Optical rotation and electromagnetically induced transparency in a chiral metamaterial with C4 symmetry.

We design and fabricate a double-layered chiral metamaterial with 4-fold rotational symmetry, which simultaneously exhibits optical rotation and electromagnetically induced transparency (EIT) effects. Using analytical equivalent circuit model and Lorentz's coupled oscillator model, we interpret the physical mechanisms and derive material equations. Importantly, we find that magnetic dipole and electric quadrupole play important roles in optical rotation and keeping the symmetry of the material equations. Our work offers a better understanding of optical rotation in chiral metamaterials, and provides a new and simple approach to combine optical rotation and EIT effects into a single metamaterial.

Mechanism and performance analyses of optical beam splitters using all-dielectric oligomer-based metasurfaces.

Compact and planar optical beam splitters are highly desirable in various optical and photonic applications. Here, we investigate two kinds of optical beam splitters by using oligomer-based metasurfaces, one is trimer-based metasurface for 3-dB beam splitting, and the other is pentamer-based metasurface for 1:4 beam splitting. Through electromagnetic multipole decomposition and in-depth mechanism analyses, we reveal that the electromagnetic multipolar interactions and the strong near-field coupling between neighboring nanoparticles play critical roles in beam-splitting performance. Our work offers a deeper understanding of electromagnetic coupling effect in oligomer-based metasurfaces, and provides an alternative approach to planar beam splitters.

Dual-band asymmetric optical transmission of both linearly and circularly polarized waves using bilayer coupled complementary chiral metasurface.

It is highly desirable to develop asymmetric transmission (AT) devices for both linearly and circularly polarized light. However, currently existing metamaterial-based AT devices require multi-step micro-nano fabrication processes and usually realize AT responses only for linearly or circularly polarized waves, not simultaneously for both. We here propose a dual-band AT device for both linearly and circularly polarized waves in the near-infrared region by using a bilayer coupled complementary chiral metasurface, which includes a half-gammadion-shape gold (Au) structural layer and its Babinet's complimentary copy. Unlike other multilayer AT devices working at optical frequencies, it takes less micro-nano fabrication steps. Besides, with the help of chirality and the inherent near-field coupling effect between the two complementary Au layers, the maximal AT parameters for linearly and circularly polarized waves can reach up to 0.45 and 0.56, respectively. The underlying mechanisms of dual-band AT responses are also investigated in depth from the perspectives of chirality and coupling effect. Our work offers a new and simple approach to high-performance AT devices, helps to better understand near-filed coupling effect in coupled complementary metasurfaces, and also expands their application fields.

High-performance asymmetric optical transmission based on coupled complementary subwavelength gratings.

Asymmetric transmission (AT) devices are fundamental elements for optical computing and information processing. We here propose an AT device consisting of a pair of coupled complementary subwavelength gratings. Different from previous works, asymmetric dielectric environment is employed for unidirectional excitation of surface plasmon polaritons (SPPs) and thus asymmetric optical transmission, and near-field coupling effect inherent in the coupled complementary structure is exploited to enhance forward transmission and AT behavior, and determine operation bandwidth as well. The influence of asymmetric dielectric environment, effect of vertical and lateral couplings, interactions of electric- and magnetic-dipole moments and the realization of Kerker conditions, are investigated in depth to unearth the AT mechanism and performance. High-performance AT with large forward transmittance of 0.96 and broad bandwidth of 174 nm is achieved at wavelength 1250 nm. Our work helps people to gain a better understanding of near-filed coupling effect in coupled complementary structures, expand their application fields, and it also offers an alternate way to high-performance AT devices.

Direct-tuning methods for semiconductor metamaterials.

Among various tunable optical devices, tunable metamaterials have exhibited their excellent ability to dynamically manipulate lights in an efficient manner. However, for unchangeable optical properties of metals, electromagnetic resonances of popular metallic metamaterials are usually tuned indirectly by varying the properties or structures of substrates around the resonant unit cells, and the tuning of metallic metamaterials has significantly low efficiency. In this paper, a direct-tuning method for semiconductor metamaterials is proposed. The resonance strength and resonance frequencies of the metamaterials can be significantly tuned by controlling free carriers' distributions in unit cells under an applied voltage. This direct-tuning method has been verified in both two-dimensional and three-dimensional semiconductor metamaterials. In principle, the method allows for simplifying the structure of tunable metamaterials and opens the path to applications in ultrathin, linearly-tunable, and on-chip integrated optical components (e.g., tunable ultrathin lenses, nanoscale spatial light modulators and optical cavities with resonance modes switchable).

Narrow-band and high-contrast asymmetric transmission based on metal-metal-metal asymmetric gratings.

A narrow-band and high-contrast asymmetric transmission (AT) device based on metal-metal-metal (M-M-M) asymmetric grating structure is proposed and investigated. Significantly distinct from previous reports, the upper and lower metallic silver (Ag) gratings are connected by a very thin metallic Ag film, without any dielectric spacer layer or subwavelength slit. Under forward incidence, the M-M-M structure supports efficient surface plasmon polaritons (SPPs) excitation and tunneling, more importantly, it promotes direct and thus high-efficiency SPPs decoupling, enabling high forward transmittance. While under backward incidence, the M-M-M structure offers not only high reflection by the Ag film but also a strong near-field coupling effect between the upper and lower gratings, which further suppresses backward transmittance, leading to near-zero backward transmittance. In addition, the M-M-M structure is optimized for narrow-band operation by employing grating groove depth effect and multiple interference effect. Numerical simulation results demonstrate that high-performance AT with high-quality factor (Q≈91), narrow-bandwidth (6.7 nm) and high contrast ratio is achieved, with forward transmittance of 0.72 and backward transmittance of 0.0015 at visible light (610 nm). Our work provides an alternative and simple way to high-performance AT devices.

Dual-wavelength multifunctional metadevices based on modularization design by using indium-tin-oxide.

Combining two or several functionalities into a single metadevice is of significant importance and attracts growing interest in recent years. We here introduce the concept of modularization design in dual-wavelength multifunctional metadevice, which is composed of a lower metasurface and an upper metasurface with an indium-tin-oxide (ITO) layer. Benefiting from the fact that ITO holds high infrared (IR) reflection while transparence at visible wavelengths, the metadevice can work in reflection and transmission modes at two very distinct wavelengths, one is 2365 nm in the IR band and the other 650 nm in the visible range. More interestingly and importantly, the two metasurface layers with different functionalities are easy to flexibly integrate into a series of dual-wavelength multifunctional metadevices, with negligible interaction between them and no need of re-designing or re-optimizing their structure parameters. Based on modularization design and functional integration, four kinds of dual-wavelength multifunctional metadevices are demonstrated, which can perform reflective deflection/focusing at 2365 nm and transmissive deflection/focusing at 650 nm. We believe our work may open a straight-forward and flexible way in designing multi-wavelength multifunctional metadevices and photonic integrated devices.

Polarization-controlled dynamically switchable plasmon-induced transparency in plasmonic metamaterial.

Dynamical manipulation of plasmon-induced transparency (PIT) in metamaterials promises numerous potential applications; however, previously reported approaches require complex metamaterial structures or an external stimulus, and dynamic control is limited to a single PIT transparency window. We propose here a metamaterial with a simple structure to realize a dynamically controllable PIT effect. Simply by changing the polarization direction of incident light, the number of PIT transparency windows can be increased from 1 to 2, accompanied by a tunable amplitude and a switchable resonance-wavelength. Moreover, a coupled three-level plasmonic system is employed to explain the underlying mechanism and near-field coupling between the horizontal and vertical gold bars, and the analytical results show good consistency with the numerical calculations. This work provides a simple approach for designing compact and tunable PIT devices and has potential applications in selective filtering, plasmonic switching and optical sensing.

Polarization-switchable and wavelength-controllable multi-functional metasurface for focusing and surface-plasmon-polariton wave excitation.

Realizing versatile functionalities in a single photonic device is crucial for photonic integration. We here propose a polarization-switchable and wavelength-controllable multi-functional metasurface. By changing the polarization state of incident light, its functionality can be switched between the flat focusing lens and exciting surface-plasmon-polariton (SPP) wave. Interestingly, by tuning the wavelength of incident light, the generated SPP waves can also be controlled at desired interfaces, traveling along the upper or lower interface of the metasurface, or along both of them, depending on whether the incident light satisfies the first or second Kerker condition. This polarization-switchable and wavelength-controllable multifunctional metasurface may provide flexibility in designing tunable or multifunctional metasurfaces and may find potential applications in highly integrated photonic systems.

Asymmetric optical transmission based on unidirectional excitation of surface plasmon polaritons in gradient metasurface.

Asymmetric optical transmission is fundamental and highly desirable in information processing and full manipulation of lightwave. We here propose an asymmetric optical transmission device consisting of a gradient metasurface and a one-dimensional subwavelength grating. Owing to the unidirectional excitation of surface plasmon polaritons (SPPs) by the gradient metasurface, and SPP-assisted extraordinary optical transmission, forward incident light has much higher transmission than the backward one. We combine temporal coupled mode theory and finite-difference time-domain simulations to verify its operation principle and study the performance. The results indicate that asymmetric transmission with high-contrast and large forward transmittance can be obtained around the 1.3 µm optical communication band.

Coupling-based Huygens' meta-atom utilizing bilayer complementary plasmonic structure for light manipulation.

Huygens' meta-atom is the basic building unit of Huygens' metasurfaces allowing for almost arbitrary wavefront shaping across a surface. We here present a kind of Huygens' meta-atom by coupling a nanodisk to its Babinet-complementary structure (nanohole), and develop an optical lumped nanocircuit model to analyze vertical and lateral coupling effects and resonance frequencies. Simulation results show that the tuned coupling via lateral misalignment between the two nanostructures is sufficient to shape the wavefront without changing the dimensions or orientations of antennas. By tuning the coupling via lateral misalignment, we design a reflective gradient metasurface based on one coupled mode and a high-efficiency transmissive gradient metasurface working in the spectral overlap of electric and magnetic resonances to realize beam deflection. The proposed coupling-based Huygens' meta-atom is a new building block for plasmonic metasurfaces with enhanced light-matter interactions, high-efficiency and almost arbitrary wavefront shaping over the full electromagnetic spectrum.