When considering a planar Fabry-Pérot (FP) microcavity, Â⊥ (r), Ê⊥ (r) and B⊥ (r) satisfy the boundary conditions and thus the wavevector k becomes quantized . For cavity mirrors imposing a boundary condition along z direction (see Fig. ), the z component of the wavevector k z = n π Lz with n = 1, 2, 3... as a positive integer. Note that k x and k y still remain quasicontinuous variables. These are discussed in details in Sect. VI.