The surface plasmon resonance (SPR) phenomenon [
1] has been extensively investigated both experimentally and theoretically by scholars’ tremendous efforts in recent decades. In general, the free electrons on a metal surface respond collectively by oscillating in resonance with a light wave, and the resonant interaction can constitute surface plasmons (SPs). Then, SPs are highly trapped on the interface between the metal and dielectric, and thus the strong confinement will lead to an electric field enhancement [
2]. Simultaneously, the SPR effect has the trait of being very sensitive to slight refractive index (RI) changes. Combining the advantages of field enhancement and RI sensitivity, SPR sensors have been developed in many areas such as environment monitoring, food safety, water testing, gas detection, bio-sensing, and so on [
3].
In the early stages, bulky Kretschmann prism-based SPR sensors played a certain role due to their robustness. However, the configurations with many optical and mechanical moving components are cumbersome, expensive and difficult to fabricate, so they cannot fulfill the requirements of modern sensors to be densely compact, amenable to integration, affordable and capable of remote sensing. To achieve modern sensing requirements, photonic crystal fibers (PCFs) [
4] have become one of the most prominent potential candidates. PCFs have been widely recognized in the optical transmission field since 1996. They introduce the principle that the photonic crystal can modulate the electromagnetic wave with the corresponding wavelength into the fiber; therefore, cladding micro-nano dimension air pores arranged in the form of photonic crystals can regulate light propagation in PCFs. The excellent feature of PCFs is their design flexibility, so dispersion, birefringence, nonlinearity, etc. can be tailored by different air pore arrangements [
5]. These aspects make PCFs particularly eye-catching in many areas and result in a wide range of applications in gas-based nonlinear optics, atom and particle guidance, ultrahigh nonlinearities, rare-earth doped lasers and sensing fields. Nowadays, the mature PCF fabrication widely utilized is the stack-and-draw procedure, which has proved highly versatile, allowing complex lattices to be assembled from individual stackable units of the correct size, and thereby shape, solid, empty, or doped glass regions can easily be incorporated. Because of natural microstructure channels in PCFs, gas, solid or liquid functional materials can be infiltrated or selectively infiltrated into periodic air holes, which helps to implement the manipulation of light–matter interactions [
6]. Besides, the processing of conventional optical fibers, such as side polishing, rotation, slot and coating technology, can also be applied to PCFs, which makes PCFs’ application range extremely extensive.
It turns out that the future of PCF sensors based on the SPR effect can be expected, especially since Lee et al. achieved selectively filling gold wire into a PCF’s individual holes via experiments successfully combined with finite element simulation, which shows that gold-filled PCFs can be used as in-fiber wavelength-dependent devices [
7]. A lot of novel and high performance PCF SPR sensor designs have emerged based on experimental or numerical methods. According to the processing technology for the PCF, the sensors can be roughly divided into three categories: First, selectively infiltrating metal into the PCF cladding air hole. Shuai et al. filled the central core pore with gold and designed a liquid-core PCF based on plasmonic effect with a maximum negative RI sensitivity of −5500 nm/RIU in the sensing range of 1.50–1.53 [
8]. Rifat et al. proposed a highly sensitive plasmonic sensing scheme with miniaturized PCF attributes, which yielded maximum sensitivities of 11,000 nm/RIU and 1420 RIU
−1, maximum resolutions of 9.1 × 10
−6 and 7 × 10
−6 RIU for the wavelength and amplitude sensing schemes and a maximum figure of merit (FOM) of 407 [
9]. Second, coating a metal film around a PCF. Rifat et al. proposed a simple, two-ring, hexagonal lattice PCF biosensor with an active plasmonic gold layer, and the sensor could provide a maximum wavelength sensitivity of 4000 nm/RIU and a maximum amplitude sensitivity of 320 RIU
−1 with a resolution of 3.125 × 10
−5 RIU for wavelength interrogation (WI) and amplitude interrogation (AI) [
10]. Lu et al. proposed a large-mode-area polymer PCF made of polymethyl methacrylate with the cladding having only one layer of air holes. A nanoscale metal film and analyte can be deposited on the outer side of the fiber, and an intensity sensitivity of 8.3 × 10
−5–9.4 × 10
−5 RIU can be obtained in their sensor [
11]. Islam et al. presented a novel PCF sensor based on the SPR effect in the region from visible to near-infrared (500–2000 nm) wavelength range for RI sensing. The sensor showed a maximum wavelength sensitivity of 58,000 nm/RIU for the x polarization and 62,000 nm/RIU for the y polarization for analyte RI ranging from 1.33 to 1.43, maximum sensitivities of 1415 RIU
−1 and 1293 RIU
−1 for the x and y polarizations, a maximum figure of merit (FOM) of 1140 and fine RI resolution of 1.6 × 10
−6 [
12]. Third, using a side polishing technique to make a flat plane and coating a metal film on the plane as a sensing layer. Santos et al. presented a sensing configuration of RI, based on SPR in a micro-structured D-type optical fiber with a thin gold layer [
13]. The sensor showed an improvement in sensitivity of 10 × 10
3 nm/RIU and an improvement in resolution of 9.8 × 10
−6 RIU. Luan et al. presented a D-shaped hollow core microstructured optical fiber (MOF)-based SPR sensor, achieving a spectral sensitivity of 2900 nm/RIU, a maximum amplitude sensitivity of 120 RIU
−1, and a maximum phase sensitivity of 50,300 deg/RIU/cm [
14]. What is more, other sensors with distinctive shapes and excellent performance are also desirable, with designs such as the opening-up dual-core micro-structured optical fiber-based SPR sensor by Luan et al. [
15] and the SPR sensor with two open-ring channels based on a PCF for mid-infrared detection by Liu et al. [
16], which was similarly compelling.
In practical sensing applications such as water quality monitoring, bio-medicine and chemistry, devices are required to be very sensitive to RI changes in some unknown analyte, so a highly sensitive PCF SPR sensor is proposed and numerally investigated in this paper. The sensor has a unique structure with dual-side polishing, which can help produce more obvious signals for detection to ensure sensing performance. On both planes, gold film is applied as a plasma excitation material and silicon nitride (Si
3N
4) layers are plated on top of the gold layer. Our sensor can be applied in the analyte environment to detect small RI changes. The effects of structural parameters such as the pore diameters of the inner and outer layers, pore pitch, thickness of the gold layer and Si
3N
4 layer are investigated. Furthermore, wavelength interrogation and amplitude interrogation methods [
17] are implemented, and the major performance of the sensor is simulated by the finite element method (FEM) [
18] using COMSOL software.