  We present a double-resonant Raman mode in few-layer graphene, which is able
to probe the number of graphene layers reliably. This so-called N mode on the
low-frequency side of the G mode results from a double-resonant
Stokes/anti-Stokes process combining a LO and a ZO' phonon. Simulations of the
double-resonant Raman spectra in bilayer graphene show very good agreement with
the experiments. The investigation of the out-of-plane ZO' phonon for layer
number determination is expected to be transferable to other layered materials
like boron nitride.
