SCANNING NEAR-FIELD OPTICAL MICROSCOPY - SNOM
fundamentals
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Scanning Near-Field Optical Microscopy (SNOM, also called NSOM) allows to perform 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. Hence, the sample interacts with the ‘near field’ of the optical probe. Thus, the emission of the probe is modulated and this optical signal can be recorded.

The near-field optical signal is strongly dependent on the distance between the tip of the optical probe and the sample surface: only slight changes of this distance in the nanometer range are already enough to considerably change the recorded optical signal. It is therefore important to keep the optical probe at a constant distance from the sample. Different approaches to control the tip-sample distance have been described up to date and implemented in the different attoSNOM systems to meet the customers requirements. Thus, measurements of transmission, reflection, and lateral scattering of light are possible with high stability simultaneously to topography and force measurements.

Probes - Different systems of optical probes are known. Probes are often 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. Other probes are micromachined cantilever SNOM sensors that are handled like AFM cantilevers, but offer also an optical aperture for near-field measurements. These cantilever SNOM sensors were introduced and are distributed by WITec Instruments, Germany.

The attoSNOMs are 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.

To perform low temperature microscopy, the attoSNOMs are cooled by a controlled exchange gas atmosphere in a liquid Helium bath cryostat.

attocube systems SNOMs
Two different SNOM setups implementing different approaches concerning tip-sample control and probe design were developed to meet the customers various requirements. All attocube microscope systems are compatible with vacuum environment as well as high magnetic field.

attoSNOMI:
This easy-to-use cantilever-based SNOM system uses microfabricated SNOM tips according to the WITec© design. Thus, the sample-tip distance is controlled analogous to the deflection detection in AFM. Simultaneous topographical and optical measurements can be realized using a very simple experimental setup.

attoSNOM II:
One of the key features of this system is the ease of use due to the simple tip-sample distance control using the interferometric detection scheme and at the same time high optical efficiency due to the fiber based SNOM tips.

attoSNOMIII:
This system is unique due to the collection of the reflected signal using an ellipsoidal aluminum mirror. Thus, the attoSNOM?III impresses by its outstanding sensitivity and spatial resolution. The applicable probes are optical fiber tips and the tip–sample distance control is achieved using a piezoelectric tuning fork force sensor for shear force detection (non-optical distance control).