  Optical microcavities based on zero-group-velocity surface modes in photonic
crystal slabs are studied. It is shown that high quality factors can be easily
obtained for such microcavities in photonic crystal slabs. With increasing of
the cavity length, the quality factor is gradually enhanced and the resonant
frequency converges to that of the zero-group-velocity surface mode in the
photonic crystal. The number of the resonant modes with high quality factors is
mainly determined by the number of surface modes with zero-group velocity.
