Photochemically controlled photonic crystals

We have developed photochemically controlled photonic crystals that may be useful in novel recordable and erasable memories and/or display devices. These materials can operate in the UV, visible, or near-IR spectral regions. Information is recorded and erased by exciting the photonic crystal with - 360 nm UV light or - 480 mn visible light. The information recorded is read out by measuring the photonic crystal diffraction wavelength. The active element of the device is an azobenzene-functionalized hydrogel, which contains an embedded crystalline colloidal array. UV excitation forms cis-azobenzene while visible excitation forms trans-azobenzene. The more favorable free energy of mixing of cis-azobenzene causes the hydrogel to swell and to redshift the photonic crystal diffraction. We also observe fast nanosecond, microsecond, and millisecond transient dynamics associated with fast heating lattice constant changes, refractive index changes, and thermal relaxations.