Metasurfaces are artificial interfaces consisting of subwavelength resonant units arranged in a two-dimensional lattice. Through the design of unit geometry, material properties, and spatial organization, they allow for precise control over the amplitude, phase, and polarization of light at subwavelength scales. This unique capability has been extensively applied in various research fields, including the generalized Snell's law [1], bound states in the continuum (BIC) [[2], [3], [4]], and Fano resonance [5,6]. Advances in micro-nano processing technology have enabled the precise design of metasurface microstructures, which in turn exhibit unique optical properties. These outstanding physical properties show great application potential for applications in fields such as refractive index sensing [7], metalenses [8,9], optical filtering [10], and narrowband thermal emitters [11]. Initially, most metasurfaces utilized metallic micro-nano structures to excite plasmonic resonances [12,13]. However, the inherent ohmic loss of metals leads to a broad resonance linewidth, which consequently limits the achievable Q-factor [14,15]. To overcome this limitation, low-loss all-dielectric materials have been adopted [16,17]. Their relatively low absorption coefficient leads to sharper resonances, thereby yielding a higher Q-factor for the associated Fano resonances [18]. Furthermore, all-dielectric structural units are highly compatible with CMOS processes, offer low cost, and are suitable for large-scale fabrication [[19], [20], [21]]. Owing to the synergistic advantages of high Q-factor, low loss, and integrability, all-dielectric metasurfaces have emerged as a key platform for the new generation of high-sensitivity refractive index sensors.

The physical origin of the asymmetric resonant scattering in light-matter interactions is the coherent interference between a continuum state and a discrete state [22,23]. When their energies are close, a specific coherent superposition occurs: destructive interference at energies below the discrete state gives rise to a dark mode, while constructive interference at energies above it forms a bright mode [[24], [25], [26]]. The synergy of these two interference effects ultimately produces a characteristic asymmetric Fano resonance lineshape. The traditional Fano resonance stems from the coupling effect of these two modes, but by regulating the geometric parameters, Fano resonance peaks with excellent optical response can also be excited [27]. Most Fano resonance effects are observed in the visible and near-infrared bands. For example, Wu et al. designed a high-Q quasi-BIC in a silicon tetramer cylinder nanostructure by varying the cylinder dimensions and introducing a slight perturbation, leading to highly sensitive sensing performance [28]. Similarly, Wang et al. reported a metasurface composed of nanodisk tetramers that supports dual-band symmetry-protected BICs; they excited high-Q quasi-BIC modes by introducing asymmetric air holes, thereby achieving polarization independence within the frequency band of interest [29]. Furthermore, Liao et al. demonstrated a dual-resonant mid-infrared all-dielectric metasurface sensor based on asymmetric cross-dimers, where adjusting the asymmetric parameter excited a TD and enabled dual-resonant operation [30]. These studies primarily focus on the near-infrared or terahertz bands, and the lack of exploration into the mid-infrared region hinders their application potential in key areas such as broad-spectrum molecular fingerprint identification and environmental monitoring. Furthermore, methods employed to excite quasi-BIC modes, such as introducing asymmetric air holes or precisely controlling nanoscale displacements, demand extremely high precision and uniformity in nanofabrication. This poses significant challenges for practical manufacturing, including high costs and difficulties in large-scale production.

We present an all-dielectric metasurface sensor for mid-infrared refractive index sensing, that is designed for simplified fabrication and offers broad applicability. Exploiting the narrow linewidth and strong field localization of Fano resonance [31], a sharp resonance is excited via periodic coupling, achieving a high Q factor with a simple fabrication process. This mechanism enhances the optical response, enabling simultaneous high Q factor and ultra-high sensitivity [32], which is promising for a range of advanced applications. This work presents a low-loss all-dielectric double-rod nanostructure. The metasurface consists of two symmetrically arranged germanium (Ge) nanorods, with the unit cell periodically patterned on a BaF2 substrate. Using the finite-difference time-domain (FDTD) method, we analyzed its resonance characteristics and observed a narrow Fano resonance peak with a high Q factor. Through electromagnetic field mode analysis and multipole decomposition, the resonant modes are identified as TD and MQ. Furthermore, investigation into the influence of structural parameters on the reflection spectrum and found that the resonance exhibits a blueshift with increasing incident angle. The flexible tuning of resonance peaks via geometric parameters was further explored, and the impact of conical nanocolumns on resonance peaks during fabrication was analyzed simultaneously. This sensor achieves a narrowband Fano resonance in the mid-infrared region with a high Q factor of 18544 and a reflectivity exceeding 94 %. It also demonstrates a maximum sensitivity of 1337.1 nm/RIU and the FOM of 1238. These outstanding performance metrics indicate that the proposed structure shows great potential for applications in environmental monitoring, biomedical sensing, and food safety.