Far-infrared stimulated emission from optically excited bismuth donors in silicon

Far-infrared stimulated emission from optically pumped neutral Bi donors in silicon has been obtained. Lasing with wavelengths of 52.2 and 48.6 μm from the intra-center ${\text{2p}}_{\pm }{\text{→1s(E:Γ}}_8{\text{),1s(T}}_2{\mathrm{:\Gamma }}_8)$ transitions has been realized under ${\mathrm{CO}}_2$ laser pumping. The population inversion mechanism is based on fast optical-phonon-assisted relaxation from the ${\text{2p}}_0$ and $\text{2s}$ excited states directly to the ground $\text{1s(A)}$ state leading to relatively small population in the intermediate $\text{1s(E),}$ ${\text{1s(T}}_2)$ excited states.