Description: We have used phase-sensitive ultrashort-pulse interferometry to study the modifi cation of optical
pulse propagation near the photonic band edges in colloidal crystals consisting of polystyrene spheres in
water. A strong suppression of the group velocity is found at frequencies near the L gap of the fcc lat-
tice. The group velocity dispersion diverges at the band edges and shows branches of both normal dis-
persion and anomalous dispersion, which can be interpreted as large changes in the effective mass, both
positive and negative. We obtain excellent agreement with the dynamical diffraction theory.
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