Dynamically controllable plasmon induced transparency based on hybrid metal-graphene metamaterials

Ultrasensitive optical modulation in hybrid metal-perovskite and metal-graphene metasurface THz devices

Implementation of efficient terahertz (THz) wave control is essential for THz technology development for applications including sixth-generation communications and THz sensing. Therefore, realization of tunable THz devices with large-scale intensity modulation capabilities is highly desirable. By integrating perovskite and graphene with a metallic asymmetric metasurface, two ultrasensitive devices for dynamic THz wave manipulation through low-power optical excitation are demonstrated experimentally here. The perovskite-based hybrid metadevice offers ultrasensitive modulation with a maximum modulation depth for the transmission amplitude reaching 190.2% at the low optical pump power of 5.90 mW/cm². Additionally, a maximum modulation depth of 227.11% is achieved in the graphene-based hybrid metadevice at a power density of 18.87 mW/cm². This work paves the way toward design and development of ultrasensitive devices for optical modulation of THz waves.

Actively Controllable Terahertz Metal-Graphene Metamaterial Based on Electromagnetically Induced Transparency Effect

A metal-graphene metamaterial device exhibiting a tunable, electromagnetically induced transparency (EIT) spectral response at terahertz frequencies is investigated. The metamaterial structure is composed of a strip and a ring resonator, which serve as the bright and dark mode to induce the EIT effect. By employing the variable conductivity of graphene to dampen the dark resonator, the response frequency of the device shifts dynamically over 100 GHz, which satisfies the convenient post-fabrication tunability requirement. The slow-light behavior of the proposed device is also analyzed with the maximum group delay of 1.2 ps. The sensing performance is lastly studied and the sensitivity can reach up to 100 GHz/(RIU), with a figure of merit (FOM) value exceeding 4 RIU-1. Therefore, the graphene-based metamaterial provides a new miniaturized platform to facilitate the development of terahertz modulators, sensors, and slow-light applications.

Constant frequency reconfigurable terahertz metasurface based on tunable electromagnetically induced transparency-like approach

A terahertz constant frequency reconfigurable metasurface based on tunable electromagnetically induced transparency (EIT)-like property was designed, whose transparency window frequency did not vary with Fermi energy. This structure was composed of two single-layer graphene resonators, namely, left double big rings and right double small rings. An evident transparency window (EIT-like phenomenon) was caused by the near-field coupling between bright modes of the two resonators in the transmission spectrum, in which amplitude over 80% was acquired at 1.98 THz. By individually reconfiguring the Fermi energy of each resonator, the EIT-like effects, transparency window amplitude, modulation speed and group delay could be actively controlled while the frequency of EIT-like window remained constant. Significantly, the transparency window was fully modulated without changing the frequency, and the maximum modulation depth reached 78%. Furthermore, the modulation speed also increased because the total graphene areaAwas effectively reduced in the proposed structure. Compared with other metasurface structures, the modulation properties of the proposed structure showed higher performance while the EIT-like window frequency remained static. This research provides an alternative method for developing constant frequency reconfigurable modulation terahertz devices (such as optical switches and modulators), as well as a potential approach for miniaturization of terahertz devices.

Broadband Filter and Adjustable Extinction Ratio Modulator Based on Metal-Graphene Hybrid Metamaterials

A novel multifunctional device based on a hybrid metal-graphene Electromagnetically induced transparency (EIT) metamaterial at the terahertz band is proposed. It is composed of a parallel cut wire pair (PCWP) that serves as a dark mode resonator, a vertical cut wire pair (VCWP) that serves as a bright mode resonator and a graphene ribbon that serves as a modulator. An ultra-broadband transmission window with 1.23 THz bandwidth can be obtained. The spectral extinction ratio can be tuned from 26% to 98% by changing the Fermi level of the graphene. Compared with previous work, our work has superior performance in the adjustable bandwidth of the transmission window without changing the structure of the dark and bright mode resonators, and has a high extinction ratio and dynamic adjustability. Besides, we present the specific application of the device in filters and optical modules. Therefore, we believe that such a metamaterial structure provides a new way to actively control EIT-like, which has promising applications in broadband optical filters and photoelectric intensity modulators in terahertz communications.

Reconfigurable Meta-Coupler Employing Hybrid Metal-Graphene Metasurfaces

Efficient excitation of surface wave (SW) remains one of the most challenging considerations in the photonics and plasmonics areas. Inspired by recent investigations of metasurfaces, we propose a hybrid metal-graphene transmitarray converting incident propagating wave (PW) to SW, as a solution for SW excitations-a meta-coupler. The structure comprises ultra-thin four-layer transparent metasurfaces in which H-shaped etched metal films together with graphene patches are employed, and also all four layers are identical. Full-wave simulations demonstrate that the suggested meta-coupler possesses an efficiency of 46% and a directivity of 19 dB, which is promising in the terahertz (THz) range. At the same time, in light of unique graphene characteristics, the proposed device is tunable and easily reconfigurable, i.e., the direction of converted SWs can be electrically switched from right to left and vice versa. We believe that this system responds to emerging applications such as THz communications and sensing, and furthermore the employed architecture introduce electrostatically tunable building blocks being able to develop graphene plasmonic components effectively.

Fano-Resonance in Hybrid Metal-Graphene Metamaterial and Its Application as Mid-Infrared Plasmonic Sensor

Fano resonances in nanostructures have attracted widespread research interests in the past few years for their potential applications in sensing, switching and nonlinear optics. In this paper, a mid-infrared Fano resonance in a hybrid metal-graphene metamaterial is studied. The hybrid metamaterial consists of a metallic grid enclosing with graphene nanodisks. The Fano resonance arises from the coupling of graphene and metallic plasmonic resonances and it is sharper than plasmonic resonances in pure graphene nanostructures. The resonance strength can be enhanced by increasing the number of graphene layers. The proposed metamaterial can be employed as a high-performance mid-infrared plasmonic sensor with an unprecedented sensitivity of about 7.93 μ m/RIU and figure of merit (FOM) of about 158 . 7 .

Plasmon-Induced Transparency in an Asymmetric Bowtie Structure

Plasmon-induced transparency is an efficient way to mimic electromagnetically induced transparency, which can eliminate the opaque effect of medium to the propagating electromagnetic wave. We proposed an aperture-side-coupled asymmetric bowtie structure to realize on-chip plasmon-induced transparency in optical communications band. The plasmon-induced transparency results from the strong coupling between the detuned bowtie triangular resonators. Either of the resonator works as a Fabry-Perot cavity with compact dimensions. The transparent peak wavelength can be easily controlled due to its strong linear relation with the resonator height. The ratio of absorption valley to the transparent peak can be more than 10 dB. Moreover, with excellent linearity of shifting wavelength to sensing material index, the device has great sensing performance and immunity to the structure deviations.

Hybrid Metal Graphene-Based Tunable Plasmon-Induced Transparency in Terahertz Metasurface

In this paper, we look at the work of a classical plasmon-induced transparency (PIT) based on metasurface, including a periodic lattice with a cut wire (CW) and a pair of symmetry split ring resonators (SSR). Destructive interference of the 'bright-dark' mode originated from the CW and a pair of SSRs and resulted in a pronounced transparency peak at 1.148 THz, with 85% spectral contrast ratio. In the simulation, the effects of the relative distance between the CW and the SSR pair resonator, as well as the vertical distance of the split gap, on the coupling strength of the PIT effect, have been investigated. Furthermore, we introduce a continuous graphene strip monolayer into the metamaterial and by manipulating the Fermi level of the graphene we see a complete modulation of the amplitude and line shape of the PIT transparency peak. The near-field couplings in the relative mode resonators are quantitatively understood by coupled harmonic oscillator model, which indicates that the modulation of the PIT effect result from the variation of the damping rate in the dark mode. The transmitted electric field distributions with polarization vector clearly confirmed this conclusion. Finally, a group delay t g of 5.4 ps within the transparency window is achieved. We believe that this design has practical applications in terahertz (THz) functional devices and slow light devices.

Efficient terahertz transmission modulation in plasmonic metallic slits by a graphene ribbon array

Extraordinary optical transmission is the widely known phenomenon of enhanced transmission of waves through subwavelength periodic metallic apertures. Here, we propose efficient terahertz transmission modulation in subwavelength metallic slits by a graphene plasmonic ribbon array. The extraordinary optical transmission through the subwavelength metallic slits can be tuned by coupling with the plasmonic resonance of graphene ribbon array, resulting in a deep transmittance depression. Numerical simulations show that maximal transmission modulation depth of 98.99% can be obtained at 1.03 THz via this mechanism.

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.

Active control of light slowing enabled by coupling electromagnetic metamaterials with low-lossy graphene

We theoretically investigate dynamical control of light slowing within the induced transparent window, e.g., in near-infrared frequencies, in electromagnetically induced transparent (EIT) metamaterials loaded by low-lossy graphene. Coupling with graphene enables distinctive optical responses of the "bright" and "dark" resonators in EIT metamaterials, rendering a switching of the transparent window and a modulation on light dispersion. Optical performance of the transparent window manifests continuous tuning as the graphene doping level changes. We show that the active modulation on optical properties of the transparent window enabled by low-lossy graphene is distinctive either by passively adjusting the interspacing between the building blocks of EIT metamaterials, or active tuning by high-lossy graphene. Furthermore, we report that the group refractive index can be in situ tuned dynamically over a broad range, e.g., ∼2 orders for near-infrared frequencies, together with absorption maintained at a level similar to that of the unloaded structure. Our study offers new possibilities towards chip-scale devices, such as active optical switching, filtering, and data storing.

Approaching perfect absorption of monolayer molybdenum disulfide at visible wavelengths using critical coupling

A simple perfect absorption structure is proposed to achieve the high efficiency light absorption of monolayer molybdenum disulfide (MoS₂) by the critical coupling mechanism of guided resonances. The results of numerical simulation and theoretical analysis show that the light absorption in this atomically thin layer can be as high as 98.3% at the visible wavelengths, which is over 12 times more than that of a bare monolayer MoS₂. In addition, the operating wavelength can be tuned flexibly by adjusting the radius of the air hole and the thickness of the dielectric layers, which is of great practical significance to improve the efficiency and selectivity of the absorption in monolayer MoS₂. The novel idea of using critical coupling to enhance the light-MoS₂ interaction can be also adopted in other atomically thin materials. The meaningful improvement and tunability of the absorption in monolayer MoS₂ provides a good prospect for the realization of high-performance MoS₂-based optoelectronic applications, such as photodetection and photoluminescence.