State preparation by optical pumping in erbium-doped solids using stimulated emission and spin mixing

Erbium-doped solids are potential candidates for the realization of a quantum memory for photons at telecommunication wavelengths. The implementation of quantum memory proposals in rare-earth-ion-doped solids require spectral tailoring of the inhomogeneous absorption profile by efficient population transfer between ground-state levels (spin polarization) using optical pumping. In this paper we investigate the limiting factors of efficient optical pumping between ground-state Zeeman levels in an erbium-doped ${\mathrm{Y}}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ crystal. We introduce two methods to overcome these limiting factors: stimulated emission using a second laser and spin mixing using radio frequency excitation. Both methods significantly improve the degree of spin polarization. Population transfer between two Zeeman levels with less than 10% of the total population in the initial ground state is achieved, corresponding to a spin polarization greater than 90%. In addition, we demonstrate spectral tailoring by isolating a narrow absorption peak within a large transparency window.