Supporting information for Full-color Subwavelength Printing with Gapplasmonic Optical Antennas Masashi Miyata, Hideaki Hatada, and Junichi Takahara *,, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Photonics Advanced Research Center, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan E-mail: *takahara@ap.eng.osaka-u.ac.jp 1
S1. Fabrication details Aluminum nanodisks placed on an aluminum oxide-coated aluminum film were fabricated with standard electron-beam (EB) lithography, thin-film deposition, and lift-off processes as shown in Fig. S1. An aluminum oxide/aluminum (30 nm/100 nm) layer was first deposited on a silicon substrate via EB evaporation. A positive resist (ZEP520A, ZEON Corp.) was then spincoated onto the substrate to a thickness of 150 nm. Subsequently, EB lithography (ELS-7700T, ELIONIX Inc.) was performed for patterning. Then, 40-nm-thick aluminum was deposited by resistive heating evaporation, and a lift-off process was finally performed in an acetone bath. Figure S1. Schematic view of the fabrication process. (a) Deposition of an aluminum oxide/aluminum (30 nm/100 nm) film using electron-beam (EB) evaporation. (b) EB lithography for patterning (resist thickness: 150 nm). (c) Deposition of 40-nm-thick aluminum using resistive heating evaporation. (d) Removal of EB resist layer to form aluminum nanodisks. 2
S2. Experimental setup Reflection measurements were performed using a confocal microscope coupled to a CCD color camera (Nikon DS-Fi1) and a spectrometer (QE65 Pro, Ocean Optics), as illustrated in Fig. S2. The sample was illuminated by unpolarized white light from a halogen lamp (100 W) through a 20 objective with NA = 0.45 (Nikon Tu Plan Fluor BD) for reflection measurements, or a 150 objective with NA = 0.9 (Nikon Tu Plan Apo BD) for resolution tests. The resulting reflected light was collected by the objective and then recorded by the CCD color camera for the bright-field image capture, or by the spectrometer through the confocal optics for the spectrum measurements. The spectra were corrected for the reflection spectrum of white light from a standard silver mirror. Figure S2. Schematic view of the experimental setup. A bright-field optical microscope coupled to a camera and a spectrometer was used for the reflection measurements. An objective with 20 and NA = 0.45 was used for the spectrum measurements and color image capture. An objective with 150 and NA = 0.9 was employed for the pixel resolution tests. 3
S3. Angle dependence of the reflectance properties In order to investigate the incident-angle dependence of the reflectance properties, we numerically calculated the reflection spectra of the color pixels (d = 140 nm and P = 300 nm) with systematically varied incidence angles φ for p- and s-polarizations (defined in Fig. S3a and S3b). The simulated reflectance maps as a function of both the wavelength and the angle for the p- and s-polarizations are shown in Figs. S3c and S3d, where the spectral dip at a wavelength of 710 nm remained up to φ = 60 for both polarizations. For p-polarization (Fig. S3c), an additional dip at wavelengths of 450 560 nm emerges above φ = 10, which can be attributed to the grating-assisted excitation of surface plasmons at the aluminum oxide aluminum interfaces (see the white dashed line). Further, the excitation of the second-order mode of the standingwave resonance may affect the spectral shape (the simulated resonant wavelength of this mode is ~440 nm). Since this mode has a symmetric charge distribution on resonance, the resonance dip can be observable only under the oblique illumination. It is worth noting that observation of this spectral dip is elusive in the measurements, because of the weighted average of the resonances at many angles supported by an objective. In contrast, the spectral dip was not observed for the s- polarization (Fig. S3d). This is because the incident fields with the s-polarization cannot excite surface plasmons on the film as well as the second-order mode resonance. 4
Figure S3. Incident-angle dependence of the reflectance properties. (a, b) Schematic of illumination conditions: p-polarization (a) and s-polarization (b). (c, d) Simulated reflectance maps of resonant nanodisk arrays (d = 140 nm and P = 300 nm) as a function of wavelength and incident angle for the p- (c) and s-polarizations (d). The white dashed line in (c) indicates the analytical line of diffraction-assisted excitation of surface plasmons at the aluminum oxide aluminum interface. 5
S4. Periodic dependence of the reflectance properties Figure S4 shows the periodic dependence of the reflection properties for the nanodisk arrays with the 80-nm-diameter and 120-nm-diameter nanodisks. We also obtained optical images with varying array periods as shown in the insets in Figs. S4a and S4b. The measured reflection spectra show that the period variation slightly affects the depth of the reflection dip (Figs. S4a, b). Importantly, the dip locations are identical to the absorption peak wavelengths of the individual nanodisks (see vertical dashed lines). Moreover, the optical images with different periods show that the color phase is preserved, whereas the color brightness gradually varies, which is consistent with the trend of the observed spectra. These results indicate that the array period influences the color saturation, whereas the color phase is strongly dependent on the resonant properties of the individual building blocks. Figure S4. Spectral reflection properties of resonant nanodisk arrays with different array periods. (a, b) Experimental reflection spectra of the nanodisk arrays with the 80-nm-diameter nanodisks (a) and the 120-nm-diameter nanodisks (b) with varying array periods. The vertical dashed lines indicate the absorption peak wavelengths of the constituent nanodisks. Insets: optical images of the arrays. 6
S5. Color range in a CIE 1931 color space Figure S5 shows a CIE 1931 color space with all plots measured from Fig. 2c to illustrate the range of colors in experiments. The CIE Standard illuminant D65 was used for the conversion to the chromaticity coordinate from the measured reflectance spectra. Obviously, there are points all around the achromatic point (x = 0.3127, y = 0.3290), illustrating the large degree of color tuning capability. Figure S5. All CIE plots measured from Fig. 2c in the main text. The CIE illuminant D65 was used for the conversion to the chromaticity coordinate from the measured reflectance spectra. 7
S6. Design of dark color pixels by tuning the resonances of the constituent nanodisks. Figure S6a shows how the spectral reflection properties of the dark color pixel can be tuned by changing the constituent nanodisk geometry. A single pixel is composed by two small nanodisks (diameter: d 1 ), single middle-sized nanodisk (diameter: d 2 ) and single large nanodisk (diameter: d 3 ) in 300 nm 300 nm squares. The three reflection dips observable in the spectra can be attributed to the first-order resonances of the constituent nanodisks. As shown, the longwavelength resonance dip of the spectrum can selectively be red-shifted from 660 to 700 nm by increasing d 3 from 120 to 130 nm while leaving the other resonance unaffected (see red vertical dashed lines). Similarly, a change in d 1 from 80 to 70 nm selectively shifts the dip location of short-wavelength resonance from 485 nm to 450 nm (see blue vertical dashed lines). These spectral shifts can be explained by the fact that the individual-antenna properties are preserved even in closely spaced structures. This fact can also be seen in the simulated absorption cross section spectrum of mixed nanodisks (Fig. S6b). Moreover, this result demonstrates a high degree of tunability over the reflection spectrum. Figure S6. Design of dark color pixels. (a) Simulated reflection spectra of three dark color pixels with the different constituent nanodisks. The array period is 300 nm. The vertical dashed lines represent the wavelength locations of the tunable reflection dips. Inset shows the schematic of the dark color pixel. (b) Simulated absorption cross section spectra of mixed nanodisks (80-nm- and 120-nm-diameter nanodisks, not periodic). The red and blue lines indicate simulated absorption cross section spectra for individual nanodisks (not mixed). Inset shows the schematic of the simulation model of the mixed nanodisks. 8
S7. Polarization dependence of the reflectance properties We experimentally investigated the incident-polarization dependence of the optical properties of the circular nanodisks. Figure S7 shows the measured reflection spectra for different incident-polarization directions of the color pixels. The color pixels consist of 80-nmdiameter and 300-nm-period nanodisks. As expected, the spectra show roughly isotropic resonances for all incident-polarization directions. Minor differences in the resonant wavelength locations are possibly due to the shape distortion of the fabricated nanodisks. Indeed, the SEM image (Fig. S7, inset) indicates that the nanodisks are slightly distorted and form an ellipse-like shape, giving rise to the polarization dependence. The details of the impact of this shape distortion are discussed in Supporting Information S8. Figure S7. Incident-polarization dependence of the reflectance properties. (a) Experimental reflection spectra for different incident-polarization directions in the color pixels composed by 80-nm-diameter nanodisks with a period of 300 nm. Inset: SEM image of the nanodisks with a diameter of 80 nm. The polarization direction θ pol is defined in the image. 9
S8. Reflectance properties on ellipse-shaped nanodisks The use of plasmonic antennas can provide secondary degrees of flexibility to spectral properties, such as polarization dependence. Indeed, the spectral control achievable using polarization dependence is highly attractive for high-density optical data storage as well as stereoscopic color prints. Such polarization-dependent spectral features can emerge from asymmetric nanostructures. Figure S8 shows how the nanodisk shape affects the reflection spectrum under polarized illumination. Here we consider ellipse-shaped nanodisks with minor and major axes of a and b as shown in Fig. S8a. Figure S8b presents the reflection spectra for orthogonal polarizations (x- and y-polarizations) in the nanodisks with varying b while keeping a at 140 nm. As shown, a change in b from 100 nm to 220 nm selectively red-shifts the spectral dip Figure S8. Reflectance properties on ellipse-shaped nanodisks. (a) Schematic view of the ellipse-shaped nanodisks with different lengths. (b) Simulated reflection spectra of the nanodisk arrays with different lengths under x- (blue dashed lines) and y-polarized illumination (red solid lines). 10
for the y-polarization (see red lines) while leaving the spectrum for the x-polarization unchanged (see blue lines). This spectral change is consistent with the fact that the resonant wavelength of standing-wave resonances is determined by a cavity length parallel to the incident polarization. As such, the asymmetric nanodisks can give rise a strong dependence of the reflection properties on the incident polarization. 11
S9. Impact of a dielectric protection cover To experimentally explore the impact of a dielectric cover layer on the color generation, we compared the reflection properties of the resonant nanodisks without and with polymer coating as shown in Fig. S9. We coated the nanodisks with a 400-nm-thick EB resist polymer (refractive index: ~ 1.55 at the visible wavelengths) as a dielectric cover. Figure S9a shows the optical images of the nanodisk arrays (P = 300 nm) with and without the polymer cover. It is clear that the dielectric coat influences the color phase in each pixel array such that nearly all the colors tend to become reddish. This can be understood by the fact that the resonance dips are redshifted with the presence of the dielectric cover (Fig. S9b). Such spectral red-shifts suggest that the polymer coat effectively changes the resonant condition of the nanodisks. Indeed, the cover layer can affect the reflection properties (such as a reflection amplitude and a phase pickup) for propagating surface plasmons in the metallic gap, leading to the resonant condition change of standing-wave resonances. Therefore, by redesigning the nanodisk structures to compensate for the spectral shifts, it would be possible to achieve better color creation for use with dielectric coverage Figure S9. Impact of a dielectric cover on the reflection properties. (a) Optical images of the same aluminum nanodisk arrays without (top) and with polymer coating (bottom). (b) Measured reflectance spectra of the nanodisk array (d = 80 nm and P = 300 nm) without and with polymer coating. 12