Figure 2 shows a scanning electron microscope (SEM) image of the surface of the pattern fabricated using an electron-beam lithography system and vacuum deposition. Figure 2(a) shows an SEM image of the surface of the resist pattern without Al deposition; the pattern was drawn as designed. Figure 2(b) shows an SEM image of the sample surface when Al was deposited on the resist pattern using a vacuum evaporation system, as described earlier. Compared with Fig. 2(a), the diameter of the hole is smaller, and although manufacturing errors must have occurred, we believe that the structure was successfully fabricated very closely to the intended design.
Figure 3 compares the dependence on the simulated and experimental incidence angle of a 375-nm wavelength TM-polarized light as it fell on the designed Al nanohole array structure at an azimuth angle of 0°. The solid lines show the simulated values and the scatter plots show the experimental values. The horizontal axis indicates the angle of incidence of light and the vertical axis indicates the ratio of absorptance to reflectance. The simulation showed a peak in the absorption at 10.8°, with a maximum absorption of 95%. With TE-polarized light, no such peak in the absorption or dip in the reflectance could be observed. In the experiment, an absorption peak was confirmed at ~ 11°, with a maximum absorption rate of 97.9%, which almost matched the simulation results. The difference between the simulation and experimental values is thought to be due to manufacturing errors such as the hole radius and Al film thickness.
To consider the mechanism by which a high absorption rate was obtained, we used the RCWA method to calculate the electric-field distribution when TM-polarized light was incident at a wavelength of 375 nm, an incidence angle of 10.8°, and an azimuth angle of 0°. Figure 4(a) shows the electric-field vector distribution in the x–y plane of the Al nanohole array structure. The color map shows the intensity of the electric field and the arrows indicate the direction of the electric field vector. The electrical field was concentrated at the edge of the Al nanohole, and the electric field vector adopted the direction of the x-axis. Figure 4(b) shows the intensity of the magnetic field in the direction horizontal to the x–y plane, and the color map shows its intensity. The magnetic field propagates in the x-direction. Figure 4(c) shows the distribution of the electric-field vector in the x–z plane. The red arrow in the figure represents the direction of the electric-field vector and the color bar represents its strength. At the edge of the nanohole, the electric-field vector is distributed in the x-direction. In other words, there are positive and negative charges at the edges of the nanoholes. Additionally, the electric-field vector is distributed in the z-axis direction.
From these results, the absorption mechanism of the Al nanohole array structure under consideration can be inferred, as shown in Fig. 5. First, the positive and negative charges at the edge of the structure form an electric dipole that excites the localized surface plasmons (LSP). This electric dipole then couples to a propagating surface plasmon (PSP) via an electric field along the z-axis. The coupling of these two surface plasmons is thought to induce the Fano resonance [37], causing strong absorption. To confirm the Fano resonance, the spectrum of the absorbance wavelength shown in Fig. 3 was investigated. A steep absorption peak was observed at 375 nm, although peaks were observed at other wavelengths. Typically, LSP have a broad spectrum due to the loss of energy caused by localized electronic oscillations. In contrast, the absorption spectrum of PSP is sharp, because the energy loss is very small. The absorption spectrum at 375 nm in Fig. 3 has a gradual slope at the bottom and a very steep slope at the peak apex. Along with the outline of this spectrum, this peak indicates the Fano resonance that occurs based on the LSP and PSP.
Next, we investigated the dependence of the designed Al nanohole array structure on azimuth angle. Figure 6 shows the results of the simulation of the dependence of absorption and reflectance on the incidence angle of TM-polarized and TE-polarized light with a wavelength of 375 nm and an azimuth angle of 45°. The solid and dashed lines show the simulation results for TM-polarized light and TE-polarized light, respectively. Unlike the azimuth angle of 0°, the peak absorption rate was observed at an incidence angle of 14.2°, with a maximum absorption rate of 91.5% when the TE-polarized light was incident at an azimuth angle of 45°. Therefore, the Al nanohole array structure depended on the azimuth angle.
Figure 7 shows a graph comparing the simulated and experimental values of the dependence of the absorption and reflectance on the incidence angle for TE-polarized light with an azimuth angle of 45° and a wavelength of 375 nm. Experimentally, a peak absorption rate near 14.5° and a maximum absorption rate of 91.5% were confirmed, which were in close agreement with the simulation results. The difference between the simulated and experimental values can be attributed to manufacturing errors such as the hole radius and thickness of the Al film.
We calculated the magnetic field distribution when the TE-polarized light was incident at an azimuth angle of 45° using the RCWA method. Figure 8 shows the magnetic field distribution in the x–y plane on the surface of the Al nanohole array structure. When we investigated this magnetic field distribution, we confirmed that it oscillated strongly in two directions. The diagram in Fig. 8(a) depicts the magnetic field distribution obtained by extracting only the component in the H1 direction. The magnetic field oscillates strongly in the H1 direction, implying that propagating surface plasmon (PSP) are excited in the k1 direction, specifically φ = −10° direction. Figure 8(b) shows the magnetic field distribution obtained by extracting only the component along the H2 direction. When the magnetic field oscillates strongly in the H2 direction, SPP is excited in the k2 direction, specifically in the φ = 100° direction.
The reason for the occurrence of these two SPPs has been elaborated with absorption rate mapping in Fig. 9, where the color bar denotes the intensity of absorption. Figure 9(a) shows an absorption map of the incident TM-polarized light; a strong distribution can be observed in the (1,0) and (0,1) directions. The red dotted line shows the dispersion curve of the SPP, which corresponds to a high absorption upon the incidence of TM-polarized light. These observations imply that the strong absorption of TM-polarized light is due to the resonance absorption by the SPP. Figure 9(b) depicts the absorption rate map when the TE-polarized light is incident. Here, a strong absorption distribution is observable only in the (1,1) direction, which is in good agreement with the results regarding the dependence of the absorption coefficient on the incidence angle of the TE-polarized light when the azimuth angle is 45°. This resonance point can be decomposed into resonance points, k1 and k2, when the TM-polarized light is incident. In other words, changes in the absorption characteristics of TE-polarized light, when it is incident at an azimuth angle of 45°, are thought to be caused by the two SPP modes of TM-polarized light.
The above results that by using Fano resonance in the Al nanohole array structure, it was found that perfect absorption of near-ultraviolet light at 375 nm can be achieved selectively for wavelength and polarization direction. It has been revealed that by using Fano resonance in an Al nanohole array structure, it is possible to create a perfect absorber which selective absorbs wavelengths and polarization directions. Furthermore, it was found that TE-polarized light can be completely absorbed by the combined surface plasmon resonance of the two kinds of TM polarization. This suggests that optical switching in the UV region can be realized using Fano resonance.