  Coherent engineering of landscape potential in crystalline materials is a
rapidly evolving research field. Ultrafast optical pulses can manipulate
low-frequency shear phonons in van der Waals layered materials through the
dynamical dressing of electronic structure and photoexcited carrier density. In
this work, we provide a diagrammatic formalism for nonlinear Raman force and
implement it to shear phonon dynamics in bilayer graphene. We predict a
controllable splitting of double degenerate shear phonon modes due to
light-induced phonon mixing and renormalization according to a coherent
nonlinear Raman force mechanism. Intriguingly, we obtain a light-induced shear
phonon softening that facilitates structural instability at a critical field
amplitude for which the shear phonon frequency vanishes. The phonon splitting
and instability strongly depend on the laser intensity, frequency, chemical
potential, and temperature of photoexcited electrons. This study motivates
future experimental investigation of the optical fine-tuning and regulation of
shear phonons and layer stacking order in layered van der Waals mater
