Hamburg 2001 – scientific programme

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O: Oberflächenphysik

O 3: Rastersondentechniken (I)

O 3.1: Talk

Monday, March 26, 2001, 11:15–11:30, B

A new Intermediate Contact Mode for Near-Field Optical Microscopy — •Matthias Wellhöfer and Olaf Hollricher — WITec Wissenschaftliche Instrumente und Technologie GmbH, Hörvelsinger Weg 6, D-89081 Ulm, Germany

Distance control is very crucial in near-field optical microscopy, because the optical resolution is not only determined by the size of the illuminating aperture but also by the distance between tip and sample. Therefore it is important to keep the tip-sample distance as small as possible. The resolution decreases, if the tip-sample distance gets larger than the radius of the tip aperture.
In our α-SNOM we use microfabricated cantilever sensors with incorporated hollow Al-pyramid. The light is focussed into the backside of this pyramid and illuminates the sample through a hole at its end.
Contact mode minimizes tip-sample distance and therefore leads to the best obtainable resolution with a given aperture size. Disadvantage of this mode are lateral forces which may cause problems on soft samples. Typical dynamic modes use vertical amplitudes between 30 nm and 100 nm shifting the mean tip-sample distance to 15-50 nm with subsequent loss of resolution.
We present measurements combining SNOM with the PulsedForceMode, a periodic intermediate contact mode where the tip is oscillated below resonance. Due to the fact that the contact time is short and periodic, the lateral forces are strongly reduced. To get the highest possible optical resolution, the data acquisition is done during the time of contact between tip and sample.