Highly sensitive tri-path photonic crystal fiber plasmonic sensor based on hybrid layer of gold/platinum diselenide

Platinum Diselenide, PtSe2 is becoming highly trending owing to its fascinating optoelectronic, thermoelectric and semiconductor properties. They are non-toxic, chemically inert and allow high biomolecule absorption which makes them highly applicable in sensors to boost the sensing performance. Here, we propose Surface Plasmon Resonance (SPR) based Photonic Crystal Fiber (PCF) sensor for enhanced refractive index sensing at mid infrared wavelengths. In order to achieve this, tri-path PCF coated with hybrid layer of gold/PtSe2 which allows light to travel freely through the cladding and interact with the plasmonic material to create strong coupling effect. Finite Element Method is used for numerical examination and investigation of the sensing performance for the designed tri-path sensor. The optimized proposed sensor exhibits maximum wavelength sensitivity of 42,000 nm/RIU and maximum wavelength resolution of 2.4 × 10−6 within the analyte range from 1.33 to 1.38, which can be excellent detection of unknown chemical, biochemical and biological analytes. Further, we achieve very low loss and unique design to accomplish high sensitivity which makes it applicable to be a future candidate in various sensing applications.


Introduction
Photonic Crystal Fiber (PCF) is highly trending in photonics which is a single material providing a platform for sensing (Qin et al. 2013). The main purpose of using PCF as sensing platform is design flexibility and regulation of core effective index which is accomplished by altering the air holes present (Monfared et al. 2019). They design flexibility produces high birefringence, high confinement field, endlessly single material, tunable dispersion, compact sensing and many more. When this PCF material is combined with plasmonic science, high sensitivity results will be achieved which paves way for being a potential future candidate in various fields (Vigneswaran et al. 2018). Surface Plasmon Resonance (SPR) is a promising sensing technique that detects the minimum variation in refractive index (RI) when there is an interaction with the metal film. They expose high sensitivity results which makes it trending and highly applicable in various sensing fields. As SPR is label free and real time detection process which produces high sensitivity, intensive research curiosity for the scientists has been highly stimulated for making more advancements in the sensors (Rahman et al. 2020). They are growing tremendously from the scientific region as there are extensive applications that also include temperature monitoring (Luan and Yao 2017), multi analyte sensing (Otupiri et al. 2015), biosensing (Akowuah et al. 2012), liquid sensing (Kaur and Singh 2019), disease diagnosis (Jabin et al. 2019) and many more. The most popularly used plasmonic is gold as it is chemically stable, provide long resonant shift, biocompatible and is prone to oxidation (Esfahani Monfared 2020; Hasan et al. 2018b).
Lately, along with the plasmonics, flimsy oxide layers are added such as aluminium oxide (Al 2 O 3 ), indium-tin oxide (ITO), titanium dioxide (TiO 2 ) to elevate the sensing performance of the sensor. They assist in robust SPR effect which aid to improve the sensing range along with sensitivity for the sensor (Mahfuz et al. 2019). But these plasmonics and additional coating layers also face a drawback where they have very low biomolecule absorption capability in biosensors. To defeat this drawback, it was suggested to use biomolecule recognition elements as Transition Metal Dichalcogenide (TMDC) materials along with the plasmonics (Singh and Prajapati 2020). TMDC materials are also known as 2D materials which is a huge family that have intriguing features and unique structure that have made them highly attractive and significant in various industrial applications owing to its capability for biomolecule recognition and sensitivity enhancement (Wu et al. 2017). 2D materials limit the transport of heat and charge in their unique layered structure which is an exceptional property capable, making them enticing for optoelectronic and electronic applications (Gong et al. 2020). One of the leading factors for determining the properties and applications in 2D materials is the bandgap. These 2D materials have tunable finite band gap which make them to be a promising candidate in numerous applications . These materials exhibit exclusive properties as large absorption of light, high charge mobility and transitional behavior which build them more appropriate for fabricating high performance electronic and optoelectronic devices (Guo et al. 2020). Graphene was the first man-made and discovered 2D nanomaterial in 2004. Due to its inter-band transition, it exposes good optical conductivity from near to mid infrared frequency, produces huge surface to volume ratio aiding high biocompatibility (Singh and Prajapati 2020). There are many other 2D materials as MoS 2 (Radisavljevic et al. 2011), WO 3 (Wang et al. 2014), WS 2 (AlaguVibisha et al. 2020), PtSe 2 (Xie et al. 2019), etc. which are recently trending in the research field. These materials have excellent properties as optical, thermal, electronics, optoelectronics, catalytic, super conductivity and energy-storage . The investigation and analysis of the 2D materials along with its properties was first initiated by coating them over the prism as sensors (Ouyang et al. 2016). In 2016, highly sensitive prism based SPR sensor was reported using 2D materials. It is heterostructured configuration where the arrangement was made as MoS 2 /Aluminium/MoS 2 /Graphene. The sensor exposes a maximum sensitivity of 190.83 ͦ /RIU (Wu et al. 2017). Specifically, Platinum Diselenide (PtSe 2) is a group ten monolayer material in 1 T phase and it is highly preferred owing to their exclusive features (Guo et al. 2020). They resemble the structure of phosphorene and graphene and also reveals admirable thermoelectric, optoelectronic and semiconductor properties making it more unique. Owing to its robust interlayer interaction, the 2D material PtSe 2 exposes high tunable bandgap. Moreover, this material is observed to have good chemical stability with also less toxic which makes it highly applicable for practical usage (Jia et al. 2020). Recently, prism sensor was proposed where PtSe 2 was selected to attain maximum Goos-Hanchen shift. The sensor has gold and titanium coating along with PtSe 2 and three layers of graphene is added in BK7 prism. The results revealed the Goos-Hanchen shift sensitivity shows an increment four times when compared with the conventional gold coating. The detection limit is very low as 5 × 10 −7 RIU which is diminished by 2 order of magnitude and sensitivity elevates by 1000-fold when compared with conventional gold coating (Guo et al. 2020). Hence, 2D material based SPR sensors are recommended for RI sensing as they exhibit highly improved sensitivity (Jia et al. 2020).
The PCF combined with SPR sensing method along with various additional coating materials were investigated extensively as it was perceived to reveal superior sensitivity for the sensors. In 2016, SPR combined with D-shape PCF was mentioned with plasmonic metal employed as gold and additional coating layer of ITO for RI ranging from 1.28 to 1.34. The sensor gains a high sensitivity of 6000 nm/RIU (Huang 2017). In 2018, PCF combined with SPR sensor was reported with bimetallic coating of new plasmonic material as niobium nanofilm along with additional covering of flimsy Al 2 O 3 for organic and biochemical sensing. The sensor functions for analyte RI 1.36 to 1.41 and reveals a maximum sensitivity of 8000 nm/RIU (Hasan et al. 2018a). Highly birefringent and simple design PCF-SPR was presented with gold as plasmonic and TiO 2 as additional thin layer forming the bimetallic structure. The sensor operates for RI range from 1.33 to 1.38 and exposes a high sensitivity of 25,000 nm/RIU making it highly suitable in numerous sensing applications . Recently, dual core PCF-SPR sensor with bimetallic coating layers of gold and TiO 2 was described for numerous biochemical analytes and bio-organic molecules RI detection. The sensor functions for analyte RI range of 1.33 to 1.42, disclosing a maximum sensitivity of 28,000 nm/RIU (Mahfuz et al. 2020). But these plasmonics with additional layers have low capability of biomolecule absorption. So, 2D materials were recommended to be used as additional covering layers along with plasmonics over PCF as they are excellent in biomolecule recognition (Singh and Prajapati 2020). The real time PCF based modal interferometer was mentioned for auditing the concentration of dissolved hydrogen in the transformer oil. The sensor was coated with Pd/WO 3 film over PCF by employing dip-coating technique. For the range 0-10 000 µl/l, the sensitivity acquired is 0.109 µl/l with response time below 33 min (Zhang et al. 2016). In 2019, PCF sensor combined with SPR was presented where thin film of gold was employed as the plasmonic along with flimsy graphene layer for detection of RI in liquid sample. The sensor operates for analyte RI range from 1.33 to 1.38. With the addition of graphene layer, the sensor improvises by 84.66% and reveals a maximum sensitivity of 8600 nm/RIU (Lou et al. 2019). Lately, a D-shaped PCF-SPR sensor with coating layers of gold/MoS 2 /graphene was reported. The sensor was applicable for biomedical and biochemical analyte detection for the analyte RI ranging from 1.33 to 1.40. The sensor exposes a maximum sensitivity of 14,933.34 nm/RIU (Singh and Prajapati 2020). The above reported papers along with additional layer covering exhibit high loss and less sensitivity. So, for the first time in this work, 2D material PtSe 2 is integrated through plasmonics over the outer surface of PCF to upsurge the sensitivity which reveals the novelty for this work along with which the designed tri-path sensor exposes a low loss.
We present a tri-path PCF sensor which is integrated with PtSe 2 through plasmonic gold to achieve high sensitivity for the designed sensor. The tri-path structure is introduced for the evanescent field to move freely through the cladding and interact with the metal region to create strong plasmonic coupling effect. This assists in reaching enhanced sensitivity for the designed sensor. External sensing is approached which removes the fabrication complexities for the sensor. Gold along with PtSe 2 is employed as it reveals excellent sensitivity and unique properties which make the sensor a potential future candidate in many fields. The next, Sect. 2 discusses about the design and its novelty. Further, in Sect. 3, the coupling characteristics and results will be mentioned. Finally, in Sect. 4, the optimizations done for the geometrical parameters will be elaborately discussed.

Structural design
The schematic view of designed tri-path PCF sensor is displayed in Fig. 1. There are two air hole rings present which are organized in hexagonal lattice. The center air hole along with three air holes from first ring are removed. Further, three air holes from second ring are also scaled down. This arrangement is done for light to travel in an effective path. This arrangement leads to the formation of tripath structure which paves way for the light to travel easily. The guided evanescent field from core move freely through the cladding and strikes the metal region, creating strong coupling effect. This boosts the sensing performance of the designed tri-path PCF sensor making the sensor to attain high sensitivity. The diameter of big sized air hole, d is 1.4 µm and the diameter of scaled down air hole is symbolized as d s which is 0.2 µm. Pitch is represented as Λ which is the distance from center of one air hole to another.
Silica is used as the background element for the designed sensor whose RI is dependent on wavelength and expressed using Sellmeier equation ; where refractive index of silica is denoted by n, λ i and N i are the appropriate constants, where λ 1 = 0.0684043, λ 2 = 0.1162414, λ 3 = 9.8961611 and = 0.6961663, N 2 = 0.4079426, N 3 = 0.8974794. The plasmonic material preferred for the designed sensor is gold whose thickness is denoted as t g = 40 nm. Drude Lorentz model is used to obtain the dielectric gold constant which is expressed as (Vial et al. 2005); (1) where the permittivity of gold is ε gold , angular frequency is ω = 6.283c/λ, ε ∞ = 5.9673 is high frequency permittivity, and Δε = 1.09 is the weighing factor. The damping frequency and plasma is denoted as γ D = 2π *15.92 THz and ω D = 2π *2113.6 THz respectively. Moreover, oscillator strength is Ω L = 2π*650.07 THz and the spectral width is Γ L = 2π*104.86 THz of the Lorentz oscillators. Gold is applied with the help of various methods as thermal evaporation (Barnes et al. 2000), wet chemistry deposition (Sioss and Keating 2005) and radio sputtering technique (Armelao et al. 2005). These methods result in extreme surface roughness which diminishes the usage of these coating methods. To solve the issue of rough uneven surface, another coating method known as chemical vapour deposition (CVD) (Sazio et al. 2006) is employed which offers minimum roughness along with even coating on the surface. Further, PtSe 2 was glazed over the plasmonic gold metal whose thickness is taken to be 4 nm. The RI for PtSe 2 is obtained from the cited paper (Guo et al. 2020) which involves both real and imaginary parts. Gold is applied at the outer region of PCF along with PtSe 2 and sensing analyte by approaching external sensing technique which removes the fabrication complexities. Perfectly Matched Layer (PML) along with scattering boundary conditions are added which protects from removing the radiant energy from fiber axis. Further, numerical study is done through Finite Element Method (FEM) which is associated with COMSOL Multiphysics software and results are discussed in upcoming sections.

Results and analysis
The proposed model sensing mechanism is based on the coupled mode theory ) which takes place amongst the evanescent field from the core mode that interacts with plasmonic mode at gold/Ptse 2 layer to produce robust coupling effect. Figure 2 illustrates the electric field distribution where Fig. 2a as well as (b) displays the field distribution for fundamental core mode and plasmonic mode. Figure 2c reveals the coupling effect for the analyte 1.36. The designed tri-path sensor functions for the sensing range from 1.33 to 1.38 where the shift in wavelength and variation in peak intensity is observed when the analyte RI is varied. Figure 3 illustrates the dispersion relation and loss spectrum of core mode and plasmonic mode. When the interaction takes place amidst the fundamental core and SPP mode, phase matching condition occurs at a specific wavelength which is termed as resonant wavelength. The confinement loss is calculated by (Xie et al. 2019); where λ indicates the input wavelength in microns, the effective RI of the imaginary part is signified by Im (n eff ) and k 0 = 2π/λ denotes free space number. Maximum loss peak is attained in this resonant wavelength and while shift occurs in this wavelength towards shorter or longer wavelength, the anonymous sample can be detected. By the collision within core and SPP mode for the designed tri-path sensor, the maximum peak reached is 14.10 dB/cm at the wavelength 0.84 µm for the analyte 1.36. Figure 4 depicts the loss spectrum for analyte RI varied from 1.33 to 1.38. The peak loss attained for the designed tri-path sensor with gold/PtSe 2 layer coating is 1.36 dB/cm, 2.14 dB/ cm, 2.76 dB/cm, 2.97 dB/cm, 4.55 dB/cm and 12.82 dB/cm at their corresponding resonance wavelength 0.72 µm, 0.74 µm, 0.79 µm, 0.85 µm, 0.96 µm and 1.38 µm respectively. This shows that the loss attained for the designed sensor is also very low making it more efficient and practical. Figure 5 portrays the polynomial fit characteristic analysis for analyte RI altered from 1.33 to 1.38 for the designed tri-path sensor with coating of gold/PtSe 2 . For this range, the degree relation amidst the resonant wavelength and analyte RI is estimated by R 2 . The R 2 value acquired for the designed tri-path sensor is 0.91438. High polynomial regression charecteristic is revealed which proves the quality of the designed tri-path sensor that can be employed for precise analyte RI identification. Wavelength ssensitivity is significant parameter that is employed to calculate the sensing performance for the designed tri-path sensor. The wavelength sensitivity is estimated by (Dhinakaran et al. 2021 where Δn a is change in RI of analyte and Δλ peak is the discrepancy in shift of wavelength peak. The wavelength sensitivity calculated and obtained for the designed tri-path sensor with gold/PtSe 2 coating is 2000 nm/RIU, 5000 nm/RIU, 6000 nm/RIU, 11,000 nm/RIU and 42,000 nm/RIU for its corresponding RI analyte 1.33, 1.34, 1.35, 1.36 and 1.37. Further, resolution is another important parameter that is needed to measure the sensing performance of the designed sensor. It is the efficiency to sense slight alterations in RI of analyte and is realized by Haider et al. (2019); where Δλ min is 0.1 nm which denotes the minimum spectral resolution which, Δλ peak denotes the difference in wavelength amongst the loss peak shift and Δn a is 0.01 which denotes the analyte RI alteration. From simulation and result analysis, we observe that the (5) R (RIU) = Δn a Δλ min Δλ peak Fig. 4 Loss spectrum for analyte RI varied from 1.33 to 1.38 with gold/PtSe 2 coating layer

Fig. 5
Polynomial fit curve analysis for analyte RI altered from 1.33 to 1.38 with gold/ PtSe 2 coating layer maximum sensitivity attained for designed tri-path sensor is 42,000 nm/RIU along with maximum wavelength resolution of 2.4 × 10 −6 RIU for the analyte 1.37. Table 1 gives the synopsis of sensing performance of designed tripath sensor for the RI of analyte altered from 1.33 to 1.38. Further optimizations and its analysis are done with coating of gold/PtSe 2 layer where the geometrical parameters are scrutinized and adjusted to attain high sensitivity for the designed tri-path sensor. The scrutiny of optimization and its results are discussed elaborately below. Primarily, the values of parameters were considered scaled down air hole diameter (d s ) as 0.2 µm, as diameter of big air hole is 1.4 µm, gold layer thickness of gold coating (t g ) as 40 nm and PtSe 2 thickness as 4 nm. These parameters are scrutinized and investigated to achieve maximum sensitivity for the designed tri-path sensor.
The scaled down air hole diameter is optimized as 0.2 µm, 0.3 µm and 0.4 µm for analyte RI 1.36 and 1.37 respectively. The effect of this alteration is illustrated in Fig. 6. The obtained loss peak for analyte 1.36 is 37.68 dB/cm, 14.10 dB/cm and 6.24 dB/cm and for analyte 1.37 it is attained as 50.47 dB/cm, 20.54 dB/cm and 9.27 dB/cm. It is evident from this investigation that the loss spectrum for analyte 1.36 transpires at 0.84 µm and for analyte 1.37 it occurs at 0.93 µm. It is noticed that the loss occurs at the same resonant So, the size of the scaled down air hole cannot be incremented above certain value as it will diminish the space for the light to pass through which will weaken the coupling effect and reduce the sensing performance of the designed sensor. Thus, we choose optimum value of 0.4 µm as the scaled down air hole diameter which is low peak loss for the designed tripath sensor. Next, the ratio amongst the diameter of big air hole and pitch (d/Λ), was modified and analyzed for analyte RI 1.36 and 1.37 as 0.8 µm, 0.85 µm and 0.9 µm. The examinations were done for the mentioned modifications where loss peak of 5.67 dB/cm, 4.97 dB/cm and 6.24 dB/cm was attained for the analyte RI 1.36 at wavelength of 0.76 µm, 0.79 µm and 0.84 µm. Similarly, for analyte 1.37, the loss peak attained is 8.22 dB/cm, 7.38 dB/cm and 9.27 dB/cm at wavelength of about 0.81 µm, 0.85 µm and 0.93 µm respectively. The modification in d/Λ is depicted in Fig. 7. From this scrutiny, we can observe that the wavelength swings towards longer wavelength which is termed as red shift. The shift takes place from 0.76 to 0.84 µm and 0.81 µm to 0.93 µm for analyte RI 1.36 and 1.37 respectively. The obtained wavelength sensitvity is 5000 nm/RIU for 0.8 µm, 6000 nm/RIU for 0.85 µm and 9000 nm/RIU for 0.9 µm. We can conclude from the results that the highest wavelength sensitivity is reached for 0.9 µm as 9000 nm/RIU which is decided to be the optimum value. Further modifications and elevations cannot be done for d/Λ as the designed tri-path sensor does not allow design flexibilty above this. So, from scrutiny, we opt 0.9 µm as the ratio amongst diameter of big air hole and pitch for further study.
The further study analysis is done for changes in thickness of gold layer t g . For analyte RI 1.36 and 1.37, the changes done in t g is 30 nm, 40 nm and 50 nm. The results are displayed in Fig. 8. It is noticed that the loss peak obtained is 10.91 dB/cm, 6.24 dB/ cm and 2.97 dB/cm for analyte RI 1.36 at 0.8 µm, 0.84 µm and 0.85 µm respectively. This shows that as thickness of gold rises, the loss decreases correspondingly. Further, for analyte 1.37, the loss peak reached is 16.81 dB/cm, 9.27 dB/cm and 4.55 dB/cm at 0.88 µm, 0.93 µm and 0.96 µm respectively. The shift is observed to move towards larger wavelength from 0.8 to 0.85 µm and from 0.88 to 0.96 µm for analyte RI 1.36 Fig. 7 Loss peak of ratio amongst diameter of big airhole and pitch, d p which is modified from 0.8 to 0.9 µm and 1.37 respectively. This exposes that red shift takes place and loss peak correspondingly diminishes when thickness of plasmonic gold covering layer is incremented. For the changes made in t g as 30 nm, 40 nm and 50 nm, the wavelength sensitivity gained is 8,000 nm/RIU, 9,000 nm/RIU and 11,000 nm/RIU. So it is revealed from the investigation that highest wavelength sensitivity is gained as 11,000 nm/RIU for 50 nm thickness which is preferred as the optimum value after optimization analysis and study. The thickness should be selected in right amount for efficient functioning of the sensor. If the thicknesss is too low, then the plasmonic effect will be very less which will not be applicable for coupling to transpire. Similarly, if the thickness is surged too high, then the plasmonic effect would get very robust which would weaken the coupling effect as the evanescent field decays while reaching the thick plasmonic coating and highly increment the loss. Thus, precise thickness should be opted for the designed sensor to make the sensor operate efficiently. Hence, we select 50 nm as thickness of gold coating for further analysis.
The final scrutinization is for the thickness of PtSe 2 material. The thickness of PtSe 2 is adjusted as 2 nm, 4 nm and 6 nm for analyte RI 1.36 and 1.37. The loss acquired is 2.66 dB/cm, 2.97 dB/cm and 3.27 dB/cm for the analyte RI 1.36 at 0.78 µm, 0.85 µm and 1 µm respectively. Similarly, at 0.82 µm, 0.96 µm and 1.06 µm the loss peak attained is 4.26 dB/cm, 4.55 dB/cm, 4.43 dB/cm for analyte 1.37 respectively. This is portrayed in the Fig. 9. It is noticed that the wavelength swings towards larger wavelength for both RI of analyte 1.36 and 1.37 from 0.78 to 1 µm and from 0.82 to 1.06 µm respectively which depicts the red shift occurance. Further, wavelength sensitivity has been estimated for the adjustmaents made with thickness of PtSe 2 material as 2 nm, 4 nm and 6 nm as 4000 nm/ RIU, 11,000 nm/RIU and 6000 nm/RIU. From this we can conclude that for 4 nm of PtSe 2 thickness, maximum sensitivity of 11,000 nm/RIU is attained. The thickness of 2D material, PtSe 2 is fixed to be 4 nm as it exposes the maximum sensitivity for the designed tripath sensor. The other optimizations of 2 nm and 6 nm reveal a lower sensitivity when compared to 4 nm. Hence, we decide 4 nm as thickness of PtSe 2 material. Finally, we have completed the optimization analysis and have selected the optimized parameters as d s = 0.4 µm, d/Λ = 0.9 µm, t g = 50 nm and Ptse 2 thickness = 4 nm with which the designed tri-path sensor reaches maximum sensitivity as 42,000 nm/RIU. Moreover, we have examined the sensing performance of the designed tri-path sensor by eliminating the PtSe 2 coating layer. Figure 10 portrays the loss spectrum for analyte RI varied from 1.33 to 1.38 for the designed tri-path sensor without PtSe 2 coating. At wavelength 0.63 µm, 0.64 µm, 0.66 µm, 0.69 µm, 0.74 µm and 0.84 µm the loss obtained for the designed sensor when PtSe 2 material is detached is 1.47 dB/cm, 1.79 dB/cm, 1.90 dB/cm, 3.07 dB/cm, 3.47 dB/cm and 5.78 dB/cm respectively. Figure 11 portrays the polynomial fitting analysis for analyte RI altered from 1.33 to 1.38 for the designed tri-path sensor when PtSe 2 material is removed and attained R 2 value is 0.98267. The wavelength sensitivity is 1000 nm/RIU, 2000 nm/RIU, 3000 nm/RIU, 4000 nm/RIU and 11,000 nm/RIU for analyte RI 1.33, 1.34, 1.35, 1.36 and 1.37 respectively. Through this simulation and study analysis, we can witness that the sensitivity attained for designed tri-path sensor with no PtSe 2 material is 11,000 nm/RIU with maximum wavelength resolution of 1 × 10 −5 for the analyte 1.37. Table 2 gives an overall sensing performance of designed tripath sensor when PtSe 2 material is removed for the RI analyte ranging from 1.33 to 1.38. While comparing both the performances of designed tri-path sensor with gold/PtSe 2 layer and when PtSe 2 coating layer is removed, we can conclude from the investigation done that the sensing performance is highly enhanced while 2D material PtSe 2 is added. While gold/PtSe 2 layer is glazed, the sensitivity reaches much higher sensitivity of 42,000 nm/RIU. Further, there is also a shift that takes place towards larger wavelength with gold/PtSe 2 coating layer. The entire wavelength spectrum for gold/PtSe 2 layer coating ranges from 0.72 to 1.38 µm which aids the sensor to operate in mid infrared frequency owing to its inter-band transition. The wavelength spectrum range when PtSe 2 coating layer is eliminated for the designed tri-path sensor occurs from 0.63 to 0.84 µm respectively. Table 3 is displayed below which gives a ephemeral comparitive study about previously done research works with various materials used for additional coating layer along with our designed tri-path sensor which is intergrated with PtSe 2 material through plasmonic gold. From this study analysis, we can conclude that the designed tri-path sensor reaches maximum sensitivity along with low loss which makes it to be anticipated as a budding applicant in various sensing domains.

Conclusion
We propose a tri-path PCF intergrated with SPR sensor along with gold/PtSe 2 coating layer. PtSe 2 2D material is added over gold coating owing to its fascinating optoelectronic properties which assist in elevating the sensitivity for the proposed tripath sensor. The sensor is simple in design where tri-path is formed to make the light pass through the cladding in the scaled down path and interact with the plasmonic surface which produces enhanced coupling effect. FEM is employed for analyzing the sensing performance and numerical simulation investigation for the deisgned tri-path sensor. Gold/PtSe 2 coating layer for the designed tri-path sensor reveals superior sensing performance when compared with removal of PtSe 2 coating layer. They are externally coated over the PCF which makes the sensor economical and reduces the difficulties faced by fabrication. The proposed sensor shows an excessive rise in wavelength sensitivity as 42,000 nm/RIU with maximum resolution of 2.4 × 10 −6 RIU in range of RI analyte 1.33 to 1.38. Further, when study was done where PtSe 2 layer coating was removed, the sensor attains sensitivity of 11,000 nm/RIU along with resolution of 1 × 10 −5 RIU. The simple designed tri-path sensor also achieves low loss which makes it highly applicable and potential candidate in various fields.