Disentangling Second Harmonic Generation from Multiphoton Photoluminescence in Halide Perovskites using Multidimensional Harmonic Generation

Layered
two-dimensional Ruddlesden–Popper (RP) halide perovskites
are an intriguing class of semiconductors being explored for their
linear and nonlinear optical and ferroelectric properties. Second
harmonic generation (SHG) is commonly used to screen for noncentrosymmetric
and ferroelectric materials. However, SHG measurements of perovskites
can be obscured by their intense multiphoton photoluminescence (mPL).
Here, we apply multidimensional harmonic generation as a method to
eliminate the complications from mPL. By scanning and correlating
both excitation and emission frequencies, we unambiguously assess
whether a material supports SHG by examining if an emission feature
scales as twice the excitation frequency. Measurements of a series
of n = 2, 3 RP perovskites reveal that, contrary
to previous belief, n-butylammonium (BA) RP perovskites
are not SHG-active and thus centrosymmetric, but RP perovskites with
phenylethylammonium (PEA) and 2-thiophenemethylammonium (TPMA) spacer
cations display SHG. This work establishes multidimensional harmonic
generation as a definitive method to measure SHG in halide perovskites.