Efficient Upper-Excited State Fluorescence in an Organic Hyperbolic Metamaterial

Upper-excited state emission is not usually observed from molecules owing to competition with much faster nonradiative relaxation pathways; however, it can be made more efficient by modifying the photonic density of states to enhance the radiative decay rate. Here, we show that embedding the small molecule zinc tetraphenylporphyrin (ZnTPP) in a hyperbolic metamaterial enables an ∼18-fold increase in fluorescence intensity from the second singlet excited state (S₂) relative to that from the lowest singlet excited state (S₁). By varying the number of periods in the HMM stack, we are able to systematically tune the ZnTPP fluorescence spectrum from red (dominated by emission from S₁) to blue (dominated by emission from S₂) with an instrument-limited decay lifetime <10 ps. Our results are consistent with a broadband Purcell enhancement in the radiative rate of both transitions predicted via transfer matrix modeling and point to a general opportunity to harness upper-excited states for spectrally tunable, ultrafast fluorescence via radiative decay engineering.