Towards the origin of the shear force in near-field microscopy

The shear force from a gold or a graphite sample acting on an approaching n ear-field optical probe is studied in detail. The adiabatic and dissipative contributions to the force are clearly distinguished by monitoring the amp litude as well as the phase of the tip vibration when the tip approaches th e surfaces. We also take into account that not only the damping and the res onance frequency but also the mass of the system changes when the tip appro aches the surface. The relative strength of the contributions to the force varies differently but characteristically with the distance of the two samp les, starting at a much larger distance in the case of graphite. Tile adiab atic contribution is larger in the case of the gold sample. Measurements at various temperatures are performed using the gold sample, showing a depend ence of the shear force on the varying conditions.