Tunable multicolor photonic crystal/long persistent phosphor composites for dual-mode display in safety sign and information encryption

Photonic crystals (PCs) can generate vivid structural colors and show great potential in color-related applications such as smart displays, information encryption, and anti-counterfeiting. However, the structural colors depend on the reflection of light source, resulting in invisibility in the dark. Although PCs are incorporated with luminophores to extend their application in darkness, the luminescence color is usually determined by the luminophores which are embedded in PCs, resulting in only one emission color output each time. In this work, we fabricate opal PCs on long persistent phosphor/poly(methyl methacrylate) (LPP) substrates to obtain dual-mode display PC/LPP composites that showed variable color outputs both in daylight and in darkness. By matching the photonic band gap of PC with the emission band of long persistent phosphors (LPs), e.g. Eu, Dy doped strontium aluminate (LP1), or Eu, Dy, Ti, Y doped strontium aluminate (LP2), the PC constructed by polystyrene microspheres was used as a tunable filter. In the daylight, the PC/LPP composites showed high quality angle-depended structural color when the PC thickness was 5.4 μm. While in the darkness, the emission wavelength of PC/LPP composites became tunable. The PC/LPP2 composites with LP2 as the phosphor showed obvious luminous color change (megenta, light pink, yellow-green and blue-green) under different viewing angles (0°, 15°, 30° and 45°). Especially, being excited by the transmitted light in daylight, the PC/LPP composites showed persistent luminescence and the afterglow intensity of 5.4 μm-PC/LPP2 composites achieved 4.90 cd/m². Due to the angle-depended multicolor output properties at different light conditions, PC/LPP composites can be used in 24-h safety sign and information encryption.