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

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.