Table : Summary of the space groups and spectroscopic characterization of the six 2D-RP perovskites. RNH 3 and n correspond to the chemical formula of (RNH 3 ) 2 (MA) n-1 Pb n I 3n+1 . The peak values for photoluminescence correspond to the vertical bars shown in Figure . I SHG /I THG was determined from the data shown in Figure for ω = 1.03 (eV) with a fluence of ∼1400 µJ/cm 2 . Note, that without considering phase-matching, absorption, and instrument response effects, comparison by a ratio of intensities is problematic. However, we report ratios here in order to compare to past literature (e.g. ref. ), and because we are comparing materials with similar bandgaps at identical emission frequencies these ratios offer a fairly self-consistent comparison. attribute the redshift to a photon recycling effect. Multiphoton absorption can create excited carriers much deeper into a sample than single photon absorption. The deeper that carriers are excited in a sample, the more likely their photoluminescence will be reabsorbed to create other carriers. The Stokes-shift associated with the photoluminescence will lead to the reddest photons selectively being able to exit the sample.