Enhancing atomic force microscopy stability through second harmonic optical fibre cavity control

The interatomic potential is key to understanding all properties of materials. Yet, conventional atomic force microscopes do not usually measure the interatomic potential, because they lack control of the tip-sample distance. Here, we propose a simple methodology for measuring tip-surface interactions directly as a function of tip-sample distance, rather than deflection as a function of sample displacement. We use an AC interferometer to monitor the absolute tip displacement. When a force is applied on the tip a negative feedback loop displaces the cantilever anchoring point, deflecting the lever such that the tip position remains constant. This feedback loop actively maintains the tip at a distance from an optical fibre where the interferometer sensitivity is maximum. As a result, the tip-sample distance or the indentation is directly given by the sample motion and the tip does not jump-to-contact.