Atomic force microscopy optical detection mechanism

Atomic Force Microscopy Atomic force microscopy is a direct descendant of STM and was first described in 1986 [254], The basic principle behind AFM is straightforward. An atomically sharp tip extending down from the end of a cantilever is scanned over the sample surface using a piezoelectric scanner. Built-in feedback mechanisms enable the tip to be maintained above the sample surface either at constant force (which allows height information to be obtained) or at constant height (to enable force information to be obtained). The detection system is usually optical whereby the upper surface of the cantilever is reflective, upon which a laser is focused which then reflects off into a dual-element photodiode, according to the motion of the cantilever as the tip is scanned across the sample surface. The tip is usually constructed from silicon or silicon nitride, and more recently carbon nanotubes have been used as very effective and highly sensitive tips. 

The method applied to measure the depth of the ablated area or the removed mass can also have an influence on the ablation parameters. If profilometric measurements (optical interferometry, mechanical stylus [34], or atomic force microscopy [35]) are used to calculate the ablation rate, a sharp ablation threshold can be defined. This is also supported by reflectivity [36] and acoustic measurements [37], In mass loss measurements, such as mass spectrometry or with a quartz crystal microbalance (QCM), the so-called Arrhenius tail [38] has been observed for certain conditions. The Arrhenius tail describes a region in the very low fluence range, where a linear increase of detected ablation products is observed, which is followed by a much faster increase, that coincides with removal rates of the profilometric measurements [39]. 

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