We present theoretical and experimental results showing the effects of incoherent population mixing on two-dimensional (2D) coherent excitation spectra that are measured via a time-integrated population and phase-sensitive detection. The technique uses four collinear ultrashort pulses and phase modulation to acquire two-dimensional spectra by isolating specific nonlinear contributions to the photoluminescence or photocurrent excitation signal. We demonstrate that an incoherent contribution to the measured line shape, arising from nonlinear population dynamics over the entire photoexcitation lifetime, generates a similar line shape to the expected 2D coherent spectra in condensed-phase systems. In those systems, photoexcitations are mobile such that inter-particle interactions are important on any time scale, including those long compared with the 2D coherent experiment. Measurements on a semicrystalline polymeric semiconductor film at low temperatures show that, in some conditions in which multi-exciton interactions are suppressed, the technique predominantly detects coherent signals and can be used, in our example, to extract homogeneous line widths. The same method used on a lead-halide perovskite photovoltaic cell shows that incoherent population mixing of mobile photocarriers can dominate the measured signal since carrier-carrier bimolecular scattering is active even at low excitation densities, which hides the coherent contribution to the spectral line shape. In this example, the intensity dependence of the signal matches the theoretical predictions over more than two orders of magnitude, confirming the incoherent nature of the signal. While these effects are typically not significant in dilute solution environments, we demonstrate the necessity to characterize, in condensed-phase materials systems, the extent of nonlinear population dynamics of photoexcitations (excitons, charge carriers, etc.) in the execution of this powerful population-detected coherent spectroscopy technique.

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非相干种群混合对相位调制二维相干激发光谱的贡献

我们目前的理论和实验结果显示了非相干种群混合对二维（2D）相干激发光谱的影响，该光谱通过时间积分种群和相敏检测来测量。该技术使用四个共线超短脉冲和相位调制，通过隔离对光致发光或光电流激发信号的特定非线性贡献来获取二维光谱。我们证明了在整个光激发寿命中由非线性总体动力学引起的对测量线形的不相干贡献，产生了与凝聚相系统中预期的二维相干谱相似的线形。在那些系统中，光激发是可移动的，因此粒子间的相互作用在任何时间尺度上都很重要，包括那些与2D相干实验比较长的实验。在低温下对半结晶聚合物半导体薄膜进行的测量表明，在某些抑制多激子相互作用的条件下，该技术主要检测相干信号，在我们的示例中，该技术可用于提取均匀的线宽。卤化钙钛矿光伏电池上使用的相同方法表明，移动式载流子的不相干人口混合可以主导测量的信号，因为即使在低激发密度下载流子-载流子双分子散射也很活跃，这掩盖了对谱线形状的相干贡献。在此示例中，信号的强度依赖性在两个以上的数量级上与理论预测相匹配，从而确认了信号的非相干性。