Spontaneous emission spectra S(δ_k) (in units of ) each as a function of detuning frequency δ_k for different amplitudes of three coupling fields. (a) |Ω₁| = 0, Ω₂ = 1, Ω₃ = 1; (b) |Ω₁ | = 0.25, Ω₂ = 1, Ω₃ = 1; (c) |Ω₁ | = 0.5, Ω₂ = 1, Ω₃ = 1; (d) |Ω₁| = 2, Ω₂ = 1, Ω₃ = 1; (e) |Ω₁| = 1, Ω₂ = 2, Ω₃ = 1; (f) |Ω₁| = 1, Ω₂ = 1, Ω₃ = 2. The other parameters are set to be θ = 0.2π, A₁(0), A₂(0) = 1, A₃(0) = 0, δ_14c1 = − 0.1, and δ_14c2 = 0.1.