Atomic Force Microscopy
We analyze the nonlinear oscillations of an AFM cantilever to reveal the forces between the tip and the surface. The cantilever is driven with two pure tones near resonance. The nonlinear tip-surface force gives rise to frequency mixing, or intermodulation of the drive tones. With a multifrequency intermodulation lockin we capture in real-time the amplitude and phase at many intermodulation products as we scan. The method gives multiple amplitude and phase images in one scan, with each image showing rich and varying contrast. We have developed several different algorithms for interpreting the information in these multiple amplitude and phase images, to reconstruct the interaction between the AFM tip and the surface.
Here is a link to a movie which demonstrates intermodulation as the oscillating AFM tip approaches a surface.