is the dimensionless coupling constant proportional to the density N / V of the dielectric with the Lorentz frequency ω ω (tight-binding charge wave dispersion). One may notice that unlike in the vacuum of the electromagnetic field without matter the expectation value of the average photon number < a λ λ k + a λ λ k > is non zero in the ground state of the polaritonic Hamiltonian C k ± ± | 0 >= 0 similarly to the Hawking radiation in the neighbourhood of the black hole because of the Unruh-Davies effect. One may readily notice that the lower eigenfrequency Ω Ω − − becomes imaginary when the coupling constant becomes critical at g > 1 which suggests that Hopfield dielectric will undergo the superradiant phase transition.