Structural color systems with the combinations of silicon nanowire arrays and ultra-thin 1D resonators

Abstract

Structural colors, which can occur by the interaction between visible light and exceptional photonic structures, have rapidly emerged as a key alternative to the traditional dyes or pigments because of their conspicuous advantages as following: 1) outstanding spatial resolution, 2) durability under harsh environments, 3) versatile utilizations, 4) compactness, 5) eco-friendly materials, and 6) spectral selectivity. With these strong points, various structural colors including plasmonic nanostructures, metal-dielectric multilayers, and photonic crystals have attempted to substitute with the conventional dyes and pigments. Among them, silicon (Si) nanostructures are considered as the most suitable candidates because of the low-cost/mature fabrication process and excellent optical constants (i.e., high refractive index and low absorption loss compared to metals). Using the aforementioned properties, multicolor generation by Si nanowire arrays (Si NWAs) has been successfully demonstrated for reflective/transmissive structural color filters. However, the restricted color presentation of currently-reported structural colors has hindered the wide spreading of the promising structural color printing. Here we propose a new class of reflective color filters to enlarge color gamut by attaching a sticker form of Si NWA on ultra-thin one-dimensional resonators which are in nanoscale. To implement the sticker form of Si NWAs, we fabricate transferable Si NWAs (T-SiNAs) embedded in polydimethylsiloxane (PDMS) which is a visibly transparent polymer. The nanoscale structures of each photonic configuration allow an exceptional mechanical softness, hence it enables flexibility and reusability without mechanical failure. First, we design and fabricate the T-SiNAs to possesses optical resonances covering from visible to near-infrared (NIR) ranges (i.e., 400 to 1000 nm). The resonance dip positions are shifted from short to long wavelengths by increasing the diameter of T-SiNAs. The fabricated T-SiNAs have the optimized structural parameters as follows: height = 2 μm, diameter = 50 to 150 nm with 10-nm-step, and period = 1250, 900, and 600 nm. To demonstrate the widening ability in color gamut, we selected metal-insulator-metal (MIM; Ag-SiO2-Ag) structure as ultra-thin 1D resonator. Generally, MIM structures exhibit subtractive primary colors such as cyan, magenta, and yellow. However, by transferring T-SiNAs on MIMs, we enlarge the pristine color gamut of MIMs and experimentally realize additive primary colors such as red, green, and blue. Furthermore, such intriguing optical and mechanical characteristics facilitate a novel optical anti-counterfeiting sticker.

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