Towards the Origin of the Shear Force in Near-Field Microscopy

Abstract

The shear force from a gold or a graphite sample acting on an approaching near-field optical probe is studied in detail. The adiabatic and dissipative contributions to the force are clearly distinguished by monitoring the amplitude as well as the phase of the tip vibration when the tip approaches the surfaces. We also take into account that not only the damping and the resonance frequency but also the mass of the system changes when the tip approaches the surface. The relative strength of the contributions to the force varies differently but characteristically with the distance of the two samples, starting at a much larger distance in the case of graphite. The adiabatic contribution is larger in the case of the gold sample. Measurements at various temperatures are performed using the gold sample, showing a dependence of the shear force on the varying conditions.

The Enhancement Effect at Local Reflectance and Emission Back to Apertureless SNOM Tips in the Shear-Force Gap

“…The fiber (quality-or Q-factor is about 100) is pulled to < 10-20 nm end radius with a laser puller such as the P-2000 micropipette puller (Sutter Instruments, Navato, CA). The depolarized (checked with a polarizer) light intensity at the end of the tip on an about 1 m wide spot may reach 60-80 W. The system operates with optical shear-force control (2) [26,27]. The tip vibrations are in the range of 100-400 kHz with free amplitudes of 5 or 10 nm and the damping settings are fixed between 30 % and 60 % depending on the materials.…”

“…Therefore, inclusion of the merging concept should be considered for any modeling or simulation of the experimentally secured enhancement effect [1,8,24,30]. Unfortunately, the shearforce is still not fully understood [27] due to numerous factors and very diverse situations that are also reflected by the strong chemical contrasts in apertureless shear-force SNOM. Variations in the damping efficiency are clearly part of the optical contrast, as different distances are installed for different materials.…”

Spectroscopy of the shear force interaction in scanning near-field optical microscopy

“…this approach, but little is known about the origin of the involved forces. Some investigations in this field are made under ambient conditions [7][8][9][10] or low temperatures [11][12][13], and only a few under high [14] or ultra-high vacuum (UHV) [15] conditions. In recent years, the fundamental properties of the forces involved in tip-sample interaction were discussed by Refs.…”