Plasmonic analogue of electromagnetically induced transparency at the Drude damping limit

Self-assembled skin-like metamaterials for dual-band camouflage

Skin-like soft optical metamaterials with broadband modulation have been long pursued for practical applications, such as cloaking and camouflage. Here, we propose a skin-like metamaterial for dual-band camouflage based on unique Au nanoparticles assembled hollow pillars (NPAHP), which are implemented by the bottom-up template-assisted self-assembly processes. This dual-band camouflage realizes simultaneously high visible absorptivity (~0.947) and low infrared emissivity (~0.074/0.045 for mid-/long-wavelength infrared bands), ideal for visible and infrared dual-band camouflage at night or in outer space. In addition, this self-assembled metamaterial, with a micrometer thickness and periodic through-holes, demonstrates superior skin-like attachability and permeability, allowing close attachment to a wide range of surfaces including the human body. Last but not least, benefiting from the extremely low infrared emissivity, the skin-like metamaterial exhibits excellent high-temperature camouflage performance, with radiation temperature reduction from 678 to 353 kelvin. This work provides a new paradigm for skin-like metamaterials with flexible multiband modulation for multiple application scenarios.

Quasi-bound states in the continuum for electromagnetic induced transparency and strong excitonic coupling

Advancing on previous reports, we utilize quasi-bound states in the continuum (q-BICs) supported by a metasurface of TiO2 meta-atoms with broken inversion symmetry on an SiO2 substrate, for two possible applications. Firstly, we demonstrate that by tuning the metasurface's asymmetric parameter, a spectral overlap between a broad q-BIC and a narrow magnetic dipole resonance is achieved, yielding an electromagnetic induced transparency analogue with a 50 μs group delay. Secondly, we have found that, due to the strong coupling between the q-BIC and WS2 exciton at room temperature and normal incidence, by integrating a single layer of WS2 to the metasurface, a 37.9 meV Rabi splitting in the absorptance spectrum with 50% absorption efficiency is obtained. These findings promise feasible two-port devices for visible range slow-light characteristics or nanoscale excitonic coupling.

A Polarization-Insensitive, Vanadium Dioxide-Based Dynamically Tunable Multiband Terahertz Metamaterial Absorber

A tunable multiband terahertz metamaterial absorber, based on vanadium dioxide (VO2), is demonstrated. The absorber comprises a three-layer metal-insulator-metal (MIM) configuration with a split ring and slots of VO2 on the uppermost layer, a middle dielectric substrate based on silicon dioxide (SiO2), and a gold reflector on the back. The simulation results indicate that, when VO2 is in the metallic state, the proposed metamaterial exhibits nearly perfect absorption at six distinct frequencies. The design achieves an average absorption of 98.2%. The absorptivity of the metamaterial can be dynamically tuned from 4% to 100% by varying the temperature-controlled conductivity of VO2. The proposed metamaterial absorber exhibits the advantages of polarization insensitivity and maintains its absorption over 80% under different incident angle conditions. The underlying physical mechanism of absorption is explained through impedance matching theory, interference theory, and the distribution of electric fields. The ability to achieve multiband absorption with tunable characteristics makes the proposed absorber a promising candidate for applications in terahertz sensing, imaging, communication, and detection. The polarization insensitivity further enhances its practicality in various scenarios, allowing for versatile and reliable performance in terahertz systems.

Terahertz stretchable metamaterials with deformable dolmen resonators for uniaxial strain measurement

In this study, we propose a terahertz stretchable metamaterial that can measure uniaxial strain. Gold dolmen resonators formed on a sheet of polydimethylsiloxane (PDMS) is deformed by strain, and its resonance peak exhibits the gradual decrease in reflectance without a frequency shift, which is suitable for imaging applications at a single frequency. The metamaterial was designed by mechanical and electromagnetic simulations and fabricated by microfabrication including a transfer process of gold structures from a glass substrate to a PDMS sheet. By measuring the reflectance and observing the deformation under different strains, the reflectance decrease was obtained at 0.292 THz despite the appearance of wrinkles on gold structures. Linear response and repeatability up to 20% strain were also confirmed. Furthermore, the strain measurement through a sheet of paper was demonstrated, suggesting that our method can be applied even in situations where opaque obstacles in the visible region exist.

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

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

Magneto-optical heterostructures with second resonance of transverse magneto-optical Kerr effect

Two conventional magneto-plasmonic (MP) structures are firstly superimposed with mirror symmetry to form a symmetric MP heterostructure. These two MP components are separated from each other by a noble metallic layer. The unique feature of this novel heterostructure is that both magneto-plasmon modes of the up and down MP portions can be coupled as the spacer becomes thinner. This intertwining effect leads to appearance of a new peak in the angular transverse magneto-optical Kerr effect (TMOKE) curve of the heterostructure. This new peak which is reported for the first time in the TMOKE signal, is generally similar to plasmon induced transparency (PIT) phenomenon observed in plasmonic multilayered structures. We entitle this novel effect as "second resonance of TMOKE signal". More importantly, the occurrence angle and magnitude of the second peak can be controlled by varying the thickness and material of separating layer between two MP parts. Also, the dispersion diagram of the heterostructure shows this coupling so that two branches convert into four branches by reducing the thickness of spacer. Furthermore, coupled oscillators model confirms emergence of the second peak in the TMOKE signal. These results can offer great promise for increasing sensitivity of conventional magneto-optical refractive index sensors.

Directive giant upconversion by supercritical bound states in the continuum

Photonic bound states in the continuum (BICs), embedded in the spectrum of free-space waves1,2 with diverging radiative quality factor, are topologically non-trivial dark modes in open-cavity resonators that have enabled important advances in photonics3,4. However, it is particularly challenging to achieve maximum near-field enhancement, as this requires matching radiative and non-radiative losses. Here we propose the concept of supercritical coupling, drawing inspiration from electromagnetically induced transparency in near-field coupled resonances close to the Friedrich-Wintgen condition2. Supercritical coupling occurs when the near-field coupling between dark and bright modes compensates for the negligible direct far-field coupling with the dark mode. This enables a quasi-BIC field to reach maximum enhancement imposed by non-radiative loss, even when the radiative quality factor is divergent. Our experimental design consists of a photonic-crystal nanoslab covered with upconversion nanoparticles. Near-field coupling is finely tuned at the nanostructure edge, in which a coherent upconversion luminescence enhanced by eight orders of magnitude is observed. The emission shows negligible divergence, narrow width at the microscale and controllable directivity through input focusing and polarization. This approach is relevant to various physical processes, with potential applications for light-source development, energy harvesting and photochemical catalysis.

Ultrahigh-Q Metasurface Transparency Band Induced by Collective-Collective Coupling

The metasurface analogue of electromagnetically induced transparency (EIT) provides a chip-scale platform for achieving light delay and storage, high Q factors, and greatly enhanced optical fields. However, the literature relies on the coupling between localized and localized or localized and collective resonances, limiting the Q factor and related performance. Here, we report a novel approach for realizing collective EIT-like bands with a measured Q factor reaching 2750 in silicon metasurfaces in the near-infrared regime, exceeding the state of the art by more than 5 times. It employs the coupling between two collective resonances, the Mie electric dipole surface lattice resonance (SLR) and the out-of-plane/in-plane electric quadrupole SLR (EQ-SLR). Remarkably, the collective EIT-like resonance can have diverging Q factor and group delay due to the bound state in the continuum characteristics of the in-plane EQ-SLR. With these findings, our study opens a new route for tailoring light flow in metasurfaces.

Multifunctional graphene metamaterials based on polarization-insensitive plasmon-induced transparency

In this paper, a 4L-shaped graphene patterned polarization-insensitive plasmon-induced transparency (PIT) metamaterial structure is proposed. The photoelectric switch based on this structure supports a variety of light sources, such as linearly polarized light with different polarization directions, left rotation circularly polarized light (LCP) and right rotation circularly polarized light (RCP). And the switch has excellent performance in the case of different light sources, the amplitude modulation is as high as 99.01%, and the insertion loss is as low as 0.04 dB. In addition, the PIT metamaterial has a high refractive index sensitivity of up to 49156 nm/RIU. The group index of the PIT metamaterial is as high as 980, which can achieve excellent slow light effect. This study provides a scheme and guidance for the design of optoelectronic devices.

Terahertz Metamaterials for Biosensing Applications: A Review

In recent decades, THz metamaterials have emerged as a promising technology for biosensing by extracting useful information (composition, structure and dynamics) of biological samples from the interaction between the THz wave and the biological samples. Advantages of biosensing with THz metamaterials include label-free and non-invasive detection with high sensitivity. In this review, we first summarize different THz sensing principles modulated by the metamaterial for bio-analyte detection. Then, we compare various resonance modes induced in the THz range for biosensing enhancement. In addition, non-conventional materials used in the THz metamaterial to improve the biosensing performance are evaluated. We categorize and review different types of bio-analyte detection using THz metamaterials. Finally, we discuss the future perspective of THz metamaterial in biosensing.

Design and experimental realization of triple-band electromagnetically induced transparency terahertz metamaterials employing two big-bright modes for sensing applications

Multi-band electromagnetically induced transparency (EIT) effects have attracted widespread attention due to their great application prospects. However, their realization is mainly based on the coupling of multiple sub-resonators that typically exceed the number of transparency peaks, resulting in complex structural designs and cumbersome preparation procedures. This paper reports a simple design of a terahertz metamaterial that can produce the triple-band EIT effect using two "big-bright" mode coupling of two sub-resonators. The design adopts the classical two-layer structure. A U-shaped split-ring resonator and a fork-shaped resonator form a periodic array on the surface of the flexible organic polymer material. Three transparency peaks around 0.59 THz, 1.07 THz, and 1.34 THz are experimentally realized, and their formation mechanisms are explored. Furthermore, the triple-band EIT metamaterial was prepared by the photolithography technology and characterized by terahertz time-domain spectroscopy. Theoretical simulation results agree well with experimental results. Sensing characteristics and slow light effects of the terahertz metamaterial are further discussed experimentally. The proposed triple-band EIT metamaterial having excellent properties, including thin size, good flexibility, simple and compact structure, and high sensing sensitivity, could provide guidance for the subsequent design and implementation of multifunctional multi-band EIT metamaterials.

A One-Bit Programmable Multi-Functional Metasurface for Microwave Beam Shaping

In this paper, we demonstrate a multi-functional metasurface for microwave beam-shaping application. The metasurface consists of an array of programmable unit cells, and each unit cell is integrated with one varactor diode. By turning the electrical bias on the diode on and off, the phase delay of the microwave reflected by the metasurface can be switched between 0 and π at a 6.2 GHz frequency, which makes the metasurface 1-bit-coded. By programming the 1-bit-coded metasurface, the generation of a single-focus beam, a double-focus beam and a focused vortex beam was experimentally demonstrated. Furthermore, the single-focus beam with tunable focal lengths of 54 mm, 103 mm and 152 mm was experimentally observed at 5.7 GHz. The proposed programmable metasurface manifests robust and flexible beam-shaping ability which allows its application to microwave imaging, information transmission and sensing applications.

Terahertz narrowband filter metasurfaces based on bound states in the continuum

The electromagnetically induced transparency (EIT) effect realized by metasurfaces have potential for narrowband filtering due to their narrow bandwidth. In optics, bound states in the continuum (BIC) can produce strong localized resonances, which means that light can be trapped and stored for long periods of time to produce very high Q-factors. This has potential applications in designing highly efficient sensors and narrow bandpass filters. Here, we present two metal-flexible dielectric metasurfaces consisting of copper structures and polyimide substrates. Quasi BICs are obtained by breaking C2 symmetry of the metal structures. Resonance-captured quasi-BICs with ultra-high q-factor resonances satisfy the dark modes required to realize the EIT and couple to the bright modes in the structure to achieve narrowband filtering. The peak transmission rates are around 0.9 at 0.29 THz-0.32 THz and 0.23 THz-0.27 THz, respectively. Our results have practical implications for the realization of low-frequency terahertz communications.

Surface-acoustic-wave-driven graphene plasmonic sensor for fingerprinting ultrathin biolayers down to the monolayer limit

Surface plasmon polaritons in graphene can enhance the performance of mid-infrared spectroscopy, which is key for the study of both the composition and the conformation of organic molecules via their vibrational resonances. In this paper, a plasmonic biosensor using a graphene-based van der Waals heterostructure on a piezoelectric substrate is theoretically demonstrated, where far-field light is coupled to surface plasmon-phonon polaritons (SPPPs) through a surface acoustic wave (SAW). The SAW creates an electrically-controlled virtual diffraction grating, suppressing the need for patterning the 2D materials, that limits the polariton lifetime, and enabling differential measurement schemes, which increase the signal-to-noise ratio and allow a quick commutation between reference and sample signals. A transfer matrix method has been used for simulating the SPPPs propagating in the system, which are electrically tuned to interact with the vibrational resonances of the analytes. Furthermore, the analysis of the sensor response with a coupled oscillators model has proven its capability of fingerprinting ultrathin biolayers, even when the interaction is too weak to induce a Fano interference pattern, with a sensitivity down to the monolayer limit, as tested with a protein bilayer or a peptide monolayer. The proposed device paves the way for the development of advanced SAW-assisted lab-on-chip systems combining the existing SAW-mediated physical sensing and microfluidic functionalities with the chemical fingerprinting capability of this novel SAW-driven plasmonic approach.

Bidirectional planar absorber with polarization-selective absorption and reflection capabilities

In this study, we developed a novel planar bidirectional perfect metamaterial absorber (PMA) with polarization-selective absorption and reflection capabilities. The proposed bidirectional PMA has near-perfect absorption for y- and x-polarized waves propagating in the -z and + z directions. It also reflects x- and y-polarized waves propagating in the -z and + z directions. We used full-wave simulations and Fabry-Perot cavity models to evaluate the performance of the proposed bidirectional PMA. We also used a free-space method to measure the fabricated sample. To demonstrate the potential of the proposed PMA in multiband systems, we extended our PMA design to a dual-band bidirectional absorber.

Research Progress in Surface-Enhanced Infrared Absorption Spectroscopy: From Performance Optimization, Sensing Applications, to System Integration

Infrared absorption spectroscopy is an effective tool for the detection and identification of molecules. However, its application is limited by the low infrared absorption cross-section of the molecule, resulting in low sensitivity and a poor signal-to-noise ratio. Surface-Enhanced Infrared Absorption (SEIRA) spectroscopy is a breakthrough technique that exploits the field-enhancing properties of periodic nanostructures to amplify the vibrational signals of trace molecules. The fascinating properties of SEIRA technology have aroused great interest, driving diverse sensing applications. In this review, we first discuss three ways for SEIRA performance optimization, including material selection, sensitivity enhancement, and bandwidth improvement. Subsequently, we discuss the potential applications of SEIRA technology in fields such as biomedicine and environmental monitoring. In recent years, we have ushered in a new era characterized by the Internet of Things, sensor networks, and wearable devices. These new demands spurred the pursuit of miniaturized and consolidated infrared spectroscopy systems and chips. In addition, the rise of machine learning has injected new vitality into SEIRA, bringing smart device design and data analysis to the foreground. The final section of this review explores the anticipated trajectory that SEIRA technology might take, highlighting future trends and possibilities.

Slot driven dielectric electromagnetically induced transparency metasurface

The control of resonant metasurface for electromagnetically induced transparency (EIT) offers unprecedented opportunities to tailor lightwave coupling at the nanoscale leading to many important applications including slow light devices, optical filters, chemical and biosensors. However, the realization of EIT relies on the high degree of structural asymmetry by positional displacement of optically resonant structures, which usually lead to low quality factor (Q-factor) responses due to the light leakage from structural discontinuity from asymmetric displacements. In this work, we demonstrate a new pathway to create high quality EIT metasurface without any displacement of constituent resonator elements. The mechanism is based on the detuning of the resonator modes which generate dark-bright mode interference by simply introducing a slot in metasurface unit cells (meta-atoms). More importantly, the slot diameter and position on the meta-atom can be modulated to tune the transmittance and quality factor (Q-factor) of the metasurface, leading to a Q-factor of 1190 and near unity transmission at the same time. Our work provides a new degree of freedom in designing optically resonant elements for metamaterials and metasurfaces with tailored wave propagation and properties.

Nonlocality-Enabled Pulse Management in Epsilon-Near-Zero Metamaterials

Ultrashort optical pulses are integral to probing various physical, chemical, and biological phenomena and feature in a whole host of applications, not least in data communications. Super- and subluminal pulse propagation and dispersion management (DM) are two of the greatest challenges in producing or counteracting modifications of ultrashort optical pulses when precise control over pulse characteristics is required. Progress in modern photonics toward integrated solutions and applications has intensified this need for greater control of ultrafast pulses in nanoscale dimensions. Metamaterials, with their unique ability to provide designed optical properties, offer a new avenue for temporal pulse engineering. Here an epsilon-near-zero metamaterial is employed, exhibiting strong nonlocal (spatial dispersion) effects, to temporally shape optical pulses. The authors experimentally demonstrate, over a wide bandwidth of tens of THz, the ability to switch from sub to superluminal and further to "backward" pulse propagation (±c/20) in the same metamaterial device by simply controlling the angle of illumination. Both the amplitude and phase of a 10 ps pulse can be controlled through DM in this subwavelength device. Shaping ultrashort optical pulses with metamaterials promises to be advantageous in laser physics, optical communications, imaging, and spectroscopy applications using both integrated and free-standing devices.

Ultrasensitive Molecular Fingerprint Retrieval Using Strongly Detuned Overcoupled Plasmonic Nanoantennas

Tailoring light-matter interactions via plasmonic nanoantennas (PNAs) has emerged as a breakthrough technology for spectroscopic applications. The detuning between molecular vibrations and plasmonic resonances, as a fundamental and inevitable optical phenomenon in light-matter interactions, reduces the interaction efficiency, resulting in a weak molecule sensing signal at the strong detuning state. Here, it is demonstrated that the low interaction efficiency from detuning can be tackled by overcoupled PNAs (OC-PNAs) with a high ratio of the radiative to intrinsic loss rates, which can be used for ultrasensitive spectroscopy at strong plasmonic-molecular detuning. In OC-PNAs, the ultrasensitive molecule signals are achieved within a wavelength detuning range of 248 cm-1 , which is 173 cm-1 wider than previous works. Meanwhile, the OC-PNAs are immune to the distortion of molecular signals and maintain a lineshape consistent with the molecular signature fingerprint. This strategy allows a single device to enhance and capture the full and complex fingerprint vibrations in the mid-infrared range. In the proof-of-concept demonstration, 13 kinds of molecules with some vibration fingerprints strongly detuning by the OC-PNAs are identified with 100% accuracy with the assistance of machine-learning algorithms. This work gains new insights into detuning-state nanophotonics for potential applications including spectroscopy and sensors.

Switchable and Tunable Terahertz Metamaterial Based on Vanadium Dioxide and Photosensitive Silicon

A switchable and tunable terahertz (THz) metamaterial based on photosensitive silicon and Vanadium dioxide (VO₂) was proposed. By using a finite-difference time-domain (FDTD) method, the transmission and reflective properties of the metamaterial were investigated theoretically. The results imply that the metamaterial can realize a dual electromagnetically induced transparency (EIT) or two narrow-band absorptions depending on the temperature of the VO₂. Additionally, the magnitude of the EIT and two narrow-band absorptions can be tuned by varying the conductivity of photosensitive silicon (PSi) via pumping light. Correspondingly, the slow-light effect accompanying the EIT can also be adjusted.

Terahertz biosensor integrated with Au nanoparticles to improve the sensing performance

A terahertz (THz) sensor is presented based on a metasurface integrated with Au nanoparticles (AuNPs). It was used to improve the sensitivity of the detection of whey protein and to enhance the sensing index of group delay. This demonstrates that AuNPs can improve the sensing performance of the biosensor. The internal mechanism can be explained by the modified perturbation theory and the coupled harmonic oscillator model. This study provides a means of enhancing the sensitivity of the THz biosensor.

Mode-Specific Coupling of Nanoparticle-on-Mirror Cavities with Cylindrical Vector Beams

Nanocavities formed by ultrathin metallic gaps permit the reproducible engineering and enhancement of light-matter interaction, with mode volumes reaching the smallest values allowed by quantum mechanics. While the enhanced vacuum field in metallic nanogaps has been firmly evidenced, fewer experimental reports have examined the far-field to near-field input coupling under strongly focused laser beam. Here, we experimentally demonstrate selective excitation of nanocavity modes controlled by the polarization and frequency of the laser beam. We reveal mode selectivity by recording confocal maps of Raman scattering excited by cylindrical vector beams, which are compared to the known excitation near-field patterns. Our measurements reveal the transverse vs longitudinal polarization of the excited antenna mode and how the input coupling rate depends on laser wavelength. The method introduced here is easily applicable to other experimental scenarios, and our results help connect far-field with near-field parameters in quantitative models of nanocavity-enhanced phenomena.

Active control of terahertz waves based on hybrid VO2 periodic corrugated waveguides

We describe a method for the active control of terahertz (THz) waves using hybrid vanadium dioxide (VO₂) periodic corrugated waveguide. Unlike liquid crystals, graphene and semiconductors and other active materials, VO₂ exhibits a unique insulator-metal transition characteristic by the electric fields, optical, and thermal pumps, resulting in five orders of magnitude changes in its conductivity. Our waveguide consists of two gold coated plates with the VO₂-embedded periodic grooves, which are placed in parallel with the grooves face to face. Simulations show that this waveguide can realize mode switching by changing the conductivity of the embedded VO₂ pads, whose mechanism is attributed to the local resonance induced by defect mode. Such a VO₂-embedded hybrid THz waveguide is favorable in practical applications such as THz modulators, sensors and optical switches, and provides an innovative technique for manipulating THz waves.

Slow-light effects based on the tunable Fano resonance in a Tamm state coupled graphene surface plasmon system

We theoretically realize the tunable Fano resonance in a hybrid structure that allows the coupling between Tamm plasmon-polaritons (TPPs) and graphene surface plasmon-polaritons (SPPs). In this coupling system, a distributed Bragg reflector (DBR)/Ag structure is designed to generate the TPP with a narrow resonance, and the graphene SPP is excited by grating coupling with a broad resonance. The overlap of these two kinds of resonances results in the Fano resonance with a high-quality factor close to 1500. The behaviors of the Fano resonance are discussed carefully, and the results show that both the graphene Fermi level and the incidence angle can actively tune the profile of the Fano resonance. Owing to the ultrasharp spectrum of the tunable Fano resonance, our design may offer an alternative strategy for developing various optoelectronic devices such as filters, sensors, and nonlinear and slow-light devices. Finally, as an example of the potential applications, we apply the tunable Fano resonance to the slow-light effect, a high performance slow-light effect can be achieved, and the group delay can reach up to 52 ps.

Classical Analog and Hybrid Metamaterials of Tunable Multiple-Band Electromagnetic Induced Transparency

The electromagnetic induced transparency (EIT) effect originates from the destructive interference in an atomic system, which contributes to the transparency window in its response spectrum. The implementation of EIT requires highly demanding laboratory conditions, which greatly limits its acceptance and application. In this paper, an improved harmonic spring oscillation (HSO) model with four oscillators is proposed as a classical analog for the tunable triple-band EIT effect. A more general HSO model including more oscillators is also given, and the analyses of the power absorption in the HSO model conclude a formula, which is more innovative and useful for the study of the multiple-band EIT effect. To further inspect the analogizing ability of the HSO model, a hybrid unit cell containing an electric dipole and toroidal dipoles in the metamaterials is proposed. The highly comparable transmission spectra based on the HSO model and metamaterials indicate the validity of the classical analog in illustrating the formation process of the multiple-band EIT effect in metamaterials. Hence, the HSO model, as a classical analog, is a valid and powerful theoretical tool that can mimic the multiple-band EIT effect in metamaterials.

Optimization of metamaterials and metamaterial-microcavity based on deep neural networks

Computational inverse-design and forward prediction approaches provide promising pathways for on-demand nanophotonics. Here, we use a deep-learning method to optimize the design of split-ring metamaterials and metamaterial-microcavities. Once the deep neural network is trained, it can predict the optical response of the split-ring metamaterial in a second which is much faster than conventional simulation methods. The pretrained neural network can also be used for the inverse design of split-ring metamaterials and metamaterial-microcavities. We use this method for the design of the metamaterial-microcavity with the absorptance peak at 1310 nm. Experimental results verified that the deep-learning method is a fast, robust, and accurate method for designing metamaterials with complex nanostructures.

Propagation of Gaussian vortex beams in electromagnetically induced transparency media

Electromagnetically induced transparency (EIT) is an important phenomenon in quantum optics, and has a wide range of applications in the fields of quantum information processing and quantum precision metrology. Recently, with the rapid progress of the generation and detection of structured light, the EIT with structured light has attracted enormous interests and offers new and novel functionalities and applications. Here, we theoretically study the propagation and evolution of Gaussian vortex beams, a typical type of structured light, in an EIT medium with Λ-type three-level atoms. Based on the generalized Huygens-Fresnel principle, we derive the analytical expressions of fully and partially coherent Gaussian vortex beams propagating in the EIT medium, and study the evolution of the intensity and phase distributions of the beams and their dependencies on parameters such topological charge, coherence length, Rabi frequency, etc. It is shown that both the fully and partially coherent Gaussian vortex beams undergo focusing and diverging periodically during propagation. The phase singularity of the fully coherent beam keeps unchanged, while the phase singularity of the partially coherent beam experiences splitting and recombination periodically. In addition, new phase singularities with opposite topological charge are generated in the latter case. Our results not only advance the study of the interaction between structured light and coherent media, but also pave the avenue for manipulating structured light via EIT.

Ultra-wideband transmission filter based on guided-mode resonances in two terahertz metasurfaces

This paper reports on a broadband transmission filter that employs the guided mode resonances pertaining to a terahertz metasurface composed of metallic gold disks with a quartz slab. Unlike structures involving conventional metasurfaces, two identical metasurfaces are placed on the upper and lower sides of a thick quartz slab. This structure can excite both even and odd guided mode resonances. The interaction of the two resonances at similar frequencies produces a broadband transmission peak. The sharp spectral feature of each resonance leads to the abrupt degradation of the transmission at the spectral edge, which can enable the development of the filter application. The proposed scheme can facilitate practical applications such as those of broadband filters at a terahertz frequency.

Dual Tunable Electromagnetically Induced Transparency Based on a Grating-Assisted Double-Layer Graphene Hybrid Structure at Terahertz Frequencies

We propose a graphene plasmonic structure by applying two graphene layers mingled with a thin gold layer in a silicon grating. By utilizing the finite-difference time-domain (FDTD) method, we investigate the optical response of the system, and observe that the design achieves dual tunable electromagnetically induced transparency (EIT)-like effect at terahertz frequencies. The EIT-like effect arises from the destructive interference between the grapheme-layer bright modes and the gold-layer dark mode. The EIT-like phenomenon can be adjusted by the Fermi level, which is related to the applied voltage. The results show that the group delay of the present structure reaches 0.62 ps in the terahertz band, the group refractive index exceeds 1200, the maximum delay-bandwidth product is 0.972, and the EIT-like peak frequency transmittance is up to 0.89. This indicates that the device has good slow light performance. The proposed structure might enable promising applications in slow-light devices.

Dynamically electrical/thermal-tunable perfect absorber for a high-performance terahertz modulation

We present a high-performance functional perfect absorber in a wide range of terahertz (THz) wave based on a hybrid structure of graphene and vanadium dioxide (VO₂) resonators. Dynamically electrical and thermal tunable absorption is achieved due to the management on the resonant properties via the external surroundings. Multifunctional manipulations can be further realized within such absorber platform. For instance, a wide-frequency terahertz perfect absorber with the operation frequency range covering from 1.594 THz to 3.272 THz can be realized when the conductivity of VO₂ is set to 100000 S/m (metal phase) and the Fermi level of graphene is 0.01 eV. The absorption can be dynamically changed from 0 to 99.98% and in verse by adjusting the conductivity of VO₂. The impedance matching theory is introduced to analyze and elucidate the wideband absorption rate. In addition, the absorber can be changed from wideband absorption to dual-band absorption by adjusting the Fermi level of graphene from 0.01 eV to 0.7 eV when the conductivity of VO₂ is fixed at 100000 S/m. Besides, the analysis of the chiral characteristics of the helical structure shows that the extinction cross-section has a circular dichroic response under the excitation of two different circularly polarized lights (CPL). Our study proposes approaches to manipulate the wide-band terahertz wave with multiple ways, paving the way for the development of technologies in the fields of switches, modulators, and imaging devices.

An Active Electromagnetically Induced Transparency (EIT) Metamaterial Based on Conductive Coupling

In this paper, we demonstrate an active metamaterial manifesting electromagnetically induced transparency (EIT) effect in the microwave regime. The metamaterial unit cell consists of a double-cross structure, between which a varactor diode is integrated. The capacitance of the diode is controlled by a reversed electrical bias voltage supplied through two connected strip lines. The diode behaves as a radiative resonant mode and the strip lines as a non-radiative resonant mode. The two modes destructively interference with each other through conductive coupling, which leads to a transmission peak in EIT effect. Through electrical control of the diode capacitance, the transmission peak frequency is shifted from 7.4 GHz to 8.7 GHz, and the peak-to-dip ratio is tuned from 1.02 to 1.66, demonstrating a significant tunability.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Transition metal dichalcogenide metaphotonic and self-coupled polaritonic platform grown by chemical vapor deposition

Transition metal dichalcogenides (TMDCs) have recently attracted growing attention in the fields of dielectric nanophotonics because of their high refractive index and excitonic resonances. Despite the recent realizations of Mie resonances by patterning exfoliated TMDC flakes, it is still challenging to achieve large-scale TMDC-based photonic structures with a controllable thickness. Here, we report a bulk MoS₂ metaphotonic platform realized by a chemical vapor deposition (CVD) bottom-up method, supporting both pronounced dielectric optical modes and self-coupled polaritons. Magnetic surface lattice resonances (M-SLRs) and their energy-momentum dispersions are demonstrated in 1D MoS₂ gratings. Anticrossing behaviors with Rabi splitting up to 170 meV are observed when the M-SLRs are hybridized with the excitons in multilayer MoS₂. In addition, distinct Mie modes and anapole-exciton polaritons are also experimentally demonstrated in 2D MoS₂ disk arrays. We believe that the CVD bottom-up method would open up many possibilities to achieve large-scale TMDC-based photonic devices and enrich the toolbox of engineering exciton-photon interactions in TMDCs.

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

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

Terahertz Asymmetric S-Shaped Complementary Metasurface Biosensor for Glucose Concentration

In this article, we present a free-standing terahertz metasurface based on asymmetric S-shaped complementary resonators under normal incidence in transmission mode configuration. Each unit cell of the metasurface consists of two arms of mirrored S-shaped slots. We investigate the frequency response at different geometrical asymmetry via modifying the dimensions of one arm of the resonator. This configuration enables the excitation of asymmetric quasi-bound states in the continuum resonance and, hence, features very good field confinement that is very important for biosensing applications. Moreover, the performance of this configuration as a biosensor was examined for glucose concentration levels from 54 mg/dL to 342 mg/dL. This range covers hypoglycemia, normal, and hyperglycemia diabetes mellitus conditions. Two sample coating scenarios were considered, namely the top layer when the sample covers the metasurface and the top and bottom layers when the metasurface is sandwiched between the two layers. This strategy enabled very large resonance frequency redshifts of 236.1 and 286.6 GHz that were observed for the two scenarios for a 342 mg/dL concentration level and a layer thickness of 20 μm. Furthermore, for the second scenario and the same thickness, a wavelength sensitivity of 322,749 nm/RIU was found, which represents a factor of 2.3 enhancement compared to previous studies. The suggested terahertz metasurface biosensor in this paper could be used in the future for identifying hypoglycaemia and hyperglycemia conditions.

Advances in Two-Dimensional Materials for Optoelectronics Applications

The past one and a half decades have witnessed the tremendous progress of two-dimensional (2D) crystals, including graphene, transition-metal dichalcogenides, black phosphorus, MXenes, hexagonal boron nitride, etc., in a variety of fields. The key to their success is their unique structural, electrical, mechanical and optical properties. Herein, this paper gives a comprehensive summary on the recent advances in 2D materials for optoelectronic approaches with the emphasis on the morphology and structure, optical properties, synthesis methods, as well as detailed optoelectronic applications. Additionally, the challenges and perspectives in the current development of 2D materials are also summarized and indicated. Therefore, this review can provide a reference for further explorations and innovations of 2D material-based optoelectronics devices.

Constant frequency reconfigurable terahertz metasurface based on tunable electromagnetically induced transparency-like approach

A terahertz constant frequency reconfigurable metasurface based on tunable electromagnetically induced transparency (EIT)-like property was designed, whose transparency window frequency did not vary with Fermi energy. This structure was composed of two single-layer graphene resonators, namely, left double big rings and right double small rings. An evident transparency window (EIT-like phenomenon) was caused by the near-field coupling between bright modes of the two resonators in the transmission spectrum, in which amplitude over 80% was acquired at 1.98 THz. By individually reconfiguring the Fermi energy of each resonator, the EIT-like effects, transparency window amplitude, modulation speed and group delay could be actively controlled while the frequency of EIT-like window remained constant. Significantly, the transparency window was fully modulated without changing the frequency, and the maximum modulation depth reached 78%. Furthermore, the modulation speed also increased because the total graphene areaAwas effectively reduced in the proposed structure. Compared with other metasurface structures, the modulation properties of the proposed structure showed higher performance while the EIT-like window frequency remained static. This research provides an alternative method for developing constant frequency reconfigurable modulation terahertz devices (such as optical switches and modulators), as well as a potential approach for miniaturization of terahertz devices.

Tunable Electromagnetically Induced Transparent Window of Terahertz Metamaterials and Its Sensing Performance

The electromagnetically induced transparency effect of terahertz metamaterials exhibits excellent modulation and sensing properties, and it is critical to investigate the modulation effect of the transparent window by optimizing structural parameters. In this work, a unilateral symmetrical metamaterial structure based on the cut-wire resonator and the U-shaped split ring resonator is demonstrated to achieve electromagnetically induced transparency-like (EIT-like) effect. Based on the symmetrical structure, by changing the structural parameters of the split ring, an asymmetric structure metamaterial is also studied to obtain better tuning and sensing characteristics. The parameters for controlling the transparent window of the metamaterial are investigated in both passive and active modulation modes. In addition, the metamaterial structure based on the cut-wire resonator, unilateral symmetric and asymmetric configurations are investigated for high performance refractive index sensing purposes, and it is found that the first two metamaterial structures can achieve sensitivity responses of 63.6 GHz/RIU and 84.4 GHz/RIU, respectively, while the asymmetric metamaterial is up to 102.3 GHz/RIU. The high sensitivity frequency response of the proposed metamaterial structures makes them good candidates for various chemical and biomedical sensing applications.

Guiding-mode-assisted double-BICs in an all-dielectric metasurface

The electromagnetically induced transparency (EIT) effect realized in a metasurface is potential for slow light applications for its extreme dispersion variation in the transparency window. Herein, we propose an all-dielectric metasurface to generate a double resonance-trapped quasi bound states in the continuum (BICs) in the form of EIT or Fano resonance through selectively exciting the guiding modes with the grating. The group delay of the EIT is effectively improved up to 2113 ps attributing to the ultrahigh Q-factor resonance carried by the resonance-trapped quasi-BIC. The coupled harmonic oscillator model and a full multipole decomposition are utilized to analyze the physical mechanism of EIT-based quasi-BIC. In addition, the BIC based on Fano and EIT resonance can simultaneously exist at different wavelengths. These findings provide a new feasible platform for slow light devices in the near-infrared region.

Terahertz graphene metasurfaces for cross-polarized deflection, focusing, and orbital angular momentum

Polarization is an important characteristic of electromagnetic wave. Due to novel optical properties, graphene-based anisotropic structure is widely used to control polarization state of electromagnetic wave. In this work, four graphene-based meta-atoms are designed to regulate polarization state of terahertz wave by changing Fermi energy level of graphene. When Fermi energy level is 0.01 eV, cross-polarized wave is emitted by four meta-atoms with phase difference of 90° at 1.18 THz, and the corresponding polarization conversion ratio reaches ∼90%. When Fermi energy level is adjusted to 0.70 eV, linear phase gradient will disappear, and cross-polarized wave almost disappears. Using four selected elements, three dynamic metasurfaces are designed for controlling wavefront of reflected beam, and they are gradient metasurface, metalens, and vortex beam generator. The designed metasurfaces successfully combine wavefront control and polarization manipulation, and greatly improve the ability to control electromagnetic wave. Our designs may have many potential applications, such as terahertz switching, imaging, and polarization beam splitter.

Unified model for plasmon-induced transparency with direct and indirect coupling in borophene-integrated metamaterials

We propose, both numerically and theoretically, a uniform model to investigate the plasmonically induced transparency effect in plasmonic metamaterial consisting of dual-layer spatially separated borophene nanoribbons array. The dynamic transfer properties of light between two borophene resonators can be effectively described by the proposed model, with which we can distinguish and connect the direct and indirect coupling schemes in the metamaterial system. By adjusting the electron density and separation of two borophene ribbons, the proposed metamaterials enable a narrow band in the near-infrared region to reach high transmission. It provides a new, to the best of our knowledge, platform for optoelectronic integrated high-performance devices in the communication band.

All-optical switching based on plasmon-induced Enhancement of Index of Refraction

In quantum optical Enhancement of Index of Refraction (EIR), coherence and quantum interference render the atomic systems to exhibit orders of magnitude higher susceptibilities with vanishing or even negative absorption at their resonances. Here we show the plasmonic analogue of the quantum optical EIR effect in an optical system and further implement this in a linear all-optical switching mechanism. We realize plasmon-induced EIR using a particular plasmonic metasurface consisting of a square array of L-shaped meta-molecules. In contrast to the conventional methods, this approach provides a scheme to modulate the amplitude of incident signals by coherent control of absorption without implementing gain materials or nonlinear processes. Therefore, light is controlled by applying ultra-low intensity at the extreme levels of spatiotemporal localization. In the pursuit of potential applications of linear all-optical switching devices, this scheme may introduce an effective tool for improving the modulation strength of optical modulators and switches through the amplification of input signals at ultra-low power.

mm-wave Rydberg-Rydberg transitions gauge intermolecular coupling in a molecular ultracold plasma

Out-of-equilibrium, strong correlation in a many-body system can trigger emergent properties that act to constrain the natural dissipation of energy and matter. Signs of such self-organization appear in the avalanche, bifurcation, and quench of a state-selected Rydberg gas of nitric oxide to form an ultracold, strongly correlated ultracold plasma. Work reported here focuses on initial stages of avalanche and quench, and uses the mm-wave spectroscopy of an embedded quantum probe to characterize the intermolecular interaction dynamics associated with the evolution to plasma. Double-resonance excitation prepares a Rydberg gas of nitric oxide composed of a single selected state of principal quantum number, n0. Penning ionization, followed by an avalanche of electron-Rydberg collisions, forms a plasma of NO+ ions and weakly bound electrons, in which a residual population of n0 Rydberg molecules evolves to a state of high orbital angular momentum, l. Predissociation depletes the plasma of low- l molecules. Relaxation ceases and n0l(2) molecules with l {greater than or equal to} 4 persist for very long times. At short times, varying excitation spectra of mm-wave Rydberg-Rydberg transitions mark the rate of electron-collisional l-mixing. Deep depletion resonances that persist for long times signal energy redistribution in the basis of central-field Rydberg states. The widths and asymmetries of Fano lineshapes witness the degree to which coupling in the arrested bath i) broadens the allowed transition and ii) mixes the local network of levels in the ensemble.

Selective broadband absorption by mode splitting for radiative cooling

A plasmonic-photonic structure based on colloidal lithography was designed for a scalable radiative cooling system and its absorption properties were theoretically investigated. The structure comprises a SiO₂ core, which is on top of an Au reflector and partially covered by an indium tin oxide (ITO) shell. This simple and scalable structure possesses a strong selective absorption in the primary atmospheric transparency window (8-13 µm). The strong selective absorption is attributed to a mode splitting of the localized surface plasmon (LSP) of the ITO shell. To understand the mechanisms of the mode splitting, a quantitative analysis was conducted using a coupled-oscillator model and a coupled-dipole method. The analysis revealed that the mode splitting is induced by a strong coupling between the LSP of the ITO shell and a magnetic dipole Mie resonance of the SiO₂ core.

Ultra-Broadband Tunable Terahertz Metamaterial Absorber Based on Double-Layer Vanadium Dioxide Square Ring Arrays

Based on the phase transition of vanadium dioxide(VO₂), an ultra-broadband tunable terahertz metamaterial absorber is proposed. The absorber consists of bilayer VO₂ square ring arrays with different sizes, which are completely wrapped in Topas and placed on gold substrate. The simulation results show that the absorption greater than 90% has frequencies ranging from 1.63 THz to 12.39 THz, which provides an absorption frequency bandwidth of 10.76 THz, and a relative bandwidth of 153.5%. By changing the electrical conductivity of VO₂, the absorption intensity can be dynamically adjusted between 4.4% and 99.9%. The physical mechanism of complete absorption is elucidated by the impedance matching theory and field distribution. The proposed absorber has demonstrated its properties of polarization insensitivity and wide-angle absorption, and therefore has a variety of application prospects in the terahertz range, such as stealth, modulation, and sensing.

Principle and Applications of Multimode Strong Coupling Based on Surface Plasmons

In the past decade, strong coupling between light and matter has transitioned from a theoretical idea to an experimental reality. This represents a new field of quantum light–matter interaction, which makes the coupling strength comparable to the transition frequencies in the system. In addition, the achievement of multimode strong coupling has led to such applications as quantum information processing, lasers, and quantum sensors. This paper introduces the theoretical principle of multimode strong coupling based on surface plasmons and reviews the research related to the multimode interactions between light and matter. Perspectives on the future development of plasmonic multimode coupling are also discussed.

Terahertz multimode modulator based on tunable triple-plasmon-induced transparency in monolayer graphene metamaterials

A simple monolayer graphene metamaterial based on silicon/silica substrates is proposed, and typical triple-plasmon-induced transparency (PIT) is realized in the terahertz band. The physical mechanism is analyzed by coupled mode theory (CMT), and the results of CMT agree well with the finite-difference time-domain simulation. A multimode electro-optical switch can be designed by dynamic tuning, and the modulation degrees of its resonant frequencies are 84.0%, 87.3%, 83.0%, 88.1%, and 76.7%. In addition, triple-PIT gradually degenerates into dual-PIT with a decrease in the length of one bright mode. Interestingly, the group index can reach 770 at Ef=0.8eV, which shows that it can be designed as a slow light device with extraordinary ability. Therefore, the results of this paper are of great significance to the research and design of electro-optical switches and slow light devices in the terahertz band.

High-Q and tunable analog of electromagnetically induced transparency in terahertz all-dielectric metamaterial

We propose a novel all-dielectric metamaterial (ADMM), to the best of our knowledge, with a simple structure to achieve the analog of electromagnetically induced transparency (EIT) in the terahertz range. The ADMM is constructed by unit cells with two same silicon bar resonators on a quartz substrate. By breaking the symmetrical array of silicon resonators, the guided-mode resonance can be excited in the substrate, and the destructive interference between a broadband electric-dipole resonance and a narrowband guided-mode resonance gives rise to an EIT-like response. The EIT window can reach a high quality factor (Q-factor) over 1500 by carefully adjusting the asymmetry degree within the unit cell. A dynamically tunable ADMM was further developed by employing photoactive doped silicon. By varying the carrier density of the doped silicon through optical pump, the strength of the EIT-like resonance can be actively modulated, enabling an on-to-off switch of the slow-light effect. The designed ADMM can achieve a high-Q EIT-like response and dynamic modulation, which may give potential applications in bio/chemical sensing, optical switching, and slow-light devices.

Full manipulation of transparency and absorption through direct tuning of dark modes in high-Q Fano metamaterials

Controlling the line shape of Fano resonance has continued to attract significant research attention in recent years owing to its practical applications such as lasing, biosensing, and slow-light devices. However, controllable Fano resonances always require stringent alignment of complex symmetry-breaking structures; therefore, the manipulation can only be performed with limited degrees of freedom and a narrow tuning range. This work demonstrates dark-mode excitation tuning independent of the bright mode for the first time, to the authors' knowledge, in asymmetric Fano metamaterials. Metallic subwavelength slits are arranged to form asymmetric unit cells and generate a broad and bright (radiative) Fabry-Perot mode and a sharp and dark (non-radiative) surface mode. The introduction of the independent radial and angular asymmetries realizes independent control of the Fano phase (q) and quality factor (Q). This tunability provides a dynamic phase shift while maintaining a high-quality factor, enabling switching between nearly perfect transmission and absorption, which is confirmed both numerically and experimentally. The proposed scheme for fully controlled Fano systems can aid practical applications such as phase-sensitive switching devices.

Toroidal electromagnetically induced transparency based meta-surfaces and its applications

The vigorous research on low-loss photonic devices has brought significance to a new kind of electromagnetic excitation, known as toroidal resonances. Toroidal excitation, possessing high-quality factor and narrow linewidth of the resonances, has found profound applications in metamaterial (MM) devices. By the coupling of toroidal dipolar resonance to traditional electric/magnetic resonances, a metamaterial analogue of electromagnetically induced transparency effect (EIT) has been developed. Toroidal induced EIT has demonstrated intriguing properties including steep linear dispersion in transparency windows, often leading to elevated group refractive index in the material. This review summarizes the brief history and properties of the toroidal resonance, its identification in metamaterials, and their applications. Further, numerous theoretical and experimental demonstrations of single and multiband EIT effects in toroidal-dipole-based metamaterials and its applications are discussed. The study of toroidal-based EIT has numerous potential applications in the development of biomolecular sensing, slow light systems, switches, and refractive index sensing.