Untargeted effects in organic exciton-polariton transient spectroscopy: A cautionary tale.

Scott Renken,Zhen Shen,Andrew J Musser,Akshay Rao,Rahul Jayaprakash,Kyriacos Georgiou,David G Lidzey,Lizhi Gai,Raj Pandya

doi:10.1063/5.0063173

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 modeling 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.

Highlights

There has long been substantial interest in using light-matter interactions to alter the photophysical dynamics of molecular materials
We have explored the time-resolved spectroscopy of strongly coupled microcavities containing the organic dye BODIPY-R
Our transient transmission data revealed many of the same types of features— derivative-like lineshapes centred at the polariton resonances—described in previous organic polariton systems.[33,36,50,51,52,54,56]

Summary

Introduction

There has long been substantial interest in using light-matter interactions to alter the photophysical dynamics of molecular materials. Polaritons inherit some of the dispersion from their parent photonic state, yielding angle-dependent upper and lower polariton bands which anti-cross at the parent exciton energy This anticrossing is the hallmark of the strong coupling regime, and its magnitude defines the Rabi splitting which is used to benchmark the light-matter interaction strength.[39] The dispersion of the lower polariton can be observed in photoluminescence spectroscopy.[40] Following non-resonant excitation (i.e. at energies above the lower polariton), the distribution of emission intensity along the lower polariton dispersion, and the energetic distribution of polariton population, depends sensitively on the energetic separation of the cavity photon and exciton. Considering that, in the exciton-polaritons field, this stringent condition has only ever been met in the first transient absorption study,[50] these results call for careful re-evaluation of the dynamics of organic exciton-polaritons

Methods

Findings

Conclusions