Dark Mode Resonators Research Articles

Engineering of the Fano resonance spectral response with non-Hermitian metasurfaces by navigating between exceptional point and bound states in the continuum conditions.

We address the engineering of Fano resonances and metasurfaces, by placing it in the general context of open non-Hermitian systems composed of coupled antenna-type resonators. We show that eigenfrequency solutions obtained for a particular case of scattering matrix are general and valid for arbitrary antenna radiative rates, thanks to an appropriate transformation of parametric space by simple linear expansion and rotation. We provide evidence that Parity-Time symmetry phase transition path and bound states in continuum (BIC) path represent the natural axis of universal scattering matrix solutions in this parametric coupling-detuning plane and determine the main characteristics of Fano resonance. Specifically, we demonstrate the control of asymmetry and sharpness of Fano resonance through navigation between BIC and PT-symmetric phase transition exceptional point. In particular, we demonstrate a fully symmetric Fano resonance in a system of two coupled bright and dark mode resonators. This result goes beyond current wisdom on this topic and demonstrates the universality of scattering matrix eigenfrequency solutions highlighted in our study. The validity of our approach is corroborated through comparison with experimental and full 3D numerical simulations results published in the literature making it thus possible to grasp a large body of experimental work carried out in this field. The detrimental impact of absorption losses on the contrast of the Fano resonance, which must be two orders of magnitude lower than the radiative losses, is also evidenced.

All-dielectric Si metamaterials with electromagnetically induced transparency and strong gap-mode electric field enhancement

We designed all-dielectric π-shaped Si metamaterials and numerically demonstrated their electromagnetically induced transparency (EIT) behavior. Each unit of the metamaterial comprises one horizontal Si nanobar and two vertical Si nanobars with feed-nanogaps between them. A narrow EIT peak with a transmittance of 99% and a Q-factor of 2436 is observed due to the destructive interference between the horizontal bright and vertical dark mode resonators. The EIT can be tunable by changing the nanogap width, and the narrowest EIT window with the largest Q-factor is obtained as the nanogap width is 20 nm. Strong electric field enhancement exceeding 104 in magnitude is also realized inside the 20-nm nanogap. Theoretical fittings according to the coupled Lorentz oscillator model exhibit a well consistency with the simulation spectra, validating the accuracy of the numerical simulation. The designed metamaterial is sensitive to the surround refractive index change with a figure-of-merit of 460, which far exceeds Fano-resonant plasmonic metal sensors. The designed Si metamaterial with a sharp EIT response and a strong electric field enhancement is an ideal platform for applications in optical sensing, low-loss slow-light devices, fluorescence enhancement, and nonlinear optics.

Tailoring dual-band electromagnetically induced transparency with polarization conversions in a dielectric-metal hybrid metastructure.

Metastructure analogs of electromagnetically induced transparency (EIT) provide a new approach for engineering realizations of nonlinear optical manipulations regardless of harsh conditions; further can be employed in polarization conversions for its low-loss transmission and phase modulation. In this work, dual-band EIT in a dielectric-metal hybrid metasurface achieved via providing different coupling channels is theoretically investigated with a maximum group delay of 404 ps. The linear-to-circular polarization conversion (LCPC) behaviors are observed respectively holding the transmittance of 0.58 at 0.68 THz, 0.73 at 0.76 THz, 0.61 at 0.90 THz, 0.53 at 0.99 THz, owning to the asymmetric EIT responses in the transverse magnetic (TM) and transverse electric (TE) modes incidence. On the other hand, phase-transition VO2 is doped to perturb the dark mode resonances. With its conductivity σ = 105 S/m, dual transparency peaks transform into unimodal broadband transmission windows with relative bandwidths of 17.1% and 9.1% under the TE and TM excitations apart. Induced LCPC possesses a bandwidth of 10.4% centered at 0.76 THz attributed to the drastic dispersion. The as-proposed design exploits pattern asymmetry of EIT responses to realize LCPC, promising the wide prospect of reconfigurable multiplexings.

Three-band plasmon induced transparency effect based on four-disk resonator coupled waveguide system

In order to reduce power consumption and realize ultrafast response time and dynamic tunability, a plasmonic waveguide system based on four disk resonators is designed. A plasmon induced transparency effect is theoretically analyzed by using two different methods: one is the direct destructive interference between bright mode resonator and dark mode resonator, and the other is the indirect coupling through a plasmonic waveguide. Owing to the giant effective nonlinear Kerr coefficient of the graphene-Ag composite material structure and the enhancement characteristics of slow light response to optical Kerr effect, the pump intensity of PIT system for changing the phase shift of transmission spectrum is greatly reduced. An ultrafast response time of 1 ps is achieved, and 0.4π, 0.8π, 1.2π, 1.6π and 2π-phase shift of the transmission spectrum in the plasmon induced transparency system are achieved with the intensity of the pump light as low as 2.34, 4.68, 7.02, 9.36, 11.7 MW/cm<sup>–2</sup>, respectively. In this work, a plasmonic waveguide coupled directly by two small disk resonators is employed, because two small disk resonators play a role of the slit between the waveguide and the resonators, and also act as two separate resonators side-coupled with a plasmonic waveguide, which leads to the more efficient coupling of electromagnetic energy in the waveguide into the big disk resonators to form resonance and easier storage of light in the resonator. The triple-band plasmon induced transparency (PIT) effect and slow light properties of the model are analyzed by the expression of the deduced theoretical transmittance based on the coupled mode theory, indicating that they are very consistent with the finite-difference time-domain simulations. The results show that the transmission peak of the system is over 80% and the maximum group index is as high as 368. Furthermore, the disk resonators are easy to fabricate and the size of the entire PIT structure is < 0.5 μm<sup>2</sup>, which is beneficial to the design of optoelectronic device on-chip integration. The research results have important application prospects in highly integrating optical circuits and networks, and also provide the ideas for the design and fabrication of multi-channel optical filter and light storage devices with low power consumption, ultrafast nonlinear response, ultracompact and dynamical tunability.

Plasmonic induced reflection based on Al2O3 nanoslit side coupled with silicon nanodisk resonator

plasmonic induced reflection (PIR) effect has potential applications in many nanophotonics devices, Such as slow light, absorber and filter. To realize plasmonic induced interference, in this paper, we present PIR effect in a periodic subwavelength array of nanoslit side-coupled to nanodisk resonator. Both finite-difference time-domain (FDTD) and coupled-mode theory (CMT) method indicate that the destructive interference between the bright mode resonator (nanosilt) and dark mode resonator (nanodisk) leads to PIR effect. Double-band absorption with maximum absorbance up to 96% and 90% is achieved 1440 nm and 1661 nm, respectively. The effect of structural parameters on reflection and the field distribution are also studied in detail. Moreover, multiple PIR effect can be realized by introducing more dark mode resonators. The results will pave the way towards the design of double band PIR, which may have potential application in slow light technology, plasmonic modulator, plasmonic filter and multiband plasmonic absorbers.

Active modulation of electromagnetically induced transparency analog in graphene-based microwave metamaterial

Metamaterial analogs of electromagnetically induced transparency (EIT) enable a unique route to enhance light-matter interactions without quantum approach, showing remarkable potentials for slow-light devices and highly sensitive sensors. In particular, dynamic control of EIT in metamaterials opens new avenues for optical networks and electromagnetic communications. Here, we experimentally present, for the first time, the active modulation of EIT analog by integrating graphene into a microwave metamaterial. Experimental results demonstrate that the EIT peak can be dynamically controlled under a relatively low bias voltage applied on graphene. A coupled harmonic oscillation model is exploited to analyze the physical mechanism which lies in the active tuning of the damping rate of the dark mode resonator through varying the graphene's sheet resistance. Via variably dampening the dark resonator using graphene, the coupling condition could be electrically modulated and the continuous tuning of the EIT resonance strength is achieved. In addition, the controllable slow-wave propagation is also performed theoretically based on the tunable phase delay of the transmitted waves. This work opens up promising possibilities for slow-wave manipulation with applications in smart and multifunctional devices.

Active thermal modulation of electromagnetically induced transparency at terahertz frequencies

We present thermal control of electromagnetically induced transparency (EIT) by actively modulating the dark mode in terahertz (THz) metamaterials, including a cut wire and a split-ring resonator (SRR). By integrating indium antimonide (InSb) into the SRR and increasing the temperature, the active modulation of EIT is realized. The coupling mechanism is numerically analyzed through the coupled oscillator model, and the result of fitting the EIT intensity agrees well with the simulation results when the temperature changes from 200 to 240 K. By analyzing the electric field distribution, the physical mechanism is the change in the damping rate of the dark mode resonator due to the increase in InSb temperature. Our work has practical significance in designing tunable THz functional 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.

Selective excitation and enhancement of multipolar resonances in dielectric nanospheres using cylindrical vector beams

Resonant excitation and manipulation of complex interactions among two or more resonances in high-index dielectric nanostructures provide great opportunities for engineering novel optical phenomena and applications. However, difficulties often arise when interpreting the observed spectra because of the overlap of the broad resonances contributed by many factors such as particle size, shape, and background index. Therefore, selective excitation of resonances that spectrally overlap with each other provides a gateway towards an improved understanding of the complex interactions. Here, we demonstrate selective excitation and enhancement of multipolar resonances of silicon nanospheres using cylindrical vector beams (CVBs) with different diameters of nanospheres and numerical apertures (NAs) of the excitations. By combining single particle spectroscopy and electrodynamic simulations, we show that the radially polarized beam can selectively excite the electric multipoles, whereas the azimuthally polarized beam can selectively excite the magnetic multipoles even though multipolar resonances are convoluted together due to their spectral overlap. Moreover, focusing the CVBs with high NA can lead to a dominant longitudinal polarization of the electric or magnetic field. We show that the enhanced longitudinal polarization with increasing NA of the radially and azimuthally polarized beams can selectively enhance the electric and magnetic multipolar resonances, respectively. Our approach can be used as a spectroscopy tool to enhance and identify multipolar resonances leading to a better understanding of light-matter interactions in other dielectric nanostructures as well as serve as a first step toward excitation of dark mode and Fano resonances in dielectric oligomers by breaking the symmetry of the nanostructures.

Novel terahertz metasurfaces based on complementary coupled split ring resonators

In this paper, we propose a terahertz metasurface unit cell based on an efficient coupling between two completely symmetric complementary split-ring resonators via a coupling slot. This coupling allows the excitation of a dark eigenmode resonance that is otherwise forbidden. In turn, the radiation losses are highly suppressed and the excited eigenmode features a quite large quality factor. Interestingly, this kind of coupling leads to the restoration of the symmetry of the unit cell of the coupled resonators with respect to the terahertz field illumination. More importantly, we show that the quality factor of this dark mode resonance can be tuned by modifying the coupling slot length and reach a high value of 67.4. Furthermore, a laser beam machining technique can be easily exploited during the fabrication process of this design. Besides using this design as a sharp bandpass filter, it can be utilized as an efficient biosensor. We show that a sensitivity level of 6.3 × 104 nm/RIU is indeed doable using the proposed structure. This design may emerge as a potential candidate for future biosensors and hence utilized during the development of new cancer biomarkers.

Active control of electromagnetically induced transparency based on terahertz hybrid metal-graphene metamaterials for slow light applications

Recently, the dynamically controlled electromagnetically induced transparency (EIT) metamaterials have been widely used in the slow light effect field to enhance the light-matter interactions and nonlinear effects, which has aroused the great attraction of many researchers. The emergence of two-dimensional materials has pushed this research to a new research hot again. Therefore, a terahertz hybrid metal-graphene metamaterial, consisting of a bi-layer metallic EIT metamaterial, an unpatterned monolayer graphene, and an encapsulated ion-gel layer, was proposed to actively control the EIT resonance. In this structure, the transparency peak of the bi-layer metallic EIT metamaterial can realize on-to-off switch through shifting the Fermi energy of graphene covering onto the dark mode resonators. Theoretical analysis and surface current distributions reveal that the active modulation can be attributed to the increasing damping rate of the dark mode caused by the actively controllable conductivity of the graphene. In addition, the active control of the group delay in the hybrid metamaterial is also demonstrated for the slow-light applications. Therefore, this work provides an alternative way to design compact slow-light devices for future terahertz wireless communications.

Ultra-thin subwavelength film sensing through the excitation of dark modes in THz metasurfaces

We demonstrate ultra-subwavelength thin film sensing through experimental and numerical studies of the transmission characteristics of several symmetric and asymmetric planar split ring resonator (SRR) based terahertz metasurfaces coated with thin film of analyte layer(s). A sharp narrow-band dark mode resonance excited in the asymmetric metasurfaces is seen to be particularly very interesting to be exploited in thin film sensing, which is otherwise absent in the symmetric case. For this mode, our experiments clearly have demonstrated sensing of extreme subwavelength (close to thousandth part of the wavelength) thickness film. With the increasing thickness of the analyte layer a gradual red-shift of the resonance mode is observed. The sensitivities and figure of merit (FoM) of all the metasurfaces are derived and compared. The resonant dark modes in asymmetric metasurfaces are found to display substantially better FoM as compared to the typical dipolar modes.

Plasmonic Properties of Close-Packed Metallic Nanoparticle Mono- and Bilayers

The self-assembly of metallic nanoparticles is a promising route to metasurfaces with unique properties for many optical applications, such as surface-enhanced spectroscopy, light manipulation, and sensing. We present an in-depth theoretical study of the optical properties of mono- and bilayers assembled from gold and silver nanoparticles. With finite-difference time-domain simulations, we predict the occurrence of two plasmon modes, a bright and a dark mode, which exhibit symmetric and antisymmetric dipole configurations between the layers, respectively. The dark mode resonance energy depends sensitively on the size of the particles and the interparticle gaps. Hotspots with a nearfield intensity enhancement of up to 3000 are expected, which, together with the fact that the dark mode is roughly four times narrower than the bright mode, reveals how promising these materials are for spectroscopy purposes.

Dynamical switching of electromagnetically induced reflectance in complementary terahertz metamaterials

Active manipulation of group delay of electromagnetic waves is highly desirable for telecommunications and information processing. However, most control means of the group delay currently demonstrated experimentally in the artificial metamaterials require different external excitations necessitating complex setups. Here, electrical manipulation of resonance strength of electromagnetically induced reflectance (EIR) are theoretically and numerically demonstrated in a hybrid metal-graphene complementary metamaterial for actively tunable slow light applications. Combining a monolayer graphene with a metal-based complementary metamaterial composed of a dipole slot and a quadrupole slot, a sharp EIR window in the complementary structure is excited due to strong coupling between two resonators. As shifting the Fermi energy of graphene implanted under the dark mode resonator via electrostatic doping, the EIR window can realize an on-to-off switch, and the modulation depth of 73.3% for the EIR resonance strength is achieved with actively switchable group delay of 6.44 ps. Further investigations show that the active modulation can be attributed to the change in the damping rate of the dark mode caused by the tunable conductivity of graphene. This work provides an alternative way for innovative design and realization of dynamically controllable terahertz slow light devices, which may show potential applications in future terahertz wireless communications.

Dark mode tailored electromagnetically induced transparency in terahertz metamaterials

In this paper, a novel terahertz metamaterial structure composed of a pair of sub-wavelength reverse U-shaped split ring resonators (RUSRs) and cut wire (CW) resonator is designed to realize electromagnetically induced transparency (EIT) effect in weak coupling region. Theoretical and simulated results show that by modulating the relative coupling distance between CW and SRR or mutual distance between SRR pair, the EIT-like phenomenon can be tailored. Furthermore, by introducing photosensitive silicon (Si) cell into the units of the dark mode resonator, actively optical control of the EIT-like effect is realized through increasing the dark mode damping rate. The present work provides an alternative method to design ultrasensitive sensors, filters and slow-light devices in the THz regime.

Dynamically Tunable Plasmon-Induced Transparency Based on Radiative–Radiative-Coupling in a Terahertz Metal–Graphene Metamaterial

New technologies and materials with superior characteristics impel great development of functional devices in the terahertz field. The dynamically tunable plasmon-induced transparency (PIT) based on radiative–radiative-coupling in terahertz hybrid metal–graphene metamaterial is numerically investigated in this paper. For the active manipulation of the PIT device, the single-layer graphene is integrated into the proposed structure consisting of the split-ring-resonator (SRR) and the closed-ring-resonator (CRR). Dynamically adjusting Fermi energy in graphene leads to modulation of the PIT window, allowing for the active control of the group delay. From the simulated electrical field distributions and effective circuit model to analyze, the transmission spectrum modulation can be attributed to the altering in the energy loss of the dark mode resonator through the conduction effect of the graphene layer. Our work offers theoretical references for the development of slow light terahertz devices in the future.

Analogue of tunable electromagnetically induced transparency in terahertz metal-graphene metamaterial

In this paper, a novel EIT analogue consisting of metal-based resonator integrated with graphene is proposed, which can achieve tunable EIT-like effects in terahertz frequencies. For practical applications, a thin Si layer is grown on the resonator to provide a flat surface to transfer monolayer graphene using a wet transfer method. Numerical and theoretical investigations on the tunable EIT-like effect are carried out. The simulations reveal that EIT-like phenomenon is induced by coupling of bright-dark mode of metal-based resonator. Further investigations reveal that the amplitude of EIT window could be modulated by changing the Fermi level of graphene meanwhile the resonant frequencies exhibits slight shift. The physical mechanism underling the modulation phenomenon is mainly attributed to the damping rate of the dark mode and bright mode resonators, which is verified by theoretical analysis of classical two-particle model and absorption of proposed EIT-like analogue. The retrieved effective permittivity and permeability of the proposed metamaterial cell clearly exhibit dispersion in dependence of Fermi level of graphene. This work will offer a new perspective application in terahertz modulation and slow light devices.

Actively tunable slow light in a terahertz hybrid metal-graphene metamaterial

We theoretically and numerically demonstrate an actively tunable slow light in a hybrid metal-graphene metamaterial in the terahertz (THz) regime. In the unit cell, the near field coupling between the metallic elements including the bright cut wire resonator and the dark double split-ring resonator gives rises to a pronounced transmission peak. By positioning a monolayer graphene under the dark mode resonator, an active modulation of the near field coupling is achieved via shifting the Fermi level of graphene. The physical origin can be attributed to the variation in the damping rate of the dark mode resonator arising from the conductive effect of graphene. Accompanied with the actively tunable near filed coupling effect is the dynamically controllable phase dispersion, allowing for the highly tunable slow light effect. This work offers an alternative way to design compact slow light devices in the THz regime for future optical signal processing applications.

Dynamically Tunable Resonant Strength in Electromagnetically Induced Transparency (EIT) Analogue by Hybrid Metal-Graphene Metamaterials

In this paper, a novel method to realize a dynamically tunable analogue of EIT for the resonance strength rather than the resonance frequency is proposed in the terahertz spectrum. The introduced method is composed of a metal EIT-like structure, in which a distinct EIT phenomenon resulting from the near field coupling between bright and dark mode resonators can be obtained, as well as an integrated monolayer graphene ribbon under the dark mode resonator that can continuously adjust the resonance strength of transparency peak by changing the Fermi level of the graphene. Comparing structures that need to be modulated individually for each unit cell of the metamaterials, the proposed modulation mechanism was convenient for achieving synchronous operations for all unit cells. This work demonstrates a new platform of modulating the EIT analogue and paves the way to design terahertz functional devices which meet the needs of optical networks and terahertz communications.

Independently tunable dual-spectral electromagnetically induced transparency in a terahertz metal–graphene metamaterial

We theoretically investigate the interaction between the conductive graphene layer with the dual-spectral electromagnetically induced transparency (EIT) metamaterial and achieve independent amplitude modulation of the transmission peaks in the terahertz (THz) regime. The dual-spectral EIT resonance results from the strong near field coupling effects between the bright cut wire resonator in the middle and two dark double-split ring resonators on the two sides. By integrating monolayer graphene under the dark mode resonators, the two transmission peaks of the EIT resonance can exhibit independent amplitude modulation via the shifting of the Fermi level of the corresponding graphene layer. The physical mechanism of the modulation can be attributed to the variation of damping factors of the dark mode resonators arising from the tunable conductivity of graphene. This work shows great prospects in designing multiple-spectral THz functional devices with highly flexible tunability and implies promising applications in multi-channel selective switching, modulation and slow light.