  Electron energy-loss spectroscopy studies have been performed in order to get
a deeper insight into the electronic structure and elementary excitations of
the two-leg ladder system Ca$_{11}$Sr$_3$Cu$_{24}$O$_{41}$. We find a strong
anisotropy of the loss function for momentum transfers along the a and
c-crystallographic axis, and a remarkable linear plasmon dispersion for a
momentum transfer parallel to the legs of the ladders. The investigated
spectral features are attributed to localized and delocalized charge-transfer
excitations and the charge carrier plasmon. The charge carrier plasmon position
and dispersion in the long wave-length limit agree well with expectations based
upon the band structure of the two-leg ladder, while the observed quasi-linear
plasmon dispersion might be related to the peculiar properties of underdoped
cuprates in general.
