How two-dimensional brick layer J-aggregates differ from linear ones: excitonic properties and line broadening mechanisms

TL;DR

This paper investigates how two-dimensional brick layer J-aggregates differ from linear ones in excitonic properties and line broadening, revealing larger spectral shifts and the dominance of pure dephasing in two-dimensional systems.

Contribution

It provides a detailed analysis of excitonic couplings, spectral shifts, and dephasing mechanisms in two-dimensional versus linear J-aggregates, highlighting the dominance of pure dephasing in 2D structures.

Findings

Two-dimensional aggregates show larger spectral shifts for the same coupling.

Pure dephasing dominates line broadening in 2D aggregates up to a crossover temperature.

Reduced density of states at the band edge decreases population relaxation contributions.

Abstract

We study the excitonic coupling and homogeneous spectral line width of brick layer J-aggregate films. We begin by analysing the structural information revealed by the two-exciton states probed in two-dimensional spectra. Our first main result is that the relation between the excitonic couplings and the spectral shift in a two-dimensional structure is different (larger shift for the same nearest neighbour coupling) from that in a one-dimensional structure, which leads to an estimation of dipolar coupling in two-dimensional lattices. We next investigate the mechanisms of homogeneous broadening - population relaxation and pure dephasing - and evaluate their relative importance in linear and two-dimensional aggregates. Our second main result is that pure dephasing dominates the line width in two-dimensional systems up to a crossover temperature, which explains the linear temperature dependence of the homogeneous line width. This is directly related to the decreased density of states at the band edge when compared with linear aggregates, thus reducing the contribution of population relaxation to dephasing. Pump-probe experiments are suggested to directly measure the lifetime of the bright state and can therefore support the proposed model.