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Full-color Subwavelength Printing with Gap-plasmonic Optical Antennas Masashi Miyata, Hideaki Hatada, and Junichi Takahara Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b00500 • Publication Date (Web): 18 Apr 2016 Downloaded from http://pubs.acs.org on April 21, 2016

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Full-color Subwavelength Printing with Gapplasmonic Optical Antennas Masashi Miyata,†,§ Hideaki Hatada,†,§ and Junichi Takahara*,†,‡ †

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

Japan ‡

Photonics Advanced Research Center, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-

0871, Japan KEYWORDS: Color printing, structural color, optical antenna, gap plasmon, standing-wave resonance

ACS Paragon Plus Environment

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ABSTRACT: Metallic nanostructures can be designed to effectively reflect different colors at deepsubwavelength scales. Such color manipulation is attractive for applications such as subwavelength color printing; however, challenges remain in creating saturated colors with a general and intuitive design rule. Here, we propose a simple design approach based on allaluminum gap-plasmonic nanoantennas, which is capable of designing colors using knowledge of the optical properties of the individual antennas. We demonstrate that the individual-antenna properties that feature strong light absorption at two distinct frequencies can be encoded into a single subwavelength-pixel, enabling the creation of saturated colors, as well as a dark color in reflection, at the optical diffraction limit. The suitability of the designed color pixels for subwavelength printing applications is demonstrated by showing microscopic letters in color, the incident polarization and angle insensitivity, and color durability. Coupled with the low cost and long-term stability of aluminum, the proposed design strategy could be useful in creating microscale images for security purposes, high-density optical data storage, and nanoscale optical elements.


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Metallic nanostructures that involve collective electronic resonances (known as surface plasmon resonances) offer the unique optical property of the manipulation of light absorption and/or scattering beyond the optical diffraction limit.1,2 This unprecedented ability provides a new pathway to generate structural colors at deep-subwavelength scales.3 They have thus served as the foundation for micro/nanoscale color applications such as transmission color filters,4-8 reflective color printing,9-16 and scattering-based color displays.17 One of the most conspicuous applications empowered by such “plasmonic colors” is subwavelength color printing based on metallic nanostructures. This printing technology has the advantage of individual color generation at a diffraction-limited pixel (a square of 250~300 nm in the visible light range). This feature can dramatically improve printing resolutions from ~1000 dots per inch (dpi) with modern pigment-based printing to ~100,000 dpi with plasmonic printing.9 In addition, plasmonic color prints composed of ultrathin metallic structures (that are < ~100 nm in thickness) can potentially reduce the fabrication and materials cost of many printing applications. Ultrathin structures naturally also bring an increased mechanical flexibility that is required in a variety of printing technologies. Plasmonic color printing with subwavelength resolutions could therefore be useful in creating microscale images for security purposes, high-density optical data storage, and nanoscale optical elements. However, plasmonic color printing technology retains significant challenges that need to be addressed for practical use. For example, a general rule for color design of a single subwavelength-pixel has not yet been established. This can be explained by the fact that most studies on plasmonic color printing have relied on optical resonant properties that greatly depend on the periodicity or spacing of plasmonic building blocks.9,10,12-16 Although color generation at microscales has been successfully demonstrated with such design approaches, they cannot easily


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be transferred to the color design of an individual subwavelength-pixel. Additionally, the color variety of aluminum-based plasmonic prints is still greatly limited. Aluminum is currently the most promising material for plasmonic colors because of its lower cost and higher stability over an extended period of time, compared with other plasmonic materials such as silver and gold.12,17 However, the low Q-factor plasmon resonances arising from the higher loss of aluminum18 severely reduce the purity of the colors,12,13,16 resulting in a limited color variety in plasmonic printing. Thus, in the case of aluminum color printing, a new color design approach to compensate for the low Q-factor is highly desirable. The creation of a black color (near-unity light absorption over a broadband spectrum), which is important for achieving full-color printing, is challenging. In addition to these issues, simple constituent structures suitable for low-cost and large-area fabrication techniques, incident polarization and angle insensitivity of the color theme, and color durability for ambient use are necessary for practical utilization. Here, we propose and demonstrate a simple and general color design strategy that is capable of addressing these practical issues. This is accomplished by employing all-aluminum metal−insulator−metal (MIM) plasmonic nanoantennas as individual color elements (Fig. 1a). It has been previously reported that gold MIM nanoantennas can be used to generate colors in subwavelength printing applications;11 however, it is still challenging to create full colors on a single subwavelength-pixel with an intuitive design rule. In this study, we further develop this approach by highly utilizing our knowledge of the optical properties of the individual aluminum nanoantennas. The proposed strategy relies on the fact that such nanostructures exhibit strong standing-wave resonances in which light is effectively confined into a metallic nanogap.19-23 We start by exploring how the individual plasmonic antennas behave within their array structures with a subwavelength periodicity. We demonstrate that the optical resonant properties of the


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antenna periodic arrays directly reflect those of the individual antennas. This implies that the optical interactions between closely spaced building blocks are very weak, which is a completely different feature from that of periodicity-based plasmonic structures such as metallic-holearrays.5,6,14,16 Such behavior allows us to encode a color into a single subwavelength-pixel using knowledge of the resonant properties of the individual antennas. Furthermore, near-unity optical absorption within a single pixel can be achieved at two distinct wavelengths that can be tuned across the entire visible spectrum. This spectral feature leads to the appearance of a single spectral peak between two deep resonant dips in reflection, contributing to the creation of various primary colors with high color purity and saturation, in spite of the use of aluminum structures. A dark color can also be designed by mixing the building blocks within a unit cell. We illustrate the usefulness of the proposed strategy for subwavelength printing applications by imaging microscopic letters in full-color, as well as their feasibility for practical use. Figure 1a schematically shows the proposed color pixel element consisting of a 40-nm-thick aluminum nanodisk placed on top of a 30-nm-thick aluminum oxide (Al2O3)-coated aluminum film. To begin with, we use full-field simulations to analyze the optical resonant properties of the individual color-elements (not periodic). The simulations were performed with three-dimensional finite element method calculations (COMSOL Multiphysics). In the simulations, we used the dielectric constant of aluminum obtained by linear interpolation of the experimental data.24 Further, we used a disk structure with square corners as a calculation model in order to save the calculation time. Figure 1b shows a simulated absorption cross-section map quantifying the predicted resonances as a function of both the wavelength and the nanodisk diameter. The two red−orange bands in the map show that strong light absorption occurs at two different wavelengths that feature red-shifts with increasing disk diameters. The electric field distributions


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at the peak wavelengths of 370 nm (A) and 815 nm (B) (Fig. 1c) provide insight into this light absorption. For the 815 nm wavelength, there is one node in the field in the metallic gap, indicative of the excitation of the first-order standing-wave resonance for surface plasmons.19-23 Similarly, for the 370 nm wavelength, there are three nodes in the field, showing the excitation of the third-order standing-wave resonance.19,21,23 For such resonances, the resonant condition can be determined by the net cavity length for the standing-wave, that is, the disk diameter in our case. The optical absorption properties of the nanodisks can thereby be tuned by engineering the nanodisk size.

It is important to note that the absorption cross-sections σabs of both resonance modes can exceed their geometrical cross-section σdisk (for example, σdisk = 0.314 × 105 nm2 with a 200-nmdiameter nanodisk) and they are as large as a single subwavelength-pixel-area σpixel (σpixel = 0.9 × 105 nm2 with a 300 nm-pitch pixel). This fact indicates that in theory the individual antenna can simultaneously show near-unity light absorption at two distinct wavelengths within a single subwavelength-pixel. This can give rise to a single spectral peak in reflection, effectively leading to the generation of saturated colors at an individual pixel. Also from the field maps, it is evident that the nanodisks have the dominant fraction of the optical field internal to their metallic gap on resonance. This feature suggests that, when making color prints with nanodisk arrays, the optical interactions between closely spaced nanodisks can be very weak, and each nanodisk can individually act as a single color-element without periodicity dependence. Color design of subwavelength pixels can therefore be possible by using knowledge of the optical properties of the individual building blocks.


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To experimentally explore how the individual nanodisks behave within their array structures with a subwavelength periodicity, we fabricated aluminum nanodisk arrays with systematic variations of their diameter d and periodicity P (Fig. 2a). Aluminum nanodisks with a thickness of 40 nm were patterned on aluminum oxide (30 nm thick) coated aluminum film (100 nm thick) using standard electron-beam lithography, thin-film deposition, and lift-off processes as detailed in Supporting Information S1. Scanning electron microscopy (SEM) images of a small area of the fabricated nanodisk array with d = 80 nm and P = 300 nm are shown in Fig. 2b. We characterized the nanodisk arrays using a confocal reflection microscope coupled to a CCD color camera and a spectrometer (see Supporting Information S2).

Figure 2c presents a bright-field image of the nanodisk arrays under unpolarized white light illumination, showing various brilliant colors including red, green, and blue (RGB). From these full-color arrays, it becomes clear that (i) the color phase is strongly dependent on the nanodisk diameters, and (ii) that the color saturation gradually varies as the periodicity changes. Figures 3a and 3b respectively show optical images and their corresponding reflection spectra for the 300nm-period row. In these spectra, we observed a single peak between two deep dips at distinct wavelengths that can be tuned across the visible wavelength range by varying the nanodisk diameter (see blue and green triangles). It is also evident that low reflection (< 25 %) is obtained for the two dips, indicative of near-unity light absorption. These measurements are in qualitative agreement with the rigorous coupled wave analysis (RCWA) simulations for the arrays shown in Fig. 3b. It is worth noting that small spectral dips are observed between the two deep dips in the experiments. These small dips can be ascribed to the influence of oblique light incidence supported by an objective. In Supporting Information S3, we show the dependence of the optical properties on the incident angle. We find that the oblique incidence can potentially give rise to


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additional spectral dips because of the grating-assisted excitation of surface plasmons13 at the aluminum oxide−aluminum interfaces, as well as the excitation of the second-order resonance in the gap. Similarly, structural variations in fabrication (the fabricated nanodisks are a slightly elliptical shape, see Fig. 2b), as well as the native oxide layer of aluminum,25 may affect the spectral shape and cause subtle differences between the experimental and simulated spectra. In order to better understand how the resonant optical properties of the nanodisk array emerge from those of the individual nanodisks, we generated a two-dimensional reflectance map as a function of both the wavelength and the diameter, while keeping the period at P = 300 nm (Fig. 4a). The reflectance of the array was calculated using RCWA simulations similar to those in Figure 3b. The black dual bands show that near-unity absorption at two distinct wavelengths can be achieved across the whole range of visible wavelengths (400–800 nm) by varying the disk diameter. These strong absorption bands closely track the simulated absorption peaks of the individual nanodisks shown by the dashed white lines, which feature a red-shift with increasing nanodisk diameters. Above a diameter of 250 nm, the reflectance dip of the array diverges and weakens. For these diameters, plasmonic coupling between neighboring nanodisks would be pronounced because inter-nanodisk spacing is less than 100 nm at an array period of 300 nm. Such coupling can give rise to the resonance mode splitting,26,27 potentially leading to the divergence of the spectral dip observed in the reflectance map. Similarly, minor differences between the absorption peaks for individual nanodisks and the minimum in the reflection spectra for the high order mode may be ascribed to the appearance of weak interactions between neighboring nanodisks in the arrays. The blue dots and green diamonds present the experimentally obtained minimum points in reflection for different diameters with a 300 nm period. They show good agreement with simulations, meaning that there is a large range of


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nanodisk sizes for which the resonant optical properties of the individual antennas are preserved in the optical properties of their array with a subwavelength period. To further analyze the link between the resonance properties of the constituent antennas and the arrays, we explored the periodic dependence of the reflectance properties of the nanodisk arrays. The measurements show that period variation only influences the brightness of the reflected light, while keeping the reflection dip wavelengths identical to the absorption peaks of the individual antennas (see Supporting Information S4). This again implies that the resonance properties of the individual antennas are preserved in their array structures. The period will affect the fraction of the surface area covered by the absorption cross-section of the individual antenna, indicative of the color saturation change from varying the period. Therefore, subwavelengthpitch pixels that reflect saturated colors can be achieved when the absorption cross-section matches the single-pixel area. In our case, plasmonic pixels consisting of 300 nm × 300 nm squares (P = 300 nm) would be the best choice to simultaneously achieve brilliant color generation and high-resolutions up to the optical diffraction limit. To experimentally verify that a single nanodisk can act as an individual color-element, we considered the checkerboard structure with alternating colors shown in Fig. 4b. These figures show an SEM image of the checkerboard, consisting of 100-nm-diameter and 180-nm-diameter nanodisks (top), and the corresponding optical image obtained through a × 150 and 0.9 NA objective (bottom). The array period is 300 nm, which corresponds to half the wavelength for visible light, assuming 600 nm as the mid-spectrum wavelength. The checkerboard pattern is clearly observable in the optical image, while preserving the color scheme seen in Fig. 3a, showing that each nanodisk can reflect the individual colors even in an array with a subwavelength period. Furthermore, considering that a diffraction-limited optical microscope


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was used for imaging, the observable pattern demonstrates that each nanodisk can support color generation at the optical diffraction limit. These results show that simple and intuitive color design of the single subwavelength-pixel can be achieved using knowledge of the optical properties of the individual antennas, highlighting the usefulness of our strategy. The color variety capability of nanostructure-design is critical for subwavelength color printing applications. To quantify the colors generated by the proposed approach, we converted the measured reflection spectra for P = 300 nm, 340 nm, and 380 nm into the CIE1931 color space, which is commonly used to quantify perceived colors. The results are summarized in Fig. 4c. It is important to note that there are points all around the achromatic point, illustrating the large degree of color tuning possible with our approach (in Supporting Information S5, we present a CIE color map with all plots measured from Fig. 2c, showing the range of colors with this approach). These color variations, including RGB colors, can be achieved by varying only the nanodisk diameter, with a constant periodicity. This is a great advantage for primary color generation at a constant resolution of the diffraction limit. Further, the plot circle around the achromatic point becomes smaller with increasing period, indicating the color saturation decrease. This change occurs while roughly maintaining the color phase (see the evolution of the CIE color plots highlighted by the black dashed lines in Fig. 4c). This implies that the color saturation (in a microscale pixel-array) can be determined by the array period, while the color phase can be tailored with the resonant wavelengths governed by the nanodisk diameter. In a single subwavelength-pixel, it can be expected that near-unity light absorption occurs at target wavelengths, at least within the area of the absorption cross section. Thus, when the absorption cross section matches the pixel area, it will be possible to generate a color with the color phase and saturation similar to those of P = 300 nm in Fig. 4c even in a single subwavelength-pixel.


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In addition to primary colors such as RGB, the creation of a black color at the diffraction limit is highly desirable for full-color printing. Achieving strong light absorption over a broad spectral range (that is, black in reflection) on a metallic film has recently been demonstrated by M. G. Nielsen et al.,28 in which the contribution of multiple plasmonic antennas and the intrinsic losses of a gold film leads to broadband light absorption over the visible wavelengths. More recently, S. J. Kim et al. have demonstrated broadband absorption using differently sized Ge optical antennas on a gold film.29 In such studies, individual optical antennas with distinct resonance frequencies are closely placed on a substrate to achieve strong light absorption over the whole visible range. Drawing inspiration from these studies, we designed black subwavelength pixels by mixing differently sized gap-plasmonic antennas within a pixel. For broadband near-unity absorption, it is highly desirable to choose antennas that feature spectrally non-overlapping resonances. It is also necessary that each resonance has an absorption cross-section that can cover a single pixel area. Figure 5a shows an SEM image of optimized pixels that achieve these requirements. A unit cell consists of two 80-nm-diameter nanodisks, a 100-nm-diameter nanodisk, and a 120-nm-diameter nanodisk in a 300 nm × 300 nm square. According to Fig. 1b, each antenna can exhibit near-unity absorption at distinct resonance frequencies, covering a large part of the visible spectrum. An optical image of this pixel array is shown in Figure 5b, clearly demonstrating the creation of a dark color. Figure 5c presents the measured reflection spectrum for the dark color pixels under unpolarized light illumination, showing qualitative agreement with the numerical simulations. Minor differences in the spectral shapes are possibly due to the shape distortion of the fabricated nanodisks as well as small discrepancies between the experimental and simulated nanodisk size. From the spectrum, it is clear that the pixel array exhibits three observable spectral dips, indicating that the resonant


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properties of each individual building block can be preserved even though they are closely spaced. It is also clear that strong absorption (> 70%) can be achieved over a large bandwidth (~ 220 nm) with this approach. We also demonstrated that the spectral reflection properties of dark color pixels can be tuned by varying the diameters of the constituent disks (see Supporting Information S6). To experimentally demonstrate that the dark color pixel is also capable of realizing subwavelength resolutions, we fabricated a checkerboard pattern with alternating colors (dark color and white) similar to those in Fig. 4b. Figure 5d shows an SEM image (left) and an optical image (right) of the checkerboard consisting of the dark color pixels and the pixels without antennas. The pixel size is a 300 nm × 300 nm square, matching that of Fig. 4b. As shown in Fig. 5d, the pattern with alternating dark color and white was resolved by an optical microscope with a × 150 and 0.9 NA objective, showing that a single pixel can support a dark color at the diffraction limit. To demonstrate the creation of subwavelength color prints, we now show a microscopic color image with a resolution of the diffraction limit. We prepared colored printed letters of “Nano” with dark color and colored pixels (violet: d = 100 nm, blue: d = 120 nm, green: d = 140 nm), with single pixels of 300 nm × 300 nm square. The fabricated letters are displayed in Fig. 6. Remarkably, the letters of “Nano” are accurately reproduced in the optical image with bright and saturated colors (Fig. 6a). Each color can be encoded into an individual pixel without neighboring pixel-elements (Fig. 6b,c), again showing that a single nanodisk can act as a single color element. It is noteworthy that each color pixel well reproduces the color phase and saturation of a color palette in Fig. 3a. This fact enables us to intuitively design the color prints with the color palette govern by the resonant properties of the individual antennas and it thus


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provides a high degree of design flexibility to create arbitrary full-color subwavelength images or letters. Finally, we investigated the feasibility of the proposed approach for use in practical color printing applications. To this end, we first explored the incident polarization sensitivity of the color pixels. In Supporting Information S7, we demonstrate that, under normal incidence, our nanodisk structures exhibit almost identical performance for all incident polarization directions. This characteristic would be useful for color generation with randomly polarized light sources such as LEDs and sunlight. In addition, a strong dependence on incident polarization can emerge by reshaping the antenna from a circle to an ellipse (Supporting Information S8). This could lead to the realization of stereoscopic color prints30 and high-density optical data storage. Further, the simulations of the color pixel array show that the reflection properties are generally insensitive to the incident angle, which is important for practical use (Supporting Information S3). This angle insensitivity is consistent with the fact that the color phases obtained by different NA objectives show no significant differences (see Fig. 3a and Fig. 6a). The color stability and mechanical durability of the pixels are especially important for use in ambient conditions. As our structures are composed from aluminum, one can expect their color stability over an extended time period.12 In fact, the color pixel arrays (shown in Fig. 2c) exhibited no signs of color deterioration under ambient conditions for at least 3 months, demonstrating good color stability for ambient use. Furthermore, we coated the pixel array with an overlying transparent dielectric for mechanical and chemical protection, and investigated its impact on the reflection properties of the pixels. The measurements show that the reflection dips are red-shifted by the presence of the dielectric cover (Supporting Information S9). This


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indicates that our structures can endure the dielectric protection cover by redesigning the nanodisk diameter specifically for use with dielectric coverage. An additional appealing feature of the developed approach is its ease of transfer to low-cost and large-area fabrication methods, such as nanoimprint lithography. Indeed, similar nanodisk arrays have previously been fabricated via such high-throughput methods.31,32 Constituent structures composed of ultrathin aluminum/aluminum oxide layers will also contribute to the reduction of fabrication and material costs, highlighting their suitability for use in plasmonic printing. In summary, we have demonstrated an intuitive and general design approach based on individual gap-plasmonic antennas to create full-color printed images and letters with a resolution of the diffraction limit of light. Individual antenna properties that exhibit strong light absorption at two distinct frequencies could be encoded into single subwavelength-pixels, enabling saturated color generation in reflection at the diffraction limit. A black color could also be created by mixing differently sized antennas into a single pixel to achieve strong absorption over the whole visible light range. The suitability of these color pixels for subwavelength printing applications was demonstrated by showing microscopic letters in color, the incident polarization and angle insensitivity, and color durability. We believe that the proposed design strategy provides a general platform for the practical utilization of subwavelength-scale plasmonic colors.


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FIGURES

Figure 1. Optical properties of an individual aluminum nanodisk placed on a metallic back reflector. (a) Schematic of the top view (left) and the cross-section (right) of an aluminum nanodisk placed on an aluminum oxide-coated aluminum film. (b) Simulated absorption map of individual aluminum nanodisks (not periodic) with different nanodisk diameters. (c) Electric field distributions near the nanodisk (d = 160 nm) illuminated at wavelengths of 370 nm (left) and 815 nm (right). The electric field is normalized to the incident field |Eo|.


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Figure 2. Full-color generation on resonant aluminum nanodisk arrays. (a) Schematic of the top view of aluminum nanodisks placed on an aluminum oxide-coated aluminum film. (b) Scanning electron microscopy (SEM) images of a fabricated nanodisk array with a diameter of 80 nm and a period of 300 nm. Scale bar, 200 nm. (c) Optical image of the nanodisk arrays with varying diameters d from 80 to 280nm in 10 nm increments, and periods P from 200 to 400nm in 20 nm increments. The image was obtained with a × 20 objective (NA = 0.45) under unpolarized white light illumination.


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Figure 3. Spectral reflection properties of resonant nanodisk arrays with a period of 300 nm. (a) Optical images of the arrays with varying nanodisk diameters. Scale bar, 5 µm. (b) Experimental (solid lines) and simulated reflection spectra (dashed lines) of the arrays with different nanodisk diameters. The blue and green triangles indicate the resonant dips.


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Figure 4. Color design using the optical properties of individual nanodisks. (a) Simulated reflection map for nanodisk arrays with different disk diameters at a constant period of 300 nm. Overlaid white lines present the wavelength of maximum absorption for individual nanodisks. The blue dots and green diamonds are the experimentally measured minimum reflection wavelengths. (b) SEM (top) and optical images (bottom) of a checkerboard pattern. The pattern consists of 100-nm-diameter and 180-nm-diameter nanodisks. The optical image was obtained with a ×150 and 0.9 NA objective. (c) CIE 1931 chromaticity coordinate for the measured reflection spectra of 300-nm-period, 340-nm-period, and 380-nm-period nanodisk arrays. The CIE illuminant D65 was used for the conversion to the chromaticity coordinate. Black dashed lines present evolution of CIE color plots (d = 80 nm and d =140 nm) with an array period.


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Figure 5. Design of dark color pixels. (a) SEM image of a fabricated dark color pixel array constructed from three differently sized nanodisks of 80 nm, 100 nm, and 120 nm. Each single pixel consists of a 300 nm × 300 nm square. (b) Optical image of the dark color pixels. The image was obtained in the same way as in Fig. 2c. (c) Experimental (solid lines) and simulated spectra (dashed lines) of the dark color pixel array. The green dots indicate the resonant dips. (d) SEM (left) and optical images (right) of a checkerboard resolution test pattern consisting of the dark color pixels and the pixels lacking the nanodisks. The optical image was obtained in the same way as in Fig. 4b.


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Figure 6. Subwavelength color print of “Nano”. (a) Optical micrograph of the “Nano” letters in color. The patterns are composed of dark color and colored pixels (violet: d = 100 nm, blue: d = 120 nm, green: d = 140 nm). Each single pixel is a 300 nm × 300 nm square. The images were obtained through an objective of ×150 and 0.9 NA. Scale bar, 2 µm. (b, c) SEM images of enlarged regions of “N” with the dark color pixels (b) and “a” with the violet pixels (c). Scale bar, 500 nm.


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ASSOCIATED CONTENT Supporting Information. Fabrication details, experimental setup details, angle-, periodicity-, and polarization-dependence of reflectance properties, color range in a CIE 1931 color space, design of dark color pixels, reflectance properties on ellipse-shaped nanodisks, and impact of a dielectric protection cover. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions M.M. and H.H. conceived the idea of creating full-color subwavelength prints with optical antennas. J.T. supervised the project. M.M. and H.H. performed the numerical simulations. H.H. fabricated the samples. M.M. and H.H. performed the optical characterization of the samples. M.M. wrote the initial draft of the manuscript. All the authors analyzed and discussed the results and contributed to the writing of the manuscript. §These authors contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT One of the authors (M. Miyata) is supported by Research Fellowships of Japan Society for the Promotion of Science (JSPS) for Young Scientists. A part of this work was supported by


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“Nanotechnology Platform Project (Nanotechnology Open Facilities in Osaka University)” of Ministry of Education, Culture, Sports, Science and Technology, Japan [No.: F-15-OS-0015]. We also thank Yusuke Nagasaki and Akira Kaijima for helpful discussions.


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Nanoimprint


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Table of Contents Graphic


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Full-Color Subwavelength Printing with Gap ... - ACS Publications

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