STM induced second harmonic generation: towards near-field nonlinear optical microscopy

We propose an original technique which takes profit of Second Harmonic Generation (SHG) effects in molecular solutions. Our technique exploits the specificities of molecular contributions. We show that we can use the electric field present inside a Scanning Tunneling Microscope (STM) junction towards creating a local non-centrosymmetry via molecular orientation under the tip. Experiments were performed inside a STM junction immersed in concentrated solutions of azo-dyes molecules chosen for their highly nonlinear properties and the possibility to generate a local SHG signal from those molecules was demonstrated. More particularly, the quadratic dependence of the SHG signal intensity with the voltage applied between the tip and the substrate unambiguously shows that it comes from an electric field induced molecular polarization under the tip. The dependence of the signal with the tip height or size is reported and discussed. This approach opens the way to a new and original near field optical microscopy technique.