Er³⁺ excited state absorption and the low fraction of nanocluster-excitable Er³⁺ in SiO_x

Silicon nanoclusters (Si-nc) can be efficient sensitisers of surrounding Er ions, making Er-doped silicon-rich silica material very appealing for 1.5µm amplifiers and lasers pumped by broadband sources. Recent reports of optical signal enhancement in waveguides based in this system have increased the interest even more. However, there is currently a debate on how many surrounding Er ions can one single nanocrystal excite. In fact, there is considerable experimental evidence that the fraction of Er ions excited by Si-nc is actually very low, of the order of few percent. We point out that excited state absorption (ESA) is a major cause for this low fraction of indirectly excitable Er