In this section, we study tunable characteristic for proposed 1DPC structure as (Si/SiO2)3Metal/Defect/Metal/(Si/SiO2)3 embed in air. Here, the defect layer is introduced between two metal layers and entered at the middle of a regular 1DPC structure constructed with a periodic layer of Silicon and SiO2. The width of Si and SiO2 layer are taken as quarter wavelength with reference wavelength as λ0 = 800 nm. The quarter-wave stacked 1DPC has an advantage of wide PBG at the desired reference wavelength. Here, we are interested to study the tunable characteristic with a metal layer, which is considered as silver. Since, the dielectric constant for metal is a complex as equation (3), the plasma frequency and damping constant of silver is 2.18 pHz and 4.35 THz [22], respectively. The defect layer refractive index and width are considered as 1.35 and 500 nm, along with the metal layer width as 5 nm for primary study. The transmission, reflection and absorption for the proposed structure are calculated by TMM and plotted as shown in Fig. 2.
Fig. 2 shows the transmission, reflection and absorption values achieved as 46.5%, 25%, and 28.5% at wavelength 779.5 nm. The transmission is corresponding to defect mode due to the defect layer along with metal layers. Here, the metal layer besides the defect layer causes surface plasmon wave (SPW) at metal-dielectric’s interfaces. The incident frequency shows shifting to excite the surface plasmon resonance (SPR) which results a decrease in transmission as shown in Fig. 2. The sensitivity of surface plasmon is high and shows the variation with the value of refractive index and width, which gives useful application as optical sensor and filter. The transmission is calculated with the wavelength on the incident electromagnetic wave and metal layer width and plotted as shown in Fig. 3(a). Here, the transmission is calculated at zero incident angle. The transmission, reflection, and absorption with the metal layer width (dm) are plotted at normal incidence in Fig. 3 (b).
Fig. 3(a) shows, the defect mode transmission is shifted from higher to lower wavelength side and the value of transmission is reduced with the metal layer width. Fig. 3(b) shows, the transmission peak is sharply reduced with the metal layer width, while the reflection and absorption increases with the metal layer width. Because of the metal layers on either sides of defect, there is a sharp absorption peak and a decrease in transmission as results of localized SPW at metal interface. Therefore, the width of the metal layer can be useful below 20 nm for the tunable characteristic or sensing application with a good transmission.
The transmission colormap is plotted with incident angle and wavelength along with transmission, reflection and absorption are plotted with incident angle in both cases of TM mode and TE mode in Fig. 4 (a-b). The above transmission is considered at defect layer width as 570 nm with metal layer width as 5 nm. Fig. 4 (a-b) shows, the transmission peak moves towards from higher to lower wavelength side with incident angle in both cases of TM and TE modes. The redshift of localised SPW results from the incident angle increasing the effective width of the defect as well as the metal layer. For the TM mode, transmission peak increases with incident angle and becomes maximum at 600 as shown in Fig. 4 (a). Fig. 4(b) shows that the shifting of transmission is high when compared to the TM mode, but the transmission value decreases with incident angle. Therefore, the TM mode is more effective to study tunable characteristics or sensing applications for localized mode.
Here, the width of defect layer is considered as 2.5 μm for investigate the effect of refractive index. The colormap transmission is plotted with refractive index value 1.35 to 1.45 of defect layer with wavelength in Fig. 5(a-b). Fig. 5 (a) is plotted for the width of metal layer as dL = 5 nm and Fig. (b) is plotted for dL = 15nm.
From Fig. 5, it is seen that the transmission peak shows a redshift with the defect layer refractive index in all cases. The peak shifts linearly as the defect layer refractive index changes, which can be used for tunable or sensor applications. The transmission peak shifting with the defect layer refractive index improves as the incident angle increases in both TE and TM modes, as shown in Fig. 5 (a-b). In the case of TE mode, the shifting increases with the defect layer refractive index but the transmission peak decreases, which limits its application as a tunable filter and sensor. In the TM mode, the shifting of defect mode is slow compared to TE mode, but the transmission increases with the defect layer refractive index. Therefore, TM mode can be more useful for sensor-based applications at a large value of incident angle to improve the shifting of defect mode. Fig. 5 (a-b) reflects the shifting of defect mode with the refractive index increases with an increase the width of metal layers used on both sides of defect. Such improvement of shifting results of localized SPW with change the defect layer refractive index and width as well as the metal layer width. The large width of the metal layer improves shifting but decreasing transmission.
The proposed structure exhibits excellent tuning with a change in the defect layer refractive index with a large metal layer and at a high incident angle. Therefore, the proposed 1DPC structure has a potential application as a refractive index sensor. The bio-component shows a variation in refractive index at various stages or concentrations. Since the refractive index is a physical parameter for bio-component to detect. Therefore, the proposed 1DPC shows a useful application as an optical biosensor device. We have already studied a 1DPC structure with simple defect used as an infected malaria blood sample to diagnose in the early stage, which achieved a sensitivity of about 495.73 nm/RIU [23]. The proposed device can be used to improve the sensitivity using SPW with metal layers.
For the optical biosensor application study, the defect layer width is considered as 7 μm (used in our previous study [23]) and malaria infected blood sample is used as a defect. Since, it has been studied that the shift in defect mode observed much better in TE mode compared with TM mode, while the transmission peak reduces sharply in TE mode. Therefore, the TM mode is more effective to study the shifting in transmission peak at large angle. The wavelength shift per unit refractive index is defined as sensitivity, is plotted with a large incident angle as shown in Fig. 6 along with the transmission peak value.
Fig 6 shows that the sensitivity of proposed structure increases with incident angle and obtains a maximum value at incident angle of 86o, while the average value of transmission peak decreases with incident angle. Therefore, the optimum value of sensitivity for the proposed 1DPC structure is achieved as 1114 nm/RIU with the defect layer width as 7 μm along with the metal layer width as 15 nm. Ankita et al. [23] reported the sensitivity as 495.73 nm/RIU for 1DPC structure of the same defect layer width as 7μm. In this way, the sensitivity is improved more than twice by introducing metal layer and angular variation. The refractive values of different stages of malaria samples, defect mode wavelength, quality factor, and transmission peak are tabulated as shown in table 1.
Table 1. Various parameters of malaria infected blood sample with defect mode wavelength, quality factor and transmission peak
Stages of Malaria
|
Refractive Index
|
Wavelength (nm)
|
Quality Factor
|
Transmission (%)
|
Schizont stage
|
1.371
|
785.11
|
487.9063
|
33.70
|
Trophozoite stage
|
1.381
|
796.50
|
570.8274
|
43.61
|
Ring stage
|
1.396
|
813.25
|
681.7981
|
59.77
|
Normal Stage
|
1.408
|
826.33
|
719.9373
|
69.75
|
From table 1, it is clear the transmission values are quite well as 33.7 % to 69.75 %. Therefore, the proposed 1DPC structure can be used as a biosensor application with high sensitivity about 1114 nm/RIU for malaria diagnosis and good to use for Cancer diagnosis too. In general, the proposed 1DPC structure with a metal layer has good tunable characteristics that can be used for tuning narrow filter device applications. Since, the refractive index of material has dependent on environmental parameters like temperature, pressure, humidity, etc. Therefore, this can be used to construct externally controlled tunable filters or optical switching devices using the proposed 1DPC structure.