High-Q mid-infrared refractive index sensor based on Fano resonance in an all-dielectric double-rod structure

Light-molecular vibration coupling on metasurfaces induces complex optical phenomena, such as Fano resonance and Rabi splitting, which complicate molecular identification via conventional spectroscopic analysis. In this study, we developed an analytical system that uses deep feature learning to directly extract molecule-specific absorption information from these complex spectra. For a molecular vibration model with a single peak, we evaluated a DenseNet-169 convolutional neural network (CNN) model using 1,496 spectra generated via finite-difference time-domain (FDTD) simulations; however, the results were suboptimal. In contrast, for a two-peak molecular vibration model, we trained multiple CNN models, including DenseNet-169, on a dataset of 80,267 spectra. Consequently, we successfully reconstructed absorption coefficients with an extremely high accuracy, achieving a mean coefficient of determination ([Formula: see text]) of 0.9209, even in complex systems with overlapping vibrational peaks. This approach demonstrates significant potential as a foundational technology for next-generation molecular sensing.

Superconducting nanowire single photon detectors (SNSPDs) exhibit excellent performance in the near-infrared band, but their application range is limited by the detection efficiency and detection speed. In this paper, we propose a high-efficient and ultralow-filling-factor design scheme based on Fano resonance which is excited by a one-dimensional silicon dielectric grating. This design can reduce the kinetic inductance of the nanowires while maintaining the active sensing area of SNSPDs, which is beneficial for improving the recovery time of the detector. Meanwhile, the absorption efficiency of the nanowires can be enhanced by the Fano resonance. Taking λ = 1550nm as an example, numerical simulations are performed using commercial simulation software based on the finite-difference time-domain method. Under idealized structural conditions, the absorption efficiency of the Niobium Nitride (NbN) superconducting nanowires can exceed 98% when the filling factor of the nanowire is only 10%.

Conventional ab initio approaches are unable to describe phonon time-reversal symmetry ([Formula: see text]) breaking. Here, we develop an ab initio framework, grounded in molecular Berry curvature (MBC) theory, which captures electronic-order-driven symmetry breaking in lattice dynamics. Using Co3Sn2S2 as a model system, our ab initio framework yields phonon spectra that break both [Formula: see text] and mirror symmetries, quantitatively reproduce the observed phonon splittings observed in experiments, and reveal distinct microscopic origins for the [Formula: see text] and [Formula: see text] modes: [Formula: see text] splitting is governed by MBC and is accurately captured by our algorithm, whereas [Formula: see text] splitting is enhanced by the Fano resonance and matches the experimental data once the Fano-factor correction is included. Leveraging this algorithm, we predict several candidate materials with nonzero electronic-order-driven symmetry breaking in lattice dynamics, establishing a first-principles route to understand electron-phonon coupling, phonon magnetism, and related Hall-type lattice responses.

Spatially-localized Fano resonance in acoustic metamaterials and its application to frequency-selective earmuffs

Raman spectroscopy, since its discovery almost a century ago, has been one of the most widely used techniques, mainly due to continuous advancements in its instrumentation and variants. Apart from its "spectroscopy" aspect, Raman mapping/imaging has emerged as a powerful and noninvasive technique for spatially resolved analysis of structural, electronic, and vibrational properties across a wide range of materials, including nanomaterials and device-relevant systems. This Perspective highlights applications such as mapping anharmonic vibrational dynamics in carbon nanotubes through temperature-dependent Raman line-shape changes, along with magneto-Raman imaging of defect evolution and field-dependent phonon behavior in MoS2, polarization-resolved crystal orientation mapping in anisotropic materials, strain visualization in MXenes via Raman mode shifts, and phonon confinement and Fano resonance in inhomogeneous silicon nanowires. Recent relevance to quantum and modern device architectures is also briefly discussed. At the same time, key limitations such as diffraction-limited spatial resolution, weak signal strength, fluorescence interference, and possible laser-induced heating effects (including potential cell or tissue damage in biological samples) are addressed. Overall, Raman imaging offers combined spatial and spectral insight beyond conventional techniques, making it a promising tool for applications in optoelectronics, photonics, and emerging quantum technologies.

We report the design and multiphysics simulation of a monolithically integrated optoelectronic methane (CH[Formula: see text]) sensor that operates without external electrical bias or spectroscopic instrumentation. The device comprises a methane-selective cryptophane-A-infiltrated GaAs photonic crystal slab, engineered to support a sharp Fano resonance at [Formula: see text]nm, monolithically integrated with an InP/In[Formula: see text]Ga[Formula: see text]As/InP p-i-n photodiode. Methane adsorption reduces the refractive index of the cryptophane-A layer, inducing a blueshift of the Fano resonance that is directly transduced into a photocurrent change in the underlying detector. Simulations predict a sensitivity of [Formula: see text](A/cm[Formula: see text])/%CH[Formula: see text] with excellent linearity ([Formula: see text]) in self-powered mode, and an ultrafast photoresponse with [Formula: see text]GHz bandwidth-orders of magnitude faster than the methane adsorption kinetics. The InP/InGaAs heterojunction design, employing a wide-bandgap p-InP anode rather than conventional p-InGaAs, suppresses dark current to [Formula: see text]A/cm[Formula: see text] at zero bias while maintaining [Formula: see text] absorption efficiency in the 3μm intrinsic layer. Fabrication tolerance analysis demonstrates robust performance against [Formula: see text]nm variations in hole radius and [Formula: see text]nm variations in analyte thickness, with sensitivity degradation below [Formula: see text]. Unlike passive ultrahigh-Q resonators that require external spectroscopy, this fully integrated platform combines high optical sensitivity, efficient photocarrier generation, and zero static power consumption in a compact, scalable architecture suitable for distributed environmental monitoring, industrial safety, and IoT applications.

The mass law is a cornerstone in predicting sound transmission loss, yet it neglects the constraints of causal dispersion. Current causality-based theories, such as the Rozanov limit, are applicable only to one-port reflective absorbers. Here, we derive a universal sum rule governing causal scattering in acoustic systems, establishing a rigorous analogy to the Baldin sum rule in quantum field theory. This relation reveals that the integral of the extinction cross section is fundamentally locked by the scatterer's static effective mass and stiffness, which is validated numerically using seminal examples of underwater metamaterials. Furthermore, the proposed sum rule predicts an optimal condition for an anomalously broadened transmission loss bandwidth, as experimentally observed through the spectral shaping effect of an acoustic Fano resonator. Our findings open up an unexplored avenue for enhancing the scattering bandwidth of passive metamaterials.

A high-sensitivity on-chip complex refractive index sensor based on fano resonances in U-shaped nested slot microring of GeSbSe glass

A micro-nano optical sensor consisting of a rectangular waveguide integrated with a built-in baffle and a rectangular resonant cavity is proposed in this paper. Surface plasmon polaritons (SPPs) are excited by incident light, and surface plasmon resonance (SPR) is achieved via the SPPs stimulated by the rectangular coupling cavity and the baffle, thereby forming a quasi-bound state in the continuum (q-BIC) and further inducing Fano resonance. After structural parameter optimization, the sensor exhibits a full width at half maximum (FWHM) of 53 nm, a Q-factor of 21, and a sensitivity of 1133 nm/RIU. The sensor is further employed for sensing detection of cells infected with four types of viruses, namely Herpes Virus type-1, Influenza A, HIV-1, and M13 bacteriophage. Experimental results reveal that different viruses correspond to distinct resonant wavelengths in their resonance peak responses, which validates that the proposed sensor is capable of detecting cells infected with the above four viruses. This work offers a promising strategy for future developments in biomedical sensing.

Abstract We propose and theoretically investigate a refractometric sensing scheme that harnesses a giant photonic spin Hall effect mediated by long-range surface phonon polaritons (LRSPhPs) in the mid-infrared. Coupling the photonic waveguide mode to the LRSPhP induces strong mode hybridization, giving rise to a high-Q Fano resonance. By judiciously tuning the structural parameters, the resonance conditions can be manipulated, resulting in a deep and polarization-selective dip in reflectance for different polarizations of light. The resulted cooperative amplitude-phase response produces sign-reversible PSHE displacements reaching 10 2 -10 3 micrometres. Owing to the strong penetration of the LRSPhP field into the analyte, minute refractiveindex changes drive large variations in the spin shifts. In particular, the intensitybased sensitivity for H polarization attains 5.125 × 10 6 µm/RIU under the optimal parameter setup. Our results establish LRSPhP-mediated PSHE as a powerful route toward ultra-sensitive, label-free mid-infrared refractometric sensing and provide clear design guidelines for future experimental implementations.

Higher-order topological photonic crystals offer a promising route for robust light confinement. This work presents a Fano resonance sensing system via near-field coupling between two topological corner states (TCSs) in a Kagome photonic crystal. The coupling generates an odd-symmetric supermode with suppressed radiative loss and an ultra-high eigenmode quality factor. Integrating the coupled TCSs with a topological edge waveguide forms a cavity-waveguide system supporting sharp Fano resonance. The proposed system exhibits excellent performance, with a sensitivity of 562.75nm/RIU and a maximum figure of merit of 1.22×107RIU-1 achieved for the odd-symmetric supermode. This platform paves the way for advanced higher-order topology applications in integrated photonics and sensing.

Engineered magnetoplasmonic nanostructures offer an efficient route to enhance the magneto-optical (MO) activity of materials, greatly improving their practical utility. Especially, three-dimensional (3D) magnetoplasmonic nanostructures offer richer MO properties through multi-mode excitations, providing new paradigms for miniaturized non-reciprocal photonic devices. However, the complex geometrical configurations of 3D nanostructures impose significant fabrication challenges compared to conventional planar nanostructures, which creates substantial obstacles for practical implementation. Here, we present free-standing metallic hole–vertical nanoplate (MH-VNP) nanostructures fabricated by high-precision focused ion beam (FIB) technology on Au/Co/Au trilayers. Under y-polarized excitation, MH-VNPs exhibit an unusual Fano resonance arising from in-phase conductive coupling between the nanoplate and metallic hole, thereby forming 3D currents. This resonance induces strong magnetic field localization, causing a larger Faraday rotation (FR) and enabling FR sign inversion. Systematic modulation of geometric parameters allows precise control of FR magnitude and resonance positions, offering versatile tunability. The self-supported architecture maximizes exposure to the surrounding medium, achieving an exceptional refractive index sensitivity of up to 1500 nm/RIU. Overall, this work demonstrates an environmentally friendly fabrication route for 3D magnetoplasmonic structures, offering a versatile approach for tailoring MO responses in nonreciprocal photonic and sensing applications.

In this paper, we report on the influence of structural and thermal decoherence on the Raman line shape undergoing Fano interference. Using BaTiO3 as a model sample, we examine its well-known ∼180 cm-1 Raman mode that originates due to the Fano interference between a discrete phonon (A1(TO1)) at 172 cm-1, and the continuum formed by the 250 cm-1 (A1(TO2)) phonons. Additionally, the asymmetric ∼521 cm-1 mode in p-type doped silicon is examined. The decoherence is demonstrated by introducing random phase fluctuation to the energy-dependent scattering phase Δ(ε) appearing in the typical Fano treatment with the help of a stochastic Monte Carlo approach, and a complete methodology is presented to explain the changes in the Raman line shape. Our approach successfully explains observed changes in the intensity and line shape of mode, suggesting that decoherence is a crucial factor controlling the Raman profiles.

Raman signature corresponding to quasi elastic scattering (QES) has been discovered where the local vibrational mode (LVM) B11, present in heavily doped p-type (boron doped) silicon (Si), participate in Raman scattering. The LVM manifests itself as an asymmetric Raman line-shape due to Fano resonance which gets weakened on increasing temperature as revealed through temperature dependent Raman spectroscopy and Raman thermal mapping. The temperature dependent Raman experiment, combined with the modified Balkanski model for the LVM, reveals that the increase in Raman line width (fwhm) with temperature does not originate from the direct anharmonic phonon decay rather QES is taking place. In particular, the presence of Fano coupling between the local vibrational mode B11 and the electronic continuum serves as a significant probe for exploring the quantum behavior of such material. The observed reduction in Fano coupling strength, duly quantified by Fano parameter, accompanied by an increase in fwhm of the Raman line shape with rising temperature, has been analyzed to confirm the presence of QES. Evidence of the phonons' dephasing (loss of coherence) process with temperature in LVM has been identified. This dephasing arises from the suppression of quantum interference between the discrete localized mode B11 and the electronic continuum. Overall the findings present an improved insight about the physical processes taking place at the microscopic level in a heavily doped semiconductor.

Enhanced bidirectional neural network for tailoring high-Q Fano resonances in a Y-shaped metasurface array

Fano resonances in thin-film optics exhibit strong angular dispersion, which severely degrades the Q factors and significantly distorts the lineshapes. In this Letter, we theoretically demonstrate an angle-dispersion-free Fano resonance in a heterostructure composed of a plasmonic layer and two one-dimensional photonic hypercrystals. This fascinating property of the Fano resonance originates from the interference between an angle-dispersion-free continuum and an angle-dispersion-free optical Tamm state. As the incident angle increases from 0 to 80 degrees, the Fano dip exhibits an extremely small wavelength shift of less than 0.1%, while its Q factor remains highly stable. Our work breaks the angular dispersion limit of Fano resonances in thin-film optics.

Realizing high-quality-factor (high-Q) plasmonic resonances in the visible regime is critical for enhancing light-matter interactions and advancing biochemical sensing. However, traditional localized surface plasmon resonances (LSPRs) typically suffer from broad spectral linewidths due to severe radiative damping. In this work, we propose a simple two-dimensional symmetric gold nanohole-array metasurface that supports a symmetry-protected bound state in the continuum (SP-BIC) at normal incidence. By introducing extrinsic symmetry breaking via oblique incidence, this non-radiative dark state is successfully transformed into an observable high-Q quasi-BIC Fano resonance. Cartesian multipole decomposition reveals that this sharp mode (λ≈688 nm) is predominantly driven by a tightly confined Magnetic Dipole (MD) excitation, which drastically suppresses radiative leakage compared to the highly damped Electric Dipole (ED)-dominated LSPR. Consequently, the quasi-BIC mode exhibits an ultra-narrow spectral linewidth (FWHM≈17.4 nm). While its bulk sensitivity (236.9 nm/RIU) is slightly lower than that of the LSPR mode, the exceptionally sharp resonance yields a remarkably low Limit of Detection (LOD) of 7.35×10−3 RIU, achieving a nearly five-fold improvement over the traditional LSPR. Furthermore, the quasi-BIC mode maintains an outstanding Figure of Merit (FOM up to ∼19.7 RIU−1) across the entire sensing range. By eliminating the need for complex asymmetric nanofabrication, this robust angle-tuned design strategy provides a highly promising platform for the development of high-resolution, low-cost optical biosensors.

Advances in microwave photonics call for the investigation of Fano resonances in the microwave domain for efficient signal generation and processing. While emerging integrated acousto-optic (AO) technology provides a promising approach for this purpose, Fano resonances in these systems have seldom been reported. Here, based on a polymer-loaded lithium niobate on insulator (LNOI) platform, we achieve efficient AO and electro-optic (EO) coupling in a well-designed photonic crystal nanobeam cavity (PCNBC), enabling Fano resonance in the microwave domain within a single AO modulator. The characteristics of the Fano resonance are strongly determined by the acoustic modes. By adjusting the gap between the PCNBC and the interdigital transducer, the Fano resonance can be periodically tuned for a given acoustic mode. Additionally, its dependence on radio-frequency power is demonstrated, achieving a high extinction ratio up to 44 dB at a low radio-frequency power of -25 dBm. This work provides an approach to achieve the Fano resonance in the microwave domain, promoting the development of the AO-based devices in microwave photonics.

We demonstrate electrically tunable control of the radiative and nonradiative decay rates of a fluorescent molecule through a Fano-resonant transparency embedded in the plasmonic local density of optical states (LDOSs). An auxiliary quantum object (QO) placed at the hotspot of a plasmonic nanoparticle suppresses the plasmonic excitation at its transition frequencyωQO, thereby creating a narrow transparency window and reducing the LDOS atω=ωQO. When the fluorescence frequency of a nearby emitter overlaps this window, the plasmon-induced enhancement of both radiative and nonradiative decay is strongly suppressed. BecauseωQOcan be shifted electrically, the transparency can be moved reversibly across the fluorescence line, enabling continuous voltage control of the decay rates. Three-dimensional Maxwell simulations predict tuning of the radiative and nonradiative channels by up to two orders of magnitude. The proposed mechanism offers a compact route toward fast, reversible control of light-matter interaction in integrated photonics, with potential applications in single-photon sources, electrically programmable quantum devices, super-resolution microscopy, and surface-enhanced Raman spectroscopy.