Ultrafast dynamics in light-matter strong coupling state

　Atoms and molecules are always affected by their interaction with the radiation field. In free space, because of the numerous modes of radiation field, the excited state monotonically decays by a spontaneous emission. However, in an environment where only a specific standing wave of light is allowed, such as in a microcavity, an energy splitting (vacuum Rabi splitting ) proportional to the reciprocal of the time required for coherent energy transfer between the molecule and the radiation field occurs. When this splitting is larger than the absorption line width of the molecule in free space, it is called as in a strong light-matter coupling regime.

　Because the adiabatic potential energy surface (PES) of the light-matter coupled state (polariton) is a quantum mixture of the "ground state PES shifted by the photon energy of the standing wave" and the "electronically excited state PES in a free space", the excited state dynamics including a non-adabatic transition rates can be significantly modified.

　We studied the excited state dynamics of organic thin films in microcavities fabricated by vacuum deposition. We found that the singlet fission rate in rubrene amorphous film changes depending on the cavity thickness[1]. While there have been many preceding studies on the polariton chemistry, the microscopic mechanisms have not been fully understood. Particularly, no studies have experimentally clarified how the PES is modulated by the polariton formation. We studied the energy structure of the polariton from the analysis of the absorption spectrum by using the Holstein-Tavis-Cummings model, and found that the polaron decoupling in the electronic excited state plays a major role in the modulation of the singlet fission[1].

　In addition, we studied the excited state spectral diffusion dynamics in a polariton state of an amorphous film of tetraphenyldibenzoperifrantene, which is known as a promising donor molecule in organic photovoltaics. Two-dimensional electronic spectroscopy revealed that the vibronic couplings with a thermal bath that causes the energy fluctuations of excitons is significantly reduced with the strong light-matter coupling [2].