In our study, we aimed to comprehensively evaluate a performance of a thin film based on surface plasmon resonance (SPR) sensing. We focused on key parameters such as sensitivity, full width at half maximum (FWHM), and figure of merit (FOM). The FOM, in particular, plays a crucial role in assessing the sensor's quality, as a higher FOM indicates better performance of the film.
Figure 3 illustrates the correlation between incident light absorption and the incidence angle on the metal surfaces for a wavelength of 500 nm. By analyzing the absorption data obtained from the simulations, we were able to determine the optimal metal thickness and refractive index change for achieving the highest sensitivity and FOM, thereby ensuring superior chip performance.
In Fig. 3, we observed that when the incident light beam loses all its energy, it is completely absorbed by the metal layers, resulting in the excitation of surface plasmons (SPs). The absorption curve depicted in the figure exhibits a gradual increase from low values until reaching a maximum at the resonance angle. Shifting the refractive index by ∆n = 0.03 or 0.06 causes a change in the resonant angle (θSPR). This indicates that alterations in the layer structure lead to a different angle of absorption. For gold (Au), the optimal thickness for achieving maximum absorption is found to be d = 40 nm, resulting in absorption values of A = 0.988, 0.987, and 0.989 (a.u.) for ∆n = 0.00, 0.03, and 0.06, respectively. The waves reflected from surfaces of the film interfere, leading to an enhanced electric field within the film. This increased electric field can lead to greater absorption of light in the case of silver (Ag), the best absorption occurs in layers with a thickness of d = 40 nm, resulting in absorption values of A = 0.9935 and 0.9933 (a.u.) for ∆n = 0.00 and 0.03, respectively. However, at ∆n = 0.06, the optimal absorption occurs at a thickness of d = 50 nm, with a value of A = 0.933 (a.u.). As for copper (Cu), the highest absorption occurs at a thickness of d = 30 nm, yielding absorption values of A = 0.999, 0.998, and 0.999 (a.u.) for ∆n = 0.00, 0.03, and 0.06, respectively.
Based on the observations from Fig. 4, the optimal absorption for gold (Au) occurs at a thickness of d = 40 nm with an absorption value of A = 0.977 (a.u.) when ∆n = 0.00. For silver (Ag), the best absorption is achieved at a thickness of d = 50 nm with an absorption value of A = 0.961 (a.u.) when ∆n = 0.00. On the other hand, copper (Cu) exhibits the highest absorption at a thickness of d = 40 nm with an absorption value of A = 0.985 (a.u.) when ∆n = 0.06. Thus, copper demonstrates the best absorption among the three metals. The optimal absorption values and thicknesses for these metals are a result of the interaction between incident light and the plasmonic oscillations of free electrons within the metal films. The specific values depend on the material properties and the surrounding medium's refractive index, which affect the plasmon resonance conditions. These findings have implications in various applications, including plasmonic devices, sensors, and optoelectronics.
Another important parameter is a full-width at half maximum (FWHM). At a wavelength of 500 nm, gold has an FWHM of 3.8 nm with a thickness of d = 40 nm and ∆n = 0.00. This indicates that the plasmonic resonance in gold is not as sharp or narrow. The broader FWHM suggests that there is a wider range of wavelengths around 500 nm over which the plasmon resonance of gold is significant. This broader resonance could be due to the material's properties and the chosen thickness and refractive index conditions. Silver, on the other hand, has an FWHM of 0.2 nm with a thickness of d = 50 nm and ∆n = 0.00. Silver exhibits an extremely narrow FWHM of 0.2 nm at 500 nm. This indicates an exceptionally sharp and focused plasmon resonance in the vicinity of 500 nm. The narrow FWHM suggests that silver's plasmonic response is highly specific to the chosen wavelength and the provided conditions. Silver's unique properties result in a very confined spectral range where the plasmon resonance is prominent.
Copper exhibits an FWHM of 19.6 nm with a thickness of d = 30 nm and ∆n = 0.06 this indicates a wider spectral range over which copper's plasmonic resonance is effective. The combination of a dielectric layer (as indicated by the refractive index difference) and the chosen thickness contributes to the broader resonance observed in copper.
When the wavelength is 600 nm, the gold layer shows an FWHM of 5.6 nm at d = 40 nm and ∆n = 0.00. Silver demonstrates an FWHM of 0.2 nm with a thickness of d = 50 nm and ∆n = 0.00. Copper, with ∆n = 0.06, exhibits an FWHM of 17.5 nm. The FWHM measurements offer understanding regarding the range of wavelengths in which plasmonic resonance takes place for each metal at a 600 nm wavelength. These FWHM values are shaped by the properties of the materials, their thicknesses, and the differences in refractive indices. Together, these factors define the interaction between plasmonic resonance and incoming light. The differences in FWHM values signify distinctions in the resonance behaviors among gold, silver, and copper in the given settings.
Table 1presents the sensitivity of metal thin films (Au, Ag, and Cu) at wavelengths 500 nm and 600 nm, providing further details on their performance.
Table 1
sensitivity of (Au, Ag, Cu) metals layer at wavelengths (500,600) nm.
Metals | θ∆ | n∆ | S RIU− 1 | nmλ |
Au | 3 | 0.03 | 100 | 500 |
Ag | 2.66 | 0.03 | 88.66 |
Cu | 3.05 | 0.03 | 101.6 |
Au | 2.95 | 0.03 | 98.33 | 600 |
Ag | 2.7 | 0.03 | 90 |
Cu | 3.05 | 0.03 | 101.6 |
Table 1illustrates the top sensitivity (S) readings acquired from copper (Cu), registering at 101.6 RIU− 1 for both λ = 500 nm and 600 nm. Gold (Au) achieves the second-highest sensitivity value at λ = 500 nm, measuring S = 100 RIU− 1. These sensitivity findings align with the outcomes observed by N. Miguel et al, who utilized gold and silver, along with additional coatings, to enhance sensor sensitivity [17]. Furthermore, Farah Jawad Kadhum and associates employed gold-PVA as a biosensor [18]. Sensitivity holds significant importance in assessing sensor performance, serving as an indicator of the change in resonance angle for every unit change in refractive index.
To further assess the sensor performance, the figure of merit (FOM) can be calculated based on the sensitivity and full-width at half maximum (FWHM) values of the metal layers. Table 2displays the FOM values for gold (Au), silver (Ag), and copper (Cu) at wavelengths 500 nm and 600 nm.
Table 2
FOM for metal layers (Au, Ag, Cu) for λ = 500 and 600 nm
Thin film | S | FWHM | FOM | nmλ |
Au | 100 | 3.8 | 26.31 | 500 |
Ag | 88.6 | 0.2 | 443 |
Cu | 101.6 | 19.6 | 5.18 |
Au | 98.33 | 5.6 | 17.55 | 600 |
Ag | 90 | 0.2 | 451 |
Cu | 101.6 | 17.5 | 5.77 |
From Table 2, we can observe the optimal figures of merit (FOM) for silver (Ag) with values of 443 at a wavelength of λ = 500 nm and 451 at λ = 600 nm. In the case of gold (Au), the FOM is 26.3 at λ = 500 nm and 17.55 at λ = 600 nm, while for copper (Cu), the FOM is 5.18 at λ = 500 nm and 5.77 at λ = 600 nm. These results indicate that silver exhibits superior sensor performance compared to gold and copper. Silver's superior sensor performance can be attributed to its enhanced sensitivity, narrow resonance peaks, strong field enhancement, versatility, and applications in various fields. These properties make silver a preferred choice for plasmonic-based sensors that require high sensitivity, selectivity, and accuracy in detecting molecular interactions and changes in the surrounding environment.