“…We can assume the following values: the average refraction index is n 0 ≈ 3.6, the wavelength (in the medium) λ ≈ 232 nm, which corresponds to the angular frequency ω c ≈ 2.26×10 15 s −1 . Excitons in quantum wells can, under certain conditions (such as low densities and proximity of the operating frequency to an excitonic resonance, see Khitrova, Gibbs, Jahnke, Kira and Koch [1999]) may be regarded as effective two-level systems (TLS's). We consider their surface density to be ≈ 10 10 -10 11 cm −2 , which corresponds to a bulk density ρ 0 ≈ 10 15 -10 16 cm −3 .…”