Sensitivity maximized near-field scanning optical microscope with dithering sample stage

Abstract

We developed a new scheme for a higher sensitivity near-field scanning optical microscope (NSOM) by using a dithering sample stage rather than a dithering probe for the constant gap control between probe and sample. In a conventional NSOM, which use tip dithering feedback mechanism, the Q factor drastically decreases from 7783 to 1000 (13%) or even to 100 (1%) because harmonic oscillating characteristic is deteriorated owing to the large change of stiffness and mass of one prong of tuning fork when a probe is attached to it. In our proposed scheme, on the other hand, we use sample dithering feedback mechanism, where the probe is not attached to the tuning fork and the sample is loaded directly onto the surface of dithering tuning fork. Thus, the Q factor does not decrease significantly, from only 7783 to 7480 (96%), because the loaded sample hardly changes the stiffness and mass of tuning fork. Accordingly, gap control between the immobile fiber probe and the dithering sample is performed precisely by detecting the shear force with high sensitivity. Consequently, the extremely high Q factor enables clear observation of graphene sheets with sub-nanometer vertical resolution, which is not possible with a conventional NSOM setup.

Operation of a wet near-field scanning optical microscope in stable zones by minimizing the resonance change of tuning forks

“…The practical difficulty in manifesting a wet-NSOM system is the instability of the tuning fork vibration at resonance whereas the resonance frequency in the conventional NSOM system is more definite and stable [21,22]. The reliable response of the tuning fork is critical in the stable operation of the NSOM system because its resonant oscillation is used to monitor the shear force between the tip and samples and thus to control the gap between them precisely [23,24]. In liquid environments, the resonance frequency shifts to a lower frequency and its quality factor (Q factor) deteriorates due to the high viscosity and surface tension of the liquid surrounding the tip [25][26][27][28][29].…”