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Abstract
Structural-color nanoprinting, which can generate vivid colors with spatial resolution at subwavelength level, possesses potential market in optical anticounterfeiting and information encryption. Herein, we propose an ultracompact metasurface with a single-cell design strategy to establish three independent information channels for simultaneous watermarked structural-color nanoprinting and holographic imaging. Dual-channel spectrum manipulation and single-channel phase manipulation are combined together by elaborately introducing the orientation degeneracy into the design of variable dielectric nanobricks. Hence, a structural-color nanoprinting image covered with polarization-dependent watermarks and a holographic image can be respectively generated under different decoded environments. The proposed metasurface shows a flexible method for tri-channel image display with high information capacity, and exhibits dual-mode anticounterfeiting with double safeguards, i.e., polarization-controlled watermarks and a far-field holographic image. This study provides a feasible route to develop multifunctional metasurfaces for applications including optical anticounterfeiting, information encryption and security, information multiplexing, etc.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In traditional color printing, using pigment is a common method to record color-image information, but it has unavoidable disadvantages like limited resolution, color fading and environmental pollution, which cannot face the challenges of next-generation color printing towards ultra-compactness, long-life and zero-pollution [1]. Metasurfaces, which possess the capability of precisely manipulating optical waves at the subwavelength resolution [2–10], provide a new approach to achieve information recording with both high-capacity and high-resolution [11–18]. In recent years, scientists have found that the resonant effect inside or around nanostructures of a metasurface can be controlled to modulate spectrum by adjusting geometric sizes of metallic or dielectric nanostructures, which can help to create a variety of vivid structural-colors for printing [19–24]. Different from the traditional color printing using dyes or inks, nanostructures with different geometric parameters will influence the spectrum of transmitted or reflected light directly, making the metasurface exhibit different colors on its surface. Moreover, researchers have found that changing the orientation angles of anisotropic nanostructures can also influence the transmission/reflection of light according to Malus’s law [25–27], thus generating grayscale nanoprinting images or vivid color images.
One of the most potential applications of nanoprinting is anticounterfeiting and information encryption [28,29]. To further increase the information capacity and security, metasurface-based nanoprinting has been developed from previous single-channel [22–26,30,31] and dual-channel [32–40] to current multifunctional integration such as nanoprinting and holography [41–50]. For example, a tri-channel metasurface was proposed, with which a structural-color image under unpolarized natural light, a nanoprinting image decoded by linearly polarized light and a holographic image under circularly polarized (CP) light are independently displayed [44]. Regarding anticounterfeiting application, a hidden watermarked pattern covered on a structural-color nanoprinting image was proposed, and it presents an exclusive dual-mode visual effect which is quite different from conventional anticounterfeiting approaches [32]. In this paper, we found that the design degree of freedom (DOF) demonstrated in Ref. [32] has not been fully exploited since each nanostructure has two orientation options. The DOF of the two orientation options can be employed to generate an additional two-step Pancharatnam-Berry (P-B) phase and project a Fourier holographic image in the far-field. Therefore, the newly proposed metasurface can not only display a nanoprinting image covered with polarization-dependent watermark pattern, but also reconstruct an independent holographic image, thus making the information delivery safer.
Schematic illustration of the proposed tri-channel metasurface is shown in Fig. 1. Three operating channels are established to display images separately by designing the geometric sizes and orientations of nanobricks to change the spectrum and phase profile of light. Here, channels 1 and 2 are related: channel 1 is used to record a structural-color nanoprinting image and channel 2 is employed to record the same image but covered with a watermark pattern (transmission mode). The two channels can be readily switched by changing the outgoing polarization state. Channel 3 is independent of channels 1 and 2, thus a different holographic image can be projected in the far-field (reflection mode). With advantages of high resolution, good fidelity, high information capacity and no crosstalk, the proposed approach provides a practical method for application in multiple-folded anticounterfeiting and information encryption.
Fig. 1. Schematic illustration of the tri-channel metasurface for watermarked structural-color nanoprinting and holographic imaging. Nanobricks with different sizes and orientations are arranged on the transparent substrate. By changing the illuminating condition and outgoing polarization of light, three types of images can be produced with the simple metasurface. Under the irradiation of unpolarized natural light, a structural-color nanoprinting image of landscape appears on the surface of the metasurface (channel 1). When an analyzer is inserted to control the polarization direction of the emergent light, the identical structural-color nanoprinting image but covered with a watermark pattern can be observed (channel 2). Additionally, a centrosymmetric holographic image can be projected in the far-field under the laser illumination at 620 nm (channel 3).
2. Design and methods
Mie resonance, which often occurs inside dielectric nanostructures, carries electromagnetic enhancement for ultra-high reflection or transmission at specific frequencies [51–54], and has become a proper choice for structural-color design. These resonant nanostructures can be called Mie nano-resonators. By controlling geometric parameters of the nano-resonator, its resonant wavelengths and corresponding reflected/transmitted spectrum can be effectively modulated, which finally accounts for different structural-colors. Here, in order to accomplish simultaneous watermarked structural-color nanoprinting and holographic imaging, we consider the spectrum manipulation as well as the P-B phase manipulation of anisotropic nano-resonators, and finally present a theoretical model for the tri-channel nanobrick:
According to the theoretical model shown in Eqs. (1)–(3), we designed nanobrick structures acting as nano-resonators. CST Studio Suite commercial software was employed for nanobrick optimization, and the unit-cell structure is shown in Fig. 2(a). Silicon on sapphire (SOS, with a fixed silicon layer height) material is employed for nano-resonator design, so height (H) of the nanobrick structure is set to be 230 nm. To avoid high-order diffraction and coupling effect between neighboring nanobricks, the period size (S) of the nanobrick is fixed at 400 nm. The length (L) changed from 90 nm to 250 nm, while the width (W) changed from 60 nm to 170 nm. The gap of L and W variations was set to be 10 nm. To establish dual channels for watermarked structural-color nanoprinting, we need to control the resonant difference along the long- and short-axis directions. In another words, nanobricks with distinct differences between L and W are more preferable to generate different structural-colors under different polarization controls. Besides, we should also consider the diffraction efficiency of the holographic imaging under channel 3, which depends on the polarization conversion efficiency of the nanobrick [ηPCE(λ) = |rl(λ)-rs(λ)|2/4]. Thus, we finally selected 11 silicon nanobricks with different sizes to establish the tri-channel metasurface. The simulated results are shown in Fig. 2(b) and Table 1. Here, we set 620 nm to be the operating wavelength of channel 3, and selected the nanobrick with L = 140 nm and W = 100 nm (N11, whose reflected polarization conversion efficiency reaches 45.7%) to lay the background of the structural-color nanoprinting image. The other 10 nanobricks with variable sizes are used to design landscape patterns. Corresponding calculated structural-colors are shown in Fig. 2(c), C1 and C2 stand for the structural-color profiles under unpolarized natural illumination with an analyzer of α = 0. C1 is calculated with the nanobrick of θ = π/4 while C2 is with θ = π/2. Clearly, the distinct transmission spectral difference between long- and short-axis directions finally accounts for different structural-colors under two different situations.
Fig. 2. Schematic diagram and simulated results of the tri-channel metasurface. (a) Schematic diagram of the metasurface. The metasurface is composed of nanobricks with variable dimensions and orientations. Each nanobrick acts as a unit-cell. (b) The simulated results of 11 different nanobricks. The black and red curves represent the transmissivity versus wavelength along long- and short-axis directions respectively. (c) Calculated structural-color profiles with nanobricks of θ = π/4 (C1) and π/2 (C2) under unpolarized natural incidence. An analyzer with α = 0 is inserted.
Table 1. Propagation phase (β) and polarization conversion efficiency (ηPCE) of nanobricks at 620 nm in reflection
According to Eq. (2), nanobricks with θ = π/4 or 3π/4 can be used to realize identical Sout2(λ, θ) when the analyzer is set to polarization angle of α = 0. In addition, since the cosine square function [cos2(θ - α)] and the sine square function [sin2(θ - α)] take the same values of 1/2 in this case (α = 0, θ = π/4 or 3π/4), so Sout1(λ) = 2Sout2(λ, θ) is satisfied. It means that C1 can also be used to represent the calculated structural-colors of nanobricks with θ = 3π/4 under unpolarized natural illumination and an analyzer of α = 0, or the calculated structural-color hue of nanobricks with arbitrary θ under unpolarized natural illumination. Therefore, three orientation states (θ =π/4, 3π/4 and π/2) were capable to be assembled for watermarked structural-color nanoprinting and holographic imaging. Importantly, the propagation phase of the cross-polarized sub-beam (β(λ) = arg [rl(λ)-rs(λ)]) in the channel 3 should be considered carefully when designing the holographic image because it changes with geometric sizes of the nanobrick and the operating wavelength. Besides, the polarization conversion efficiency should also be taken into account to calculate the reflected amplitude distribution. The whole design flow chart of the tri-channel metasurface is exhibited in Fig. 3, and there are three main steps to design the nanobrick size and orientation distribution. Firstly, a target structural-color image is recorded into the preliminary nanobrick arrangement with 11 nanobricks of different sizes and initial orientation angles (i.e., each nanobrick satisfies θ = 0). Secondly, considering the watermark pattern displayed under channel 2, nanobricks with orientation angles of π/4 are used to generate identical colors under channels 1 and 2, while nanobricks with orientation angles of π/2 to generate different colors under the two channels for watermark recording. The polarization control of output light brings more energy loss under identical natural illumination, so the target structural-color image has double brightness of the watermarked one. Finally, taking orientation degeneracy into consideration, nanobricks of θ = π/4 and 3π/4 are employed to cause the same structural-colors under previous two channels but two-step P-B phase delays (Ψ = 2θ) for LCP incidence under channel 3, which helps to design the holographic image by using simulated annealing algorithm [41,44]. In the hologram design, total phase delay of light after passing through a nanobrick structure [β(λ)+Ψ] is employed for phase optimization, and it depends on the dispersion characteristic of SOS material. In addition, since the two-step phase-modulated hologram possesses twin-image effect, a centrosymmetric pattern is designed as the target holographic image. After these steps, a final optimized phase distribution (shown in the right black dotted box of Fig. 3) can be acquired to record the holographic image under the premise of keeping both structural-color image and watermark pattern unchanged.
Fig. 3. Design flow chart of the tri-channel metasurface for watermarked structural-color nanoprinting and holographic imaging. By arranging the 11 nanobricks of different dimensions, a target structural-color image can be recorded under channel 1. By designing orientation distribution with two orientation candidates (π/4 and π/2), a watermark pattern is recorded under channel 2. By introducing the orientation degeneracy, two-step P-B phase manipulation (Ψ = π/2 and 3π/2) is utilized to realize a hologram for LCP incidence under channel 3. The final optimized phase distribution is shown in the right black dotted box.
3. Results and discussion
To experimentally demonstrate the simultaneous watermarked structural-color nanoprinting and holographic imaging, we fabricated a metasurface sample by using standard electron beam lithography. The sample is with dimensions of 500 × 500 pixels (200 µm × 200 µm), and its scanning electron microscopy (SEM) photos are shown in Figs. 4(a) and (b). The experimental setups for all channels are exhibited in Figs. 4(c)–4(e). In the experiment, we first observed the structural-color nanoprinting images under channels 1 and 2. The structural-color nanoprinting images with polarization-dependent watermarks can be displayed on the surface of the sample, thus a microscope was used for observation. The captured experimental images are shown in Fig. 5. Obviously, all the landscape elements (clouds, birds, moon, building, mountains, rivers and fog) have clear edges and bright colors. The experimental images under channel 1 are brighter than those under channel 2 because the inserted analyzer decreases the output light intensity. Compared with the theoretical results shown in Fig. 4(c), the experimental images under unpolarized natural illuminations possess similar color profiles. However, distinct tone differences existed in areas I, II and III because of lattice resonance effect [55,56], slight metasurface sample fabrication error and some experimental condition errors. Beyond that, it can be clearly seen that the captured watermark patterns exhibited in Figs. 5(d)-(f) are readily distinguished, which shows good validity of our design.
Fig. 4. SEM photos and experimental setups of the metasurface sample. (a) SEM photo of the sample (partial view). The scale bar is 10 µm. (b) SEM photo in zoom-in view. The scale bar is 2 µm. (c) General sketch and (d) detailed illustration of the microscope (Motic BA310Met) to observe the structural-color nanoprinting images under channels 1 and 2. A quartz halogen lamp is used to illuminate the sample while an analyzer is employed to control the polarization direction of the output light under channel 2. The calculated structural-color nanoprinting images under both channels are shown in the blue dotted box of (c). (e) The experimental setup to observe the holographic image under channel 3. The optical path is composed of a continuum laser (YSL SC-pro), an iris, a lens, the sample and an optical screen.
Fig. 5. Experimentally captured structural-color nanoprinting images under channels 1 and 2. (a) The observed unwatermarked structural-color nanoprinting image and (b,c) corresponding enlarged views under channel 1. (d) The observed watermarked structural-color nanoprinting image and (e,f) corresponding enlarged views under channel 2. The white arrows in the upper right of (d-f) represent the polarization angle of the analyzer (α = 0). The scale bars are 50 µm in (a,d) and 20 µm in (b,c,e,f). (a,d) are with size of 200 µm × 200 µm while (b,c,e,f) are with size of 80 µm × 80 µm.
Then, we rotated the analyzer to study polarization characteristics of the metasurface, corresponding experimental results and the calculated theoretical ones are shown in Fig. 6. The watermark pattern changes when α varies, which exhibits strong polarization sensitivity. From the observation, it can be clearly seen that watermark patterns shown in Figs. 6(a)-(c) are consistent with the theoretical results calculated by Eq. (2) [shown in Figs. 6(d)-(f)]. Thus, the proposed theoretical model can be used for scientific analysis. Specially, the watermark pattern shown in Fig. 6(b) is with bright hues, this reversal phenomenon is caused due to the spectrum reallocation, which can also be interpreted by Eq. (2). It is readily found that the structural-color distribution in each element of the experimental images is nearly even when α = 0 and π/2 while distinctly uneven at other values. The reason is that nanobricks with θ = π/4 and 3π/4 possess different spectral responses when α ≠ 0 or π/2. Based on these comparisons and analyses, the watermarked structural-color nanoprinting is experimentally demonstrated with high polarization sensitivity, clear visual and bright color effects.
Fig. 6. Experimental and theoretical results when rotating the analyzer. (a-c) Experimentally captured structural-color nanoprinting images. (d-f) Corresponding theoretical results calculated by Eq. (2). The white arrows on the upper right of images represent the polarization direction of the analyzer (π/4, π/2 and 3π/4 respectively). The scale bars are 50 µm in all experimental images.
A holographic observation optical path shown in Fig. 4(e) was established to catch the holographic image projected in the far-field. In our experiment, a hole was dug out in the center of the optical screen to filter the zero-order light of the reflected light. Then we used a commercial camera (Nikon 5100) to capture holographic images under channel 3. Experimental results at different wavelengths are shown in Fig. 7. Obviously, the captured holographic image at design wavelength (620 nm) has the highest fidelity. The holographic image can also be readily recognized when the wavelength changes, this is due to the wavelength independence of P-B phase manipulation and good robustness of phase-modulated hologram. Some image noises exist in the experimental results because the propagation phase distribution variation with the wavelength could decrease the holographic image quality. Although the influence of the propagation phase variation is unignored, the designed metasurface can operate under channel 3 in a broad wavelength range.
Fig. 7. Experimentally captured holographic images at different wavelengths under channel 3. (a) Enlarged holographic image at the design wavelength of 620 nm. (b) Experimentally captured holographic images at different wavelengths (from 480 nm to 650 nm with a gap of 10 nm).
The proposed tri-channel metasurface possesses several distinctive advantages. First of all, the metasurface can accomplish high-resolution watermarked structural-color nanoprinting with theoretical resolution of 63,500 dots per inch, i.e., the watermark pattern can be readily hidden into the structural-color nanoprinting image by designing orientations of nanobricks and each nanobrick acts as a minimum operating unit. Secondly, an extra holographic image can be projected in the far-field since the two orientation candidates (π/4 and 3π/4) are flexibly arranged for two-step P-B phase manipulation. Both watermarked structural-color nanoprinting and holographic imaging can be independently designed because the orientation selecting strategies are independent. Importantly, our tri-channel metasurface possesses improved information capacity and functionality since a single-cell design strategy is established. In addition, we designed and fabricated the tri-channel metasurface by employing SOS material, which is widely used in semiconductor industry, so it is hopeful to achieve integrated anticounterfeiting applications for some ultra-portable devices.
4. Conclusions
In summary, we propose and experimentally demonstrate a tri-channel metasurface which can simultaneously realize watermarked structural-color nanoprinting and holographic imaging. In our design, the structural-color nanoprinting image is recorded into the nanobrick size arrangement while the watermark pattern as well as the holographic image are recorded into the orientation distribution of nanobrick arrays. Subtly, the concept of orientation degeneracy is exploited so that two orientation design strategies for the watermark pattern and the holographic image do not interfere, hence one more DOF is employed to realize independent design of all images. The experimentally demonstrated metasurface generates watermarked structural-color nanoprinting images under unpolarized natural light with polarization control, and reconstructs a holographic image by using a coherent laser light. All experimental results prove our theoretical design. Additionally, the tri-channel metasurface exhibits a dual-mode anticounterfeiting method with double safeguards of both watermark pattern and holographic image. Therefore, this study can pave a new way toward multifunctional metasurface for potential applications including information encryption and security, high-end optical anticounterfeiting and many other related fields.
Funding
National Key Research and Development Program of China (2021YFE0205800); National Natural Science Foundation of China (12204359, 12174292, 11904267, 91950110); China Postdoctoral Science Foundation (2022TQ0243, 2022M712455); Fundamental Research Funds for the Central Universities (2042022kf1013, 2042022kf0024, 2042021kf0011).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper is not publicly available at this time but may be obtained from the authors upon reasonable request.
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