Organic two-dimensional photonic bandgap structures (2D PBG) have been fabricated by spin-coating a thin polymer film onto a nano-patterned SiO2 circular-grating surface-emitting distributed Bragg reflectors (CG-SE-DBR). When optically pumped and for certain grating parameters, these structures exhibit a peak inside the stop band that
leads to lasing with a reduced threshold. An analytical model based on the transfer-matrix method has been developed to investigate the origin of this peak. The theoretical results are in excellent agreement with the experimental findings.