We developed a robust nanosecond photonic crystal switching material by using poly(N-isopropylacrylamide) (PNIPAM) nanogel colloidal particles that self-assemble into crystalline colloidal arrays (CCAs). The CCA was polymerized into a loose-knit hydrogel which permits the individual embedded nanogel PNIPAM particles to coherently and synchronously undergo their thermally induced volume phase transitions. A laser T-jump from 30 to 35 degrees C actuates the nanogel particle shrinkage; the resulting increased diffraction decreases light transmission within 900 ns. Additional transmission decreases occur with characteristic times of 19 and 130 ns. Individual NIPAM sphere volume switching occurs in the approximately 100 ns time regime. These nanogel nanosecond phenomena may be useful in the design of fast photonic crystal switches and optical limiting materials. Smaller nanogels will show even faster volume phase transitions.

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 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‐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 red‐shift 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.

We demonstrate azobenzene photochemically driven diffraction switching of a photonic crystal consisting of a crystalline colloidal array (CCA) polymerized within a hydrogel matrix. A novel azobenzene derivative that has a large ground-state activation barrier between the cis and trans forms in water is used. The system is actuated by excitation with UV light (wavelength of 365 nm), which photoisomerizes the azobenzene trans state to the cis ground state. The increased dipole moment of the cis state increases the free energy of mixing, causing a hydrogel swelling, which red-shifts the embedded CCA diffraction. Excitation with visible light photoisomerizes the cis state to the trans state, which resets the diffraction. This material acts as a memory storage material. Information is recorded and erased by exciting the photonic crystal in the UV or visible spectral region. The written information is read out completely and nondestructively by the wavelength of the Bragg diffraction (in this case, in the red).Many groups are developing fabrication methods to produce photonic crystals with band gaps in the visible, infrared (IR), and microwave spectral regions. [1][2][3][4][5][6] Photonic crystals are materials with periodic variations in their optical dielectric constants. The resulting periodic variations in the refractive index lead to the diffraction of light and to the occurrence of photonic band gaps. Light with frequencies within the band gaps cannot propagate within the photonic crystal materials. If the refractive index is purely real, then all of the light is diffracted. Photonic crystal materials provide the opportunity to control the flow of light. Photonic crystals may be the key elements of all-optical integrated circuits. 7 The earliest chemical approach to fabricating photonic crystals was through the self-assembly of highly charged monodisperse colloidal particles into crystalline colloidal arrays (CCAs). 8 These CCAs are complex liquids that self-assemble because of long-range electrostatic repulsions between particles into fluid, plastic, face-centered cubic (fcc) crystalline arrays. The CCAs diffract light (Bragg diffraction) in the ultraviolet (UV), visible, or near-IR range, depending on the colloidal particle array spacings. 8,9 Robust semisolid polymerized crystalline colloidal array (PCCA) materials were fabricated by imbedding the CCA into a hydrogel network (see Figure 1). 9 PCCAs can be fabricated from stimuli-responsive polymer networks, 10,11 where appropriate physical or chemical stimuli alter the PCCA volume, which alters the resulting CCA plane spacings and diffraction wavelengths. 12-14 Previously, we described photochemically controlled PCCA photonic crystals, which operated in organic solvents such as dimethylsulfoxide (DMSO), which utilized azobenzene and spirobenzopyran photochemistry. 15 In our azobenzene-functionalized PCCA, the photochemistry involved photoisomerization between the trans and cis isomers in an organic medium. In the spiropyran-modified PCCA, the photochromi...

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.