  We show that, taking a two-dimensional photonic-crystal slab system as an
example, surprisingly high quality factors (Q) over 10^5 are achievable, even
in the absence of a rigorous photonic-band-gap. We find that the density of
in-plane Bloch modes can be controlled by creating additional photon feedback
from a finite-size photonic-crystal boundary that serves as a low-Q resonator.
This mechanism enables significant reduction in the coupling strength between
the bound state and the extended Bloch modes by more than a factor of 40.
