where ω c is the photon frequency of the quantized direction (z-direction) in the cavity, used in the single mode approximation of the cavity QED (see Eq. 42, Eq. 44, θ (degree) θ (degree) θ (degree) and Eq. 56 in Sec. 2.3). Further, under the single mode approximation (by setting k x = 0) the photonic momentum k (or the field propagation direction) will be perpendicular to the cavity mirror. Note that in principle, the Fabry-Pérot cavity has an infinite set of possible k z that satisfy the mirror boundary conditions (Eq. 86). 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. 86) 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 ).