Photoactivated Second Harmonic Generation in Centrosymmetric Double Perovskites

We report the first observation of second harmonic generation (SHG) from halide double perovskites single crystals, a promising class of materials for low-cost and versatile optoelectronic applications, owing to their enormous structural flexibility and environmental friendliness. We show that the SHG efficiency of these materials with centrosymmetric crystalline structures critically depends on the measurement temperature. At high temperatures, it is determined by a surface contribution but increases by up to 3 orders of magnitude at low temperatures (T < 137 K for Cs2NaFeCl6 and T < 250 K for Cs2AgBiBr6) under light illumination within several minutes. We attribute this enhancement to the build-up of a light-induced electric field within the near-surface region, which generates an additional contribution to the SHG process. This DC electric field is found to be predominantly oriented orthogonally to the sample surface, as deduced from the six-fold rotational symmetry of the SHG azimuthal pattern. The electric field formation is explained by photoinduced charge transfer from deep surface-related states to traps in the bulk region or vice versa, mainly driven by diffusion. Furthermore, the inscribed electric field can be maintained for hours at low temperatures and can only be erased by raising the temperature due to carrier detrapping. Our findings, therefore, highlight the importance of the surface states in double perovskites, which could be utilized for enhancing the nonlinear properties of these centrosymmetric materials.