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)₃NH₃)₂(CH₃NH₃) _n-1Pb _n I_3n+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, (CH₃NH₃)PbI₃. Surprisingly, however, the impact of dimensional reduction on two-photon absorption, which is the Kramers-Kronig conjugate of n₂, 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.

While noncentrosymmetric ${\mathrm{MoS}}_2$ monolayer is known to exhibit efficient second-harmonic generation (SHG), there is currently no agreement on its absolute nonlinear susceptibility ${\chi }^{\left(2\right)}$, varying over three orders of magnitude according to recent experiments. In order to resolve this conflicting issue, we have studied the nonlinear optical properties of ${\mathrm{MoS}}_2$ monolayer grown by chemical vapor deposition. The polycrystalline nature of the monolayer was directly probed by the SHG polarization dependence across the grain boundaries using femtosecond pulses. Broadband wavelength-dependent SHG response $\left(\lambda =1.1–2.0\mu \mathrm{m}\right)$ using picosecond pulses was studied by comparing the relative SHG counts of ${\mathrm{MoS}}_2$ to quartz and incorporating the structural and optical characteristics of the monolayer. Significant nonlinear optical dispersion gives rise to ${\chi }^{\left(2\right)}\sim 430$ pm/V at 580 nm, where SHG is neither affected by any excitonic absorption/resonance nor by fundamental absorption. We also show that ${\chi }^{\left(2\right)}$ must be derived from a thin bulk (sheet) optical nonlinearity and that the previous measurements are in fact all consistent, together with our measurements and first-principle calculations.