Energy relaxation pathways between light-matter states revealed by coherent two-dimensional spectroscopy

Lars Mewes,Majed Chergui,Mao Wang,Karl Börjesson,Rebecca A Ingle

doi:10.1038/s42005-020-00424-z

Abstract

Coupling matter excitations to electromagnetic modes inside nano-scale optical resonators leads to the formation of hybrid light-matter states, so-called polaritons, allowing the controlled manipulation of material properties. Here, we investigate the photo-induced dynamics of a prototypical strongly-coupled molecular exciton-microcavity system using broadband two-dimensional Fourier transform spectroscopy and unravel the mechanistic details of its ultrafast photo-induced dynamics. We find evidence for a direct energy relaxation pathway from the upper to the lower polariton state that initially bypasses the excitonic manifold of states, which is often assumed to act as an intermediate energy reservoir, under certain experimental conditions. This observation provides new insight into polariton photophysics and could potentially aid the development of applications that rely on controlling the energy relaxation mechanism, such as in solar energy harvesting, manipulating chemical reactivity, the creation of Bose–Einstein condensates and quantum computing.

Highlights

Coupling matter excitations to electromagnetic modes inside nano-scale optical resonators leads to the formation of hybrid light-matter states, so-called polaritons, allowing the controlled manipulation of material properties
We demonstrate here how broadband visible (500–750 nm; 13,300–20,000 cm−1) 2DFT spectroscopy can be used to follow the ultrafast dynamics in molecular cavity quantum electrodynamics (cQED) systems via cross-peak dynamics and find branching energy relaxation pathways that depend on different sample properties, such as cavity tuning, Rabi-splitting and k∥
The angular signal dependence is given with respect to the angle from the surface normal (φ) and the inset shows the molecular structure of TDBC. c The pulse sequence employed for two-dimensional Fourier transform spectroscopy

Summary

Introduction

Coupling matter excitations to electromagnetic modes inside nano-scale optical resonators leads to the formation of hybrid light-matter states, so-called polaritons, allowing the controlled manipulation of material properties. While the former has been awarded for the observation of strong light-atom coupling[7] and the latter is based on strong coupling with inorganic quantum mechanical objects (artificial atoms, Josephson junctions, etc.) at low temperatures, strong excitonphoton coupling using organic materials is a more recent development[8,9] It allows the manipulation of physical and chemical properties of matter—such as the interaction of a system with its environment[10] or energy ordering between singlet and triplet states11,12—at room temperature due to the large transition dipole moment of organic molecules and their aggregates, which leads to larger coupling strengths with the electromagnetic field compared to individual atoms or inorganic semiconductors[13]. Subtleties such as disorder-induced photonic intensity borrowing[27,28,29,30,31,32,33] and the presence of “bright” uncoupled excitonic states with allowed transitions from the ground state[20,30] are important for the systems’ photo-induced dynamics, as arguably the excitonic states can act as intermediates during the energy relaxation[26,29,30,31,32,34,35]

Methods

Results

Conclusion