Light propagation in photonic crystal with gain: Applicability of the negative loss approximation

Abstract The applicability of the concept of permittivity with Im ɛ gain N of elementary cells will sooner or later lead to lasing; (ii) if the frequency of the pump lies in the band gap, then lasing at band gap frequencies may occur in a sample with a low N before the band gap has been formed. Nevertheless, lasing will be necessarily suppressed by a further increase in N . In any case, for sufficiently large number of layers due to a finite line width of ɛ gain ( ω ), lasing appears in the pass band even if the pump frequency is in the band gap.