The width of the THG and SHG features along the ω m axis is determined by the frequency bandwidth of our ultrafast driving laser. When recording the data shown in Figure we were not always able to fully reject mPL with our aperture. This mPL appears as broad features (upper left corner of each spectrum) whose emission frequency does not depend on the excitation frequency. The SHG and THG features exhibit structure which is not merely due to the non-uniform spectrum of our driving laser (Figure ). For instance, the SHG from TPMA n = 2 shows a sharp peak which may due to some form of excitonic enhancement. Additionally, projections of the THG spectra corrected for the excitation laser power (Figure ) demonstrate prominent peaks between ω m = 2.9 -3.2 eV-these enhancements are at significantly bluer frequencies compared to the enhancements at excitonic resonances observed by refs. in similar 2D-RP perovskites. Because the present work is focused on the existence of SHG in materials, we elect to not provide further discussion of the structure present in the SHG and THG spectra.