  We demonstrate dynamic control of the lattice by THz light by exploiting the
coupling of phonon modes. The low-energy sliding phonon modes in the
spin-ladder system, Sr$_{14}$Cu$_{24}$O$_{41}$, are excited using THz radiation
with high electric fields. Due to the nonlinearities induced by the THz
electric fields, the low-energy phonon mode couples to the higher energy,
silent optical phonon modes at $\approx$ 1.17 THz. This indirect excitation of
the silent phonon mode is reflected as an enhancement of the THz transmission
near 1.17 THz. Our results demonstrate that it is possible to indirectly
control inaccessible phonon modes with THz frequency electromagnetic radiation,
which provides opportunities to alter such material systems' electronic and
magnetic properties dynamically using THz radiation.
