As a novel surface mode, Tamm plasmon polaritons (TPPs) can be directly generated by the incident light with any polarization on the interface between metal and a distributed Bragg reflector (DBR) because its dispersion curve lies inside the light cone. Significantly enhanced energy distribution on the metal-DBR interface makes TPPs a potential candidate for nanoscale sensor devices. However, highly localized energy also prevents TPPs from touching the outside medium. In order to improve the sensing sensitivity of TPPs to the ambient medium, a triple-layer combinative structure has been proposed in this study, which is constituted by a metal film sandwiched between a metal grating and a DBR section. In this configuration, TPPs can be effectively excited on the interface between DBR and the metal film with a proper thickness, and a fraction of the localized energy induced by TPPs can penetrate the metal film into the grating slits to produce the surface plasmon polariton (SPP) modes supported by the metal slits. A quasi-Fabry-Perot (F-P) resonance of SPPs can be generated by a proper incident wavelength, and the highly localized energy accumulated through the F-P resonance can be employed to sense the refractive index of the ambient medium.

The DBR section in this study is formed by the alternating dielectric layers of TiO₂ with a thickness of 121 nm and ZnO with a thickness of 156 nm, which ensure a Bragg wavelength to be 1 μm. Meantime, the metal gratings and film are made of silver, and the corresponding frequency-dependent complex relative permittivity is described by the Drude-Lorentz model. Due to the periodicity of metal gratings and the uniform distribution of the proposed structure along the slit direction, the three-dimensional triple-layer structure can be simplified to a two-dimensional plane unit cell model combined with the periodic boundary condition. The modal distribution and transmission characteristics of the cell model have been calculated numerically by the finite element method. The transmittance spectra of the proposed model have been obtained by changing the incident wavelength successively. The influence of the film thickness on the peak transmittance, peak wavelength, and transmission bandwidth has been analyzed in detail by varying the film thickness from 4 to 20 nm. In addition, by changing the refractive index of the medium filled in the grating slits, the sensing performance of the proposed structure has been evaluated by calculating the transmittance spectra of the triple-layer structure with different structural parameters, such as film thickness, duty cycle, and grating height.

At first, the transmittance spectra of the triple-layer structure with varied film thickness have been analyzed in detail by employing the finite element method. The numerical results demonstrate that the peak transmittance first increases for small values of the film thickness and then decreases when the film thickness is greater than 12 nm. As a result, a maximum transmittance of 0.712 is obtained when the film thickness equals 12 nm, which represents an improvement of nearly 29% in the peak transmittance compared with the filmless case [Fig. 3(a)]. Meantime, the full width at half maximum (FWHM) of transmission peak decreases monotonously with increasing film thickness, and the peak wavelength gradually approaches to the fixed value of 1040 nm, which is the central wavelength of the typical TPPs in the interface between the semi-infinite silver layer and DBR [Fig. 3(b)]. In addition, the sensitivity of the refractive index sensing has been calculated by changing the refractive index of the medium filled in the metal slits, and it is found that the sensitivity decreases monotonously with the increase in film thickness (Fig. 5). Therefore,a film thickness of 8 nm provides the most balanced performance in the transmittance enhancement and highly sensitive refractive index sensing, which can increase the peak transmittance by about 16% and the sensitive by nearly 50% compared with the filmless case. When the ambient media refractive index changes continuously from 1.0 to 2.2, the resonance order of the FP resonances occurring in the grating slits can be changed from third to fifth order (Fig. 6). Numerical results demonstrate that the detection range of the third, fourth, and fifth order resonances are 1.10-1.26, 1.49-1.65, and 1.86-1.99, respectively, and the sensitivity and FOM values associated with three resonant modes are monotonically increasing with the resonant order (Table 1). On this basis, by changing the duty cycle and height of the metal grating, the refractive index sensing performance of the transmission peaks corresponding to the third, fourth, and fifth-order F-P resonances in the grating slits is analyzed in detail. The results show that as the duty cycle decreases, the sensitivity will increase significantly, and the sensing sensitivities of the transmission peaks induced by the fourth and fifth-order resonances occurring in the grating slits are 171.2 nm/RIU and 178.35 nm/RIU, respectively, when the duty cycle of grating equals 0.6 (Fig. 7). Meantime, the refractive index detection range can be shifted by a nearly linear manner by adjusting the grating height. According to third, fourth, and fifth-order resonant modes in the grating slits, the detection range of the proposed structure can effectively cover the values ranging from 1.00 to 2.27 by tuning the height of metal grating from 900 nm to 1200 nm (Fig. 8).

In this study, a triple-layer composite structure has been proposed to detect the refractive index of the ambient medium based on the F-P resonance induced by the TPPs. Research results indicate that introducing a silver film between the metal grating and DBR can effectively improve the excitation efficiency of TPPs, thereby enhancing the field intensity of SPP modes within the grating slits and the amplitude of the transmittance peak. Especially when the duty cycle is reduced to improve the sensitivity, the introduction of silver films can avoid the signal degradation induced by the lower duty cycle. This configuration can thus assure the high sensitivity to the refractive index of the filling medium in the gratings slits and the satisfied excitation efficiency of TPPs in the metal film-DBR interface. Moreover, the proposed structure can adjust the detection range in a nearly linear manner by changing the grating height. In this study, the detection range of the refractive index can be extended from 1.00 to 2.27 by adjusting the grating height from 900 nm to 1200 nm. The research results of this study provide an effective design idea for the TPPs-based refractive index sensor.