Wide-Gamut Plasmonic Color Palettes with Constant Subwavelength

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Wide-Gamut Plasmonic Color Palettes with Constant Subwavelength Resolution Soroosh Daqiqeh Rezaei, Ray Jia Hong Ng, Zhaogang Dong, Jinfa Ho, Eleen H. H. Koay, Seeram Ramakrishna, and Joel K.W. Yang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b00139 • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 9, 2019

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Wide-Gamut Plasmonic Color Palettes with Constant Subwavelength Resolution Soroosh Daqiqeh Rezaei 1, 2, Ray Jia Hong Ng 2, 3, Zhaogang Dong 2, Jinfa Ho 2, Eleen H. H. Koay 2, Seeram Ramakrishna 1, and Joel K.W. Yang 2, 3* 1

Department of Mechanical Engineering, Faculty of Engineering, National University of Singapore, 117575, Singapore 2

Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, 138634, Singapore 3

Pillar of Engineering Product Development, Singapore University of Technology and Design (SUTD), 8 Somapah Road, 487372, Singapore Abstract Unlike dye-based colorants for which dilution results in a decrease in color saturation, a reduction of nanostructure density in plasmonic prints could increase color saturation instead. This interesting observation can be explained by the absorption cross-section of the nanostructure being larger than its physical cross-section. In this paper, we demonstrate the correlation between absorption cross-section and nanostructure density, and use it to realize saturated colors by fabricating metal-insulator-metal aluminum nanostructures that support gapsurface plasmons (GSPs). We obtained structures with absorption cross-sections that exceed ten times their physical cross-sections. The large absorption cross-sections of the GSP structures heralds a color mixing scheme where nanostructures of different hues are combined within subpixels at a constant pitch. The pitch is chosen such the total absorption cross-sections of individual constituents of the cell occupies the unit size area. Using a constant pitch of 320 nm, hence preserving the print resolution, our structures exhibit 45% coverage of the sRGB color space. By employing absorption cross-sections of the nanostructures, we produced black and saturated green pixels, which have been challenging to achieve in plasmonic color printing. The effects of square and hexagonal arrangements on color saturation are investigated, and point mixing effects are observed between individual nanostructures.

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Keywords: nanoplasmonics; color printing; high-resolution; nanoantenna; electron-beam lithography

Plasmonic color generation has evolved rapidly during the last few years due to interesting properties such as fade-resistant colors and high resolution at the optical diffraction limit of light. These attractive properties have been studied for a broad range of applications, such as anticounterfeiting, high-density optical storage, and coloring consumer products.1–4 Various systems such as metal-insulator-metal (MIM),5–8 hole-array,9 all-metallic,10–12 and alldielectric structures,13–16 and different materials including silver (Ag),17–20 aluminum (Al),6– 9,18,19,21

gold (Au),5 copper (Cu),22 titanium oxide (TiO2),23 and silicon (Si)13,24–28 have been

employed to produce plasmonic and structural colors. Although very large color gamuts, even beyond the sRGB color space,13 has been achieved by dielectric nanostructures due to their high quality factor resonances, achieving highly saturated colors remains a challenge in plasmonic structures. As plasmonics allows for stronger field enhancements, they remain promising for high-resolution prints and colorimetric sensing applications. One way to circumvent the poor color coverage of metallic systems is through color mixing,2 which is also a practical approach in printing to extend colors. We have previously shown that stacking of colors can result in unintended colors due to the inter-layer coupling of the plasmonic structures.29 Instead, our planar nanostructure array modification approach in this paper allows us to achieve the intended colors. Also, unlike dye-based pigments where their dilution reduces the color saturation, plasmonic arrays can even have their color saturation increased when their density is reduced due to their absorption cross-section size.1,30 Metallic particles can possess large absorption cross-sections compared to their physical crosssections. The absorption cross-section for a 13-nm-diameter spherical Al nanoparticle is 18 times


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of its geometrical cross-section at the illumination wavelength of  = 140 nm.31 Such a large absorption cross-section facilitates light manipulation beyond the physical boundaries of a resonator. It has been reported that MIM structures that support gap-surface plasmon resonances (GSPRs) can possess absorption cross-sections that exceed their geometrical size.32 To take advantage of the large absorption cross-sections of metallic nanostructures, we use an Al-based MIM structure that supports GSPRs, which have a large absorption cross-section area of ~10 times that of the structure’s physical extent in the case of a single resonator at the wavelength corresponding to maximum absorption (for diameters less than ~125 nm). Al and aluminum oxide (Al2O3) are used for the MIM structure due to their earth abundance, CMOS compatibility of materials and Al’s stability due to the formation of self-protecting native oxide.33 In this paper, the increase in color saturation by diluting the nanostructures is explained through the correlation between absorption cross-section and plasmonic array density. Moreover, we carry out color mixing at the subpixel level to realize a wide color gamut in an Al-based MIM plasmonic color palette. This design is achieved by employing GSP resonators with large absorption cross-sections to absorb the intended wavelength of light on a pixel by sparsely arranged subpixel resonators. The pixel pitch is selected such that most of its area is covered with the absorption cross-section of the individual constituents. An areal coverage of 45% is realized over the sRGB color space using a fixed pitch of 320 nm, which is the most extensive color gamut achieved using reflective subwavelength metallic structures reported in the literature to date. Such an approach would maintain the color print’s spatial resolution throughout its entire area and facilitate the realization of additive colors including saturated green which is difficult to produce in plasmonic color printing, and even black color. Furthermore, by investigating the


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optical micrograph of individual resonators, color mixing effects between individual resonators are observed, stemming from the overlap of their absorption cross-sections. Results & discussion The schematic of the gap-surface plasmon structure used throughout this work is depicted in Figure 1a. A 30-nm-thick Al2O3 film is sandwiched between a 100-nm-thick Al back-reflector and 40-nm-thick Al disks (see Figure S1 in the supporting information for detailed fabrication process). The disk diameter d is varied from 60 nm to 150 nm and the gap size between resonators g changes from 80 nm to 270 nm. Therefore, the periodicity P = d + g ranges from 140 nm to 420 nm. By mapping out the absorption cross-section for various diameters of an individual resonator, we can identify two resonance modes. We observe that the absorption cross-section increases as the diameter is increased, as the resonator strength is linked to the volume of the disks to determine the absorbed power and scattered field. However the absorption efficiency, i.e. the absorption cross-section normalized by the size of the disk, decreases with increasing diameter (see Figure S2 in the supporting information).34 If we look at the charge density distribution of a resonator shown in Figure 1c and Figure 1d with d = 150 nm (shown with magenta and cyan triangles in Figure 1b), we can see that resonance modes in the absorption map correspond to the fundamental and third order modes of the resonance as the number of nodes formed on the inner disk surface are one and three for the fundamental and third-order mode respectively.35 The charge distribution produces a current loop between the disk and the substrate which acts as a magnetic dipole resonance with high absorption and minimal scattering compared to an electric dipole.32,36 Figure 1e shows that when the gap-surface plasmon resonator is on resonance the electric field is localized inside the dielectric gap and the incoming light energy flow is funneled


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into this gap. The electric field profile follows the pattern of the charge distribution in Figure 1c as it is maximum at the edges of the gap and there is a node in the middle of the gap. In this case, all of the light energy at the wavelength of 550 nm is absorbed by the structure which is equivalent to an area of more than 10 times of the resonator geometrical area (according to Figure S2b for disks with diameters less than ~125 nm, absorption cross-section is about 10 times the physical size of the corresponding disks). Later on, we show how this large absorption cross-section can be employed to realize color mixing. When the resonator is off resonance (Figure 1f), it can barely absorb the light energy incident on its geometrical area, and the electric field is maximized on top of the disk rather than inside the gap which shows that the gap-surface plasmon mode is not excited. These two points are shown with magenta and cyan circles in Figure 1b.


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Figure 1. Schematic and simulation of the optical characteristics of the MIM nanostructures with gap-plasmons. (a) Schematic of gap-surface plasmon periodic array illuminated with an xpolarized plane wave at normal incidence. (b) Absorption cross-section map for various disk diameters. Fundamental (solid blue line) and third-order (solid green line) gap-surface resonance modes are shown. Magenta and cyan triangles show a 150-nm diameter disk at the third-order and fundamental resonances respectively. Magenta and cyan circles show a 100-nm diameter disk when it is on and off resonance respectively. (c), (d) Charge density map for a resonator with d = 150 nm. Scale bar, 50 nm. (e), (f) E-field distribution overlaid with light power flow (Poynting vector) lines of the incoming light when the resonator is on and off resonance, respectively. Scale bar, 50 nm. Patterning disks with diameters of 60 nm to 150 nm and inter-disk gaps of 80 nm to 270 nm produce a basic color palette, as shown in Figure 2a. The colors of the bottom two rows are


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hardly visible, in particular for large pitches, as their resonance is not entirely within the visible range but rather in the ultraviolet (UV) spectrum. With increasing gap size, colors start to increase and peak in saturation. Beyond this optimal gap size, further increase causes the colors to become less saturated. Moreover, the two top rows do not have sharp colors since the fundamental resonance mode is almost outside the visible range. The experimentally obtained colors match very well with the simulated color palette (see Figure S3 in the supporting information). The arrays in the red box correspond to the pitches that have very low saturation due to coupling between the disks, and the arrays in the blue box have the highest saturation. No observable color shift can be seen in the optical micrograph of the sample with a tilt angle (see Figure S4 in the supporting information) thus demonstrating the color insensitivity to illumination and viewing angles. The resonance of the fundamental mode redshifts as the disk diameter is increased, as shown in the reflection spectra in Figure 2c for three different diameters obtained from experiments and simulations. The redshift results in a color change from yellow to cyan when the diameter is increased. As can be seen from the experimental reflection spectra, when the particle is on resonance it has a high absorption of about 90%, which results in saturated colors, and in fact as shown in the CIE 1931 plot in Figure 2d, three points of the basic color palette lie outside the sRGB color space which is hard to achieve with Al since their resonances are broad and not sharp enough, particularly when compared to Ag.37 The simulated absorption is about 100% as fabrication imperfections such as disk imperfection and surface roughness were not simulated. The reflection data are transformed into the CIE 1931 color space,38 as shown in Figure 2d. Our basic color palette has more color coverage than any other subwavelength Al color palettes in literature.


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Unlike dye-based colors where the color saturation decreases with dilution, plasmonic colors do not necessarily lose their saturation when the resonators are diluted (decreasing the fill factor).1,30 To study color saturation in our color palette, the colors are transformed into the HSB color space.39 The reason is that in the sRGB and CIE 1931 XYZ color spaces, saturation is not one of the main coordinates, but in the HSB color space, color saturation is readily defined as one of the color coordinates. HSB color space is a cylindrical representation of the sRGB color space, and it uses three parameters, namely hue (H), saturation (S), and brightness (B), to define a point corresponding to a color in the color space.39 In addition, by using the HSB color space, we can study color saturation (equivalent to color purity) and brightness in particular (higher color brightness will increase color readability under bright conditions). Figure 2e shows the experimentally measured hue, saturation, and brightness for the row of the color palette with d = 125 nm. As can be seen in the experimental data, the color saturation increases with pitch increase, and after reaching a maximum, it decreases. We can use a simple geometrical model to predict the pitch range where the saturation is maximized (the optimum pitch).30 Based on this model, the saturation depends on the percentage of the unit cell that is covered by the absorption cross-section (Figure 1b) of the resonator. According to the mentioned model, Pu highlighted in Figure 2e is the upper pitch (largest pitch possible) for which the saturation is maintained and Pl is the lowest pitch at which a high saturation can be achieved. The unit cell coverages by these two pitches are 78% and 100% respectively. Saturation is maximized between these two pitches as can be seen from the experimental saturation in Figure 2e and also by looking at the d = 125 nm row in Figure 2a. This effect is highlighted by the blue box in Figure 2a. The minimum pitch Pm, which is the pitch at which the absorption cross sections of the resonators start to interact with neighboring


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resonators, is highlighted in Figure 2e. The coupling between resonators for pitches smaller than Pm causes absorption by resonators to decrease, and saturation reaches very low values, resulting in washed out colors. This effect can be notably observed in Figure 2a for d = 125 nm where the colors for arrays with pitches smaller than 250 nm (highlighted by the red box) have lost their sharpness and look washed out. According to Figure 2e, the color brightness and hue are not as significantly affected by pitch variation as compared to saturation. Color brightness is also high which makes colors visible even under intense illumination.

Figure 2. (a) Bright-field optical micrograph of the basic color palette captured by a 20 objective lens (NA: 0.45). The gap was varied from 90 nm to 270 nm in increments of 10 nm, and disk diameter ranges from 60 nm to 150 nm in increments of 5 nm. The arrays with maximum saturation Pl < P < Pu and deteriorated saturation P < Pm are highlighted with the blue and red boxes for d = 125 nm, respectively. (b) SEM image for the array with P = 395 nm and d = 125 nm (c) experimental and simulated reflection spectra for disks with d = 80 nm, P = 280 nm, d = 100 nm, P = 330 nm, and d = 125 nm, P = 365 nm (d) CIE 1931 plot for experimental and simulated color palettes under the CIE D65 illumination condition. The black triangle shows


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the sRGB color space. (e) Experimental and simulated hue, saturation, and brightness variations for different pitches for d = 125 nm. The upper limit, lower limit, and the minimum pitches Pu, Pl, Pm are highlighted. The scale bars in (a) and (b) are 9 m, and 100 nm respectively. We next investigate how different arrangements of resonators affect the color parameters (hue, saturation, and brightness). To do so, arrays with three diameters (d = 80 nm, d = 100 nm, and d = 125 nm) corresponding to the primary subtractive colors (yellow, magenta, and cyan, respectively) were fabricated in square and hexagonal arrangements with pitches ranging from 240 nm to 440 nm with 20 nm steps, as shown in the optical micrograph of Figure 3a. SEM images of hexagonal and square arrangements are also shown. According to Figure 3a, the color saturation of hexagonal arrays with large pitches is higher compared to their square counterparts. Figure 3b depicts the reflection spectra for d = 80 nm, d = 100 nm, and d = 125 nm at P = 440 nm. Reflectance plots from hexagonal arrangements have deeper dips and larger maximum to minimum reflectance ratios which causes higher saturation for the same pitch. To obtain a quantitative understanding of color saturation, we transformed them into the HSB color space as illustrated in Figure 3c-e. As can be seen, the maximum saturation peak for the hexagonal arrangement occurs at a higher pitch compared to the square arrangement. This effect is due to higher cell coverage by the absorption cross-section for the same pitch (~91% coverage for the hexagonal arrangement and ~79% for the square arrangement). Since the pitch is proportional to the square root of the area covered by the absorption cross-section, we would expect a ~7% increase in the pitch that corresponds to the saturation maximum.30 The increase in the pitch corresponding to maximum saturation is ~7% and ~6% for d = 80, 100 nm respectively. Although for d = 125 nm, there is no increase in the pitch corresponding to maximum saturation for hexagonal arrangement, the saturation is higher for P > 360 nm compared to square arrangement.


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According to Figure 3c-e, the brightness for square arrangement is higher in all cases. The higher brightness can be attributed to the poor absorption and hence higher reflection from square arrays. The color hues for both arrangements are very close in value which highlights the fact that absorption cross-sections do not interact; consequently, the main reflection dips that dictate the color hue occur at the same wavelength for both arrangements. Another interesting effect is observed when the two arrangements were observed under high magnification (see Figure S5 in the supporting information), where the gaps between the structures are visible for the square arrangement while for hexagonal arrangement the gaps are only visible for the largest pitch. This observation shows that the hexagonal arrangement has a higher unit cell coverage by the absorption cross-section of the resonators.

Figure 3. (a) Bright-field optical micrograph for disk diameters with d = 80 nm, d = 100 nm, and d = 125 nm and pitches in the range of 240 nm to 440 nm for square and hexagonal arrangements, captured by a 20 objective lens (NA: 0.45). SEM micrographs are displayed for both arrangements. (b) Reflection spectra of d = 80 nm, d = 100 nm, and d = 125 nm at constant pitch of 440 nm for square and hexagonal arrangements (c-e) Hue, saturation, and brightness of disks with d = 80 nm, d = 100 nm, and d = 125 nm for various pitches and arrangements.


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We have shown that by covering the whole or a considerable area of a unit cell area by total absorption cross-sections of individual constituents, the color saturation is maximized.30 Here we use the same concept to realize color mixing, in particular, to create black color. Black color has been previously achieved by incorporating differently-sized disks supporting GSPRs with very high absorption in visible.40 Here, By placing multiple elements in one unit-cell, we perform subpixel mixing as illustrated in Figure 4a, such that the surface area of each unit cell is covered with the absorption cross-section of all the constituent elements. Such a design guarantees high color saturation and minimum hue shift due to coupling effect as previously shown in Figure 2. Consequently, a color mixing procedure may be used which simplifies color mixing in plasmonic printing. Figure 4a shows the optical micrograph of a printed image with different subpixel arrangements. Only three disk sizes (yellow: d = 80 nm, magenta: d = 100 nm, cyan: d = 120 nm) at a constant pitch of 320 nm (spatial resolution of ~80,000 dots per inch (d.p.i.)) are used throughout this plasmonic print. Red, green, and blue colors are the result of mixing using two different disk sizes within a unit cell scheme as shown in SEM images and schematics. Black is realized by using four disks in one unit cell. The smallest disk (yellow: d = 80 nm) is repeated twice so that its absorption cross-section covers the whole unit cell. By looking at the experimental reflection spectra of the primary subtractive colors (yellow: d = 80 nm, magenta: d = 100 nm, cyan: d = 120 nm) in Figure 4b and the colors resulted from mixing (red, green, blue, and black) in Figure 4c one notes that the mixed colors are simply the superpositions of the primary subtractive colors with a small blue-shift in dip position. It is well-known that the performance of gap-surface plasmons is reduced in dense arrays due to near-field coupling,41 but interestingly, by properly choosing the pitch through employing the correlation between absorption cross-section and cell size, broadband absorption that is sufficient to produce black is


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achieved. Figure 4d-e shows that in the black pixel, at the resonance wavelengths of the primary additive colors (blue, green, red), light is funneled only into the disk which is on resonance. Since the pitch is chosen such that the absorption cross-sections from individual constituents cover the whole pixel, it can be seen that almost all of the energy in the visible spectrum is redirected toward the gap which results in broadband absorption, producing the black color. Another advantage of using such an approach is that a constant resolution is used throughout the print (constant pitch) without the need for a change in pixel size that changes the resolution.

Figure 4. Demonstration of subpixel color mixing. (a) Bright-field optical micrograph of a plasmonic microprint at the constant pitch of P = 320 nm through 50 objective (NA: 0.8) lens and various subpixel arrangements for color mixing. The SEM image and schematics show the subpixel arrangement. Only three disk diameters (yellow: d = 80 nm, magenta: d = 100 nm, cyan: d = 120 nm) are used in this print. (b-c) Experimental reflection spectra from primary subtractive colors (yellow, magenta, cyan) and mixing colors (red, green, blue, black) (d-f) Efield distributions at different planes of the black pixel illuminated with an x-polarized plane wave at normal incident at  = 450 nm,  = 530 nm, and  = 575 nm overlaid with Poynting vectors. The planes in the black pixel at which the E-field is plotted are shown in the insets. Scale bar, 50 nm.


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Although primary additive colors (red, green, and blue) and black were realized by only using primary subtractive colors (yellow, magenta, and cyan), we are not limited to these three diameters for mixing. If we perform the mixing by employing various disk sizes ranging from 60 nm to 150 nm at a constant pitch of 320 nm, the colors shown in Figure 5a can be achieved. The schematic of the subpixel mixing and SEM micrograph for a typical array are also displayed in the insets. As shown in the CIE plot in Figure 5b, a considerable color coverage that even exceeds the sRGB at five points and has a coverage of 45% of the sRGB color space was achieved, which is considerable for subwavelength reflective structures fabricated with Al and even Ag and Au. The color palette covers a considerable color space on the green area, and it includes very bright and saturated green hues which are proven to be difficult to generate by plasmonic color printing and surpasses other works.2,4,7,8,17 Reflection spectra for three primary additive colors (red, green, and blue) as highlighted in the extended palette in Figure 5a are shown in Figure 5c. As can be seen, all distinct pairs of reflection spectra have one dip at almost the same position. That is because they have one disk with the same diameter (d = 75 nm and d = 130 nm) as can be observed in Figure 5a. It can be noticed that by changing the size of one disk in the pixel the dip position for the other disk remains almost the same with a small shift. (see Figure S6 in the supporting information) In addition, very high absorption at the dip is achieved (more than 92%) which improves the color saturation. As can be observed in Figure 5a red, green, and blue appear to be more saturated than these colors in Figure 4a. This color deterioration is caused by electron beam lithography proximity effects induced by the resonators with considerably different structural densities (black has four times higher structural density than yellow, magenta, and cyan, and also two times higher density than green, red, and blue) in Figure 4a compared to the uniform densities in each array of Figure 5a. To see if the color


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mixing can be observed between individual pixels, the layouts in Figure 5d were fabricated. As highlighted by the arrows in Figure 5d there is subtle color mixing taking place in the central ring of the structures (reddish color between yellow and magenta; greenish color between cyan and yellow; bluish color between cyan and magenta) although we are limited by the diffraction limit of light at such a small size. By looking at the optical micrograph and comparing it to SEM images one can observe that in optical micrograph the disks seem larger which is again due to their large absorption cross-section which exceeds their geometrical surface.

Figure 5. (a) Bright-field optical micrograph of the mixing palette for various diameters at a constant pitch of 320 nm. The schematic of the subpixel mixing and SEM micrographs are shown as well. The most saturated red (r), green (g), and blue (b) arrays are highlighted. (b) CIE 1931 plot of the mixing (extended) palette colors under the CIE D65 illumination condition. sRGB color space is highlighted in the plot. (c) Reflection spectra for the red, green, and blue arrays as highlighted by the dashed box in the extended palette (d) Optical and SEM micrographs of the structures used to demonstrate color mixing between individual pixels. The color mixing effect is highlighted by arrows of the same mixed color. The optical image is captured through a 150 objective (NA: 0.9) lens. Scale bars in (d) are 500 nm.


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Conclusions We have experimentally shown how decreasing the areal density of nanostructures can result in the non-intuitive increase in color saturation. There is a range of optimum pitches that maximizes the saturation, and it can be determined from the absorption cross-sections of individual structures. The effect of square and hexagonal arrangements on color saturation was analyzed, and we found that that the maximum saturation at a higher pitch can be achieved in the hexagonal arrangement compared to the square arrangement. By performing subpixel mixing and choosing the pixel size such that the whole pixel is covered by the absorption cross-section of the individual constituents, we demonstrated how primary additive colors (red, green, blue) and black can be achieved by changing the disk diameters inside each unit cell without any change in pitch. Furthermore, we analyzed the light energy flow in such unit cells and showed how each resonator can absorb the light energy from over almost the entire unit cell area when it is at resonance. Finally, by changing disk diameters in the subpixel arrangement over a wide range, (from 60 nm to 150 nm) we realized a wide color gamut covering 45% of the sRGB color space at a constant pitch. Color mixing effect between single GSP resonators was also observed which again emphasizes how an individual resonator can manipulate light beyond its physical boundary. The approach presented here can be used to create saturated colors along with the use of abundant materials such as Al. Methods Nanofabrication of Al gap-surface plasmon structures. The periodic Al gap-surface plasmon sample was fabricated using electron beam lithography (EBL), and the fabrication process is shown in Figure S1a-b in the supporting information. Figure S1a shows the sample profile before EBL. A 100-nm-thick Al layer was first deposited by


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using an electron-beam evaporator (Denton Explorer, 5×10-7 torr, evaporation rate of 0.5 Å/s) onto a Si substrate. The thickness and evaporation rates were measured in situ with a quartz balance. An Al2O3 film with a thickness of 27 nm was then evaporated onto the Al layer by atomic layer deposition (Beneq TFS 200) with a chamber temperature of 200 °C and a base pressure of 0.1 Torr prior to precursor injection. Trimethylaluminum was utilized as the precursor while deionized water acted as the oxygen source. A linear growth rate was obtained with an average deposition rate of 0.098 nm/cycle. A Si sample was placed along the samples in the reaction chamber for subsequent film thickness evaluation by ellipsometry. Next, polymethyl methacrylate (PMMA) resist (950k molecular weight, 3.3% wt. in anisole) was spin coated onto Al-coated Si substrate at a spin speed of 3k revolutions-per-minute (rpm) to give a ~100-nmthick PMMA layer. The PMMA resist was then baked at 180 oC for 120 seconds to remove residual stress and solvent. After that, the sample was exposed by EBL (Elionix ELS-7000, electron acceleration voltage of 100 keV). Then, the sample was developed as described in Ref. 40. Evaporation of 40 nm Al was carried out using an electron-beam evaporator (pressure of 5x10-5 Pa, and an evaporation rate of 0.5 Å/s), followed by a lift-off process (N-methyl-2pyrrolidone at 150 oC).

Ellipsometry, Microscopy and Spectroscopy. Ellipsometry was employed to measure thicknesses of deposited films. A Woollam wvase32 variable angle spectroscopic ellipsometer was utilized to collect the spectra across three different angles. Film thicknesses were obtained by fitting the data. Film growth was obtained from measurements on the aforementioned Si witness sample. The optical reflectance spectrum of the fabricated Aluminum nanostructures was measured using a CRAIC UV-VIS-NIR micro-spectrophotometer QDI 2010 (×36 objective lens with a numerical


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aperture of 0.5), with a polarized broadband light source. The absolute reflectance spectra were obtained, since reflectance measurements were calibrated with respect to a certified standard calibration sample from CRAIC Technologies.42 Moreover, for the image acquisition of all optical micrographs Figure 2a, Figure 3a, Figure 4a, Figure 5a, and Figure 5d a white color balancing was first carried out on a 200-nm-thick aluminum film as evaporated onto silicon substrate by e-beam evaporation (Denton evaporator, 5×10-7 torr, evaporation rate of 2 Å/s). Next, the optical microscope images were taken using an Olympus microscope (MX61) with the software “analySIS”, a ×10 objective lens (MPlanFL N, NA=0.3), a camera (Olympus SC30) with an integration time of 20 ms, broadband halogen light source (U LH100 3, 100 Watts) with a linear polarizer (U-AN360-3). The optical microscope image in Figure 5d was captured by a ×150 objective lens. The scanning electron microscope (SEM) images were taken with an electron acceleration voltage of 5 keV with a JSM-7600F Schottky Field Emission Scanning Electron Microscope from JEOL.

Numerical Calculations. Throughout this paper, the results for the absorption cross-sections, reflection spectra, Poynting vectors, charge density distributions, and electric field distributions were calculated using the LUMERICAL FDTD software package.43 To calculate the absorption cross-section of an individual resonator, the total field scattered field (TFSF) analysis was used. For periodic array simulations, a standard plane wave was employed with periodic boundary conditions along the xand y-axes. Due to structural symmetry, symmetry conditions were used to reduce the computational time. Perfectly matched layers (PML) were used along the propagation direction at the top and bottom boundaries. The source spans the wavelength range of 380 nm to 800 nm. The permittivity data of aluminum and aluminum oxide were obtained from Palik.44


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Associated Content The authors declare no competing financial interest. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. (S1) Fabrication process of the periodic Al gap-surface plasmon arrays. (S2) FDTD Absorption cross-section and absorption efficiency maps of individual gap-surface plasmon structures. (S3) Simulated and experimental basic color palettes of the gap-surface plasmon structures. (S4) Influence of the substrate tilt angle on basic color palette. (S5) Square vs. hexagonal arrangements of gap-surface plasmon arrays. (S6) Influence of the subpixel disk diameters on the reflection spectra of the arrays in extended color palette. Author Information Corresponding Authors *E-mail: [email protected] ORCID Soroosh Daqiqeh Rezaei: 0000-0002-9807-2074 Zhaogang Dong: 0000-0002-0929-7723 Jinfa Ho: 0000-0001-6884-4785 Seeram Ramakrishna: 0000-0001-8479-8686 Joel K.W. Yang: 0000-0003-3301-1040 Author Contributions S.D.R. Z.D. J.H., and J.K.W.Y. conceived the concept. S.D.R. and J.K.W.Y. wrote the manuscript. Z.D., R.N., S.D.R., and J.H. performed the EBL writing. S.D.R. deposited aluminum oxide films using ALD. S.D.R., R.N., and E.H.H.K. deposited aluminum using EBE. S.D.R.


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performed the SEM and optical imaging and spectroscopic measurements of the samples. S.D.R. performed the FDTD simulations and the calculation of the CIE color coordinate. All authors analyzed the data, read and corrected the manuscript before the submission. Acknowledgments S.D.R acknowledges the financial support provided by Agency for Science, Technology and Research (A*STAR) through Singapore International Graduate Award (SINGA) program. We would like to acknowledge the funding support from Agency for Science, Technology and Research (A*STAR) SERC Pharos project (Grant Number 1527300025), A*STAR Young Investigatorship (Grant Number 0926030138), SERC (Grant Number 092154099), National Research Foundation Grant Award No. NRF-CRP001-021, NRF-CRP 8-2011-07, and A*STARJCO under Project Number 1437C00135. References (1) (2) (3) (4) (5) (6) (7) (8) (9)

Kumar, K.; Duan, H.; Hegde, R. S.; Koh, S. C. W.; Wei, J. N.; Yang, J. K. W. Printing Colour at the Optical Diffraction Limit. Nat. Nanotechnol. 2012, 7, 557–561. Tan, S. J.; Zhang, L.; Zhu, D.; Goh, X. M.; Wang, Y. M.; Kumar, K.; Qiu, C.-W.; Yang, J. K. W. Plasmonic Color Palettes for Photorealistic Printing with Aluminum Nanostructures. Nano Lett. 2014, 14, 4023–4029. Goh, X. M.; Zheng, Y.; Tan, S. J.; Zhang, L.; Kumar, K.; Qiu, C. W.; Yang, J. K. W. Three-Dimensional Plasmonic Stereoscopic Prints in Full Colour. Nat. Commun. 2015, 5, 5361. Kristensen, A.; Yang, J. K. W.; Bozhevolnyi, S. I.; Link, S.; Nordlander, P.; Halas, N. J.; Mortensen, N. A. Plasmonic Colour Generation. Nat. Rev. Mater. Nature Publishing Group January 22, 2016, p 16088. Roberts, A. S.; Pors, A.; Albrektsen, O.; Bozhevolnyi, S. I. Subwavelength Plasmonic Color Printing Protected for Ambient Use. Nano Lett. 2014, 14, 783–787. Clausen, J. S.; Højlund-Nielsen, E.; Christiansen, A. B.; Yazdi, S.; Grajower, M.; Taha, H.; Levy, U.; Kristensen, A.; Mortensen, N. A. Plasmonic Metasurfaces for Coloration of Plastic Consumer Products. Nano Lett. 2014, 14, 4499–4504. James, T. D.; Mulvaney, P.; Roberts, A. The Plasmonic Pixel: Large Area, Wide Gamut Color Reproduction Using Aluminum Nanostructures. Nano Lett. 2016, 16, 3817–3823. Miyata, M.; Hatada, H.; Takahara, J. Full-Color Subwavelength Printing with GapPlasmonic Optical Antennas. Nano Lett. 2016, 16, 3166–3172. Li, Z.; Clark, A. W.; Cooper, J. M. Dual Color Plasmonic Pixels Create a Polarization


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(10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21)

(22) (23) (24) (25) (26)

Controlled Nano Color Palette. ACS Nano 2016, 10, 492–498. Ng, R. J. H.; Goh, X. M.; Yang, J. K. W. All-Metal Nanostructured Substrates as Subtractive Color Reflectors with Near-Perfect Absorptance. Opt. Express 2015, 23, 32597–32605. Goh, X. M.; Ng, R. J. H.; Wang, S.; Tan, S. J.; Yang, J. K. W. Comparative Study of Plasmonic Colors from All-Metal Structures of Posts and Pits. ACS Photonics 2016, 3, 1000–1009. Li, Z.; Wang, W.; Rosenmann, D.; Czaplewski, D. A.; Yang, X.; Gao, J. All-Metal Structural Color Printing Based on Aluminum Plasmonic Metasurfaces. Opt. Express 2016, 24, 20472. Dong, Z.; Ho, J.; Yu, Y. F.; Fu, Y. H.; Paniagua-Dominguez, R.; Wang, S.; Kuznetsov, A. I.; Yang, J. K. W. Printing beyond SRGB Color Gamut by Mimicking Silicon Nanostructures in Free-Space. Nano Lett. 2017, 17, 7620–7628. Nagasaki, Y.; Suzuki, M.; Takahara, J. All-Dielectric Dual-Color Pixel with Subwavelength Resolution. Nano Lett. 2017, 17, 7500–7506. Vashistha, V.; Vaidya, G.; Hegde, R. S.; Serebryannikov, A. E.; Bonod, N.; Krawczyk, M. All-Dielectric Metasurfaces Based on Cross-Shaped Resonators for Color Pixels with Extended Gamut. ACS Photonics 2017, 4, 1076–1082. Wood, T.; Naffouti, M.; Berthelot, J.; David, T.; Claude, J. B.; Métayer, L.; Delobbe, A.; Favre, L.; Ronda, A.; Berbezier, I.; Bonod, N.; Abbarchi, M. All-Dielectric Color Filters Using SiGe-Based Mie Resonator Arrays. ACS Photonics 2017, 4, 873–883. Wang, H.; Wang, X.; Yan, C.; Zhao, H.; Zhang, J.; Santschi, C.; Martin, O. J. F. Full Color Generation Using Silver Tandem Nanodisks. ACS Nano 2017, 11, 4419–4427. Li, Z.; Butun, S.; Aydin, K. Large-Area, Lithography-Free Super Absorbers and Color Filters at Visible Frequencies Using Ultrathin Metallic Films. ACS Photonics 2015, 2, 183–188 Duempelmann, L.; Luu-Dinh, A.; Gallinet, B.; Novotny, L. Four-Fold Color Filter Based on Plasmonic Phase Retarder. ACS Photonics 2016, 3, 190–196. Zhang, Y.; Zhang, Q.; Ouyang, X.; Lei, D. Y.; Zhang, A. P.; Tam, H. Y. Ultrafast LightControlled Growth of Silver Nanoparticles for Direct Plasmonic Color Printing. ACS Nano 2018, 12, 9913–9921. Lee, Y.; Park, M. K.; Kim, S.; Shin, J. H.; Moon, C.; Hwang, J. Y.; Choi, J. C.; Park, H.; Kim, H. R.; Jang, J. E. Electrical Broad Tuning of Plasmonic Color Filter Employing an Asymmetric-Lattice Nanohole Array of Metasurface Controlled by Polarization Rotator. ACS Photonics 2017, 4, 1954–1966. Xiong, K.; Tordera, D.; Emilsson, G.; Olsson, O.; Linderhed, U.; Jonsson, M. P.; Dahlin, A. B. Switchable Plasmonic Metasurfaces with High Chromaticity Containing Only Abundant Metals. Nano Lett. 2017, 17, 7033–7039. Sun, S.; Zhou, Z.; Zhang, C.; Gao, Y.; Duan, Z.; Xiao, S.; Song, Q. All-Dielectric FullColor Printing with TiO2 Metasurfaces. ACS Nano 2017, 11, 4445–4452. Flauraud, V.; Reyes, M.; Paniagua-Domínguez, R.; Kuznetsov, A. I.; Brugger, J. Silicon Nanostructures for Bright Field Full Color Prints. ACS Photonics 2017, 4, 1913–1919. Nagasaki, Y.; Suzuki, M.; Hotta, I.; Takahara, J. Control of Si-Based All-Dielectric Printing Color through Oxidation. ACS Photonics 2018, 5, 1460–1466. Li, S. Q.; Song, W.; Ye, M.; Crozier, K. B. Generalized Method of Images and Reflective Color Generation from Ultrathin Multipole Resonators. ACS Photonics 2018, 5, 2374–


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(27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44)

Page 22 of 23

2383. Nagasaki, Y.; Hotta, I.; Suzuki, M.; Takahara, J. Metal-Masked Mie-Resonant Full-Color Printing for Achieving Free-Space Resolution Limit. ACS Photonics 2018, 5, 3849–3855. Shen, Y.; Rinnerbauer, V.; Wang, I.; Stelmakh, V.; Joannopoulos, J. D.; Soljačić, M. Structural Colors from Fano Resonances. ACS Photonics 2015, 2, 27–32. Jalali, M.; Yu, Y.; Xu, K.; Ng, R. J. H.; Dong, Z.; Wang, L.; Safari Dinachali, S.; Hong, M.; Yang, J. K. W. Stacking of Colors in Exfoliable Plasmonic Superlattices. Nanoscale 2016, 8, 18228–18234. Rezaei, S. D.; Ho, J.; Ng, R. J. H.; Ramakrishna, S.; Yang, J. K. W. On the Correlation of Absorption Cross-Section with Plasmonic Color Generation. Opt. Express 2017, 25, 27652. Paul, H.; Fischer, R. Comment on ‘“How Can a Particle Absorb More than the Light Incident on It?”’ Am. J. Phys. 1983, 51, 323–327. Pors, A.; Bozhevolnyi, S. I. Plasmonic Metasurfaces for Efficient Phase Control in Reflection. Opt. Express 2013, 21, 27438. Langhammer, C.; Schwind, M.; Kasemo, B.; Zoric, I. Localized Surface Plasmon Resonances in Aluminum Nanodisks. Nano Lett. 2008, 8, 1461–1471. Craig, F. B.; Bohren, F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; John Wiley & Sons, 1983. Harutyunyan, H.; Martinson, A. B. F.; Rosenmann, D.; Khorashad, L. K.; Besteiro, L. V.; Govorov, A. O.; Wiederrecht, G. P. Anomalous Ultrafast Dynamics of Hot Plasmonic Electrons in Nanostructures with Hot Spots. Nat. Nanotechnol. 2015, 10, 770–774. Bozhevolnyi, S. I.; Søndergaard, T. General Properties of Slow-Plasmon Resonant Nanostructures: Nano-Antennas and Resonators. Opt. Express 2007, 15, 10869. Knight, M. W.; King, N. S.; Liu, L.; Everitt, H. O.; Nordlander, P.; Halas, N. J. Aluminum for Plasmonics. ACS Nano 2014, 8, 834–840. Smith, T.; Guild, J. The C.I.E. Colorimetric Standards and Their Use. Transactions of the Optical Society. IOP Publishing January 1, 1931, pp 73–134. Shapiro, L.; Stockman, G. C. Computer Vision. 2001. ed Prentice Hall 2001. Nielsen, M. G.; Pors, A.; Albrektsen, O.; Bozhevolnyi, S. I. Efficient Absorption of Visible Radiation by Gap Plasmon Resonators. Opt. Express 2012, 20, 13311. Deshpande, R.; Zenin, V. A.; Ding, F.; Mortensen, N. A.; Bozhevolnyi, S. I. Direct Characterization of Near-Field Coupling in Gap Plasmon-Based Metasurfaces. Nano Lett. 2018, 18, 6265–6270. Microspectra. http://www.microspectra.com (accessed Dec 01, 2018). Lumerical. https://www.lumerical.com (accessed Dec 01, 2018). Palik, E. D. Handbook of Optical Constants of Solids; Academic press, 1998; Vol. 3.

For Table of Contents Use Only


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Title: Wide-Gamut Plasmonic Color Palettes with Constant Subwavelength Resolution Authors: Soroosh Daqiqeh Rezaei, Ray Jia Hong Ng, Zhaogang Dong, Jinfa Ho, Eleen H. H. Koay, Seeram Ramakrishna, and Joel K.W. Yang

The table of contents graphic depicts the subpixel mixing by placing multiple elements in one unit-cell. The pitch is chosen such that each unit cell area is covered with the absorption crosssection of its constituent elements. Only three disk sizes (yellow: d = 80 nm, magenta: d = 100 nm, cyan: d = 120 nm) at a constant pitch of 320 nm are used throughout the right-side plasmonic print to produce seven colors. By such an approach, color coverage of plasmonic colors is enhanced as shown on the CIE color space and broadband absorption can be realized to achieve black color which is challenging to produce. Another advantage is that spatial resolution remains constant throughout the plasmonic print by using constant pitch unit cells.


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Wide-Gamut Plasmonic Color Palettes with Constant Subwavelength

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