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Abstract
Structural color printing based on plasmonic metasurfaces has been recognized as a promising alternative to the conventional dye colorants, though the color brightness and polarization tolerance are still a great challenge for practical applications. In this work, we report a novel plasmonic metasurface for subtractive color printing employing the ultrathin hexagonal nanodisk-nanohole hybrid structure arrays. Through both the experimental and numerical investigations, the subtractive color thus generated taking advantages of extraordinary low transmission (ELT) exhibits high brightness, polarization independence and wide color tunability by varying key geometrical parameters. In addition, other regular patterns including square, pentagonal and circular shapes are also surveyed, and reveal a high color brightness, wide gamut and polarization independence as well. These results indicate that the demonstrated plasmonic metasurface has various potential applications in high-definition displays, high-density optical data storage, imaging and filtering technologies.
© 2017 Optical Society of America
1. Introduction
Color printing technology has great significance in digital imaging, information storage, cryptography, as well as product-branding applications [1–4]. Compared with conventional colors consisting of chemical pigments or dyes, structural colors featured by plasmonic metasurfaces have been widely studied owing to their distinct merits, e.g. durability, radiation resistance, and high resolution [5]. Therefore, color printing based on subwavelength-scale structures becomes an important research direction in recent years.
Since the discovery of extraordinary optical transmission (EOT) through periodic subwavelength hole arrays perforated in a metal film in 1998 [6], many kinds of plasmonic structural colors have been demonstrated, such as periodic metallic subwavelength hole arrays [7–9], plasmonic nanoparticles [10–14], metal-insulator-metal (MIM) structures [15], nanohole with nanodisk structures [16], and Fabry-Perot (FP) cavity structures [17–19]. Besides the largely explored EOT phenomenon based on optically thick metal films, another interesting phenomenon known as extraordinary low transmission (ELT) through an ultrathin nanopatterned metal film was also proposed theoretically [20, 21]. Compared with the EOT-based additive color, the subtractive color taking advantages of ELT are found to be a beneficial complement due to the higher optical transmission efficiency, which could generate brighter colors. For color printing and displays, color brightness and polarization independence are necessarily required. D. Inoue et al. presented three kinds of structural colors with periodic subwavelength hole arrays of circular, square and triangular shapes [7], but the brightness of these colors was dim due to their low optical efficiency. B. Zeng et al. proposed a kind of one-dimensional ultrathin metal nanograting with a high transmission [22], but it was sensitive to the light polarization. X. M. Goh et al. proposed a three-dimensional stereoscopic color microprinting method, composed of isolated nanoellipses or nanosquare dimers, it was also polarization-sensitive [23].
A novel plasmonic metasurface for subtractive color printing based on ultrathin hexagonal nanodisk-nanohole hybrid structure arrays is proposed in this research. Through both the experimental and numerical investigations, the subtractive color generated by this metasurface reveals a high brightness with the polarization independence. Moreover, an illustrative palette of subtractive colors is obtained by changing the key geometrical parameters, including the three primary component colors of cyan, magenta, and yellow (CMY). As a proof-of-concept demonstration, a color printing image of the university logo generated by the suggested structure arrays is presented. Additionally, other regular patterns including square, pentagonal and circular shapes are also surveyed, and reveal a high color brightness, wide gamut and polarization independence as well. The plasmonic metasurface as proposed would provide various potential applications in highly integrated optoelectronic devices, such as high-resolution color filters and displays.
2. Design and fabrication
The proposed plasmonic metasurfaces are transmissive triangular-lattice hexagonal nanodisk-nanohole hybrid structure arrays on quartz, as shown in Fig. 1. The metallic plasmonic structures composed of 25 nm Ag were directly evaporated onto the 100 nm HSQ pillars with 1 nm Cr as the adhesion layer. The light source is illuminated along the + z-axis. The triangular-lattice array has been proved to reveal a higher color saturation compared with the square lattice [24].
Fig. 1 Illustrations of the hexagonal nanodisk-nanohole hybrid structure array on quartz, (a) an overall view and (b) the cross section of one nanopillar
Figure. 2 shows the schematics of the fabrication process for the nanostructures as suggested. A hydrogen silsesquioxane (HSQ, XR-1541-006, Dow Corning) resist film with the thickness of 100 nm was spin-coated onto the quartz substrate (as shown in Fig. 2(a-i)). The HSQ nanopillar templates were exposured by the Raith-150two electron-beam lithography systems with an accelerating voltage of 30 kV and a beam current of 310 pA. Then, the sample was developed by the salty developer (1% NaOH + 4% NaCl) for 1 min and then rinsed in deionized (DI) water for 1 min, and finally, blow-dried by N2 stream (as shown in Fig. 2(a-ii)). An adhesion layer of Cr (1 nm thick) and an Ag film (25 nm thick) were deposited by an electron-beam evaporator system (Lab-line, Kurt J Lesker) (as shown in Fig. 2(a-iii)). Figure. 2(b) shows the SEM images of the ultimately achieved hexagonal nanodisk-nanohole hybrid structure array.
Fig. 2 (a) The schematics of the fabrication process for the designed nanostructures. (b) SEM images of the nanostructure arrays with the period of 410 nm and side length of 130 nm. The inset gives an enlarged view. The scale bars are 2 μm (left) and 200 nm (right), respectively.
3. Results and discussions
3.1 Wide gamut and high brightness
The subtractive color generated by hexagonal nanodisk-nanohole hybrid structure arrays is due to the ELT phenomenon. Similar to its EOT counterpart, the wavelength of transmission valley λmin depends on the periodicity and configuration of the array. For the triangular-lattice array, the valley position λmin of the transmission spectrum at the normal incidence can be approximated by [25]
where εd and εm(λ) are the permittivities of the dielectric and metal layers, respectively, and i, j are integers signifying the order of resonance.Based on the above equation, the palette of experimentally transmitted subtractive colors displayed in Fig. 3(a) is obtained by changing the period P and side length a of the hexagonal nanodisk-nanohole hybrid structure arrays. Corresponding positions of these colors are plotted in the CIE 1931 color space, as shown in Fig. 3(b). The dots distribute as a semicircle, confirming the capability for achieving the main colors ranging from cyan to magenta to yellow, i.e. the CMY color system. The drawbacks such as uneven color shown in some squares of the color palette are caused by the manufacturing deficiency. For example, as the inset in Fig. 3(a), during the evaporation process, the metal material cannot attach to the substrate at some special places, which has been denoted in the red circle. The transmission of the arrays was characterized by a microscopic system (Olympus-BX51) with the visible wavelength ranging from 400 nm to 800 nm using a NOVA-EX spectrometer. A halogen lamp was used as the white light source. The transmission signal was collected by an objective lens (Zeiss Epiplan, NA = 0.9, 100x). Figure. 3(c) presents the transmission spectra of the experimental samples, with the period varying from 120 nm to 360 nm and the duty cycle fixed at sqrt(3)/6. The valleys have a redshift from 437 nm to 635 nm, and the transmission maxima of all spectra are around 60%, which demonstrates a high color brightness.
Fig. 3 (a) The color palette of experimentally transmitted subtractive colors is revealed, with a square size of 10 μm in the array under the unpolarized white light illumination, as the period changing from 110 nm to 410 nm in a 10 nm increment and the side length changing from 40 nm to 130 nm also in a 10 nm increment. The inset gives the enlarged SEM image of the nanostructure arrays with the period of 300 nm and side length of 100 nm. (b) CIE1931 chromaticity diagram overlaid with the points corresponding to the colors in (a). Experimental (c) and simulated (d) transmission spectra of the structure arrays with different geometrical parameters. For example, ‘80-240’ means a = 80 nm, P = 240 nm. (e) Comparison of transmission valley positions obtained by simulation (red circle) and experiment (green triangle). (f) Contour map of the experimental transmission spectra as a function of the incident wavelength and period. The white dots refer to the valleys’ positions (λmin). The white solid line refers to the fitted straight line with the corresponding valleys.
Moreover, for the same structures, the simulated transmission spectra shown in Fig. 3(d) are in qualitative agreement with the corresponding experimental results, where valleys red-shift with the increasing P and a. Figure. 3(e) plots the transmission valleys’ locations of the simulated (red circles) and experimental (green triangles) results, which exhibits an appreciable consistency although a little difference still exists. The difference is mainly caused by the following reasons: (1) dimensional errors from nanofabrication; (2) non-uniform HSQ photoresist thickness; (3) surface roughness of Ag film; (4) Ag oxidization. The contour map of the experimental transmission spectra is plotted in Fig. 3(f). The white dots refer to the transmission valleys, respectively, and the fitted straight line shows that the spectral dependence on the period, which is in accordance with the prediction by Eq. (1).
3.2 Polarization independence
With the finite-difference time-domain (FDTD) simulation of hexagonal nanodisk-nanohole hybrid structure arrays, polarization independence of the transmission has been observed. A broadband plane wave covering the wavelength of 400-800 nm was normally incident along the + z-axis. Periodic boundary conditions were applied in the x and y directions. Perfect matched layer (PML) boundary conditions were applied in the z direction. The mesh size was set to 2 nm in all directions [26], as shown in Fig. 4(a). In this research, the polarization angle is defined as the angle between the polarization direction and + x-axis. Figure. 4(b) shows the spectral results of the light transmission as the polarization degree varying from 0 to 90 degrees. As the polarization angle increases, the transmission spectra are nearly unchanged. Thus, it reveals that the transmission through hexagonal nanodisk-nanohole hybrid structure arrays presents a strong polarization independence.
Fig. 4 (a) FDTD simulation model of the hexagonal nanodisk-nanohole hybrid structure array. (b) Simulated spectral results of the light transmission varying with different polarization degrees for the structure array with a = 80 nm, P = 240 nm. For clarity, the spectral curves are shifted in the transmission axis.
3.3 Physical mechanism
The counterintuitive ELT phenomenon can be observed through an ultrathin nanopatterned metal film, when its film thickness is comparable to or below the skin depth. In order to analyze the underlying physical mechanism, the electric-field intensity distributions were calculated by using the FDTD numerical simulations. Figure. 5 presents the simulation results at transmission valleys of the hexagonal (a) nanodisk, (b) nanohole, (c) nanodisk-nanohole hybrid structure arrays with the side length a = 100 nm and the period P = 300 nm. For nanodisks in Fig. 5(a), we see the strong electric-field distribution around the disks. For nanoholes in Fig. 5(b), the electric-field intensity is confined at the quartz/metal and metal/air interfaces and edges of the nanoholes. As shown in Fig. 5(c), nanodisk-nanohole hybrid structure is the combination of nanoholes and nanodisks, but the electric-field energy is mainly confined at the nanohole part. The electric-field distribution in the film means that the translucent Ag film can be penetrated by the incident light. Strong confinement of light at the edges of nanoholes and nanodisks is obvious, which is corresponding to the localized surface plasmon resonance (LSPR) mode. The electric-field intensity on the surface of Ag film demonstrates the excitation of the short-range surface plasmon polariton (SRSPP). Accordingly, the resonant electromagnetic modes in the ultrathin Ag nanostructures have the properties of hybrid LSPR and SRSPP modes. Excitation of SRSPP and LSPR in ultrathin Ag hexagonal nanodisk-nanohole hybrid structure arrays causes the enhanced absorption and reflection that affect the transmission valleys. Thus, both LSPR and SRSPP modes contribute to the ELT effect for the designed structures.
Fig. 5 Cross sections (y = 0) showing the simulated electric intensity distribution illuminated by the wavelengths of transmission valleys for the hexagonal (a) nanodisk, (b) nanohole, (c) nanodisk-nanohole hybrid structure arrays with side a = 100 nm, and P = 300 nm. The white dashed lines are the boundaries of the structures. The structural illustration is at the bottom right corner of each figure.
3.4 Optical performance research on other regular patterns
Regular patterns including square, pentagonal and circular nanodisk-nanohole hybird structure arrays are also surveyed, and reveal a high color brightness, wide gamut and polarization independence as well, as shown in Fig. 6. A series of square, pentagonal and circular nanodisk-nanohole hybird structure arrays were fabricated by adopting the same fabrication process as given in Fig. 2(a). Figures. 6(a1)-(c1) present the SEM images as fabricated. Figures. 6(a2)-(c2) depict the experimental spectra and structural colors of the arrays. For the three kinds of structures, with the period varying from 180 nm to 390 nm and the duty cycle fixed at sqrt(3)/6, the valleys all have a redshift. The transmission maxima are all around 60%. In addition, they all exhibit a wide color gamut. Figures. 6(a3)-(c3) have demonstrated that, the transmission spectra are nearly unchanged with increased polarization angle. Thus, these three kinds of structures also possess a polarization independence. These properties are all similar with those possessed by hexagonal nanodisk-nanohole hybrid structure arrays.
Fig. 6 SEM images and experimental spectra of (a1,a2) square, (b1,b2) pentagonal and (c1,c2) circular nanodisk-nanohole hybrid structure arrays, respectively. The colors on the right side of (a2)-(c2) are corresponding to the spectra from top to bottom, respectively. Simulated spectral results for (a3) square, (b3) pentagonal, and (c3) circular nanodisk-nanohole hybrid structure arrays all with the side length a = 80 nm (for circular, a is radius) and period P = 240 nm of the light transmission varying with different polarization degrees. For clarity, the spectral curves are shifted in the transmission axis. The scale bars in (a1)-(c1) are all 200 nm.
Figure 7 shows the experimental spectral results for square, pentagonal, hexagonal, circular nanodisk-nanohole hybird structure arrays in the case of the same transmissive area and structural period. The results demonstrate that accompanied by the increasing number of edges, the transmission valleys have a blueshift and cause the light transmittance increase.
Fig. 7 Experimental spectral results demonstrate the transmission for square, pentagonal, hexagonal, circular nanodisk-nanohole hybrid structure arrays in the case of the same tranmissive area and structural period.
3.5 Application for color printing
The Northwestern Polytechnical University (NPU) logo with the size of 400 μm × 400 μm was printed by the hexagonal nanodisk-nanohole hybrid structure arrays. Figure. 8(c) depicts its transmission-mode optical microscope image, illuminated by the unpolarized white light. Due to the size effect [27], the same structure in these two white dashed circles in Fig. 8(c) displays two different kinds of colors (the bigger one is orange, the other one is yellow). Figures. 8(a) and 8(b) show the SEM images of the regions outlined in Fig. 8(c). Bright and distinct colors are observed, even at the sites of sharp corners and edges of the logo, which reveals that this printing scheme has a high resolution. As shown in Fig. 8(a), the end of a local region with only one unit (period of 330 nm, side length of 110 nm) shows blue color clearly. Therefore, the estimated dots per inch (DPI) can be close to 77,000. The vividly colorful pattern also indicates that this printing scheme can be readily applied to the high-resolution color display.
Fig. 8 Colorful image of the NPU university logo with the size of 400 μm × 400 μm printed by the hexagonal nanodisk-nanohole hybrid structure arrays. (a,b) SEM images of the regions outlined in (c). The scale bars in (a)-(c) are 1 μm, 2 μm, 100 μm, respectively. (Logo printed with the permission from Northwestern Polytechnical University. Copyright 2017 Northwestern Polytechnical University.)
4. Conclusion
In summary, the subtractive color generated by the plasmonic metasuface based on ultrathin hexagonal nanodisk-nanohole hybrid structure arrays exhibits the high brightness, polarization independence and wide gamut. With this printing scheme, vividly colorful image has been acquired. Bright and distinct colors observed even at the sites of sharp corners and edges of the patterns reveal the high resolution of this printing scheme. In addition, other regular patterns including square, pentagonal, and circular shapes are also surveyed, and reveal a high color brightness, wide gamut and polarization independence as well. The plasmonic metasurface as proposed will be highly attractive for diverse applications, covering high-definition display, high-resolution color printing, high-density optical data storage, cryptography, as well as product-branding applications.
5. Funding
National Natural Science Foundation of China (No. 51622509, No. 51375400); The Specific Project for the National Excellent Doctorial Dissertations (No. 201430); The Fundamental Research Funds for the Central Universities (No. 3102017jg02007); The 111 Project (No. B13044).
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