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 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. For simplicity, let us focus on the TE mode, and set k y = 0. For a field propagation direction k (see Fig. ), the total energy of the photon is