1. Introduction

The wavelength range of 200–400 nm is crucial for missile warning detection [1–3], flame detection [4–6], UV communication [7–9], and astronomical imaging [10–12]. For these applications, a UV filter with a high transmission performance in the narrowband, appropriate out-of-band rejection, and easy integration with a UV detector, is essential.

Several types of UV filters have been reported in previous studies. Wood’s UV filter consists of a single layer of continuous metal film [13–15], in which the plasma frequency lies in the UV band, and the out-of-band rejection can reach up to 10⁷. However, the cut-off wavelength intrinsically depends on the metal properties and cannot be adjusted. Widely used interference band-pass UV filters are produced by alternately stacking multi-layer high/low-refractive-index metal–dielectric thin films [16–20]. The center wavelength can be adjusted by changing the layer thickness. With the layer thickness determined, the center wavelength remains unchanged. In recent years, two-dimensional (2D) Al nanohole arrays have been designed to operate as bandpass filters in the 200–400 nm waveband [21–27]. The transmission wavelength can be flexibly tuned by varying the lateral structural parameters of the arrays. This wavelength adjustment straightforwardly allows for fabrication of multiple filters with different center wavelength on a same substrate that is important to downsized multiband imaging system, compared with the Wood’s and interference UV filter. Furthermore, Al material is cheap and COMS compatible, making the Al nanohole array a competitive candidate for UV filters. Nevertheless, the transmission of Al nanohole arrays in the UV band is generally lower than 30%, seriously limiting its application. The low transmission is because of the simplicity of the 2D nanohole array that always provides only one mode to select the transmission wavelength. More specifically, Wen-Di et al. reported an Al nanogrid on a glass substrate with a transmission peak of 29% at 290 nm [22] that only supported the wavelength selectivity of the waveguide cut-off of nanoholes; Ahmed et al. designed an Al nanohole array with a transmission peak in the range of 10%–30% [25] that only supported the wavelength selectivity of the surface plasmon resonance (SPR).

Furthermore, several layers of metasurfaces are stacked to design a laminated metasurface [28–31] that can support multiple modes and provide a better performance. For example, laminated perfect absorbers show an ultrabroad absorption band owing to the synergy of guided mode resonances, localized surface plasmons, and cavity modes [31]. Therefore, in this study, we attempted to design an Al laminated nanostructure that supports multiple modes to produce a high-transmission narrowband UV filter, based on a stacking scheme. The laminated nanostructure is mainly composed of an Al nanohole array, and each Al nanohole has a coaxial Al nanoring at the bottom. This UV filter shows a single dominant peak with a high transmission over 50% and narrow bandwidth less than 80 nm in the 200–400 nm waveband. We analyzed the physical mechanism of laminated nanostructure, showing the good optical performance is achieved based on the synergy of surface plasmon resonance (SPR) and localized surface plasmon resonance (LSPR). The designed filter is instructive and will further facilitate the application of Al nanostructure in UV detectors.

2. Structure design and simulation

Figure 1 depicts the schematic of the proposed high-transmission narrowband UV filter based on an Al laminated nanostructure on a glass substrate. The laminated nanostructure is mainly composed of an Al nanohole array, and each Al nanohole has a coaxial Al nanoring at the bottom. The nanoholes are arranged in a hexagonal lattice for polarization insensitivity, wherein the period is T and the depth is h. The nanorings of the Al laminated nanostructure were embedded on the surface of the glass substrate. One Al nanohole and its coaxial Al nanoring can be treated as a unit cell of the Al laminated nanostructure, as depicted in Fig. 1(b). The diameter of the nanohole, inner diameter, depth, and ridge width of the nanorings are denoted as D, d, C_h, and C_r, respectively.

 

Fig. 1. (a) Schematic of the Al laminated nanostructure, mainly composed of an Al nanohole array in hexagonal lattice (period T, depth h), and each Al nanohole has a coaxial Al nanoring at the bottom. The nanorings of the Al laminated nanostructure are embedded in the surface of the glass substrate. (b) Schematic graph of a unit cell of the Al laminated nanostructure. The diameter of the nanohole, inner diameter, depth, and ridge width of the nanoring are denoted as D, d, C_h, and C_r, respectively.

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The optical performance of the proposed UV filter was calculated via the finite-difference time-domain (FDTD) method [32]. The basic calculated unit is the green dashed rectangle shown in Fig. 1(a). The periodic boundary condition is applied in the x and y directions because the device can be considered to be infinitely long in the two directions. The perfectly matched layers (PML) were applied in the z direction. The mesh grid size was set to 2 nm. The dielectric constant of Al was set as the value recommended by Palik [33], and the refractive index of the glass n = 1.45. The plane wave (denoted by the orange arrows in Fig. 1(a)) was injected from the side of the Al nanohole array in air, and the transmission was recorded in the glass substrate.

3. Results and discussion

Figure 2(a) shows the calculated transmission spectra of the proposed UV filter (green solid line) and 2D Al nanohole array (black solid line) with the same structural parameters at normal incidence. The period T, depth h, and diameter D of the nanoholes were 210 nm, 90 nm, and 126 nm, respectively. The inner diameter, depth, and ridge width of the nanorings were 126 nm (d), 15 nm (C_h), and 10 nm (C_r), respectively. One transmission peak of 47% at 250 nm (marked by orange dashed line) and one subpeak of 23% at 330 nm (marked by the green dashed line) were observed in the 2D Al nanohole array, resulting from the fundamental surface plasmon at the Al–air and Al–glass interfaces [25]. However, only a single transmission peak is required in narrowband UV filters. In Ref. [22] and [25], an Al nanogrid was designed on a glass substrate and an Al nanohole array was designed on a Si substrate to acquire a single peak. However, their simulation results showed that the maximum transmission was less than 40%. In our study, by integrating the Al nanorings at the bottom of the Al nanoholes, a single dominant peak with a transmission of up to 50% was observed in the 200–400 nm waveband that was higher than that observed in the 2D Al nanohole arrays reported previously. Meanwhile, a small subpeak of 3% (marked by the gray dashed line in Fig. 2(a)) was observed at 415 nm that could be suppressed by optimizing the structural parameters further. Figure 2(b) shows the transmission spectra of the Al laminated nanostructure with different periods T. The depth h was 90 nm. The depth and ridge width of the nanorings were set to 25 nm and 10 nm, respectively. The relationships of D/T = 0.6 and d = D were maintained, with the period T increasing from 210 to 250 nm. A single dominant transmission peak was obtained for each period T. The center wavelength increased from 250 nm to 300 nm, and the bandwidth was maintained below 80 nm with increasing T. Furthermore, no small subpeaks were observed. Owing to the single peak with a high transmission, tunable center wavelength, and narrow bandwidth of the transmission spectrum, the Al laminated nanostructure is superior to the 2D Al nanohole arrays and more efficient in narrowband UV filters.

 

Fig. 2. (a) Transmission spectra of the Al laminated nanostructure (green solid line) and 2D Al nanohole array (black solid line) with the same structural parameters. Here, T = 210 nm, h = 90 nm, D = 126 nm, d = 126 nm, C_h = 15 nm, and C_r = 10 nm. (b) Transmission spectra of the Al laminated nanostructure with different periods T. Here, h = 90 nm, C_h = 25 nm, and C_r = 10 nm. The relationships of D/T = 0.6 and d = D are maintained with the period T increasing from 210 to 250 nm.

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To reveal the single-peak optical response mechanism of the Al laminated nanostructure, we plotted the electric field (|E|) distribution, as shown in Fig. 3, to show the transmission spectrum at 250 nm corresponding to the distinct single peak (marked by the orange dashed line in Fig. 2(a)) and 415 nm corresponding to the extremely small subpeak (marked by the gray dashed line in Fig. 2(a)). As shown in Fig. 3(a), the intense electric field was mainly distributed in the nanohole and reached a maximum value at the upper edge of the nanohole. This indicates that the excitation of surface plasmons at the Al–air interface of the 2D Al nanohole array and its fundamental mode is mainly responsible for the dominant transmission peak of the Al laminated nanostructure [25]. At normal incidence, the SPR in a periodically patterned metallic film is governed by [34]

 

Fig. 3. Electric field |E| distributions of the transmission spectrum at 250 nm (a) and at 415 nm (b) for the Al laminated nanostructure. Here, T = 210 nm, h = 90 nm, D = 126 nm, d = 126 nm, C_h = 15 nm, and C_r = 10 nm.

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The relatively intense electric field (|E|) at 415 nm (Fig. 3(b)) is mainly distributed in the air above, indicating a low transmittance of the Al laminated nanostructure at 415 nm. However, a non-negligible electric field enhancement occurs at the bottom of the nanoring (marked by the white dashed circles in Fig. 3(b)), and the profiles signify the appearance of the localized surface plasmon (LSP) [36]. The light transmitted from the Al nanohole excites the localized surface plasmon on the Al nanoring, causing an extremely small subpeak at 415 nm (marked by the gray dashed line in Fig. 2(a)). The resonance wavelength of the localized surface plasmon can be tuned by the depth (C_h) and ridge width (C_r) of the nanoring. Figure 4 shows the transmission spectra of the Al laminated nanostructure with different values of C_h and C_r. The period T, depth h, nanohole diameter D, and nanoring inner diameter d were set to 210 nm, 90 nm, 126 nm, and 126 nm, respectively. The wavelength of the subpeak exhibits a redshift with increasing C_h from 5 nm to 25 nm, as shown in Fig. 4(a), while C_r decreases from 27.3 nm to 6.3 nm, as shown in Fig. 4(b). These wavelength responses agree well with those in Ref. [37–39], the resonant wavelength of the localized surface plasmon on the gold (Au) nanoring shows a redshift with increasing C_h and decreasing C_r in the quasi-static approximation. The agreement of the optical wavelength response properties indicates the independence of the resonance wavelength of the surface plasmon and localized surface plasmon in the Al laminated nanostructure.

 

Fig. 4. Transmission spectra of the Al laminated nanostructure with different (a) nanoring depth C_h and (b) nanoring ridge width C_r. Here, T = 250 nm, h = 90 nm, D = 126 nm, C_h = 15 nm in (b), and C_r = 10 nm in (a).

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Thus, excitation in two optical response modes is realized in the Al laminated nanostructure. The surface plasmon is supported by the Al nanohole array that determines the dominant transmission peak, and the localized surface plasmon excited on the nanoring corresponds to the suppressible subpeak. The synergy of these two modes produces a single dominant transmission peak that is crucial for the design of the narrowband UV filter.

To further investigate the ability of the Al laminated nanostructure in suppressing the Al nanohole subpeak with a transmission of 23% at 330 nm (marked by the green dashed line in Fig. 2(a)), we compared the electric field (|E|) distributions at 330 nm with the same structural parameters in Fig. 5. The period T, depth h, nanohole diameter D, and nanoring inner diameter d were set to 210 nm, 90 nm, 126 nm, and 126 nm, respectively. Figure 5(a) shows the electric-field distribution in the Al laminated nanostructure. It was found that the electric field was mainly distributed at the entrance of the nanohole or in the air above the Al laminated nanostructure, indicating a low transmission at 330 nm. Figure 5(b) shows the electric field distribution in the 2D Al nanohole array. The electric field is mainly distributed at the entrance of the nanohole or in the air above the Al nanohole array as well as at the lower edge of the nanohole. The differences between the electric field profiles in Fig. 5(a) and 5(b) indicate that the surface plasmon is excited at the Al–glass interface of the 2D Al nanohole array [25] and results in an unwanted subpeak at 330 nm, but fails to be excited in the Al laminated nanostructure. This can be attributed to the Al nanoring destroyed the flatness of the Al–glass interface, which degenerated its continuity of the effective complex index indispensable for exciting SPR on it, and thus suppressed the subpeak at 330 nm. It is the Al nanoring that enable the Al laminated nanostructure to obtain a single dominant transmission peak by suppressing the SPR at Al–glass interface and supporting a sufficiently weak LSPR instead.

 

Fig. 5. Electric field |E| distributions in (a) the Al laminated nanostructure and (b) 2D Al nanohole array at 330 nm with the same structural parameters. Here, T = 210 nm, h = 90 nm, D = 126 nm, d = 126 nm, C_h = 15 nm, and C_r = 10 nm.

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For the laminated nanostructure with the Al nanohole array, we further explored the impact of the materials of the nanoring on the suppression of the subpeak at 330 nm. The structural parameters were T = 210 nm, h = 90 nm, D = 126 nm, d = 126 nm, C_h = 15 nm, and C_r = 10 nm. The calculated transmission spectra are shown in Fig. 6. The silicon nitride (Si₃N₄) nanoring (blue solid line) still present a distinct subpeak at 330 nm, indicating that the Si₃N₄ nanoring fails to suppress the SPR at the Al–glass interface. The silver (Ag), silicon (Si), index = 5, and perfect electric conductor (PEC) nanorings can suppress the SPR at 330 nm, but appear as new subpeaks at 321 nm (black solid line), 430 nm (purple solid line), 410 nm (green solid line), and 320 nm (red solid line), respectively. These new subpeaks imply material-dependent properties. For example, the subpeak wavelength of the Ag nanoring of 321 nm is close to the plasmonic resonant wavelength of bulk Ag [40]. The Au nanoring (brown solid line) successfully suppressed the SPR at the Al–glass interface without the appearance of a new subpeak, indicating the potential of the single dominant peak of the heteroid laminated nanostructure. In addition, it should be noted that for non-absorbed materials, the difference in the refractive index between index = 5 or PEC (index = ∞) and glass substrate (set to 1.45) is significantly large compared with that between Si₃N₄ (index = 2.1) and glass substrate. For absorbed materials, the difference in the absorption coefficient between Ag (∼ 0.77541 + 0.41562i at 321 nm) or Au (∼1.6921 + 1.9122i at 330 nm) or Si (5.0070 + 0.21462i) and glass substrate is also significantly large compared with that between Si₃N₄ and glass substrate. Thus, we suggest that a sufficiently large difference in the complex refractive index between the nanoring and glass substrate is necessary to suppress the SPR at the Al–glass interface, wherein the complex refractive index continuity is disintegrated. Germanium (Ge, ∼3.288 + 3.063i at 350 nm) and chromium (Cr, ∼1.8133 + 2.6200i at 350 nm) are recommended for the nanoring of a heteroid laminated nanostructure having 2D Al nanohole array.

 

Fig. 6. Transmission spectra of the laminated nanostructure with different nanoring materials. The structural parameters are T = 250 nm, h = 90 nm, D = 126 nm, d = 126 nm, C_h = 15 nm and C_r = 10 nm, respectively. The material of nanoring is set to Si₃N₄ (blue solid line), Ag (black solid line), Si (purple solid line), PEC (red solid line), index = 5 (green solid line), and Au (brown solid line).

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Furthermore, we investigated the tuning of the dominant peak of the laminated nanostructure. It is worth noting that the wavelength of the dominant peak is almost maintained at 250 nm at a different depth (C_h) and ridge width (C_r) of the nanoring, as shown in Fig. 4, and with different materials of the nanoring in Fig. 6. Through intensive simulation based on various structural parameters and materials, we suggest that the wavelengths of the dominant peak and the subpeak can be tuned independently. The dominant peak can be tuned by the period T, dielectric environment, and material of the 2D metallic nanohole array according to Eq. (1). The subpeak can be tuned by the depth C_h, ridge width C_r, dielectric environment, and nanoring material. The independence allows one to obtain more laminated nanostructures with a single dominant transmission peak.

Finally, we studied the influence of the difference between D and d on the optical performance of the Al laminated nanostructure. The difference is defined as Θ = d–D. Figure 7(a) shows the transmission spectra of the Al laminated nanostructure with different Θ. The period T, depth h, nanoring depth C_h, and nanoring ridge width C_r are 210 nm, 90 nm, 15 nm, and 10 nm, respectively. The inner diameter d of the nanoring increased from 86 nm to 166 nm. The difference Θ increased from –40 to 40 nm because the diameter D of the nanohole was fixed at 126 nm. It is found that the single dominant transmission peak degenerates with increasing absolute value of Θ. This degeneration can be attributed to the introduced new strong optical response in the Al laminated nanostructure, as shown in Fig. 7(b)–(e). We suggest that the value of the difference Θ be in the range of –5 nm and 10 nm for the Al laminated nanostructure, to achieve a single dominant transmission peak.

 

Fig. 7. (a) Transmission spectra of Al laminated nanostructure with different Θ (defined as Θ = d–D). Here, T = 250 nm, h = 90 nm, C_h = 15 nm, and C_r = 10 nm. The nanoring inner diameter d is varied from 86 to 166 nm, and the nanohole diameter D is fixed at 126 nm. Therefore, the difference Θ increases from –40 to 40 nm. (b)–(e) show the electric field distributions at (b) 330 nm, (c) 315 nm, (d) 269 nm, and (e) 285 nm marked by the red, blue, cyan and brown dashed lines in (a), respectively.

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The transmission spectra of the Al laminated nanostructure at different values of depth h are shown in Fig. 8. The transmittance decreases with increasing h. Obviously, choosing an appropriate value of h is important to obtain a high transmission for the Al laminated nanostructure.

 

Fig. 8. Transmission spectra of the Al laminated nanostructure with different depths. Here, T = 250 nm, D = 126 nm, d = 126 nm, C_h = 15 nm, and C_r = 10 nm. Depth h increases from 70 to 130 nm.

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4. Conclusions

We present a highly transmitted narrowband UV filter with an Al laminated nanostructure that is mainly composed of an Al nanohole array and a coaxial Al nanoring at its bottom. The proposed structure shows a single dominant transmission peak with a high transmission over 50% and a narrow bandwidth less than 80 nm in the 200–400 nm waveband. The electric field distributions indicate that a single dominant peak is achieved by blending the strong SPR excited at the Al–air interface of the Al nanohole array and the weak LSPR excited in the Al nanoring. The strong SPR selects the dominant transmission peak, and the weak LSPR results in a suppressible subpeak. Nanoring plays an important role in suppressing the SPR at the Al–glass interface, as it disintegrates the refractive index continuity of the interface. A standard single dominant peak is achieved, while the absolute value of the difference Θ is maintained at (–5 nm, 10 nm). The resonance wavelengths of the SPR and LSPR can be tuned independently, thereby improving the flexibility of the design of the laminated nanostructure. The proposed UV filter shows immense potential for application in UV detectors.