Slow Light Devices Research Articles

Ultra-wideband terahertz fingerprint enhancement sensing and inversion model supported by single-pixel reconfigurable graphene metasurface

AbstractThe molecular fingerprint sensing technology based on metasurface has unique attraction in the biomedical field. However, in the terahertz (THz) band, existing metasurface designs based on multi-pixel or angle multiplexing usually require more analyte amount or possess a narrower tuning bandwidth. Here, we propose a novel single-pixel graphene metasurface. Based on the synchronous voltage tuning, this metasurface enables ultra-wideband ($$\sim$$ ∼ 1.5 THz) fingerprint enhancement sensing of trace analytes, including chiral optical isomers, with a limit of detection (LoD) ≤ 0.64 μg/mm2. The enhancement of the fingerprint signal ($$\sim$$ ∼ 17.4 dB) originates from the electromagnetically induced transparency (EIT) effect excited by the metasurface, and the ideal overlap between the light field constrained by single-layer graphene (SLG) and ultra-thin analyte. Meanwhile, due to the unique nonlinear enhancement mechanism in graphene tuning, the absorption envelope distortion is inevitable. To solve this problem, a universal fingerprint spectrum inversion model is developed for the first time, and the restoration of standard fingerprints reaches Rmax2 ≥ 0.99. In addition, the asynchronous voltage tuning of the metasurface provides an opportunity for realizing the dynamic reconfiguration of EIT resonance and the slow light modulation in the broadband range. This work builds a bridge for ultra-wideband THz fingerprint sensing of trace analytes, and has potential applications in active spatial light modulators, slow light devices and dynamic imaging equipments.

Single and dual-band electromagnetically induced transparency in a strongly near field coupled planar toroidal terahertz metamaterial

We demonstrate electromagnetically induced transparency (EIT) in a strongly coupled planar terahertz toroidal metasurface through simulations, theory and experiments. The near-field interaction between a unique toroidal resonator enveloped by a 2-arc resonator leads to the excitation of EIT in the metamaterial. A modulation in the EIT response of the metasurface is observed by increasing the gap between the resonators, resulting a redshift of the transmission response. Furthermore, we report the excitation of dual-band EIT in the metasurface by introducing an asymmetry in the 2-arc resonator. A theoretical oscillator model is used to validate the experimental EIT effect, and to gain an in-depth understanding of the coupling mechanism of the proposed geometry. Metasurfaces were fabricated and characterized for each individual resonator geometry as well as the geometries corresponding to the single-EIT design and the dual-EIT design. This study can contribute to the development of slow light devices and ultrafast switches in the terahertz regime.

Multiple Brillouin Zone Winding of Topological Chiral Edge States for Slow Light Applications.

Photonic Chern insulators are known for their topological chiral edge states (CESs), whose absolute existence is determined by the bulk band topology, but concrete dispersion can be engineered to exhibit various properties. For example, the previous theory suggested that the edge dispersion can wind many times around the Brillouin zone to slow down light, which can potentially overcome fundamental limitations in conventional slow-light devices: narrow bandwidth and keen sensitivity to fabrication imperfection. Here, we report the first experimental demonstration of this idea, achieved by coupling CESs with resonance-induced nearly flat bands. We show that the backscattering-immune hybridized CESs are significantly slowed down over a relatively broad bandwidth. Our work thus paves an avenue to broadband topological slow-light devices.

Monolayer actively tunable dual-frequency switch based on photosensitive silicon metamaterial

We propose a monolayer actively tunable dual-frequency switch in the terahertz (THz) range based on photosensitive silicon (PSi) metamaterial. The designed unit cell consists of hollow cross-shaped slots and four L-shaped slots made of a perfect electric conductor (PEC), with PSi embedded in both the cross-shaped and four L-shaped slots of identical dimensions. The PEC serves as the dielectric substrate. By varying the intensity of the pump beam to adjust the conductivity of the PSi, tuning of the transmittance at the resonant frequencies of the entire structure is achieved, enabling the switch to function in both ON and OFF states. Under vertical incidence of THz waves in the range of 1 THz to 3 THz, in the absence of pump beam, the PSi is in an insulating state with a conductivity of approximately 1 S/m. The proposed terahertz dual-frequency switch is in the ON state, with transmittances of 97.0% at the first resonance frequency (f1=1.736THz) and 98.3% at the second resonance frequency (f2=2.176THz), and group delays of 4.397 ps and 2.540 ps, respectively. When the pump beam intensity is 294.6 μj/cm2, the PSi is in a metallic state with a conductivity of approximately 1 × 105 S/m. The switch is in the OFF state, with zero transmittance in the range of 1 THz to 3 THz and a group delay of 0.327 ps. The calculated modulation degrees of amplitude at f1 and f2 for this terahertz dual-frequency switch are both 100%, demonstrating a high-performance switch. The two resonant frequencies f1 and f2 of the terahertz dual-frequency switch are generated by weakly coupled superposition between the embedded cross-shaped and four L-shaped PSi resonators (CSPR and FLSPR), exhibiting polarization-insensitive characteristics under vertical incidence of THz waves. We believe that this compact structure of terahertz metamaterial dual-frequency switch holds great application prospects in terahertz filtering, slow light devices, terahertz modulation, and the future 6G communication field.

A 2-bit graphene encoder based on the plasmon-induced transparency effect and its sensing characteristics

A 2-bit encoder based on the plasmon-induced transparency (PIT) effect was proposed and investigated in the range from 2 to 7 THz. The Lorentz oscillation coupling model was utilized to confirm the simulation. By tuning the Fermi levels of cross1 and cross2, the 2-bit encoder was achieved. And the minimum MD was 94.73 %, the minimum ER was 12.77 dB, and the maximum IL was 0.32 dB. As a sensor, the structure had a sensitivity of up to 2.33 THz/RIU. In addition, due to the insensitivities to incidence angle and polarization angle, the encoder could be utilized to complex environments. Therefore, these results reveals that our work is significant in terahertz encoders, optical switches, sensors, slow light devices, and modulators.

Versatile terahertz metasurface: dynamic switching between electromagnetically induced transparency and perfect absorption

This paper introduces a versatile metasurface based on vanadium dioxide (VO2) and graphene that seamlessly transitions between electromagnetically induced transparency (EIT) and multi-band absorption through VO2’s phase change property. When VO2 is in a dielectric state, the device can generate EIT. This configuration allows dynamic tuning of the central frequency by adjusting the graphene’s Fermi levels (E f ), achieving a remarkable group delay of 1.42 ps. When VO2 is in a metallic state, the structure facilitates a Fabry–Perot resonance between the VO2 layer at the bottom and the graphene layer at the top, leading to exceptional light absorption. Specifically, absorptivity of 99.8% and 99.4% is achieved at 1.66 THz and 2.87 THz, respectively. In addition, these two resonance peaks can also be dynamically adjusted by modulating E f . Furthermore, the device serves as a highly sensitive sensor with sensitivity up to 0.65 THz/RIU. Notably, both absorption and EIT modes are polarization-insensitive and exhibit tolerance to a wide range of incidence angles. Consequently, the proposed device holds significant promise across various applications within the electromagnetic field, including tunable devices, absorbers, sensors, slow-light devices, and so on.

Graphene metamaterials-based plasmon-induced terahertz modulator for high-performance multiband filtering and slow light applications.

We proposed multilayered graphene (Gr)-based surface plasmon resonance-induced high-performance terahertz (THz) modulators with tunable resonance frequencies. Previously reported Gr metamaterials-based THz plasmonic modulators had small group delay, low extinction ratio (ER), and difficult-to-tune resonant frequency without changing structural parameters in the THz range. A comprehensive investigation employing the finite-difference time-domain (FDTD) simulation technique revealed high group delay, broad tunability independent of structural parameters, and large ER for our proposed quadband and pentaband plasmonic modulators. We obtained tunable group delays with a maximum of 1.02 ps and 1.41 ps for our proposed quadband and pentaband plasmonic modulators, respectively, which are substantially greater compared to previously reported Gr-based metamaterial structures. The maximum ER of 22.3 dB was obtained, which was substantially high compared to previous reports. Our proposed modulators were sensitive to the polarization angle of incident light; therefore, the transmittance at resonant frequencies was increased while the polarization angle varied from 0° to 180°. These high-performance plasmonic modulators have emerging potential for the design of optical buffers, slow light devices, multistop band filters, integrated photonic circuits, and various optoelectronic systems.

SOI Based metasurface for broadband perfect reflection in visible spectrum

We propose modeling and design of a low-loss all-dielectric metasurface (DM), comprised of Silicon on Insulator (SiO2) substrate to demonstrate a perfect reflector in the visible spectrum. The proposed metasurface unit cell consists of V and W shapes arranged in a mirror image configuration, with nanometre-sized gaps (g) between them. A narrow peak with a nearly 100% reflectance and a broad perfect reflectance spectrum is observed within the visible region (400–700 nm) of the electromagnetic spectrum. The effective electromagnetic parameters were also analyzed for electric and magnetic dipole resonance. The electric and magnetic field distributions at the resonant wavelength were also analyzed for the proposed structure. By altering the gap region ‘g’, the thickness of the dielectric Silica layer (ts ), and the Si resonator (t m), the proposed structure exhibits tunable characteristics. We have successfully illustrated the consistent position of the scattering parameter’s response, regardless of the structure’s rotation, concluding the homogeneity of the designed structure across the entire visible spectrum. The all-DM exhibits a unique combination of features, including a distinct and wide reflectance spectrum as well as a tuned and enhanced electric field which makes it an ideal platform for the applications in filters, color printing, low-loss slow-light devices, and nonlinear optics.

Analog electromagnetic induced transparency of T-type Si-based metamaterial and its applications

A T-type silicon-based metamaterial is proposed, which realizes electromagnetically induced transparency (EIT) by using the asymmetry of its structure. This dielectric metamaterial exhibits an ultranarrow EIT transparent window, with a transmittance of 91% and a Q factor of 180. Measuring its sensing performance, a refractive index sensor with a sensitivity of 466 nm RIU−1 is obtained. In addition, by analyzing the dispersion characteristics of the structure, the maximum group delay value is 2.84 ps, and the corresponding group refractive index is 4250. Therefore, dielectric metamaterials with this structure are expected to be used in refractive index sensing and slow light devices.

Experimental validation of group delay in a multi-window electromagnetically induced transparency metasurface

Metasurface analogs of electromagnetically induced transparency (EIT) are attracting sustaining attention due to their ability to maintain transparency windows accompanied by extreme dispersion of propagating waves, which are important for slow light devices and highly sensitive optical sensors. In this paper, we study theoretically, numerically, and experimentally the conditions for the existence of multi-band transparency windows in the metasurface supported by the interaction of dipole modes in an asymmetric unit cell. The unit cell is composed of a single bright resonator and several dark resonators made in the form of rectangular metal patches. The manifestation of EIT is studied for different metasurface configurations by varying the number and positions of resonators used within the unit cell. To validate the slow-down effect caused by EIT, a prototype of the metasurface is fabricated and tested, providing a measurement of the group delay and bandwidth-delay product features. The obtained results clearly confirm the presence of four EIT-like transparency windows in the metasurface transmission spectra originating from the coupling between either quasi-TE or quasi-TM modes of the resonators.

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.