  Based on temperature dependent far infrared transmission spectra of YMn2O5
and TbMn2O5 single crystals, we report the observation of electric
dipole-active magnetic excitations, or electromagnons, in these multiferroics.
Electromagnons are found to be directly responsible for the step-like anomaly
of the static dielectric constant at the commensurate--incommensurate magnetic
transition and are the origin of the colossal magneto-dielectric effect
reported in these multiferroics.
