Toroidal dipole bound states in the continuum metasurfaces for terahertz nanofilm sensing

Terahertz binary computing in a coupled toroidal metasurface

The applications of terahertz metamaterials are being actively explored in recent times for applications in high-speed communication devices, miniature photonic circuits, and bio-chemical devices because of their wide advantages. The toroidal resonance, a new type of metasurface resonance, has been examined with great interest to utilize its properties in terahertz metasurface applications. This study reports a proof of concept design of a toroidal metasurface that experimentally demonstrates binary computing operations in the terahertz frequency regime. The analog computing of binary operations is achieved by the passive tuning of distance between the split ring resonators comprising the meta-molecule. The amplitude modulation is utilized as a method of determining the Boolean logic outputs of the system. The proposed metasurface could be further optimized for high amplitude modulations and active logic gate operations using tunable materials including graphene and ITO. The proposed metasurface consists of three split-ring resonators, and the near-field coupling between the adjacent resonators dictates the Boolean operations. A multipole analysis of the scattered powers of terahertz radiation determines the toroidal excitation in the metasurface. The proposed metasurfaces experimentally define AND Boolean logic operation at 0.89 terahertz, and OR Boolean logic operation at 0.97 terahertz. Numerical simulations support the experimentally obtained results. Additionally, we numerically report the excitation of NAND operation at 0.87 THz. Such toroidal analog computing metasurfaces could find applications in digitized terahertz circuits and integrated photonic devices.

Photonic Bound States in the Continuum in Nanostructures

Bound states in the continuum (BIC) have garnered considerable attention recently for their unique capacity to confine electromagnetic waves within an open or non-Hermitian system. Utilizing a variety of light confinement mechanisms, nanostructures can achieve ultra-high quality factors and intense field localization with BIC, offering advantages such as long-living resonance modes, adaptable light control, and enhanced light-matter interactions, paving the way for innovative developments in photonics. This review outlines novel functionality and performance enhancements by synergizing optical BIC with diverse nanostructures, delivering an in-depth analysis of BIC designs in gratings, photonic crystals, waveguides, and metasurfaces. Additionally, we showcase the latest advancements of BIC in 2D material platforms and suggest potential trajectories for future research.

Achiral nanoparticle trapping and chiral nanoparticle separating with quasi-BIC metasurface

Dielectric metasurfaces based on quasi-bound states in the continuum (quasi-BICs) are a promising approach for manipulating light-matter interactions. In this study, we numerically demonstrate the potential of silicon elliptical tetramer dielectric metasurfaces for achirality nanoparticle trapping and chiral nanoparticle separation. We first analyze a symmetric tetramer metasurface, which exhibits dual resonances (P1 and P2) with high electromagnetic field intensity enhancement and a high-quality factor (Q-factor). This metasurface can trap achiral nanoparticles with a maximum optical trapping force of 35 pN for 20 nm particles at an input intensity of 100 mW. We then investigate an asymmetric tetramer metasurface, which can identify and separate enantiomers under the excitation of left-handed circularly polarized (LCP) light. Results show that the chiral optical force can push one enantiomer towards regions of the quasi-BIC system while removing the other. In addition, the proposed asymmetric tetramer metasurface can provide multiple Fano resonances (ranging from R1 to R5) and high trap potential wells of up to 33 kBT. Our results demonstrate that the proposed all-dielectric metasurface has high performance in nanoparticle detection, with potential applications in biology, life science, and applied physics.

Ultrasensitive Terahertz Biodetection Enabled by Quasi-BIC-Based Metasensors

Advanced sensing devices, highly sensitive, and reliable in detecting ultralow concentrations of circulating biomarkers, are extremely desirable and hold great promise for early diagnostics and real-time progression monitoring of diseases. Nowadays, the most commonly used clinical methods for diagnosing biomarkers suffer from complicated procedures and being time consumption. Here, a chip-based portable ultra-sensitive THz metasensor is reported by exploring quasi-bound states in the continuum (quasi-BICs) and demonstrate its capability for sensing low-concentration analytes. The designed metasensor is made of the designed split-ring resonator metasurface which supports magnetic dipole quasi-BIC combining functionalized gold nanoparticles (AuNPs) conjugated with the specific antibody. Attributed to the strong near-field enhancement near the surface of the microstructure enabled by the quasi-BICs, light-analyte interactions are greatly enhanced, and thus the device's sensitivity is boosted significantly. The system sensitivity slope is up to 674 GHz/RIU, allowing for repeatable resolving detecting ultralow concentration of C-reactive protein (CRP) and Serum Amyloid A (SAA), respectively, down to 1 pM. The results touch a range that cannot be achieved by ordinary immunological assays alone, offering a novel non-destructive and rapid trace measured approach for next-generation biomedical quantitative detection systems.

Terahertz asymmetric metallic hole arrays with polarization-independent quasi-bound states in the continuum for membrane sensing

Resonances with both high-quality factor and polarization-independent characteristics are highly desirable for terahertz (THz) sensing. Here, THz sensors based on asymmetric metallic hole arrays (AMHAs) are experimentally demonstrated. Such sensors consisting of four-hole arrays support polarization-independent quasi-bound states in the continuum (BICs). The induced quasi-BIC presents a quality factor exceeding 2000, which enables enhanced sensing for thin membranes. Results show that the frequency shift is 97.5 GHz for the 25-µm thick polyimide (PI), corresponding to a sensitivity of 147.7 GHz/RIU. The sensing performance strongly relates to the enhanced field originating from sharp quasi-BICs. A maximum field enhancement of 15.88 in contrast to the incident field is achieved. When the PI thickness is large than the decay length of enhanced fields, the interaction strength of field-PI becomes weak, resulting in a saturation effect for the shift of quasi-BICs. The proposed sensor possessing polarization-independent quasi-BICs has great potential for practical sensing applications in real-time chemical and biomolecular.

Polarization-selective narrow band dual-toroidal-dipole resonances in a symmetry-broken dielectric tetramer metamaterial

Here we propose a metasurface consisting of symmetry-broken dielectric tetramer arrays, which can generate polarization-selective dual-band toroidal dipole resonances (TDR) with ultra-narrow linewidth in the near-infrared region. We found, by breaking the C_4v symmetry of the tetramer arrays, two narrow-band TDRs can be created with the linewidth reaching ∼ 1.5 nm. Multipolar decomposition of scattering power and electromagnetic field distribution calculations confirm the nature of TDRs. A 100% modulation depth in light absorption and selective field confinement has been demonstrated theoretically by simply changing the polarization orientation of the exciting light. Intriguingly, it is also found that absorption responses of TDRs on polarization angle follow the equation of Malus' law in this metasurface. Furthermore, the dual-band toroidal resonances are proposed to sense the birefringence of an anisotropic medium. Such polarization-tunable dual toroidal dipole resonances with ultra-narrow bandwidth offered by this structure may find potential applications in optical switching, storage, polarization detection, and light emitting devices.

Excitation of multiple Fano resonances on all-dielectric nanoparticle arrays

In this paper, an all-dielectric metasurface consisting of a unit cell containing a nanocube array and organized periodically on a silicon dioxide substrate is designed and analyzed. By introducing asymmetric parameters that can excite the quasi-bound states in the continuum, three Fano resonances with high Q-factor and high modulation depth may be produced in the near-infrared range. Three Fano resonance peaks are excited by magnetic dipole and toroidal dipole, respectively, in conjunction with the distributive features of electromagnetism. The simulation results indicate that the discussed structure can be utilized as a refractive index sensor with a sensitivity of around 434 nm/RIU, a maximum Q factor of 3327, and a modulation depth equal to 100%. The proposed structure has been designed and experimentally investigated, and its maximum sensitivity is 227 nm/RIU. At the same time, the modulation depth of the resonance peak at λ = 1185.81 nm is nearly 100% when the polarization angle of the incident light is 0 °. Therefore, the suggested metasurface has applications in optical switches, nonlinear optics, and biological sensors.

Ultra-high-Q substrate-mode coupled resonances in complementary THz metamaterial

Achieving high-Q resonances in the THz frequency range is significant for applications such as sensors, filters, and emitters. A promising approach for obtaining such resonances is by using metamaterials. However, high-Q resonances in THz metamaterials are usually limited by metallic radiation losses in the meta-atoms. In this Letter, we investigate both experimentally and numerically a complementary metallic disk-hole array (CMA) that uses the coupling between lattice resonances and Fabry-Pérot cavity resonances, and features in-substrate modes with experimentally obtained record breaking Q-factors of up to 750. To the best of our knowledge, this is the highest quality factor measured for free-space-coupled metallic metamaterial structure at THz frequencies.

Highly sensitive terahertz sensing with 3D-printed metasurfaces empowered by a toroidal dipole

Highly sensitive terahertz (THz) sensing with metasurfaces has attracted considerable attention recently. However, ultrahigh sensing sensitivity remains a huge challenge for practical applications. To improve the sensitivity of these devices, herein we have proposed an out-of-plane metasurface-assisted THz sensor consisting of periodically arranged bar-like meta-atoms. Benefiting from elaborate out-of-plane structures, the proposed THz sensor with high sensing sensitivity of 325 GHz/RIU can be easily fabricated via a simple three-step fabrication process, and the maximum sensing sensitivity can be ascribed to toroidal dipole resonance-enhanced THz-matter interactions. The sensing ability of the fabricated sensor is experimentally characterized by the detection of three types of analytes. It is believed that the proposed THz sensor with ultrahigh sensing sensitivity and its fabrication method might provide great potential in emerging THz sensing applications.

THz absorbers with an ultrahigh Q-factor empowered by the quasi-bound states in the continuum for sensing application

The exceptional resonances excited by symmetry-protected quasi-bound states in the continuum (QBICs) have provided significant potential in high-sensitive sensing applications. Herein, we have proposed a type of metal-insulator-metal (MIM) absorbers supported by QBIC-induced resonances, and the ideal Q-factors of QBIC-induced resonances can be enhanced up to 10⁵ in the THz regime. The coupled mode theory and the multipole scattering theory are employed to thoroughly interpret the QBIC-induced absorption mechanism. Furthermore, the refractive index sensing capacities of the as-presented absorbers have been investigated, where the maximum values of the sensing sensitivity and figure of merit (FOM) can reach up to 187 GHz per refractive index unit and 286, respectively. Therefore, it is believed that the proposed absorbers enabled by QBIC-induced resonances hold promising potential in a broad range of highly demanding sensing applications.

Excitation of Asymmetric Resonance with Symmetric Split-Ring Resonator

In this paper, a new approach to excite sharp asymmetric resonances using a single completely symmetric split-ring resonator (SRR) inside a rectangular waveguide is proposed. The method is based on an asymmetry in the excitation of a symmetric split-ring resonator by placing it away from the center of the waveguide along its horizontal axis. In turn, a prominent asymmetric resonance was observed in the transmission amplitude of both the simulated results and the measured data. Using a single symmetric SRR with an asymmetric distance of 6 mm from the center of a rectangular waveguide led to the excitation of a sharp resonance with a Q-factor of 314 at 6.9 GHz. More importantly, a parametric study simulating different overlayer analytes with various refractive indices revealed a wavelength sensitivity of 579,710 nm/RIU for 150 μm analyte thickness.

A pixelated frequency-agile metasurface for broadband terahertz molecular fingerprint sensing

Terahertz (THz) plasmonic resonance based on an arbitrarily designed resonance metasurface is the key technique of choice for enhancing fingerprint absorption spectroscopy identification of biomolecules. Here, we report a broadband THz micro-photonics sensor based on a pixelated frequency-agile metasurface and illustrate its application ability to enhance and differentiate the detection of broadband absorption fingerprint spectra. The design uses symmetrical metal C-shape resonators with the functional graphene micro-ribbons selectively patterned into the gaps. A strong electric resonance with a high quality factor was formed, consisting of an electric dipole mode associated with the excitation of a dark toroidal dipole (TD) mode through the coupling from the electric dipole moment of the individual frequency-agile meta-unit. The resonance positions are nearly linearly modulated with the varying Fermi level of graphene. The configuration arranges a certain metapixel of the metasurface to multiple response spectra assembling a one-to-many mapping between spatial and spectral information which is instrumental in greatly shrinking the actual size of the sensor. By the synchronous regulation of graphene and C-shape rings, we have obtained highly surface-sensitive resonances over a wide spectral range (∼1.5 THz) with a spectral resolution less than 20 GHz. The target multiple enhanced absorption spectrum of glucose molecules is read out in a broadband region with high sensitivity. More importantly, the design can be extended to cover a larger spectral region by altering the range of geometrical parameters. Our microphotonic technique can resolve absorption fingerprints without the need for spectrometry and frequency scanning, thereby providing an approach for highly sensitive and versatile miniaturized THz spectroscopy devices.

All-Optical Tuning of Fano Resonance for Quasi-BIC and Terahertz Sensing Applications

The bound states in the continuum (BIC) support anomalous resonances in the optical or terahertz band with a theoretically infinite quality factor. Therefore, it has great application prospects in the field of sensors. However, the current regulation of BIC mainly relies on the asymmetry of the material structure, which requires high processing technology. The structure can hardly be effectively adjusted once it is formed. In this work, we propose a new metasurface consisting of an array rectangular hole structure combined with aluminum and photosensitive silicon, which supports quasi-BIC to achieve ultrasensitive sensing in the terahertz range. By introducing photosensitive silicon, the asymmetry of the structure is efficiently controlled by the light field, thus realizing the bidirectional continuous control from quasi-BIC to BIC-like states. Through the optimization of the structure, a class of highly sensitive terahertz sensing based on optical tuning is finally proposed. The narrow-band quasi-BIC resonance is sensitive to medium thickness and refractive index, and compared with pure metal structure, the sensitivity and dynamic range can be increased by 2.60 times and 2.63 times, respectively. Due to the high slope of the Fano lineshape, sensitivity can reach 9.41 GHz/RIU and 0.65 GHz/μm, respectively. Furthermore, this feasible and practical structure provides an ideal platform for highly sensitive sensing.

Electric Fano resonance-based terahertz metasensors

An ultra-sensitive THz metasensor is presented based on quasi-BIC Fano resonance, which can distinguish extremely dilute concentrations (nM) of solutions. It provides a nondestructive sensing approach for disease prevention and diagnosis. However, the main drawback limiting the performance of THz-based bio-chemical sensors is the weak interaction between the optical field and the analyte, the characteristic scale of which is mismatched with the THz wavelength, leading to low sensitivity. Herein, we present an ultra-sensitive THz metasensor based on an electric Fano resonant metasurface which consists of three gold microrods arranged periodically. The designed electric Fano resonance provides a strong near-field enhancement near the surface of the microstructure, significantly boosting the light-analyte interactions and thus the sensitivity. Such an electric Fano resonance is formed by the interference between a leaky electric dipole resonance and a bound toroidal dipole mode which is a symmetry-protected bound state in the continuum supported by the sub-diffractive periodic system here. Owing to the strong electric fields generated near the interface of our microstructure around the toroidal dipole BIC, the proposed structure can distinguish extremely dilute concentrations (nM) of solutions. Importantly, by controlling the degree of geometrical asymmetry, the BIC-inspired mechanism provides an important and simple tool to engineer and tailor the linewidth and Q-factor of our proposed electric Fano resonance, indicating the ability to realize different biosensors for different optical regimes. Our results open new possibilities to realize a non-destructive and non-contact quantitative inspection of low-concentration solutions, providing a useful sensing approach for disease prevention and diagnosis.

Terahertz high-Q quasi-bound states in the continuum in laser-fabricated metallic double-slit arrays

A laser-fabricated metallic resonator based on a double-slit array (DSA) is numerically and experimentally demonstrated at terahertz frequencies. Such free-standing resonators achieve a sharp resonance with high quality (Q) factor, arising from a distortion of symmetry-protected bound states in the continuum (BIC). By breaking the structural symmetry of DSAs, the BIC with infinite Q-factor can be transformed into quasi-BICs, and the Q-factors decrease gradually as the asymmetry parameter increases. We analyzed the influence of the imperfection in experimental samples such as the round edge and the trapezoid shape on the transmission properties of DSAs. Different from the DSAs composed of ideal perfect electrical conductors, copper DSAs show lower Q-factor because of the Ohmic loss. The effect of metal thickness on the quasi-BICs for DSAs is also investigated. Results exhibit that thinner resonators can achieve sharper quasi-BICs. These findings suggest that such metallic resonators with high Q-factors have great potential for practical applications in electromagnetic wave filtering and biomolecular sensing.

Simultaneous Measurements of Refractive Index and Methane Concentration through Electromagnetic Fano Resonance Coupling in All-Dielectric Metasurface

Dual-parameter measurements of refractive index and methane concentration based on electromagnetic Fano resonance are proposed. Two independent Fano resonances can be produced through electric dipole and toroidal dipole resonance in an all-dielectric metasurface separately. The linear relationship between the spectral peak-shifts and the parameters to be measured will be obtained directly. The refractive index (RI) sensitivity and gas sensitivity are 1305.6 nm/refractive index unit (RIU), −0.295 nm/% for one resonance peak (dip1), and 456.6 nm/RIU, −0.61 nm/% for another resonance peak (dip2). Such a metasurface has simpler structure and higher sensitivity, which is beneficial for environmental gas monitoring or multi-parameter measurements.

Multiple Fano resonances excitation on all-dielectric nanohole arrays metasurfaces

Both toroidal dipoles, electric dipoles and magnetic dipoles belong to one type of electromagnetic excitation. In this paper, we present an all-dielectric metasurface composed of an array of square nanoholes. It can simultaneously generate four resonance responses excited by TD, EQ and MD in the continuous near-infrared band. By introducing the in-plane symmetry breaking of the unit cell, asymmetric dielectric nanohole arrays are used to achieve two quasi-BIC resonance modes with high Q-factors excited by EQ and MD. The paper theoretically analyzes and demonstrates the relationship between structural asymmetry and the radiative Q-factor of two Fano resonances, that are governed by symmetry-protected BICs. And multipole decomposition and near-field analysis are performed to demonstrate the dominant role of various electromagnetic excitations in the four modes. The spectra response is also calculated for different incident polarization angles and medium refractive indices. The proposed metasurface is more feasible and practical compared to other complex nanostructures, which may open avenues for the development of applications such as biochemical sensing, optical switches and optical modulators, and provide a reference for the design of devices with polarization-independent properties.