  The terahertz (THz) excitations in the quantum spin-ladder system Sr14Cu24O41
have been determined along the c-axis using THz time-domain, Raman and infrared
spectroscopy. Low-frequency infrared and Raman active modes are observed above
and below the charge-ordering temperature T_{co} ~ 200 K over a narrow interval
of 1 - 2 meV . A new infrared mode around 1 meV develops below ~ 100 K. The
temperature dependence of these modes shows that they are coupled to the
charge- and spin-density-wave correlations in this system. These low-energy
features are conjectured to originate in the gapped sliding-motion of the chain
and ladder sub-systems, which are both incommensurate and charged.
