Coupled molecular excited states form unstable spatial modes in an optical λ/2-microresonator

We have investigated the radiation patterns formed by a quasi-planar optical λ/2-microresonator enclosing fluorescent dye molecules that were immobilized in a polymer film between two silver mirrors. Using time-resolved widefield imaging microscopy, we observed laterally confined transversal modes that occurred in the optically pumped microresonator area, exhibiting strong intensity fluctuations. The measured diameter of the isolated spatial modes was found to be 0.5 μm in agreement with theoretical predictions. The instability of the spatial mode emission patterns originates from the triplet-state-induced fluorescence intensity fluctuations of cavity-coupled collective molecular excited states.