1. Introduction

With the development of nano-fabrication technology, metamaterial absorbers [1–4] have attracted significant interest of the research community in many application fields, such as thermal emitters [5–11], photovoltaics [12–14], microbolometers and sensors [15–18]. Most of these applications greatly benefit from novel functions and extraordinary properties of metamaterial nanostructures. There are amounts of different shapes of metamaterial absorbers that have been designed. A majority of these metamaterial absorbers are based on two kinds of structures, the metamaterial resonator structure [19,20] and the metal-dielectric-metal structure [21–23]. The metamaterial resonator structure can couple separately to electric and magnetic fields so as to absorb incident radiation [19]. The metal-dielectric-metal structure can localize the electromagnetic energy inside and gradually dissipate it. However, most of these designs only exhibit one resonant absorption peak at certain wavelength range [24,25]. In order to achieve multiple resonances in a single structure, several absorber structures operating at different wavelengths are integrated to form a composite nanostructure [26–28]. However, this design strategy restricts the dimension and efficiency of absorber devices.

To circumvent these above disadvantages, the stacked multilayer metal-insulator structure is a good candidate to replace a composite nanostructure. In this paper, we design and fabricate a dual-band absorber based on a five-layer stacked structure for TM polarization light. By optimizing structure parameters, there are two absorption peaks obtained with over 95% efficiency at 450 and 584 nm at normal incidence. These two absorption peak respectively originates from the symmetric mode of IMI structure and anti-symmetric mode of MIM structure. The experimental results show good consistency with simulations. The nearly perfect absorption of our designed structure is very attractive for the design of multi-wavelength photo-thermal device.

2. Structure design and simulations

Figure 1(a) depicts the schematic diagram of the MIM stacked array. The structure consists of five alternating layers of Aluminum (Al) and silicon dioxide (SiO₂) deposited on quartz substrate, where periodic narrow slits are etched through the first three-layer structure. The designed structure can be treated as two MIM structures stacked along the z direction. The Al film at the bottom has a 200 nm thickness, opaque to incident light. The thickness of other Al and SiO₂ films are 40 nm and 87 nm, respectively. And the array has a periodicity of 370 nm and slit width of 120 nm. Al is chosen to be the metallic material due to its chemical stability and low-loss optical response in the visible range. These structural parameters have been optimized to obtain high absorption under the TM polarized light (electric field parallel to x direction) by using the full-wave simulations based on the finite-difference time-domain (FDTD) algorithm. In this simulation, the periodic boundary conditions are applied in the x direction and perfectly matched layers are used in z direction. The permittivity of Al in the visible region takes from Johnson and Christy [29] and the refractive index of the SiO₂ film is 1.46. Figure 1(b) shows the simulated reflection (R) and absorption (A) spectra of the designed structure with above optimized structure parameters. The absorption of structure is determined as A = 1- R due to the opaque Al film at the bottom preventing light transmission. There are two absorption peaks observed at the wavelengths of 450 nm and 584 nm, where the absorption efficiencies at both resonant wavelengths are over 95% in the visible range. The full width at half-maximum values of the two absorption peaks are less than 30 nm, which means that the designed structure has high quality factor, different from previous reported hyperbolic metamaterial absorber [30–32].

 

Fig. 1 (a) Schematic of the metamaterial absorber consisting of the five-layer MIM stacked array. The electric field of incident light is perpendicular to metal grating. (b) Simulated absorption spectrum (red solid line) and reflection spectrum (blue dash line).

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In order to gain the physical origin of two absorption peaks, we calculate the normalized magnetic field intensity and electric displacement in one unit-cell at two resonant wavelengths, as shown in Fig. 2. At the wavelength of 584 nm in Fig. 2(a), it can be clearly seen that the magnetic field intensity is mainly localized inside the two SiO₂ waveguide region. In each waveguide region, the magnetic field profile satisfies mirror symmetry with respect to the middle of the MIM waveguide layer, which is the typical characteristic of anti-symmetric mode. The electric displacement in the top and bottom Al films of each SiO₂ waveguide layer is opposite to each other and forms a loop, which generates a significant magnetic resonance. In addition, these magnetic responses formed by electric loops in top and bottom SiO₂ waveguide layers are parallel to each other which greatly enhance the absorption. Therefore, the designed structure at this wavelength can be treated as two-cascaded MIM structure with the induced magnetic resonance along the same direction. Due to the structural asymmetry, the magnetic response of the bottom SiO₂ waveguide layer is stronger than that of top waveguide layer.

 

Fig. 2 Simulated magnetic field intensity (color map) and electric displacement (grey arrow) distribution at the wavelengths of (a) 584 nm and (b) 450 nm corresponding two absorption peaks.

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On the other side, at the wavelength of 450 nm in Fig. 2(b), the calculated magnetic field intensity clearly exhibits the characteristic of symmetric mode in the middle IMI structure consisting of two SiO₂ layers and Al layer between them. The magnetic profile satisfies approximately central symmetry with respect to the middle of the second Al layer. The difference of magnetic field intensity on the top and boundaries of the second Al layer is resulted from structural asymmetry. Although the magnetic resonance in the top and bottom SiO₂ waveguide layers also are excited, the magnetic responses of them is exactly opposite, which decreases greatly the role of magnetic resonance for the generation of absorption peak at 450 nm.

To further confirm the physical origins of above two absorption peaks, we investigate the influence of slit width on the two absorption peaks in Fig. 3(a). The two absorption peaks have red shifts with the decrease of silt width, which can be explained qualitatively by using waveguide mode theory. The absorption peak at the longer wavelength is from the two–cascade anti-symmetric waveguide mode. With fixed periodicity but decreasing slit width, the effective permittivity of the top SiO₂ waveguide layer increases, which has an important impact on the anti-symmetric mode in the top SiO₂ waveguide layer resulting in a redshift of the absorption wavelength based on three-layer waveguide theory. For absorption peak at the shorter wavelength, the increase of the effective permittivity of the top SiO₂ waveguide layer in the middle IMI structure also results in the red shift of symmetric mode. Figure 3(b) also depicts the dependence of the two absorption peaks on the periodicity with fixed slit width. It can be clearly seen that the two absorption peaks have red shifts as the structure periodicity increases. Compared to that of the absorption peak of shorter wavelength, the absorption peak at the longer wavelength has a larger red shifts with the increase of periodicity. This is because the transverse wavevector of waveguide mode in the two SiO₂ layer is provided by the first order scattering of incident light by the grating array, given as β = ± 2π/P + k₀sinθ (where P is the array grating and θ is the incident angle). The different structure periodicity can couple the incident light with the specific resonant wavelength into waveguide modes. According to the plasmon dispersion of the three-layer waveguide theory [33], we can find that with the increase of transverse wavevector, the change rate of resonance wavelength for the anti-symmetric mode is faster than that for symmetric mode. Therefore, the absorption peak at the longer wavelength as a larger red shift than that at shorter wavelength, which further demonstrates the physical mechanism of two absorption peaks

 

Fig. 3 The dependence of two absorption peaks on (a) slit width and (b) periodicity of the array.

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3. Experiment fabrication

In order to verify the rationality of the proposed design, we perform the corresponding experiment. The fabrication process of the designed sample mainly includes the electron beam (EB) evaporation and focus-ion-beam (FIB) milling. It starts with a 10 mm × 10 mm polishing quartz plate used as the substrate of structure. Then 40 nm Al layer and 87 nm SiO₂ layer is deposited alternately on the quartz substrate pre-coated with a 200 nm-thick Al layer by EB evaporation in the same vacuum chamber. The deposition rate for Al and SiO₂ is R_Ag ≈0.2 nm/s and R_SiO2 ≈0.05 nm/s, respectively. Finally, the 20 μm × 20 μm grating array is patterned by FIB milling with a dual-beam (FIB/SEM) system (Ga + ions, 40 pA beam current, 30k eV beam energy). The milling depth is controlled by the milling time. To prevent the oxidation of Al layer in the air, we store the samples in a nitrogen gas holder before optical measurement. The scanning electron microscopy (SEM) image of the fabricated stacked array is shown in Fig. 4(a). The inset clearly show that the fabricated structure is composed with five-layer metal/dielectric alternate films.

 

Fig. 4 (a) The top-view SEM image of the fabricated five-layer MIM stacked array. Scale bar: 5 μm. Inset shows a zoomed oblique-view SEM image. Scale bar: 200 nm. (b) Experimental absorption spectra for different structure periodicity revealing the five-layer metal/dielectric structure.

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Optical absorption spectra of the fabricated sample with different periodicity are measured by an ultraviolet/visible/near-infrared microspectrometer (PV20/30 from CRAIC Technologies). In the measurement, broadband xenon lamp is used to generate a broadband illumination. Measurement is set to an area about 20 μm × 20 μm using a 4 × microscope objective lens with a numerical aperture (NA) of 0.1. The measurement spectrum is calibrated using the thick Al film with high reflection. The small NA objective lens used in the measurement reduces the influence of the non-zero incident angle on the experimental results. The results are shown in Fig. 4(b). For the case of structure periodicity 370 nm, there are two obvious absorption peaks observed at the wavelengths of 450 nm and 578 nm, where the absorption efficiencies of both absorption peaks are both over 88%. Moreover, it can be also clearly seen that the absorption peak at the longer wavelength has a larger red shifts with the increase of periodicity compared to that at the shorter wavelength. These experimental results agree well with the above numerical simulations. There are some small discrepancies between experimental and simulated results, which may come from the dimensional or morphological deviations in the structure of the fabricated sample compared with that of the ideal model structure, and a nonzero NA objective lens in the measurement. Therefore, our dual-band absorber metamaterial not only has a high absorptivity, but also can be adjusted in a broad frequency range, making it a good choice for sensing and multi-wavelength photo-thermal device.

4. Conclusions

In summary, we present the theoretical and experimental demonstration of a dual-band absorber based on a five-layer stacked structure in the visible region. The design structure exhibits two obvious absorption peaks with high efficiency under normal incidence, corresponding to the symmetric mode of IMI structure and anti-symmetric mode of MIM structure. The experimental results show good consistency with that of numerical simulation. Furthermore, the designed structure can also be extended to a two-dimensional structure to realize a polarization-independent dual-band absorber. The nearly perfect absorption of our designed structure is very attractive for the design of high performance photovoltaics and thermal emitters devices.