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Structural coloration with hourglass-shaped vertical silicon nanopillar arrays

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Abstract

We demonstrate that arrays of hourglass-shaped nanopillars patterned into crystalline silicon substrates exhibit vibrant, highly controllable reflective structural coloration. Unlike structures with uniform sidewall profiles, the hourglass profile defines two separate regions on the pillar: a head and a body. The head acts as a suspended Mie resonator and is responsible for resonant reflectance, while the body acts to suppress broadband reflections from the surface. The combination of these effects gives rise to vibrant colors. The size of the nanopillars can be tuned to provide a variety of additive colors, including the RGB primaries. Experimental results are shown for nanopillar arrays fabricated using nanoimprint lithography and plasma etching. A finite difference time domain (FDTD) model is validated against these results and is used to elucidate the electromagnetic response of the nanopillars. Furthermore, a COMSOL model is used to investigate the angle dependence of the reflectance. In view of display applications, a genetic algorithm is used to optimize the nanopillar geometries for RGB color reflective pixels, showing that nearly all of the sRGB color space and most of the Adobe RGB color space can be covered with this technique.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Figures (11)

Fig. 1
Fig. 1 Schematic showing the nanopillar fabrication process.
Fig. 2
Fig. 2 (a) Tilted cross-sectional SEM (Scanning Electron Microscopy) images (scale bar 400 nm), (b) 5X microscope images of samples (scale bar 100 µm), and (c) Experimental and simulated reflectance spectra (solid black and red dashed lines, respectively) of the corresponding samples (by row). In (b), the large squares contain the nanopillar arrays, and the “streets” in between the squares contain bare Si and occasionally some alignment structures. The bare Si appears yellow in the images due to the yellow hue of the source lamp in the microscope. Geometric parameters for the simulations in (c) are taken from cross-sectional SEMs as detailed in Fig. 3. A thin oxide layer is added to the simulation to account for surface etch roughness and oxidation (5 nm for blue, 3 nm for green, 0 nm for red). In (c) the simulation wavelength range is from 360 to 830 nm, but the experimental measurement was limited to 400-700 nm.
Fig. 3
Fig. 3 (a) Schematic showing the structural parameters of the nanopillars and their corresponding labels. (b) SEM measurements (in nm) of the structural parameters for the three experimental structures shown in Fig. 2.
Fig. 4
Fig. 4 (a) Electric field plots for the experimental green nanopillar array (left) and the corresponding floating head array (right). (b) Magnetic field plots for the nanopillar array (left) and floating head array (right). Background color plots are absolute value of the field and arrows are field vectors. (c) 3D model of the hourglass-shaped nanopillar array. (d) 3D model of the floating head array. (e) Reflectance of the two different structures (solid black is the nanopillar array and dashed red is the floating head array).
Fig. 5
Fig. 5 (a) Photographs of the green sample at various angles taken inside an integrating sphere illuminated with white light (scale bar = 5 mm). Circular spots on the samples are areas of missing pattern caused by particles during imprinting. In order to correct for different exposure times and white points, each photo was adjusted to have the same RGB values at the same pixel corresponding to a point on the inner wall of the integrating sphere which was visible in each photograph. (b) Simulated angle-dependence of the nanopillar structure reflectance for TE polarized illumination. (c) Simulated angle-dependence of the nanopillar structure reflectance for TM polarized illumination. (d) Simulated angle-dependence of the nanopillar structure reflectance for 45° polarized illumination shown from 400 to 700 nm to match with experimental results. (e) Experimental angle-dependent reflectance of the nanopillar structure (unpolarized illumination). The range of angles corresponding to the unpolarized experimental measurement (15° to 65°) is shown bounded by the dotted black lines in (d) and (e). Experimental measurements were taken at 15°, 25 o, 30 o, 35 o, 45 o, 60 o, 65 o and the plot was smoothed using interpolation.
Fig. 6
Fig. 6 Results of the genetic algorithm optimization. (a) Plots showing the optimized geometries for the pillars corresponding to red, green, and blue (top to bottom). There are two results for green: one for sRGB (left) and one for Adobe RGB (right). (b) Simulated reflectance corresponding (by row) to the optimized pillar geometries. For the green result, the Adobe RGB reflectance curve is shown by the darker, dotted green line. (c) CIE chromaticity plot with sRGB and Adobe RGB color spaces outlined by the black triangles, the D65 white point shown by the filled white circle in the middle, the three experimental results shown by the black empty circles, and the four optimization results shown by the filled black circles. (d) Table of the optimized parameter values (in nm).
Fig. 7
Fig. 7 (a) Electric field plots for the nanopillar array optimized for the Adobe RGB green primary point (left) and corresponding floating head array (right). (b) Magnetic field plots for the nanopillar array (left) and floating head array (right). (c) Reflectance of the two different structures (solid black is the nanopillar array and dashed red is the floating head array).
Fig. 8
Fig. 8 (a) Schematic of the simulation with TM polarized illumination. (b) Magnetic (left) and electric (right) field plots of the nanopillar when θ = 0° and λ = 525 nm. (c) Magnetic (left) and electric (right) field plots of the nanopillar when θ = 50° and λ = 525 nm. Background color plots are absolute value of the field and arrows are field vectors.
Fig. 9
Fig. 9 (a) Schematic of the simulation with TE polarized illumination. (b) Magnetic (left) and electric (right) field plots of the nanopillar when θ = 50° and λ = 485 nm. (c) Magnetic (left) and electric (right) field plots of the nanopillar when θ = 50° and λ = 525 nm. Background color plots are absolute value of the field and arrows are field vectors.
Fig. 10
Fig. 10 (a) Plot showing the geometry of the three nanopillars (red, green, and blue left to right). (b) Simulated reflectance curves of the three structures. (c) CIE chromaticity plot with sRGB and Adobe RGB color spaces outlined by the black triangles, the D65 white point shown by the filled white circle in the middle, the three GA results shown by the black filled circles, and the three constant height results shown by the open white circles.
Fig. 11
Fig. 11 Photographs of fabricated 4” wafers.
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