Fano resonance, a new kind of resonance with sharp asymmetry lineshape, has drawn huge attentions in many optical applications such as slow light, biochemical sensing, and optical modulation, considering that its narrow and asymmetric characteristics can improve and enhance the sensitivity [[1], [2], [3], [4], [5], [6], [7], [8]]. It originates from the coherent interference between discrete state and continuous state, therefore inducing asymmetric spectrum relative to the traditional lorentzian system with symmetric lineshape. Recently, nano-plasmonic structure-based Fano resonance has also been researched deeply because of the fact that surface plasmon polaritons (SPPs) have the capability of overcoming optical diffraction limit and confining light in the sub-wavelength region [[9], [10], [11], [12]]. In fact, SPPs can propagate through metal-dielectric interface associated with exponential decay along the normal direction of interface due to the interaction and coupling between optical waves and electron oscillations. Fano resonance can be easily demonstrated by applying symmetric-breaking plasmonic nanostructures, such as microring or microdisk cavities with defects around their edges [13,14], asymmetric stub pair in MIM waveguide [15,16], and asymmetric plasmonic nanoclusters [17,18]. Furthermore, symmetric plasmonic structures can also be used for Fano resonances, including cavities interference [19], and metallic gratting with narrow slits [20]. Especially, Fano resonances in the MIM structure-based plasmonic devices arouse many researchers’ interests due to their huge advantages, such as compact structure, strong confinement of SPPs, long propagation distance, and low band loss [[21], [22], [23], [24], [25], [26]]. Meanwhile, combination of Fano resonance and MIM waveguide can utilize both advantages of sharp asymmetric lineshape of Fano resonance and excellent MIM structures.

Because of excellent performance of ultra-high quality factor and small mode volume, WGM microcavity has been applied in many optical fields such as ultra-low threshold microlasers and high-sensitivity microsensors [[27], [28], [29], [30]]. Therefore, MIM waveguide coupled with WGM cavity is a good potential to realize high-performance Fano resonance [31,32]. For a long time, single Fano resonance with one asymmetric lineshape has been obtained much attention, while few reports focus on multiple Fano resonances, for instance dual Fano resonances. Actually, multiple Fano resonances become more important and have great advantages in the optical applications, such as enhanced biochemical sensing, multi-color nonlinear processes, and multi-wavelength spectroscopy [[32], [33], [34]]. For example, double Fano resonances are studied based on the asymmetrically split ring cavities [35], or the combination of MIM waveguide and rectangular cavity [36]. Furthermore, the tunability of dual Fano resonances is very important, especially the independent tuning of each Fano resonance. Whereas, it faces some difficulties in MIM waveguide coupling WGM cavity because plasmonic system has the collective behavior and all WGMs would be affected simultaneously in the traditional WGM cavity.

Recently, Taiji resonator has been obtained huge attentions due to its particular configuration of an S-shaped additional branch inside the ring cavity. This additional branch, as an independent section, can tailor the propagating modes into their counter-propagating directions, finally tuning the number of WGMs in Taiji resonator. It has been applied to various optical fields, including topological laser, unidirectional laser, and multiple Fano resonances [32,[37], [38], [39]]. However, that application of multiple Fano resonances has not involved the capability of independently tuning each Fano resonance. Relative to traditional WGM microcavity, Taiji resonator has good potential to independently tune Fano resonances because of the additional section of S-shaped crossover branch, while not any reports focus on it.

In this paper, we propose and demonstrate independently tunable dual Fano resonances based on a MIM Sagnac loop with embedded Taiji resonator. An S-shaped branch inside Taiji resonator can modify the number of WGMs in Taiji resonator. Both Fano resonances originate from the interference between the discrete state generated from WGMs in Taiji resonator and continuous state from modes propagating in the plasmonic Sagnac loop. By stuffing a SCA with different sizes or locating at different places in the S-shaped branch, dual Fano resonances can be tuned independently. The physical mechanism is that Fano resonance would be influenced strongly when SCA locates at the energy intensity valley of WGM, while without any influence as SCA situates at the energy intensity peak of WGM. As a potential application of refractometric sensing, the sensitivity of proposed plasmonic device can be up to 1899 nm/RIU with FOM of 100/RIU. Furthermore, it's also used for temperature sensing of ethanol with the maximal sensitivity of −0.793 nm/°C.