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

Materials displaying intrinsic optical bistability (IOB), i.e,, allowing th e coexistence of two stable steady-state excitation rates for a single give n excitation power, are of interest for potential technological application s related to optical data switching and manipulation. The properties of the unusual IOB previously observed in Yb3+-doped Cs3Lu2Br9 and CsCdBr3 host m aterials are studied here using absorption and luminescence spectroscopies. The IOB phenomenon is concluded to derive ultimately from laser hearing ef fects. 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 i n both power- and temperature-sweep experiments. A simple model describing this effect in terms of rates of sample heating and cooling in the irradiat ed volume reproduces the power, temperature, concentration, and excitation- energy dependence of the Yb3+ IOB using only the experimental absorption da ta as input. The temperature dependence of the absorption cross section is correlated with thermal changes in the monomeric YbBr63- geometry, which be comes more asymmetric as the temperature is elevated. The IOB observed in Y b3+-doped Cs3Lu2Br9, and CsCdBr3 host lattices is therefore a property of t he monomeric Yb3+ ion in these materials, and not a dimer property as was p reviously believed. These results also emphasize the more general conclusio n that laser hearing may contribute significantly to the shape or slope of an excitation power dependence curve, and may even be the dominant aspect o f that curve when absorption cross sections are strongly dependent on tempe rature.