Linear wave vector dependence of low-frequency Raman modes in two uniaxial gyrotropic quartz-type materials: α-GaPO4 and α-AlPO4

Linear wave vector dependence of low-frequency E modes was investigated in two quartz-type materials: α-GaPO4 and α-AlPO4. It was found that the splitting of doubly degenerate optical E-modes caused by spatial dispersion is similar to that previously measured for the 132 cm−1 E-mode in α-quartz by Pine and Dresselhaus. Somewhat surprisingly, spatial dispersion does not occur for all E-modes studied. This behaviour is discussed in terms of the different origin of these modes.