We have observed strongly modified spontaneous-emission spectra from laser
dye in photonic crystals made of inverse opals in titania (TiO2). We identi
fy stop bands with large relative widths of 15%, comparable to the dyes' em
ission spectrum, and 75% attenuation of the emission strength. For a wide r
ange of angles, two stop bands are simultaneously manifested; their angle-d
ependent frequencies display an avoided crossing that is corroborated by re
flectivity experiments. The edges of the stop bands agree well with band-st
ructure calculations, but differ from simple Bragg diffraction as a result
of multiple wave coupling. The strongly reduced dispersion of the Bloch mod
es likely causes a significant change in the density of states.