Rotational Periodicity Display of the Tunable Wettability Pattern in a Photoswitch Based on a Response Bilayer Photonic Crystal

Abstract

Although the forward diffraction of the threedimensional (3D) photonic crystal is easily applied to a photoswitch, backward diffraction rainbows are rarely reported. The first rotational photoswitch based on a bilayer 3D photonic crystal with backward diffractions similar to those of twodimensional photonic crystals was fabricated by vertically combining different thicknesses of nanoparticle templates. When rotating the bilayer photonic crystal, the opening or closing of the rotational photoswitch shows periodic reproducibility values of 30 and 60°. Different periods are regulated by the thickness and scattering effect of the top layer. Moreover, invisible patterns can be encoded and erased by changing the wettability via pH. Because of the decreasing of the refractive index differentials, it will be revealed rapidly when immersed in water. The revealed pattern can be periodically turned on and off by rotating the bilayer photonic crystal. It has great application prospects in optical prism, warning board, anti-counterfeiting, steganography, watermarking, and complex information coding.

Retroreflection and Wettability Controlled Smart Indicator Based on Responsive Bilayer Photonic Crystals for Traffic Warning

“…In our previous reports, [ 36 ] due to the Bragg diffractions, [ 37 ] 2D out‐of‐plane diffractions, [ 38–40 ] and diffuse scattering [ 37,41 ] of the photonic crystals, the light is diffracted back in its direction of incidence, resulting in the grating diffract‐like retroreflection. [ 42–44 ] According to the 2D diffraction relationship, [ 45 ] , where m is the diffraction order, λ is the diffracted wavelength (in vacuum), d is the lattice spacing, and θ is the incident angle.…”

“…Preparation of Monodispersed SiO 2 Nanospheres, SiO 2 Arrays, Monolayer Inverse Opals, and Bilayer Films: SiO 2 nanospheres, SiO 2 arrays, and monolayer inverse opals were prepared under a modified Stöber method (detailed information is listed in Figures S1-3, and Table S1, Supporting Information). [36,54] For the preparation of bilayer photonic crystals, "template A" and "template B" were combined into the sandwich structure after dislocation of 90°. [36] By utilizing the capillary force, the interstitials in-between SiO 2 microspheres of the "sandwich" template were filled up with oligomer mixtures consisting of ETPTA, PEGDA (200), and BMA with varying volumetric ratios from 1:4:0.25 to 1:6:0.25.…”

“…[36,54] For the preparation of bilayer photonic crystals, "template A" and "template B" were combined into the sandwich structure after dislocation of 90°. [36] By utilizing the capillary force, the interstitials in-between SiO 2 microspheres of the "sandwich" template were filled up with oligomer mixtures consisting of ETPTA, PEGDA (200), and BMA with varying volumetric ratios from 1:4:0.25 to 1:6:0.25. Darocur 1173 (2-hydroxy-2-methyl-1-phenyl-1-propanone, BASF, 1 wt%) was added as a photoinitiator.…”

“…In our previous reports, [36] due to the Bragg diffractions, [37] 2D out-of-plane diffractions, [38][39][40] and diffuse scattering [37,41] of the photonic crystals, the light is diffracted back in its direction of incidence, resulting in the grating diffract-like retroreflection. [42][43][44] According to the 2D diffraction relationship, [45] 3 sin…”

“…The retroreflection is designed for landslide, swerve, and sidewalk warning. By using the rotational periodic diffraction of the “Layer B,” [ 36 ] the traffic warning can be set up or disarmed by rotating the film. Moreover, flickering structural colors were manufactured and used for speed limits warning.…”