Abstract: Defect modes in a photonic crystal slab cavity are calculated with finite difference time domain method and group theory. Using symmetry boundary eondition.all frequencies of defect modes are obtained through dipole driving-sources with a broadband pulse.For one given frequency of defect mode,we use narrowband pulse to excite one single defect mode.Numerical results show that all defect modes with different symmetries can be obtained with this method.

Key words: defect mode of photonic crystal slab cavity, effective mode volume, finite difference time domain