Fig. 2

a TPEF from the pump (dashed line) and from the Stokes (dotted line) beams, both contribute to the total CARS signal (solid line) within the fingerprint range. Despite the TPEF background, the noticeable “dip” at 1585 cm⁻¹ (Exc: pump 30 μW/Stokes 100 μW) is well seen in the CARS spectrum of SLG. The dip at the G-band frequency is clearly manifested in the spectrum of MLG. b The contribution of TPEF to the background (~ 50% of amplitude) was the same for single-layer and multi-layer graphene. CARS images of SLG and MLG respectively recorded at 1585 cm⁻¹ (Exc: pump 310 μW/Stokes 530 μW) are presented in c and d