Metal-Masked Mie-Resonant Full-Color Printing for Achieving Free

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Metal-masked Mie-resonant Full Color Printing for Achieving Free-space Resolution Limit Yusuke Nagasaki, Ikuto Hotta, Masafumi Suzuki, and Junichi Takahara ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00895 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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Metal-masked Mie-resonant Full Color Printing for Achieving Free-space Resolution Limit Yusuke Nagasaki†, Ikuto Hotta†, Masafumi Suzuki†, and Junichi Takahara*,†,‡ †

Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871,

Japan ‡

Photonics Center, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita,

Osaka 565-0871, Japan

ACS Paragon Plus Environment

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ABSTRACT: Mie resonance wavelengths of a dielectric structure are strongly dependent on the inherent material property and structural geometry. In particular, a high-index nanostructure enables light confinement within itself over the range of visible wavelengths, which allows the Mie resonator to be applied to a pixel in color printing of subwavelength-scale resolution. However, if the Mie resonator is packed into a smaller area in order to achieve better resolution, the interaction between adjacent resonators occurs depending on these spatial distances, leading to unexpected color changes. Here, we demonstrate metal-masked Mie-resonant color printing for suppressing undesirable color changes. We observed that the interaction between monocrystalline Si resonators can be suppressed by the addition of a Cr mask. The pixels with this functionality can produce individual colors even if operating as a single element or in other periodic arrays, resulting in the realization of higher-resolution encoding. The coincidence of resonance peak positions derived from excited electric/magnetic dipoles enables the demonstration of brilliant full color printing with higher color purity. Furthermore, a vivid printing image with a resolution of more than 100,000 dpi was achieved using the designed subwavelength pixels. This study can contribute not only to the improvement of the resolution of color printing, but also to the suppression of unwanted interactions of Mie resonance in optical devices. KEYWORDS: Mie resonance, structural color, color printing, metal-mask, metamaterial, Si nanostructure


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Over the past decade, we could precisely manipulate the nature of electromagnetic waves by using a high-index dielectric nanostructure acting as a resonator, which is the so-called “Mie resonator.”1–4 This resonator has a unique resonance property driven by the simultaneous excitation of electric and magnetic multipoles when incident light is trapped and confined inside the nanostructure.5–8 The resonance wavelengths strongly depend on the structural geometry,9,10 and the nanostructure strongly scatters only specific wavelengths. Depending on the presence of multipoles excited inside the nanostructure, the scattering direction can also be tailored.11,12 These interesting properties are observed in the periodical system of Mie resonators, which behaves as an artificial material (all-dielectric metamaterial/metasurface),13,14 facilitating the development of applicable devices, including flat lenses,15,16 color filters,17,18 and detectors.19 In particular, by utilizing the characteristic of strong reflection of specific wavelengths, Mie resonance-based color printing with high reflectance and purity has been recently demonstrated.20-27 An important property for constructing a color pixel in printing is its spatial resolution, as the number of pixels where adjacent colors can be distinguished. As the structural dimension of a Mie resonator is approximately 100 nm in the visible wavelength region, the realization of high-resolution printing can be expected. However, the presence of other closely spaced resonators leads to non-negligible resonant wavelength shifts owing to coupling with the neighboring structures,28-31 arising from the overlapping of leaky electromagnetic fields generated by the nanostructures.32 In the visible wavelength region, shrinking the gap could significantly influence the generation of an unexpected color at subwavelength resolution. The perceived color change depending on the arrangement of structures such as photonic crystals strongly limits the importance of Mie resonator color pixels. If a single pixel exhibits the same


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color as it does in an array, we can directly use the current printing design guideline that any image can be drawn by appropriately arranging the structure as long as the color of a single pixel is known. Therefore, the realization of high-resolution pixels producing an individual color without undesirable color changes is highly necessary. Our previous study25 has demonstrated the resolution of approximately 85,000 dpi (dot per inch) by using single-crystalline Si Mie resonators. However, in order to achieve 100,000 dpi, which is close to the free-space resolution limit, it is necessary to further pack the pixels closer to each other, resulting in non-negligible interaction. Weakening the interaction between the resonators enables not only dense pixel integration, but also the further development of various metasurface devices, including color filters and phase elements. In this study, we propose a metal-masked Mie resonator for weakening the coupling between the Mie resonators at subwavelength resolution. By adding a Cr mask to a monocrystalline Si nanopatch structure, the reflection peak becomes sharp and an undesirable peak shift is suppressed. The fabricated nanostructures generate various vivid colors including RGB, depending on the physical dimensions of each nanostructure. The obtained reflection spectra are quantitatively evaluated using a simulation, indicating that the colors strongly depend on the diameter of the nanostructure, whereas the peak position, even for a single Mie resonator, is hardly affected by the periodicity. By converting the obtained spectra to a color space, the feature of full color generation around the monochromatic point is revealed. The weak interaction with the neighboring structures supports the development of full-color images of resolution exceeding 100,000 dpi using the proposed pixels. Our approach for suppressing undesirable color changes would also facilitate unprecedented metasurface applications


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including propagation light manipulation, high-density nonvolatile data storage, and security certificate. Non-negligible resonant peak shifts between single and periodically arrayed nanostructures are due to the coupling of individual high-index building blocks with other closely spaced structures. Therefore, it is important to compare the spectra of the individual and periodic systems. Figure 1a shows the schematic of a designed Si nanostructure with a diameter d of 100, 150, and 200 nm and height h of 150 nm placed on a quartz substrate. The period P for the array is fixed at 250 nm. Si was selected as the dielectric part of the Mie resonator because it is a high-index material with low loss at visible wavelengths, and is capable of confining a sufficient amount of light in the nanostructure. The relative permittivity of Si was experimentally determined using an ellipsometry measurement (see Supporting Information S1), and the refractive index of quartz was fixed at 1.46. Figure 1b shows the backscattering cross-section, which is associated with the origin of the reflectance. For spectral calculations, linear polarized light is illuminated on the Si nanostructure normal to the surface (see calculation details in Supporting Information S2). There are strong scattering peaks and the peak position dramatically red-shifts as d increases. This is due to Mie resonant wavelength shifts, which are caused by various excited resonance modes in the Si nanostructure depending on the structural geometry. The distinctive spectral shape consisted of two peaks observed on the longer wavelength side of each spectrum, originating from the magnetic and electric dipoles excited in the nanostructure8. As the nanostructure is further enlarged, different peaks attributed to the high-order/multi-dipole mode excitations are observed. As reflection is related to the scattering properties, the scattering spectra should also exhibit a strong peak at similar positions. As shown in Figure 1c, the peak position in the reflection


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spectra for d = 100 nm is almost the same as that of the scattering spectra, but as d increases, the reflection peak positions blue-shift compared with the scattering peak positions and the spectral shape is changed. Crammed Si nanostructures can cause undesirable spectral shifts, leading to color changes depending on the pixel arrangement. However, metal-masked Mie resonators show a different behavior. The proposed metal-masked nanostructure, as shown in Figure 1d, is composed of Si nanopatch structures with a 30-nm-thick Cr layer on the top. Cr was chosen because it is reliable when used in combination with Si such as a victim metallic mask for Si nanofabrication. The relative permittivity of Cr was obtained from experimental data.33 As shown in Figure 1ef, a strong reflection peak at the wavelength λ = 520 nm is generated for d = 100 nm, and the peak position dramatically red-shifts as d increases, which is the same trend as Figure 1bc. Compared with the optical properties of the array without metallic masks, the shape of the peak becomes sharp—i.e., the Q-value increases. This contradicts our physical intuition that a lossy metal decreases the Q-value of a resonator. To reveal the mechanism of this spectral shape change, we performed multipole decomposition analysis using a commercially available finite element package. Supporting information S3 shows the contribution of electric and magnetic multipoles to the scattering spectra of no-mask and Cr-masked Si nanopatch structures. On the one hand, for the no-mask nanostructure, there are two main scattering peaks driven by electric and magnetic dipoles with approximately the same scattering contribution, which is the origin of the dual peak shape. On the other hand, multipoles excited in a Cr-masked nanostructure show unexpectedly interesting behavior. The peaks caused by both electric and magnetic dipoles are at the same wavelength. This peak correspondence is relevant to the phase delay at the Si/Cr interface, because the permittivity of a


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part of the external area in contact with a Si nanostructure has been changed from positive (air) to negative (metal). In addition, these peak positions can be controlled by the Cr layer thickness (see Supporting Information S4), which changes the condition of the phase retardation. This peak position correspondence of the dipoles produces a single peak shape of the backscattering and reflection. Moreover, spectral peak position shifts between backscattering and reflection hardly occur. Supporting information S5 presents the difference in peak positions of backscattering and reflection spectra with/without a Cr layer. In both cases, the peaks remain at the same positions for small d. As d increases, in the case of only Si nanostructures, undesirable peak shift reaches more than 100 nm, but by adding a Cr layer, it is possible to suppress the shift to less than half of the above value. These results suggest that the color generated from a single metal-masked Mie resonator does not change significantly, even if the nanostructures are periodically arranged owing to weak optical coupling between adjacent nanostructures. The origin of the suppression can be qualitatively understood by the field distribution at the peak. Supporting information S6 shows the electric field distribution of the Si nanopatch structure without/with a Cr mask. By attaching a Cr mask, the leakage of the electric field to the outside of the nanostructure is evidently suppressed, indicating the origin of weak coupling with adjacent nanostructures. These color generation features are capable of encoding an independent individual color into a single pixel with a subwavelength resolution not easily affected by the periodicity such as that for a photonic crystal. In addition, as apparent from the resonance field, the resonance behavior differs significantly from that of plasmonic color printing.34, 35 The real part of the dielectric constant of Cr is negative, but close to 0, and the imaginary part is larger than those of conventional metals,


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resulting in the occurrence of very weak plasmon resonances even on a thin layer in the visible light region. Metal masks can suppress undesirable peak shifts for arraying Mie resonators.

Figure 1. Optical properties of the proposed metal-masked Si nanostructure. (a) Schematic of a Si nanopatch structure on a quartz substrate. (b) Backscattering cross-section of a single Si nanopatch and (c) reflection spectra of a Si nanopatch array for d = 100, 150, 200 nm, represented by the blue, green, and red lines, respectively. (d) Schematic of a Cr-masked Si nanopatch structure. (e) Backscattering cross-section of a single Cr-masked Si nanopatch and (f) reflection spectra of a Cr-masked Si nanopatch array.


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To experimentally demonstrate metal-masked Mie-resonant color printing, Cr-covered Si nanopatch arrays were fabricated by using a monocrystalline silicon-on-quartz substrate. 35-nm-thick Cr patterns were designed and lithographically fabricated on an monocrystalline [100] Si layer with a thickness of 150 nm on a quartz substrate (Shin-Etsu Chemical Co., Ltd.). The Si layer was thereafter selectively etched with plasma SF6 and C4F8 gases in a reactive-ion etching chamber. The fabrication details are described in our previous work.25 Figure 2a shows a scanning ion microscope (SIM) image of the typical Cr-masked Si nanostructure arrays. Although reactive etching selectively removes specific material i.e., Si, the Cr layer is also damaged slightly owing to the physical sputtering. Accordingly, the film thickness of the Cr layer after the fabrication process was confirmed to be 30 nm. The surface property of the Cr mask was changed after fabrication, but it hardly affected the resonance responses (Supporting Information S7). Figure 2b shows a bright-field optical image of the fabricated nanopatch structure arrays under linearly polarized white-light illumination through a ×20 objective (numerical aperture NA of 0.45). Optical measurements were performed using a confocal reflection microscope with an objective lens connected to a charge-coupled device camera and spectroscope (Supporting Information S8). By changing d and P, distinct vivid color pixels exhibiting red, green, and blue can be obtained from each area. The color phase strongly depends on the physical geometry of the nanopatch structure, but the array period mainly affects the color saturation. Metal-masked Mie-resonant color pixels can generate various brilliant reflection colors.


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Figure 2. Experimentally observed reflection properties of metal-masked Mie-resonant color printing. (a) Oblique SIM and (b) optical microscope image of the fabricated Cr-masked Mie-resonator arrays through a ×20 objective (NA: 0.45) irradiated with linear polarized white light. The diameter d and periodicity P are changed from 70 to 250 nm in increments of 5 nm, and from 150 to 350 nm in increments of 20 nm, respectively. Each individual color area is 10 × 10 µm2. Scale bar = 20 µm. The black frames indicate unstructured regions.

To further investigate the reflection properties of the fabricated nanopatch structure arrays, the experimental reflection spectra for each nanostructure diameter were quantitatively compared with the corresponding spectra simulated using three-dimensional rigorous coupled-wave analysis calculations. Figure 3 shows the variation in the reflection spectra for different d by fixing P at 250 nm. The experimental spectra can be tuned across all visible wavelengths and good consistency is obtained between the experimental data and calculations. The reflection spectra are not dependent on the incident polarization direction, indicating that the nanostructure arrays were precisely fabricated in a cylinder shape (see Supporting information S9). Strong reflection peaks for d = 80 nm around λ = 470 nm are observed in the spectra, and there is a


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red-shift in the peak positions as d increases. These features are attributed to both electric and magnetic dipoles of Mie resonances excited inside the high-index dielectric nanostructure, as discussed above. Upon tuning the physical dimensions of each nanostructure, the Mie resonance wavelengths shift, resulting in color changes. In contrast, decreasing P from 350 nm to 150 nm results in an increase in the reflection intensities at the resonant peak (see Supporting information S10). This feature is related to the dependence of the color saturation on the periodicity. For large value of P (~ 350 nm), the color phase is slightly changed, because reflection spectra can be influenced by the diffraction, which does not occur for the small value of P. Therefore, it is necessary to carefully select the periodic conditions that are affected neither by the coupling with adjacent nanostructures nor the diffraction, in order to generate vivid colors while suppressing unwanted color changes. The minor deviation between the numerical and experimental spectra may arise from the influence of the oblique illumination through a microscope objective (see Supporting Information S11), which can cause undesired spectral shifts and reflection dips. In addition, the oxidation of the Cr mask in air (formation of film of thickness 1–2 nm at room temperature36, 37) may influence the spectra. By tailoring the structural geometry of the pixels, colors can widely be designed and controlled with subwavelength resolution.


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Figure 3. Reflection properties of the Cr-masked Si nanopatch structure arrays. Reflection spectra upon changing d at P = 250 nm. The solid black and broken red lines show the experimental and simulated spectra, respectively. Insets show optical microscope images obtained from Figure 2b with an area of 5 × 5 µm2 corresponding to the size of the nanostructure.

Cr-masked Si nanostructure arrays exhibit brilliant colors including RGB, which depend on their structure geometry. In order to quantitatively evaluate the color tunability of the designed nanostructure, the measured reflection spectra were systematically converted into the CIE1931 color space as shown in Figure 4. The conversion method used to represent perceived colors as quantitative values was described in our previous work.25 The black dots show converted values of the reflection spectra of the fabricated nanostructure arrays for P = 250 nm. Full-color variety can be achieved by using the designed nanostructure owing to the sharp reflection dips based on the peak position correspondence of excited electric/magnetic dipoles of a metal-masked Mie resonator. In the case of Si color generation with only Mie resonances, it may be difficult to generate magenta, which is a composite color of red and blue.20,21,25,27 This is because peak positions derived from the electric/magnetic dipoles of the lowest order mode are apart, resulting in a relatively broad reflection spectral shape characterized by a dual peak (see Supporting Information S3). In order to generate magenta in this broad reflection peak, it is necessary to shift the peak derived from the lowest order electric dipole to approximately 630 nm, which is the wavelength of red. However, the higher order mode originating in the magnetic resonances shifts to green color at that time, and multiple higher order modes are also excited in the blue


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region, resulting in a color close to white. With the addition of a Cr mask, when the coincided dipole peak shifts to red, the peak of higher order modes is produced around blue, resulting in the generation of magenta. By matching the peaks of the electric/magnetic dipoles, it is possible to expand the color space.

Figure 4. CIE1931 color space for demonstrating full-color tunability of the designed Cr-masked Si nanostructure arrays. Black dots represent the converted xy value from the spectra for P = 250 nm, with d varied from 70 nm to 200 nm in increments of 5 nm.

The suppression of undesirable color change is directly related to the improvement of resolution, which is one of the essential parameters of color printing. Figure 5ab shows the SIM and bright-field microscope images of checkered patterns consisting of alternating nanopatch structures of two different sizes with the diameters of 80 nm and 200 nm through a ×150 objective (NA: 0.9) in order to demonstrate the achievement of subwavelength resolution. Individual blue and magenta pixels for P = 250 nm are distinguishable while the color phase


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corresponding to the selected size is retained, indicating the production of color without color mixing even at subwavelength resolution. The suppression of undesired color changes between the pixels of a single nanostructure and periodic arrays enables the use of a Mie resonator as an “ink.” It is only necessary to know the color of a single pixel and thereafter create the image because an individual Cr-masked Si nanostructure can produce an individual color even in an array (see Supporting Information S12). Here, we introduce a picture created with the designed pixels. Figure 5c shows a microscale image of “fireworks” through a × 50 objective (NA: 0.8). The imported original image was diced into pixels and systematically converted to the diameter at which Cr-masked Si nanostructures generate the corresponding colors, and thereafter exported as a CAD file for lithography. The converted image consists of pixels of various diameters shown in Figure 2 with a diced area of 250 × 250 nm2. As the pixels preserve the color phase independent of the pixel arrangement, the displayed image exhibits brilliant colors including RGB at any part. As shown by the inset SIM image, it is evident that each color is produced by every nanostructure, which indicates that the resolution of this printing picture is more than 100,000 dpi. Color printing with subwavelength resolution can be realized by using the designed Cr-masked Si Mie resonators.


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Figure 5. Demonstration of Cr-masked Si color printing by using a subwavelength pixel. (a) SIM and (b) optical microscope images of a checkered pattern consisting of alternating nanopatch structures of two different sizes with d = 80 nm and 200 nm. (c) Optical microscope image of “fireworks” created by using Cr-masked Si nanopatch pixels. The black background indicates unstructured region. Inset shows an oblique SIM image of a part of the fabricated area.

The metal-masked Mie resonator can be designed according to the application. For example, its resonance wavelength can be controlled by changing the Si domain thickness, similar to the diameter (see Supporting Information S13). The proposed resonator can also have degrees of freedom depending on the metallic film thickness (see Supporting Information S14) and metal selection (see Supporting Information S15 and S16). By combining them, it is possible to design pixels with higher purity while weakening the periodic dependence. In addition, the interaction


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suppression can be principally applied not only to Si but also to other dielectric Mie resonators. In particular, by adding a metallic mask to nanostructures that are made of a lossless material and have strong reflection characteristics but with strong periodic dependence, such as GaN, AlAs, and TiO2 nanostructures, brighter color printing with subwavelength resolution would be realized. Recently, some researchers have demonstrated the tunable control of color generation.38,39 Combining these color activation techniques with our designed nanostructure will enable the realization of high resolution tunable color pixels, further accelerating the practical applications of Mie resonant color printing. In conclusion, we have experimentally demonstrated metal-masked Mie resonators for achieving subwavelength resolution. Through numerical analysis, we revealed that, by adding Cr masks, it was possible to suppress unwanted color changes caused by the adjacent Si nanostructures. The fabricated Cr-masked Si nanopatch structure arrays exhibited distinct brilliant colors, which were strongly dependent on the physical geometry. Quantitative evaluation of the obtained reflection spectra indicated a wide tunable color range including RGB, and vivid colors originated from the peak position coincidence with excited electric/magnetic dipoles. Subwavelength individual pixels in a checkered pattern could be distinguished without unwanted color mixing. Additionally, color printing with our designed nanostructure achieved a resolution of ~100,000 dpi, which is near the free-space diffraction limit. We believe that our approach of suppressing interaction between Mie resonators can be utilized for further extending color pixel design for subwavelength printing and other potential applications such as security certification and optical data storage. Moreover, this will benefit not only reflective devices but also transmission devices such as flat lenses and filters.


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ASSOCIATED CONTENT Supporting Information. Supporting Information is available free of charge on the ACS Publications website at DOI:xxxxxx Optical constants of Si; Calculation details; multipole contribution; dependence of Cr layer thickness; suppression of peak shift; field distribution; Cr film property; optical setup; incident polarization dependence; periodic dependence; incident angle dependence; reflection spectra of a single nanoblock; Si and Cr thickness dependence in reflection spectra; resonant property with other metal masks (PDF).

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions Y.N. and I.H. contributed equally to this work. Y.N. conceived the idea of metal-masked Si color printing and designed the structures. J.T. supervised the project. Y.N. and M.S. performed the numerical simulations. I.H. and Y.N. performed the experiments. I.H. wrote the program code of the full color image. Y.N. analyzed the experimental data and wrote the initial draft of the manuscript. All the authors discussed the results and contributed to the writing of the manuscript.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by the Photonics Center at Osaka University and JSPS Core-to-Core Program, A. Advanced Research Networks. Y.N. is supported by a Research Fellowship from the Japan Society for the Promotion of Science (JSPS) for Young Scientists. A part of this work was supported by the “Nanotechnology Platform Project (Nanotechnology Open Facilities in Osaka University)” of the Ministry of Education, Culture, Sports, Science and Technology, Japan [No. F-17-OS-0011, and S-17-OS-0011]. We would like to thank Dr. Mai Higuchi for her helpful discussion and Mr. Toshinori Khono for his helpful assistance. We also would like to thank Shin-Etsu Chemical Co., Ltd. for donating high-quality silicon-on-quartz substrates.


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Metal-Masked Mie-Resonant Full-Color Printing for Achieving Free

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