An Ultrahigh Narrowband Absorber Close to the Information Communication Window

In this paper, we demonstrate a plasmonic ultrahigh narrowband perfect absorber, which realizes an absorption intensity of up to 99.99% in the near-infrared electromagnetic spectrum regime. Different dimensional effects on absorption properties are studied using computer simulation technology (CST) with finite element method (FEM) solver. For both transverse electric (TE) and magnetic (TM) polarization, the absorber shows high stability over a wide range of incident angles. This ultrahigh absorption is attributed to synergy effect of magnetic resonance and surface plasmon resonance. Furthermore, the sensitivity goes to 300 nm/RIU for various refractive indices of different analytes. Our proposed absorber working wavelength is very close to the communication window of information; therefore, except for remarkable sensing abilities, it can also be well utilized in optoelectronic applications like optical switching, amplifiers, and all-optical plasmonic modulators.


Introduction
Metamaterials and metasurfaces are artificially patterned materials consisting of periodically arranged subwavelengths "metaatoms." The meta-atoms design and geometry flexibility have branded metamaterial as designable permittivity and permeability materials [1][2][3][4] and make them widely used in electromagnetic (EM) induced transparency (EIT) [5][6][7], imaging [8][9][10], EM wave detections [11], ultrahigh sensitive sensors [12][13][14], EM invisibility cloaking [15], and solar cells [16][17][18][19]. Because of the huge investment in the solar energy harvesting systems, the plasmonic and metamaterial perfect absorbers have aroused great attention in various fields of technologies. The perfect absorbers are categorized into two, the narrow-and broad-band absorbers [20], where broadband absorbers have prominent applications in ultrashort pulse generation [21], photovoltaics [22], and thermal emitters [23], and narrowband absorbers have numerous applications in nonlinear photonics like optical modulators [24], and switches [25]. Moreover, the ultra-narrow band perfect absorbers in both visible and nearinfrared regions are also of great importance in refractive index sensors. Although some narrow-band absorbers have been studied [26][27][28][29], most of them have polarization or angle sensitivity or lower absorption intensity, which limit their potential applications. In this work, we propose an ultrahigh narrowband absorber with absorbance up to 99.99% close to the information telecommunication window. The absorber also reveals an outstanding performance in polarization and angle independence. The sensitivity can achieve 300 nm/RIU for various refractive indices of different analytes.

Result and Discussion
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 goldalumina nanoring with a common air hole of radius Rin.
The thicknesses of dielectric alumina Al 2 O 3 and plasmonic gold is h 1 = 30 nm and h 2 = 35 nm, with inner and outer radii Rin = 65 nm and Rout = 130 nm, respectively. The bottom gold film with thickness h 3 = 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 Harris et al. [30], while noble gold is obtained from Johnson and Christy [31]. 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. 1b 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 Fig. 1 a The schematic diagram of the proposed metasurface device consisting of goldalumina ring resonators on gold mirror backplane. b The absorption A and reflection R at normal incidence of the electromagnetic waves.

Fig. 2
Electric and magnetic field distribution in both front and side view. a, b The absolute amplitude of electric field distribution at resonance peak 1490 nm while c, d the absolute value of H-field at resonance peak value 1490 nm of the absorption spectrum 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. 2a, b. The strongly enhanced magnetic field shown in Fig. 2c, 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 "h 1 ," and the results are plotted in Fig. 3a. The results show that 30 nm thick Al 2 O 3 can provide the optimal absorption results at the normal incidence of electromagnetic waves. For the spacer of a constant thickness (i.e., h 1 = 30 nm), the varied thickness of top gold nanodisk is studied from h 2 = 25 to 45 nm, and we can see  Fig. 4 The absorption spectra at a different periodicity of the unit cell of the metasurface 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. 3c, d the effect of both radii separately. From Fig. 3c, we observe that the absorption intensity for a solid disk or ring with R in = 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. 3c. As the inner radius increases, the plasmon resonance position shifts to a higher wavelength, and the absorption intensity increases up to 99.99% at R in = 50 nm for R out = 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. 3c. As for as the outer radius is concern, for constant R in , the lower R out gives lower absorption, but bandwidth is sharper, as shown in Fig. 3d. The optimum value of R out 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-tocenter 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 5a 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 0° to 70°. 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. 5b. Here, the average absorption intensity is about 99.89% until 65 0 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 5c, 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. 5c Fig. 6a 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 S n 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. 6b. 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 S n = △ ∕ △ n Fig. 6 The resonance wavelength as a function of refractive index a wavelength shift versus the refractive index and b sensitivity of the device for the various analyst of refractive index having a range of 1.302-1.352 of incidence and polarization. Therefore, it has advantages over a prism-based sensor whose work is good only at oblique incidence.

Conclusion
In summary, we proposed and theoretically designed a perfect metamaterial absorber whose absorption intensity reaches 99.99% in the near-infrared range. The ultrahigh absorption of the device is attributed to the plasmon resonance of the plasmonic nanostructure and magnetic resonance at the spacer alumina region. The device geometrical effects have been investigated for various dimensions and found the optimum thicknesses, periodicity, and inner and outer radii at which ultrahigh absorption can achieved. Furthermore, the absorber ability at both TE and TM polarization was also evaluated at a very long range of the incidence angles and found no critical change in absorption intensity as well as the plasmon resonance position. Finally, the device is tested for the sensing property for analytes with different refractive indices and to get the sensitivity of the system which reached 300 nm/RIU. The proposed plasmonic device can also be utilized in the hot electron-based plasmonic modulators and switches because of the narrow perfect absorption close to the information's communication window.