Lirong Huang

Continuous metasurface for high-performance anomalous reflection

A new type of a metasurface, known as a continuous metasurface, having only one trapezoidal antenna within a super cell, is proposed. Markedly different from previously reported discrete metasurfaces having multiple discrete antennas within a super cell, continuous metasurfaces can provide a continuously varying phase response for anomalous reflection following the generalized Snell’s law. The inherent spatial continuity of the phase response and the elimination of near-field coupling among neighboring antennas enable the continuous metasurface to achieve low-distortion, high-efficiency, and ultrawide-band anomalous reflection. The concept of continuous metasurfaces offers a new alternative to design flat plasmonic optical components.

Manipulating the wavefront of light by plasmonic metasurfaces operating in high order modes

In this work, plasmonic metasurfaces with abrupt phase discontinuities operating in high order modes are investigated for manipulating the wavefront of light. We first design two types of meta-super-cells consisting of V-shaped antennas with the phase shift coverage larger than 2π. And then, we create two linear gradient phased metasurfaces using the designed cells, which exhibit exceptional abilities for light-steering functioned as meta-echelette gratings operating in high order diffraction modes, may be valuable for using in high resolution spectrographs and advantage to achieve high numerical aperture plasmonic lenses. Based on the new designed super cells we further build another two azimuthal gradient phased metasurfaces that are able to generate high order optical vortex beams. Our results could lead to wide applications in photonic research.

All-optical power equalization based on a two-section reflective semiconductor optical amplifier.

A simple all-optical power equalization scheme based on a single two-section reflective semiconductor optical amplifier (RSOA) is presented. Double optical path and non-uniform injection current density in the two sections easily saturate the RSOA and suppress pattern effect, thereby significantly reducing packet-to-packet power fluctuation while maintaining better signal quality. The mechanism of the two-section RSOA-based power equalizer is investigated and it is indicated that the two-section RSOA biased at proper current density functions as three cascaded SOAs, including a preamplifying SOA, a gain-saturated SOA and a third SOA. The performance dependence on driven current and structural parameters is also studied.

Perspective on resonances of metamaterials.

Electromagnetic resonance as the most important characteristic of metamaterials enables lots of exotic phenomena, such as invisible, negative refraction, man-made magnetism, etc. Conventional LC-resonance circuit model as the most authoritative and classic model is good at explaining and predicting the fundamental resonance wavelength of a metamaterial, while feels hard for high-order resonances, especially for resonance intensity (strength of resonance, determining on the performance and efficiency of metamaterial-based devices). In present work, via an easy-to-understand mass-spring model, we present a different and comprehensive insight for the resonance mechanism of metamaterials, through which both the resonance wavelengths (including the fundamental and high-order resonance wavelengths) and resonance intensities of metamaterials can be better understood. This developed theory has been well verified by different-material and different-structure resonators. This perspective will provide a broader space for exploring novel optical devices based on metamaterials (or metasurfaces).

Dual Metamaterial With Large Birefringence

Strong birefringence and a large phase delay are highly desirable for high-efficiency and compact wave plates. In order to enhance the phase delay, we combine a semiconductor metamaterial (SMM) and split-ring resonators (SRRs) into a novel dual metamaterial, which exhibits two types of resonance response, i.e., one type is mainly induced by the SMM for one polarization component of the incident wave, and the other is mainly induced by the SRRs for the other orthogonal polarization component. The unique resonance characteristics lead to large birefringence and, hence, a considerable phase delay, enabling the realization of a very compact quarter-wave plate in the middle-infrared spectrum. The concept of the dual metamaterial has the potential for developing ultracompact high-performance microoptical/nanooptical components.

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.

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.

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.

Incidence Ways of Electromagnetic Wave and Their Influences on the Absorption and Resonant Wavelength of Split-Ring Resonators

We systematically investigate the classification of incidence ways (represented by different field polarization and propagation directions of the impinging electromagnetic (EM) wave) and their influences on the EM resonant characteristics of split-ring resonators (SRRs). It is found that the resonance absorption varies noticeably with incidence ways and the variation could reach as large as 60xa0%, while transmittance almost keeps unchanged. When resonant electric response and resonant magnetic response are both excited in SRRs, EM resonance characteristics are found to be mainly dependent upon electric response rather than on magnetic response. The results also indicate that magnetic resonant wavelength (λH) induced by coupled magnetic field H component of the EM wave is longer than electric resonant wavelength (λE) induced by coupled electric field E, and the mechanism is discussed.

Asymmetric electromagnetic wave transmitter based on one-way excitation of surface plasmon polaritons in gradient metasurface

By employing the one-way excitation of surface plasmon polaritons (SPPs), we propose a novel metasurface-based asymmetric electromagnetic wave transmitter with large working bandwidth and high contrast ratio. Temporal coupled mode theory and simulations are used to study its mechanism and performances.