Bond model of second harmonic generation in tetragonal and orthorhombic perovskite structures

We apply the simplified-bond-hyperpolarizability model (SBHM) to investigate second harmonic generation (SHG) from tetragonal and orthorhombic methylammonium (MA) lead (Pb) iodide (I) perovskite structures. Assuming that the SHG radiation comes from the anharmonic charges parallel to the Pb-I covalent bond vectors, we show explicitly how the reflective rotational anysotropy SHG (RASHG) intensity profiles arise via dipole superposition from the perovskite surface and that their bulk contribution is zero for both the tetragonal and orthorhombic bulk structures if the contribution from MA molecules can be neglected. In addition, we compare the third order susceptibility tensor obtained from group theory and SBHM and calculate the values for the nonzero components as well as derive the SHG intensity field formulas. We found that the tetragonal and orthorhombic perovskite SHG intensity profiles are different. Furthermore, our results show that even in relative complex systems such as metal halide perovskites (MHPs), RASHG experimental data can be fitted using only one independent fitting parameter, which is the effective Pb-I nonlinear hyperpolarizability. Therefore, the analysis of perovskite using an anharmonic bond model can lead to better efficiencies of photovoltaic devices, while it will also pave a way for the use of SHG in MHP crystals as frequency conversion and optical signal processing.