Evidence for Polariton-Mediated Biexciton Transition in Single-Walled Carbon Nanotubes

TL;DR

This study uses ultrafast spectroscopy to provide direct evidence of a polariton-mediated biexciton transition in single-walled carbon nanotubes within microcavities, revealing new insights into polariton dynamics beyond their short lifetime.

Contribution

It introduces a protocol combining spectroscopy and genetic algorithm simulations to identify and analyze polariton-biexciton transitions in strongly-coupled nanotube microcavities.

Findings

Identified a polariton to biexciton transition energy.

Demonstrated instantaneity of the transition under resonant excitation.

Provided evidence of intrinsic polariton transitions beyond bright state lifetime.

Abstract

Strong coupling of excitonic resonances with a cavity gives rise to exciton-polaritons which possess a modified energy landscape compared to the uncoupled emitter. However, due to the femtosecond lifetime of the so-called bright polariton states and transient changes of the cavity reflectivity under excitation, it is challenging to directly measure the polariton excited state dynamics. Here, near-infrared pump-probe spectroscopy is used to investigate the ultrafast dynamics of exciton-polaritons based on strongly-coupled (6,5) single-walled carbon nanotubes in metal-clad microcavities. We present a protocol for fitting the reflectivity-associated response of the cavity using genetic algorithm-assisted transfer matrix simulations. With this approach are able to identify an absorptive exciton-polariton feature in the transient transmission data. This feature appears instantaneously under resonant excitation of the upper polariton but is delayed for off-resonant excitation. The observed transition energy and detuning dependence point toward a direct upper polariton to biexciton transition. Our results provide direct evidence for exciton-polariton intrinsic transitions beyond the bright polariton lifetime in strongly-coupled microcavities.