Optical microscopy beyond the diffraction limit

Abstract

The near-field optical interaction between a sharp probe and a sample of interest can be exploited to image, spectroscopically probe, or modify surfaces at a resolution (currently down to ~12 nm with visible light) well beyond the diffraction limited response of traditional far-field systems. The resulting technique, near-field scanning optical microscopy (NSOM), still retains many of the attractive features of conventional optics, including non-invasiveness, low cost, reliability, and ease of use. Most importantly, NSOM takes advantage of the numerous and powerful contrast mechanisms and associated sample preparation procedures developed for optical microscopy over the last three centuries.Several applications have been investigated to demonstrate the versatility arising from these contrast mechanisms. For example, polarization contrast has been coupled with near-field surface modification capabilities to image and record domains in thin-film magneto-optic materials (Fig. 1). This might form the basis for a new method of ultra-high density optical data storage, or could be used in a study of domain wall energetics in magnetic systems.In the biological arena, NSOM has been used to image fluorescently labeled cytoskeletal actin in fixed mouse fibroblast cells (Fig. 2), and applied in a study of the actin organization within wound healing protrusions. The resulting images were also compared to topographic data arising from the shear force mechanism which regulates the probe-to-sample separation.