Chunmei Ouyang

Broadband Metasurfaces with Simultaneous Control of Phase and Amplitude

By combining the freedom of both the structural design and the orientation of split ring resonator antennas, we demonstrate terahertz metasurfaces that are capable of controlling both the phase and amplitude profiles over a very broad bandwidth. As an example, we show that the phase-amplitude metasurfaces can be engineered to control the diffraction orders arbitrarily.

Efficient flat metasurface lens for terahertz imaging

Metamaterials offer exciting opportunities that enable precise control of amplitude, polarization and phase of the light beam at a subwavelength scale. A gradient metasurface consists of a class of anisotropic subwavelength metamaterial resonators that offer abrupt amplitude and phase changes, thus enabling new applications in optical device design such as ultrathin flat lenses. We propose a highly efficient gradient metasurface lens based on a metal-dielectric-metal structure that operates in the terahertz regime. The proposed structure consists of slotted metallic resonator arrays on two sides of a thin dielectric spacer. By varying the geometrical parameters, the metasurface lens efficiently manipulates the spatial distribution of the terahertz field and focuses the beam to a spot size on the order of a wavelength. The proposed flat metasurface lens design is polarization insensitive and works efficiently even at wide angles of incidence.

Dynamic mode coupling in terahertz metamaterials

The near and far field coupling behavior in plasmonic and metamaterial systems have been extensively studied over last few years. However, most of the coupling mechanisms reported in the past have been passive in nature which actually fail to control the coupling mechanism dynamically in the plasmonic metamaterial lattice array. Here, we demonstrate a dynamic mode coupling between resonators in a hybrid metal-semiconductor metamaterial comprised of metallic concentric rings that are physically connected with silicon bridges. The dielectric function of silicon can be instantaneously modified by photodoped carriers thus tailoring the coupling characteristics between the metallic resonators. Based on the experimental results, a theoretical model is developed, which shows that the optical responses depend on mode coupling that originates from the variation of the damping rate and coupling coefficient of the resonance modes. This particular scheme enables an in-depth understanding of the fundamental coupling mechanism and, therefore, the dynamic coupling enables functionalities and applications for designing on-demand reconfigurable metamaterial and plasmonic devices.

Asymmetric excitation of surface plasmons by dark mode coupling

Asymmetric excitation of surface plasmons is achieved by classical dark mode coupling, promising metadevices with unique functionalities. Control over surface plasmons (SPs) is essential in a variety of cutting-edge applications, such as highly integrated photonic signal processing systems, deep-subwavelength lasing, high-resolution imaging, and ultrasensitive biomedical detection. Recently, asymmetric excitation of SPs has attracted enormous interest. In free space, the analog of electromagnetically induced transparency (EIT) in metamaterials has been widely investigated to uniquely manipulate the electromagnetic waves. In the near field, we show that the dark mode coupling mechanism of the classical EIT effect enables an exotic and straightforward excitation of SPs in a metasurface system. This leads to not only resonant excitation of asymmetric SPs but also controllable exotic SP focusing by the use of the Huygens-Fresnel principle. Our experimental findings manifest the potential of developing plasmonic metadevices with unique functionalities.

Frequency-agile electromagnetically induced transparency analogue in terahertz metamaterials

Recently reported active metamaterial analogues of electromagnetically induced transparency (EIT) are promising in developing novel optical components, such as active slow light devices. However, most of the previous works have focused on manipulating the EIT resonance strength at a fixed characteristic frequency and, therefore, realized on-to-off switching responses. To further extend the functionalities of the EIT effect, here we present a frequency tunable EIT analogue in the terahertz regime by integrating photoactive silicon into the metamaterial unit cell. A tuning range from 0.82 to 0.74 THz for the EIT resonance frequency is experimentally observed by optical pump-terahertz probe measurements, allowing a frequency tunable group delay of the terahertz pulses. This straightforward approach delivers frequency agility of the EIT resonance and may enable novel ultrafast tunable devices for integrated plasmonic circuits.

Active metasurface terahertz deflector with phase discontinuities

Metasurfaces provide great flexibility in tailoring light beams and reveal unprecedented prospects on novel functional components. However, techniques to dynamically control and manipulate the properties of metasurfaces are lagging behind. Here, for the first time to our knowledge, we present an active wave deflector made from a metasurface with phase discontinuities. The active metasurface is capable of delivering efficient real-time control and amplitude manipulation of broadband anomalous diffraction in the terahertz regime. The device consists of complementary C-shape split-ring resonator elements fabricated on a doped semiconductor substrate. Due to the Schottky diode effect formed by the hybrid metal-semiconductor, the real-time conductivity of the doped semiconductor substrate is modified by applying an external voltage bias, thereby effectively manipulating the intensity of the anomalous deflected terahertz wave. A modulation depth of up to 46% was achieved, while the characteristics of broadband frequency responses and constant deflected angles were well maintained during the modulation process. The modulation speed of diffraction amplitude reaches several kilohertz, limited by the capacitance and resistance of the depletion region. The scheme proposed here opens up a novel approach to develop tunable metasurfaces.

Mapping the near-field propagation of surface plasmons on terahertz metasurfaces

Controlling the propagation of surface plasmon polaritons is essential in developing highly integrated photonic devices. By using near-field scanning terahertz microscopy, we experimentally demonstrate that polarization-controlled tunable surface plasmons (SPs) could be directionally excited on a metal surface with carved columns of aperture resonators under special arrangement. The experimental results reveal that terahertz SPs could be unidirectionally launched in opposite directions owning to destructive and constructive interferences on the two sides with circularly polarized incident waves of opposite handedness. Meanwhile, the linearly polarized wave is able to excite the terahertz SPs along either side of the structures. The presented results would be useful to implement functional terahertz plasmonic devices.

Pancharatnam–Berry Phase Induced Spin-Selective Transmission in Herringbone Dielectric Metamaterials

A dielectric metamaterial approach for achieving spin-selective transmission of electromagnetic waves is proposed. The design is based on spin-controlled constructive or destructive interference between propagating phase and Pancharatnam-Berry phase. The dielectric metamaterial, consisting of monolithic silicon herringbone structures, exhibits a broadband operation in the terahertz regime.

Broadband Terahertz Transparency in a Switchable Metasurface

Plasmon-induced transparency in terahertz metamaterials markedly modifies the dispersive properties of an otherwise opaque medium and reveals unprecedented prospects on novel functional components. However, plasmon-induced transparency in metamaterials so far exists in a narrow frequency band or without actively tunable abilities. Here, we demonstrate optical control of a broadband plasmon-induced transparency in a hybrid metamaterial made from integrated silicon-metal unit cells. Attributed to the modification in damping rate of the dark mode resonators under optical excitation, a giant dynamic amplitude modulation of the broadband transparency window is observed. The scheme suggested here is promising in developing broadband active slow-light devices and realizing on-to-off switching responses of the terahertz radiation at room temperature.

Plasmonic Analog of Electromagnetically Induced Transparency in Stereo Metamaterials

Plasmon-induced transparency (PIT) is a key addition to mimicking the quantum phenomena of electromagnetically induced transparency (EIT) in atomic systems. So far, various metamaterial structures have been proposed to excite and manipulate the PIT effect. However, most of the reported works were based on 2-D metal structures, and consequently, the PIT phenomena often arise from their electric responses. Here, we propose a novel PIT metamaterial scheme based on three vertically placed split ring resonators (SRRs) working at terahertz frequencies. This stereo structure, with a typical EIT-like transmission, couples to both the electric and magnetic fields of the normally incident wave. We numerically demonstrate that the coupling between the radiative and subradiative elements can be modulated not only by their mutual separation but also by the vertical height of the SRRs. In addition, a classical Fano resonance model is applied to explain the coupling effects of EIT-like transmission spectra, which is in good accordance with the numerical results. Considering the higher design freedom of the stereo metamaterials, our work provides a promising way for PIT metamaterial and terahertz slow light device research.