Slow Light Devices Research Articles

Double plasmon-induced transparency 3 bit graphene encoder

In this paper, a 3 bit encoder was proposed which was based on double plasmon-induced transparency (PIT) effect. This structure consists of the bright-bright mode and realizes a double PIT window in the 0.1–3 THz range. Electric field distributions reveal that the double PIT effect is originated from the strong destructive interference between three bright modes, the Lorentz oscillation coupling model is utilized to confirm the finite-integration time-domain (FITD) simulation regardless of the x-polarization direction or y-polarization direction. When the incident light is x-polarized, encoding frequencies were at 0.976 THz, 1.525 THz and 2.069 THz, respectively; while in y-polarization, encoding frequencies were at 1.579 THz, 1.935 THz and 2.589 THz, respectively. Results depict that the Modulation Depth (MD) could reach 98 % regardless of the x-polarization direction or the y-polarization direction. As an electro-optical switch, the maximum MD could reach ≥98 %, the minimum IL could reach 0.35 dB, the maximum ER could reach ≥14 dB. Besides the encoder, the proposed structure is of great importance for the design of optical switches, terahertz modulators and slow light devices.

The construction of versatile terahertz metamaterial having tunable dual-band electromagnetically induced transparency based on two falling asymmetric T-shaped graphene resonators

The combination of graphene and metamaterials has played an important role in studying tunable terahertz functional devices, with electromagnetically induced transparency (EIT) being one of the typical examples. In this work, we present a versatile terahertz meta-device consisting of two asymmetric T-shaped graphene resonators, which could produce a dual-band EIT effect. The strong interaction between the two T-shaped graphene resonators leads to a dual-band EIT effect, and the electric field and surface current distributions are provided to reveal their generation mechanisms. Results also indicate that the dual-band EIT effect is not only influenced by size variations of T-shaped graphene resonators themselves, but also strongly depends on the Fermi energy levels they utilized. Interestingly, multi-frequency switching with a maximum modulation depth of 85.13 % could be further realized when the Fermi energy levels are precisely controlled. The group delay near two transmission peaks can reach 2.87 ps and 4.20 ps, which is important for the study of slow light devices. Besides, the sensitivity of the graphene-based dual-band EIT device as a sensor reaches 109 GHz·RIU−1 and 145 GHz·RIU−1, indicating that the design has excellent sensing performance and is expected to be useful for the detection of biomolecules in biochemistry, medicine and other industries.

Bound states in continuum domain of double resonant ring metal metasurfaces

Metasurfaces have found extensive applications in microwave, terahertz, and optical range, serving different purposes such as filters, sensors, slow light devices, and nonlinear devices due to their distinctive electromagnetic response characteristics. Recent development requires metasurface devices to exhibit enhanced monochromaticity and stronger light interaction. Consequently, there is a growing interest in designing metasurfaces with high-quality factor (<i>Q</i>-factor) resonances, considering their crucial role in achieving sharp resonances through constructing bound states in the continuum (BIC) mode. The utilization of BIC has emerged as a prominent method of designing metasurfaces with high <i>Q</i>-factor resonances. Due to the fact that the changes in the structural parameters of metasurfaces can simultaneously affect the resonance of two components of q-BICs, it is difficult to achieve on-demand design of operating frequency, bandwidth, and <i>Q</i>-factor. In this work, we investigate a novel THz metasurfaces supporting q-BIC resonance. We optimize the geometric parameters of two split ring resonators (SRRs) to tailor the operating frequencies of intrinsic resonance, and tune the coupling between different resonance modes to form the q-BIC mode resonance. The dominant modes are demonstrated by the results of multipolar decomposition calculations of the electromagnetic field distributions and scattered power at different resonant operating frequencies. In <i>x</i>-polarized and <i>y</i>-polarized incident electromagnetic wave, the normalized coupling strength ratio between the two modes are calculated by Jaynes-Cummings model to be 0.54% (<i>x-</i>polarized) and 4.42% (<i>y-</i>polarized) respectively, which explains the law that the resonant frequency of different modes changes with the structural parameters of SRRs device. In order to analyze the refractive index sensing capabilities of our designed metasurfaces under the incident electromagnetic waves with different polarizations, we investigate the variations of the transmitted spectrum of the metasurface with refractive index of matters. The calculated results show that the sensitivity of the metasurface is 151 GHz/RIU when the incident wave is <i>y</i>-polarized and 108 GHz/RIU when the incident wave is <i>x</i>-polarized. We realize the effective control of the operating frequency, bandwidth, and <i>Q</i>-factor of the q-BIC mode resonance in the transmission spectrum of the metasurface, which provides a new idea for the practical designing of terahertz metasurfaces with high <i>Q</i>-factor.

Exploring the tuning mechanism of multipolar quasi-continuous domain bound states in tetramer metasurface

High-quality factor (high-<i>Q</i>) resonance has broad prospects in applications such as in narrow-band filtering, slow-light devices, and nonlinear optics interaction enhanced to highly sensitive sensing. Previous methods of designing high-<i>Q</i> resonance suffered intrinsic drawbacks such as high-volume cavities or large-scale bending radii. However, recently, a new approach to designing high-<i>Q</i> resonances has begun to attract public attention on the basis of asymmetric metasurfaces that are related to the bound states in the continuum (BIC) phenomenon. Constructing BIC resonance in electromagnetic metasurface can generate sharp resonant transmission peak. Therefore, there is growing interest in utilizing BIC to achieve metasurface with high-<i>Q</i>. However, most of existing studies are based on single BIC, and few studies focusing on multiband BICs and multiple forms of symmetry breaking. In this work, we propose an all-dielectric metasurface composed of tetrameric cuboids. By etching two elliptical cylinders in each cuboid, the metasurface can simultaneously support in-plane symmetry breaking, displacement perturbations and periodic perturbations. We first use multipole calculations to analyze the physical mechanism by which the metasurface generates quasi-BIC under these three conditions. It is confirmed that the <i>Q</i> factor and resonant peak position of quasi-BIC can be controlled by adjusting the asymmetry parameters. Subsequently, we introduce the in-plane symmetry breaking, displacement perturbations and periodic perturbations into the metasurface simultaneously and generate five quasi-BIC modes, whose numbers and positions can be flexibly adjusted, and the largest <i>Q</i> factor is 58039. In summary, this work provides a new practical design concept for realizing high-<i>Q</i> all-dielectric metasurfaces, which can be used to improve the sensitivity of multi-parameter sensors.

Anapole assisted self-hybridized exciton-polaritons in perovskite metasurfaces.

The exciton-polaritons in a lead halide perovskite not only have great significance for macroscopic quantum effects but also possess vital potential for applications in ultralow-threshold polariton lasers, integrated photonics, slow-light devices, and quantum light sources. In this study, we have successfully demonstrated strong coupling with huge Rabi splitting of 553 meV between perovskite excitons and anapole modes in the perovskite metasurface at room temperature. This outcome is achieved by introducing anapole modes to suppress radiative losses, thereby confining light to the perovskite metasurface and subsequently hybridizing it with excitons in the same material. Our results indicate the formation of self-hybridized exciton-polaritons within the perovskite metasurface, which may pave the way towards achieving high coupling strengths that could potentially bring exciting phenomena to fruition, such as Bose-Einstein condensation as well as enabling applications such as efficient light-emitting diodes and lasers.

Perfect absorption frequency modulation, optical switching and slow-light multifunctional integrated device based on plasmon-induced absorption

Perfect absorption frequency modulation, optical switching and slow-light multifunctional integrated device based on plasmon-induced absorption

Dynamically tunable terahertz slow light device based on triple plasmonic induced transparency

Dynamically tunable terahertz slow light device based on triple plasmonic induced transparency

Thermally controlled electromagnetically induced transparency metamaterial through the near-field coupling of electric and toroidal resonances

We investigate a thermally controlled electromagnetically induced transparency terahertz metamaterial through the near-field coupling of electric and toroidal resonances. The fundamental unit consists of a composite design incorporating both metal and vanadium dioxide components aimed at inducing toroidal resonance, along with a pair of metal strips generating electric resonance. Simulation results authenticate the coupling mechanism and illustrate that the envisioned EIT phenomenon can be dynamically adjusted by temperature. In a coupled oscillator model analysis, the control over coupling strength primarily emerges from the fluctuating damping rate of the bright-mode oscillator. Moreover, the displacement of the EIT peak is linked to alterations in the inherent resonant frequency of the bright-mode oscillator. This study not only broadens the potential applications for toroidal terahertz metamaterials but also enhances the range of EIT methodologies available, providing practical approaches for the utilization of terahertz slow-light devices, sensors, and switch devices.

Triple frequency bands terahertz metasurface sensor based on EIT and BIC effects

How to improve the sensitivity of terahertz wave metasurface biosensors has always been a research hotspot in the field of biosensors. In this article, we propose a well-shaped all dielectric metasurface, which can simultaneously excite electromagnetic induced transparency (EIT) and bound states in the continuum (BIC) effects, achieving high sensitivity sensing at three frequency points. The maximum Q value of refractive index sensing is 69238, with a sensitivity of 535 GHz/RIU and the figure of merits (FoM) of 85468. Moreover, the structure exhibits EIT group delay effect, with a maximum group delay of 5.57ps. The results indicate that the well-shaped all dielectric metasurface enhances the interaction between terahertz waves and matter, which can not only achieve multi frequency terahertz high sensitivity sensing, but also be used for the design of new slow light devices.

High-Q and modulation-depth PIT-like effect through the phase coupling between guided modes and Tamm state in a hybrid Tamm plasmon system

High-quality (high-Q) resonators play a crucial role in optical-wave manipulation. It is challenging to excite guide modes in 1-D photonic crystals (PhC guided modes (PCGMs)) because of their high Q factors and complex patterns. To address this, a hybrid cavity Tamm plasmon (HCTP) structure composed of a metal film, cavity, and 1-D photonic crystal (1-D PhC) was designed. Subsequently, a grating was incorporated into the HCTP structure to diffract the electric fields of the HCTP resonance mode. This efficiently excited the PCGMs. When the detuning between the PCGM and HCTP modes approached zero, a plasmon-induced transparency (PIT)-like effect occurred. Under normal incidence, the PIT-like effect exhibited an impressively high Q factor (5015.5). The physical properties of the PIT-like effect were analyzed using the coupling mode theory. The results show that there is a strong phase coupling between PCGM and HCTP modes. This resulted in a modulation depth of 97.4 %. Under an oblique incidence, the degenerated PCGMs split into two modes. Both these were coupled with the HCTP mode. This resulted in a double PIT-like effect with a high modulation depth. The Q factor was enhanced further to 8957.7. Active modulation of the PIT-like spectra was achieved by modifying the incident or polarization angle. The coupling also exhibited slow-light properties, with a group delay of 17.54 ps under inclined incidence. The strong coupling between PCGM and HCTP resulted in the generation of high-Q and deep-modulation PIT-like effect. These hold high potential for various applications such as nanosensing, slow-light devices, nanolasers, and angle detectors.

Terahertz sensor based on plasmon-induced transparency in a carbon nanotube metamaterial

In this paper, the plasmon-induced transparency (PIT) effect based on a carbon nanotube (CNT) resonator structure is achieved. An array of two split ring resonators (SRRs) and a cut wire (CW) resonator are utilized to form the proposed metamaterial. A PIT transparency window is achieved under a TM polarization terahertz light. Results show that the PIT effect is originated from the near-field coupled of the bright mode and dark mode. A coupled harmonic oscillator model is used to describe the near-field coupling between the bright mode and subradiant mode, and the results agree well with the FDTD simulation. The effect of geometrical sizes, like structure period, the radius and the splitting degree of the split ring resonator, the length of the cut wire resonator, and the coupling distance on the PIT window is analyzed in detail. Besides, the sensing and slow light performance of the proposed CNT metamaterial are studied, a maximum sensitivity of 0.74 THz RIU, and a time delay of 0.54 ps are obtained. Therefore, the proposed CNT-based device can be applied to the PIT effect, near-infrared modulators, slow light devices, sensors, and other fields.

Dynamically tunable multiple plasmon-induced transparency effect based on monolayer graphene structure system with rectangular defect cavities

In this article, a dynamically tunable multiple plasmon-induced transparency (PIT) effect in monolayer graphene structure system with rectangular defect cavities is investigated both theoretically and numerically. Because the graphene of our structure exists in a continuous form, the Fermi level of the graphene can be dynamically tuned by simply applying a bias voltage. The expressions of the theoretical transmittance are correctly deduced, and the fitting theoretical results are very consistent with the numerical simulation data. When the Fermi level of the graphene is increased from 0.8 eV to 1.2 eV, the group index of the dual-PIT system is controlled between 383 and 766. Alternatively, the group index of the triple-PIT system is maintained between 445 and 812. Moreover, the maximum group index can reach 812 at 1.2 eV, which shows that it can be designed as an excellent slow light device. Therefore, the proposed structures and results may provide strong guidance towards multichannel optical filters, dynamically tunable and excellent slow light and light storage devices.

Electromagnetically induced transparency in a strongly coupled orthogonal polarization-insensitive planar terahertz metamaterial

We demonstrate experimentally and numerically the electromagnetically induced transparency (EIT) effect in a strongly coupled planar terahertz (THz) metamaterial. The circular-shaped four-arc geometry combined with cross resonators ensures the polarization-insensitive response of the EIT for the orthogonal polarization incident THz. The EIT response can be varied by changing the gap between the arc and cross-shaped resonators. The field profiles indicate a strong coupling between the resonators leading to the EIT effect. In order to understand the underlying physical mechanism, we employed a coupled harmonic oscillator model, which suggests an increase in coupling when the distance between resonators is reduced. THz time-domain spectroscopy of the fabricated samples with the same shape and size of the simulated structures was used to verify the numerical findings. Our study uses a symmetric and easy-to-fabricate planar metasurface that can pave the way for the design and construction of THz photonic components, such as optical switches and slow light devices.

Tunable plasmon refractive index sensor and a slow-light device optimized by an improved genetic algorithm based on a MIM waveguide

A structure is proposed in this article consisting of a stub metal–insulator–metal (MIM) waveguide coupled with an embedded T-shaped square ring resonator (ETSRR). The transmission characteristics and magnetic field distribution of the design are analyzed in detail using the finite element method (FEM). Furthermore, the geometric parameters are optimized using an improved genetic algorithm to enhance device performance. The proposed structure is therefore an ideal candidate for realizing a refractive index sensor and slow-light device, with potential applications in various fields such as sensing and communication. Moreover, our study provides valuable insights to the design of surface plasmon polarition (SPP) waveguides with computer assistance.

Switchable high-Q electromagnetically induced transparency based on the Ge2Sb2Te5 nanodisk dimers

Bound states in the continuum (BICs) based on metasurfaces have gained significant attention recently to enhance the strength of light-matter interaction. However, most BIC metasurfaces possess fixed optical properties once they have been fabricated. In this study, we introduce the concept of BICs to the phase-change metasurfaces composed of Ge2Sb2Te5 nanodisk dimers. The Ge2Sb2Te5 nanodisk dimers can support a low-Q transverse magnetic dipole resonance, a high-Q toroidal dipole resonance and a high-Q longitudinal magnetic dipole resonance. By adjusting geometric parameter, the high-Q longitudinal magnetic dipole resonance can be converted into a BIC mode. Notably, when Ge2Sb2Te5 is in the amorphous phase, high-Q electromagnetically induced transparency (EIT)-like resonances can be achieved due to the interaction between a low-Q transverse magnetic dipole mode and either a high-Q toroidal dipole mode or a high-Q longitudinal magnetic dipole mode. However, the EIT resonances are switched off when Ge2Sb2Te5 is transformed into the crystalline phase. Furthermore, the high-Q EIT resonances enable the realization of an ultrahigh group refractive index, and can be tuned through the phase transition of Ge2Sb2Te5. The switchable high-Q EIT resonances hold potential applications in slow-light devices, optical modulators, and biosensors.

Dual-function tunable metasurface for polarization-insensitive electromagnetic induction transparency and dual-band absorption

In this paper, we propose a dual-operating mode metasurface based on graphene and vanadium dioxide (VO2), which can switch operating modes by changing the temperature. At room temperature (25 °C), the metasurface can generates a polarization-insensitive electromagnetically induced transparency (EIT)-like effect that can be modulated by changing the Fermi energy level (E F) of graphene (through adding external voltage). In addition, the theoretical results derived from the two-particle model are in good agreement with the simulation results based on the finite element method. At high temperature (68 °C), the metasurface mode of operation can be changed to a dual-band absorber, providing absorption of 78.6% and 99.9% at 1.13 THz and 2.16 THz, respectively. Both absorption peaks can be dynamically tuned by changing the E F of graphene. The metasurface is also simultaneously polarization insensitive and has a wide incidence angle. The proposed metasurface can be used as a slow light device with a maximum group delay of 0.5 ps at room temperature and as a refractive index sensor with a maximum sensitivity of 0.5 THz/RIU at high temperature. The designed metasurface offers a new way for designing multifunctional terahertz devices, slow light devices, and refractive index sensors.

Origin of plasmonic Fano resonance in metal-hole/split-ring-resonator metamaterials disclosed by temporal coupled-mode theory.

In plasmonic Fano resonance, the interaction between a discrete plasmonic mode and a continuum of plasmonic mode gives rise to an asymmetric line shape in the scattering or absorption spectrum, enabling a wide range of applications such as sensing, switching, and slow light devices. Here, we establish a theoretical solution in the framework of temporal coupled-mode theory (TCMT) to study the three-dimensional (3D) and two-dimensional (2D) Fano resonances induced by strong coupling between metal hole (MH) and split ring resonator (SRR) array. We first separately analyze the transmission spectra of the MH array and SRR array under different polarized light excitation. We further investigate the electromagnetic field and charge density distribution corresponding to the resonant modes at the peak or valley wavelength of the transmission spectrum and figure out the electric/magnetic dipole feature of these resonance modes. We then establish a theoretical solution by TCMT for Fano resonances arising from the coupling of these modes. The calculated transmission spectrum is closely matching with the numerically simulated transmission spectrum for these Fano resonances in the MH-SRR array, which effectively elucidates that the asymmetry of the Fano resonances is caused by the coupling between bright and dark plasmonic modes involved in the two structures. Our results can help to understand the profound physics in such coupled plasmonic systems.

Tunable slow light device based on a graphene metasurface.

Slow light devices have significant applications in memory, switching, and quantum optics. However, the design and fabrication of slow light devices with large tunable group delay are still challenging. Here, a graphene-based slow light device that can electrically modulate the group delay of terahertz (THz) waves is proposed and experimentally demonstrated. The unit cell of the device consists of a U-shaped metal resonator and an Ω-shaped metal resonator, with three graphene ribbons embedded between the two resonators. Under electrical stimuli, a relatively high amplitude modulation depth of 74% is achieved and the maximum transmission amplitude is as high as 0.7 at the transmission peak of 0.6 THz. Most importantly, the maximum group delay variation reaches 5 ps at 0.76 THz and the maximum group delay amplitude is as high as 8.8 ps. The experiment shows good agreement with simulation. This study paves a new way for developing novel switchable nanophotonic devices and slow light devices.

Exploring the influence of nanocavity alignment on slow light generation via multiple EIT and Fano resonances in square lattice plasmonic silver nanostructures

In this paper, we present a comprehensive investigation of multiple electromagnetic induced transparency (EIT) and Fano resonances in a square lattice plasmonic nanostructure, which is composed of four silver slabs arranged in a square configuration, with each slab featuring a cylindrical nanocavity at its center. Initially, symmetric structures were analyzed to explore the potential for achieving EIT effects. Subsequently, we introduce deliberate symmetry breaking by precisely aligning the nanocavity in a controlled manner, first within a single slab, then in combinations of two, three, and all four slabs simultaneously. This controlled alignment strategy enabled the relaxation of dipole coupling selection rules, leading to the mixing of dipole and higher-order modes. The interaction between these modes resulted in the generation of multiple EITs and Fano resonances in the optical spectrum. Furthermore, the effective group index was evaluated for the optimal results obtained in the single, double, triple, and four symmetry reduced structures. High group index values were observed in the vicinity of the EIT and Fano resonances, with a remarkable maximum group index value of 6900 achieved within the EIT window. These findings highlight the significant potential of these structures in the design of slow light devices and sensitive sensors.

Tunable topological slow-light in gyromagnetic photonic crystal waveguides with unified magnetic field.

We have proposed a tunable topological slow-light in a photonic crystal (PC) waveguide with unified magnetic field. The waveguide is constructed by bringing close two gyromagnetic photonic crystals (GPCs) with different structural parameters and introducing a row of Al2O3 rods as the coupling layer. The two GPCs are applied with a unified external static magnetic field (ESMF) instead of two opposite ESMFs. Such waveguide supports a slow-light state originated from the coupling effect of two one-way edge states on both sides of the waveguide. By simply changing the strength of ESMF, one can achieve a tunable slow-light state with large normalized delay-bandwidth product (NDBP) (0.36< NDBP <0.84). Based on these excellent properties, we further design an optical delayer with a compact structure and expansibility simultaneously. This unique topological slow-light state with simple unified magnetic condition, high maneuverability and strong immunity to defects holds promise for many fields such as signal processing, optical modulation, and the design of various slow-light devices.