Modulated shear-force distance control in near-field scanning optical microscopy

The tip-sample distance in near-field scanning optical microscopy is typica lly controlled by the shear-force interaction between the laterally vibrati ng tip and sample. In this article, a mode of shear-force feedback is descr ibed in which an additional vertical modulation is introduced. Similar to t he tapping mode applied in atomic force microscopy, the modulated shear-for ce technique deals with problem due to the snap to contact and therefore im proves the mapping of soft and ductile materials, such as biological sample s and soft polymers. The imaging properties of the modulated shear-force mo de is demonstrated on structures of a soft polymer blend. Additionally, the modulated shear-force technique allows a simple comparison between effects in the optical far field and in the optical near field. (C) 2000 American Institute of Physics. [S0034-6748(00)02403-5].