Optically Unstable Phase from Ion-Ion Interactions in an Erbium-Doped Crystal

We demonstrate an optical unstable phase for a laser driven erbium ion ensemble. The transmitted light through an erbium-doped yttrium orthosillicate crystal becomes dynamically unstable when illuminated by a strong continuous-wave laser. Transient net gain was recorded if the light passes the sample twice. The experimental results are understood in the framework of a many-body system interacting with a classical field, where the ion-ion interaction becomes significant as a result of the high erbium concentration. A Bloch-equation model that includes the excitation-induced frequency-shift is introduced to discuss the conditions of the instabilities.