where 𝑐 is the speed of light in vacuum, 𝑛 c is the refractive index of the cavity, 𝑐/𝑛 c is the speed of the light inside the cavity, and 𝜃 is commonly referred to as the incident angle, which is the angle of the photonic mode wavevector k relative to the norm direction of the mirrors (see the top panel of Fig. for a schematic illustration). In most of the VSC experiments, 𝑛 c ≈ 1.5 for the solution used inside the microcavity. Because 𝑛 c ≈ 1, it will not influence the order of the magnitude of our discussion. For simplicity, we explicitly drop 𝑛 c throughout this paper. Later, whenever we write 𝑐 in an expression we should replace it with 𝑐/𝑛 c , in principle. When 𝑘 ‖ = 0 (or 𝜃 = 0), the photon frequency is