Homogeneous Optical Line Widths in Hybrid Ruddlesden–Popper Metal Halides Can <i>Only</i> Be Measured Using Nonlinear Spectroscopy

Abstract

The homogeneous photoluminescence spectral line width in semiconductors carries a wealth of information on the coupling of primary photoexcitations with their dynamic environment as well as between multiparticles. In the limit in which inhomogeneous broadening dominates the total optical line widths, the inhomogeneous and homogeneous contributions can be rigorously separated by temperature-dependent steady-state photoluminescence spectroscopy. This is possible because the only temperature-dependent phenomenon is optical dephasing, which defines the homogeneous line width, since this process is mediated by scattering with phonons. However, if the homogeneous and inhomogeneous line widths are comparable, as is the case in hybrid Ruddlesden–Popper metal halides, the temperature dependence of linear spectral measurement cannot separate rigorously the homogeneous and inhomogeneous contributions to the total line width because the line shape does not contain purely Lorentzian components that can be isolated by varying the temperature. Furthermore, the inhomogeneous contribution to the steady-state photoluminescence line shape 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. Here, we will discuss our perspective on how the coherent line shapes of Ruddlesden–Popper metal halides can be effectively rationalized within an exciton polaron framework.

Topics & Concepts