“Triggering” effect of second harmonic generation in centrosymmetric α-BaB2O4 crystals

Abstract We have discovered an occurrence of optical second harmonic generation at 1064 nm fundamental wavelength for the α-BaB2O4 (ABO) crystal illuminated by 7 ns nanosecond nitrogen laser of 337 nm. The effect is observed only during illumination and achieves the maximum SHG output after 15–20 min of treatment. After switching off of the UV laser treatment the SHG decreases during about 5 min. The maximum second order susceptibility achieved was equal to about 2.1 pm/V at 1064 nm after about 20 min of UV laser treatment. The observed SHG is explained by local photothermal heating of the crystals and photo stimulated photothermal anisotropy that favors an occurrence non-centrosymmetry of the local charge density. An additional investigations of the low-power (cw) He–Ne laser light indicate the absence of additional light scattering, which unambiguously indicates an absence of any additional microcrystalline states including non-cenrosymmetrical β-BaB2O4 phases. Probably photostimulated enhancement of thermal expansion anisotropy the main reason for the observed non-centrosymmetry.