Exciton-Polariton Relaxation and Emission in Carbon Nanotube Microcavities with Varied Quality Factors

Single-walled carbon nanotubes (SWCNTs) are an interesting material for investigating strong light-matter coupling in the near-infrared and at room temperature due to their large exciton binding energies and stable emissions. In this work, using thin films of monochiral (6,5) SWCNTs as emitters, we study the strong light-matter coupling in three types of well-designed Fabry-Pérot microcavities with a gradual increase in the quality factor (Q factor) from ∼20 to ∼1000. We observe sharp polariton emissions in the near-infrared with a full width at half-maximum down to ∼1 meV. In the structure, exciton-like subradiant states resulting from the strong coupling manifest themselves through the relaxation dynamics of the exciton reservoir (ER). Our time-resolved photoluminescence (PL) measurements indicate that the coherence of these states can be tuned by the Q factor, which enables a high ratio of bright excitons above ∼90% relative to that of the intrinsically dark excitons in SWCNTs. With increasing Q factor, we also show that the population transfer from the ER to the lower polaritons (LPs) can be systematically enhanced. Furthermore, our angle-resolved PL spectra show a narrow distribution of the polariton emission centered around the LP ground state, which is necessary to realize the polariton condensation. These results broaden our understanding of the photophysics of both the polaritonic and subradiant states in the strongly coupled SWCNT microcavity, which will be critical for further studies on the polariton condensation and the engineering of polaritonic devices based on SWCNTs.