  Improving and controlling the efficiency of a gain medium is one of the most
challenging problems of laser research. By measuring the gain length in an opal
based photonic crystal doped with laser dye, we demonstrate that optical
amplification is more than twenty-fold enhanced along the Gamma-K symmetry
directions of the face centered cubic photonic crystal. These results are
theoretically explained by directional variations of the density of states,
providing a quantitative connection between density of the states and light
amplification.
