  Recent spectroscopic studies at terahertz frequencies for a variety of
multiferroics endowed with both ferroelectric and magnetic orders have revealed
the possible emergence of a new collective excitation, frequently referred to
as electromagnon. It is magnetic origin, but becomes active in response to the
electric field component of light. Here we give an overview on our recent
advance in the terahertz time-domain spectroscopy of electromagnons or
electric-dipole active magnetic resonances, focused on perovskite
manganites--$R$MnO$_3$ ($R$ denotes rare-earth ions). The respective electric
and magnetic contributions to the observed magnetic resonance are firmly
identified by the measurements of the light-polarization dependence using a
complete set of the crystal orientations. We extract general optical features
in a variety of the spin ordered phases, including the $A$-type
antiferromagnetic, collinear spin ordered, and ferroelectric $bc$ and $ab$
spiral spin ordered phases, which are realized by tuning the chemical
composition of $R$, temperature, and external magnetic field. In addition to
the antiferromagnetic resonances of Mn ions driven by the magnetic field
component of light, we clarify that the electromagnon appears only for light
polarized along the a-axis even in the collinear spin ordered phase and grows
in intensity with evolution of the spiral spin order, but independent of the
direction of the spiral spin plane ($bc$ or $ab$) or equivalently the direction
of the ferroelectric polarization $P_{\rm s}$ ($P_{\rm s}\| c$ or $P_{\rm s}\|
a$). A possible origin of the observed magnetic resonances at terahertz
frequencies is discussed by comparing the systematic experimental data
presented here with theoretical considerations based on Heisenberg model.
