A nonoptical tip–sample distance control method for near-field scanning optical microscopy using impedance changes in an electromechanical system

We present a nonoptical shear-force feedback method to regulate tip–sample distance for near-field scanning optical microscopy. In the shear force setup, the dither piezo and the attached fiber tip form an electromechanical system, whose power dissipation on resonance is sensitive to the change in damping force as the tip approaches and interacts with the sample. At the frequencies of interest (∼10–100 kHz), the change in power dissipation is conveniently manifested as a change in the electrical impedance of the dither piezo. We demonstrate that tip–sample distance feedback control can be achieved by measuring this change in dither piezo impedance. The sensitivity is currently ∼0.5 Å. This new technique is compared to other methods currently used for distance control in near-field scanning optical microscopy.