The schematic representation of the tunable hybrid metasurface is shown in Fig. 1 as a three-dimensional structure diagram. The device's topmost layer is made of gold-alumina nanoring with a common air hole of radius Rin. The thicknesses of dielectric alumina Al2O3 and plasmonic gold is h1 = 30 nm and h2 = 35 nm, with inner and outer radii Rin = 65 nm and Rout = 130 nm, respectively. The bottom gold film with thickness h3 = 150 nm is used as a ground layer to guarantee total reflection and eradicate any incident EM wave transmission. The periodicity of the unit cell of metamaterial is P = 500 nm. Alumina's dielectric function is taken from Malitson and Dodge , while noble gold is obtained from P. B. Johnson and R. W. Christy . The whole structure is illuminated with plane electromagnetic waves of propagation constant K directed in negative z-axis. Under the FEM solver for the Maxwell equation, here CST microwave studio is employed to simulate and study the response of planner EM waves for both TE and TM polarization at various incident angles. For the meta-atom simulation purposes, the unit cell boundary conditions are employed periodically in the x and y direction and open (add space) in the z-direction.
Evaluating the proposed hybrid metasurface's ability in both absorption A and reflection R, Fig. 1(b) gives the optical responses at normal incident of plane EM waves. The absorption intensity can reach 99.99% at 1490 nm wavelength, which is also very close to the communication window (1550 nm). In this device, the suppression of the reflectance R at normal incidence is because of the electric and magnetic dipole matching or, in other words, the matching of the impedance of the device with free space at the frequency of operation.
The meta-device ensures near-perfect absorption of the EM waves in a specific regime from zero reflectance and transmittance. Figure 2 displays the absolute values of electric and magnetic field distribution of the proposed device at normal incidence to the plane of the device. As the device has a metal-insulator-metal (MIM) configuration, therefore the Fabry Perot cavity in the alumina region has a high tendency of the field confinement (at appropriate thickness), as shown in Fig. 2(a,b). The strongly enhanced magnetic field shown in Fig. 2(c) and Fig. 2(d) gives a clear view of magnetic resonance inside the alumina spacer and at the edges of the ring hole. To ensure the best matching of the impedance, we simulated various dielectric alumina thickness ‘h1’, and the results are plotted in Fig. 3(a). The results show that 30 nm thick Al2O3 can provide the optimal absorption results at the normal incidence of electromagnetic waves. For the spacer of a constant thickness (i.e., h1 = 30 nm), the varied thickness of top gold nanodisk is studied from h2 = 25 to 45 nm, and we can see 35 nm is the optimum choice for 30 nm alumina, which makes almost completely matching of the impedance and improves the absorption intensity nearly to 100%. In both cases, we can observe that as the thickness decrease of either nanostructure, there is a red shift in the absorption peaks. The thinner dielectric enhances image charge distribution on the bottom layer due to the top metallic nanostructure's plasmonic resonance. Similarly, a thinner gold layer provides more EM transmission to the cavity region, and hence the development of standing waves enhances the total absorption ability of the device.
As our device consists of the gold-alumina disk with common inner and outer radii, therefore we investigate in Fig. 3(c, d) the effect of both radii separately. From Fig. 3(c), we observe that the absorption intensity for a solid disk or ring with Rin=0 nm reaches 90% at resonance peak 1300 nm of the EM wave spectrum, not as good as the optimum ring resonator. This amount of absorption is high enough to be utilized in different types of applications, like sensing and imaging. To further enhance the absorption intensity, we introduce an air hole in the nanodisk to make more chances of field confinement as elucidated for different inner radius values in Fig. 3(c). As the inner radius increases, the plasmon resonance position shifts to a higher wavelength, and the absorption intensity increases up to 99.99% at Rin=50 nm for Rout=130 nm. After 50 nm, a further increase in inner radius reduces the absorption intensity, and at 100 nm, the intensity reduces to 70%, as shown in Fig. 3(c). As for as the outer radius is concern, for constant Rin, the lower Rout gives lower absorption, but bandwidth is sharper, as shown in Fig. 3(d). The optimum value of Rout is 130 nm, and with the further increase, the absorption intensity decreases.
The proposed device is further investigated for the unit cell's periodicity in Fig. 4 for five different values starting from 300 nm up to 700 nm. Among this 500 nm, center-to-center separation of the periodic layer gives the optimum result of absorption. The plasmon resonance position at all periods is nearly the same with slightly different intensity of the absorption.
In various applications, whether the absorber in a broad or narrow band range, the most important property is the less sensitivity to incidence angle and the polarization of the impinging EM waves. Here we investigate the angular stability of the proposed absorbing device for a very broad range of the incidence angles (in Fig. 5) for both TE and TM polarization. Figure 5(a) represents the absorption at 1490 nm for different incidence angles when the incident waves have TE polarization, and the device is insensitive to the whole incident angles from 0⁰ to 70⁰. Furthermore, there is no critical shifting of the resonance position in this wide range at all, and such property of the absorber is very suitable for biological sensing applications. Nearly the same response is observed for TM polarization, as shown in Fig. 5(b). Here the average absorption intensity is about 99.89% until 65⁰ of the incidence angle, and after that, there is a minor decline to 97% at the same resonance wavelength. Thus, the absorber overall response in both TE and TM polarization at a broad range of the incidence angle is nearly independent. Because of such high absorption and tunability, this proposed configuration is highly recommended for many optical applications, especially for refractive index ultrahigh sensitivity.
Finally, we are going to discuss the refractive index sensing ability of our ultrahigh plasmonic metamaterial absorber. The analytes here used with different refractive indices represent different concentrations of the glucose solution. Figure 5(c, d) displays the proposed metamaterial structure's sensing ability for various refractive indices from 1.302 to 1.352, and water is considered as a reference medium. As the refractive index changes a little bit, there is a very clear red shift in the absorption spectra's resonance peaks with no critical change in its intensity, as shown in Fig. 5(c) and Fig. 5(d). Maximum absorption achieved is 99.82%, 99.81%, 99.80%, 99.79%, and 99.77% at the resonance wavelength 1549 nm, 1552 nm, 1554 nm, 1556 nm, and 1559 nm for 1.302–1.352 broad range of the refractive indices of analytes. The analysis of the resonant wavelength versus the refractive index in Fig. 6(a) gives a straight line, which indicates that there is no critical change in the absorption intensity and the plasmon resonance of the nanostructure is highly sensitive to the change in the surrounding medium. Sensitivity Sn of refractive index based sensor can be calculated by using the following relations between wavelength resonance shift ∆λ and refractive index ∆n
The sensitivity of our proposed structure reached 300 nm/RIU, as shown in Fig. 6(b). We observe that the proposed device has a high sensitivity for very low refractive index values and high linearity in sensing. It is not far from practical realization and implementation due to the advanced electron beam lithographic technologies. Furthermore, our proposed absorber is insensitive to the angle of incidence and polarization. Therefore, it has advantages over a prism-based sensor whose work is good only at oblique incidence.