There has been recent interest in the utility and physical information content of higher harmonics in Atomic Force Microscopy: theoretically as a way to better understand the dynamical behavior of the cantilever-sample system and experimentally both as feedback signals to enhance scanning stability under liquid and as indicators of the properties of the tip-sample interaction force such as the Hamaker constant and Young's Modulus. However, the great majority of investigations of cantilever motion have assumed the cantilever position detection system to be linear in cantilever position when in fact it is not. The output of both beam-bounce and interferometric detection systems are nonlinear and this nonlinearity leads to harmonic distortion of the output signal. Importantly, the magnitude of this distortion can easily be equal to or greater than the actual harmonic components of the cantilever motion, thus rendering attempts to use harmonic signals for feedback or as keys to the reconstruction of the tip-sample interaction force non-quantitative.
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