A dual-band polarization insensitive metamaterial absorber with a single metal square patch for sensing application

We proposed a dual-band polarization-insensitive metamaterial absorber consisting of merely the metal square patch and a continuous metal ground separated by a middle dielectric layer. Two resonance peaks derived from `the fundamental resonance (with 97% absorbance) and the surface lattice resonance (with 99% absorbance) are realized. It is different from previous work the dual-band response is obtained by combining two resonances of different sizes. Moreover, a first-order diffraction mode of grating predicted the resonance wavelength of the proposed absorber. The surface electromagnetic field distributions of the unit-cell structure reveal the physical origin of the dual-band absorption. Importantly, the first absorption peak result from surface lattice resonance with narrow line-width has large sensitivity perform and high quality factor, which has significant potential in the application of biosensors and monitoring.

However, the reported approaches face two challenging problems: large unit size problem and the difficulties of manufacturing technology at higher frequencies such as terahertz, infrared, and visible region.
In this paper, we demonstrate a polarization-insensitive dual-band metamaterial absorber constituted by merely a metal square patch, a dielectric space layer and a metal ground. It is found that there are two distinct absorption bands whose peak absorption about 97% in the fundamental mode and peak absorption about 99% with surface lattice resonance. The proposed absorber used the single pattered metallic structure by combining the fundamental resonance and surface lattice resonance, which is different from the results of previous report by overlapping or connecting various sized subunits to obtain dual-band response. Thus, it makes the proposed absorber much easier to construct than previously reported structures.
Importantly, the absorption peak stem from surface lattice resonance has narrow line-width and large sensitivity perform. These characteristic make it exhibit prominent sensing application. Furthermore, we demonstrate the proposed absorber can keep high absorption level at large angles of polarization.

Structure and design
The proposed structure of the dual-band metamaterial absorber is shown in Fig. 1, which is consists of three layers. The entire structure is fabricated on a glass substrate.
The top layer is a gold square patch. The middle layer is the dielectric layer, and the bottom layer is gold film. The thickness of gold square patch is t=57 nm, which have same side lengths w=0.366 um along the x and y axis. The middle dielectric layer had a dielectric constant of 1.96 [22] and a thickness h=35 nm. The thickness of the ground layer gold film is L=0.1um, and the lattice constant is P=0.825 um. The dielectric constant of gold is from reference [33,34]. The finite difference time domain (FDTD) method is used to get the exact results. In the simulation process, the periodic boundary condition is applied along the x and y direction, and perfectly matched layers are set along the z direction. The absorption of the structure can be obtained by A=1-R-T, where R and T is reflectivity and transmissivity, respectively.
The transmissivity 0 T  , while the thickness of the metal film (L=0.1um) is much larger than the skin depth. The perfect absorption can be achieved when the reflectivity R approach to zero (i.e. the equivalent impedance of the proposed absorber structure matched to air). Fig. 2 shows the reflectivity, transmittance and absorption of the dual-band metamaterial absorber. It is obvious that the reflectivity curve has two significant dips result from two distinct type resonances. The transmittance T is zero owing to the presence of the metallic bottom layer, which blocks the transmission of the incident beam. Therefore, two discrete nearly perfect absorption peaks appear, the absorption of which is 99.2% (i.e., peak 1) and 97% (i.e., peak 2), respectively. The peak 1 at 723 nm has the absorption line-width of 16 nm, which is about one-fifteenth of the peak at the 1254 nm. The absorption line-width in this paper refers to the full width at half maximum (FWHM). Furthermore, the quality factor is defined as

The absorption property under normal incidence
where  is the wavelength of the absorption peak [22,23]. The Q value of peak 1 is about 46, which is 7.2 times larger than that of peak 2. It is known that the narrow line-width and high Q value are promised for sensing performance. This result is confirmed in below Fig. 7 and Fig. 8. It is demonstrated that the peak 1 has the sensitivity 550 nm/RIU and figure of merit (FOM) 35, which are superior to better than that of the many sensing devices, see below Table 1.
The peak 1 with narrower absorption line-width and higher Q come from the surface lattice resonance of the basic unit, while the peak 2 arises from the fundamental resonance. To give intuitional evidence, Fig. 3 shows absorption spectra with the different lattice constant P. It can be found that the peak 1 shift to the larger wavelength with the increase of the lattice constant P, while the shift of peak 2 is neglected. Therefore，the physical mechanism of peak 1 is attributed to surface lattice resonance, which is distinct from the previously reported sandwich structure with only electromagnetic resonance or interference mechanisms [24,25]. The surface lattice resonance can be predicted approximately by the first-order diffraction mode of grating [23]. The resonance wavelength can by expressed as Where b is a numerical factor, and n is the refractive index of surrounding the device, i and j are the grating diffraction orders, and P is the grating constant (or the unit period). The functional relationship of the resonance wavelength on the lattice period P is shown in Fig. 4. It can be seen that the resonance wavelength linearly rises with the lattice period P increasing, which is same as shown in Fig. 3. The theoretically predicted resonance frequency indicated by asterisk agrees roughly with the result of simulation when the lattice period P varies from 0.765um to 0.825 um. The resonance wavelength of surface lattice resonance in simulation results has a little deviation for the theoretical value of the one-order diffraction modes when the lattice period is smaller. This is because a surface lattice resonance involve the interplay of the dipole resonance and the first-order diffraction mode [22,23].
To reveal the absorption mechanisms of the two resonance peaks, Fig

The polarization property of the proposed absorber
In many situations, it is desirable to design a polarization insensitive absorber [23,24]. Figs. 6(a) and (b) show the dependence of the proposed dual-band absorber on the polarization angle for the TE and TM modes, respectively. It is found that the location of absorption peaks and absorbance remained unchanged as the polarization angle change from 0 to 90 degree at normal incidence. It is easily understood because of the high degree of symmetry for the metallic square patch in Fig 1.

The sensing performance of the proposed absorber
Since the first absorption peak has a high Q value, it is promising for sensing application [22,27]. For verification, Fig. 7 shows the dependence of the absorption spectrum when the surrounding refractive index (RI) from n=1.0 (air) to1.08 with an interval of 0.02. From Fig. 7, we can find that the peak 1 strongly depend on the RI (n), while the peak 2 has a weak effect. The functional relationship of the resonance peak 1 on the RI (n) is shown in Fig. 8. It is shown that resonant peak redshift linearly with the increasing the RI of surrounding. Additionally, the absorption bandwidth and absorption are almost unchanged due to the impedance of the proposed absorber matched to the free space.
To evaluate the sensing performance of the proposed absorber, the sensitivity (S) and the figure of merit (FOM ) are defined as follows [22,23]: where   and n  are the changes of the resonance wavelength and the RI, respectively. FWHM is the full-width at the half-maximum of the proposed device.

Conclusion
A polarization-insensitive dual-band metamaterial absorber composed by only a metal square patch and a continuous metal ground separated by a middle dielectric layer is reported. Two distinct absorption peaks derived from `the fundamental resonance and the surface lattice resonance are presented. Moreover, a first-order diffraction mode of the grating predicted the resonance wavelength of the proposed absorber. In addition, proposed dual-band absorber is insensitivity for all polarization angles of both TE and TM modes on normal incidence. Importantly, the absorption peak stem from surface lattice resonance has high Q and a large sensitivity, which has significant potential in the application of biosensors.