Pra.Ma.

At present the most-used method of probe-sample distance regulation relies on the detection of shear forces between the end of near-field probe and the sample [1]. Shear Force based system allows simultaneous Shear Force and Near-Filed imaging, including Transmission mode for transparent samples, Reflection mode for opaque samples and Luminescence mode for obtaining additional characterisation of samples.
At the heart of nonoptical method for obtaining of information about surface lies idea to use response of quartz tuning fork attached to optical fiber on interaction with surface. System fiber-quartz is excited in transverse vibrations with help of external feed element on quartz resonance frequency. Further piezoeffect is used: in the presence of mechanical oscillations electrical outputs of quartz have voltage response, which is used as information signal about amplitude of fiber oscillation.
Transmission mode of SNOM is realized simultaneously with Shear Force Microscopy, which in turn is realized in the following way. Piezodriver via quartz tuning fork excite oscillations of the fiber probe with some initial amplitude. Suitable output value of quartz is A_o. After approaching sample surface the amplitude of fiber probe oscillations reaches some set-point value and quartz output reaches value A. After that scanning of the sample surface is conducted with maintaining this value by the feedback system.
Under the scanning the sample is illuminated by the fiber probe and the passed through sample light via objective is directed on the photomultiplier tube.

References

1. Appl. Phys. Lett. 60, 2484 (1992).