Photodetectors using photonic crystals (PhCs) in polysilicon film that include an in-plane resonant defect. A biatomic photodetector includes an optical defect mode that is confined from all directions in the plane of the PhC by the photonic bandgap structure. The coupling of the resonance (or defect) mode to out-of-plane radiation can be adjusted by the design of the defect. Further, a “guided-mode resonance” (GMR) photodetector provides in-plane resonance through a second-order grating effect in the PhC. Absorption of an illumination field can be enhanced through this resonance.