Photoactivated Second Harmonic Generation in Centrosymmetric Double Perovskites

We report the firstobservation of second harmonic generation (SHG)from halide double perovskites single crystals, a promising classof 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 centrosymmetriccrystalline structures critically depends on the measurement temperature.At high temperatures, it is determined by a surface contribution butincreases by up to 3 orders of magnitude at low temperatures (T < 137 K for Cs2NaFeCl6 and T < 250 K for Cs2AgBiBr6) underlight illumination within several minutes. We attribute this enhancementto the build-up of a light-induced electric field within the near-surfaceregion, which generates an additional contribution to the SHG process.This DC electric field is found to be predominantly oriented orthogonallyto the sample surface, as deduced from the six-fold rotational symmetryof the SHG azimuthal pattern. The electric field formation is explainedby photoinduced charge transfer from deep surface-related states totraps in the bulk region or vice versa, mainly driven by diffusion.Furthermore, the inscribed electric field can be maintained for hoursat low temperatures and can only be erased by raising the temperaturedue to carrier detrapping. Our findings, therefore, highlight theimportance of the surface states in double perovskites, which couldbe utilized for enhancing the nonlinear properties of these centrosymmetricmaterials.