SCANNING NEAR-FIELD OPTICAL MICROSCOPY - SNOM
fundamentals

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Scanning Near-Field Optical Microscopy (SNOM, also called NSOM) allows optical microscopy with a spatial resolution better than 100 nm, i.e. beyond the diffraction limit in the visible spectrum. In principle, a small nanometer sized optical probe is scanned very close to the sample surface. The sample interacts with the ‘near field’ of the optical probe, leading to a modulation in reflected or transmitted signal intensity.

The near-field optical signal depends strongly on the separation of optical probe and sample surface: only slight changes of this distance in the nanometer range are sufficient to considerably alter the recorded optical signal. It is therefore important to keep the optical probe and the sample at constant separation. In case of the attoSNOM III, a tuning fork sensor connected to an electronic feedback loop achieves this task with highest precision. As a result, not only measurements of transmission, reflection, and lateral scattering of light become possible with highest stability, but also simultaneous topographic and force measurements can now be conducted.
Probes - Different systems of optical probes are known. attocube systems‘ probes are mainly fabricated from an optical fiber, which has been tapered to reduce its size and coated with an opaque metal layer from the sides leaving only a small aperture at its very end.
The attoSNOM III is designed particularly for the use at extreme environmental conditions such as ultra low temperature, high magnetic field, and high vacuum. Reliable functionality at these extreme conditions is provided by implementing the outstanding attocube systems nanopositioning modules.

ip-Sample Distance Control:
The tuning fork sensor as a tip-sample distance control mechanism is a non-optical method for measuring small vibrations of the SNOM probe by means of a quartz tuning fork. In general, the glass fiber tip is glued onto one leg of a small quartz tuning fork. The fiber based tip is excited in horizontal direction with an amplitude of typically 50 pm. As the tip approaches the sample in the nanometer range, the vibrational amplitude of the tip decreases. This reduction in amplitude caused by lateral forces, the so-called shear forces, is monitored and kept constant, leading to a constant force and therefore separation between SNOM probe and sample surface. The sensor allows measurement of the tip–sample friction and shear forces ranging down to approximately 0.1 pN. In this configuration, the whole system behaves like a simple forced harmonic oscillator.

attoSNOM III:
The tip–sample distance control is achieved by using a non-optical piezoelectric tuning fork sensor for shear force detection (non-optical distance control). The applicable probes are optical fiber tips guaranteeing highest light throughput and sensitivity. This approach is suitable for measurements in transmission and reflection mode.