Multiplexing Optical Images for Steganography by Single Metasurfaces.

Image steganography based on intelligent devices is one of the effective routes for safely and quickly transferring secret information. However, optical image steganography has attracted far less attention than digital one due to the state-of-the-art technology limitations of high-resolution optical imaging in integrated devices. Optical metasurfaces, composed of ultrathin subwavelength meta-atoms, are extensively considered for flat optical-imaging nano-components with high-resolutions as competitive candidates for next-generation miniaturized devices. Here, multiplex imaging metasurfaces composed of single nanorods are proposed under a detailed strategy to realize optical image steganography. The simulation and experimental results demonstrate that an optical steganographic metasurface can simultaneously transfer independent secret image information to two receivers with special keys, without raising suspicions for the general public under the cloak of a cover image. The proposed optical steganographic strategy by metasurfaces can arbitrarily distribute a continuous grayscale image together with a black-and-white image in separate channels, implying the distinguishing feature of high-density information capacity for integration and miniaturization in optical meta-devices.

Spin-selected bifunctional metasurface for grayscale image and metalens.

With the extensive research on the Pancharatnam-Berry phase, metasurfaces have been widely designed as various cross-polarized nanodevices for circularly polarized (CP) illumination. However, co- and cross-polarized lights are rarely co-modulated by the metasurface. To fully utilize the transmitted light, we propose a spin-selected bifunctional metasurface composed of arrayed silver nanorods, integrating an amplitude-based grayscale imaging for co-polarized transmission and a phase-based metalens for cross-polarized transmission, under left-handed CP incidence. Moreover, such dual functionalities work well under right-handed CP incidence. Both experiments and simulations demonstrate the bifunctional performance as potential meta-devices.

Engineering multimode resonances for tunable multifrequency superscattering.

We demonstrate a rigorous multimode engineering method to achieve multifrequency superscattering with flexible controllability in a subwavelength graphene/hexagonal boron nitride (hBN) cylindrical system. Through delicately tuning the chemical potential of graphene, different resonance channels of the proposed stucture can be spectrally overlapped to construct the multiple superscattering points. Consequently, the scattering cross section is enhanced effectively and the so-called superscattering beyond the single-channel scattering limit can be attained. Numerical calculations on scattering spectra, near-field, and far-field distributions are performed to confirm the scattering enhancement. The general principles presented here may suggest an accurate and efficient approach to actively tune the light-matter interaction at the subwavelength scale.

Optical Pulling Forces Enabled by Hyperbolic Metamaterials.

We propose a novel approach to generating optical pulling forces on a gold nanowire, which are placed inside or above a hyperbolic metamaterial and subjected to plane wave illumination. Two mechanisms are found to induce the optical pulling force, including the concave isofrequency contour of the hyperbolic metamaterial and the excitation of directional surface plasmon polaritons. We systematically study the optical forces under various conditions, including the wavelength, the angle of incidence of light, and the nanowire radius. It is shown that the optical pulling force enabled by hyperbolic metamaterials is broadband and insensitive to the angle of incidence. The mechanisms and results reported here open a new avenue to manipulating nanoscale objects.

Grayscale image for broadband linear polarization measurement by an ultracompact metasurface.

The polarization of light, the vector nature of electromagnetic waves, is one of the fundamental parameters. Finding a direct and efficient method to measure the state of polarized light is extremely urgent for nano-optical applications. Based on Malus's law, we design an ultracompact metasurface composed of silver nanorods, which is demonstrated to directly measure the state of linear polarization by a grayscale image. Using an ultrathin metasurface, we generate grayscale images with gradient grayscale levels which are linked directly to the polarization state of the incident light. The direction of the linear polarization of incident light can be conveniently and efficiently obtained through extracting the angle of the brightest area of the grayscale image. The ultrathin metasurface operates in the broadband 750-1100 nm spectral range. It is a novel and significant method to analyze the linear polarization state of light, which provides opportunities for various applications, such as polarimetric multispectral imaging and miniaturized polarimeter.

Dual-wavelength complementary grayscale imaging by an ultrathin metasurface.

It is important to arbitrarily manipulate optical intensity, an important degree of freedom to light, on microscales, which is a fundamental requirement for integration and miniaturization of optical devices. Metasurfaces have shown unprecedented capabilities for manipulating light in terms of phase, intensity, and polarization. Here, an ultrathin metasurface composed of silver nanorods illuminated by linearly polarized light is demonstrated to manipulate optical intensity in subwavelength scales. By modulating rod orientations, gradient reflectance of light can be achieved on dual-wavelength regimes with contrast reflection intensities. Further, a nanorod metasurface, embedded with a picture of a panda profile, is experimentally designed for grayscale imaging, and the measurements demonstrate that two complementary grayscale images can be displayed at 633 and 900 nm. The grayscale imaging by a proposed ultrathin metasurface with dual-wavelength, complementary, and subwavelength-resolution characteristics provides a simple but efficient way for tailoring optical intensity on subwavelength scales, which is promising for a variety of applications such as encryption and decryption, display, information security, and optical communication.

Spin-dependent dual-wavelength multiplexing metalens.

The Pancharatnam-Berry (PB) phase is generally utilized to realize a single wavelength spin-dependent function or dual-wavelength functions but operating only in one spin state. A dual-wavelength multifunctional metasurface relying on both spins has been rarely designed due to the rather complicated degrees of freedom to be considered. In this Letter, both dynamic and PB phases are adopted, instead of a pure PB phase, to propose a multiplexing metasurface that can independently and simultaneously manipulate left- and right-handed circularly polarized incidences at dual wavelengths. It is demonstrated experimentally as well as numerically that such spin-dependent dual-wavelength metalenses can make circularly polarized incidences of different wavelengths split into and focus at multi-dimensional positions. Our work demonstrates a new avenue in designing spin-dependent dual-wavelength multifunctional optical devices.

Dual-mode subwavelength trapping by plasmonic tweezers based on V-type nanoantennas.

We propose novel plasmonic tweezers based on silver V-type nanoantennas placed on a conducting ground layer, which can effectively mitigate the plasmonic heating effect and thus enable subwavelength plasmonic trapping in the near-infrared region. Using the centroid algorithm to analyze the motion of trapped spheres, we can experimentally extract the value of optical trapping potential. The result confirms that the plasmonic tweezers have a dual-mode subwavelength trapping capability when the incident laser beam is linearly polarized along two orthogonal directions. We have also performed full-wave simulations, which agree with the experimental data very well in terms of spectral response and trapping potential. It is expected that the dual-mode subwavelength trapping can be used in non-contact manipulations of a single nanoscale object, such as a biomolecule or quantum dot, and find important applications in biology, life science, and applied physics.

Enhanced asymmetric transmissions attributed to the cavity coupling hybrid resonance in a continuous omega-shaped metamaterial.

In this paper, the infinite-length metallic bar is folded to a continuous omega-shaped resonator and then arranged as a bi-layer metamaterial, which presents a hybrid resonance and a Fabry-Perot-like cavity mode. The asymmetric transmission (AT) for linearly polarized light is powerfully enhanced at a near-infrared regime by strongly coupling the hybrid resonance to the cavity, with the maximum value of the high-efficiency AT effect reaching 0.8 at around 1364 nm. At this near-infrared band, such a high-efficiency AT effect has never been realized previously by a bi-layer metamaterial. More importantly, we demonstrate that our design is robust to the misalignments, which greatly decreases the difficulties in sample fabrications. Accordingly, the proposed omega-shaped metamaterial provides potential applications in designing polarization filters, polarization switches, and other nano-devices.

Optical non-reciprocity induced by asymmetrical dispersion of Tamm plasmon polaritons in terahertz magnetoplasmonic crystals.

Nonreciprocal light phenomena, including one-way wave propagation along an interface and one-way optical tunneling, are presented at terahertz frequencies in a system of magnetically controlled multi-layered structure. By varying the surface termination and the surrounding medium, it is found that the nonreciprocal bound or radiative Tamm plasmon polartions can be supported, manipulated, and well excited. Two different types of contributions to the non-reciprocity are analyzed, including the direct effect of magnetization-dependent surface terminating layer as well as violation of the periodicity in truncated multi-layered systems. Calculations on the asymmetrical dispersion relation of surface modes, field distribution, and transmission spectra through the structure are employed to confirm the theoretical results, which may potentially impact the design of tunable and compact optical isolators.

Experimental verification of asymmetric transmission in continuous omega-shaped metamaterials.

A bi-layer continuous omega-shaped metamaterial was proposed and fabricated to measure the asymmetric transmission (AT) effect of a linearly polarized light at near-infrared region. The metamaterial was fabricated by the electron-beam lithography method, and the AT effect was demonstrated by the difference between total transmittances in the two opposite propagation directions for x-/y-polarized incident light. The experimental results were confirmed by the full-wave simulated results. Importantly, we also experimentally demonstrated that the AT effect is robust against the misalignments between the first and the second omega-shaped layers. Accordingly, the successfully prepared sample and its characterization provide a bright future for applications in light-controlled switchers and optical diodes in on-chip optical systems and information communication systems.

Super-radiating manipulation of a nano-emitter by active toroidal metamaterials.

The far-field radiation of a single dipolar emitter can be controlled by coupling to toroidal dipole resonance attached to metallic double flat rings, realizing a conversion from non- to super-radiating. The underlying physical mechanism is the hybridization interference of toroidal and electric dipoles under an asymmetric configuration by introducing a radial displacement of the dipolar emitter. By embedding gain medium in the gap spacer between double flat rings, the directional far-field super-radiating power can achieve a tremendous enhancement with a moderate requirement on the gain coefficient, promoting light-matter interaction manipulation.

Optical force enhancement and annular trapping by plasmonic toroidal resonance in a double-disk metastructure.

Optical forces can be enhanced by surface plasmon resonances with various interesting characteristics. Here, we numerically calculated the optical forces enhanced by a new kind of toroidal dipolar resonance in a double-disk metastructure. The results show that this kind of optical force is competitive with ordinary plasmonic forces and typically can reach-182.5pNµm2mW-1. Influences of geometric parameters are discussed for the enhancement characteristic of optical force. Finally, we make a contrastive investigation on the optical trapping characteristic on a 5-nm-diameter nanoparticle, and show that the unique annular trapping region can be utilized for nanoscale applications.

From non- to super-radiating manipulation of a dipolar emitter coupled to a toroidal metastructure.

Toroidal dipolar response in a metallic metastructure, composed of double flat rings, is utilized to manipulate the radiation pattern of a single dipolar emitter (e.g., florescent molecule/atom or quantum dot). Strong Fano-type radiation spectrum can be obtained when these two coupling dipoles are spatially overlapped, leading to significant radiation suppression (so-called nonradiating source) attributed to the dipolar destructive interference. Moreover, this nonradiating configuration will become a directionally super-radiating nanoantenna after a radial displacement of the emitter with respect to the toroidal flat-ring geometry, which emits linearly polarized radiation with orders of power enhancement in a particular orientation. The demonstrated radiation characteristics from a toroidal-dipole-mediated dipolar emitter indicate a promising manipulation capability of the dipolar emission source by intriguing toroidal dipolar response.

Toroidal dipolar response by a dielectric microtube metamaterial in the terahertz regime.

Due to metal losses in plasmonic metamaterials, high-refractive-index dielectrics are promising to improve optical performances of their metallic counterparts. In this paper, a LiTaO(3) microtube metamaterial is numerically investigated to explore the toroidal dipolar resonance based on the multipole expansion theory. The local field strength probed on the central axis of the microtube is greatly enhanced for the toroidal dipolar mode, forming a strong hot spot concentrated in the deep-subwavelength scale. Furthermore, we also show the influences of geometrical parameter on the quality (Q) factor of the toroidal mode. The high Q factor and strongly concentrated field strength in the toroidal microtube metamaterial offer application potentials such as sensing, energy havesting, particle trapping, and nonlinear optical effects.

Flexible transformation plasmonics using graphene.

The flexible control of surface plasmon polaritons (SPPs) is important and intriguing due to its wide application in novel plasmonic devices. Transformation optics (TO) offers the capability either to confine the SPP propagation on rigid curved/uneven surfaces, or to control the flow of SPPs on planar surfaces. However, TO has not permitted us to confine, manipulate, and control SPP waves on flexible curved surfaces. Here, we propose to confine and freely control flexible SPPs using TO and graphene. We show that SPP waves can be naturally confined and propagate on curved or uneven graphene surfaces with little bending and radiation losses, and the confined SPPs are further manipulated and controlled using TO. Flexible plasmonic devices are presented, including the bending waveguides, wave splitter, and Luneburg lens on curved surfaces. Together with the intrinsic flexibility, graphene can be served as a good platform for flexible transformation plasmonics.

Toroidal dipole response in a multifold double-ring metamaterial.

The toroidal response is numerically investigated in a multifold double-ring metamaterials at the antibonding magnetic-dipole mode (i.e., antiparallel magnetic dipoles in one double-ring fold). This intriguing toroidal resonance in metamaterials is considered as a result of the magnetoelectric effect due to the broken balance of the electric near-field environment. We demonstrate that the toroidal dipole response in metamaterials can improve the quality factor of the resonance spectrum. In viewing of the design flexibility on the double-ring geometry, such toroidal metamaterials will offer advantages in application potentials of toroidal dipolar moment.

Transparency window for the absorptive dipole resonance in a symmetry-reduced grating structure.

We demonstrate that a transparency window can be obtained within the absorptive dipole resonant regime, by slightly reducing the symmetric arrangement of a dipole-like bar grating covered by a waveguiding layer. The physical understanding is that, under the condition of reducing the grating symmetry, the lossy dipole plasmon resonance can be completely transferred into the waveguide mode in a way of destructive interference. In accompany with the tunable transparency window modulated by the symmetry-reduced displacement, an ultra high group index (slowing down the light) as well as a vortex distribution of the electromagnetic field is found.

Role of asymmetric environment on the dark mode excitation in metamaterial analogue of electromagnetically-induced transparency.

An otherwise dark magnetic dipole resonance in a split-ring resonator can be excited electrically with a Fano-type profile once the symmetric environment for this resonator is broken with respect to the polarized electric-field direction of incident waves. When this asymmetrically induced narrow resonance coincides with a broad dipolar resonance at an identical frequency regime, the metamaterial analogue of electromagnetically-induced transparency (EIT) window can be formed. We demonstrate that this environmental-asymmetry condition can be introduced dielectrically as well as plasmonically, either resonantly or nonresonantly, which indicates the plasmon coupling between different resonant modes is not responsible for the dark mode excitation. Thus, this result should contribute to the physical understanding on dark-mode excitation pathway for EIT-like phenomenon in plasmonic metamaterials.

Plasmonically induced transparent magnetic resonance in a metallic metamaterial composed of asymmetric double bars.

We demonstrate that the trapped magnetic resonance mode can be induced in an asymmetric double-bar structure for electromagnetic waves normally incident onto the double-bar plane, which mode otherwise cannot be excited if the double bars are equal in length. By adjusting the structural geometry, the trapped magnetic resonance becomes transparent with little resonance absorption when it happens in the dipolar resonance regime, a phenomenon so-called plasmonic analogue of electromagnetically induced transparency (EIT). This planar EIT-like metamaterial offers a great geometry simplification by combining the radiant and subradiant resonant modes in a single double-bar resonator.