Figure 1(a) shows the schematic of the proposed plasmonic device using a conventional Kretschmann configuration, wherein CaF2 (n = 1.426) glass prism is considered as the substrate and Al is considered as the plasmonic metal. Due to better rejection of errors during mechanical setup as well as a higher threshold for laser damage, the CaF2 prism is used as the material substrate for angle interrogation. Over a 25nm thick Al film (tm1), BTO (tBTO = 10nm) is deposited as a dielectric material to increase the sensitivity. Thereafter, the second film of Al (tm2 = 10nm) is deposited again to get the MDM configuration, thus intensifying field confinement. MoS2 is used as a biorecognition element (BRE) layer for the analyte under test. The overall fabrication procedure for the proposed device is easy to achieve. With techniques such as e-beam evaporation/thermal evaporation, Al can be deposited on the CaF2 prism. In between two metal layers of Al, BTO can be deposited using methods like MOCVD or sol-gel method, etc. There are detailed experimental reports for the deposition of BTO over Al films for the application of thin-film capacitors (Ramesh et al. 2003). Above the second Al metal layer, monolayer MoS2 can be deposited using techniques such as chemical vapor deposition.
Drude’s model is used to calculate the dielectric constant for Al using Eq. (1)
\({\varepsilon }_{m}\) (λ) = 1-\(\left(\frac{{\lambda }^{2}{\lambda }_{c}}{{\lambda }_{p}^{2} \left({\lambda }_{c+}j\lambda \right)}\right)\) (1)
where λ is the wavelength in µm, \({\lambda }_{p}\) = 1.0657\({e}^{-7}\)m and \({\lambda }_{c}\)= 2.4511 \({e}^{-5}\)m denote the plasma wavelength and collision wavelength respectively (Arora and Shukla 2020). The refractive index for BTO is calculated from the experimental values reported by Cardona (1965). The refractive index for the MoS2 monolayer (0.65nm) is extracted from Ref. (Shukla and Arora 2021). The analyte ‘na’ is considered as water over which the change in SPR angle is observed. When a TM-polarized light strikes the metal surface, thereafter, under momentum matching conditions, maximum absorption of incident light is accounted for with points of reflection minima. The reflection characteristics for the proposed device calculated using TMM Method are shown in Fig. 1 (b) where the dip in the reflectivity curve is due to the SP excitation at the interface. A redshift in the SPR curve is noticed with the increase in the analyte’s refractive index from 1.33 to 1.34. To quantify the performance parameters of the proposed device, Full Width at Half Maximum (FWHM) and Sensitivity (Smax) are calculated which corresponds to the linewidth of the SPR curves and shift in SPR angle w.r.t the change in the refractive index of the analyte (na), respectively. Finally, considering both the parameters, FOM, i.e. ratio of Smax and FWHM is found to be 540.9 RIU−1.
To get the optimized parameter for all the intermediate layers of the proposed device, several simulations are carried out using the TMM method considering low FWHM of SPR curves, and high signal contrast (low value of Rmin) as decisive parameters. Firstly, a conventional Kretschmann configuration with thin Al metal (tm1) coated with an analyte is considered as shown in the inset of Fig. 2(a), and the thickness of Al film (tm1) is varied from 15nm to 35nm in steps of 10nm. The calculated reflection characteristics are shown in Fig. 2(a). It is observed that the reflectivity dip corresponding to 25 nm strives for efficient SP excitation due to high signal contrast (low value of Rmin) whereas the value of tm1 = 15 nm and 35 nm leads to poor signal contrast. Figure 2(b), shows the electric field distribution for tm1 = 25 nm, where the excitation of SP mode can be observed with stronger field enhancement at the interface as compared to tm1 = 15 nm and 35 nm (not shown here).
Next, a high dielectric constant material ‘BTO’ is considered on the top of the Al thin metal layer (tm1 = 25nm), and reflection characteristics are calculated with the variation in the thickness of BTO material (tBTO) over a range of 5nm to 15nm as shown in Fig. 3(a). It can be seen that the signal contrast (value of Rmin) is almost constant with the variation in the thickness of the BTO layer. Since the high dielectric constant of BTO leads to enhanced sensitivity, the sensitivity and FOM are chosen to be the decisive parameters to get the optimized value of tBTO. To calculate the sensitivity and FWHM from SPR curves, the refractive index of the analyte is varied from 1.33 to 1.34. An increase in the redshift is observed with the changing analyte’s index as we keep on increasing the BTO thickness. It is also observed that with the increase in the BTO thickness (tBTO), the FWHM is also increasing. Therefore, a trade-off is established between sensitivity (Smax) and FOM over a varying thickness of BTO (tBTO) as shown in Fig. 3(b). A value of 10 nm for the thickness of the BTO layer is found to be the optimized one for a decent value of sensitivity and FOM. The comparison among the respective values of sensitivity and FOM with the variation in tBTO can be seen subsequently in Table 1 below.
Table 1
COMPARISON OF PERFORMANCE PARAMETERS WITH INCREASE IN THICKNESS OF BTO LAYER
t BTO [nm] | Smax [°/RIU] | FWHM [°] | FOM [RIU− 1] | Rmin |
5 | 121 | 0.51 | 237.2 | 0.026 |
10 | 124 | 0.54 | 229.6 | 0.028 |
15 | 128 | 0.64 | 200 | 0.030 |
After optimizing the thicknesses of the Aluminum thin metal layer (tm1 = 25nm), and BTO layer (tBTO =10nm), we have considered another Al metal layer of thickness tm2 deposited on the BTO layer to achieve MDM configuration. The reflection characteristics are calculated with the variation in the thickness of the second Al metal layer (tm2) over a range of 5nm to 15nm, as shown in Fig. 4(a). To calculate the sensitivity and FWHM from SPR curves, the refractive index of the analyte is varied from 1.33 to 1.34. Although the sensitivity is observed to be almost constant with the increase in thickness of the second Al metal layer (tm2), the FWHM is found to be decreasing, resulting in improved FOM or the resolution of the sensor. A decrease in the signal contrast (value of Rmin) is also observed with increasing the second Al metal layer thickness(tm2). To have a decent value of signal contrast and FWHM, a trade-off between the Rmin and FWHM is established with the variation in the thickness of the second metal layer (tm2) as shown in Fig. 4(b). An optimized value of tm2 = 10 nm is noticed for a decent value of signal contrast and FWHM for the proposed MDM configuration. The comparison among the respective values of Rmin, FWHM, and FOM with the variation in tm2 can be seen subsequently in Table 2 below.
Table 2
COMPARISON OF PERFORMANCE PARAMETERS WITH INCREASE IN THICKNESS OF SECOND Al METAL LAYER (tm2)
tm2 [nm] | Smax [°/RIU] | FWHM [°] | FOM [RIU− 1] | Rmin |
5 | 120 | 0.31 | 390.3 | 0.049 |
10 | 119 | 0.22 | 540.9 | 0.24 |
15 | 120 | 0.18 | 655.5 | 0.45 |
The reason for an increased resolution or FOM of the proposed MDM configuration-based plasmonic device can be explained with the help of calculated electric field distribution as shown in Fig. 5. Figure 5(a) shows the electric field distribution for conventional Kretschmann configuration (glass prism + Al metal + analyte), whereas Fig. 5(b) shows the electric field distribution for the proposed Al-based multi-layered plasmonic device with MDM configuration respectively. An enhanced electric field observed in the vicinity of the metal-analyte interface is attributed to the SP mode excitation after the maximum energy transfer from the incident light to SPs (Fig. 5(a)). Eventually, this field interaction gets more localized as soon as the tm2 layer is added above the BTO thin film to form an MDM configuration (Fig. 5(b)). The strong field localization results in lower FWHM of the SP curve in the case of MDM configuration (Arora and Krishnan 2013). Such field distribution is directly related to the penetration depth that defines the interaction length of SPs. As a result, a reduced penetration depth in the MDM configuration (a few tens of nanometers) leads to reduced FWHM and thus larger FOM with respect to conventional Kretschmann plasmonic devices having penetration depth of about hundreds of nanometers.
Table 3 compares the effect of each of the intermediate layers in between the glass prism and analyte. Compared to the conventional Kretschmann configuration (Case -I), with the successive addition of the BTO layer (Case-II), sensitivity is improved but at the cost of poor FOM, as already reported in the literature. In the proposed Al-based multi-layered plasmonic device with MDM configuration (Case-III), an enhancement in FOM or resolution is observed due to a reduction in the FWHM of the SP curve at 1550nm.
Table 3
EFFECT OF DIFFERENT LAYERS ON THE PERFORMANCE PARAMETERS OF THE PROPOSED DEVICE
Configurations | Smax [°/RIU] | FWHM [°] | FOM [RIU− 1] |
Case I | 118 | 0.42 | 280.95 |
Case II | 124 | 0.54 | 229.62 |
Case III | 119 | 0.22 | 540.9 |
Finally, to demonstrate biosensing application for the proposed plasmonic device, a monolayer MoS2 is considered on the proposed device (Fig. 1(a)) as BRE to bind the biomolecules. The better absorptivity nature of this bio-analyte enables its extensive use for biosensing purposes. Since pure Al is not viable for biosensing applications, thus, the MoS2 layer protects the Al metal layer from oxidation. Figure 6 presents the reflection characteristics of different blood components in normal and infected stages as a bio-analyte on the proposed plasmonic device. The refractive index values for the different blood components are taken from Ref. (Sharma et al. 2021). A blueshift in the SPR curves is observed for all the blood components due to a decrease in the refractive index in the infected stage as compared to the normal stage.
At last, the results obtained from the proposed plasmonic device are compared with the previously published results as shown in Table 4. The obtained FOM is much higher than the recently published results where FOM was largely ignored. Utilizing the cost-effective Al for resolution/FOM enhancement with the proposed MDM configuration in the prism-based plasmonic device can be used as a promising SPR sensor for future applications in the NIR region.
Table 4
FOM COMPARISON OF THE PROPOSED PLASMONIC DEVICE WITH PREVIOUSLY REPORTED WORKS
Schematic | Wavelength [nm] | FOM [RIU− 1] | References |
Cu + BTO + Cu + BP | 633 | 145 | (Karki et al. 2022a) |
Ag + BTO + Gr | 633 | 45.05 | (Sun et al. 2019) |
Ag + Au + BTO + Gr | 633 | 42.13 | (Karki et al. 2022b) |
Ag + BTO + Ag + Gr | 633 | 54 | (Pal and Jha 2021) |
Ag + BTO + BP/MoS2 | 633 | 60.52 | (setareh and Kaatuzian 2021) |
CaF2 prism + Al + BTO + Al + MoS2 | 1550 | 540.9 | Proposed work |