▎ 摘　　要

We present and discuss a double-resonant Raman mode in few-layer graphene that is able to probe the number of graphene layers. This so-called N mode on the low-frequency side of the G mode results from a double-resonant Stokes-anti-Stokes process combining an longitudinal optical (LO) and an out-of-plane (ZO') phonon. Simulations of the double-resonant Raman spectra in bilayer graphene show very good agreement with the experiments.