Photochemically Controlled Cross-Linking in Polymerized Crystalline Colloidal Array Photonic Crystals

Abstract

We developed photochemically controlled photonic crystals which may be useful in novel recordable and erasable memories and/or display devices. Information is recorded and erased by exciting the photonic crystal with 360 nm UV light or 480 nm visible light. The information recorded is read out by measuring the photonic crystal diffraction wavelength. The active element of the device is an azobenzene cross-linked hydrogel which contains an embedded crystalline colloidal array. UV excitation forms cis-azobenzene cross-links while visible excitation forms trans-azobenzene cross-links. The less favorable free energy of mixing of cis-azobenzene cross-linked species causes the hydrogel to shrink and blue-shift the photonic crystal diffraction. This is completely the opposite behavior as observed from pendant azobenzene groups we reported previously. We also observe fast nano-, micro-, and millisecond transient dynamics associated with fast heating lattice constant changes, refractive index changes, and thermal relaxations.