Non‐Radiative Energy Transfer Mediated by Hybrid Light‐Matter States

We present direct evidence of enhanced non‐radiative energy transfer between two J‐aggregated cyanine dyes strongly coupled to the vacuum field of a cavity. Excitation spectroscopy and femtosecond pump–probe measurements show that the energy transfer is highly efficient when both the donor and acceptor form light‐matter hybrid states with the vacuum field. The rate of energy transfer is increased by a factor of seven under those conditions as compared to the normal situation outside the cavity, with a corresponding effect on the energy transfer efficiency. The delocalized hybrid states connect the donor and acceptor molecules and clearly play the role of a bridge to enhance the rate of energy transfer. This finding has fundamental implications for coherent energy transport and light‐energy harvesting.