Homogeneous optical linewidths in hybrid Ruddlesden-Popper metal halides can only be measured using nonlinear spectroscopy

TL;DR

This paper demonstrates that nonlinear spectroscopy is essential for accurately measuring homogeneous linewidths in hybrid Ruddlesden-Popper metal halides, revealing insights into exciton interactions that linear methods cannot provide.

Contribution

The study shows that nonlinear coherent optical spectroscopies are necessary to separate homogeneous and inhomogeneous linewidths in hybrid Ruddlesden-Popper metal halides, especially when their contributions are comparable.

Findings

Nonlinear spectroscopy enables accurate linewidth separation in these materials.

Homogeneous linewidths are linked to exciton-phonon and exciton-exciton interactions.

Linear spectral measurements cannot rigorously distinguish linewidth contributions in this regime.

Abstract

The homogeneous photoluminescence spectral linewidth in semiconductors carries a wealth of information on the coupling of primary photoexcitations with their dynamic environment as well as between multi-particles. In the limit in which inhomogeneous broadening dominates the total optical linewidths, the inhomogeneous and homogeneous contributions can be rigorously separated by temperature-dependent %linear spectral measurements such as steady-state photoluminescence spectroscopy. This is possible because the only temperature-dependent phenomenon is optical dephasing, which defines the homogeneous linewidth, since this process is mediated by scattering with phonons. However, if the homogeneous and inhomogeneous linewidths are comparable, as is the case in hybrid Ruddlesden-Popper metal halides, the temperature dependence of linear spectral measurement \emph{cannot} separate rigorously the homogeneous and inhomogeneous contributions to the total linewidth because the lineshape does \emph{not} contain purely Lorentzian components that can be isolated by varying the temperature. Furthermore, the inhomogeneous contribution to the steady-state photoluminescence lineshape is not necessarily temperature independent if driven by diffusion-limited processes, particularly if measured by photoluminescence. Nonlinear coherent optical spectroscopies, on the other hand, do permit separation of homogeneous and inhomogeneous line broadening contributions in all regimes of inhomogeneity. Consequently, these offer insights into the nature of many-body interactions that are entirely inaccessible to temperature-dependent linear spectroscopies. When applied to Ruddlesden-Popper metal halides, these techniques have indeed enabled us to quantitatively assess the exciton-phonon and exciton-exciton scattering mechanisms.