Distance control in near-field optical microscopy with piezoelectrical shear-force detection suitable for imaging in liquids

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Abstract
We introduce an improved piezoelectric shear-force feedback system for tip-sample distance control in a scanning near-field optical microscope. A tapered glass fiber is glued into a metal tube and both are integrated in a mounting, sandwiched between two piezosegments. One of the piezoelements excites the fiber tip at mechanical resonance while the other one is used for detection. During surface approach the fiber resonance is damped by shear forces, which is registered by the second piezoelement and used for distance control. The main attractions of this setup are its simplicity, its compactness, and the lack of disturbing light sources. The fiber is easy accessible and tip exchange is simple. With an acceptable fiber amplitude of 5–10 nm (peak to peak) we obtained a topographical resolution of 5 pm/Hz. The geometry also allows the measurement of samples covered with a few millimeters of liquid, which is important for applications in biology and medicine.