We present investigations of the optical gain cross section of $1.54\mu \text{m}$ Er-related emission at 4.2 K in Si/Si:Er molecular-beam-epitaxy-grown multinanolayers. This ultranarrow (full width at half maximum below $8\mu \text{eV}$) emission originating from the unique Er-related optical complex, Er-1 center, ensures the best condition to achieve stimulated emission. The experiments were carried out using a combination of the variable stripe length and shifting excitation spot techniques under pulsed and continuous-wave excitations. The comparison of results for both excitation regimes enables to estimate an optical gain within the strong optical losses due to the free carrier absorption. Based on these measurements, the upper limit of the optical gain cross section of ${10}^{-17}{\text{cm}}^2$ and the gain coefficient of $8.8{\text{cm}}^{-1}$ are deduced.

We present evidence of a mechanism for the indirect excitation of Er3+ ions in a SiO2 matrix sensitized with Si quantum dots (SiQDs). The proposed process enables the simultaneous and rapid excitation of two proximal Er3+ ions upon absorption of a single high-energy photon in a SiQD. The experimental evidence leading to the identification of this energy-transfer path is obtained from investigations of the photoluminescence quantum yield of two Er-related emission bands at 1.54 and 0.98 $\mu${}m in SiO2 layers doped with small ($\sim${}2-nm-diam) SiQDs and a high concentration of Er3+ ions, prepared by sputtering on hot substrates. In contrast to the previously considered mechanisms for the indirect excitation of Er3+ ions in SiO2, this excitation process offers an advantage of efficient suppression of the most important channels of nonradiative deexcitation\char22{}the Auger energy transfer to free carriers as well as the so-called energy back transfer of excitation reversal. This feature revives hopes for the practical application of Er-doped SiO2 sensitized by SiQDs.