Population of Subradiant States in Carbon Nanotube Microcavities in the Ultrastrong Light–Matter Coupling Regime
Ryan T. Allen, Abitha Dhavamani, Minjung Son, Stéphane Kéna‐Cohen, Martin T. Zanni, Michael S. Arnold
Abstract
Strong light–matter coupling results in eigenstates called polaritons which share the properties of both light and matter and provide a useful way to engineer electronic energies and behaviors. In this work, we study nearly monochiral (6,5) semiconducting carbon nanotubes (CNTs) in a Fabry–Pérot microcavity. Light–matter coupling leads to the formation of three bands of bright polariton states (upper, middle, and lower─resulting from coupling to the bright S11 CNT exciton and the X1 phonon sideband of the K-momentum dark exciton state). The structure also supports many exciton-like subradiant states at the bright S11 and X1 energies. Here, ultrafast transient reflection spectroscopy is used to study the dynamics and spectral signatures of excited subradiant-state polariton populations and the pathways by which they are populated. After a pump pulse, the excited subradiant-state population is revealed by (i) spectral signatures with relaxation times (∼5 ps) similar to those of CNT S11 band gap excitons outside of the cavity and (ii) a Rabi contraction of the lower polariton energy, whose magnitude quantifies the excited subradiant-state population. Data show that, following the excitation of the upper polariton (UP), the excited subradiant-state population is maximized at a sample position with a detuning of 118 meV, light–matter coupling of 336 meV, and UP transition energy of 1.52 eV. The excited subradiant-state population is reduced for other detunings. The X1 Hopfield coefficient of the UP also peaks at the same energy, revealing UP to X1 scattering as a potentially efficient relaxation pathway. These results will be important for understanding and controlling energy relaxation and transport in future CNT polariton devices.