Cooperative Bistability in Dense, Excited Atomic Systems

We report a new mechanism of intrinsic bistability, operative in excited atomic systems with substantial atom-atom coupling. Predictions and observations of bistable luminescence from ${\mathrm{Yb}}^{3+}$ pairs in crystalline ${\mathrm{Cs}}_3$ ${\mathrm{Y}}_2$ ${\mathrm{Br}}_9$: ${\mathrm{Yb}}^{3+}$ are in good agreement and have interesting implications for local field effects in solids and dense, ultracold gases.