  Missing second-order nonlinearity in centrosymmetric graphene overshadows its
intriguing optical attribute. Here, we report redox-governed charge doping
could effectively break the centrosymmetry of bilayer graphene (BLG), enabling
a strong second harmonic generation (SHG) with a strength close to that of the
well-known monolayer MoS2. Verified from control experiments with in situ
electrical current annealing and electrically gate-controlled SHG, the required
centrosymmetry breaking of the emerging SHG arises from the charge-doping on
the bottom layer of BLG by the oxygen/water redox couple. Our results not only
reveal that charge doping is an effective way to break the inversion symmetry
of BLG despite its strong interlayer coupling but also indicate that SHG
spectroscopy is a valid technique to probe molecular doping on two-dimensional
materials.
