The idea of apertureless near-field optical microscopy challenged many researchers to use atomic force microscopes (AFM) which are based on the optical monitoring of the cantilever. The critical issue of the tip to sample gap (TSG), its accuracy and real time control, is the main difficulty. The problem of harmless approaching a sharp tip to the solid surface and holding it at a distance of only a few nanometers is still topical. The apparent scheme, to raise a little the tip after contact approach, does not work for several reasons: 1) Upon lifting, the soft cantilever "jumps up" farther than the desired few nm; 2) Sharp tips carried by stiff cantilevers are known not to survive the contact approach; 3) The large susceptibility of the ceramic bodies of the scanners to temperature and humidity makes it practically impossible to maintain fixed TSG for a reasonable time without constant verification. On the other hand, the traditional AFM non-contact operational mode is characterized by the unsuitably large mean distance between the tip and sample surface.
We propose some new solution. In this research, we consider the commercial instrument, equipped with the commercial probe, working under standard ambient environment, as a part of the optical set-up, open for the run-time design and adjustments. Using the intensity of the scattered evanescent field above a glass prism as a reliable sensor of TSG, we measured simultaneously different signal components from AFM position sensitive photo detector (PSPD) in response to the lateral sample and vertical probe oscillations. We found out, that the abrupt strong change of the phase of the low amplitude (<1 nm) nonresonant oscillations of a stiff cantilever normally used for the non-contact measurements, in close proximity to the surface, is an excellent monitor tool for the TSG control. We realized a close loop, driving the probe oscillator at the frequency far from the main resonance and holding the given phase shift between driver and PSPD response by controlling the z-scanner. We succeeded to approach repeatedly without damage the fragile tip from afar to the 5±2 nm gap and hold it for an arbitrarily long time. As an attestation of the method, we accomplished the femto-laser near-field nano ablation of the metal film.
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