In principle, the Fabry-Pérot cavity has an infinite set of possible k z that satisfy the mirror boundary conditions (Eq. 65). Often, one only considers the k z that is close to the matter excitation energy. However, when E ph is much smaller than the matter excitation energy, multiple modes that contain various k z (Eq. 65) in the range of matter-energy and a given range of θ have to be considered. In this review, we only consider the case for a single k z (such that k z = π/L z ).