where ê is the electric field unit polarization vector, V is the cavity volume, and ϵ 0 is the permittivity of free space. Note that in Sec. 2, we have used the magnitude of the vector potential, A 0 , itself as the coupling strength. On the other hand, when coupling solid state materials, the total dipole operator is no longer well defined, and Ĥp•A (Eq. 45) is often used. For example, in Ref. 144 and Ref. 145, the polariton states of a 2D TMD coupled to an optical cavity are computed based on Ĥp•A , where one needs to evaluate the matrix elements of the matter momentum operator.