Nonlinearity-induced frequency mixing in AFM: novel contrast imaging with machine learning

On complex thin films pertinent to lubrication and superhydrophilicity, dynamic AFM methods were used to sense both the elastic and viscous response to an AFM tip. A novel strategy in "multifrequency" AFM -- vibrating the AFM microcantilever (to which the tip is attached) at two tones near the fundamental flexural resonance -- generates dozens of intermodulation tones of response due to nonlinear tip-sample interaction (a well-known concept in electrical engineering usually in the context of AC signal distortion). This method¹ both expands contrast mechanisms -- with images of amplitude and phase at each mixing tone via 40 FPGA lock-in amplifiers -- and the ability to reconstruct the distance dependence of conservative and dissipative response at each image pixel via a 40-term discrete Fourier transform of tip motion. This reconstruction is an unprecedented capability in "force spectroscopy" (probing distance dependent interactions). Machine learning, to cluster force-spectroscopic fingerprints and thereby generate higher signal-to-noise images, is further discussed.

¹Intermodulation Products AB, Stockholm.