for E g being the material's optical bandgap, j being the number of photons interacting in the multiphoton absorption process, and assuming no saturation effects or Stokes shift. Initially, we attempted to measure the SHG capabilities of a library of RP perovskites as part of a structureproperty study. The broad, intense mPL from the perovskites swamped our initial (conventional) SHG measurement and obscured the weak SHG signals (see Figure ). To address this challenge, here we report a multidimensional harmonic generation method which relies on a tunable excitation laser, spatial filtering, and a spectrally resolved output to ensure that we can definitively determine whether a material supports SHG by correlating the excitation and emission frequencies. We call this a multidimensional technique because it is analogous to coherent 2D spectroscopies which resolve both emission and excitation frequencies. As illustrated in Figure , we can confidently assess whether a material supports SHG (or THG) because SHG (THG) will have an emission frequency that scales as twice (thrice) the excitation frequency (Equation , blue line, for SHG and Equation , magenta line, for SHG), in contrast to the mPL emission frequency which is not dependent on excitation frequency (Equation 3, green band). In this way, mPL emission features cannot obscure the definitive assignment of SHG features regardless of the relative intensities or peak positions of mPL vs. SHG. We use this technique to conclusively determine the SHG responses of RP perovskites with different n values, spacer cations, and A-site cations, all of which have intense mPL, thus elucidating and clarifying their respective crystal symmetries.