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Abstract
Nanostructured metasurfaces applied in structural-color nanoprinting and holography have been extensively investigated in the past several years. Recently, merging them together is becoming an emerging approach to improve the information capacity and functionality of metasurfaces. However, current approaches, e.g., segmenting, interleaving and stacking schemes for function merging, suffer from crosstalk, low information density, design and fabrication difficulties. Herein, we employ a single-celled approach to design and experimentally demonstrate a high-density multifunctional metasurface merging nanoprinting and holography, i.e., each nanostructure in the metasurface can simultaneously manipulate the spectra (enabled with varied dimensions of nanostructures) and geometric phase (enabled with varied orientation angles of nanostructures) of incident light. Hence, with different decoding strategies, a structural-color nanoprinting image emerges right at the metasurface plane under white light illumination, while a holographic image is reconstructed in the Fraunhofer diffraction zone under circularly polarized laser light incidence. And the two images have no crosstalk since they are independently designed and presented at different distances. Our proposal suggests a space-multiplexing scheme to develop advanced metasurfaces and one can find their markets in high-density information storage, optical information encryption, multi-channel image display, etc.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
There is a long history of observing and investigating colors, and most researches usually focus on studying the property of colors and how to obtain them. In order to get desired colors, people have tried many methods, among them, dyes and pigments are the most commonly used. However, due to the particle size limitation of dyes and pigments, traditional printing can hardly achieve high resolution. Besides, the color gamut is hard to be expanded because it is determined by the properties of the matters that chosen to be dyes and cannot be designed arbitrarily. Structural-color, which generates as a result of the interaction of nanostructures with incident light, possess many advantages such as environmental friendship, subwavelength resolution, zero pollution, low cost, and high stability [1–10]. Structural-color metasurfaces have drawn more and more attention in recent decades. Recently, many researchers have made great efforts to improve the performance of the structural-color nanoprinting, which brings a wider color gamut, more grayscale levels, higher resolution and so on [11–18]. Further, by extending the degrees of freedom in nanostructured metasurface design, multiplexing structural-color nanoprints can be created, which further increases the information capacity of metasurfaces [19–23].
Recently, merging structural-color nanoprinting and other image display schemes, such as meta-holography [24–30], into a single metasurface becomes an emerging approach to realize information multiplexing and encrypting [31–37]. However, because there are fundamental differences in the principle of spectrum control and phase modulation, the current schemes merging nanoprinting and holography usually adopt artifices like segmenting [32], interleaving [33] or stacking schemes [33–36]. The structural colors generate from the spectral differences of the nanostructures, and the holographic images are usually reconstructed by the phase modulation of metasurfaces such as geometric phase, propagation phase and even detour phase. To produce different structural-colors, the nanostructures are usually designed to obtain narrow-band spectral responses [32,33]. Although this scheme can realize structural-color nanoprinting and color holography, it complicates both the design and fabrication of metasurfaces, and can only generate a few structural-colors of which the amount is difficult to increase. What is worse, not all the nanostructures of a metasurface contribute to the holographic image at one particular wavelength because of their narrow-band spectral responses, which results in an inevitable deterioration of image quality. A scheme of dual-mode metasurface is proposed to introduce all the nanostructures to reconstruct one holographic image by optimizing the dimensions of two types of nanobricks that have different spectrum responses while possess the same amplitude modulations at a specific wavelength [37]. Limited by the amplitude uniformity of the phase-modulated hologram, this method applying complex unit design can hardly improve the quantity of the structural colors and the quality of the structural-color nanoprinting-image.
In this paper, with the capability of complex-amplitude modulation, we design and experimentally demonstrate a dielectric metasurface with single-celled and single-layered configuration to implement both structural-color nanoprinting and holography, as shown in Fig. 1. By carefully optimizing the geometric dimensions and arrangement of the silicon nanobricks, a structural-color nanoprinting-image can be seen right at the metasurface plane under the illumination of un-polarized white light. The holographic image is formed mainly by geometric phase modulation [38], assisted by orientation variations of nanostructures. Due to the uneven amplitude modulation caused by different nanobricks, we employ the simulated annealing algorithm (SAA) to optimize the orientation profile of the nanobricks and obtain the appropriate phase distribution under the constraint of the non-uniformed amplitude to reproduce high-quality holographic image without sacrificing the performance of structural-color nanoprinting. The SAA is a random optimization algorithm based on the Monte-Carlo iterative solution strategy. It can accept new solutions with probability and probabilistically jump out of the local optimal solution, and it eventually converges to the global optimal solution [39]. Therefore, it is useful to optimize the phase distribution for the metasurface with a large number of nanobricks whose orientation angles can be continuously adjusted. Hence, our single-celled and single-layered configuration can not only relief the fabrication difficulty, but also provide an alternative scheme for information multiplexing and information hiding.
Fig. 1. Schematic illustration of the single-celled and single-layered dielectric metasurface implementing structural-color nanoprinting and holography simultaneously. Each unit-cell of the metasurface contains only one nanobrick, designed with different geometric sizes and orientation angle. When illuminated with un-polarized white light, a structural-color nanoprinting image can be observed at the metasurface plane. When circularly polarized (CP) laser light is incident, the holographic image is projected into the Fraunhofer diffraction zone.
2. Unit-cell design and simulation
The proposed metasurface made of silicon-on-sapphire (SOS) material contains two layers: the silicon nanobrick arrays and the sapphire substrate. Based on resonant mechanisms of dielectric nanostructures [40–44], the spectra of incident light can be modulated which is strongly dependent on the dimensions of nanostructures. Figure 2(a) illustrates one unit-cell structure of the metasurface. Each nanobrick has five structural parameters: height H, cell sizes C × C, length L, width W and orientation angle α. The height H of the nanobrick is fixed at 230 nm, which is determined by the SOS wafer we chose. To reduce the near-field coupling effect between adjacent nanobricks and improve the conversion efficiency, the cell sizes should be larger; however, to avoid the high diffraction orders of the meta-holograms, the cell sizes should be smaller. Here, we carefully choose the cell sizes C × C to be 400 nm × 400 nm, which are smaller than the specific working wavelength (550 nm) to extinct high diffraction orders. Next, in order to generate different structural colors, we utilize the CST Microwave Studio software to implement the numerical simulations and scan the dimensional parameters of the nanobrick to investigate its spectral response and anisotropy. Figure 2(b) shows the reflected spectra of several selected nanobricks with different dimensions under un-polarized plane wave incidence. In the simulation, the lengths of the nanobricks range from 90 nm to 200 nm and the widths range from 60 nm to 70 nm with a gap of 10 nm. As the length of the nanobrick increases, the peak of the reflectivity exhibits a red shift, which leads to the change of the color perceived by the human eyes. The colors of the selected nanobricks under the illumination of the standard light source D65 are shown in Fig. 2(c). Since the structural-color of the nanobricks is determined by their spectral response and the illumination light, the color is closely related to the light source, which will be discussed later. Meanwhile, in the principle of the geometric phase [38], when CP light passes through an anisotropic nanostructure, the output light contains the useful cross-polarized (cro-pol) part, of which the complex amplitude can be manipulated by the nanostructure. The incident CP light can be described by the Jones vector $\left[ {\begin{array}{{c}} 1\\ {\sigma i} \end{array}} \right]$, and the complex-amplitude of output light is:
Fig. 2. (a) Schematic diagram of a metasurface unit-cell based on SOS material. Each nanobrick in a unit-cell is located in a square lattice substrate. The orientation angle α is defined as the angle between the x-axis and the long axis of the nanobrick. (b) Simulated reflectivities of the nanobricks, under the normally incidence of un-polarized plane wave. (c) Simulated structural colors when illuminating the metasurface with the standard light source D65.
In Eq. (1), the amplitude of the cro-pol part is determined by $\; \textrm{abs}(\frac{{\textrm{A - B}}}{\textrm{2}}\textrm{)}$, and the phase of the cro-pol part composes of two parts: the propagation phase $\; \textrm{arg}(\frac{{\textrm{A - B}}}{\textrm{2}}\textrm{)}$ and the geometric phase 2σα. Thus, we can arrange the orientation angle of each nanobrick to precisely modulate the phase of the cro-pol CP light to reshape the wave-front and reconstruct the desired holographic image in the Fraunhofer diffraction zone.
3. Design of the single-celled metasurface for simultaneous structural-color nanoprinting and holography
Since the dielectric nanobricks with different sizes have different spectral responses, they can exhibit different colors under the illumination of white light. Besides, we can manipulate the complex amplitude of the light at a single wavelength by carefully arranging the orientation angles of all the nanobricks, and therefore reconstruct an additional holographic image in the far-field. Considering both structural-color nanoprinting and holography, Fig. 3 shows the detailed design flow chart.
Fig. 3. Detailed design flow chart of the single-celled metasurface for simultaneous structural-color nanoprinting and holography.
First of all, the electromagnetic simulation should be carried out to obtain the spectral responses of the nanobricks with the swept dimensions, and the corresponding structural colors when illuminated by the standard light source D65 can be calculated. Then we can select the appropriate sizes for the nanobrick at each position according to the color profile of the target structural-color nanoprinting image, and the nanobrick arrays with specific dimension configuration are determined. Once the dimensions of the nanobricks are chosen, the amplitude modulation can be deduced, which leads to an uneven amplitude distribution. On the other hand, the phase modulation of each nanobrick consists of both propagation phase and geometric phase: the propagation phase is a constant with fixed dimensions, while the geometric phase can be changed with different orientation angles. Hence, we can design the geometric phase distribution of the metasurface to reshape the wave-front and reconstruct the holographic image in the Fraunhofer diffraction zone. Taking the uneven amplitude distribution into consideration, we utilize SAA to optimize the desired phase distribution to reconstruct the target holographic image, and the geometric phase distribution can be obtained afterwards. Then, the orientation angle of each nanobrick is determined. Combining the geometric dimensions and orientation angle, all the structural parameters of each nanobrick are suitable for both the nanoprinting and holography. Finally, the metasurface is fabricated by the standard electron beam lithography (EBL), which has been demonstrated by many previous literatures [45–48].
4. Experiments and discussion
To prove the metasurface’s ability of displaying both holographic image and structural-color nanoprinting image, we designed and fabricated two metasurface samples labeled S1 and S2, which have the same structural-color nanoprinting pattern but different holographic images. The two designed samples have dimensions of 400 × 400 µm2 (1000 × 1000 pixels). Since the nanoprinting image appears right at the metasurface plane, an optical microscope (Motic BA310Met) was used to observe the experimental images, as shown in Fig. 4(a). When the two metasurface samples are illuminated with a white-light LED and a halogen lamp, the reflective images captured by the microscope are shown in Figs. 4(d)–4(e), 4(i)–4(j), respectively. When illuminated by white-light LED, both of the metasurfaces can display color nanoprinting images, of which the colors are bright and high-contrast. What’s more, the resolution of the structural-color nanoprinting images are up to 63,500 dpi (dots per inch) theoretically, which is hardly achieved by traditional printing. Compared with the design target image, the color nanoprinting images captured by the microscope show a slightly changed tint. On the one hand, this phenomenon arises from the differences of luminous spectra between lighting sources: during the design process the standard light source D65 is used to calculate the colors, while a white-light LED with a color temperature of 6000 K is used to illuminate the metasurfaces. Moreover, the differences between the spectral response of the CCD and the human eye also cause the disparities between the calculated results and the experimental results. On the other hand, the inevitable fabrication errors of the metasurfaces will also cause color deviation. To verify the influence of the light source on the structural-color nanoprinting images, we also use a halogen lamp to illuminate the metasurface samples and observe the images at the metasurface plane by the same microscope. Because the halogen lamp has higher spectral components in the long wave band than the LED with a color temperature of 6000 K, structural-color nanoprinting-images when illuminated the metasurfaces with the halogen lamp is generally biased towards warm colors. Due to the fact that the color is related to the spectra of the light sources, the response of the metasurface and the response characteristics of the receiver, the structural colors of the nanoprinting patterns change with the light sources, but the resolution remains at a high level. Furthermore, there is no obvious difference between the structural-color nanoprinting images of the two samples, which proves that the structural-color nanoprinting image and the holographic image can be designed independently.
Fig. 4. Experimental setups and results of the single-celled multifunctional metasurfaces for simultaneous structural-color nanoprinting in the near field and holography in the far field. (a) Sketch of the optical microscope for observing the structural-color nanoprinting-images. An objective of 10× (N.A. = 0.25) is used to capture the nanoprinting images. (b) Partial scanning electron microscope (SEM) image of the fabricated SOS metasurface. The fabricated nanobricks with different dimensions are marked by different colors. (c) Holographic experimental setup. (d-m) Experimentally captured images of the two metasurface samples. The structural-color nanoprinting-images of the sample S1 and S2 are illuminated with a white LED light (d,i) and a halogen lamp (e,j), respectively. The holographic images of the sample S1 and S2 are presented with the incident polarization states of LCP (f,k), RCP (g,l) and LP (h,m), respectively.
To display the holographic images, we employ a super-continuum laser source (YSL SC-pro) with a fixed wavelength of 550 nm to illuminate the metasurface samples and a white screen placed 150 mm away from the sample is utilized to receive the holographic image. As the results shown in Fig. 4, both the two samples can be used to reconstruct holographic images with high quality under the designed CP light incidence. When the incident laser light is RCP, there are also holographic images but in a centrosymmetric position. Since the linearly polarized (LP) light can be decomposed into a superposition of LCP and RCP light, two centrosymmetric images appear on the screen simultaneously when LP light is incident. Interestingly, the complex amplitude distributions of the two circularly polarized states are not conjugated due to the spatially dependent propagation phase, but the holograms work well for both LCP and RCP light incidence. This is mainly because the propagation phases of the designed nanobricks have a small contribution to the total phase modulation. According to our calculation, the average phase variation of the propagation phase is only 0.16π. Even though the holograms in our work are designed to work under LCP incident light, but when irradiated with RCP light, the holographic images appear in a centrosymmetric position and the image quality still remains high.
After that, we used the same laser source to investigate the wideband characteristics of the designed metasurfaces. The output wavelength of the laser source ranges from 500 nm to 600 nm with the steps of 10 nm and the experimental results of the sample S1 and S2 are shown in Fig. 5 and Fig. 6, respectively. It can be seen that if the wavelength deviates too much from the designed wavelength of 550 nm, the holographic image quality still remains high, which means the image is always clear and the background noise has been suppressed to a very low level. Furthermore, greater geometric distortion of the holographic images emerges when the wavelength deviates more, which can be readily interpreted by the diffraction theory. Besides, we can see that the variation of propagation phase has little influence on the holographic images when the wavelength ranges from 500 nm to 600 nm. The reason lies that the average phase deviation in our design is less than 0.25π in above wavelength range (more details can be seen in Appendix B). Although the propagation phase is wavelength dependent, the designed metasurfaces can work well in a broad wavelength range.
Fig. 5. Experimentally captured holographic images generated by illuminating the sample S1 with a laser source ranging from 500 nm to 600 nm in steps of 10 nm.
Fig. 6. Experimentally captured holographic images generated by illuminating the sample S2 with a laser source ranging from 500 nm to 600 nm in steps of 10 nm.
The proposed single-celled dielectric metasurface represents several distinctive advantages. Above all, the metasurface can achieve high-performance nanoprinting-image, which can hardly be affected by the holography, i.e., there is no trade-off between the quality of the nanoprinting and the holography. This means the nanoprinting can be designed completely unrestrained as long as the nanobrick is anisotropic. Secondly, the holographic image can also be freely designed since the orientation angles of nanobricks can arbitrarily varies from zero to π, leading to a continuous phase ranging from zero to 2π. The nanobricks consisting the metasurface all contribute to the holographic image, which can improve the image quality compared to the other design methods. Besides, our design approach shows good robustness and wideband characteristics. Last but not least, the fabrication of the SOS material is compatible with the modern mature semiconductor processing technology, so it is hopeful to achieve light-integrated applications.
5. Conclusion
In summary, we propose and demonstrate a single-celled dielectric metasurface that can simultaneously display both a structural-color nanoprinting image and a holographic image. In our design, the nanoprinting and the holography do not interfere with each other, which permits us to design arbitrary holographic images without affecting the function of nanoprinting via optimizing the orientation angles of the nanobricks, which greatly increases the design flexibility and the number of structural colors of the merging image displays. The experimentally demonstrated metasurface can display a vivid structural-color nanoprinting-image at the metasurface plane under the illumination of un-polarized white light. And the holographic image can be reconstructed by a CP coherent light incidence. All the experimental results are in good agreement with the theoretical designs. The proposed merging scheme provides high information density and security, which has broad prospects in information multiplexing, encryption, anti-counterfeiting, image display and many other related fields.
Appendix A: experimentally measured holographic efficiency
The holographic efficiency is defined as the ratio of holographic imaging light intensity to the incident light intensity, and the experimental measurement results are shown in Fig. 7.
Appendix B: the average phase deviation in the wavelength range of 500 nm to 600 nm
In the wavelength range of 500 nm to 600 nm, we calculate the average deviation of the propagation phase distribution from the design wavelength of 550 nm, and the results are shown in Fig. 8. It can be seen that the average phase deviation in this wavelength range is less than 0.25π, which means the designed metasurfaces can work well in a broad wavelength range.
Funding
National Natural Science Foundation of China (91950110, 11774273, 11904267); Fundamental Research Funds for the Central Universities (2042020kf1050); China Postdoctoral Science Foundation (2019M652688); Natural Science Foundation of Jiangsu Province (BK20190211); Open Fund of the Key Laboratory for Metallurgical Equipment and Control Technology of Ministry of Education in Wuhan University of Science and Technology (MECOF2020A01).
Disclosures
The authors declare no conflicts of interest.
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