Near-Field Optical Microscopy: Imaging beyond the Diffraction Limit

The wealth of optical phenomena offers many opportunities for the investigation of individual nanometer-sized structures if, and only if, the relevant radiation can be confined to a sufficiently small extension. This can be achieved with material structures having nanometer-sized dimensions themselves. Small apertures, small particles and pointed tips are particularly suited for this purpose. When integrated appropriately into a scanning probe microscope, they provide optical scan images with resolution in the 10 to 100 nm regime. The most successful near-field optical (NFO) microscope to date is the aperture scanning NFO microscope (SNOM): It can be operated in transmission or reflection mode, with amplitude, phase and polarization contrast. Its most intriguing mode of operation, however, is fluorescence microscopy, of utmost importance for biological and medical applications. In practical work with SNOM, various types of artifacts have to be avoided that can mask the true near-field images to a large extent. The theoretical description of the imaging process requires the numerical solution of Maxwell's equations for complicated boundary conditions, a (computer-)time consuming but rewarding task.