The value of k z is explicitly quantized, due to the boundary condition imposed by two mirrors, where L z is the distance between the two mirrors. In the literature, k z is often denoted as k ⊥ because it is perpendicular to both mirrors (not to be confused with the transverse component of the field in Eq. 37a). There are two more degenerate wavevectors, k x and k y , with their directions parallel to the mirror, and are commonly denoted as k ∥ in the literature (not to be confused with the longitudinal component of the field, such as Eq. 36).