Spectroscopy of silica layers containing Si nanocrystals: Experimental evidence of optical birefringence

We report an unusual case of spectral filtering by a silica waveguide containing Si nanocrystals (Si-nc’s) deposited on a silica plate. For a number of Si-rich silica (SiOx) slab waveguides annealed at 1100°C, the TE and TM waveguide mode cutoff positions are found in the inversed order with respect to the classical waveguide theory for an isotropic material. Using the cutoff and m-line spectra, this unusual behavior was explained assuming an optical birefringence of the material. For the highest Si content (x∼1.5), we estimated a maximal positive birefringence of ∼8%. The cutoff spectrum simulated with the optical parameters extracted from the m-line measurements corresponds well to the cutoff spectrum directly obtained by measuring waveguided luminescence. This agreement shows that the spectral filtering effect of silica layers containing Si-nc can be described within the quantitative model of delocalized waveguide modes. The possible origin for the observed birefringence is discussed.