Nanosecond Switchable Polymerized Crystalline Colloidal Array Bragg Diffracting Materials

Abstract

We fabricated a mesoscopically periodic array of novel colloidal particles containing absorbing dye embedded in a polyacrylamide hydrogel through the use of crystalline colloidal self-assembly. These colloidal particles are highly fluorinated (see accompanying paper) and have a very low refractive index, which can be easily refractive index matched to a predominantly aqueous medium. Thus, we were able to prepare a material where the real part of the refractive index was matched, while preserving a periodic modulation of the imaginary part of the refractive index. Previously we theoretically predicted that such a material could be used as a nanosecond optical switch (Kasavamoorthy, Super, and Asher, J. Appl. Phys. 1992, 71, 1116), and more recently experimentally demonstrated this optical switching (Pan et al., Phys. Rev. Lett. 1997, 78, 3860). Under low light intensities the crystalline colloidal arrays (CCA) is refractive index matched to the medium and does not diffract. However, high incident intensity illumination within the dye absorption band heats the particles within nanoseconds to decrease their refractive index. This results in a mesoscopically periodic refractive index modulation with the periodicity of the CCA lattice. The array “pops up” to diffract light within 2.5 ns. These intelligent CCA hydrogels may have applications in optical limiting, optical computing, and nanosecond fast optical switching devices, etc.