𝜔 2 k -𝜔 2 c term in Eq. 23, such that the integral only survives and gives a finite value at 𝜔 k = 𝜔 c , and at 𝜔 k > 𝜔 c , the integral becomes vanishingly small. However, directly extending this simple consideration for the DOS cannot explain the normal incidence condition for a 2D FP cavity (when only considering the TE polarization direction). This is because the 2D DOS 𝑔 2D (𝜔 k ) does not have any singularity. Specifically, the DOS for the photonic modes inside a 2D FP cavity is expressed as