We propose an ultrahigh quality factor (Q) photonic crystal slab nanocavity created by the local width modulation of a line defect. We show numerically that this nanocavity has an intrinsic Q value of up to 7× 107. Transmission measurements for fabricated Si photonic-crystal-slab nanocavities directly coupled to input/output waveguides have exhibited a loaded Q value of ∼800 000. These theoretical and experimental Q values are very high for photonic crystal nanocavities. In addition, we demonstrate that simply shifting two holes away from a line defect is sufficient to achieve an ultrahigh Q value both theoretically and experimentally.
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