Molecular second harmonic generation induced at a metallic tip

We report the measurement of efficient second harmonic generation at a metallic tip immersed in a molecular solution of highly nonlinear molecules. The dependence of the signal either with the tip-to-substrate applied voltage or with the tip-to-substrate distance is studied. We show that the signal originates from the orientation of a limited number of molecules following local centrosymmetry breaking under the tip. The results are discussed within the context of scanning probe microscopy.