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. B3a). 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. B2). Both k x and k y are in principle, quasi-continuous because the boundary length for the lateral directions (x and y in Fig. ) are generally much larger than the mirror distance L z . The cavity quantization volume is V = S • L z , where S represents the effective quantization area at which molecules are coupled to the cavity. Using the experimentally measured Ω R and V, one can estimate how many molecules N are effectively coupled to the cavity. Overall, this leads to many photonic modes that can be energetically close to a matter state transition, such as electronic excitations 87,99-105 or vibrational excitations. For these cavities, the photonic dispersion relations are the same for both the transverse electric (TE) and transverse magnetic (TM) polarizations, and experimentally, one can easily access both 105,108,109 .