Additionally, compared to Fig. , here 𝒜(𝜔) tails toward the higher energy. This is because the weighting factor 𝜔 k Γ 01 /(Γ 10 + Γ ′ 10 ) is not truly singular at 𝜔 c . The smaller the 𝒟/𝑐 value, the sharper 𝒜(𝜔) will be. When taking the limit of 𝒟/𝑐 → 0, Fig. reduces back to Fig. . On the other hand, when 𝒟/𝑐 → ∞, there will be no loss in the in-plane direction, corresponding to a much wider 𝒜(𝜔) (see the dark blue curve in Figs. and), which is only bounded by 𝑒 -𝛽ℏ𝜔 k . Under this condition, 𝒜(𝜔) still peaks at a particular frequency, but with 𝜔 c > 𝜔 0 . Fig. -f corresponds to the red-detuned case under oblique incidence, where 𝜔 c = 0.85𝜔 0 .