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Reducing the dimensionality of three-dimensional hybrid metal halide perovskites can improve their optoelectronic properties. Here, we show that the third-order optical nonlinearity,  n   2 , of hybrid lead iodide perovskites is enhanced in the two-dimensional Ruddlesden-Popper series, (CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 )  n -1 Pb  n  I 3 n +1  ( n  = 1–4), where the layer number ( n ) is engineered for bandgap tuning from  E   g  = 1.60 eV ( n  = ∞; bulk) to 2.40 eV ( n  = 1). Despite the unfavorable relation,    \documentclass[12pt]{minimal} 				\usepackage{amsmath} 				\usepackage{wasysym}  				\usepackage{amsfonts}  				\usepackage{amssymb}  				\usepackage{amsbsy} 				\usepackage{mathrsfs} 				\usepackage{upgreek} 				\setlength{\oddsidemargin}{-69pt} 				\begin{document}$${n_2} \propto E_{\rm{g}}^{ - 4}$$\end{document}n    2    ∝    E    g    -  4, strong quantum confinement causes these two-dimensional perovskites to exhibit four times stronger third harmonic generation at mid-infrared when compared with the three-dimensional counterpart, (CH 3 NH 3 )PbI 3 . Surprisingly, however, the impact of dimensional reduction on two-photon absorption, which is the Kramers-Kronig conjugate of  n   2 , is rather insignificant as demonstrated by broadband two-photon spectroscopy. The concomitant increase of bandgap and optical nonlinearity is truly remarkable in these novel perovskites, where the former increases the laser-induced damage threshold for high-power nonlinear optical applications.

Selective enhancement of optical nonlinearity in two-dimensional organic-inorganic lead iodide perovskites