Quasi-dark Mode Research Articles

Manipulating near-field enhancement using hybrid bowtie nanoantenna for SERS

Abstract The metallic bowtie antenna has shown great potential to enhance SERS through LSPR. However, it rely heavily on narrow nanogaps to excite strong dipole-dipole interactions, which induce field enhancement effects and cause significant light scattering issues. According to theoretical analysis and numerical simulations, this study proposes a method to improve near-field enhancement in hybrid bowtie nanoantennas by exciting dark plasmon modes. Specifically, this dark plasmon mode is achieved by coupling large metal-insulator-metal (MIM) structures with small arrays of silver (Ag) nanorods in the nanogap. This quasi-dark plasmon mode not only significantly reduces scattering loss but also supports stronger near-field enhancement. MIM-BNA nanostructures with arrays of Ag nanorods exhibit a local electric field amplitude approximately eight times greater than that of a pure Ag-BNA nanoantenna with the same nanogap width. This demonstrates the great potential of the improved BNA nanoantenna for SERS applications

A polarization-insensitive dual plasmon-induced transparency terahertz sensor based on graphene metamaterial

We present a polarization-insensitive dual plasmon-induced transparency (PIT) sensor based on graphene metamaterial in the terahertz band. The double PIT window results from the coupling of the bright mode and quasi-dark modes, and the theoretical results fit well with the simulated results by using the three-particle coupling system. The sensor exhibits excellent sensing performance with sensitivity and the figure of merit (FOM) of the two transmission peaks can reach 1.3 THz/RIU and 6.259 RIU−1. The sensor is insensitive to the changes in the polarization angle, and the transmission spectra of the sensor remain essentially constant in two polarization directions for incident angles less than 60°, which reduces the impact of the experimental environment on the results. The resonant frequency of the PIT window can be turned by adjusting the Fermi level of graphene. The sensor proposed in this paper is an active advance in improving the reliability of analyte detection results.

Quasi-dark resonances with antiferromagnetic order in silicon metasurfaces

Quasi-dark resonances exhibiting antiferromagnetic order are theoretically investigated in a near-infrared metasurface composed of square slotted rings etched in a thin silicon layer on glass substrate. Access to the quasi-dark mode is achieved by reducing the symmetry of the metasurface according to the findings of a detailed group theory analysis. A thorough finite-element study reveals the key optical properties of the antiferromagnetic order quasi-dark mode, namely resonant wavelengths, quality factors, angular dispersion, and its robustness against optical extinction losses. It is demonstrated that the thickness of the silicon metasurface can adjust the asymmetry degree of the resonant Fano lineshape without affecting substantially its quality factor. Furthermore, tuning of the resonant wavelength can be achieved without significant modification of the Fano lineshape by controlling the angle of incidence of the impinging planewave. Overall, the work presents an all-dielectric, near-infrared metasurface for the excitation of sharp resonances with antiferromagnetic order, which can find use in emerging applications based on this particular configuration of artificial optical magnetism and/or strong field confinement and light-matter interaction.

A Tunable Terahertz Graphene Metamaterial Sensor Based on Dual Polarized Plasmon-Induced Transparency

We present a terahertz graphene metamaterial sensor based on dual polarized plasmon-induced transparency caused by the destructive interference between the bright mode and the two quasi-dark modes in the terahertz (THz) band. In addition, the sensor can achieve frequency modulation by adjusting the polarization direction of the THz wave and the Fermi level of graphene. Importantly, the sensor also has high sensitivity at each polarization direction, and the sensitivity of dual transmission peaks with x-polarization direction can reach about 1.1 THz/RIU (Refractive Index Unit). Furthermore, the theoretical results of the proposed three-particle model are in good agreement with the simulated transmission spectra. The sensor is also insensitive to the change of incident angles, and the transmission spectra of the sensor can remain roughly unchanged with the incident angle less than 60°, which is beneficial to the high-speed and high-sensitivity detection in a complex environment. Therefore, the proposed graphene metamaterial sensor exhibits numerous potential applications in THz biochemical sensing.

Tunable dual-band linear-to-circular polarization conversion based on the electromagnetically induced transparency utilizing the graphene metamaterial

A tunable dual-band linear-to-circular polarization conversion (LTCPC) related to electromagnetically induced transparency (EIT) is theoretically proposed by employing the graphene metamaterial in the terahertz (THz) regime. Since the electric resonance (bright mode) and the magnetic resonance (quasi-dark mode) for the x - and y -polarized (TM and TE) waves are mutually coupled, the EIT behavior is realized due to the destructive interference. Two transparent EIT windows have emerged in 1.452–1.661 THz and 1.348–1.683 THz when the TM and TE waves are incidents. The corresponding values of the maximum group delay and group index respectively are 225 ps, 162 ps, 1636, and 1177. The LTCPC is achieved at 1.432 THz and 1.676 THz when the graphene does not exist. While the graphene is introduced, the LTCPC is dynamically adjustable by controlling the Fermi energy ( E f ) of the graphene. The optimal ARs can dynamically change between 1.438 (1.689) THz and 1.452 (1.696) THz while the E f changes from 0.1 eV to 0.9 eV. A theoretical investigation of two-oscillator model further confirms the effectiveness and consistency of the simulation results. The presented metamaterial with a thickness of subwavelength and high transparency for the electromagnetic waves opens a new path to the applications in beam steering and polarization controls. • The dual-band linear-to-circular PC of EIT phenomenon can be simultaneously realized. • The structure is equipped with graphene which can adjust the EIT behavior and circular polarization features. • The presented structure is great transparency for the EM wave, and its thickness is subwavelength. • The proposed structure provides a new idea to manipulate the polarization state of the EM wave. • The device is stable for the circular polarization feature within 10°.

Field manipulation of electromagnetically induced transparency analogue in terahertz metamaterials for enhancing liquid sensing

Terahertz (THz) metamaterial sensing, an emerging technology in biomedical sciences, has made considerable progress with unique characteristics, such as label-free, non-invasive, non-destructive, and efficient. However, excellent sensing performance involves the need for extremely strong light-matter interaction that is difficult to implement for the intrinsic divergent electric field excited on the surface of metamaterials. Here, an innovative method of manipulating the electric field to realize maximumly light-matter interaction for enhancing liquid sensing at THz frequencies is reported. The electric field of electromagnetically induced transparency (EIT) resonance is tightly confined inside the structure around the quasi-dark mode by optimizing the fully-enclosed metamaterial structure design. More importantly, the electric field is enhanced and the part that permeates into the substrate is exploited, together with the formation of inartificial fluid loading units by the effective regional substrate etching, exhibiting the enhanced interaction between the polar liquids and the THz waves. The measurements reveal a significantly improve in sensitivity of 0.312 THz/RIU, which is 9.8 times that without etching by contrast. The strong light-matter interaction implements in THz metamaterial via the electric field manipulation holds bright promises for realizing highly sensitive THz sensing, forging a route for real-time supervising trace biomolecules.

Achieving polarization control by utilizing electromagnetically induced transparency based on metasurface

In this study, a design of transmissive linear-to-circular polarization conversion (LCPC) related to electromagnetically induced transparency (EIT) by arranging four sets of asymmetric resonators on a metasurface in a cross-shaped distribution is theoretically reported in the terahertz (THz) regime. It has numerically demonstrated that the bright modes and quasi-dark modes can also be mutually coupled when the x-polarized (TM) and y-polarized (TE) waves are incidents, resulting in a famous EIT phenomenon. Subsequently, when the direction of the electric fields of the electromagnetic (EM) waves are incident along the x-axis with an angle of 45°, the electric fields can be decomposed into the x- and y-directions. At this time, the corresponding amplitude and phase meet the conditions of the LCPC. Furthermore, for the TM and TE waves, the values of the maximum transmission coefficients respectively reach up to 0.910 at 1.384 THz and 0.890 at 1.277 THz. Correspondingly, two brand-new bright transparent windows are located in 1.276-1.454 THz for the TM waves and 1.206-1.373 THz for the TE waves. Concurrently, the band of the maximum 3 dB axial ratio is obtained from 1.308 THz to 1.343 THz with a relative bandwidth of 3%.

Investigating on the electromagnetically induced absorption metamaterial in the terahertz region realized by the multilayer structure

In recent years, electromagnetically induced absorption (EIA) has had potential application value in nonlinear optics and other fields, which attracts more and more attention. Based on the bright-dark-quasi-dark theory, an electromagnetically induced transparency (EIT) metamaterial is proposed which realizes the transition from EIT to EIA by adjusting the coupling distance. At the same time, a substantial enhancement in absorption has been achieved by adding four nested split-ring resonators (SRRs) as the quasi dark modes which are obtained by rotating the first nested split resonator 90° in turn, and the absorption peak can reach 0.8969 at 0.412 THz. The study proves that the inter-layer coupling causes magnetic resonances which could lead to enhanced absorption. The structure also has polarization-insensitive characteristics. The influences of different structural parameters on absorption performance are further discussed in this paper. This absorption enhancement method has universal applicability and has huge potential application value in many fields such as optical signal processing, optical storage, quantum switching, and optical sensing, which also can be extended to microwave and infrared bands.

Ultrafast all-optical switching of dual-band plasmon-induced transparency in terahertz metamaterials

An active ultrafast formation and modulation of dual-band plasmon-induced transparency (PIT) effect is theoretically and experimentally studied in a novel metaphotonic device operating in the terahertz regime, for the first time, to the best of our knowledge. Specifically, we designed and fabricated a triatomic metamaterial hybridized with silicon islands following a newly proposed modulating mechanism. In this mechanism, a localized surface plasmon resonance is induced by the broken symmetry of a C2 structure, acting as the quasi-dark mode. Excited by exterior laser pumps, the photo-induced carriers in silicon promote the quasi-dark mode, which shields the near-field coupling between the dark mode and bright mode supported by the triatomic metamaterial, leading to the dynamical modulation of terahertz waves from individual-band into dual-band PIT effects, with a decay constant of 493 ps. Moreover, a remarkable slow light effect occurs in the modulating process, accompanied by the dual-transparent windows. The dynamical switching technique of the dual-band PIT effect introduced in this work highlights the potential usefulness of this metaphotonic device in optical information processing and communication, including multi-frequency filtering, tunable sensors, and optical storage.

The slow electromagnetic wave effect induced by the interaction of dark and quasi-dark modes in microwave metamaterials

The slow electromagnetic wave effect induced by the interaction of dark and quasi-dark modes in microwave metamaterials

Quasi-Dark Resonances in Silicon Metasurface for Refractometric Sensing and Tunable Notch Filtering

A silicon metasurface with a symmetry-protected quasi-dark resonant mode is designed and demonstrated as a refractometric sensor and tunable notch filter in the near-infrared spectrum. The resonant mode is excited by perturbing a periodic array of Si cuboids patterned on a glass substrate. Thanks to the delocalized nature of the resonant mode, intense light-matter interaction with the overlayer material is manifested. Such interaction leads to strong dependence of the resonant wavelength on the overlayer refractive index and high achievable quality factors, including the case of lossy materials. High sensitivity and figure of merit is calculated for operation of the metasurface as a refractometric sensor. In a complementary approach, three types of tunable notch filters are demonstrated, based on the use of thermo-optic, electro-optic polymers, and nematic liquid crystals.

Tunable dual plasmon-induced transparency based on a monolayer graphene metamaterial and its terahertz sensing performance.

In this paper, tunable dual plasmon-induced transparency (PIT) is achieved by using a monolayer graphene metamaterial in the terahertz region, which consists of two graphene strips of different sizes and a graphene ring. As the dual PIT effect is induced by the destructive interference between the two quasi-dark modes and the bright mode, we propose a four-level plasmonic system based on the linearly coupled Lorentzian oscillators to explain the mechanism behind the dual PIT. It is proved that the theoretical results agree well with the simulation results. Most importantly, the sensing properties of the designed device have been investigated in detail and we found that it can exhibit high sensitivities and figure of merit (FOM). Furthermore, the dual PIT windows can be effectively modulated by changing the Fermi energy of the graphene layer and the angle of incidence. Thus, the proposed graphene-based metamaterial can hold wide applications for switches, modulators, and multi-band refractive index sensors in the terahertz region.

Actively mode tunable electromagnetically induced transparency in a polarization-dependent terahertz metamaterial

Electromagnetically induced transparency (EIT) is a quantum destructive interference phenomenon in three-level atomic systems, which can slow down the light velocity and has application prospects in information storage and processing. However, the EIT effect in atomic systems requires harsh experimental conditions. This problem can be solved by employing an EIT metamaterial, where destructive interference occurs between a bright mode and a dark mode or a quasi-dark mode, inducing a transparency window accompanied by the slow light effect. Here, we propose an actively mode tunable electromagnetically induced transparency terahertz metamaterial, which is comprised of a T-type resonator, a split-ring resonator (SRR), and coupled split-ring resonators (CSRRs). When the external electric field is vertical to the gap of the SRR (x-polarization), there is a single EIT mode accompanied by one slow light wave packet. On the other hand, when the external electric field is parallel to the gap of the SRR (y-polarization), there are two EIT modes accompanied by two slow light wave packets. Therefore, an active switch from a single EIT mode to dual EIT modes controlled by changing the polarization is demonstrated, which can find explanation from the electric field intensity distributions. This work offers a strategy to realize the mode tunable EIT, which may achieve potential applications in active filters, modulators, and slow light devices.

Further enhancement of the near-field on Au nanogap dimers using quasi-dark plasmon modes

Metallic nanogap dimers are extremely useful for enhancing surface-enhanced Raman scattering and various nonlinear optical effects employing near-field enhancement effects induced by the localized surface plasmon resonance. However, the metallic nanogap dimers exhibit an intense light scattering due to the strong dipole-dipole interaction between two metallic nanostructures and, therefore, are not necessarily a structural design that exhibits the highest near-field enhancement due to the radiation loss. Here, we propose further enhancement of the near-field on metallic nanogap dimers using quasi-dark plasmon modes. By coupling with gold (Au) nanorods having the same plasmon resonant wavelength, but completely different sizes, a quasi-dark plasmon mode, which reduces the radiation loss slightly, is induced, resulting in the elongation of the plasmon dephasing time. As a result, the signal of surface-enhanced Raman scattering of crystal violet molecules adsorbed on the Au nanogap dimer is enhanced up to about three times as compared to that measured using the Au nanogap dimer without the Au nanorods. Scattering spectrum measurements as well as electromagnetic simulations were performed to clarify the mechanism for further enhancement of the near-field. The proposed coupled plasmonic system is expected to be advantageous, especially in enhancing nonlinear optical effects using plasmonic enhancement effects.

Electromagnetic induced transparency in graphene waveguide structure for Terahertz application

Electromagnetic Induced transparency (EIT) has been considered as a possible manner to implement plasmonic devices for slow light and sensing applications. In this paper, we proposed a novel design of graphene waveguide structure, which provides a tunable EIT response. Numerical results reveal that the plasmon induced transparency spectral response occurs due to the interference between quasi-dark mode and quasi-bright mode which exist in the loaded graphene resonator of the device. The EIT peak frequency can be tuned by changing the Fermi level of the graphene. An elaborate analysis of the relationship between the transparency frequency and graphene’s Fermi level, in turn, proved the two-dimensional nature of the graphene plasmon. Compared with previous designs, the proposed design can work in the THz frequency and provides a much sharper EIT window which is quite useful for sensing applications.

Eigenmode hybridization enables lattice-induced transparency in symmetric terahertz metasurfaces for slow light applications

Traditional lattice-induced transparency demonstrations are activated by varying the meta-atom lattice constant such that the plasmonic and lattice modes interfere. Here we report on the observation of enhanced coupling between two eigenmodes (first- and second-order) by varying the lattice parameter in a symmetric split ring resonator design. The higher-order quasi-dark mode blueshifts, introducing strong coupling with the fundamental bright mode for periods above 345 μm. Numerical simulations are verified experimentally and supplemented with a two-oscillator model showing good agreement. Furthermore, larger positive group delay values are achieved in the vicinity of the transparency window with minimal absorption, indicating a strong potential for slow light applications.

Comparing Q-factor of electromagnetically induced transparency based on different space distribution quasi-dark mode resonator

Comparing the Q-factor of electromagnetically induced transparency based on a symmetric and asymmetric quasi-dark resonator has been numerically and experimentally demonstrated. The results show that the Q-factor of electromagnetically induced transparency based on the symmetric quasi-dark mode resonator is an order of magnitude larger than that based on the asymmetric quasi-dark mode resonator. The reason for the improved Q-factor is ascribed to the low radiative loss of the symmetric quasi-dark mode resonator. Furthermore, the proposed way holds promise to obtain a different Q-factor of electromagnetically induced transparency in the microwave, terahertz, and optical frequency range.

Tailoring polarization of electromagnetically induced transparency based on non-centrosymmetric metasurfaces

In this manuscript, tailoring polarization of analogy of electromagnetically induced transparency (EIT-like) based on non-centrosymmetric metasurfaces has been numerically and experimentally demonstrated. The EIT-like metamaterial is composed of a rectangle ring and two cut wires. The rectangle ring and the cut wire are chosen as the bright mode and the quasi-dark mode, respectively. Under the incident electromagnetic wave excitation, a polarization insensitive EIT-like transmission window can be observed at specific polarization angles. Within the transmission window, the phase steeply changes, which leads to the large group index. Tailoring polarization of EIT-like metamaterial with large group index at specific polarization angles may have potential application in slow light devices.

Quantum optomechanical straight-twin engine

We propose a realization of a quantum heat engine in a hybrid microwave-optomechanical system that is the analog of a classical straight-twin engine. It exploits a pair of polariton modes that operate as out-of-phase quantum Otto cycles. A third polariton mode that is essential in the coupling of the optical and microwave fields is maintained in a quasidark mode to suppress disturbances from the mechanical noise. We also find that the fluctuations in the contributions to the total work from the two polariton modes are characterized by quantum correlations that generally lead to a reduction in the extractable work compared to its classical version.

Deep-subwavelength lithography via graphene plasmons

We propose a realization of a quantum heat engine in a hybrid microwave-optomechanical system that is the analog of a classical straight-twin engine. It exploits a pair of polariton modes that operate as out-of-phase quantum Otto cycles. A third polariton mode that is essential in the coupling of the optical and microwave fields is maintained in a quasi-dark mode to suppress disturbances from the mechanical noise. We also find that the fluctuations in the contributions to the total work from the two polariton modes are characterized by quantum correlations that generally lead to a reduction in the extractable work compared to its classical version.