  In this study, we address the selection rules with respect to the
polarization of the optical excitation of two colour centres in 4H-SiC and
6H-SiC with potential for applications in quantum technology, the divacancy and
the nitrogen-vacancy pair. We show that the photoluminescence (PL) of the axial
configurations of higher symmetry (C3v) than the basal ones (C1h) can be
cancelled using any excitation (resonant or non-resonant) with polarization
parallel to the crystal axis (EL||c). The polarization selection rules are
determined using group-theoretical analysis and simple physical arguments
showing that phonon-assisted absorption with EL||c is prohibited despite being
formally allowed by group theory. A comparison with the selection rules for the
silicon vacancy, another defect with C3v symmetry, is also carried out. Using
the selection rules, we demonstrate selective excitation of only one basal
divacancy configuration in 4H-SiC, the P3 line and discuss the higher contrast
and increased Debye-Waller factor in the selectively excited spectrum.
