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

Abstract 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, $${n_2} \propto E_{\rm{g}}^{ - 4}$$ 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.