  Ultrafast atomic vibrations mediate heat transport, serve as fingerprints for
chemical bonds and drive phase transitions in condensed matter systems. Light
pulses shorter than the atomic oscillation period can not only probe, but even
stimulate and control collective excitations. In general, such interactions are
performed with free-propagating pulses. Here, we demonstrate intra-cavity
excitation and time-domain sampling of coherent optical phonons inside an
active laser oscillator. Employing real-time spectral interferometry, we reveal
that Terahertz beats of Raman-active optical phonons can bind temporal solitons
to bound-states - termed "Soliton molecules" - and we resolve a coherent
coupling mechanism of phonon and intra-cavity soliton motion. Concurring
electronic and nuclear nonlinearities generate distinct soliton trajectories
and, effectively, enhance the coherent Raman signal. As a result, we observe
that the inter-soliton motion automatically performs highspeed time-domain
Raman spectroscopy of the gain medium. In particular, our results pinpoint the
impact of Raman-induced soliton interactions in crystalline laser media and
microresonators, and offer unique perspectives towards ultrafast nonlinear
phononics by exploiting the enhanced coupling of atomic motion and solitons
inside a cavity.
