Wavelength-selective optical waveguiding of photoluminescence in a thermally annealed Si/SiO2 superlattice

We investigate the long-distance propagation of broadband photoluminescence (PL) light inside an annealed Si/SiO2 superlattice (SL) with similar to600 repeats of 1.5 nm Si and 2 nm SiO2 layers deposited on a silica substrate. The SL material annealed at 1150degreesC contains Si nanocrystals with diameters of 34 nm as estimated by Raman spectroscopy, and the SL layer was estimated to be overall 1.75 mum thick with an effective refractive index of 1.65. As measured in the conventional transverse detection geometry, the SL material exhibits a broad PL band with a maximum at similar to780 nm (1.6 eV). Our measurements in the waveguiding detection geometry show wavelength selectivity of optical waveguiding by the SL layer. In fact, efficient narrowing of the PL spectrum (down to 16 meV) takes place upon the propagation of the PL light inside the SL layer for wavelengths of 710 and 960 nm. We observed the guiding of PL light by the SL layer to distances above 5 mm, and the propagation losses for the guided light at 710 and 960 nm were estimated to be 6.2 and 4.7 cm(-1), respectively. The attenuation of PL light outside the guided wavelengths is much larger (e.g., similar to11 cm(-1) at 780 nm) demonstrating that Si/SiO2 SL-based waveguides can be used as optical filters. The transmittance peaks can be tuned by changing the SL layer optical thickness.