Design and simulation of electromagnetically induced transparent structure based on terahertz metamaterials

<sec>The phenomenon of electromagnetically induced transparency (EIT)-like in terahertz (THz) metasurfaces facilitates agile manipulation of electromagnetic wave transmission windows and the deceleration of light, rendering it suitable for applications in modulators, absorbers, slow light devices, and more. Traditional design methodologies focus on the coupling between bright-dark modes and bright-bright modes within the unit cell, leveraging interference cancellation effects to regulate electromagnetic wave transmission. Notably, the periodicity of the array structure also plays a pivotal role in modulating the amplitude and resonance intensity of the transparent window, a phenomenon termed lattice-induced transparency (LIT). In this paper, we introduce a gold nanorod structure and an S-shaped gold split-ring resonator supported on a vanadium dioxide (<inline-formula><tex-math id="M1">\begin{document}${\rm{VO}}_{2}$\end{document}</tex-math></inline-formula>) thin film to investigate LIT. Unlike conventional structures that solely consider single bright-bright or bright-dark mode coupling, our proposed structure incorporates both bright-bright and bright-dark modes coupling. Furthermore, the dark mode in our structure is not a conventional multipolar mode but rather a surface lattice resonance (SLR) arising from the coupling between lattice modes and the localized surface plasmon resonance (LSPR) of the structure itself.</sec><sec>Through the analysis of simulated transmission spectra for the individual gold nanorod and S-shaped split-ring structures, we observed that the gold nanorod exhibits LSPR at 0.985 THz, whereas the S-shaped split-ring structure demonstrates LSPR and SLR at 0.51 THz and 1.025 THz, respectively. When combined, these structures form transparent windows with transmission rates of 66.03% and 59.4% at 0.643 THz and 1.01 THz due to the interplay of bright-bright and bright-dark modes coupling. Upon examining the electric field distribution in the x-y plane, we found that the electric field energy is predominantly concentrated on the S-shaped split-ring.</sec><sec>To gain deeper insights into each resonance mode, we employed multipolar decomposition to quantify resonance scattering energy. Our findings revealed that both transparent windows are predominantly governed by electric dipole scattering energy. Further investigations showed that as the array structure’s period varies from 60 μm to 95 μm, the lattice mode progressively couples into the high frequency transmission valley (1.031 THz), giving rise to a high frequency hybrid mode (HFHM). The <i>Q</i> value of this mode initially increases and then decreases, peaking at 27 when the period is 84 μm. Similarly, as the period continues to increase, the lattice mode couples into the low frequency resonance valley (0.76 THz), forming a low frequency hybrid mode (LFHM) with a <i>Q</i> value that reaches a maximum of 51 at 115 μm—approximately an order of magnitude higher than that at a period of 60 μm. Additionally, as the periodicity increases, the near field coupling effect between adjacent units diminishes, leading to the gradual disappearance of the two transparent windows.</sec><sec>To achieve active control over these transparent windows, we varied the conductivity of <inline-formula><tex-math id="M2">\begin{document}${\rm{VO}}_{2}$\end{document}</tex-math></inline-formula> from 20 S/m to 30000 S/m, resulting in a decrease in the transmission amplitudes of the two transparent windows to 37.58% and 3.39%, respectively. Finally, we investigated the slow light effect of the two transparent windows, comparing the maximum group delay between them, which was found to be 8.1 ps. The terahertz metasurface proposed in this study opens up avenues for the design of dynamically tunable sensing and slow light devices in the future.</sec>

The bright–bright and bright–dark mode coupling-based planar metamaterial for plasmonic EIT-like effect

In this paper, the author studies, through numerical simulation, the classical analog of the electromagnetically induced absorption/re∞ection (EIA) in a planar metamaterial structure in the near infrared spectral region. The structure is designed by transforming an electromagnetically induced transparency (EIT) structure into an EIA structure using Babinet's principle. The structure exhibits a coupling between a bright mode (a complementary ring resonator (CRR)) and a dark mode (pair of parallel straight slits) imprinted on a glass substrate. A narrow absorption window, induced in a wide transparent window, is achieved by the structure and the strength of coupling is tuned by the degree of breaking symmetry and relative displacement of the two mode elements. Electromagnetically induced transparency (EIT) is a quantum mechanical phenomenon in which an absorption is transformed into a transparency, and this is associated with strong dispersion that reduces the group velocity (1{3). This phenomena has many potential applications in signal processing, optical flltering and sensing technologies (4{7). The classical analog of the EIT has been achieved and reported by many researchers using a planar metamaterial structures that are generally composed of two metallic elements imprinted on a dielectric substrate. The two elements are metallic resonators that act either as two bright modes or a bright and dark mode. A bright mode element couples strongly with the incident excitation fleld while the dark element does not couple with the incident fleld but couples with the magnetic fleld induced by the bright element. Several EIT metamaterial structures have been employed and reported in the microwave, terahertz and optical frequency regions (8{14). In this paper, we propose a new EIT structure that achieves a more pronounced narrow transparency window sandwiched between two absorption bands compared with a similar structure proposed in (14). Also compared with (14), the structure proposed here is independent of the polarization orientation with respect to the bright mode element. Then we transform this structure into an EIA structure using Babinet's principle (15,16), where a narrow absorption/re∞ection window is sandwiched between two transparency bands. The structures proposed here may be used as obscurant particles that allow the passage of a speciflc window/windows of the electromagnetic spectrum and obscure the neighboring frequencies, for this reason the transmission spectrum will be discussed and addressed for the potential obscurant applications. Although the orientational average response of the structure is what matters, it is of good beneflt to get the behavior in a given orientation. The EIT structure is composed of a metallic ring resonator (RR) which acts as a bright mode and a pair of metallic rods that act as a dark mode. Here we overlap the RR wide antenna resonance and the narrow line width quadrupole rod resonance, which in turn produces the EIT-like efiect. The structure is transformed into an EIA structure by replacing the metallic RR and the pair of rods resonator with complementary ring resonator (CRR) and

A broad range of terahertz (THz) metamaterials have been developed for refractive index sensing. However, most of these metamaterials barely make sufficient use of the excited electric field which is crucial to achieve high sensitivity. Here, we proposed a metamaterial sensor possessing electromagnetically induced transparency (EIT) resonance that is formed by the interference of dipole and quadrupole resonance. In particular, the strengthening of light-matter interaction is realized through substrate etching, leading to a remarkable improvement in sensitivity. Hence, three kinds of etching mode were presented to maximize the utilization of the electric field, and the corresponding highest sensitivity is enhanced by up to ~2.2-fold, from 0.260 to 0.826 THz/RIU. The proposed idea to etch substrate with a strong light-matter interaction can be extended to other metamaterial sensors and possesses potential applications in integrating metamaterial and microfluid for biosensing.

Slow light with electromagnetically induced transparency (EIT) in the core of optical fibre containing three-level atoms is investigated. The guided modes are treated in the weakly guiding approximation which renders the analysis into a manageable form. The transparency window and permittivity profile of the core due to the strong pump field in the EIT scheme is calculated. For a specific permittivity profile of the core due to EIT, the propagation constant of the weak signal field and spatial shape of fundamental guided mode are calculated by solving the vector wave equation using the finite difference method. It is found that the transparency window and slow light field can be controlled via the optical fibre parameters. The reduced group velocity of slow light in this configuration is useful for many technological applications such as optical memories, effective control of single photon fields, optical buffers and delay lines.

A terahertz metamaterial structure consisting of two U-shaped split-ring resonators and a horizontal cut-line resonator is designed for realizing the electromagnetically induced transparency (EIT) effect and the Fano resonance effect. The genesis of the EIT and Fano resonance bifunctionality is illustrated by combining the near-field distribution analysis of transparent windows and transmission dips. Interestingly, the bifunctional dual-band transparency effect could transform into a single-functional single-band transparency effect when we change the position of the square metal sheet in the vertical direction of the composited terahertz metamaterial structure. In addition, based on the high-quality factor of Fano resonance response, the proposed bifunctional terahertz metamaterial device has good refractive index sensing sensitivity. These results indicate that our proposed terahertz metamaterial can provide guidance for the design of subsequent multifunctional and integrated metamaterials and optoelectronic devices.

This article investigates and improves optical properties of slow light devices. The device structure consists GaAs/AlGaAs [110] strained quantum wells (QWs) based on V-type electromagnetically induced transparency (EIT). In this paper, we use an analytical model in fractional dimension in order to simulate and analysis slow light device. Effects of well width variation have been investigated on exciton oscillator strength (EOS) and fractional dimension parameter. Influences of these parameters alterations have been illustrated in optical properties of slow light system such as slow down factor (SDF) and central energy of the device. Based on obtained results, we can adjust optical variables of slow light system with controlling the well thickness. Using proposed method, we have also estimated the maximum value of SDF about 9×103.

Guided mode resonance with extremely high Q-factors in terahertz metamaterials

A patterned laser lift-off technique is adopted for fabricating terahertz (THz) metamaterials with micrometer precision. A shaped UV laser pulse is used to remove low-viscosity silver nanopaste from a quartz substrate. Various types of metal THz metamaterials, including gradient metamaterials, can be obtained flexibly by movement of the sample and programming of the shaped beam. Two THz metamaterials, which are based on bridged double split ring resonator and its complementary resonator, respectively, are fabricated and tested. The results show that patterned laser lift-off based on a low-viscosity silver nanopaste film can supply a low-cost and easy-operation route in the fabrication of various THz metamaterials.

Triple-band graphene-based tunable electromagnetically induced transparency terahertz metamaterial with multi-frequency optical switching

A metal-perovskite hybrid metamaterial, comprising a gold cross and two gold arcs, is proposed. An electromagnetically induced transparency (EIT) based on bright–bright coupling model leads to a transparent window at 0.75 THz. A multipole scattering theory indicates that the EIT effect is generated by the destructive interference between the electric dipole and the toroidal dipole. By tuning the conductivity of perovskite, the EIT effect is dynamically controlled and shows the modulation depth of 34% at 0.75 THz. Moreover, the theoretical results calculated by the two-particle model are in good agreement with the simulated results. The group delay of the proposed EIT metamaterial can also be dynamically tuned and the maximum value of group delay is 3.53 ps. The sensitivity of the proposed EIT metamaterial is 0.182 THz/RIU. Compared with other sensors, our EIT metamaterial shows excellent sensitivity in the terahertz band. Our work provides a novel method for tunable EIT metamaterials and shows a great practical application value of modulators, slow light devices, and sensors.

In this paper, a tunable low power slow light photonic crystal device with a silicon-on-insulator platform is proposed based on the combination of an asymmetric defects coupled-cavity waveguide and the electromagnetically induced transparency (EIT) phenomenon. Modulating the refractive index of special regions in the suggested structure by the EIT phenomenon leads to a relatively wideband slow light device with adjustable group index in the same structure. Using this feature, a small and compact delay line is introduced that has many applications in optical telecommunications, especially in buffers. The numerical calculations show that the group index of 80-98 over the slow light bandwidth from 3.2 to 2.6 nm is achievable for the central wavelength of 1546-1555 nm, respectively. The device malfunction, due to fabrication errors, is modeled, and the tunable characteristics of the proposed structure are verified.

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.

The reduction in the group velocity of a light pulse due to a resonance is termed slow light. A velocity reduction to 17 m/s was demonstrated using electromagnetically induced transparency (EIT) [1]. It was later demonstrated that light could be stopped completely [2–3]. Whilst slow light EIT experiments have been ground breaking, they require a vapour cloud cooled close to 0 K. Here we present a possible alternative using gratings and a time-dependent refractive index.

The propagation of light in moving media is dragged by atomic motion. The light-drag effect can be dramatically enhanced by reducing the group velocity with electromagnetically induced transparency (EIT). We develop a systematic procedure to accurately reconstruct the complex wavefront of the slow light with single-shot measurements, enabling precise, photon shot-noise-limited spectroscopic measurements of atomic response across EIT even in the presence of generic atomic number fluctuations. Applying the technique to an expanding cloud of cold atoms, we demonstrate simultaneous inference of the atomic density distribution and the velocity field from the complex imaging data. This inline imaging technique may assist a wide range of cold-atom experiments to access spectroscopic and phase-space information with in situ and minimally destructive measurements.