An analysis of erbium excited state absorption in silicon-rich silica

Abstract

Erbium-doped silicon-rich silica is a material that has generated great interest, as it holds considerable promise for practical silicon-based optical sources. There are numerous reports of sensitization of erbium 1.5 μm luminescence by silicon nanoclusters (Si nc), and it is now accepted that this process increases the effective absorption cross-section of Er ions by three orders of magnitude. There have been reports of optical gain in a planar waveguide pumped at wavelengths away from Er absorption bands. However, no lasing action has yet been achieved. In this paper, we discuss a key interaction—excited state absorption (ESA)—which has been neglected in this material. Typically, Si nc responsible for sensitization have a broad photoluminescence peak centred around 800 nm. Moreover, spectral hole-burning studies suggest that resonant energy transfer occurs from the Si to the Er in this wavelength region. However, this energy transfer route may be problematic, due to an ESA transition known from early work on erbium-doped fibre amplifiers (EDFAs). Excitation around 800 nm causes the Er metastable state to be excited to higher-level states. Such transitions serve no useful purpose, but constitute an energy drain: EDFAs pumped at 800 nm require an order of magnitude more pump power to achieve the same gain as those pumped at 980 nm. We have conducted an analysis of the erbium-doped silicon-rich silica system, incorporating ESA in the model. We compare the results of our analysis with reported experimental data and extract the Si nc–Er excited state energy transfer coefficient, yielding a value of 1×10 −15 cm 3/s. This is comparable to the Si nc–Er ground state transfer coefficient, confirming that ESA is potentially significant.