All-Dielectric Colored Metasurfaces with Silicon Mie Resonators Julien Proust,†,‡ Frédéric Bedu,§ Bruno Gallas,⊥ Igor Ozerov,§ and Nicolas Bonod*,† †
Aix Marseille Univ, CNRS, Centrale Marseille, Institut Fresnel, 13013 Marseille, France Université de Technologie de Troyes, CNRS UMR 6281, Laboratoire de Nanotechnologie et d’Instrumentation Optique, ICD, 10004 Troyes, France § AAix Marseille Univ, CNRS, CINAM, 13288 Marseille, France ⊥ Sorbonne Universités, UPMC Univ Paris 06, CNRS, Institut des NanoSciences de Paris, UMR7588, 75005 Paris, France ‡
S Supporting Information *
ABSTRACT: The photonic resonances hosted by nanostructures provide vivid colors that can be used as color filters instead of organic colors and pigments in photodetectors and printing technology. Metallic nanostructures have been widely studied due to their ability to sustain surface plasmons that resonantly interact with light. Most of the metallic nanoparticles behave as point-like electric multipoles. However, the needs of an another degree of freedom to tune the color of the photonic nanostructure together with the use of a reliable and cost-effective material are growing. Here, we report a technique to imprint colored images based on silicon nanoparticles that host low-order electric and magnetic Mie resonances. The interplay between the electric and magnetic resonances leads to a large palette of colors. This all-dielectric fabrication technique offers the advantage to use cost-effective, reliable, and sustainable materials to provide vivid color spanning the whole visible spectrum. The interest and potential of this all-dielectric printing technique are highlighted by reproducing at a micrometer scale a Mondrian painting. KEYWORDS: all-dielectric metasurfaces, color printing, structural colors, silicon nanophotonics, Mie resonances light into photocurrent.21 Lamellar diffraction gratings made of silicon pillars embedded in a flexible membrane also proved to yield tunable coloration.22 Tunable color filters were obtained with a single silicon layer on an aluminum substrate.23 Interestingly, subwavelength silicon particles exhibit welldefined electric and magnetic Mie resonances in the visible spectrum.24−27 When observed in dark-field spectroscopy, silicon particles provide structural colors that can be controlled when modifying the shape and the crystallinity of silicon.26−32 Their extinction spectra exhibit several peaks in the visible spectrum related to the resonant excitation of their first-order magnetic and electric modes. This property brings great opportunities to manipulate the scattering of light33,34 or to design optical antennas with high gains in directivity.35,36 The Mie resonances in silicon particles allow for the design of alldielectric optical antennas able to enhance the electric and magnetic decay rates of quantum emitters,37−41 to tailor the chirality of light emisison,42 or to enhance the electric or
V
ivid colors result from a resonant interaction between light and matter structured at a subwavelength scale. Complex and highly efficient photonic structures can be found in nature, such as the iridescent wings of Morpho butterflies.1−5 Photonics engineering can be used to imprint colors without the use of pigments or organic colors.6 The color is encoded in the photonic resonator and can be tuned via the shape and composition of the nanostructures. Subwavelength plasmonic particles were recently proposed to downscale structural color printing at the diffraction limit.7,8 Different strategies have been proposed to improve the verstatility, sustainability, and reliability of this technique.9−12 The quest of cost-effective materials started with the rise of aluminum plasmonics, which featured excellent properties for nanocolor printing13−18 or quantum dots.19 Besides aluminum, silicon would also be of high interest for nanoprinting technology since it is reliable, cost-effective, and easy to integrate into optoelectronic devices. Its high refractive index, around 3.8 in the visible range of the spectrum,20 offers plenty of possibilities to tailor light at subwavelength scales. Silicon nanowires with integrated photodetectors were for example recently used as color filters able to convert absorbed © 2016 American Chemical Society
Received: May 14, 2016 Accepted: July 26, 2016 Published: July 26, 2016 7761
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Figure 1. (a) Sketch of the sample composed of arrays of silicon nanoparticles etched on a glass substrate. The height of the particles is 175 nm, the pitch is p = 1 μm, and the diameter is a free parameter. (b) SEM image of one array (diameter 100 nm) evidencing the homogeneity of the structuring. (c) Experimental dark-field images of arrays of nanoparticles when particle diameters increase from 70 nm to 210 nm (with a silicon thickness of 175 nm). (d) Simulated dark-field images corresponding to the case considered (h = 175 nm, p = 1 μm). The different colors are reconstructed following the method detailed in Figure 3.
magnetic near fields.43 They can also lead to the design of allsilicon metasurfaces acting as optical mirrors44−46 or antireflective coatings.47−49 Here, we introduce an all-dielectric colored metasurface based on the electric and magnetic morphologic resonances hosted by silicon nanoparticles. For that purpose, we consider a single silicon layer coated on a glass substrate that is etched at a period fixed at 1 μm. This period, much larger than the wavelength in the visible spectrum, ensures a negligible coupling between the silicon particles. The resonant light scattering yielded by individual silicon particles provides the structural color of the pixel. The interest in silicon Mie resonators is that the relative strength of the electric and magnetic resonances can be easily tuned with the aspect ratio of the particle. Unlike metallic particles, which feature mainly electric-like resonances, silicon Mie resonances offer an additional degree of control of the structural colors with magnetic-like resonances of dipolar and quadrupolar orders.50,51
Figure 1b) and a diameter of 210 nm for zone 14. The sizes of the particles were confirmed by SEM measurements. The zones are imaged with a dark-field microscope (magnification 50×, numerical aperture NA = 0.7) (see next section and the Methods section). First, it can be observed that each zone features a specific color. Second, the color is tuned from the dark blue for the 70 nm diameter, through the red for 130 nm, to finally the light gray for the largest particles (190 and 210 nm). Finally, spanning the diameter from 70 to 210 nm allows tuning the color through the whole visible spectrum and even to yield white light. Let us now investigate thoroughly the link between the colors and the morphology of the silicon particles (see Figure 1d). Figure 2a shows the scattering spectra of silicon particles with diameters ranging from 70 to 210 nm. These spectra have been thoroughly investigated in numerical and experimental studies.26−32 The smallest particles feature a single peak in the extinction spectrum. This peak red-shifts when increasing the diameter until splitting into two distinct peaks. When further increasing the diameter, the scattering cross section increases and a new peak corresponding to the excitation of a quadrupolar mode appears in the blue part of the spectrum. We can even observe the formation of two peaks for the 170 nm diameter (aspect ratio close to 1) in the blue part of the spectrum, corresponding to the excitation of electric and magnetic quadrupolar modes, in addition to the longwavelength dipolar magnetic and electric resonances (Figure 2a). The position of the different modes varies with the diameter of the nanoparticles (Figure 2a). The scattering spectra of individual silicon resonators on a fused silica substrate are also calculated with a simulation software (CST Microwave Studio).
RESULTS AND DISCUSSION The sample is fabricated by evaporating a 175 nm thick silicon layer over a 1 mm thick fused silica substrate. The particles are designed by electron beam lithography (EBL) and etched by reactive ion etching (RIE). We consider a square lattice of period p = 1 μm (Figure 1a). All the particles have the same height, 175 nm, and the colors are controlled via the diameter of the particle only. We define 14 areas of size 38 × 38 μm2 composed of 39 × 39 identical resonators (see Figure 1b,c). Each zone corresponds to a specific diameter, with diameters increasing from 70 nm for zone 1 to 190 nm for zone 13, with an increment of 10 nm between two neighboring zones (see 7762
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spectrum the scattering spectrum weighted by the colormatching functions times the spectral response of the source, S(λ), which can be cast for the X parameter: X = ∫ 780 380 xS(λ)σ scat ̅ dλ. These parameters need to be converted to an RGB coordinate system to be displayed by a monitor. This conversion is obtained through a 3 × 3 transformation matrix M: [R,G,B] = M−1[X,Y,Z]. The elements of M are calculated using the white point coordinates of the source [XS,YS,ZS] and assuming a standard phosphor RGB emitter (sRGB).53 For example, in zone 3, for which the diameter of the particles is 90 nm, we obtain with this method the sRGB coordinates [131,171,114], corresponding to a green color. We have applied this method to the 14 zones by calculating the scattering spectra of Si particles of height 175 nm on a fused silica substrate and of diameters ranging between 70 and 210 nm. The 14 corresponding colors obtained are displayed in Figure 1d. They are in very good agreement with the experimental colors observed with dark-field spectroscopy. The sRGB values are presented in Figure 3c as a function of the diameter of the particles. An equivalent color palette was created from the determined sRGB coordinates for any size using a polynomial interpolation (Figure 3c, more details on the polynomial interpolation are provided in Supporting Information). We can see that the blue dominates only for the smallest particles (70 nm in diameter). The G parameter is maximum for d = 90 nm, providing the silicon particles a pronounced green color. The red dominates in a wider range of diameters, typically in the range [90;130] nm with true red colors and not the purple ones that are commonly observed in interferential colors. When the diameters are larger than typically 170 nm, the R, G, and B parameters tend toward the same value, explaining why the color tends toward gray and white. A clear white color can be obtained by further enhancing the diameter to 210 nm. The experimental extinction spectrum of 210 nm particles displayed in Figure 2a clearly features four peaks, associated with the excitation of magnetic and electric dipolar modes near 935 and 770 nm, respectively, and with the excitation of magnetic and electric quadrupolar modes near 680 and 590 nm, respectively. The calculated x and y chromaticities of the Si nanoparticles are presented in a gamut plotted with the Color toolbox in Matlab (Figure 4a). The positions of the standard R, G, and B parameters are also indicated: the triangle defined by these three points defines the colors that can be displayed with sRGB phosphors. The chromaticities (x,y) of the Si nanoparticles were obtained from the (X,Y,Z) parameters calculated using the scattering spectra (see above) as x = X/(X + Y + Z) and y = Y/ (X + Y + Z) (white circles). The perimeter given by the solid line was obtained from the polynomial interpolation of the calculated RGB of the Si nanoparticles (see Table 1 in the Supporting Information and Figure 3c). This plot delineates the colors that can be rendered with the Si nanoparticles of height 175 nm. The internal color of the white circle in Figure 4a was coded from the experimental RGB measured on the images of Figure 1c. However, the gamut is a projection in a plane at constant luminance Y of a 3D plot. The experimental RGB values were then normalized to the same luminance before being converted to chromaticity. Obviously, the structural colors are not saturated because of the existence of the multiple resonant modes that contribute to the scattering when the diameter of the nanoparticles increases (see Figure 2).
Figure 2. (a) Experimental scattering measurements performed on silicon nanoparticles with a variable diameter (from 70 nm on the bottom to 210 nm on the top). Each spectrum corresponds to a signal from 9 to 12 similar Si particles in the collection area of diameter 4 μm. The dotted lines follow the evolution of the peaks when the particle size increases. The nature of the modes is indicated in the spectrum associated with the 180 nm particle: MD: magnetic dipole; ED: electric dipole; MQ: magnetic quadrupole; EQ: electric quadrupole. (b) FDTD-simulated scattering spectra of a silicon nanoparticle of diameters ranging between 70 nm (purple line) and 210 nm (gray line) on a fused silica substrate.
The colors perceived by the human eye result from the interplay between the spectrum of the scattered light, and the sensitivity of the three eye cone cells, the spectral emission of the source, and, in our case, of the RGB (red, green, and blue) emitters of the displaying screen. The experimental and simulated scattering spectra are displayed in Figure 3a for the case of a 90 nm particle. The standard RGB (sRGB) color space coordinates associated with the scattered spectra must be calculated. The sensitivity of human cone cells of the eye at short, middle, and long wavelength is described by three colormatching functions, x,̅ y,̅ and z,̅ which were established from experiments on a standard observer by the International Commission on Illumination (CIE) in 1931.52 The color matching functions are presented in Figure 3b together with the scattering spectrum of a 90 nm particle and the spectral response of the halogen source S(λ) (Methods section). The X, Y, Z components are obtained by integrating over the visible 7763
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Figure 3. Determination of the RGB values needed to reconstruct the visualized color from the scattering spectrum of silicon particles of Figure 1d. (a) Scattering spectra measured on the 90 nm nanoparticles (full blue line) with simulated scattering spectra obtained by FDTD (dotted green line). (b) Color-matching functions x̅ (red line), y ̅ (green line), and z̅ (blue line) displayed with respect to the wavelength, together with the source emission of the halogen lamp (dashed dotted line, arbitrary units) and the scattering cross section of the 90 nm nanoparticle (arbitrary units). The y-axis corresponds to the color-matching functions. The scattering spectra of the particles and the emission spectra of the halogen lamp are in arbitrary units. The sRGB values obtained with these functions are R = 131, G = 171, and B = 114, leading to the simulated colored square on the top right, directly comparable with the experimental image (square on the bottom right). (c) sRGB values calculated for the 14 particle diameters. The polynomial fits (red, green, and blue lines) are used to create the color palette displayed on top of the figure.
It can be observed in Figure 4a that the experimental colors, given by the internal color of the white circles, differ from the colors calculated using the scattering spectra, given by the coordinates of the white circles in the gamut. This difference can be quantified using the intrinsic color difference parameter ΔE.54 The values of ΔE between the calculated and measured colors as a function of the diameter are presented in Figure 4b. For the smallest diameter, the color difference is just at the limit of the just noticeable difference of 2.3. For larger diameters, the colors calculated and measured differ by approximately ΔE = 12. This difference mostly originates from the difficulty in estimating accurately the scattering spectra of nanoparticles when many modes are excited. The potential of silicon Mie resonators will be assessed in the next section to create an all-dielectric micropainting. We propose to reproduce at a micrometer scale, and with dielectric materials only, i.e., silicon particles on a transparent substrate, the famous painting by Piet Mondrian called “Composition in red, yellow, blue and black” (1921) (Figure 5 left). This painting is exhibited at the Haags Gemeentemuseum (The Hague, The Netherlands). It was chosen since it has
been in the public domain since January 1, 2015, and because it contains around 20 rectangular areas of different colors and contrasts: yellow, blue, red, white, and black. The original painting is 59.5 × 59.5 cm2, while the reproduction will be only 500 × 500 μm2, meaning that the painting will be reproduced at a reduction scale factor of 1:1200. Each pixel is defined by a unit cell of the square lattice, 1 × 1 μm2, containing a single silicon Mie resonator of 175 nm height with diameters ranging between 70 and 140 nm. We choose in the color palette displayed in Figure 1 the diameters that will be used for the composition: the white will be obtained with a 210 nm diameter (R = 193, G = 205, B = 214), the dark yellow is approximated with a 100 nm diameter (R = 151, G = 139, B = 84), the blue is approximated with a diameter of 70 nm (R = 49, G = 97, B = 128), and the red is approximated with a diameter of 130 nm (R = 209, G = 122, B = 117). The black color is simply obtained with unstructured surfaces. The micropainting is observed by an optical microscope (magnification 50×, numerical aperture NA = 0.7) mounted in a darkfield spectroscopy configuration. We etched with RIE (see the Methods section) the painting on the same sample, i.e., a 175 7764
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individual silicon particles hosting low-order Mie resonances. The rich extinction spectrum offered by the excitation of electric and magnetic dipolar and quadrupolar resonances provides a versatile tool to create structural colors. This allows us to create a large palette of colors when considering the same particle height and simply tuning the aspect ratio of the particle by varying the diameter. The considered periodic lattice of 1 μm avoids significant coupling between neighboring silicon particles, which allows us to carefully study the link between the color and the scattering spectrum of individual particles. The diversity of colors offered by this technique could be enriched by decreasing the period to a few hundreds of nanometers to couple the different modes hosted by individual particles. This all-dielectric technique could also be further developed with cost-effective fabrication techniques and other dielectric materials. These results prove that the rich spectrum yielded by magnetic and electric resonant scatterers offers a suitable platform to print colors at a subwavelength scale.
Figure 4. (a) Gamut presenting the calculated colors of the Si nanoparticles as a function of their diameter. The solid line delineates the domain of colors accessible with Si nanoparticles of height 175 nm. The positions of the standard R, G, and B are indicated. The white disk labeled W presents the white point used, and the yellow circle indicates the chromaticity of the yellow patch in Mondrian’s painting. (b) ΔE determined from the calculated and measured RGB coordinates as a function of the particle diameter. The dotted line indicates the just noticeable difference limit at ΔE = 2.3.
METHODS Fabrication. Thin amorphous silicon films with a nominal thickness of 175 nm were deposited in high vacuum on 1 in. diameter UV-grade fused silica substrates using e-beam evaporation of a solid source. The substrates were 1 mm thick with two sides polished. Prior to introduction in the chamber, the substrates were prepared in a clean room environment. They were degreased in acetone in an ultrasonic bath, rinsed using distilled and deionized water, and dried using dustfree nitrogen. The substrates were then introduced in the growth chamber, where they underwent a smooth plasma cleaning to remove any surface contamination. The substrates were maintained at room temperature during growth. The growth rate was monitored during evaporation using a crystal quartz microbalance, which, after a calibration procedure, allowed control of the deposited thickness. After growth, the substrates were transferred in a tubular furnace, where a thermal annealing at 600 °C was performed for 1 h under vacuum in order to yield dense silicon films. The optical constants of the annealed films were determined ex situ from analysis of spectroscopic ellipsometry measurements (Supporting Information S1). After deposition, we cleaned the samples in successive ultrasound baths in acetone and isopropyl alcohol (IPA, propan-2-ol), dried under clean nitrogen flow and exposed to oxygen plasma at 150 °C (Nanoplas, France) for 10 min in order to enhance the adhesion of the e-beam resist on the films. The positive poly(methyl methacrylate)
nm thick amorphous silicon layer coated on a fused silica substrate. The Mondrian copy obtained with structural colors is displayed in the right panel of Figure 5, next to the original painting to facilitate the comparison. The implication of the low saturation of the colors is illustrated in the case of the yellow color found in the painting by Mondrian (RGB = [210,165,32]). The corresponding (x,y) chromaticity is indicated in Figure 4a by a yellow circle. It is located close to the line joining the sR and the sG points, explaining the difference observed in Figure 5. Despite a slight difference observed for the yellow, we can observe the remarkable quality of this all-dielectric micropainting.
CONCLUSION To conclude, we report an all-dielectric printing technique based on the resonant interaction between white light and
Figure 5. On the left, the original Mondrian painting “Composition in red, yellow, blue and black,” 1921 (59.5 × 59.5 cm2). On the right, the micropainting reduction (1:1200, 500 × 500 μm2) made with silicon nanoparticles on glass. 7765
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ACS Nano (PMMA) resist (ARP-679, from Allresist, Germany) was spin-coated onto the silicon surface. A second conducting polymer layer (SX ARPC 5000/90.1 from Allresist GmBH, Germany) was spin-coated on the first PMMA e-beam resist and baked for 2 min at 90 °C. We used an EBL tool (Pioneer, Raith, Germany) equipped with a field emission gun (FEG) operating with typical exposure parameters (acceleration voltage 20 kV, aperture 15 μm), and write field 500 μm in order to avoid stitching defects. After exposure, the conducting layer was removed in a deionized water bath for 30 s. The PMMA was developed in a commercial solution (AR 600-55 from Allresist GmBH, Germany). The development was stopped by putting the sample into an IPA bath. A nickel mask was evaporated on the sample under vacuum (Auto 306 tool from Edwards, UK). After metallization, a liftoff process was performed in ethyl lactate using an ultrasonic cleaning bath for 48 h. During the lift-off process, the remaining e-beam resist was removed as well as the excess nickel in the areas of the sample that were not irradiated with the electron beam. Finally, the sample was rinsed in IPA and in deionized water and dried under a nitrogen flow. Then the nickel mask pattern was transferred into the polycrystalline thin silicon film by reactive ion etching. We used nickel as a mask material because it is very resistant to fluorine attack.55 The unprotected areas were etched in an RIE tool (MG-200, Plassys, France) by a gas mixture containing SF6, O2, and CHF3 (respective fluxes 20, 8, and 5 sccm) for several (10) seconds, alternated with a pure O2 plasma (5 s). Excited SF6 is known to efficiently etch the silicon, and the admixture of CHF3 gas was used both to passivate the vertical feature walls and to etch the silicon oxide forming during the process on the very reactive silicon surfaces.55 This process allows obtaining a very good etching anisotropy and nearly vertical walls of the structures. This sequence was repeated 7−9 times in order to completely etch the silicon layer on the areas exposed to plasma. The complete removal was confirmed by mechanical profilometer measurements (Dektak XT, Bruker, Germany). The remaining nickel was removed chemically in the acid aqueous solution of HCl and FeCl3. Finally, the samples were rinsed in deionized water and dried under nitrogen flow. A sketch of our fabrication process can be found in Figure 6 of ref 49. Optical Characterization. Optical spectroscopy was performed in reflection mode with a dark-field microscope. A white lamp (Halogen 4100K, Philips) was focused on the sample using a 50× dark-field objective (NA = 0.7, Nikon). The collection of the signal was performed by the same objective. The collected light was then focused by a tube lens (200 mm) on an optical fiber (200 μm diameter) placed in the confocal plane of the sample. This configuration allows for a spatial filtering in the three dimensions of space. The collection area is then 4 μm diameter, corresponding to 9−12 nanoparticles in the spot.
processes were performed in a PLANETE CT PACA cleanroom facility.
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ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b03207. Refractive index of silicon, numerical simulations of scattering cross sections of silicon monomers and dimers, polynomial fit of the RGB parameters (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was carried out with the support of the A*MIDEX project (No. ANR-11-IDEX-0001-02) funded by the Investissements d’Avenir French Government program and managed by the French National Research Agency (ANR). Nanofabrication 7766
DOI: 10.1021/acsnano.6b03207 ACS Nano 2016, 10, 7761−7767
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DOI: 10.1021/acsnano.6b03207 ACS Nano 2016, 10, 7761−7767
