Ultrafast relaxation dynamics of semiconducting carbon nanotube exciton polaritons

Strong light-matter coupling results in new eigenstates called polaritons which share properties of both light and matter and provide a useful way of modifying electronic energies. The energies of the new eigenstates depend on the concentration of molecules in the cavity. In this work, we change the concentration of Carbon Nanotubes (CNTs) in a Fabry-Perot cavity and achieve a maximum Rabi splitting of 4000 cm-1. The effects of concentration are studied with Transient Reflection (TR) spectroscopy, and we find that transfer rates between polariton states are enhanced by two orders of magnitude when their energy differences are resonant with the CNT G-band phonon energy of 1580 cm-1. The G-band phonon mode is also known to be important for exciton transfer between CNTs. The relaxation times of the system closely resemble those of CNTs outside of a cavity (~ 5 ps) which we attribute to the short lifetime of the cavity modes. The short-lived cavity modes cause the effects of the phonon mode resonances to be observed through peak intensities rather than kinetic traces. These results show that the G-band phonon mode is important for increasing excitation density in CNT polariton devices.