Photonic crystal flakes

Photonic crystals (PCs) have been traditionally produced on rigid substrates. Here, we report the development of free-standing one-dimensional (1D) slanted PC flakes. A single pulse of a 5 ns Nd:YAG laser (λ = 532 nm, 350 mJ) was used to organize silver nanoparticles (10–50 nm) into multilayer gratings embedded in ∼10 μm poly(2-hydroxyethyl methacrylate-co-methacrylic acid) hydrogel films. The 1D PC flakes had narrow-band diffraction peak at ∼510 nm. Ionization of the carboxylic acid groups in the hydrogel produced Donnan osmotic pressure and modulated the Bragg peak. In response to pH (4–7), the PC flakes shifted their diffraction wavelength from 500 to 620 nm, exhibiting 0.1 pH unit sensitivity. The color changes were visible to the eye in the entire visible spectrum. The optical characteristics of the 1D PC flakes were also analyzed by finite element method simulations. Free-standing PC flakes may have application in spray deposition of functional materials.