Optically nonlinear Bragg diffracting nanosecond optical switches.

摘要

We prepared low refractive index crystalline colloidal arrays (CCA) from highly charged fluorinated monodisperse spherical particles synthesized by emulsion polymerization of 1H,1H-heptafluorobutyl methacrylate. We have also covalently attached dyes to the fluorinated particles to prepare absorbing CCA. We photopolymerized these dyed CCA within a polyacrylamide matrix to form a polymerized crystalline colloidal array (PCCA). These semi-solid PCCA can withstand vibrations, ionic impurity addition and thermal shocks while maintaining the CCA ordering. The medium within the PCCA can easily be exchanged to exactly refractive index match the CCA.; 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. Under low light intensities the 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 nsec 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 nsec. These intelligent CCA hydrogels may have applications in optical limiting, optical computing and nsec fast optical switching devices, etc.; We have also measured the polarization dependence of the Bragg diffraction efficiency of a CCA and compared the experimental results to that predicted by theory. The diffraction efficiency is maximized for {dollar}sigma{dollar} polarization light at Bragg angle {dollar}rm(thetasb{lcub}B{rcub}){dollar} of 90{dollar}spcirc{dollar} and minimized to zero for {dollar}pi{dollar} polarized light at {dollar}rmthetasb{lcub}B{rcub}=45spcirc.{dollar} Our experimental diffraction and transmission results quantitatively agree with the predictions of Dynamical Diffraction Theory.