The role of laser heating in the intrinsic optical bistability of Yb3+-doped bromide lattices

Materials displaying intrinsic optical bistability (IOB), i.e., allowing the coexistence of two stable steadystate excitation rates for a single given excitation power, are of interest for potential technological applications related to optical data switching and manipulation. The properties of the unusual IOB previously observed in Yb-doped CsLuBr and CsCdBr host materials are studied here using absorption and luminescence spectroscopies. The IOB phenomenon is concluded to derive ultimately from laser heating effects. When combined with a strongly increasing and nonlinear dependence of the material's absorbance on internal temperature, laser heating leads to a positive-feedback absorption amplification process showing a hysteresis in both power- and temperature-sweep experiments. A simple model describing this effect in terms of rates of sample heating and cooling in the irradiated volume reproduces the power, temperature, concentration, and excitation-energy dependence of the Yb IOB using only the experimental absorption data as input. The temperature dependence of the absorption cross section is correlated with thermal changes in the monomeric YbBr geometry, which becomes more asymmetric as the temperature is elevated. The IOB observed in Yb-doped CsLuBr and CsCdBr host lattices is therefore a property of the monomeric Yb ion in these materials, and not a dimer property as was previously believed. These results also emphasize the more general conclusion that laser heating may contribute significantly to the shape or slope of an excitation power dependence curve, and may even be the dominant aspect of that curve when absorption cross sections are strongly dependent on temperature