Defect-free two-dimensional-photonic crystal (PC) structures have been patterned by nanoimprint lithography onto a nonlinear optical (NLO) polymer to fully control the photonic band structure. Defed-ftee PC structures are obtained in the case of samples from which a sufficient amount of solvent has been removed, and sharp dips are observed in the reflectance spectrum. In samples with defects, the dips of the spectrum are broadened due to the disruption of the periodicity. Results suggest that the spatial distribution and amount of the residual solvent in the NLO polymer needs to be controlled to reduce the defect density. (c) 2007 American Vacuum Society.