Untargeted Effects in Organic Exciton-Polariton Transient Spectroscopy: A Cautionary Tale

TL;DR

This paper warns that transient spectroscopy signals in organic exciton-polariton systems may be misleading, as they can originate from effects other than polariton states, emphasizing the need for more precise measurements.

Contribution

It demonstrates that long-lived spectral effects in transient spectroscopy are often due to photoexcitation effects unrelated to polariton states, challenging previous interpretations.

Findings

Most transient signals are from photoexcitation effects, not polariton states.

Complex optical properties can produce misleading spectral signatures.

High-time-resolution measurements are needed for accurate polariton dynamics.

Abstract

Strong light-matter coupling to form exciton- and vibropolaritons is increasingly touted as a powerful tool to alter the fundamental properties of organic materials. It is proposed that these states and their facile tunability can be used to rewrite molecular potential energy landscapes and redirect photophysical pathways, with applications from catalysis to electronic devices. Crucial to their photophysical properties is the exchange of energy between coherent, bright polaritons and incoherent dark states. One of the most potent tools to explore this interplay is transient absorption/reflectance spectroscopy. Previous studies have revealed unexpectedly long lifetimes of the coherent polariton states, for which there is no theoretical explanation. Applying these transient methods to a series of strong-coupled organic microcavities, we recover similar long-lived spectral effects. Based on transfer-matrix modelling of the transient experiment, we find that virtually the entire photoresponse results from photoexcitation effects other than the generation of polariton states. Our results suggest that the complex optical properties of polaritonic systems make them especially prone to misleading optical signatures, and that more challenging high-time-resolution measurements on high-quality microcavities are necessary to uniquely distinguish the coherent polariton dynamics.