  Degenerate states in condensed matter are frequently the cause of unwanted
fluctuations, which prevent the formation of ordered phases and reduce their
functionalities. Removing these degeneracies has been a common theme in
materials design, pursued for example by strain engineering at interfaces.
Here, we explore a non-equilibrium approach to lift degeneracies in solids. We
show that coherent driving of the crystal lattice in bi- and multilayer
graphene, boosts the coupling between two doubly-degenerate modes of E1u and
E2g symmetry, which are virtually uncoupled at equilibrium. New vibronic states
result from anharmonic driving of the E1u mode to large amplitdues, boosting
its coupling to the E2g mode. The vibrational structure of the driven state is
probed with time-resolved Raman scattering, which reveals laser-field dependent
mode splitting and enhanced lifetimes. We expect this phenomenon to be
generally observable in many materials systems, affecting the non-equilibrium
emergent phases in matter.
