1. Introduction

All-dielectric metasurfaces have attracted great interest in recent years [1–4]. Advances in fabrication and design of all-dielectric metasurfaces result in rapid development of compact optical metadevices, such as holograms [5–7], deflectors [8–11], beam converters [12–14], and flat lenses [15–18]. Low losses allow all-dielectric metasurface to possess magnetic dipole (MD) and electric dipole (ED) resonances with a higher efficiency compared with their plasmonic counterparts [19]. In contrast to plasmonic metasurfaces that intrinsically exhibit strong ohmic loss at optical frequency, all-dielectric metasurfaces operating via displacement currents lead to a high converting efficiency even at optical frequency [20]. However, one challenge of all-dielectric metasurface is the fabrication of nanostructures with dimension below several hundred nanometers. Most of the presented metasurfaces suffer from high cost and small scale, which are usually achieved by the focused ion beam (FIB) lithography or electron beam lithography (EBL). Moreover, the conventional high refractive index materials are suffering from strong material loss at visible frequencies. Up to now, there are few large-area and low-cost fabrication methods for titanium dioxide (TiO₂) dielectric metasurface [21].

In this paper, for the first time to our knowledge, an anodized aluminum oxide (AAO) template is combined with TiO₂ metasurface fabrication with advantages of large area and low cost. Using the ordered AAO as an evaporation mask, a TiO₂ nanocylinder array is deposited by electron beam evaporation with almost uniform spacing and size. Our studies show that the TiO₂ metasurface exhibits remarkable magnetic dipole (MD) and electric dipole (ED) resonances at visible frequencies. Moreover, CsPbBr_1.5I_1.5 quantum dot (QD) photoluminescence (PL) coupled with the TiO₂ metasurface is studied and we achieve a pronounced PL enhancement of 25% at 596.0 nm. Due to its inherent simplicity, our method offers great ease for experimental realization. Specifically, TiO₂ metasurface with 2.25-cm²-large area using AAO template method has never been attempted so far.

2. Method

In the following, we describe the fabrication procedure of TiO₂ nanocylinder metasurface. Figures 1(a)–1(f) show a stepwise schematic of the fabrication process. Figure 1(g) shows scanning electron microscopy (SEM) image of AAO templates with top view [corresponding to Fig. 1(c)]. The photograph of metasurface with a square area of 1.5 ×1.5 cm² on SiO₂ substrate is shown in Fig. 1(h). The AAO template is finally removed by transparent tape, leaving a nanostructured surface as shown in Figs. 1(i) and 1(j) after step 1(e).

 

Fig. 1. Schematic of large-area metasurface fabrication steps. (a) Anodized aluminum oxide (AAO) template supported by Polymethyl methacrylate (PMMA). (b) PMMA dissolution in acetone solution. (c) AAO transfer using SiO₂ substrate. (d) TiO₂ vapor deposition by electron beam evaporation. (e) AAO template detachment by adhesive tape. (f) large-area TiO₂ nanocylinder metasurface. (g) Scanning electron microscopy (SEM) image of AAO template with top view. (h) photograph of metasurface on SiO₂ substrate (i) SEM image of TiO₂ metasurface fabricated as described above. (j) Enlarged SEM image of TiO₂ nanocylinder with 45° titled view.

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In this paper, highly ordered porous AAO is used as evaporation template for fabricating nanocylinder. Generally, the AAO is fabricated by a multi-step anodization and wet etching process in an acidic solution with a proper direct-current (DC) voltage [22,23]. Here, the AAO supported by Polymethyl methacrylate (PMMA), as shown in Fig. 1(a), is purchased from WUXI YIRI advanced materials technology Co Ltd. The mean interpore distance and diameter of the pores are 450 and 260 nm, respectively. Thereafter, the PMMA is dissolved in acetone solution, as visible in Fig. 1(b). After complete dissolution of PMMA, the AAO in Fig. 1(c) is taken out by a piece of SiO₂ substrate, which acts as a new support substrate. TiO₂ vapors are then deposited into the pores of AAO by electron beam evaporation. A TiO₂ layer of 60 nm is deposited by evaporation [Fig. 1(d)]. After TiO₂ deposition, the template and excess TiO₂ are removed from SiO₂ substrate by adhesive tape, as shown in Fig. 1(e). Finally, a large-area (1.5 cm ×1.5 cm) of TiO₂ metasurface is obtained. Figures 1(i) and 1(j) show SEM images of TiO₂ metasurfaces fabricated as described above. The fabricated TiO₂ nanocylinders have a height of about 60 nm and radius of around 130 nm, as shown in Fig. 1(j). The center-to-center spacing between adjacent nanocylinder is around 450 nm.

3. Result and discussion

For measurement of the fabricated metasurface transmission spectra, a customized optical spectroscopic system is used and the transmission spectra is measured by a fiber-coupled spectrometer (Ocean optics). The schematic of the measurement setup is shown in Fig. 2(a). The TiO₂ metasurfaces are characterized optically with a white light source. A fiber and a collimating lens (74-UV) are used to guide and collimate light onto the metasurface. The fiber and the collimating lens are connected with a fiber coupler (SMA905). The metasurface is mounted on a stage between two collimating lenses. Another collimating lens after the metasurface is used to focus and couple light into the spectrometer. Finally, the transmission spectra is measured by the fiber-coupled spectrometer (Ocean optics USB 2000; 350.0–1000.0 nm).

 

Fig. 2. (a) Transmittance spectrum measurement with a fiber-coupled spectrometer. (b) Transmittance performed by simulation (blue solid line) and experiment (red solid line). (c) Electric field distribution for electric dipole resonance at 433.1 nm. (d) Electric field distribution corresponding to magnetic dipole resonance at 546.6 nm. (e) Magnetic field distribution for electric dipole resonance at 433.1 nm. (f) Magnetic field distribution corresponding to magnetic dipole resonance at 546.6 nm.

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To compare our experimental results with simulation, we perform finite difference time domain (FDTD) of TiO₂ metasurface with a nanocylinder radius of 130 nm and height of 60 nm. In simulation, the resonance at λ=546.6 nm corresponds to the magnetic dipole (MD) resonance, whereas the resonance at λ=433.1 nm is the electric dipole (ED) resonance of the TiO₂ nanocylinders, as shown in Fig. 2(b). Experimentally, the MD resonance and ED resonance are centered on 567.5 and 402.9 nm, respectively. The overall trend of experimental spectra is consistent with that of the simulation. ED position of Mie resonance in experiment matches well with that in simulation. However, the MD resonance in experiment reaches a minimum at 567.5 nm and maintains its elongated shape to about 600 nm, as shown in Fig. 2(b). Moreover, the MD feature is less pronounced compared with that of ED resonance. This could be due to fabrication tolerances, including diameter deviations and array defect, as well as SiO₂ substrate thickness. The 0.7-mm-thick SiO₂ substrate in experiment is replaced by one with 200 nm thickness in simulation in order to keep the size of the computational domain sufficiently small to allow for realistic computation times. To some extent, the variations of AAO template diameter limits our accuracy in defining the diameter of the TiO₂ nanocylinders.

The simulated electric field distributions corresponding to resonant wavelengths are also shown in Figs. 2(c) and 2(d). We also calculate the magnetic field distributions in the vertical cross-section for the resonant wavelengths. These results are shown in Figs. 2(e) and 2(f). For MD resonance, it can be seen that the electric field is highly confined to the lateral surfaces of the nanocylinder with enhancement factors about 2. Similar to the previous study [19,20,24–27], the MD response in a dielectric nanoparticle originates from a circular current of the electric field inside the nanoparticles, as shown in Fig. 2(d). An enhanced magnetic field appears in the cylinder in Fig. 2(f). Because of the relatively short height of 60 nm, the MD resonance is less pronounced in this paper. Enhancements of ED resonance are observed as well, but these are mostly confined inside the TiO₂ nanocylinder and SiO₂ substrate, as shown in Fig. 2(c). The ED resonance originates from the collective polarization induced in the dielectric nanoparticle, as discussed before [19,20,24–27]. The magnetic field in Fig. 2(e) circulates around the electric field in Fig. 2(c), clearly demonstrating the formation of electric dipole (ED) resonance.

Experimentally, the MD reaches a minimum at 567.5 nm, but maintaining its elongated shape to about 600 nm, as shown in Fig. 2(b). To investigate the effect of metasurface on quantum dot (QD) photoluminescence (PL) enhancement, CsPbBr_1.5I_1.5 QDs are spin-coated on the substrates with and without metasurface. CsPbBr_1.5I_1.5 QDs are synthesized from the corresponding metal-oleic acid complex. After the release of X, the CsPbX₃ nucleates and grows. The resulting QDs have an average diameter of around 10 nm and an emission peak at 596.0 nm, which matches well with the MD resonance in experiment. The PL spectra are measured by exciting the QDs with 501.0 nm laser diode focused on the metasurface. For reference, we also measure the CsPbBr_1.5I_1.5 QD PL emission from the bare SiO₂ substrate without TiO₂ metasurface. As illustrated in Fig. 3(b), the CsPbBr_1.5I_1.5 QD PL with metasurface is enhanced significantly compared to the reference, especially at wavelengths where the MD mode is resonant (2.5/2). As described in Fig. 2, TiO₂ nanocylinder can also be considered as dielectric resonators to trap external electromagnetic fields, which enhances the emission rate. However, the TiO₂ metasurface can not only enhance the emission rate through the Purcell effect [28–30], but can also out-couple the radiation from CsPbBr_1.5I_1.5 QDs in an upward direction more effectively [31,32]. The reshaping and redistribution of emission pattern caused by the Mie resonances also play a critical role in QD PL enhancement.

 

Fig. 3. (a) Sketch of TiO₂ metasurface coupled with CsPbBr_1.5I_1.5 quantum dots (QDs) (b) Measured QD photoluminescence (PL) spectra with and without metasurface.

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Different from the metasurfaces fabricated by FIB and EBL, our method offers great ease for large-area metasurface fabrication and QD PL enhancement.

4. Conclusion

In this article, we use an anodized aluminum oxide (AAO) template method to fabricate all-dielectric metasurface with area larger than 2 cm². The titanium dioxide (TiO₂) nanocylinder arrays are fabricated with advantages of large area and low cost via AAO template combined with electron beam evaporation. We study the transmission properties of TiO₂ metasurface working in the visible spectrum. Our results reveal that the TiO₂ metasurface exhibits remarkable magnetic dipole (MD) resonance and electric dipole (ED) resonances. Moreover, CsPbBr_1.5I_1.5 quantum dot (QD) photoluminescence (PL) coupled with the TiO₂ metasurface is fabricated, resulting a 25% increase of the CsPbBr_1.5I_1.5 QD emission intensity at λ=596.0 nm. A large-area metasurface fabricated by AAO template has not been attempted so far. For the first time to our knowledge, the AAO template is applied in combination with all-dielectric metasurface fabrication, which represents a step toward large-area dielectric metasurface fabrication. Our approach offers a viable bridge between high-index dielectric metasurfaces and their implementation in mass-produced devices.