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Scalable Inkjet-Based Structural Color Printing by Molding Transparent Gratings on Multilayer Nanostructured Surfaces Hao Jiang, and Bozena Kaminska ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b08580 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

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Scalable Inkjet-Based Structural Color Printing by Molding Transparent Gratings on Multilayer Nanostructured Surfaces

Hao Jiang∗ and Bozena Kaminska School of Engineering Science, Simon Fraser University, Burnaby, British Columbia, V5A1S6, Canada.

E-mail: [email protected]

Abstract To enable customized manufacturing of structural colors for commercial applications, up-scalable, low-cost, rapid, and versatile printing techniques are highly demanded. In this paper, we introduce a viable strategy for scaling up production of custom-input images by patterning individual structural colors on separate layers which are then vertically stacked and recombined into full-color images. By applying this strategy on molded-ink-on-nanostructured-surface printing, we present an industryapplicable inkjet structural color printing technique termed as multilayer molded-inkon-nanostructured-surface (M-MIONS) printing, in which structural color pixels are molded on multiple layers of nanostructured surfaces. Transparent colorless titanium dioxide nanoparticles were inkjet-printed onto three separate transparent polymer substrates and each substrate surface has one specic subwavelength grating pattern for molding the deposited nanoparticles into structural color pixels of red, green or blue primary color. After index-matching lamination, the three layers were vertically stacked and bonded to display a color image. Each primary color can be printed into a range 1

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of dierent shades controlled through half-tone process and full colors were achieved by mixing primary colors from three layers. In our experiments, image size as big as 10 cm by 10 cm was eortlessly achieved and even larger images can potentially be printed on recombined grating surfaces. In one application example, M-MIONS technique was used for printing customizable transparent color optical variable devices for protecting personalized security documents. In another example, a transparent diractive color image printed with M-MIONS technique was pasted onto a transparent panel for overlaying colorful information onto one's view of reality.

Keywords inkjet printing, structural color, titanium dioxide nanoparticle, subwavelength grating, diractive color, optical variable device, transparent display Recent years have witnessed tremendous progress in generating colors from articial nanostructures, a eld known as structural color printing. Various structural coloration mechanisms based on interaction of light with optical nanostructures have been intensively studied and demonstrated, for example, plasmonic colors from metal nanostructures, 120 colors originating from optical resonances in dielectric metasurfaces, 2125 reective colors from photonic crystals, 2635 diractive colors from nanostructure arrays, 3639 and interference colors in thin lms. 4043 Compared to conventional coloration approaches based on dyes or pigments, structural color printing has appealing advantages: structural colors can be patterned in ultra-high resolution, with smallest pixels in subwavelength scale; 4,5 structural color printing usually implements metals and dielectric materials that are environment-friendly; 9 since the colors are generated from physical structures, structural colors are usually very resistant to fading. 10,39 The benets of structural colors can be exploited towards a broad range of industrial applications, such as security and authentication, 44 colored solar cells, 45,46 product packaging, 9 and optical archival storage. 39,47 For practical industrial use, production techniques of structural colors must be economical and scalable, and such viable techniques may be mainly categorized into two types: large-quantity replication and customized printing. In 2


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techniques for large-quantity replication of same color pattern, a master stamp with surfacerelief nanostructures for the desired color pattern is rst originated and then the pattern is replicated into a massive quantity, usually through low-cost roll-to-roll nanoimprint. 4851 The origination process usually implements time-consuming nanopatterning techniques such as electron beam lithography 9,52 and laser interference lithography, 44 but the upfront cost in origination is signicantly diluted by the large number of replications. For example, in security industry, an anti-counterfeiting feature of a certain structural color pattern may be replicated into billions of pieces to be inserted into banknotes. In nowadays manufacturing industry, there is an accelerating trend of customized manufacturing of commercial products. For industrial applications, structural colors are mainly used to establish a certain appearance or to store information, and either application often requires customization of the color pattern. Furthermore, the target quantity for each color pattern could vary broadly, from a single piece to a very large volume. Therefore, techniques that can enable customized printing of structural color patterns are in demand. Such techniques must be scalable and inexpensive, and can rapidly print any custom-input image with good appearance. Optical techniques have been implemented to pattern excellent structural colors but their up-scalability in terms of production speed remains a question. 11,20,26,36,39,41,47 Inkjet-based structural color printing techniques can meet the overall demands of customized printing for industrial applications. Inkjet printing of optically variable inks, composed of akes of multilayer dielectric materials to display interference colors, has already been popularly used in security industry for decades. 53 Special inks of colloidal nanospheres that can self-assemble into photonic crystals have been used to print structural color patterns. 27,29,30,3335 Ordinary liquids can also be printed onto pre-fabricated photonic crystal substrates to tune the structural colors. 28,32 Colorless dielectric titanium dioxide (TiO2 ) nanoparticles ink has been inkjet-printed onto a holographic paper to preserve the diractive colors. 54 The same type of ink was also inkjet-printed into optical-grade thin lms to generate thickness-dependent interference colors for custom-input images. 43 Our previous 3


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work has introduced an inkjet-based molded-ink-on-nanostructured-surface (MIONS) printing which implements a nanostructured surface comprised of pixelated nanocone arrays for printing full-color images. 37,38 Silver nanoparticles ink droplets were accurately printed onto red, green and blue bands according to the desired color pattern. The printed silver thin-lm dots were molded by the underlying nanostructured surface into structural color pixels. The substrate is generic for printing all dierent patterns and can be replicated using roll-to-roll nanoimprint. MIONS technique can print any custom-input full-color image inexpensively and can enable both diractive colors and plasmonic colors. One technical challenge in scaling up MIONS printing for industrial use, is the stringent requirement on the accuracy and consistency of alignment during continuous operation of the inkjet printer. Alignment can be conveniently calibrated for a small printable area but it is dicult to be maintained over a large area. For example, when printing a big color pattern, alignment can be lost in some sub-regions due to inhomogeneous warping of the polymer substrate surface. Misalignment between the printed ink pattern and the pixel bands can cause wrong colors, referred to as `color leaking'. 38 One solution in terms of equipment that is currently under development, is a smart printing head that can optically detect pixel bands in real time and dynamically compensate alignment errors in all sub-regions.

Concepts of Multilayer MIONS Printing In this paper, we present a multilayer color separation strategy that can fundamentally solve the problem of `color leaking' caused by inconsistency of alignment in MIONS printing. Based on this strategy, we developed a technique, referred to as multilayer molded-ink-onnanostructured-surfaces (M-MIONS) printing, which implements a multilayer conguration of nanostructured surfaces for scalable inkjet printing of transparent full-color images. The concept is schematically shown in Figure 1. Each layer of nanostructured surface has only one homogeneous nanostructure for molding one type of structural color pixel structure and

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dierent colors are mixed from vertically stacked red, green and blue layers. To elucidate the concept in details, Figure 1a rst shows a 3-D schematic for molding one diractive color pixel on a nanostructured surface patterned with 1-D subwavelength grating. The grating surface is nanoimprinted into a transparent UV polymer from a stamp and the grating structures uniformly cover the entire printable area of the substrate. Ink droplets containing titanium dioxide nanoparticles (TiO2 NPs) are inkjet-printed onto the polymer grating surface according to a custom-input pattern. After drying the ink droplets, the TiO2 NPs are molded by the underlying polymer grating surface into dielectric grating structures composed of close-packed TiO2 NPs. Then the entire surface is laminated with a transparent index-matching cover lm. The refractive index of the cover lm is around 1.5, same as the polymer of the grating surface. When the cover material lls inside the grooves of gratings that are not covered with NPs, diraction from those gratings is deactivated due to index-matching whereas the molded TiO2 grating structure remains diractive because of the contrast in refractive index between TiO2 and polymer. The molded TiO2 grating structure in fact becomes a diractive color pixel encapsulated inside transparent polymer, as shown Figure 1b. TiO2 is a dielectric material with low optical loss in the visible spectrum and the molded grating pixel is therefore transparent and allows most light to transmit through the pixel. Figure 1c,d show two side-view schematics of the diraction of light from the molded grating pixel, under reection mode and transmission mode, respectively. Consider a beam of white light is incident from an angle α from a medium of refractive index n (usually air, n = 1), and the grating has grating period Λ, light of wavelength λ carried in the mth grating order is diracted into the angle β . The relationship of Λ and λ is given by Equation 1.

nΛ(sinα − sinβ) = mλ

(1)

For a given conguration of lighting and viewing angle, diractive color in red, green and blue can be displayed from three dierent gratings of appropriate grating periods calculated 5


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from the grating Equation 1. Figure 1e-g schematically show the inkjet-printing of full-color images using three layers of grating surfaces. Detailed experimental procedures in printing is provided in Section `Materials and Methods'. Subwavelength grating for red, green and blue color is patterned onto three separate layers and each layer is inkjet-printed with TiO2 NPs. After printing and drying, the three layers are laminated and stacked vertically with alignment of printed patterns. When observing from back side, a viewer sees a full-color diractive color pattern, as shown in Figure 1g. Wide range of colors are mixed by red, green and blue diractive colors from grating pixels that are molded on corresponding layers. Compared to MIONS printing based on tiled pixel bands, M-MIONS printing on multiple layers has four major advantages. Firstly, printing on three layers are completely isolated from each other and each layer has only one grating pattern. Problem of `color leaking' and bottleneck of image size are completely eliminated in M-MIONS printing. The printable image size can be easily scaled up to enable customized manufacturing of structural colors for commercial applications. Secondly, although alignment between layers is still needed, it merely requires overlaying the patterns for good visual eects and the tolerance of alignment error is at least 250 µm. In comparison, for pixel-band-based MIONS printing, alignment error must be smaller than 10 µm and be kept consistent over the entire printable area. Thirdly, the alignment between layers is carried out after printing and therefore does not interrupt the continuous operation of the printer. Fourthly, although M-MIONS technique requires three separate master stamps (one stamp for each layer), the origination of each master stamp is straightforward using laser interference lithography because the pattern is uniform on the stamp. In comparison, origination of a master stamp with layout of tiled pixel bands is more challenging, as it requires microscopic alignment between pixel bands when implementing laser interference lithography. 37

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Figure 1: Concepts of M-MIONS printing on subwavelength grating surfaces. (a) Schematic of molding one diractive pixel from a polymer subwavelength grating surface. The grating surface is rst replicated from a stamp using nanoimprint lithography. Dielectric TiO2 NPs are inkjet-printed on the grating surface and the formed TiO2 thin lm is molded by the underlying polymer grating structures to gain diractive colors. Index-matching lamination deactivates polymer gratings that are not covered by NPs to display a transparent background. (b) Schematic of diractive colors from one molded grating pixel. (c) Side-view schematic of diractive colors viewed from the backside of molded grating pixels with light illuminated from the same side of the viewer (reection mode). (d) Side-view schematic of diractive colors viewed from the backside of molded grating pixels with light illuminated from the opposite side of the viewer (transmission mode). (e) Schematic of molding red, green and blue grating pixels on three dierent subwavelength grating surfaces. (f) Schematic of stacking and laminating three layers with alignment of the patterns. (g) Schematic of the diractive full-color pattern viewed from the stacked layers.

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Results Principles of Color Mixing from Multilayer Grating Pixels

M-MIONS printing relies on mixing red, green and blue diractive pixels from vertically stacked layers to display dierent colors. The displayed color is determined by the diraction eciency of grating pixels which are separately molded on three layers, given by [ηR , ηG , ηB ] where ηR , ηG and ηB are the diraction eciency of red, green and blue pixel, respectively. The diraction eciency of each grating is further determined by the normalized diraction eciency of the molded grating structure and its lling ratio. The normalized diraction eciency is denoted as ηnr , ηng and ηnb for grating structures molded on red, green and blue grating surface, respectively. The lling ratio is dened as the percentage of the area covered by TiO2 NPs, denoted as Ar , Ag and Ab for ink printed on red, green and blue grating, respectively. The mixed color from three layers can be described in Equation 2-4, by neglecting the transmission loss from the layers stacked in front. In fact, the M-MIONS printing process is essentially to modulate the lling ratio of ink on each layer of grating surface, according to the input digital color image.

ηR = ηnr Ar

(2)

ηG = ηng Ag

(3)

ηB = ηnb Ab

(4)

Molding Grating Pixels on Single Layers

Since the colors are determined by the property of the molded grating structure and the lling ratio (ink coverage) together, in order to achieve full control of colors, the properties of molded diractive pixels must be studied rst. In experiments, we printed TiO2 NPs on single layers of polymer grating surfaces which were replicated from master stamps. The 8


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master stamps were fabricated using laser interference lithography and dry etching. Detailed fabrication procedures are provided in Section `Materials and Methods'. Scanning electron microscope (SEM) image of the replicated subwavelength grating structures on red, green and blue layer is shown in Figure 2a, b and c, respectively. Due to laser interference lithography over a large area (8-inch wafers), there are certain variations of the fabricated structures. The geometrical parameters of the gratings are schematically dened in Figure 2d with average parameters summarized in the inset table. The average grating period is 544 nm, 470 nm, and 397 nm, for red, green and blue grating, respectively. The variations in grating periods were controlled to be smaller than 25 nm. Supplementary Figure S2 shows the calculated diraction wavelengths from three dierent gratings for various angles. It should be noted that, in this work, we applied subwavelength gratings (i.e. grating periods smaller than wavelength of light) such that correct diractive colors are viewed with steep light incidence angle and the visual color in -1st grating order is more angle-robust than diractive colors from larger grating periods.

Figure 2: Replicated polymer nanostructured surfaces with subwavelength grating patterns. (a), (b) and (c) SEM image of subwavelength gratings replicated in UV polymer for red, green, and blue layer, respectively. (d) Geometrical parameters of the fabricated gratings (side view). In our experiments, we printed TiO2 NPs ink on each type of grating surface with varying lling ratio of molded gratings. In the printing process, a desktop digital inkjet color printer 9


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was used and the prepared ink material was lled into the cartridge for black channel, as will be presented in details in Section `Materials and Methods'. All input patterns are grayscale images because the ink is jetted from the black channel only. The inkjet printer is a commercial product for regular oce use for printing on mainly paper substrates. The halftone process, used by the printer, translates each shade of gray into a dot array pattern for controlling the density of printed ink droplets on the substrate. When printing on polymer grating surfaces, the resultant lling ratio of molded gratings is dierent from the density of printed ink droplets because the hydrophobic polymer substrate surface can cause merging of ink droplets, which will be investigated in details later. Nonetheless, lling ratio still depends on the printed ink density and is therefore controlled by the shade of gray in the input image. As shown in Figure 3a, 11 shades of gray, equally spaced from full white to full black, were used as the print pattern to print rectangles on the grating surfaces. A darker shade will be translated into a higher density of printed ink. Therefore, 11 dierent levels in the printed ink density were tested on the three grating surfaces. After drying and index-matching lamination, optical properties of the samples were characterized. Figure 3b shows side-view schematic of the laminated sample with specied default lighting and viewing angles used in optical characterizations. Figure 3c shows photos of the printed diractive color patterns on red, green and blue gratings. Microscope images of blue colors from selected ink density levels are shown in Figure 3d-f. The irregular shapes of the blue dots are due to merging of printed ink droplets. Measured normalized diraction spectra in Figure 3g show diraction peak centered around 640 nm, 550 nm and 460 nm wavelength, for red, green and blue pixel, respectively. These measured peaks match closely with theoretical calculations listed in supplementary Figure S2. The peak normalized diraction eciency is in the range of about 0.3% - 0.6%. It should be noted that the normalized diraction eciency is the measured diraction eciency from the spot probed through the microscope normalized with respect to the lling ratio and therefore measures the property of the pixel structures themselves 10


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regardless of the printed sizes. Figure 3h shows quantitative analysis on the printed gray-scale patterns to evaluate the brightness of diractive colors vs. printed ink density levels. Diraction spectra were measured from all rectangles shown in Figure 3c and the pixel brightness of each rectangle was calculated by integrating the diraction eciency within the spectral range corresponding to that pixel color. The pixel brightness increases monotonically with the printed ink density, however, the dependence is not linear because of merging of the printed dots. As determined from digital image processing, when printed at full ink density, the actual lling ratio on red, green and blue grating surface is about 29%, 40% and 39%, respectively. As studied in our previous work, dot merging can cause randomness in lling ratio and thus aects the accuracy of the pixel brightness. 38 When printed with denser ink droplets, the negative eect of merging is more pronounced. As controlled through the inkjet printer's half-tone process, each gray scale in the print pattern is translated into a specic dot array pattern with clearly dened drop spacings and droplet sizes. The drop spacings and droplet sizes are automatically determined by the printer for achieving a specic density of printed ink which can then be optically mixed into the wanted pixel brightness. The inkjet printer used in this work (Epson Stylus C88+) implements a special Variable Size Droplet Technology where the printer can produce a number of dierent ink droplet sizes adjusted according to the input gray-scale value. The smallest droplet size is 3 pL and the smallest drop spacing is 18 µm × 5 µm. It should be noted that the default half-tone process of the printer is optimized for printing conventional black and color inks on plain paper substrates whose surfaces tend to absorb inks. In contrast, in this work, TiO2 NPs dispersed in mixture of water and ethylene glycol were used as the ink and the polymer grating surface is a solid non-absorbing surface with low wettability for ink. When ink droplets are jetted into a well-dened array of droplets of appropriate droplet sizes, the droplets impact the solid substrate surface and some droplets randomly migrate and merge into larger dots due to the low wettability. After the ink dries up, TiO2 11


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NPs are formed into a relatively random pattern due to the merging of dots which can cause inaccuracy in the lling ratio and the brightness of diractive colors. Despite the inaccuracy, the pixel brightness for dierent printed ink density levels are suciently dierentiable to display at least 11 dierent shades of colors as shown in the diraction measurement results in Figure 3h. In addition, the visual eects of colors printed with dierent ink density levels have also been evaluated with eyes. The colors printed with ink density level 9 and 10 show big grains due to the merging of very densely printed ink droplets. For ink density level

≤ 8, the visual eects are good despite some small grains and inhomogeneities visible in the printed image. The merging of dots can certainly be avoided by modifying the half-tone process. For example, if a gray-scale value is translated into a dot array pattern with large enough drop spacings and small enough droplets, the merging of dots can be eliminated. Such a half-tone process specically optimized for M-MIONS printing with special considerations on inks and substrates can potentially be achieved by re-programming the printer's driver software or implementing a professional industrial or research-grade inkjet printer with full control of drop positions, drop spacings and droplet sizes. 37,38 Nonetheless, one goal of this work is to prove that even regular low-cost desktop inkjet printers for oce use can be used for printing photo-size structural color patterns. Advanced modications on the inkjet printer or implementation of other professional inkjet printers will be investigated in our future work for achieving more accurate control on the lling ratio. The printed TiO2 NPs on grating surfaces have been characterized with SEM imaging. For the sake of brevity, only SEM images of the printed TiO2 NPs on blue grating surface are shown in Figure 4. Results on red and green grating surface are provided in supplementary Figure S3. The irregular sizes and shapes of the merged dots can be clearly seen in Figure 4a. The printed dots tend to be in elliptical shapes (for example, the dot shown in Figure 4b) because the ink has higher wettability along the direction parallel with grating lines than the perpendicular direction. As can be seen from Figure 4c-e, NPs are mainly deposited 12


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Figure 3: Experimental results of printing TiO2 NPs on red, green and blue grating surfaces. (a) Schematic of printing 11 shades of gray on a grating surface. The density of printed ink increases as the shade of gray becomes darker. (b) Side-view schematic of the lighting angle and viewing angle for capturing color photos and measuring diraction eciency. (c) Photos of the red, green and blue grating surface after being printed with 11 dierent levels and laminated. (d), (e) and (f) optical microscope image of blue grating surface printed with ink density level 10, 8 and 5, respectively. (g) Measured normalized diraction eciency of molded red, green and blue grating pixels. (h) Brightness of diractive pixels vs. printed ink density level for red, green and blue layer. The brightness was calculated by integrating the measured diraction spectra in the red, green and blue spectral range, respectively. inside the grooves to form into dielectric structures made of close-packed NPs. At the edge of the printed dot (Figure 4d), TiO2 NPs are completely embedded inside the 317-nm-deep grooves, without any NPs on top of the grating lines. TiO2 NPs gradually thicken towards the center of the dot (Figure 4e) and eventually cover all grating lines (Figure 4c). No coeering eects were observed because we added ethylene glycol into the water-based ink which can induce an additional inward Marangoni ow within evaporating droplets to suppress

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coee-ring eects. 55,56 As shown in Figure 4f, the sizes of NPs are distributed in the range of 15 nm - 25 nm in diameter. As shown in supplementary Figure S3a, for red grating surface, TiO2 NPs can barely cover the grating lines because red grating has much deeper grooves than blue grating. At the center of the dot, TiO2 NPs were deposited into a lm of about 280 nm to 320 nm thickness embedded inside the grooves. The results on green grating surface, as shown in supplementary Figure S3b are similar with those from blue grating, because these two gratings have comparable depth of grooves.

Figure 4: SEM images of the printed TiO2 NPs on the blue grating surface for ink density level of 8. (a) Overview of the printed ink dots. (b) SEM image of one ink dot. (c)-(e) Zoom-in SEM images of (b) to illustrate the coverage of NPs on the grating surface. (f) SEM image of TiO2 NPs inside a groove.

Full-Color Images Printed Using M-MIONS Technique

Figure 5 shows a full-color pattern printed with M-MIONS technique. For a target color image shown in Figure 5a, gray-scale print patterns for red, green and blue layer are generated using custom-written MATLAB codes. As described above, ink density level ≤ 8 yields good color patterns. Therefore, the pattern generation process considers ink density level of 8 as the maximum density and the shades of gray on the print patterns were linearly scaled 14


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accordingly. Under such conguration, 9 dierent shades of gray can be printed in red, green an blue grating surface and the actual number of printable colors are 729. All the print patterns have mirrored orientations relative to the target image because the printed colors are to be viewed from back side. After printing the patterns on the corresponding grating layers and drying the ink, the printed layers were laminated layer by layer while applying visual alignment between layers. Details are presented in Section `Materials and Methods'. The bold square outlining borders on the print patterns serve as alignment marks. A photo of the produced full-color pattern is given in Figure 5b. The photo was captured by placing the printed transparent color image in front of a dark background. The size of the pattern is about 1 inch × 1 inch. The colors of the target image has been presented quite accurately on the produced sample. Figure 5c-h show optical microscope images of the diractive colors and the associated diraction spectra, which were collected from three marked spots in Figure 5b. The red-color letter `N' was displayed by gratings molded on the red layer only (Figure 5c,d). The white-color letter `O' was displayed from all three layers to mix into white color (Figure 5e,f). The magenta-color letter `F' was displayed from the red and blue layers (Figure 5g,h). The small spikes in the measured diraction spectra in red range are artifacts caused by the light source which has a spike around 650 nm. Although optical microscope images show inhomogeneous colors on individual layers, the overall viewing eects are still good despite some small noticeable grains of colors. One concern in the multilayer conguration is the transmission loss of light from the layer in front. For example, as shown in Figure 1f,g, when the color pattern from the stacked layers is to be viewed from back side under reection mode, the transmission loss through blue layer will denitely aect the brightness of green and red layer. The transmission spectra through individual printed layers are shown in supplementary Figure S4. The transmission measurements were carried out from regions printed with highest ink density (level 10). The red layer and green layer shows more than 90% transmission and the blue layer gives more than 85% transmission, across the entire visible spectrum. The transmission microscope 15


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images in supplementary Figure S4b-d also conrm that the molded gratings are very transparent. Each layer is thus considered suciently transparent and the eects of transmission loss can be neglected. The transmission spectra show dierent transmission rates for red, green and blue layers. We attribute the dierences in transmission to the coverage of TiO2 NPs on grating surfaces. On red layer, TiO2 NPs are almost completely embedded inside the grooves of grating lines, as shown in supplementary Figure S3a. On green layer, a signicant portion of TiO2 NPs are deposited on top of grating lines into a rough top layer, as shown in supplementary Figure S3b. On blue layer, the coverage of TiO2 NPs on top of grating lines is further increased into a rougher layer, as shown in Figure 4. The close-packed NPs molded inside the grooves yield dielectric grating structures with smooth sidewalls while the rough top layer scatters light and reduces the transmission. Blue layer has roughest top layer and therefore lowest transmission. In contrast, red layer has almost no rough top layer and therefore highest transmission. In addition, the transmission spectra of red, green and blue layers in supplementary Figure S4a all show lower transmission in shorter wavelength range because the scattering of light caused by surface roughness is more severe for shorter wavelengths. Another concern is the stacking order of the red, green and blue layer. In our experiments, we have tested dierent stacking orders. Two sets of red, green and blue layers were printed with the same image under the same printer conguration. These two sets were stacked and bonded in dierent stacking orders into two samples. Supplementary Figure S5 elucidates the stacking orders of the two sets of printed layers and shows the photographs captured from two samples under the same condition. The results show that the stacking order makes very little dierence on the colors, because all layers have high transparency.

Customizable Transparent Color Optical Variable Devices (OVDs)

Optical variable devices (OVDs) are security devices that exhibit distinctive color patterns with optical variable eects such as color shifting and movement eects. Since the appealing 16


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Figure 5: A full-color image printed using M-MIONS printing. (a) Process of generating print patterns for red, green and blue layers, according to the target image. (b) Photo of the printed sample captured under default lighting and viewing angle. (c)-(h) Optical characterization results on the three color spots marked onto (b). (c),(e),(g) Microscope images of the diractive color pattern. (d),(f) and (h) measured diraction spectra. visual eects from an OVD are very dicult to mimic or duplicate, OVDs are widely used as anti-counterfeiting and authentication features to protect important documents including banknotes, ID cards, and passports. 44 Traditional color OVDs are manufactured using above-mentioned large-quantity replications, based on master stamp origination and roll-toroll nanoimprint. The color patterns are xed to the master stamps, and each attempt of 17


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customization on the pattern requires originating a new master stamp. Due to the high cost in master stamp origination, it is not practical to include customized features through such traditional manufacturing approaches. In comparison, inkjet-based structural color printing techniques can conveniently enable customization on the OVDs. 27,29,33,37,43 In this work, MMIONS printing can be used for manufacturing color OVDs with customized color patterns and features. The printed images appear iridescent and the colors shift vibrantly when the images are tilted, because the colors are based on angle-dependent diraction from grating pixels. Since the printed color patterns are solely determined by the input digital images, customization of the OVDs can be achieved eortlessly by including customized features into the custom-input digital image. Figure 6a shows the concept of customizable transparent color OVDs for protecting ID cards. A transparent color OVD, with color pattern specically customized according to the owner of the ID card, can be inexpensively produced using M-MIONS printing and laminated onto the card surface. Any attempt in altering the information on the card will leave obvious traces on the laminated OVDs. Face photo and signature of the owner can also be included into the printed OVDs to enhance the complexity of the security features. The color shifting eects of the OVDs are shown in Figure 6b-d. We kept the angle between light source and viewer constant,

i.e.

45◦ , and rotated the sample. As the sample is rotated clockwise, the

diractive colors blue-shift, displaying eye-catching optical variable eects. The diraction spectra of the molded red, green and blue grating pixels under these angles are provided in supplementary Figure S6. Besides these congurations of angles, the color-shifting eects are also obvious when rotating the angles in other reasonable fashions. If we x the angle between the sample and the viewer while rotating the light source, with increasing light incidence angle, the colors red-shift. If we x the angle between the sample and the light source while rotating the viewing direction, the colors red-shift when the viewer rotates towards the light source. If the light source and the viewer are both xed while rotating the sample on an axis perpendicular to grating lines, the diractive colors turn o after the sample is rotated for a 18


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few degrees. The above-mentioned optical characterizations were carried out under directed light beam to accurately relate the colors and diraction spectra to the conguration of angles. Under lighting condition at normal environments, such as sunlight or regular room light, the light comes from wider angles and has eects on both the viewing angles and the brightness. Under such conditions, the colors can be viewed from slightly broader angles but appear a little fainter than being illuminated with directed light beam. Nonetheless, when a viewer rotates the sample by hands and examines the image with naked eyes, the iridescent bright colors with color-shifting eects can be clearly observed. Such a behavior can also be seen from the diractive colors from other commercially available color OVDs. M-MIONS printing requires stacking 3 layers of polymer substrates and the produced OVDs may be too thick. In this work, each substrate is about 250 µm thick and the total thickness of the produced OVD is around 750 µm. Such an OVD would be too thick for laminating ID cards, which are usually about 500 µm thick. In the future work, the grating surfaces will be replicated using roll-to-roll nanoimprint process onto plastic foils thinner than 50 µm, such that the total thickness of the produced OVDs will be reduced into about 150 µm to enable practical integration with ID cards.

`Print and Paste' Transparent Color Image Display Panel

Transparent displays or see-through displays can display images with attractive appearances on transparent screens and interactively overlay live information onto a viewer's perception of real world to enable an important application known as augmented reality. Commercially available transparent displays have already been realized through various digital display technologies including liquid crystal display (LCD), 57 organic light emitting diode (OLED) display, 58 and on-screen projection. 59,60 The images printed using M-MIONS technique possess three noteworthy characteristics. Firstly, the images have very high transparency. Secondly, once illuminated with a directional beam from a steep angle, the printed colors become very bright owing to the angle-specic 19


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Figure 6: Customizable transparent color OVDs manufactured using M-MIONS printing process. (a) Schematic of the customizable transparent OVDs laminated onto a personal ID card to prevent alteration of information. (b), (c) ,(d) Photo of printed color image captured from dierent angles. The angles are dened in the side-view schematics. diraction of light. Thirdly, any custom-input image can be directly printed and then pasted onto various transparent panels. These characteristics can enable a very low-cost transparent color image display panel for displaying static colorful information to a viewer. Figure 7a shows the concepts of a `print and paste' transparent color image display panel. A simplied side-view schematic with emphasis on the optical conguration is given in Figure 7b. In these schematics, a model, mimicking the Academic Quadrangle Building of Simon Fraser University, is a physical object positioned inside a display box. A transparent color image is printed with M-MIONS technique and pasted on the surface of the front glass panel. From top, a white LED lighting bar is installed and the LED emits directional white light with incidence angle between 60◦ and 80◦ . The on/o status of the LED is controlled 20


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in response to a viewer's motion. When the LED is o, the viewer directly sees through the panel to watch the object only, owing to the high transparency of the printed image; when the LED is switched on, the printed color pattern appears brightly on the panel because of the diraction of light into the viewer's eyes. In experiments, we printed a full-color pattern in the size of 10 cm × 10 cm using the same process as described in Figure 5. Then, we pasted the printed image (about 750 µm thick) onto a transparent polymer glass (6 inch× 6 inch size, 0.5 inch thick) and place the glass in front of a custom-assembled toy model. A white LED lighting bar with directional lighting was installed according to the diagram in Figure 7b. Visual eects of the transparent color image display were tested and the results are shown in Figure 7c-e. A photo of the toy model alone (the physical object only, without any glass panel or image in front) is shown in Figure 7c, to be used as a reference. Figure 7d shows the photo of the scene after the glass panel (attached with the printed transparent image) is placed in front of the object while LED is o. The existence of the printed image can barely be noticed, when comparing Figure 7c and Figure 7d. The printed transparent color image does not block or disturb the view of the object. Once the LED is switched on, the printed diractive color pattern is illuminated to display bright and colorful information, as shown in Figure 7e. Information related to the toy model, including the name, makers' names, QR code and colorful test patterns, is clearly overlaid in front of the physical object to enhance a viewer's visual experiences. On our transparent display panel, the printed color information is a static color pattern. To display a dierent information, a new color image must be printed and pasted. In comparison, other main-stream transparent displays can all play motion pictures and the live information can be interactively updated. Nonetheless, our transparent display panel has advantages in its intrinsic simplicity and is sucient for applications that can be satised with a single frame of information. Example applications include shop cabinets, shop windows, signages, and museum display boxes. In a museum display box, one transparent diractive 21


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color image can be printed for the object being exhibited and the produced image can be eortlessly pasted onto the glass. It should be noted that the pasted image does not compromise the security or safety of the display box itself, which is very important for exhibiting objects of high value. If the exhibition object is changed, a new image will be printed to replace the previous image. For ideal performance of our transparent display panel, light emitted from LED lighting bar shall only illuminate the diractive color image and the disturbance on the physical object or the viewer should be minimized. It is achieved by using subwavelength grating pixels and directional LEDs. Subwavelength grating pixels require illumination from a very steep angle (60◦ - 80◦ in this case) for the -1st order as shown in Figure 7b. With directional LED light source set up for that angle, the viewer and the object will not be illuminated as long as they keep a decent distance from the lighting bar. The viewer and object are slightly aected by the diused light from the light source but those eects are negligible. The visual experiences from our transparent color image displays are overall excellent, although the information contains only one frame. Furthermore, if a simple motion sensor is installed to control the LED, a viewer can actively turn on or o the information on the panel to gain some simple interactive experiences. A video in the Supplementary Information shows the function of one such prototype display box in response to a viewer's motion. The printed transparent image was pasted onto the front glass and a simple ultrasonic motion sensor was installed below the front glass. This sensor can detect a viewer's motion to control the white LED lighting bar installed at the ceiling of the box. The LED is switched on to illuminate the transparent color image once the waving hand is detected by ultrasonic sensor. In this work, three layers are used in printing full-color transparent color images. In fact, more layers can be stacked together to include more layers of color information. For example, stereoscopic 3-D images can be printed using M-MIONS technique on two sets of red, green and blue grating layers (6 layers in total). One printed set diracts a color pattern into a viewer's left eye and another set into the right eye, by adjusting the light diraction angle 22


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from the grating pixels. Through such 3-D eects, a viewer can potentially see virtual colorful objects superimposed onto the physical objects or scenes, and many interesting application ideas can be explored. Another potential benet of printing more layers is to improve the viewing angle of the color patterns. Due to the diractive nature of the colors from grating pixels, the colors appear brightest when a viewer perpendicularly faces the panel and become fainter as the viewer moves sideways. The limit in the angle can potentially be improved by printing images on more layers of grating surfaces with grating lines on each surface being intentionally oriented to cover wider range of diraction angles.

Figure 7: Transparent color image display panel implementing color images printed with M-MIONS technique. (a) Schematic of the function of the transparent color image display panel. A printed transparent diractive color image is pasted onto the front panel and a lighting bar is installed on top. With light o, the color image appears transparent and the viewer sees through the panel. With light on, the viewer sees the color patterns oating on the panel. (b) Side-view optical diagram of the transparent display panel. (c) Photo of a custom-assembled toy model. (d) Photo after placing a glass panel pasted with a printed image in front of the toy model while light is o. The existence of the color image is barely noticeable. (e) Photo of the setup after tuning on the light. Information related to the toy model appears colorfully on the panel. The blue circular pattern at the bottom left corner is a logo of NanoMedia Solutions Inc. (printed with permission).

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Discussion M-MIONS printing is based on a multilayer color separation strategy in which structural colors are separately printed on individual layers which are then vertically stacked and recombined into full-color images. Similarly, Jalali

et al.

have demonstrated stacking of

plasmonic colors in multilayer 3D nanostructures in which the colors can drastically change when the layers are exfoliated. 17 In M-MIONS printing, each layer is responsible for only one structural color and is patterned in a top-down fashion,

i.e.

from a large-area uniform

nanoimprinted grating surface into molded grating pixels of a certain pattern. Although our multilayer strategy increases the complexity of printing from a single layer into multiple layers, the patterning on each layer is signicantly simplied such that the overall throughput and printable image size can both be improved. Such multilayer strategy combined with top-down patterning can also potentially be applied to scale up other structural color printing techniques, for example, structural colors from all-dielectric metasurfaces. Furthermore, owing to separating colors in dierent layers, each primary color can be printed and displayed without interfering other primary colors. Rough alignment for vertically overlapping the patterns is sucient for displaying high-quality visual colors. In comparison, in structural color printing techniques that print all pixels on a single layer, arrangement of pixels in dierent colors always requires special considerations and precise controls for generating correct colors. In our previous work on pixel-band-based MIONS printing, the image is printed onto a single-layer of tiled pixel bands with microscopic alignment between the printed ink pattern and pixel bands. The alignment accuracy is the main factor that limits the image size and production speed. The printed image displays colors in a fashion resembling color LCD displays with tiled red, green and blue pixels. In this paper, M-MIONS printing implements vertically-stacked multiple layers of nanostructured surfaces to scale up the image size and the production speed towards practical industrial applications. Alignment between patterns printed on dierent layers with macroscopic accuracy is sucient which fundamentally breaks 24


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the bottleneck imposed by alignment accuracy in pixel-band-based MIONS printing. Using M-MIONS technique, the inkjet printing process can be carried out rapidly and continuously, and there is no stringent requirement on the printer. The produced image displays colors similar with a conventional color photographic lm which separates colors into verticallystacked layers of primary colors. In this work, one full-color image of 10 cm × 10 cm can be printed on three substrates within one minute using the desktop inkjet printer. The printer has a printing resolution about 180 lines per inch (LPI) for printing regular color inks on paper substrates. In our experiments, due to the merging of printed dots on polymer grating surfaces, the eective resolution is lower. The thinnest ne line printed on our polymer substrates is about 500 µm wide, equivalent to printing resolution of around 50 LPI. It should be noted that, using professional industrial inkjet printers, the merging of dots can be avoided and much higher resolution is achievable. One distinctive advantage of M-MIONS printing is that every production step can be carried out using commercially available, low-cost, and up-scalable production techniques. Based on M-MIONS technique, industrial-scale printing of transparent diractive colors for custom-input images can be realized by combining roll-to-roll replication of nanostructured substrates and roll-to-roll inkjet printing. There is no fundamental limit on the practical image size. The grating patterns on master stamps can be recombined into large-area uniform patterns and then replicated onto plastic thin foils using roll-to-roll nanoimprint. Largearea images can be directly printed onto such recombined grating patterns. In addition, the image size can be further multiplied by printing images into sections which are then combined into one image. In our experiments, we applied anti-adhesive monolayer on the replicated grating surfaces before printing to tune the wettability of the ink and removed this monolayer using oxygen plasma after printing for tight bonding of the cover lm. The treatment of this monolayer complicated the printing process and must be avoided for future industrial production. One simple solution is to replicate the grating surfaces using a UV polymer with appropriate wettability of ink droplets. 25


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In terms of image size, roll-to-roll nanoimprint can produce large images, however, it is restricted to replicating the same patterns into large quantity. 51 Plasmonic colors can be patterned into large area according to custom-input images using laser-induced coloration on metal surfaces. 20 Compared to laser coloration, M-MIONS printing has appealing advantages in patterning speed and costs of equipment and substrates, which are key attributes for commercial applications that require large image size and low manufacturing cost. In other inkjet-based techniques capable of low-cost printing in large sizes, the structural colors and other optical properties of the pixels are determined by and limited to the synthesized ink materials. 27,29,30,3335,43,53,54 In M-MIONS printing, the structural coloration mechanism is mainly controlled through the nanostructured surfaces which can be precisely engineered for dierent applications. In this work, diractive colors from subwavelength gratings were used for applications in optical variable devices and transparent color display panels. Other structural coloration mechanisms, such as resonant colors from dielectric and plasmonic metasurfaces, are also attainable through M-MIONS printing technique using specially designed nanostructured surfaces. In M-MIONS printing, broad range of visual colors were attained by simply mixing the three primary colors which were printed into dierent shades. The shades of colors were automatically controlled through the half-tone process of the digital inkjet printer according to the generated gray-scale print patterns. In this work, 11 dierent shades for each primary color were printed. In the future, much ner shades can be achieved through more accurate control of ink droplets and substrate surface property to enable high-quality color printing. For inkjet printing, the spreading of ink droplets and the shape of dried dots are mainly determined by the wettability of inks on the substrate surface. In our work, we coated a peruorooctyltriethoxysilane (PFOTS) monolayer on the grating surfaces before printing to reduce the wettability. The function of the PFOTS monolayer is elucidated in supplementary Figure S7. Two same blue grating surfaces, one without surface treatment and the other coated with PFOTS monolayer, were characterized using contact angle measurements. As 26


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schematically shown in supplementary Figure S7a, an ink droplet tends to spread preferentially along the direction parallel with the grating lines. For the blue grating surface without surface treatment (supplementary Figure S7b,c), measured contact angle is about 13◦ along the direction parallel with grating lines, and about 19◦ along the direction perpendicular to grating lines. For the blue grating surface coated with PFOTS monolayer (supplementary Figure S7d,e), measured contact angle is about 87◦ along the direction parallel with grating lines, and about 97◦ along the direction perpendicular to grating lines. Supplementary Figure S8 shows the SEM images of the TiO2 NPs printed on a blue grating surface without surface treatment. The ink droplets signicantly spread on the grating surface and merge into very big dots of irregular shapes as shown in supplementary Figure S8a. The TiO2 NPs are deposited into a very thin layer inside the grooves of the grating lines which results in very dim diractive colors after index matching lamination. In this work, TiO2 NPs are used as the high-refractive-index ink material. In fact, other transparent inks can also be potentially used to print full-color diractive images as long as the refractive index of the solute of the ink is suciently dierent from that of the polymer background. Good candidates include transparent lacquers, silica nanoparticles ink, and polystyrene nanoparticles ink.

Conclusions We demonstrated the concepts and experimental results of M-MIONS printing based on inkjet printing of TiO2 nanoparticles on multiple layers of subwavelength grating surfaces. Transparent full-color diractive images were rapidly printed according to custom-input images. Dierent colors were achieved by mixing diractive colors from vertically stacked transparent dielectric gratings that were molded on red, green and blue layers. In experiments, photo-size full-color images were printed with 729 dierent printable colors and 50 LPI resolution. Experimental results proved that the problems of `color leaking' and

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limited image size in pixel-band-based MIONS printing were both solved using M-MIONS technique. In addition, M-MIONS printing can be implemented through a broad range of inkjet printers and even a low-cost desktop color inkjet printer for regular oce use was proven capable of printing big structural color images. The printed images were applied as transparent OVDs for authenticating personalized security documents and `print and paste' transparent color image display panels for overlaying colorful information in front of physical objects. M-MIONS printing can potentially enable industrial-scale customized manufacturing of structural colors with high speed, low cost and very large image size.

Materials and Methods The process in printing a custom-input color image is mainly comprised of 4 steps: (1) replicate grating surfaces in UV polymer and apply appropriate surface treatment if necessary; (2) prepare the TiO2 NPs ink and the inkjet printer; (3) generate print patterns according to the target color image, print the patterns on the corresponding substrates, and dry the ink with applied heating; (4) index-matching lamination (oxygen plasma before lamination if the polymer surface is anti-adhesive). This section provides details about the relevant steps.

Replication of Subwavelength Grating Surfaces

A schematic of fabrication process is shown in supplementary Figure S1. Three master stamps with desired subwavelength grating patterns on quartz wafers were rst fabricated using laser interference lithography and reactive ion etching. A quartz wafer was cleaned and evaporated with aluminum (Al) thin lm. A photoresist layer was spin-coated on top of Al layer (step (i)) and then exposed with periodic nanopatterns using laser interference lithography (step (ii)). After developing the photoresist (step (iii)), the patterned photoresist layer was used as a mask to etch the pattern into the Al layer using reactive ion (step (iv)). After stripping the photoresist using oxygen plasma (step (v)), the patterned Al layer was 28


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used as a mask to etch into the quartz surface using reactive ion etching (step (vi)). Vertical side walls with taper angle larger than 80◦ were obtained from the etching. After etching, the quartz stamps were thoroughly cleaned with chemicals and oxygen plasma (step (vii)). The grating patterns on the master stamps were replicated onto transparent polymer layer in two steps. Prior to replication, the quartz stamps were rst coated with an antisticking monolayer of PFOTS for easy release. In the rst step of replication, liquid UV resin was dispensed onto a 250-µm-thick transparent polyethylene terephthalate (PET) sheet and the quartz stamp was applied on the UV resin. After removing the bubbles with pressure, the UV resin was cured with sucient UV exposure (step (viii)). After separation, a polymer stamp with nanostructures negative to the master stamp was obtained (step (ix)). This polymer stamp was then used to produce multiple replications using same UV nanoimprint process again and each replica has nanostructures identical to the master stamp (step (x) (xi)). Prior to printing, the substrate surface was treated with PFOTS monolayer to reduce the wettability of ink in order to improve printing resolution.

Preparation of TiO2 NPs Ink and the Inkjet Printer

Commercially available TiO2 nanocrystalline colloidal paste (Sigma-Aldrich, nanoparticle diameter 18 nm, concentration 16%, water-based) was used as the raw material for preparing the ink. The paste was diluted with deionized water and ethylene glycol at a volume ratio of 1:3:6. The diluted ink was sonicated for 1 hour before use. Epson Stylus C88+ was used as the inkjet printer. The printing resolution is 5760 × 1440 dots per inch (DPI) and the volume of smallest ink droplets is 3 pL. Diluted ink was slowly injected from a syringe into a rellable inkjet cartridge for black channel (Epson rellable inkjet cartridges, NovaCentrix). Another three rellable inkjet cartridges for three color channels were lled with mixed solution of water and ethylene glycol (volume ratio of 1:1). After installing all the four rellable cartridges inside the printer, a few cycles of head cleaning procedures were carried out to calibrate the quality of the printed patterns and 29


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then the printer was ready for printing TiO2 NPs on the polymer substrates.

Printing and Drying

According to the target color image, the print patterns for red, green and blue layers were generated using custom-written MATLAB codes. Each type of substrate was loaded into the printer and printed with corresponding pattern. After printing, the substrates were baked on a hotplate surface at 35◦ C for 20 mins to dry the ink. Photographs of printing process and conguration parameters of the inkjet printer are provided in supplementary Figure S9.

Index-Matching Lamination

After printing and drying, the sample surface has polymer gratings and printed TiO2 lm. Before lamination, the sample surface was treated with a gentle oxygen plasma etching for 5 seconds to remove the PFOTS monolayer and to activate the polymer surface for bonding. To laminate a single layer (red, green or blue), a transparent, low viscosity UV-curable glue (UV CURE 60-7159, Epoxies, Etc., refractive index about 1.5) was dispensed on the sample surface and a thin PET sheet was covered on top of the UV glue. After removing air bubbles with pressure, the UV glue was cured under UV exposure from both sides (10-minute exposure). For laminating three layers to produce a full-color image, the printed red layer was rst laminated on a PET sheet using the same lamination process as described above. Next, the printed green layer was laminated onto the backside of the red layer using the same procedure, with visual alignment of the patterns between two layers. Similarly, blue layer was then aligned and laminated onto the backside of green layer using same procedure.

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Optical Characterization

After printing and lamination, the samples are under optical characterization for spectral measurement and capturing photos. Supplementary Figure S10 shows a schematic of the optical setup for measuring diraction eciency, which was used in our previous work. 37,39 The sample is mounted on a rotation stage to control the light incidence angle and light collection angle. Broad-band white light from a ber-connected light source is collimated by collimator C1. The output parallel beam is then incident onto the sample surface and the diracted light is collected by a microscope objective lens (4X Olympus Plan Achromat Objective, numerical aperture = 0.10, working distance = 18.5 mm). Behind the objective lens, an iris I1 reduces the numerical aperture into about 0.06. The diracted light is split by a beam splitter BS (50:50 split ratio). Half of the diracted light is recorded by a digital color camera to give a microscope color image of the diractive colors. The other half is rst focused onto an iris I2 for selecting a circular spot from the sample surface and then the transmitted light is coupled into a ber-connected spectrometer to record an intensity spectrum IS (λ). The reference spectrum Iref (λ) is measured by loading a at mirror under the objective lens and recording the mirror reection of the source beam. The diraction eciency from a spot selected by iris I2 is given by ηROI =

IS (λ)−Idark (λ) , Iref (λ)−Idark (λ)

where

Idark (λ) is a dark spectrum recorded when the light source is switched o. The normalized diraction eciency is given by ηnorm =

ηROI , RA

where RA is the area ratio that the bright

pixels occupy within the selected spot. RA is calculated by digital image processing of the collected microscope color image. ηnorm is a property of a molded grating pixel, independent from the size. In all spectra presented in this article, the default light incident angle α = 62◦ and viewing angle β = -17◦ , unless other specied. Color photos of the samples were captured using a digital camera (Canon 50D) with a 50 mm lens. Aperture of the camera was set to be f/8 to match closely with the numerical aperture of the optical measurement setup. The sample was mounted onto a rotation stage. The camera and a halogen light source was xed at a distance about 50 cm from the sample 31


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and the angle between lighting angle and viewing angle was about 45◦ . The sample was rotated to vary viewing angle β from -5◦ to -17◦ and color photos were captured at specic angles.

Associated Contents Supporting Information Available The following les are available free of charge.

• Supplementary information document with gures on fabrication process of polymer grating surfaces, calculation of grating wavelength, SEM images of TiO2 NPs printed on red and green grating surface, transmission properties of printed layers, color shifting of grating pixels, and optical measurement setup (PDF)

• Video showing the function of a display box using a transparent diractive color image printed with M-MIONS technique (AVI) The authors declare no competing nancial interest.

Author Information Corresponding Author

*E-mail: [email protected]

Author Contributions

H.J. and B.K. conceived the concepts. H.J. conducted all experiments and analysis. Both authors reviewed the manuscript.

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Acknowledgement This work was funded by the Natural Sciences and Engineering Research Council (NSERC) of Canada, under Collaborative Research and Development (CRD) Grant No. 614108 and 569260. This work made use of the 4D LABS shared facilities supported by the Canada Foundation for Innovation (CFI), British Columbia Knowledge Development Fund (BCKDF), Western Economic Diversication Canada (WD), and Simon Fraser University (SFU). The master stamps (on quartz wafers) have been fabricated in collaboration with Moxtek Inc. (Utah, U.S.). The replications of the grating surfaces have been done in collaboration with NanoTech Security Corp. (Vancouver, British Columbia, Canada). Special thanks to Mr. Y. Wang (School of Mechatronic Systems Engineering, Simon Fraser University) for drawing the 3-D schematics and constructing the motion-activated display box.

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