  We report phonon renormalisation in bilayer graphene as a function of doping.
The Raman G peak stiffens and sharpens for both electron and hole doping, as a
result of the non-adiabatic Kohn anomaly at the $\Gamma$ point. The bilayer has
two conduction and valence subbands, with splitting dependent on the interlayer
coupling. This results in a change of slope in the variation of G peak position
with doping, which allows a direct measurement of the interlayer coupling
strength.
