Detection Defines Dephasing in Two-Dimensional Electronic Spectroscopy of Materials: Coherent Field Emission versus Incoherent Population Observables

Abstract

The homogeneous spectral linewidth associated with light-matter interactions is a fundamental descriptor of the optical properties of materials, governed by the quantum dynamics of the condensed-matter system. We discuss here that the homogeneous linewidth measured by means of two-dimensional electronic spectroscopy depends not only on microscopic coherence loss, but also on the observable through which the nonequilibrium dynamics are projected onto the measurement. In this Perspective, we develop a unified framework showing that changing the detection operator changes the operational definition of dephasing. For coherent emitted-field measurements, the observed linewidth largely retains its conventional connection to the optical coherence time $(T_2$). By contrast, in population-detected modalities such as photoluminescence-, photocurrent-, and other action-detected two-dimensional spectroscopies, the apparent linewidth can additionally encode excited-state population redistribution dynamics, leading naturally to an effective coherence time \(T_{2,\mathrm{eff}}\). Using a coupled-mode model propagated under a common Liouvillian, we show that identical microscopic dynamics yield distinct apparent dephasing times when projected onto coherent-emission and population-derived observables. We posit that the detection observable is not merely how a two-dimensional spectrum is measured, but part of what the spectrum fundamentally means as a materials probe.