Two-dimensional (2D), Ruddlesden-Popper (RP) layered perovskites have emerged as an intriguing class of halide perovskites due to their quantum-well-like structure which allows for tunable optical properties and enhanced environmental stability compared to 3D lead halide perovskites, while still being easily processable into efficient solar cells. In addition, the versatile structural chemistry of RP perovskites makes them ideal candidates for applications in non-linear optoelectronics. RP perovskites have the general formula of (RNH 3 ) 2 (A) n-1 M n X 3n+1 in which RNH 3 is a long chain ammonium cation, including n-butylammonium (BA), phenylethylammonium (PEA), and 2-thiophenemethylammonium (TPMA); A is small, monovalent cation; M is a divalent metal cation (e.g. Pb 2+ ); X is a halide anion; and n is a positive integer. RNH 3 acts as a spatial and dielectric barrier between the inorganic layers consisting of corner-sharing [MX 6 ] 4-octahedra, creating quantum wells that support large binding energy excitons with intense room-temperature PL. The value of n controls the exciton binding energy and bandgap (both binding energy and bandgap decrease as n increases). The identity of M, X and n largely determines the bandedge optical properties with the A and RNH 3 cations having a smaller influence. However, the size and shape of the RNH 3 spacer cation can impact the perovskite layers by templating the metal halide octahedral network's bonding. The choice of RNH 3 and A cation have significant impact on the symmetry of the resulting crystal structure, and can lead to non-centrosymmetric structures, which are a prerequisite to even-order non-linear optoelectronic properties. Therefore there are many recent reports of SHG active and ferroelectric layered perovskites, including diverse RP perovskites like (BA) 2 (FA)Pb 2 Br 7 (FA = formamidinium), (BA) 2 CsPb 2 Br 7 , 17 (BA) 2 (EA) 2 Pb 3 I 10 , and the biaxial (EA) 4 Pb 3 Br 10 (EA = ethylammonium). These ferroelectric materials lack inversion symmetry, exhibit an electric polarization with no applied field, and can have the direction of their polarization switched with the application of an electric field. Notably, proving that a metal halide perovskite is ferroelectric can be an arduous task. 22-28 A first step involves ensuring that a candidate material lacks inversion symmetry. Optical second harmonic generation (SHG) has commonly been employed to screen ferroelectric candidates because materials that are SHG active necessarily lack inversion symmetry. Metal halide perovskites are known to efficiently support a wide variety of nonlinear optical phenomena including SHG in special cases, third harmonic generation (THG), high-harmonic generation up to 13th harmonic, and multiphoton photoluminescence (mPL). Figure 1a diagrams these phenomena. SHG and THG are spatially coherent processes with a directional output. SHG and THG are caused by an intense electric field driving a material polarization which then emits a new electric field with frequency components not found in the original field. The output frequencies and intensities of SHG and THG scale with the driving laser color, ω, and intensity, I, as