Population of Subradiant States in Carbon Nanotube Microcavities in the Ultrastrong Light–Matter Coupling Regime

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.

Energy cascades in donor-acceptor exciton-polaritons observed by ultrafast two-dimensional white-light spectroscopy

“…The spectra exhibit derivative lineshapes with positive and negative features along the probe energy axis. These are characteristic features of Rabi contraction, where the depletion of ground-state population upon photoexcitation by the pump pulse results in a reduction in Rabi splitting, and subsequently a transient shift in peak positions 28 , 29 , 39 – 41 . Thus, the peak energies identified in the 2DWL spectra do not exactly match the energies of the eigenstates identified in the steady-state spectra.…”

Investigating Molecular Exciton Polaritons Using Ab Initio Cavity Quantum Electrodynamics

“…Figure presents the results of a single-walled carbon nanotube (SWCNT) system (∼1000 atoms) coupled to an optical cavity, which has been the subject of recent experimental interest in molecule–cavity coupling. − Semiconducting SWCNTs are known to have weak photoluminescence due to low-lying optically inactive states below the so-called band-edge E 11 exciton, which is the 6th excited state, and we denote |ψ 6 ⟩ ≡ | E 11 ⟩. Figure a provides the transition dipole matrix for the pristine (6,5) SWCNT system, showing a sparse matrix with only the ground-to-E 11 (i.e., the sixth bare molecular transition) transition to be optically allowed with a large dipole moment of ∼28 au, and small dipole matrix elements between other excited states.…”

“…Finally, we applied the pQED scheme to investigate the polaritonic properties of a single-walled carbon nanotube, a system of recent experimental interest for polaritonic applications. ,,,,,, We showed that through light–matter coupling one can red-shift the bright E 11 character of the tube below the band of dark states (which makes these materials dark to emission outside the cavity), thus enabling strong optical emission from these materials inside the cavity without the need for chemical functionalization.…”

Energy cascades in polaritonic systems with energetic disorder observed by ultrafast two-dimensional white-light spectroscopy

“…The spectra exhibit derivative lineshapes with positive and negative features along the probe energy axis. These are characteristic features of Rabi contraction, where the depletion of ground-state population upon photoexcitation by the pump pulse results in a reduction in Rabi splitting, and subsequently a transient shift in peak positions 28,29,[39][40][41] . Thus, the peak energies identified in the 2DWL spectra do not exactly match the energies of the eigenstates identified in the steady-state spectra.…”