Ultrafast Polariton Dynamics of Gold Nanocubes/Carbocyanine Aggregates

Molecular polaritons are quasiparticles that arise when the confined electromagnetic fields, for example, in a photonic cavity, are strongly coupled to a resonant molecular transition. When confined inside or near the plasmonic cavity of metal nanostructures, a small number of dye molecules show efficient polaritonic properties via localized surface plasmon resonances (LSPR). In this work, we report molecular polaritons composed of carbocyanine aggregates and gold nanocubes, and ultrafast polariton dynamics were investigated by femtosecond transient absorption spectroscopy. The excited state absorption and the ground state bleachings of the upper polariton (UP) and lower polariton (LP) states showed the common ultrafast (120-180 fs) deexcitation dynamics, which appears much faster than the excited state dynamics of carbocyanine molecule. In addition, the electron-phonon and phonon-phonon scattering dynamics of gold nanocubes were separately observed. Polarition deexciation dynamics depending on the selective excitation of the UP or LP state, and on the LSPR bands of gold nanocubes (60 vs. 100 nm in length) will be discussed. Our reports provide a fundamental experimental evidence of the ultrafast polariton dynamics, which can be strongly correlated with the to the LSPR of plasmonic metal nanostructures.