Towards Resolving Vibronic Coupling in FAPbBr₃ by Coherent Infrared Hyper-Raman Spectroscopy

Exciton-phonon coupling in metal halide perovskites is hypothesized to impact charge carrier dynamics. Recent evidence has suggested that low energy lattice vibrations couple to high energy A-site cation vibrational modes, and that high energy A-site cation vibrations couple to excitonic states in FAPbBr₃ through a vibronic coupling mechanism. Understanding which high energy vibrations couple to excitonic states could prove useful to understand how vibronic coupling in metal halide perovskites mediates carrier dynamics. Here, we use a coherent infrared-hyper-Raman based four-wave mixing spectroscopy to probe vibrational modes and their coupling to excitons in FAPbBr₃. This spectroscopy has the potential to quantify vibronic coupling strengths in lead halide perovskites. By varying detuning from the excitonic state, i.e. investigating pre-resonance response, the nature of vibronic coupling can be deduced.