FORTUITOUSLY SUPERIMPOSED LATTICE PLANE SECONDARY DIFFRACTION FROM CRYSTALLINE COLLOIDAL ARRAYS

Dispersions of monodisperse macroionic spherical particles self assemb le into bcc or fee crystalline colloidal arrays (CCA), which efficient ly Bragg diffract light in the near-IR, visible, and UV spectral regio ns, depending on the lattice constant and the crystal structure. We re port here the observation of an anomalously intense ''secondary diffra ction'', which occurs at half the wavelength (lambda(B)/2) of light di ffracted by the lattice planes with highest particle density (the bcc (110) or the fee (111) planes). This diffraction is > 10-fold more int ense than the primary diffraction at lambda(B), and is narrow for ligh t incident normal to the bcc (110) or the fee (111) planes. This secon dary diffraction results from fortuitously superimposed diffraction fr om numerous lattice planes oriented such that they diffract ca. lambda (B)/2 light. We quantitatively modeled the primary and secondary diffr action by using dynamical diffraction theory. This efficient diffracti on phenomenon should be useful for optical device fabrication.