The rapid evolution of photonic technologies has sparked growing interest in developing miniaturized optical sensors for critical applications in biomedical diagnostics, environmental surveillance, and chemical analysis [1]. Surface plasmon polaritons (SPPs), collective oscillations of electrons at metal-dielectric interfaces, have emerged as a promising platform due to their exceptional field confinement properties [[2], [3], [4], [5]]. These unique characteristics have been successfully leveraged in diverse photonic applications including absorption enhancement [[6], [7], [8]], plasmon induced transparency [9], biosensing and bioimaging [10,11]. Notably, SPP-mediated MIM waveguides demonstrate superior light manipulation capabilities that circumvent the diffraction limit [12]. This distinctive advantage, coupled with remarkable field localization, configurable design flexibility and seamless integration with photonic circuits, has positioned MIM waveguide-based sensors at the forefront of photonic research [[13], [14], [15]]. Current developments in MIM waveguide architectures have yielded various functional micro/nano-devices, ranging from wavelength filters [16], optical switches [17,18], slow-light devices [19], beam splitters [20] and so on.

MIM waveguide-coupled resonant cavity architectures have emerged as a promising platform due to their exceptional ability to confine SPPs at subwavelength scales, enabling enhanced light-matter interactions and ultrasensitive detection of minute refractive index changes in analytes [[21], [22], [23], [24], [25], [26]]. Recent studies on MIM waveguide sensors have focused on optimizing resonator geometric configurations to maximize sensitivity and specificity. For instance, in 2021, Shao et al. proposed a nanostructure consisting of MIM waveguide coupled double ring resonator, its sensitivity is 1885 nm/RIU [27]. Liu et al. proposed a structure consisting of MIM waveguide with two silver baffles and a coupled split ring, the sensitivity is 1034 nm/RIU. Both of them have prospects in the field of nano sensors [28]. Straight MIM waveguides, coupled with resonant structures like triangular-shaped resonator [14], H-shaped resonator [29], semi-elliptical ring resonator [30], split-ring and stubs [31], ring cavity [32], rectangular and ring resonator [33], baffle and circular split-ring resonator [34], concentric-ring resonator [35], U-shaped resonator [36], exploit phase shifts or resonance wavelength shifts induced by RI variations in the surrounding medium for different applications, such as human blood types detection [35,37], alcohol solution concentration detection [36], pressure measurement [38]. The coupling method is another critical factor influencing the coupling efficiency between MIM waveguides and the resonator. Central-coupled way can exhibit substantially higher sensitivity and FOM than side-coupled way due to the direct coupling effect [39]. Various design structures of central-coupled resonators have been extensively investigated theoretically and experimentally [40,41]. The integration way of MIM waveguides with different shaped resonant cavities has significant effects on plasmonic sensing performance, which requires further investigation for future high sensitivity and high integration sensor applications.

In this paper, a high sensitivity RI sensor comprising two MIM bus waveguide central-coupled two overlapping triangular resonators is proposed. The transmittance of the designed structure in infrared spectrum is performed by using FEM. The effects of structural parameters on transmittance spectra, magnetic fields and sensing inspection are investigated. Results show that a high sensitivity of 2760 nm/RIU can be obtained in the dual-overlapping triangular resonator structure within the resonant wavelength from 500 nm to 3500 nm. The sensitivity can be further enhanced by introducing symmetrical quad-overlapping triangular resonator, which attributes to extended light-matter interaction length and intensified field confinement at triangular vertices. The sensitivity can be reached 3330 nm/RIU within the resonant wavelength from 500 nm to 4000 nm, which is better than the four coupled bowtie resonators [23]. In addition, the proposed RI sensor demonstrates effective concentration detection capabilities for both glucose and plasma solutions and would be one of the promising candidates for biomedical application in nanophotonic devices.