in which χ (m) is the m th -order susceptibility. Importantly, within the electric dipole approximation, if a material has inversion symmetry, then it is necessarily the case that χ (2) = 0. 39,40 Thus, if a material supports SHG, the response is due either to a lack of bulk inversion symmetry, or the air-material interface which always lacks inversion symmetry. THG does not have this strict constraint and can occur in materials with inversion symmetry. These considerations imply that the presence of SHG can be a sensitive tool to probe whether a material is noncentrosymmetric. We consider SHG to be a complementary technique to the more definitive single-crystal X-ray diffraction (SCXRD) which sometimes requires additional confirmation of a crystal's symmetry group in order to determine its structure (as noted in ref. ). However, as noted by Govinda et al. , the strong, broad mPL present in lead halide perovskites (c.f. refs. ) can compete with weak SHG and lead to erroneous conclusions. In contrast to SHG and THG, mPL is caused by photons, usually with energy below the bandgap, undergoing multiphoton absorption to create hot carriers which then cool to the bandedge and isotropically fluoresce, just like carriers excited by normal, above-bandedge photon absorption. The output frequency and intensity of mPL will therefore be