Analysis on the Raman scattering angular dependence of KDP crystal

The Raman vibration modes of potassium dihydrogen phosphate (KDP) crystal were analyzed by the group theory in detail, and the Raman vibration modes were well assigned. The Raman spectra of type Z annealed KDP crystal in X(ZZ)X, Z(XY)X and Y(XY)X scattering geometries and type Z unannealed KDP crystal in Z(XY)Z scattering geometry were also measured by Raman spectrophotometer system. According to the selection rules we found that the Raman peaks in X(ZZ)X, Z(XY)X and Y(XY)X scattering geometries correspond to A1, B2(LO) and B2(TO) modes respectively. However, in the Z(XY)Z scattering geometry, A1 modes appear besides B2 modes, and in Y(XY)X scattering geometry there exist B2 modes only. What is more, the Raman spectra of annealed and unannealed KDP crystal in Z(XY)Z scattering geometry show no obvious difference. The results indicate that the symmetry of KDP crystal is lower and A1 modes possess angular dependence in Raman back-scattering geometry, which is determined by crystal structure and independent of lattice internal stress.