Department of Chemistry, Columbia University, New York, NY 10027.
Harvard John A. Paulson School of Engineering & Applied Sciences, Harvard University, Cambridge, Massachusetts 02138.
Proc Natl Acad Sci U S A. 2021 Feb 16;118(7). doi: 10.1073/pnas.2022268118.
The ultrafast polarization response to incident light and ensuing exciton/carrier generation are essential to outstanding optoelectronic properties of lead halide perovskites (LHPs). A large number of mechanistic studies in the LHP field to date have focused on contributions to polarizability from organic cations and the highly polarizable inorganic lattice. For a comprehensive understanding of the ultrafast polarization response, we must additionally account for the nearly instantaneous hyperpolarizability response to the propagating light field itself. While light propagation is pivotal to optoelectronics and photonics, little is known about this in LHPs in the vicinity of the bandgap where stimulated emission, polariton condensation, superfluorescence, and photon recycling may take place. Here we develop two-dimensional optical Kerr effect (2D-OKE) spectroscopy to energetically dissect broadband light propagation and dispersive nonlinear polarization responses in LHPs. In contrast to earlier interpretations, the below-bandgap OKE responses in both hybrid CHNHPbBr and all-inorganic CsPbBr perovskites are found to originate from strong hyperpolarizability and highly anisotropic dispersions. In both materials, the nonlinear mixing of anisotropically propagating light fields results in convoluted oscillatory polarization dynamics. Based on a four-wave mixing model, we quantitatively derive dispersion anisotropies, reproduce 2D-OKE frequency correlations, and establish polarization-dressed light propagation in single-crystal LHPs. Moreover, our findings highlight the importance of distinguishing the often-neglected anisotropic light propagation from underlying coherent quasiparticle responses in various forms of ultrafast spectroscopy.
超快的偏振响应和随后的激子/载流子产生对于卤铅钙钛矿（LHP）出色的光电性能至关重要。迄今为止，LHP 领域的大量机制研究都集中在有机阳离子和高极化无机晶格对极化率的贡献上。为了全面了解超快极化响应，我们还必须考虑到对传播光场本身的几乎瞬时的超极化率响应。虽然光传播对于光电子学和光子学至关重要，但在可能发生受激发射、极化激元凝聚、超荧光和光子回收的带隙附近的 LHP 中，对此知之甚少。在这里，我们开发了二维光克尔效应（2D-OKE）光谱技术，以在 LHP 中对宽带光传播和色散非线性极化响应进行能量分解。与早期的解释相反，在混合 CHNHPbBr 和全无机 CsPbBr 钙钛矿中，低于带隙的 OKE 响应都被发现源于强超极化率和高度各向异性的色散。在这两种材料中，各向异性传播光场的非线性混合导致了复杂的振荡极化动力学。基于四波混频模型，我们定量地推导出色散各向异性，再现了 2D-OKE 频率相关性，并在单晶 LHP 中建立了极化调制的光传播。此外，我们的发现强调了在各种形式的超快光谱学中，区分经常被忽视的各向异性光传播和潜在的相干准粒子响应的重要性。
