Tailoring Interfaces of All-Carbon Electromagnetic Interference Shielding Materials for Boosting Comprehensive Performance.

Electromagnetic interference (EMI) shielding materials with lightweight, high shielding effectiveness, excellent chemical stability, especially minimized secondary electromagnetic pollution, are urgently desired for integrated electronic systems operating in harsh working environments. Here in this study, by systematically engineering and matching the interfacial properties of carbon-based membrane materials, i.e., graphite paper, whisker carbon nanotube paper (WCNT paper), carbon nanotube film (CNT film), bucky paper (BP), and carbon cloth (CC) with three-dimensional (3D) porous carbon nanotube sponge (CNTS), we successfully constructed a series of multifunctional all-carbon EMI shielding materials, which exhibit excellent average shielding effectiveness of over 90 dB with a thickness of about 1 mm and dramatically minimized secondary electromagnetic reflection. Moreover, benefiting from the all-carbon nature and engineered interfaces, our CMC materials also exhibit excellent photothermal and Joule heating performances. These results not only provide guidance for designing advanced multifunctional all-carbon EMI shielding materials but also shed light on the hidden mechanism between interfaces and performances of composite materials.

Optical perfectly matched layers based on the integration of photonic crystals and material loss.

Perfectly matched layer (PML) is a virtual absorption boundary condition adopted in numerical simulations, capable of absorbing light from all incident angles, which however is still lacking in practice in the optical regime. In this work, by integrating dielectric photonic crystals and material loss, we demonstrate an optical PML design with near-omnidirectional impedance matching and customized bandwidth. The absorption efficiency exceeds 90% for incident angle up to 80°. Good consistence is found between our simulations and proof-of-principle microwave experiments. Our proposal paves the road to realize optical PMLs, and could find applications in future photonic chips.

Efficient light coupling between conventional silicon photonic waveguides and quantum valley-Hall topological interfaces.

Robust and efficient light coupling into and out of quantum valley-Hall (QVH) topological interfaces within near-infrared frequencies is demanded in order to be integrated into practical two-dimensional (2D) optical chips. Here, we numerically demonstrate efficient light coupling between a QVH interface and a pair of input/output silicon photonic waveguides in the presence of photonic crystal line defects. When the topological QVH interface is directly end-butt coupled to the silicon waveguides, the input-to-output transmission efficiency is lower than 50% and the exterior boundaries associated with a QVH interface also cause inevitable back-reflections and high-order scatterings, further reducing the transmission efficiency. The transmission efficiency is substantially increased to 95.8% (94.3%) when photonic crystal line defects are introduced between the bridge (zigzag) QVH interface and the waveguides. The buffering line defect mode, with an effective group refractive index between the interface state and the waveguide mode will ease their mode profile conversion. The design we present here brings no fabrication complexity and may be used as a guide for future implementation of on-chip 2D topological photonics.

Observation and control of pseudospin switching in a finite-width topological photonic crystal.

Finite-size effect plays a significant role in topology photonics not to mention in reality all experimental setups are in finite-size. A photonic bandgap is opened in the topological edge state dispersion if a topological photonic crystal with finite width is considered, and the bandgap size relies on the finite-size effect. Pseudospin-preserving and pseudospin-flipping processes can be realized when a selectively switch of the pseudospin of edge states are customized by our designs. Our microwave experiments also successfully demonstrate pseudospin switch-on and -off behaviors in a finite-width photonic crystal. By combining photonic crystals with finite widths, a multi-tunneling proposal of topological photonic crystals can also be achieved. Our study of the finite-size effect will provide new approaches and thoughts to improve the development of topological photonic devices in the future.

Three-Dimensional Electromagnetic Void Space.

Electromagnetic void space is a medium, while geometrically occupying a finite volume of space, optically equivalent to an infinitesimal point, in which electromagnetic waves do not experience any phase accumulation. Here, we report the first realization of three-dimensional (3D) electromagnetic void space by an all-dielectric photonic crystal possessing vanishing permittivity and permeability simultaneously. The 3D electromagnetic void space offers distinctive functionalities inaccessible to its 2D or acoustic counterparts because of the fundamental changes in topology, which comes from the ascension of dimensionality as well as the transverse nature of electromagnetic waves. In particular, we demonstrate, both theoretically and experimentally, that the transmission through such a 3D void space is unaffected by its inner boundaries, but highly sensitive to the outer boundaries. This enables many applications such as the impurity "antidoping" effect, outer-boundary-controlled switching, and 3D perfect wave steering. Our work paves a road toward 3D exotic optics of an optically infinitesimal point.

Three-Dimensional Soundproof Acoustic Metacage.

In this Letter, we theoretically propose and experimentally demonstrate a three-dimensional soundproof acoustic cage structure, hereby denoted as an acoustic metacage. The metacage is composed of six acoustic metamaterial slabs with open holes and hidden bypass space coiling tunnels connected to the holes. Band structure analysis reveals a novel physical mechanism to open a low-frequency broad partial band gap via the band folding in other directions, which can also be interpreted by an effective medium with indefinite effective mass density and negative effective modulus. Transmission loss in simulations and in the acoustic impedance tube are administered. Strikingly, we prove that the soundproofing effect of the metacage is robust against the airflow perturbation induced by a fan. Our work paves a road for low-frequency airborne soundproof structures in the presence of ventilation.

Pulse Reshaping in Double-zero-index Photonic Crystals with Dirac-like-cone Dispersion.

Triply-degenerate Dirac-like cone at the Brillouin zone center attracts much research interest in recent years. Whether the linear dispersion in such a Dirac-like cone reflects the same physics to Dirac cones at the Brillouin zone boundaries is still under investigation. In this manuscript, through microwave experiments and numerical simulations, we observe intriguing pulse reshaping phenomena in double-zero-index photonic crystals, which cannot be fully understood from their close-to-zero effective parameters. A reshaped pulse, with frequency components close to the Dirac frequency filtered, is propagating at a constant group velocity while part of these filtered frequencies appears at a much later time. In time domain measurements, we find a way to separate the effect between the linear dispersion and the extra flat band in Dirac-like cone to have a better understanding of the underneath physics. We succeed in obtaining the group velocity inside a double-zero-index photonic crystal and good consistence can be found between experiments, numerical simulations and band diagram calculations.

Gapped topological kink states and topological corner states in honeycomb lattice.

Based on the tight-binding calculations on honeycomb lattice and photonic experimental visualization on artificial graphene (AG), we report the domain-wall-induced gapped topological kink states and topological corner states. In honeycomb lattice, domain walls (DWs) with gapless topological kink states could be induced either by sublattice symmetry breaking or by lattice deformation. We find that the coexistence of these two mechanisms will induce DWs with gapped topological kink states. Significantly, the intersection of these two types of DWs gives rise to topological corner state localized at the crossing point. Through the manipulation of the DWs, we show AG with honeycomb lattice structure not only a versatile platform supporting multiple topological corner modes in a controlled manner, but also possessing promising applications such as fabricating topological quantum dots composed of gapped topological kink states and topological corner states.

Non-Abelian gauge field optics.

The concept of gauge field is a cornerstone of modern physics and the synthetic gauge field has emerged as a new way to manipulate particles in many disciplines. In optics, several schemes of Abelian synthetic gauge fields have been proposed. Here, we introduce a new platform for realizing synthetic SU(2) non-Abelian gauge fields acting on two-dimensional optical waves in a wide class of anisotropic materials and discover novel phenomena. We show that a virtual non-Abelian Lorentz force arising from material anisotropy can induce light beams to travel along Zitterbewegung trajectories even in homogeneous media. We further design an optical non-Abelian Aharonov-Bohm system which results in the exotic spin density interference effect. We can extract the Wilson loop of an arbitrary closed optical path from a series of gauge fixed points in the interference fringes. Our scheme offers a new route to study SU(2) gauge field related physics using optics.

Topological whispering gallery modes in two-dimensional photonic crystal cavities.

Utilizing the robust transport properties of the topological photonic crystal interface, we experimentally realize two-dimensional topological photonic crystal cavities, where discrete whispering gallery modes can propagate unidirectionally along the cavity circumference. Different from traditional cavities, these topological whispering galley modes are insensitive to cavity shapes. Our microwave demonstration has a good agreement with numerical simulations. Using pure dielectrics, by scaling down to the optical wavelength, an optical directional coupler based on the same topological photonic crystal scheme is also proposed. We here show that topological photonics can provide more novel designs for optical devices.

Visualization of a Unidirectional Electromagnetic Waveguide Using Topological Photonic Crystals Made of Dielectric Materials.

We demonstrate experimentally that a photonic crystal made of Al_{2}O_{3} cylinders exhibits topological time-reversal symmetric electromagnetic propagation, similar to the quantum spin Hall effect in electronic systems. A pseudospin degree of freedom in the electromagnetic system representing different states of orbital angular momentum arises due to a deformation of the photonic crystal from the ideal honeycomb lattice. It serves as the photonic analogue to the electronic Kramers pair. We visualized qualitatively and measured quantitatively that microwaves of a specific pseudospin propagate only in one direction along the interface between a topological photonic crystal and a trivial one. As only a conventional dielectric material is used and only local real-space manipulations are required, our scheme can be extended to visible light to inspire many future applications in the field of photonics and beyond.

Tunable Transmission and Deterministic Interface states in Double-zero-index Acoustic Metamaterials.

Following the seminal work by Dubois et al. (Nat. Commun. 8, 14871 (2017)), we study a double-zero-index acoustic metamaterial with triangular lattice. By varying the height and diameter of air scatterers inside a parallel-plate acoustic waveguide, acoustic dispersion of the first-order waveguide mode can be manipulated and various interesting properties are explored. With accidental degeneracy of monopolar and dipolar modes, we numerically prove the double-zero-index properties of this novel acoustic metamaterial. Acoustic waveguides with tunable and asymmetric transmission are realized with this double-zero-index acoustic metamaterial embedded. Band inversion occurs if the bulk acoustic band diagram of this acoustic metamaterial is tuned. Deterministic interface states are found to exist on the interface between two acoustic metamaterials with inverted band diagrams.

Topological Valley Transport in Two-dimensional Honeycomb Photonic Crystals.

Two-dimensional photonic crystals, in analogy to AB/BA stacking bilayer graphene in electronic system, are studied. Inequivalent valleys in the momentum space for photons can be manipulated by simply engineering diameters of cylinders in a honeycomb lattice. The inequivalent valleys in photonic crystal are selectively excited by a designed optical chiral source and bulk valley polarizations are visualized. Unidirectional valley interface states are proved to exist on a domain wall connecting two photonic crystals with different valley Chern numbers. With the similar optical vortex index, interface states can couple with bulk valley polarizations and thus valley filter and valley coupler can be designed. Our simple dielectric PC scheme can help to exploit the valley degree of freedom for future optical devices.

A hybrid invisibility cloak based on integration of transparent metasurfaces and zero-index materials.

The invisibility cloak, a long-standing fantastic dream for humans, has become more tangible with the development of metamaterials. Recently, metasurface-based invisibility cloaks have been proposed and realized with significantly reduced thickness and complexity of the cloaking shell. However, the previous scheme is based on reflection-type metasurfaces and is thus limited to reflection geometry. In this work, by integrating the wavefront tailoring functionality of transparent metasurfaces and the wave tunneling functionality of zero-index materials, we have realized a unique type of hybrid invisibility cloak that functions in transmission geometry. The principle is general and applicable to arbitrary shapes. For experimental demonstration, we constructed a rhombic double-layer cloaking shell composed of a highly transparent metasurface and a double-zero medium consisting of dielectric photonic crystals with Dirac cone dispersions. The cloaking effect is verified by both full-wave simulations and microwave experimental results. The principle also reveals exciting possibilities for realizing skin-thick ultrathin cloaking shells in transmission geometry, which can eliminate the need for spatially varying extreme parameters. Our work paves a path for novel optical and electromagnetic devices based on the integration of metasurfaces and metamaterials.

Realization of a complementary medium using dielectric photonic crystals.

By exploiting the scaling invariance of photonic band diagrams, a complementary photonic crystal slab structure is realized by stacking two uniformly scaled double-zero-index dielectric photonic crystal slabs together. The space cancellation effect in complementary photonic crystals is demonstrated in both numerical simulations and microwave experiments. The refractive index dispersion of double-zero-index dielectric photonic crystal is experimentally measured. Using pure dielectrics, our photonic crystal structure will be an ideal platform to explore various intriguing properties related to a complementary medium.

Zak phase induced multiband waveguide by two-dimensional photonic crystals.

Interface states in photonic crystals provide efficient approaches to control the flow of light. Photonic Zak phase determines the bulk band properties of photonic crystals, and, by assembling two photonic crystals with different bulk band properties together, deterministic interface states can be realized. By translating each unit cell of a photonic crystal by half the lattice constant, another photonic crystal with identical common gaps but a different Zak phase at each photonic band can be created. By assembling these two photonic crystals together, multiband waveguide can thus be easily created and then experimentally characterized. Our experimental results have good agreement with numerical simulations, and the propagation properties of these measured interface states indicate that this new type of interface state will be a good candidate for future applications of optical communications.

Achieving low-emissivity materials with high transmission for broadband radio-frequency signals.

The use of low-emissivity (low-e) materials in modern buildings is an extremely efficient way to save energy. However, such materials are coated by metallic films, which can strongly block radio-frequency signals and prevent indoor-outdoor wireless communication. Here, we demonstrate that, when specially-designed metallic metasurfaces are covered on them, the low-e materials can remain low emissivity for thermal radiation and allow very high transmission for a broad band of radio-frequency signals. It is found that the application of air-connected metasurfaces with subwavelength periods is critical to the observed high transmission. Such effects disappear if periods are comparable to wavelengths or metal-connected structures are utilized. The conclusion is supported by both simulations and experiments. Advantages such as easy to process, low cost, large-area fabrication and design versatility of the metasurface make it a promising candidate to solve the indoor outdoor communication problem.

Ultratransparent Media and Transformation Optics with Shifted Spatial Dispersions.

By using pure dielectric photonic crystals, we demonstrate the realization of ultratransparent media, which allow near 100% transmission of light for all incident angles and create aberration-free virtual images. The ultratransparency effect is well explained by spatially dispersive effective medium theory for photonic crystals, and verified by both simulations and proof-of-principle microwave experiments. Designed with shifted elliptical equal frequency contours, such ultratransparent media not only provide a low-loss and feasible platform for transformation optics devices at optical frequencies, but also enable new freedom for phase manipulation beyond the local medium framework.

Design of full-k-space flat bands in photonic crystals beyond the tight-binding picture.

Based on a band engineering method, we propose a theoretical prescription to create a full-k-space flat band in dielectric photonic crystals covering the whole Brillouin Zone. With wave functions distributed in air instead of in the dielectrics, such a flat band represents a unique mechanism for achieving flat dispersions beyond the tight-binding picture, which can enormously reduce the requirement of permittivity contrast in the system. Finally, we propose and numerically demonstrate a unique application based on the full-k-space coverage of the flat band: ultra-sensitive detection of small scatterers.

Direct Measurement of Optical Force Induced by Near-Field Plasmonic Cavity Using Dynamic Mode AFM.

Plasmonic nanostructures have attracted much attention in recent years because of their potential applications in optical manipulation through near-field enhancement. Continuing experimental efforts have been made to develop accurate techniques to directly measure the near-field optical force induced by the plasmonic nanostructures in the visible frequency range. In this work, we report a new application of dynamic mode atomic force microscopy (DM-AFM) in the measurement of the enhanced optical force acting on a nano-structured plasmonic resonant cavity. The plasmonic cavity is made of an upper gold-coated glass sphere and a lower quartz substrate patterned with an array of subwavelength gold disks. In the near-field when the sphere is positioned close to the disk array, plasmonic resonance is excited in the cavity and the induced force by a 1550 nm infrared laser is found to be increased by an order of magnitude compared with the photon pressure generated by the same laser light. The experiment demonstrates that DM-AFM is a powerful tool for the study of light induced forces and their enhancement in plasmonic nanostructures.