Optical second harmonic study of the ieicommensuirate phase in K2SeO4 and (NH4)2BeF4 crystals

Abstract Second harmonic generation was studied in the region of phase transition in K2SeO4 and (NH4)2BeF4. Symmetry-forbidden harmonic generation was observed in the incommensurate phases near Tc. The experimental results are explained under assumption that the unipolar domain-like structure with C2v symmetry appears in the incommensurate phase.