Electromagnetically Induced Transparency Window Research Articles

Controllable dual resonances of Fano and EIT in a graphene-loaded all-dielectric GaAs metasurface and its sensing and slow-light applications

Abstract Active nanophotonic metasurfaces have attracted considerable attention for their promise to develop compact, tunable optical metadevices with advanced functions. In this work, we theoretically demonstrated the dynamically controllable dual resonances of Fano and electromagnetically induced transparency (EIT) using a graphene-loaded all-dielectric metasurface with U-shaped gallium arsenide (GaAs) nanobars operating in the near-infrared region. The destructive interference between a subradiant mode (i.e., a dark mode) supported by two vertical GaAs bars and two radiative modes (i.e., two bright modes) supported by a horizontal GaAs nanobar gives rise to a Fano resonance and an EIT window with high transmission and a large quality factor (Q-factor) in the transmission spectrum. Importantly, the transmission amplitudes can be flexibly modulated by adjusting the graphene Fermi levels without rebuilding the nanostructures. This modulation results from the controllable light absorption by the loaded graphene monolayer due to its interband losses in the near-infrared spectrum. Furthermore, the peak wavelengths of the Fano resonance and EIT window with high Q-factors are highly sensitive to variations in the refractive index (RI) of the surrounding medium, giving the proposed metasurface a relatively good sensitivity of ~700 nm/RIU and a high figure of merit (FOM) of 280, making it an effective RI sensor. Additionally, the metasurface features an adjustable slow light effect, indicated by the adjusted group delay time ranging from 0.12 ps to 0.38 ps. Therefore, the metasurface system proposed in this work offers a viable platform for advanced multi-band optical sensing, low-loss slow light devices, switches, and potential applications in nonlinear optical fields.

Gain measurement of microwave antenna with heterodyne bichromatic excitation in Rydberg atoms

We propose a scheme for gain measurement of microwave (MW) antenna with heterodyne bichromatic excitation in Rydberg atoms via electromagnetically induced transparency (EIT). The Rydberg-EIT atoms serve as a frequency mixer with a strong locally oscillating MW field and a weak signal field. A large dispersion appears in the EIT windows due to the interference of two sub-EIT systems, which much narrows the transmission spectrum. The locally oscillating MW field could enhance the atomic response to the weak MW signals. The simulation results show that the gain measurement of MW antenna remains good accuracy even for weak MW fields and the minimum detectable MW field strength is about 1/12 of that of common EIT scheme. Other influences on the gain measurement are also investigated.

Terahertz metal-graphene hybrid metamaterial for active manipulation of electromagnetically induced transparency

The dynamic tuning of terahertz (THz) electromagnetically induced transparency (EIT) offers significant potential in rapidly changing THz regulation fields, such as the next generation wireless communication, switching, and sensing. In this work, we demonstrate a THz EIT metamaterial by combining a circular and a rectangular split-ring resonator (CSRR and RSRR) based on the bright-bright mode coupling. The introduction of graphene into the EIT structure enables active tuning, and reveals two distinct regulation mechanisms in the metal-graphene hybrid EIT metamaterial: 1. the internal coupling effect diminishes with the increase of the Fermi level in the CSRR-graphene integration; 2. the surface electric field undergoes redistribution in the RSRR-graphene integration. These two dynamic regulation mechanisms are well confirmed by the simulated electric field distribution. As a result, the transmittance within the EIT window of the hybrid metamaterial can be precisely modulated, exhibiting a significant modulation depth of >85%. Notably, the integration with graphene in the RSRR allows for simultaneous modulation of resonances on either side of the EIT peaks, which expands more THz modulation channels. This work enriches the design and implementation methods for active THz EIT and shows potential applications in THz dynamic application scenarios.

Electromagnetically induced transparency enabled by quasi-bound states in the continuum modulated by epsilon-near-zero materials.

Highly tunable electromagnetically induced transparency (EIT) with high-quality-factor (Q-factor) excited by combining with the quasi-bound states in the continuum (quasi-BIC) resonances is crucial for many applications. This paper describes all-dielectric metasurface composed of silicon cuboid etched with two rectangular holes into a unit cell and periodically arranged on a SiO2 substrate. By breaking the C2 rotational symmetry of the unit cell, a high-Q factor EIT and double quasi-BIC resonant modes are excited at 1224.3, 1251.9 and 1299.6 nm with quality factors of 7604, 10064 and 15503, respectively. We show that the EIT resonance is caused by destructive interference between magnetic dipole resonances and quasi-BIC dominated by electric quadrupole. Toroidal dipole (TD) and electric quadrupole (EQ) dominate the other two quasi-BICs. The EIT window can be successfully modulated with transmission intensity from 90% to 5% and modulation depths ranging from -17 to 24 dB at 1200-1250 nm by integrating the metasurface with an epsilon-near-zero (ENZ) material indium tin oxide (ITO) film. Our findings pave the way for the development of applications such as optical switches and modulators with many potential applications in nonlinear optics, filters, and multichannel biosensors.

Colossal Kerr nonlinearity without absorption in a five-level atomic medium

In this work, we present an analytical method to achieve giant Kerr nonlinearity without absorption in a five-level atomic medium. By using iterative perturbation technique on density matrix equations, we have derived the analytical expressions of nonlinear susceptibility and Kerr nonlinear coefficient in the presence of spontaneously generated coherence (SGC) and relative phase between applied laser fields. It shows that, this five-level atomic medium exhibits multiple electromagnetically induced transparency (EIT) at three different frequencies, at the same time, the Kerr nonlinear coefficient is enhanced around three transparent spectral regions; in each such EIT region appears a pair of positive–negative peaks of Kerr nonlinear coefficient. In particular, these nonlinear peaks are moved to the center of the EIT windows via SGC. This means that the Kerr nonlinear coefficient is enhanced with completely suppressed absorption at different transparency frequencies. Furthermore, the magnitude and the sign of the Kerr nonlinear coefficient are easily controlled according to the SGC strength, the coupling laser intensity, and the relative phase between applied laser fields. Such a giant nonlinear medium can be useful for photonic devices working in the resonant frequency region without absorption. As a typical application, this giant Kerr nonlinear material has been applied to an interferometer for the formation of optical bistability, and showed the appearance of OB at the resonant frequency with significantly reduced threshold intensity and OB width.

Double electromagnetically induced transparency in a four-level lambda atomic system with doppler broadening

In this paper, we study the formation of two electromagnetically induced transparency (EIT) windows in a four-level lambda atomic system with Doppler broadening. The depth and the width of the EIT windows are easily changed by adjusting the intensity of the controlling laser fields. Meanwhile, the position of the EIT windows is also easily shifted by changing the frequency of the laser fields. In particular, by turning on or off one of the two coupling laser fields, the probe field response can be switched between one or two EIT window regimes, i.e., it is possible to switch between absorption and transparency regimes, and vice versa. In addition, our analytical model also allows one to investigate the influence of temperature (or Doppler width) on EIT windows, which is necessary for experimental observations as well as related applications at room temperature.

The impact of contact and contactless interactions between the meta-atoms on terahertz bound states in the continuum

The excitation and manipulation of symmetry-protected bound states in the continuum (SP-BIC) is significantly valuable for metasurface-based biosensors. The interactions between adjacent meta-atoms determine the fundamental properties of SP-BIC; however, this topic has not been profoundly explored. In this work, we experimentally and numerically investigate the effects of contactless and contact interactions between adjacent dual-gap split-ring resonators (DSRRs) on the SP-BIC. We demonstrate that there is only one SP-BIC at 0.9 THz when the incident radiation polarization is parallel to the gap in both contactless and contact coupling conditions. When the polarization is vertical to the gap, the individual SP-BIC shifts the frequency to 0.8 THz under contactless coupling. Under contact coupling, the SP-BIC degrades to be an electromagnetically induced transparency (EIT) windows at 0.8 THz. We calculated a 3.6 ps group delay of slow light for EIT. Numerical simulations indicate that the combination of one magnetic dipole (MD) in the inner arm and another electric dipole in the outer arm of DSRR results in quasi-BIC at 0.9 THz and 0.8 THz under contactless coupling. Under contact coupling conditions, the formation of quasi-BIC at 0.9 THz is similar to contactless coupling. However, two MDs of opposite polarity results in the EIT windows at 0.8 THz. Our results reveal excitation and manipulation of terahertz SP-BIC via contactless and contact coupling, which is significant for the innovation of terahertz biosensors.

Terahertz Metamaterial Sensor Based on Electromagnetic Induced Transparency

A graphene-based terahertz electromagnetically induced transparency (EIT) metamaterial sensor is proposed and studied. The sensor is made up of two bright modes: a graphene strip resonator and a 7-shape resonator. In a terahertz metamaterial sensor based on EIT, the metamaterial structure is designed to have two resonant modes that are coupled through a common resonator. When terahertz radiation hits the metamaterial, the two resonant modes interact, creating a window of transparency in the transmission spectrum. It illuminated that the physical mechanism of the EIT effect lay in the recombination effect of the conductive resonators. By changing the carrier relaxation lifetime or the Fermi energy of the graphene, the amplitude or the location of the EIT window could be actively tuned. The terahertz metamaterial sensors based on EIT have the potential to provide highly accurate and sensitive measurements in a wide range of fields and could lead to important advances in medical diagnostics.

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.

Influence of spontaneously generated coherence on absorption, dispersion, and group index in a five-level cascade atomic system

By solving the density matrix equations in the steady state, we have derived analytical expressions for the absorption, dispersion, and group index of a five-level cascade-type atomic system as functions of laser intensity and frequency, spontaneously generated coherence (SGC), and relative phase of applied fields. The influences of SGC and relative phase on absorption, dispersion, and group index of this system under electromagnetically induced transparency (EIT) are studied. It is shown that the three EIT windows of system become deeper and narrower as the strength of SGC increases. These lead to an increase in the slope and the amplitude of the dispersion curves at three EIT windows. As a result, the amplitude of the group index at these three EIT windows also becomes larger when the strength of SGC increases. In particular, the group index can easily be switched between negative and positive values i.e., the light propagation can easily be converted between superluminal to subluminal modes by adjusting the strength of SGC, relative phase or the coupling laser intensity.

Ultrafast Modulation of THz Waves Based on MoTe2-Covered Metasurface.

The sixth generation (6G) communication will use the terahertz (THz) frequency band, which requires flexible regulation of THz waves. For the conventional metallic metasurface, its electromagnetic properties are hard to be changed once after being fabricated. To enrich the modulation of THz waves, we report an all-optically controlled reconfigurable electromagnetically induced transparency (EIT) effect in the hybrid metasurface integrated with a 10-nm thick MoTe2 film. The experimental results demonstrate that under the excitation of the 800 nm femtosecond laser pulse with pump fluence of 3200 μJ/cm2, the modulation depth of THz transmission amplitude at the EIT window can reach 77%. Moreover, a group delay variation up to 4.6 ps is observed to indicate an actively tunable slow light behavior. The suppression and recovery of the EIT resonance can be accomplished within sub-nanoseconds, enabling an ultrafast THz photo-switching and providing a promising candidate for the on-chip devices of the upcoming 6G communication.

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.

Controlling Raman gain with atomic coherence

Raman gain is the optical gain generated by stimulated Raman scattering, which makes weak light amplify possible and can play an important role in weak light communication as well as related field. In this paper, experimentally and theoretically, we have observed the controllable Raman gain under the dressing effects in a coherently prepared N-type four-level atomic configuration. By controlling the detuning of the signal field and the coupling field, the electromagnetically induced transparency (EIT) window can be observed in the signal field. With the incidence of a strong pump laser into the atomic system, the two-photon ultranarrow fluorescence signal can overlap with EIT window by changing the frequency detuning of the pump field, meanwhile, a Raman gain can be induced by placing the signal field at possible large detuning. Such Raman gain can be effectively regulated and optimized by tuning the power of the signal field, pump field and the temperature of the atomic system so as to achieve higher contrast. The multi-parameter regulated higher contrast Raman gain with atomic coherence in atomic thermal system has potential applications in optical communication, superluminal laser and biophotonics.

Active control of dual electromagnetically induced transparency in terahertz graphene-metal hybrid metasurfaces

Active control of electromagnetically induced transparency (EIT) using metasurfaces has attracted growing interests in recent years, especially the ones that have multiple EIT windows. Here, we give out a metallic metasurface design that can achieve dual EIT (D-EIT) in the terahertz (THz) regime, and propose a strategy to individually and simultaneously control the two windows by integrating graphene structures into the design. The near-field simulations indicate that the physical mechanism lies in the composite effect of conductive graphene. The theoretical analysis reveals that the active modulation is attributed to the changes in the damping rates of the dark modes and the coupling coefficients between bright mode and dark modes. The proposed graphene-metal hybrid metasurfaces provide a way for designing compact dual-band slow-light and modulation devices, which may find potential applications in dual-frequency-channel THz wireless communications.

Polarization-controlled electromagnetically induced transparency analogue based on multipolar resonances

An asymmetric metasurface capable of a dynamically controlled electromagnetically induced transparency (EIT) analogue is proposed by changing the polarization direction of the incident light. The metasurface composed of Z-L shape meta-atoms exhibits a narrowband transparency window at 1.858 THz for y-polarization incidence. This EIT window originates from the destructive interference between magnetic dipole (MD), toroidal dipole (TD) and electric quadrupole (EQ) resonances, where the coupling of MD and TD serves as bright mode and EQ acts as dark mode. By varying the polarization angle (φ) of the incident light, the transmission of the dip at 1.856/1.861 THz can be tuned from 0.90/0.95 to 0.08/0.03. Meanwhile, the Q-factor of 640.55 and the group delay of 117.21 ps are achieved at φ = 120°. This work will promote the development of novel multipolar supported EIT metasurfaces.

Frequency tunable superflash based on an electromagnetically induced transparency (EIT).

We present a new controllable superflash, a maximum transient peak at the falling edge of a square pulse, using a three-level electromagnetically induced transparency (EIT). The superflash in a two-level system occurs at a certain value of the detuning of the probe laser (Δpeak) when the optical depth (OD) of the medium is sufficiently large and constant. Here, we show the external tunability of the Δpeak for obtaining the maximum transient peak by introducing a three-level EIT. Owing to the EIT dispersion properties, we effectively tune the phase of the forward-scattered light (Es) by externally controlling the EIT coupling light intensities associated with the Rabi frequency (Ωc). When the incident light is turned off, the total transmitted field (Et) experiences an out-of-phase shift, producing a peak intensity that is three times higher than the input. We observe that this new class of superflash (Type II) occurs near the EIT window and exhibits inverse-linearity of the OD, which is in contrast with the characteristics of the previously reported two-level superflash (Type I). Furthermore, we find the quadratic nature of tunability in Δpeak(Ωc).The new tunability of the superflash by the Rabi frequency may facilitate more rapid and effective control of the superflash compared to a passive optical medium.

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

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

Tunable quantum interference effects in Floquet two- and three-level systems

Quantum interference effects in the unmodulated quantum systems with light-matter interaction have been widely studied, such as electromagnetically induced transparency (EIT) and Autler-Townes splitting (ATS). However, the similar quantum interference effects in the Floquet systems (i.e., periodically modulated systems), which might cover rich new physics, were rarely studied. In this article, we investigate the quantum interference effects in the Floquet two- and three-level systems analytically and numerically. We show a coherent destruction tunneling effect in a lotuslike multipeak spectrum with a Floquet two-level system, where the intensity of the probe field is periodically modulated with a square-wave sequence. We demonstrate that the multipeak split into multiple transparency windows with tunable quantum interference if the Floquet system is asynchronously controlled via a third level. Based on phenomenological analysis with Akaike information criterion, we show that the symmetric central transparency window has a similar mechanism to the traditional ATS or EIT depending on the choice of parameters, additional with an extra degree of freedom to control the quantum interference provided by the modulation period. The other transparent windows are shown to be asymmetric, different from the traditional ATS and EIT windows. These nontrivial quantum interference effects open up a scope to explore the applications of the Floquet systems.

Electromagnetically induced transparency, narrow absorption and transient response in a three-photon excitation process

The probe absorption characteristic of a four-level ladder-type system, in presence of two strong coherent radiation fields, is investigated analytically by following the density operator method. The parameters of 85Rb 5S1/2→5P3/2→5D5/2→63P3/2 transition are used in the simulation. In addition to the control field acting between the first excited state and the second excited state, if a switching field is applied to couple the second excited state to a Rydberg state, two-photon electromagnetically induced transparency (EIT) is converted into double EIT (DEIT) window having a strong central absorption peak. In a thermal vapor, the double EIT window is transformed into three transparency windows depending on the values of the control and switching Rabi frequencies. These multiple transparency windows vanish for certain combinations of the Rabi frequencies of the control and switching fields and the probe absorption is enhanced. It is also demonstrated how a very narrow optical band stop filter can be conceptualized by using three-photon excitation scheme. The transient optical response of the medium under three-photon transition is also investigated under both Doppler free and Doppler broadened conditions.

Switchable Metasurface with Electromagnetically Induced Transparency and Absorption Simultaneously Realizing Circular Polarization‐Insensitive Circular‐to‐Linear Polarization Conversion

AbstractA metasurface (MS) with switching features from electromagnetically induced transparency (EIT) to electromagnetically induced absorption (EIA) is theoretically investigated in the terahertz (THz) range by incorporating vanadium dioxide (VO2) when circular polarization‐insensitive circularly polarized waves are incidents, which can also simultaneously realize circular‐to‐linear polarization conversion well. While the VO2 ring is in an insulator state, two new transparent EIT windows both are gained between 0.97 THz and 1.16 THz because of destructive interference between the two metal rings that are seen as bright modes. Once the VO2 ring is tuned to the metallic state, a perfect EIA phenomenon is observed between 0.824 THz and 1.233 THz, achieving the transition from the EIT to EIA behavior well. The received electromagnetic waves are radiated to left‐handed circularly polarized (LCP) or right‐handed circularly polarized (RCP) waves with identical phases and amplitudes through the radiation patches when the LCP or RCP ones are incident. Whereby, a resultant linearly polarized wave is obtained between 0.9 THz and 1.2 THz, and relative bandwidth reaches 29%. The outcomes of maximal group index and group delay both are 1934 and 488 ps at 1.08 THz, and maximal absorption is 90.3% at 1.06 THz.