandwidths using 1L or few-layer TMDCs, and their applications for photovoltaics and photodetectors. We also review recent efforts of engineering light scattering, e.g., inducing Fano resonances, wavefront engineering in 1L or few-layer TMDCs by either integrating resonant structures, such as plasmonic/Mie resonant metasurfaces, or directly patterning monolayer/few layers TMDCs. We then overview the intriguing physical properties of different van der Waals heterostructures, and their applications in optoelectronic and photonic devices. Finally, we draw our opinion on potential opportunities and challenges in this rapidly developing field of research. 

Enhanced light–matter interaction in two-dimensional transition metal dichalcogenides

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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Tailoring Topological Transitions of Anisotropic Polaritons by Interface Engineering in Biaxial Crystals.

Polaritons in anisotropic materials result in exotic optical features, which can provide opportunities to control light at the nanoscale1-10. So far these polaritons have been limited to two classes: bulk polaritons, which propagate inside a material, and surface polaritons, which decay exponentially away from an interface. Here we report a near-field observation of ghost phonon polaritons, which propagate with in-plane hyperbolic dispersion on the surface of a polar uniaxial crystal and, at the same time, exhibit oblique wavefronts in the bulk. Ghost polaritons are an atypical non-uniform surface wave solution of Maxwell's equations, arising at the surface of uniaxial materials in which the optic axis is slanted with respect to the interface. They exhibit an unusual bi-state nature, being both propagating (phase-progressing) and evanescent (decaying) within the crystal bulk, in contrast to conventional surface waves that are purely evanescent away from the interface. Our real-space near-field imaging experiments reveal long-distance (over 20 micrometres), ray-like propagation of deeply subwavelength ghost polaritons across the surface, verifying long-range, directional and diffraction-less polariton propagation. At the same time, we show that control of the out-of-plane angle of the optic axis enables hyperbolic-to-elliptic topological transitions at fixed frequency, providing a route to tailor the band diagram topology of surface polariton waves. Our results demonstrate a polaritonic wave phenomenon with unique opportunities to tailor nanoscale light in natural anisotropic crystals.

Weiliang Ma

Papers

Interface nano-optics with van der Waals polaritons.

Ghost hyperbolic surface polaritons in bulk anisotropic crystals.