Tunable wavelength-selective waveguiding of photoluminescence in Si-rich silica optical wedges

Abstract

We report accurate tuning of spectral peaks built up upon guiding of broadband light by annealed Si-rich silica layers with varying optical thickness. The optical wedges are characterized with various methods (photoluminescence, Raman spectroscopy, visible absorption, and x-ray photoelectron spectroscopy) showing that their properties change smoothly from point to point along the sample surface. As measured in the conventional transverse detection geometry, the annealed (1100 °C) Si-rich silica material exhibits well-known broad photoluminescence (PL) with maximum at ∼800 nm conventionally connected with Si nanocrystals. In the waveguiding detection geometry, the guiding of PL light by the silica layer containing Si nanocrystals is wavelength-dependent, and the detected spectrum exhibits narrow (down to 100 cm−1) and linearly polarized spectral peaks. As the central observation of the present work, these spectral peaks can be accurately tuned over the whole PL spectral region by moving the optical wedges across the laser beam. For our samples, the tuning coefficient is ∼150 cm−1/mm. It is shown that tuning of the transmission peaks is caused by change of both refractive index (from 1.6 to 1.9) and thickness (from 1.4 to 2.0 μm) of the prepared optical wedges. The propagation loss for the guided light is found to increase for shorter wavelengths and for materials with larger Si concentration. The propagation losses reliably below 0.5 cm−1 are observed in the broad spectral interval in sample areas with smaller Si concentrations.