  Organic-inorganic hybrid halide perovskites have ignited tremendous interests
for photovoltaic applications. However, their nonlinear optical response has
not been studied although many of these structures lack the centrosymmetry and
exhibit ferroelectricity. In this work, we employ our developed large-scale
parallel, first-principles simulation tool (ArchNLO) to explore second-order
nonlinear optical properties of a typical family of organic-inorganic hybrid
halide perovskites, CH3NH3MX3 (M= Ge, Sn, Pb; X=Cl, Br, I). We find that these
hybrid perovskites exhibit second harmonic generation and linear electro-optic
effect. The nonlinear optical effects are strongly influenced by the types and
positions of cations/anions and corresponding band gaps. Particularly, the
distorted cubic phase, which is essentially triclinic, of CH3NH3SnI3 shows
significant second harmonic generation and electro-optic effect, which are
comparable with those widely used materials, such as GaAs. These second-order
optical properties of organic-inorganic hybrid halide perovskites and their
low-temperature, solution-based fabrication pave the way for achieving and
implementing nonlinear optical devices with low cost.
