Plasmon-induced Transparency Resonance Research Articles

High sensitivity terahertz biomedical sensing with graphene metamaterial

Identification of biomedical molecules by terahertz (THz) sensing is a quite promising noninvasive technique. Here, we present a THz graphene metamaterial with plasmon-induced transparency (PIT) for high sensitivity biosensing. The strong PIT effect is generated by the destructive interference between the plasmon modes excited by two graphene strips. For testing different biomedical analytes, a distinct frequency shift at PIT resonance can be observed due to the effect of surrounding refractive index variation. The metamaterial scheme for noninvasive biomedical sensing can achieve the sensitivity as high as 2.6 THz/RIU. Particular, it can not only detect the phase of red blood cell infected by malaria but also the concentration of ethanol aqueous solution. The underlying mechanism is discussed for understanding the THz sensing operation.

Bound States in the Continuum and Induced Resonances in a Simple Plasmonic Waveguide with Sensing Application

A Friedrich–Wintgen bound state in the continuum (FW-BIC) is of particular interest in the field of wave physics phenomena. It is induced via the destructive interference of two modes that belong to the same cavity. In this work, we analytically and numerically show the existence of FW-BIC in a T-shaped cavity composed of a stub of length d0 and two lateral branches of lengths d1 and d2, attached to an infinite waveguide. The whole system consists of metal–insulator–metal (MIM) plasmonic waveguides that operate in the telecommunication range. Theoretically, when d1 and d2 are commensurated, BIC is induced by these two branches. This latter is independent of d0 and the infinite waveguide, where the T structure is grafted. By breaking the BIC condition, we obtain a plasmon-induced transparency (PIT) resonance. The PIT resonance’s sensitivity to the dielectric material of the waveguide may be exploited to design a sensitive nanosensor suitable for sensing platforms, thanks to its very small footprint. A sensitivity of 1400 nm/RIU and a resolution of 1.86×10−2 RIU showed a high level of performance that the designed structure achieved. Moreover, this structure could also be used as a biosensor, in which we have studied the detection of the concentration in the human body, such as Na+, K+, and glucose solutions, and these sensitivities can reach 0.21, 0.28, and 1.74 nm dL/mg, respectively. Our designed structure advances with technology and has good application prospects, working as a biosensor to detect the blood’s hemoglobin level. The analytical results, obtained via Green’s function method, are validated via numerical simulations using Comsol Multiphysics software based on the finite element method.

Tunable terahertz slow light with hybrid coupling of a magnetic toroidal and electric dipole metasurface

We present a hybrid coupling scheme of a magnetic toroidal and electric dipole metasurface with suppressed radiation loss, which can produce the tunable plasmon-induced transparency (PIT) with an enhanced slow-light effect in the terahertz regime. The terahertz metasurface is constructed by nesting a dual-split ring resonator (DSRR) inside a ring resonator (RR) to exploit the destructive coherence of hybrid electromagnetic mode coupling at the PIT resonance. The polarization-dependence excitation performs the active tunability of a PIT-induced group slowing down by rotating the polarization angle, experimentally achieving a maximum group delay of 3.5 ps. Furthermore, the modified terahertz metasurface with a four-split ring resonator (FSRR) nested in an RR is prepared on photoconductive silicon, demonstrating the pump-controllable group delay effect at the PIT resonance. The large group delay from 2.2 to 0.9 ps is dynamically tunable by adjusting the pump power. The experimental results are in good accord with the theoretical simulations.

Graphene-enhanced hybrid terahertz metasurface sensor for ultrasensitive nortriptyline sensing and detection.

Graphene is a two-dimensional material with unique physical and chemical properties, whose excellent biocompatibility has also attracted widespread attention in the field of biosensing and medical detection. Graphene provides a novel solution for dramatically improving the sensitivity of terahertz metasurface sensors, since the electrical conductivity can be modified by contact with biomolecules. In this paper, a metal-graphene hybrid metasurface is proposed and demonstrated for high-sensitive nortriptyline sensing based on the plasmon-induced transparency (PIT) resonances. The π-π stacks between nortriptyline and graphene lead to an increase in the Fermi level of graphene and a decrease in the conductivity, thus enhancing the PIT resonance. Experimental results show that the peak-to-peak amplitude magnitude of the PIT window is enhanced up to 3.4-fold with 1 ng nortriptyline analyte, and the minimum detection limit is extended down to 0.1 ng. But no significant change is observed from the samples without graphene as a comparative experiment, which demonstrates that the presence of graphene greatly enhances the bonding to the drug molecules and improves the sensing sensitivity. This metasurface sensor has the advantages of high sensitivity, fast detection speed, label-free and steady properties, which has potential applications in the fields of trace molecular sensing and disease diagnosis.

Design Optimization and Fabrication of Graphene/J-Aggregate Kretschmann-Raether Devices for Refractive Index Sensing Using Plasmon-Induced Transparency Phenomena

Here, a novel plasmon-induced transparency (PIT) sensing platform based on a Kretschmann–Raether configuration with graphene/J-aggregate materials is proposed. The J-aggregate material, despite its dielectric optical properties, can strongly confine the surface wave-like metal layers. These features promise to highly enlarge the range of plasmonic sensing devices. Therefore, the sensing parameters have been numerically and experimentally investigated using the finite-difference time-domain (FDTD) method and atomic force microscopy (AFM). The results show that the PIT resonance of the structure has a sharp reflection, in turn, leads to high sensitivity. To deep benchmark the structure, the effects of the structural parameters and environmental variables such as temperature and magnetic field on the sensing properties of the device are analyzed in detail. The maximum sensitivity is obtained as high as 1400 angle per refractive-index unit (RIU) with an extra high figure of merit of 36 RIU−1 around the PIT resonance angle of 53°. By considering the magnetic field of 0.01 T and graphene chemical potential of μ = 0.4 eV and environmental room temperature, the proposed structure may potentially be applied in advanced off-chip PIT sensors.

Tunable Plasmon-Induced-Transparency Effect in a Simple Planar Composite Structure

A planar metamaterial structure has been designed to obtain surface plasmon-induced-transparency (PIT) effect, which is composed of a nanodisk sandwiched between double rods. From theoretical perspective, the three-level plasmonic system has been utilized to analyze the near-field coupling mechanism. Besides, the simulation results show that PIT resonance can be tailored by structure parameters. Particularly, when the length of the rods is more than twice of the diameter of the disk, two PIT windows are generated simultaneously, based on the coupling between the bright mode of the nanodisk and two multipole dark modes of rods. Furthermore, a metamaterial structure, composed by a disk and two pairs of nanorods, is suggested to achieve the polarization-independent plasmon-induced-transparency effect. As a result, this work shows great application prospect in the area of compact optical devices, such as multiband tunable filters, plasmonic switches, and slow light devices.

Mechanical control of terahertz plasmon-induced transparency in single/double-layer stretchable metamaterial

Active control of metamaterial properties in stretchable devices is crucial for advanced terahertz (THz) applications, involving large mechanical deformation or stretching. Single/double-layer mechanically tunable THz metamaterial devices based on the polymer polydimethylsiloxane are proposed and experimentally studied herein. The plasmon-induced transparency (PIT) effect is observed in the transmittance spectrum of the single-layer metamaterial, originating from the near-field coupling of two bright modes. Similarly, a bright-dark-coupled PIT system is obtained in the double-layer metamaterial. Furthermore, an external mechanical stretching could exert a switch effect on the sample. The PIT resonances of the single/double-layer metamaterials disappear when the stretching ratio is further increased, as the increased distance between the resonators blocks the coupling effect. The proposed design enriches the approaches to the modulation of the sensitive PIT effect and broadens the applications of stretchable THz equipment.

Design and verification a six-band plasmonic metasurface based on plasmon-induced transparency

Abstract In this paper, a metasurface base on plasmon-induced transparency (PIT) effect is verified and measured in 8–54 THz. This metasurface achieves six transmission peaks based on PIT resonances at resonance frequencies: 21.24 THz, 27.96 THz, 32.44 THz, 37.62 THz, 46.1 THz, and 51.12 THz, respectively. Phases near zero and positive group delays of transmission peaks result in slow light effects. In the first group experiments, structural parameter W1 decrease from 18 μ m to 14 μ m , transmission peaks move to lower frequencies and amplitudes enhance. In the second group experiments, structural parameter W2 also reduce, similar resonance behaviors obtain. This proposed PIT-based metasurface demonstrates the potential for applications in sensing, optical modulation, and slow-light devices.

Tunable and Polarization-Independent Plasmon-Induced Transparency in a Fourfold Symmetric Metal-Graphene Terahertz Metamaterial

The introduction of graphene into metamaterials allows for more flexible and convenient control of electromagnetic waves. In this paper, one simple plasmon-induced transparency (PIT) structure with tunability and polarization independence is investigated in the terahertz (THz) regime. The simulation results indicate that the transparent window can be manipulated in a wide range and even switched off by merely changing the Fermi energy of graphene. By continuously altering the resonance intensity of the dark resonator using the graphene, the PIT resonance can be actively manipulated. The behavior can be elucidated by the classical coupled two-particle model, which corresponds well to the simulation results. Owing to the fourfold symmetric structure, the proposed PIT device exhibits polarization-independent characteristics. This work provides design guidance for metal-graphene THz modulators.

Polarization-Insensitive Ultra-Narrow Plasmon-Induced Transparency and Short-range Surface Plasmon Polariton Bloch Wave in Ultra-thin Metallic Film Nanostructures

We demonstrate that polarization-insensitive ultra-narrow double plasmon-induced transparency (PIT) can be achieved in the thin-metal-film nanostructures. Ultra-narrow PIT resonance with a bandwidth of 2.5 nm at central wavelength of 768 nm was obtained (~ 1/307 of the peak wavelength). Multispectral PIT can be obtained, and its linewidth and the “on-off” state of the PIT peaks in the system can be adjusted by the metallic nanostructure thickness and particularly the waveguide layer thickness. The narrow transparency window is exhibited to be generated by the coupling and interference between a delocalized hybrid-waveguide photonics mode and the localized surface plasmon (LSP) mode. The angular-dependent dispersion of the system with double PITs is examined and investigated, where the high-order short-range surface plasmon polariton (SRSPP) Bloch waves excited for large incidence angles are indicated and revealed. The multispectral PITs with high quality in the large-area plasmonic system are promising for practical and compact nanophotonics applications such as optical buffers, ultra-sensitive sensor, plasmonic filters, and switches.

Multispectral Plasmon Induced Transparency in a Defective Metasurface Plasmonic Nanostructure

The investigation of plasmon induced transparency (PIT) resonances is performed in a system based on defective and complete silver nanostrips. The response of metasurface systems to electromagnetic fields under normal incidence is endowed by unique features, including some exquisite spatial phenomena. These phenomena are embodied in confined light-matter hybrid modes where the defective silver nanostrip excites superradiant dipole and quadrupole modes, resulting in a PIT window. Moreover, multispectral PIT resonances are observed, which involves quadrupole mode as superradiant to excite the proximity nanostrip. It can reveal much about the defective metasurface structure, such as exact tuning of the gap distance between these two nanostrips, variation in defect parameters and modulating in electric polarization angles of excitation. Still, our research realizes multiple PIT resonances in one nanostructure, which may have interesting applications of plasmonic switching devices in highly integrated circuits.

Tunable plasmon-induced transparency based on graphene nanoring coupling with graphene nanostrips

We numerically and theoretically demonstrate a plasmon-induced transparency (PIT) at the mid-infrared region with finite-difference time-domain method. The system consists of an optically bright dipole mode and a dark quadrupole mode, which are supported by the graphene nanoring and graphene nanostrips, respectively. The coupling between the two modes introduces transparency window and large group delays. The pronounced PIT resonance can be easily modified by adjusting the geometric parameters and the Fermi level of graphene nanostructure. Our results suggest that the demonstrated PIT effect may be applicated in the slow-light device, active plasmonic switching, and optical sensing.

Broadband Plasmon-Induced Transparency in Plasmonic Metasurfaces Based on Bright-Dark-Bright Mode Coupling

Plasmon-induced transparency (PIT), an analog of electromagnetically induced transparency, originates from destructive interference of plasmonic resonators with different quality factors and brings about the extreme dispersion within the narrow transparency window, promising remarkable potential for slow light, nonlinear optics and biochemical sensors. However, sometimes a broad transmission frequency band is more desirable for other applications such as bandpass filters. In general, strong coupling between bright and dark plasmon modes in coupled resonant systems leads to wide transparency bandwidth at the PIT resonance. Based on multi-oscillator coupling theory, a metasurface structure consisting of three perpendicularly connected metallic nanobars is purposefully designed and numerically demonstrated to support broadband PIT spectral response. The near-field patterns indicate that the broadening of the transparent band results from the constructive interference of dual excitations of the single non-radiative (dark) resonator by the two radiative (bright) antennas. These results show that this scheme of bright-dark-bright mode coupling is significantly beneficial for designing filters operating over a broad frequency range.

Active Multiple Plasmon-Induced Transparency with Graphene Sheets Resonators in Mid-Infrared Frequencies

A multiple plasmon-induced transparency (PIT) device operated in the mid-infrared region has been proposed. The designed model is comprised of one graphene ribbon as main waveguide and two narrow graphene sheets resonators. The phase coupling between two graphene resonators has been investigated. The multimode PIT resonances have been found in both cases and can be dynamically tuned via varying the chemical potential of graphene resonators without optimizing its geometric parameters. In addition, this structure can get multiple PIT effect by equipping extra two sheets on the symmetric positions of graphene waveguide. The simulation results based on finite element method (FEM) are in good agreement with the resonance theory. This work may pave new way for graphene-based thermal plasmonic devices applications.

Broadband plasmon-induced transparency in terahertz metamaterials via constructive interference of electric and magnetic couplings.

Plasmon-induced transparency (PIT) is a result of destructive interference of different plasmonic resonators. Due to the extreme dispersion within the narrow transparency window, PIT metamaterials are utilized to realize slow light and nonlinear effect. However, other applications such as broadband filtering more desire a broad transmission frequency band at the PIT resonance. In this paper, a broadband PIT effect is demonstrated theoretically in a planar terahertz metamaterial, consisting of a U-shaped ring (USR) supporting electric and magnetic dipole modes as the bright resonator and a cut wire pair (CWP) possessing planar electric quadrupole and magnetic dipole modes as the dark resonator. The dark resonant modes of the CWP can be excited simultaneously via near-field by both the electric and magnetic dipole modes of the USR. When the electric as well as magnetic excitation pathways constructively interact with each other, the enhanced near-field coupling between bright and dark resonators gives rise to an ultra-broad transparency window across a frequency range greater than 0.61 THz in the transmittance spectrum.

Plasmon-Induced Transparency and Dispersionless Slow Light in a Novel Metamaterial

We propose a novel metamaterial structure to realize dispersionless slow light based on plasmon-induced transparency (PIT) at optical frequency. Simply by breaking the symmetry of the metamaterial structure, two electric excitation coupling to the magnetic resonance (EEMR) modes generate and interplay with each other resulting the formation of the PIT resonance. Due to the hybridization of the two EEMR modes, an ultrabroad flat-band as large as 15 THz with average group index of $\sim 5$ is obtained. The corresponding normalized delay-bandwidth product is 0.475, which indicates the prominent feature in light slowing down without distortion. The proposed metamaterial structure is promising for practical information storage, optical memory, and optical buffers design.

Multispectral plasmon-induced transparency in triangle and nanorod(s) hybrid nanostructures

We theoretically investigate the optical properties of a hybrid nanostructure consisting of one triangle and one nanorod. A plasmon-induced transparency (PIT) resonance appears in the transmission spectrum, which is ascribed to the induced multipole plasmon mode of the nanorod. Multispectral PIT resonances are observed, when two or more nanorods are put in proximity to the triangle. It is proved that the combined effects of the induced multipole plasmon mode of nanorods and the cavity resonant mode contribute to these PIT peaks. The tunability of PIT on the geometrical parameters is also presented.