NANOSECOND SWITCHABLE POLYMERIZED CRYSTALLINE COLLOIDAL ARRAY BRAGG DIFFRACTING MATERIALS

We fabricated a mesoscopically periodic array of novel colloidal parti cles containing absorbing dye embedded in a polyacrylamide hydrogel th rough the use of crystalline colloidal self-assembly. These colloidal particles are highly fluorinated (see accompanying paper) and have a v ery low refractive index, which can be easily refractive index matched to a predominantly aqueous medium. Thus, we were able to prepare a ma terial where the real part of the refractive index was matched, while preserving a periodic modulation of the imaginary part of the refracti ve index. Previously we theoretically predicted that such a material c ould be used as a nanosecond optical switch (Kasavamoorthy, Super, and Asher, J. Appl. Phys. 1992, 71, 1116), and more recently experimental ly demonstrated this optical switching (Pan et al., Phys. Rev. Lett. 1 997, 78, 3860). Under low light intensities the crystalline colloidal arrays (CCA) is refractive index matched to the medium and does not di ffract. However, high incident intensity illumination within the dye a bsorption band heats the particles within nanoseconds to decrease thei r refractive index. This results in a mesoscopically periodic refracti ve index modulation with the periodicity of the CCA lattice. The array ''pops up'' to diffract light within 2.5 ns. These intelligent CCA hy drogels may have applications in optical limiting, optical computing, and nanosecond fast optical switching devices, etc.