Light emission and propagation in Er-doped silicon-rich oxide layers

Erbium (Er) doped photonic materials and structures continue to attract considerable attention due to possible applications in telecommunications at a wavelength of 1.54 μm. Photo-excitation of Er ions in an insulator is generally achieved by resonant high power pumping with a laser. However, efficient non-resonant excitation has been demonstrated in the presence of silicon nanocrystals (Si-nc), when both are present in the same silica substrate. The nanocrystals are believed to act as a sensitizer, absorbing incident radiation over a wide spectral range and coupling it efficiently to Er³⁺ ions. In comparison with Er-doped bulk silicon, this new medium (SiO₂:Si-nc+Er) shows a strong room temperature 1.54 μm emission due to a reduction in non-radiative recombinations. This raises the possibility of making a room-temperature optical amplifier that operates under broad band pumping. To this end, we have examined the optical properties of layers containing Si nanoclusters and Er ions under different optical pumping conditions and examined the optical properties of slab waveguides fabricated from these materials. Samples were prepared by co-implantation of silicon and erbium into thermal SiO₂ followed by a single thermal anneal, at temperatures ranging from 800 to 1100 °C, and with and without hydrogen passivation performed at 500°C. The excitation cross-section and photoluminescence of erbium in Si-rich SiO₂ was determined from time-resolved photoluminescence measurements. It has been found that the number of optically active Er ions decreases while the excitation cross section increases with annealing temperature. Optical gain measurements were performed on slab waveguides containing excess silicon in the form of nanoclusters or nanocrystals and erbium. The measurement were performed using a pump-probe technique in which a 1.54 μm probe beam was prism-coupled into the waveguide and its intensity and temporal response monitored as the waveguide was optically pumped from above with a chopped 477 nm excitation source. Induced-absorption (losses) of the 1.54 μm probe beam in erbium-doped and un-doped silicon-rich silicon oxide waveguides was observed in all cases, as seen in Figure 1. For the samples containing only nanocrystals a fast (∼60 μs) induced absorption component associated with free carriers within the silicon nanocrystals is reported, whilst for samples containing nanocrystals and erbium a much slower (>10 minutes) component is observed. The free carrier contribution is shown to be reduced by delaying the probe beam relative to the pump beam. Importantly, measurements on waveguide structures annealed at temperatures below that required for the formation of silicon nanocrystals were found to exhibit minimal induced absorption and no dependence with a delayed pump respect to the probe. In this contribution, a detailed discussion on the sensitizing effect of silicon nanoclusters will be explored as a means of minimizing induced absorption and increasing the number of optically active erbium ions as a plausible way for realizing an integrated optical amplifier