In traditional microscopy, diffraction always limits resolution capacity to around half of the wavelength of the input light. Conventional optical microscopes working with visible light can at best provide spatial resolutions of structures measuring several hundred nanometers. Such methods are therefore out of the question when undertaking nanometer-scale investigations in the infrared range, although this range is very well suited to spectral analysis of many materials, when taking a look, for example, at the excitation bands of molecules belonging to what is known as the fingerprint region. The technology behind near-field microscopy gets around these underlying limitations and allows diverse materials to be analyzed optically with a typical spatial resolution of 20 nm, regardless of the wavelength of the input laser light.
The basic element of this measuring technique is an atomic force microscope, which provides the topography of the sample by scanning its surface. A lens focuses additional laser light on the area of the probe tip. The light scattered back contains information on the sample’s optical properties and the spatial resolution is now only limited by the geometry of the probe tip.
In contrast to other measuring techniques offering resolutions on a nanometer scale, such as tunnel microscopy or electron microscopy, near-field microscopy is sensitive not only to the chemical and structural, but also the electronic properties of the sample. Here it is even possible to discern structural elements present below the surface layer that remain hidden in purely topographic surveys.
Near-field microscopy combines the high local resolution of scanning with the depth of information that comes with spectroscopical analysis techniques.