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

Reducing the dimensionality of three-dimensional hybrid metal halide perovskites can improve their optoelectronic properties. Here, we show that the third-order optical nonlinearity, n2, of hybrid lead iodide perovskites is enhanced in the two-dimensional Ruddlesden-Popper series, (CH3(CH2)(3)NH3)(2)(CH3NH3)(n-1)Pbn(3)I(n+1) (n = 1-4), where the layer number (n) is engineered for bandgap tuning from E-g = 1.60 eV (n =infinity; bulk) to 2.40 eV (n = 1). Despite the unfavorable relation, n(2) proportional to 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, (CH3NH3) PbI3. 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.