  We present the design of two-dimensional photonic crystal microcavities in
thin diamond membranes well suited for coupling of color centers in diamond. By
comparing simulated and ideal field distributions in Fourier and real space and
by according modification of air hole positions and size, we optimize the
cavity structure yielding high quality factors up to Q = 320000 with a modal
volume of V = 0.35 (lambda/n)^3. Using the very same approach we also improve
previous designs of a small modal volume microcavity in silicon, gaining a
factor of 3 in cavity Q. In view of practical realization of photonic crystals
in synthetic diamond films, it is necessary to investigate the influence of
material absorption on the quality factor. We show that this influence can be
predicted by a simple model, replacing time consuming simulations.
