1. Introduction

Since the plasmon phenomenon was observed on the subwavelength metal grating by Wood in 1902, the development of plasmon has grown rapidly in the last years [1–3]. Surface plasmon resonances (SPR) are collective oscillations of excited electrons. Localized surface plasmon resonances (LSPR) are the collective oscillations of electrons at the interface of metal nano-structures irradiated by the excited light of specific frequencies. Because of the high SPR sensitivity to the refractive index of the surrounding media, SPR has been extensively utilized to sense the molecular binding events in the surrounding media [4,5] and has gradually become a great technique in biological sensing, food safety, medicine, and other fields [6–11].

Moreover, optical metasurfaces are able to be used as nano-structure biosensors because of the strong interaction between surface plasmon polaritons (SPPs) and incident light that leads to strong enhancement of the electric and magnetic fields [12]. Recently, biosensors have been successfully used to detect different biological analytes such as the diagnosis of cancers, glucose, viruses, proteins, etc. Although invasive and clinical surgeries have been developed in recent years. Plasmonic metasurface biosensors are more effective and functional.

Over the years, numerous metasurface sensor designs have been showcased. Brolo et al. presented a plasmonic metasurface sensor that utilizes arrays of nano-holes in gold films to detect the biomolecules. This sensor demonstrates a sensitivity of 400 nm/RIU [13]. A plasmonic array of three slot strips forming a complementary planar metamaterial was presented and demonstrated a sensitivity of 560 nm/RIU [14]. In fabricated Au nano-cube arrays, sensitivity can be up to 560 nm/RIU [15]. A hybrid metasurface-based perfect absorber consists of nano-disk arrays made of amorphous silicon (a-Si) on top of the gold mirror, is able to achieve a maximum sensitivity of 350 nm/RIU within the sensing range of 1.33 to 1.41 [16]. In addition, a high refractive index sensitivity of 454.4 nm/RIU is achieved for a plasmonic metasurface sensor with gold nano-bump arrays [17]. A sensor was introduced, employing dielectric Huygens source metasurfaces to detect alterations in refractive index in a microfluidic channel. The sensitivity of this platform was experimentally determined to be 323 nm/RIU, with the figure of merit (FOM) of 5.4 [18]. Using the 3D FDTD numerical method, it was shown that the metasurface-based biosensors consisting of silver-metasurface with SiO₂ can reach a maximum sensitivity of 658 nm/RIU [19]. To achieve high sensitivity and multi-mode sensing characteristics based on the plasmon effect, a sensor structure consisting of a periodic asymmetric ring cavity array based on theoretical analysis was explored. The reflection spectrum of this structure shows six resonance modes in the wavelength range from visible to near-infrared, the second mode provides a maximum sensitivity of 440.2 nm/RIU [20]. By numerically investigating, a nano-scale sensor that comprises a dielectric-metal-dielectric waveguide, and plasmonic metasurface resonators was proposed a sensitivity of 497.8 nm/RIU for a slight change of Δn = 0.24 [21]. An ultra-subwavelength thin film sensor with symmetric and asymmetric planar split ring resonators was demonstrated to sense thin film of analyte layers in the terahertz range [22]. Yildirim et al. proposed a highly sensitive refractive-index sensor, utilizing the excitation of guided modes of a novel two-dimensional periodically modulated dielectric grating-waveguide structure and providing a maximum numerical sensitivity of 110 nm/RIU and an experimental sensitivity of 235.2 nm/RIU [23]. A high-performance graphene-dielectric metasurface sensor consisting of asymmetric dielectric disk arrays was analyzed using the finite element method and a sensitivity of as high as 550 nm/RIU was obtained [24]. An all-dielectric metasurface-based refractive index sensor is proposed in the near-infrared wavelength regime. The sensitivity of this sensor is 192.50 nm/RIU and 122.50 nm/RIU for first and second resonances, respectively. The FOM values are 2.00 and 7.42, respectively [25].

In the present study, we report on a plasmonic metasurface consisting of a periodic pattern of nano-triangle arrays. In the first Section, the theory and important parameters of the sensors and the design and structure parameters are stated. Then, the characteristics of the materials used in the structure and the numerical method used are explained. In the next section, we study the sensor's performance in different ways and the effect of various parameters on the structure. The numerical results show two resonances with high performance.

2. Theory and structure design

The interaction between the surface charges and the electromagnetic field creating the SP has two results. First, the field perpendicular to the surface decreases exponentially as it moves away from the surface, in contrast to the propagating of SPs along the surface. This perpendicular field is referred to as evanescent or near field and could be a result of the bound, nonradiative nature of SPs, which prevents power from propagating away from the surface.

The second result is that the momentum of the SP mode (ħk_SP), is greater than that of a free-space photon with the same frequency (ħk₀ where k₀=ω/c is the free-space wavevector). This momentum increase is related to the binding of the SP to the surface. By solving Maxwell’s equations, the SP dispersion relation, which is, the frequency-dependent SP wave-vector (k_SP), can be derived as Eq. (1).

Here, $\varepsilon$_m and $\varepsilon$_d are the permittivity of metal and dielectric material. In order to utilize light for the generation of SPs, the discrepancy in momentum between light and SPs needs to be overcome. There are three primary approaches to provide the missing momentum. The initial method utilizes prism coupling to amplify the momentum of the incoming light [26,27]. The second approach involves utilizing a periodic groove on the metal's surface [28]. The third technique involves scattering from a surface defect, such as a subwavelength protrusion or hole, which serves as a convenient means for locally generating SPs [29,30].

Compared to dielectric metasurfaces [31], plasmonic metasurfaces usually have a lower quality factor (Q) of resonance, but their sensitivity is usually much higher. This is because of their ability to confine the near-field, typically within the size range of various biomolecules [32]. Coupling based on special metasurface metal nano-structures has several significant advantages compared to prism coupling: improved sensing linearity, wider detection range, greater customization possibilities, and also easily detectable by a flexible and small device. Consequently, it is predicted that biomolecular sensors that rely on metasurfaces will have more potential in the clinical diagnosis of diverse diseases.

A common mechanism for the detection of biomedical analytes is the light-matter interaction through the SPR evanescent field [33]. SPR and LSPR-based metasurface sensors primarily rely on analyzing various plasmonic parameters such as wavelength, frequency, angle, phase, etc [34–37]. The performance of metasurface-based sensors in relation to both SPR and LSPR is usually expressed by two key characteristics: bulk refractive index sensitivity (S) and figure of merit (FOM). S is a significant parameter that is used to evaluate the sensing capability of the optical sensors and is determined by changes in the refractive index of the medium, which are caused by the molecular interaction on the surface of the metasurfaces. Therefore, the relationship between S and n can be described using Eq. (2) [34]:

Here, n is the refractive index and A represents the physical properties that are measured such as wavelength, angle, spectral intensity, etc. Alterations in the refractive index (n) of the analyte generally result in variations in the wavelength, angle, intensity of the spectrum, etc. Depending on structural design and material selection, the sensitivity of plasmon sensors can change.

The Q factor, also known as the quality factor, is a dimensionless parameter that characterizes the narrowness of a spectrum and is determined by dividing the resonance wavelength (λ₀) by the full width at half-maximum (FWHM) as Eq. (3).

Another significant parameter used to determine the sensor's ability to respond to small refractive index changes is the FOM. In the case of plasmonic plane metal structures, FOM is usually not large because of inherent losses. However, for metastructures, the use of diverse materials and structures can help minimize these losses. The relationship between FOM and S can be described as Eq. (4) [38]:

The schematic diagram of the designed plasmonic metasurface is shown in Fig. 1. The structure consists of a periodic lattice of nano-triangles on a glass film and a TiN mirror. As it was mentioned, plasmonic structures can effectively confine and improve the electrical field, and on the other hand, metasurfaces can overcome the challenges presented in bulk metamaterials while providing very useful functionalities and excellent label-free sensing devices, we have chosen a plasmonic metasurface structure. Furthermore, periodic structures have been preferred due to reproducible results. Gold nanostructures have unprecedented high plasmonic sensitivity, making them useful for sensing applications.

 

Fig. 1. The unit cell of the designed metasurface.

Download Full Size | PDF

Perfectly matched layer (PML) boundary conditions have been applied in the z-direction, while periodic boundary conditions have been used along the horizontal directions. PMLs serve as highly efficient ABCs (absorbing boundary conditions) and have the capability to absorb waves, even when located at corners. PML boundaries are created as an absorbing material that is impedance matched to the surrounding materials, which helps to minimize reflections. While an ideal PML boundary would have no reflections, in practice, there will be slight reflections due to the discretization of the underlying PML equations. By optimizing PML parameters, such as the thickness and conductivity of the absorbing material, the reflection errors can be decreased, leading to an enhancement in the absorption of incident light. The incident electromagnetic field is assumed to be a plane wave with electrical fields polarized along the x-axis. It radiates from the positive z direction. The periods of the unit cells are equal in both x and y directions (P_x= P_y= 550 nm). The side length of nano-triangles, the distance from the center to the inner and outer vertices, the thickness of triangles, SiO₂ spacer, and TiN mirror are L, r₁, r₂, t_Au, t_S, and t_M, respectively. The geometrical parameters of the designed metasurface sensor are shown in Table 1.

Table 1. Quantities of parameters in the proposed structure

View Table | View all tables in this article

3. Materials and methods

Planar metamaterials or metasurfaces, which have a subwavelength thickness, consist of artificial composite materials containing subwavelength metal/dielectric structures that are resonantly coupled to the electric/magnetic components of incident electromagnetic fields. These plasmonic metasurfaces are typically made using advanced metals that possess properties such as chemical inertness and bio-functional feasibility.

The resonators are made of gold. Extensive research has been conducted on noble metal nano-materials like gold, silver, and platinum across various scientific fields due to their distinct characteristics, such as excellent conductivity, plasmonic properties, and biocompatibility. These unique properties have made them a popular choice for the development of biosensors. Gold and silver, are widely used in plasmonic sensors due to their unique properties. Compared to silver nano-rods, gold nano-rods have a lower Raman signal enhancement, but they have higher stability and biocompatibility, making them more suitable for biosensing applications [39]. Gold nano-structures have an exceptional ability to absorb and scatter light. This remarkable interaction between gold and light is due to the collective oscillation of the conduction electrons on the surface of the metal when they are excited by light at specific wavelengths. This oscillation, referred to as surface plasmon resonance (SPR), leads to significantly higher absorption and scattering levels in gold nano-structures compared to non-plasmonic nano-structures of similar dimensions. Gold nano-structures are easily functionalized and can be used to develop various biosensors for molecular diagnosis. The absorption and scattering characteristics of gold nano-structures can be adjusted by controlling variables such as their size, shape, and the refractive index in their vicinity. The relevant material properties, including the permittivity √ɛ=n + ik (where n + ik represents the complex refractive index), were obtained from Palik's Handbook of optical constants [40].

A titanium nitride layer is added to the structure as a mirror to reflect transmitted light to the resonators [40]. Metal-nitrides exhibit great potential in plasmonic materials or metasurface devices. Titanium nitride (TiN) has been extensively studied and considered one of the most investigated compounds thus far. TiN has intrinsic physical, chemical, and optical properties such as low resistivity, high reflectance in the infrared spectral range, good corrosion resistance, chemical inertness, thermal stability, and high hardness making it the first-choice material in many applications. A wide variety of deposition methods that rely on vacuum technology are utilized to acquire a thin layer of TiN, including chemical vapor deposition (CVD), molecular-beam epitaxy, reactive magnetron sputtering, atomic layer deposition (ALD), or pulsed laser deposition (PLD), under a nitrogen or ammonia atmosphere.

Results have been obtained by using the discontinuous Galerkin time-domain (DGTD) method. In general, the DGTD algorithm is used to solve the Maxwell equations for isotropic, dispersive but non-magnetic materials. In the last few decades, numerous numerical techniques have been presented for resolving Maxwell's equations. The Finite-Difference Time-Domain (FDTD) and Finite Element Methods (FEM) have gained considerable popularity among them. The FDTD has emerged as a principal approach in computational electrodynamics and is a fast and simple method to solve the time-domain formulation of Maxwell’s equations though the simplicity comes only at the cost of second-order accuracy. Moreover, it is defined for an inflexible orthogonal spatial discretization. On the other hand, FEM presents a flexible discretization to solve small geometrical features or curved shapes. In addition, using higher order basis functions can make better the accuracy order. In 2002, Hesthaven and Warburton demonstrated the DGTD method to electrodynamics [41,42]. The suggested formulation relies on using an upwind numerical flux, nodal basis expansions on a triangle in two dimensions (2D), nodal basis expansions on a tetrahedron in three dimensions (3D), and a Runge-Kutta time stepping scheme. Basically, DGTD can be a type of FEM with a main difference, the basis functions are only defined on a single element without any overlap with the neighboring elements.

We use the DGTD method to study the proposed structure because it contains small geometrical features and sharp edges. The DGTD method allows the use of unstructured meshes, higher-order basis functions, and higher-order time integration methods, resulting in higher-order accuracy in geometric, spatial, and temporal discreteness. FDTD requires a fine mesh around the entire rectangular volume to cover the metal to mitigate staircasing effects. However, DGTD is able to only refine the mesh around the metal feature. It makes DGTD, a much more efficient method for non-axis aligned geometries. Designs with complex curves, sharp geometries or metal interfaces require very fine meshes to achieve high accuracy in order to reduce staircasing effects. Plasmonic metamaterial or metasurface structures, which are often not flat and non-axis aligned, use the scaling and tuning of the unstructured mesh used by DGTD when high accuracy is required.

Several techniques have been reported for the fabrication of metasurfaces, including electron beam lithography (EBL) [43], focused-ion beam (FIB) [44], interference lithography (IL) [45], and nanoimprint lithography (NIL) [46] EBL offers high nanoscale resolution when producing metasurfaces. FIB is a suitable method for rapid prototyping. IL can be used to fabricate large-scale metasurfaces. NIL is a more promising technique for combining many advantages, such as high-resolution, large-scaled production and low processing cost.

4. Results and discussion

First, spectral characteristics of the structure have been obtained. Figure 2 shows the proposed sensor's reflection, absorption, and transmission characteristics under normal incident light illumination. The TiN film in the sensor structure prevents light transmission from the resonators which helps to excite the surface plasmon by scattering light through it at the normal incident. It makes the transmission spectrum near zero and deeper reflection dip. The general equation to calculate the absorption is Absorption (A) = 1−Transmission (T)−Reflection (R). Due to the TiN layer, the transmission of the proposed structure will be negligible, and as a result, the absorption directly depends on the reflection. Hence, the minimal reflectance leads to maximum absorption.

 

Fig. 2. Spectral response of the sensor and monitored electrical field distribution.

Download Full Size | PDF

In this study, the structure's potential for high sensitivity in reflection configuration has been demonstrated. Two distinct resonance points are evidently observed at λ₁= 708 nm and λ₂= 1108 nm. At these resonances, the reflectance decreases to 44.38% and 0.54%, and correspondingly absorbance increases to the values of 55.61% and 99.45% respectively. Moreover, the full width at half maximum (FWHM) and Q-factor of the proposed structure have been also calculated according to Eq. (3). At those two resonance positions mentioned above (at 708 nm and 1108 nm), the value of FWHM is about 79 nm and 54 nm, and the corresponding Q-factor is about 9 and 20, respectively. Note that the metasurface is polarization-insensitive because it exhibits similar optical characteristics at both transverse electric (TE) and transverse magnetic (TM) modes due to the symmetry of the structure.

To verify the physics mechanism behind the observed phenomenon, the spatial distributions of electric and magnetic fields at those two reflection dips (λ₁= 708 nm and λ₂= 1108 nm) have been investigated, as shown in Fig. 3. Obviously, the spatial distribution patterns of the strong electric and magnetic fields are significantly different at various resonance frequencies, revealing the excitations of different SPR modes. However, it is evident that both electric and magnetic fields are always strongly concentrated in the interface of the gold nano-structures and surrounding medium when the resonance occurs. The second resonance (λ₂= 1108 nm) shows a more confined field, so the resulting spectrum is sharper and offers a higher quality factor. It is worth noting that magnetic field distribution is stronger beneath and on top of the nano-triangles, while the electric field is stronger at the edges of the nano-structures. In the proposed device, the suppression of the reflectance occurs due to the matching of the electric dipole and magnetic dipoles, which is also known as impedance matching. Furthermore. structures with different numbers of nano-triangles were investigated (Fig. 4). The results show that with the increase in the number of nano-triangles, the dips become clearer, and the structure with eight triangles shows the clearest dips.

 

Fig. 3. The distributions of electric and magnetic fields at reflection dips.

Download Full Size | PDF

 

Fig. 4. The reflection of various structures with different numbers of nano-triangles

Download Full Size | PDF

Using sharp edges or tips enables the attainment of an extremely high field intensity. The concept of enhancing the localized and confined electromagnetic field, known as “hot spot,” is gaining significant interest in the development of bio-molecule detection sensors. The creation of hot spots at nano-scale sharp tips allows for molecular sensing with high sensitivity. As the number of tips increases (by increasing the number of nano-triangles), the hot spots increase, the resonances become stronger, and the sensor sensitivity improves. In Table 2, the strength of absorption and suppression of reflection for the strongest resonance of each structure is stated.

Table 2. The strength of absorption and suppression of reflection for different numbers of nano-triangles

View Table | View all tables in this article

These resonances, and the corresponding reflectance dip, are highly sensitive to changes in the refractive index of the surrounding medium. In the next stage, reflection spectral responses for different refractive indices between 1 and 2 have been obtained. The reflection spectral responses for both resonances (λ₁= 708 nm and λ₂= 1108 nm) are shown in Fig. 5. Moreover, the resonance wavelengths for various refractive indices can be seen in Fig. 6. It is noticeable that for both resonances, the resonance wavelength grows almost linearly by increasing the refractive index of the medium. As can be seen in the figure, the line corresponding to the second dip has a greater slope and thus provides greater sensitivity. The first resonance provides a maximum sensitivity of about 417 nm/RIU and the second one provides a maximum sensitivity of about 735 nm/RIU. In addition, the FOM values for two resonances are obtained over 5 and 13, respectively. The results show this sensor can be used in a wide range of refractive indices and various applications such as biosensing.

 

Fig. 5. The reflection for different refractive indices (a) first and (b) second resonance.

Download Full Size | PDF

 

Fig. 6. The resonance wavelengths for different refractive indices.

Download Full Size | PDF

This broad spectrum of applications involves the use of different materials and bioparticles like the Cytosol (1.360–1.390), Nucleus (1.355–1.365), Nucleolus (1.375–1.385), Mitochondria (1.400–1.420), Lysosome (1.600) [47], and MS2 virus (1.5) [48]. Biosensors have been utilized in a wide range of applications, including medical diagnosis, environmental monitoring, pharmaceutical analysis, and food quality testing. Viruses pose a significant threat to public health and the global economy among various infectious agents due to their elusive nature and difficulty in treatment. Viruses present a major worldwide health concern. As an illustration, consider the case of Polio which is a tiny enterovirus measuring about 25-30 nm in diameter with a refractive index range of n = 1.58 ± 0.04 [49]. We have investigated the response of the sensor for poliovirus. In the case of this design, two resonances present a sensitivity of 362.43 nm/RIU and 704.15 nm/RIU, respectively. The reflection spectra have been obtained for different refractive indices around 1.58 (Fig. 7).

 

Fig. 7. The reflection spectra for refractive indices around 1.58.

Download Full Size | PDF

Since surface plasmon mode excitation depends on the incident angle, we calculated the reflectance of the proposed structure for different angles of incidence (Fig. 8). As the incident angle increases, a slight redshift is observed in the reflection dip2, and the depth of both dips decreases, which is more obvious for dip1. At smaller angles, especially angles less than 15°, the dip2 almost does not change, which can be seen in Fig. 8(b). Usually, at wavelengths which has a strong resonance, a lower angular dependence is seen and all of the reflection curves are superimposed. If the resonance is reduced, then the angle-independent behavior is lost. One reason for reducing this resonance is loss. The effects of losses appear as an angular dependence. Examining the plots, it can be seen that the angle dependence is more significant for larger angles and becomes less significant as the incident angle becomes smaller.

 

Fig. 8. The reflectance spectrum for different incident angles.

Download Full Size | PDF

Gold was used to make nano-triangles. Plasmonic sensors often rely on gold and silver because of their unique properties. The extraordinary ability of gold nano-structures to absorb and scatter light is a result of surface plasmon resonance. This phenomenon leads to significantly higher absorption and scattering levels in gold nano-structures compared to non-plasmonic nano-structures of similar sizes. Gold nano-structures are easily functionalized and can be used to develop various biosensors for molecular diagnosis. Moreover, Gold nano-structures offer greater stability and biocompatibility, making them more suitable for biosensing applications. The spectral response of the sensor for Au, Ag, and TiN nanostructures was investigated in Fig. 9. TiN was found to lack the necessary resonance strength for sensing purposes. Consequently, the reflection spectrum exhibited almost uniformity and the absence of near-zero reflection resonance, likely due to TiN's high reflectivity. By examining the structure with silver nanotriangles, it can be seen that both resonances in the structure with gold nanostructures are also seen in this structure. However, the resonances in this structure are weaker and exhibit a blue shift in resonance wavelengths.

 

Fig. 9. The spectral response for Au, Ag, and TiN nanostructures.

Download Full Size | PDF

As can be seen in Fig. 10, the resonance strength is also related to the thickness of the gold layer. By reducing the thickness of the gold nano-structures, the resonance becomes deeper, although the change in resonance strength is not significant for slight modifications in thickness, particularly for stronger resonance (λ₂= 1108 nm). Additionally, it is crucial to consider that very small thicknesses may impose limitations on the fabrication process.

 

Fig. 10. The spectral response for different thicknesses of the gold nanostructures.

Download Full Size | PDF

In addition, the complementary configuration has been studied (Fig. 11). It is formed of an array of nano-triangle holes in a gold layer. In the complementary structure, three resonances are seen which are relatively weak and close to each other, so they are not very suitable for sensing applications. Generally, metal structures can enhance reflection in various applications. Moreover, the use of metal structures, such as plasmonic metamaterials, can lead to enhanced light transmission and absorption, while minimizing reflection at specific resonance wavelengths, offering opportunities for various optical applications. The rise in reflection seen here may be attributed to the increased metal structure found in the sensor and the lack of proper tuning of the structural parameters in the complementary structure to create strong resonance conditions.

 

Fig. 11. The reflection profile of the complementary and proposed structure.

Download Full Size | PDF

The outcomes of our study suggest the promising potential of the proposed sensor for various applications. In comparison to the referenced results provided in Table 3, the performance of our structure has been significantly improved. The results show this work presents a higher FOM than most of the mentioned structures and a much higher sensitivity than all the mentioned structures. Furthermore, the absorption capabilities of the structure are comparable to those of a hybrid metasurface-based perfect absorber with amorphous silicon nano-disk arrays which achieved maximum absorption of 99.8% at a wavelength of 932 nm [16] and it is also comparable to many other absorbers [10,50].

Table 3. Comparison of the sensing parameters of the present and previous works

View Table | View all tables in this article

Plasmonic metasurface sensors have promising future prospects in various fields, including medical diagnosis and lab-on-chip applications. They offer several advantages such as high sensitivity, specificity, and low detection limits. The proposed sensor can detect minute changes in the surrounding environment by measuring variations in the reflection spectrum. Therefore, they are suitable for diverse applications particularly in biomedical sensing. Unlike traditional sensors, Plasmonic metasurface sensors can be fabricated on a chip, allowing for miniaturization and cost-effective mass production. This makes them ideal for portable and point-of-care devices, where real-time and on-site measurements are required. Furthermore, plasmonic metasurface sensors offer tunability and versatility by adjusting the design parameters of the nanostructures. However, plasmonic metasurface sensors also face challenges. Fabrication of Plasmonic metasurface sensors requires advanced nanofabrication facilities, which can make the fabrication process more complicated and expensive. In addition, they are often susceptible to a phenomenon known as surface contamination. Contaminants, such as dust or biological molecules, can adhere to the nanostructures and affect their performance. Discovering strong and dependable techniques to address and overcome these challenges will be of utmost importance.

5. Conclusion

In this study, a subwavelength plasmonic metasurface to achieve high sensitivity was suggested. As previously stated, plasmonic metasurfaces have the capability to effectively confine and enhance the electric field and offer valuable functionalities and outstanding label-free sensing devices. The proposed structure comprises a regular pattern of nano-triangle arrays on a glass layer and a TiN mirror. The structure has been proven to have the capacity for highly sensitive sensing in a reflection setup. In this structure, a TiN layer is utilized to obstruct the transmission of light from the resonators. Consequently, the transmission spectrum is significantly reduced, resulting in a deeper dip in reflection. It presented two resonances with a maximum sensitivity of approximately 429 nm/RIU and 754 nm/RIU, respectively. This sensor is not affected by polarization (TE or TM) thanks to the symmetrical structure. Additionally, it has been shown that by adding nano-triangles, the hot spots increase, resulting in an improved response. Furthermore, the investigation of the response of the structure for different angles showed that for small angles, particularly during the second resonance, the spectral response of the structure remains nearly unaffected. Moreover, by investigating the response of the structure for different refractive indices, we showed that this sensor has the potential to be used in various fields, including biosensing. Two resonances present a sensitivity of 362.43 nm/RIU and 704.15 nm/RIU, respectively for poliovirus. Finally, the performance of the structure was investigated with the structures with Ag and TiN nanotriangles and the complementary structure.