Studies of coherent backscattering from photonic opal crystals

In this research we employed the coherent backscattering (CBS) technique to study light scattering inside opal photonic crystals, which are made of self-assembled silica spheres ∼300 nm in diameter. In the fabrication process defects were introduced into the crystalline structures that results in a reduced light mean free path, l*, inside the opal crystals. By measuring CBS from these single-crystalline opals we could obtain properties related to l* for light inside the opal crystals.; We found that l* for opal photonic crystals was of order of 10 mum (∼30 layers of {lcub}111{rcub} planes) in air. At Bragg angles we observed abnormal broadening in the CBS cones due to Bragg diffraction off the {lcub}hkl{rcub} planes of the opal crystal, from which we could estimate the Bragg decay length, LB, to be 5 mum for {lcub}111{rcub} planes. Using the same technique, we also found that the relative LB of {lcub}220{rcub} planes is about 40% longer than that of the {lcub}111{rcub} planes.; Moreover we found a tilt-angle dependence for the measured CBS cones of the opal single crystals, i.e., as the incident angle was tilted away from the [111] direction, the CBS width increased. This behavior was not observed in the disordered silica suspensions, milk, and polycrystalline opal crystals. This behavior indicates that the scattering of light in opal crystals may result from large size planar defects. To check this speculation we measured the wavelength dependence of the CBS, we found very weak wavelength dependence for the mean free path indicating that the size of the scatterers is large compared with the probe light wavelength. Also SEM (scanning electron microscope) studies of the opal samples have shown the existence of large-size planar defects, in agreement with the CBS measurements.; We found that one can adjust l* by using dielectric matching solvents. By infiltrating methanol inside the opal, we observed that l* increase to >80 mum, which was much longer than that in air. This increase results from the reduction of the scattering cross-section due to index matching between the solvent and the silica balls.