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Realization of high aspect ratio metalenses by facile nanoimprint lithography using water-soluble stamps


Nanoimprint lithography (NIL) has attracted attention recently as a promising fabrication method for dielectric metalenses owing to its low cost and high throughput, however, high aspect ratio (HAR) nanostructures are required to manipulate the full 2π phase of light. Conventional NIL using a hard-polydimethylsiloxane (h-PDMS) mold inevitably incurs shear stress on the nanostructures which is inversely proportional to the surface area parallel to the direction of detachment. Therefore, HAR structures are subjected to larger shear stresses, causing structural failure. Herein, we propose a novel wet etching NIL method with no detachment process to fabricate flawless HAR metalenses. The water-soluble replica mold is fabricated with polyvinyl alcohol (PVA) which is simpler than an h-PDMS mold, and the flexibility of the PVA mold is suitable for direct printing as its high tensile modulus allows high-resolution patterning of HAR metalenses. The diffraction-limited focusing of the printed metalenses demonstrates that it operates as an ideal lens in the visible regime. This method can potentially be used for manufacturing various nanophotonic devices that require HAR nanostructures at low cost and high throughput, facilitating commercialization.


High aspect ratio (HAR) structures have been employed in various applications including biological sensing [1,2,3,4], plasmonic resonators [5,6,7,8,9,10], and nanophotonic devices [11,12,13]. These structures exhibit unusual physical properties compared to that of their bulk form owing to their unique features such as height, sharpness, and topology of HAR structures. Metasurfaces made up of an array of subwavelength nanoantennas can modulate the wavefront of light in the desired manner. Their 2-dimensional nature significantly reduces the thickness of optical devices and allows for the development of flat optics [14,15,16,17,18,19,20,21,22,23,24,25,26]. Plasmonic metasurfaces composed of metallic nanoantennas provide complete 2π phase control by the resonance of nanoantennas [27], but are limited by the intrinsic ohmic loss of metal. Therefore, dielectric materials have been exploited to increase the efficiency of metasurfaces up to 80% [24, 28, 29], while nanoparticle embedded resin (PER) has been proven for measured efficiencies over 95% [30]. Unlike metals, dielectric metasurfaces require HAR nanoantennas to control the full 2π phase of light. Accordingly, the fabrication process of HAR structures is an important consideration for high efficiency metasurfaces.

Electron-beam lithography (EBL) is mainly used for the fabrication of HAR metalenses owing to its high resolution [31], but it has limitations such as high cost and low throughput. Nanoimprint lithography (NIL) is receiving considerable attention as an alternative to EBL for metasurface manufacturing, using elastomeric stamps containing nanoscale templates [32,33,34,35,36]. Although EBL is still used to produce the master stamp for NIL, once the master stamp is produced, the replica mold can be created quickly and repeatedly at a low cost, and can itself be used as a metalens. NIL significantly reduces the manufacturing cost of metalenses; however, it requires secondary operations, including deposition of high refractive index materials such as TiO2and GaN, and HAR etching to create vertical structures [32, 33]. The metalenses fabricated via secondary operations exhibit a tapered or cone-shaped meta-atom owing to difficulties in vertical etching. Therefore, previous studies have developed the concept of PER for the single-step manufacturing of dielectric metalens without any secondary operations [37,38,39]. However, PER-based manufacturing is limited due to low aspect ratio nanostructures, small patterning areas, and low yield. This is because the fabrication process of the PER pattern involves detaching the hard-PDMS (h-PMDS) replica mold from the substrate. At this stage, shear stress is inevitably applied to the nanostructures which tend to bend or break in the direction of the applied force, deteriorating the quality of the device. Moreover, manufacturing HAR metalenses comprised of PER meta-atoms is even more challenging as the amount of shear stress is inversely proportional to the surface area in the parallel direction. Therefore, HAR structures are subjected to even larger stresses. This large shear stress in HAR structures incur the breakage and collapse of replicated patterns in the direction of the force applied during the detachment (Supplementary Note 1) [40, 41].

Herein, we propose a new NIL method for the centimeter-scale fabrication of flawless HAR metalenses consisting of PER. To mitigate the warpage and breakage of the nanostructures during mold detachment, we eliminate the detaching process and replace it with wet etching of a replica mold made of a water-soluble polymer, polyvinyl alcohol (PVA) [42,43,44]. It is notable that the proposed water-soluble mold expands the printable area from the micrometer scale to the centimeter scale. The PVA replica mold is flexible and has good contact with the substrate, making it suitable as a mold for direct patterning. It is thick enough for handling as a single layer, thereby eliminating the need for a backbone polymer. In addition, the manufacturing process is simple, and the materials used in the process, including deionized (DI) water, are harmless to the human body. As the last step of NIL, the replica mold made of PVA is completely removed via wet etching with DI water. The wet etching of PVA mold does not exert any external force on the nanostructure, leading to the fabrication of flawless HAR metalenses. The replicated nanopatterns are vertical, with an HAR and the same arrangement and size as the master stamp.

Results and discussion

Characteristics of the PVA replica mold

We prepared a replica mold using PVA and performed UV NIL to fabricate HAR metasurfaces. The PVA replica mold is suitable for sub-100 nm metasurface replication due to its high tensile modulus of 2.936 MPa (Supplementary Note 3). This prevent the “pairing” of lateral collapse with the surrounding structures that occurs at PDMS mold due to its elastomeric characteristic [45, 46]. The PVA mold has a thickness of 78 μm, which is thick enough to handle without any backbone layer (Supplementary Note 4). Therefore, the fabrication process of PVA mold is much simpler than h-PDMS mold which requires PDMS backbone layer since h-PDMS is stiff and thin. Moreover, our PVA mold eliminates the vapor self-assembled monolayer (SAM) coating process required for conventional h-PDMS molds to prevent the pattern sticking during detachment [47]. More than 3 applications of vapor phase SAM coating is required on the mold, making the preparation process longer. However, since our wet etching NIL method does not have detachment, vapor phase SAM coating is not essential on the PVA mold. The fabrication process of PVA mold is described in Supplementary Note 7 and Methods section.

Metasurface fabrication by wet etching NIL

Metasurfaces require the confinement of light in the nanostructures for the manipulation of its amplitude and phase. Polymeric resins typically used in NIL are not appropriate for metasurface fabrication owing to their low refractive index. In this study, we use TiO2 PER to enhance the refractive index. Specifically, 30 nm TiO2 in 30 wt% resin (Supplementary Note 5, 6) is dispersed in a solvent of 4-Methyl-2-pentanone (MIBK) and mixed with photoinitiator 1-Hydroxy-cyclohexyl-phenyl-ketone and dipentaerythritol penta-/hexa-acrylate to obtain TiO2 PER (Supplementary Note 7).

The characteristics of TiO2 PER are analyzed via ellipsometry. 89 wt% TiO2 PER is spin-coated on a silicon substrate and cured with UV illumination to measure the extinction coefficient and refractive index. The refractive index is recorded to be 1.8 over a range of 400 nm ≤ λ ≤ 800 nm, strengthening the confinement of light in the PER nanostructures. TiO2 PER shows an extinction coefficient of ~ 0 because TiO2 nanoparticle and polymeric resin are both low-loss materials, which increases the conversion efficiency of metalens (Fig. 1a).

Fig. 1
figure 1

Wet etching nanoimprint lithography using the PVA replica mold. a Ellipsometry analysis of TiO2 89 wt% PER. b Schematic of the wet etching NIL process. c–h Scanning electron microscopy (SEM) images of various replicated nanopatterns using PVA replica mold. Nanopillar: c Square array, Diameter (D = 200 nm), Period (P = 400 nm), Height (H = 200 nm), Aspect Ratio (AR, H/D = 1), d Hexagonal array, D = 100 nm, P = 200 nm, H = 200 nm, AR = 2. Densely packed nanocone: e Hexagonal array, D = 300 nm, P = 300 nm, H = 150 nm. Nanoline: f D = 70 nm, P = 140 nm, H = 70 nm, g D = 100 nm, P = 200 nm, H = 100 nm. High aspect ratio pillar: h Square array, D = 500 nm, P = 2 μm, H = 3 μm, AR = 6

A fabrication process of wet etching NIL is shown in Fig. 1b. TiO2 PER is dropped on the PVA replica mold and spin-coated at 3000 rpm for 30 s to form a uniform layer of TiO2 PER. A TiO2 spin-coated PVA mold is covered on the substrate, and a pressure of 5 bar is applied with UV exposure for 40 min. After PER is cured by sufficient pressure and UV illumination, the substrate attached to the PVA mold is immersed in DI water for 4 h. As PVA is a water-soluble polymer, the PVA mold covering the substrate dissolves, and only the replicated TiO2 nanostructures remain on the substrate.

Unlike conventional NIL, which inevitably introduces shear stress in cured nanostructures, the developed wet etching NIL method can replicate various patterns without faults. Figure 1c-h show SEM images of various replicated patterns using the PVA replica mold. Patterns of various shapes and sizes, such as nanopillars (Fig. 1c, d), nanocones (Fig. 1e), and nanoline (Fig. 1f, g) are successfully replicated. Notably, even HAR pillar patterns (Fig. 1h, aspect ratio (AR) = 6) that are difficult to replicate with PDMS (Supplementary Note 1a) show high fidelity.

Design and simulation of HAR metalens

The anisotropic meta-atom is finely designed to manipulate the phase of the incident light using the concept of Pancharatnam–Berry (PB) phase (Fig. 2a). The designed meta-atom functions as a half-wave plate; therefore, a full 2π phase modulation can be attained by rotation of anisotropic meta-atom. The ideal meta-atom using the PB phase has a π phase difference between the x- and y- components of the electric fields. We use rigorous coupled-wave analysis (RCWA) to calculate the optical characteristics of the anisotropic meta-atom. Conversion efficiencies are calculated at the wavelength λ = 532 nm by varying the parameters of the meta-atom—length (250–400 nm), width (50–200 nm), with a fixed height (700 nm) and period (450 nm) (Fig. 2b). We confirm that the meta-atom with a length 310 nm and width 120 nm has a conversion efficiency of 64% and a near-π phase difference between the x- and y- components of the electric fields (Fig. 2c). Moreover, the designed meta-atom can function at other wavelengths in the visible regime due to the broadband property of the PB phase (Fig. 2d). The designed meta-atom has conversion efficiencies of 48% at λ = 450 nm and 27% at λ = 635 nm.

Fig. 2
figure 2

Design and simulation of metalens. a Schematic of anisotropic meta-atom. b Simulated conversion efficiencies of meta-atoms at λ = 532 nm. (height = 700 nm, period = 450 nm) c Real part of electric field in designed meta-atom: x-component (left) and y-component (right). d Plot of conversion efficiencies in the visible wavelength range. e Simulated optical field based on the Rayleigh–Sommerfeld diffraction formula at wavelength of 450 nm (left), 532 nm (center), and 635 nm (right)

Using the designed meta-atom, a metalens is designed to follow a quadratic phase distribution to focus the cross-polarized light and the expression of the phase distribution is indicated as follows,

$$\varphi \left(x,y\right)=-\frac{2\pi }{\uplambda }\left(\sqrt{{x}^{2}+{y}^{2}+{f}^{2}}-f\right)$$

where λ = 532 nm is a target wavelength, f is the designed focal length, and x and y are coordinates of each meta-atom. The focusing properties of the designed metalens are simulated using the Rayleigh–Sommerfeld diffraction formula [48]. The simulated focal lengths of the designed metalens are 2.45 cm at 532 nm, 2.90 cm at 450 nm, and 2.05 cm at 635 nm, owing to chromatic aberration. The corresponding numerical apertures are 0.2 at 532 nm, 0.17 at 450 nm, and 0.24 at 635 nm.

Fabricated HAR-metalens via wet etching NIL

Figure 3a-c show SEM images of HAR metalens. The master stamp is fabricated via photolithography (Fig. 3a) [49]. The meta-atom is 320 nm in length, 120 nm wide, and 700 nm in height, with an aspect ratio (H/W) 5.8. The replica mold of the master stamp is fabricated with PVA resin (Fig. 3b), and TiO2 PER nanostructures are replicated through wet etching UV NIL (Fig. 3c). Although the printed metalenses have a residual layer, the efficiency is maintained with residual layers around 100 nm, which is the measured thickness of the residual layer (Supplementary Note 2). This is because the residual layer can increase transmission by acting as an antireflection layer. Moreover, the scattering effect of TiO2nanoparticles is negligible since nanoparticles have a small diameter of 30 nm (Supplementary Note 5) [30]. The PVA mold has a tensile modulus of 2.936 MPa, 1.6 times higher than commonly used PDMS, preventing sagging and pairing (Supplementary Note 3) [50]. The final PER nanostructures, replicated with the same configuration as the master stamp, exhibit the excellent resolution of the PVA mold. Our wet etching NIL method replicates HAR patterns with vertical features that are identical to the master stamp as shown in the inset of Fig. 3c. This method does not require secondary operations and the materials used in the process such as DI water and PVA are harmless to the human body.

Fig. 3
figure 3

a-c SEM images of HAR-metalens. a Master stamp of the metalens (Length = 320 nm, Width = 120 nm, Height = 700 nm, Aspect Ratio (H/W) = 5.8). b PVA replica mold. c Replicated metalens. d-g Photographs of printed 10 mm metalenses on various substrates: (d) glass substrate; (e) flexible polyethylene terephthalate (PET) film; (f) convex substrate; and (g) concave substrate

Optical properties of the metalens

A customized measurement setup is used to measure the optical characteristics of fabricated metalenses (Supplementary Note 8). The normalized intensity profiles of the focal spot are measured at different wavelengths of 450 nm, 532 nm, and 635 nm and plotted in Fig. 4a-c. Moreover, the intensity distributions of the focal point are compared with ideal diffraction-limited Airy disks at three different wavelengths (Fig. 4d-f). The measured intensity distributions agree closely with the ideal Airy disk at each wavelength, indicating that the fabricated metalenses act as a perfect lens. The measured focusing efficiencies of the fabricated metalenses are 24% at λ = 450 nm, 45% at λ = 532 nm, 14% at λ = 635 nm. The measured efficiencies of fabricated metalenses are lower than the simulated results owing to fabrication defects. These can be improved by optimizing the fabrication process.

Fig. 4
figure 4

Optical characterization. a-c Measured normalized intensity distributions of the focal planes at (a) 450 nm, (b) 532 nm, and (c) 635 nm. d-f Focusing intensity distribution with ideal Airy disks at (d) 450 nm, (e) 532 nm, and (f) 635 nm


In this study, we developed a wet etching NIL method to fabricate perfect HAR metalenses without defects. The main advantages of our method are the facile fabrication process of PVA mold compared to methods of previous studies using h-PDMS mold and large area fabrication with high pattern resolution comparable to conventional metalens fabrication using electron-beam lithography. Notably, we fabricated centimeter-scale PER metalenses for the first time. The PVA replica mold used in this method is water-soluble and can be wet-etched in DI water, which eliminates the detachment process. As wet etching of the mold does not lead to the application of external forces to the cured nanostructures, the metalenses fabricated using this method are composed of unbroken, vertical, HAR meta-atoms that perfectly mimic patterns of the master stamp. The flexibility of the PVA mold enables direct patterning on various substrates, including glass, flexible films, and curved substrates. As a proof of concept, a 10 mm diameter HAR metalens array was fabricated using the PVA replica mold. The fabricated HAR metalenses exhibit diffraction-limited focusing and a measured focusing efficiency of 28% at 532 nm, which can be improved by optimization of the fabrication process. Moreover, as the PVA mold used in this method can act as a replica mold of imprinting, it can be applied in various ways throughout the imprinting process. Although TiO2 PER was developed in previous work, large-area patterning of TiO2 PER metasurfaces has been challenging for a long time. It is notable that centimeter-scale patterning is achieved without defects in this work. Although the PVA mold is not reusable, it can be easily fabricated repeatably from the master stamp and has powerful advantage that enables large-area patterning of TiO2 PER. Moreover, metalenses can be fabricated not only with TiO2 PER, which is used in this study, but also with other high refractive index PERs such as Si and GaN. We strongly believe that the method developed in this study facilitate commercialization of metasurface fabrication with defect-free HAR nanostructures via NIL.


Preparation of master stamp

A master stamp was fabricated by patterning metalenses using photolithography on a silicon substrate [49]. High-power electron-beam lithography was used to fabricate the reticle using a positive photoresist (FEP series, Fujifilm Ltd.) and developer (tetramethylammonium: TMAH). Molybdenum silicide layer was deposited using an electron-beam evaporator to form the block layer in the reticle. Using the argon fluoride (ArF) photolithography, the reticle pattern was transferred onto a positive-tone photoresist using ArF lithography and developer (TMAH). By using the patterned photoresist as an etching mask, the master stamp was fabricated with ICP dry etcher.

SAM coating was applied to the master stamp to lower the surface energy to facilitate the separation of the mold. SAM is a 2-dimensional molecular assemblies formed on a solid surface by the adsorption and organization of an active surfactant. The driving force for spontaneous SAM formation is the chemical bond synthesis by intermolecular interactions between the substrate surface and surfactant molecules [51, 52].

A fluorine-based SAM solution was prepared by mixing heptadecafluoro-1,1,2,2-tetra-hydrodecyltricholosilane (HDFS) and hexane in a 1:1000 ratio for 10 min. Then the master stamp, which was pre-treated with UV ozone, was immersed in the SAM solution for 10 min. The SAM solution was completely removed by rinsing the master stamp with hexane and DI water. The SAM formed through this process facilitates the separation of the PVA mold by moderating the surface energy of the master stamp.

Fabrication of PVA replica mold

The PVA resin for the PVA replica mold is prepared by placing PVA pellets (Sigma Aldrich, Poly(vinyl alcohol), 96% hydrolyzed, Mw 85,000–124,000) in DI water and completely dissolving them by stirring at 90 ℃ for 24 h. Subsequently, 5 wt% PVA resin is poured onto the SAM-coated master stamp. Pores in the PVA resin are removed in a vacuum using a desiccator. After drying the resin at 30 ℃ for 24 h, a uniform PVA layer without pores is formed on the master stamp. By separating the PVA layer from the master stamp, the PVA replica mold is obtained. Separation of PVA from the master stamp is easy owing to the flexibility of the PVA mold.

Formulation of TiO2 PER

We prepared TiO2 30 wt% resin (Ditto Technology) where TiO2 NPs (30 nm size, anatase) were dispersed in a solvent of 4-Methyl-2-pentanone (MIBK). Then we mixed TiO2 30 wt% resin with 1-Hydroxy-cyclohexyl-phenyl-ketone (Sigma Aldrich) as a photoinitiator, dipentaerythritol penta-/hexa-acrylate (Sigma Aldrich) as a polymer matrix, and additional solvent of MIBK (Sigma Aldrich). The mixture of TiO2 NPs, photoinitiator, polymer matrix and solvent has 89 wt% TiO2 and we called this resin as TiO2 PER in this work.

Spin-coating of TiO2 PER on PVA replica mold

As a previous step in imprinting, we spin-coated TiO2 PER on PVA replica mold. We attached PVA replica mold to the substrate by using ketone tape and drop TiO2 PER on PVA mold, where TiO2 PER have been pre-sonicated 10 min for dispersion. Then we spin-coated TiO2 PER in 3000 rpm for 30 s to form thin layer on PVA replica mold.

UV Imprinting and wet etching process

UV-ozone was pre-treated on the substrate for 3 min, and the spin-coated PVA mold was attached to the pre-treated substrate. We applied pressure of 5 bar and UV illumination (365 nm, 1W) for 40 min to cure the TiO2 PER. After the imprinting process, we put the substrate which was covered with PVA mold in DI water (30 ℃, 200 mL in petri dish) for 2 h. To prevent the evaporation of the DI water, we sealed the petri dish with plastic wrap during the dissolution of PVA. As the PVA mold dissolved, we took the substrate out of the water where the PVA mold had eliminated. Then we performed DI water cleaning process once, and blew out the remaining DI water from the substrate by N2 blowing in room temperature.

Availability of data and materials

The datasets used or analyzed during the current study are available from the corresponding author on reasonable request.



High aspect ratio


Nanoimprint lithography




Polyvinyl alcohol


Electron-beam lithography


Argon flouride




Self-assembled monolayer




Particle embedded resin






Rigorous coupled-wave analysis


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Argon fluoride photolithography and electron-beam lithography were done in National Nano Fab Center (NNFC) and National Institute of Nanomaterials Technology (NINT), respectively, both user facilities supported by the Korean government.


H.L. acknowledges the Technology Innovation program (20016234) funded by the Ministry of Trade and Industry & Energy (MOTIE) and the National Research Foundation (NRF) grants (NRF-2019K1A4A7A02113032, NRF-2022M3H4A1A02046445, NRF-2018M3D1A1058997) funded by the Ministry of Science and ICT (MSIT) of the Korean government. J.R. acknowledges the POSCO-POSTECH-RIST Convergence Research Center program funded by POSCO, an industry-university strategic grant funded by Samsung Display, and the NRF grants (NRF-2022M3C1A3081312, NRF-2022M3H4A1A02074314, NRF-2019R1A5A8080290, CAMM-2019M3A6B3030637) funded by the MSIT of the Korean government. C.W.Q. acknowledges the support by AME Individual Research Grant (IRG) funded by A*STAR, Singapore (Grant No. A2083c0060). J.K. acknowledges the POSTECH Alchemist fellowship.

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Authors and Affiliations



J.R. and H.L. conceived the idea and initiated the project. J.K. and W.K. designed and fabricated the master stamps. H.C., C.P., N.K. and J.H. developed the PVA mold replication and imprinting processes. J.K. performed the simulations. J.K., J.S. and M.C. conducted optical characterizations of metalens. C.-W.Q. supported the materials characterizations. H.C. and J.K. wrote the manuscript. All authors confirmed the final manuscript. J.R. and H.L. guided the entire work.

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Correspondence to Junsuk Rho or Heon Lee.

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Supplementary Information

Additional file 1: Supplementary Note 1.

Replicated HAR nanostructures via PDMS and PVA mold. Supplementary Note 2. Calculated conversion efficiency by varying residual layer thickness. Supplementary Note 3. Tensile modulus of PVA mold. Supplementary Note 4. Characteristics of PVA replica mold. Supplementary Note 5. Size distribution of TiO2 PER. Supplementary Note 6. Nanoindentation test on TiO2 PER thin film. Supplementary Note 7. Fabrication process of PVA replica mold. Supplementary Note 8. Optical setup to characterize the focal spot of metalens.

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Choi, H., Kim, J., Kim, W. et al. Realization of high aspect ratio metalenses by facile nanoimprint lithography using water-soluble stamps. PhotoniX 4, 18 (2023).

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