Objects a thousand times smaller than the wavelength of infrared light (10 micrometers) are undetectable by standard far-field optical infrared microscopy since the weak nanoparticle signals would be buried far below the background level. To overcome this drawback and to achieve nanoscale spatial resolution researchers in Germany illuminate the nanoparticles by a highly intensive nanoscale infrared light spot. It is generated in the nano-gap between a laser-illuminated scanning metal tip and the substrate supporting the particles. The simple but very efficient trick finally allowing detection of sub-10 nm particles is the use of highly reflecting substrates instead of glass slides typically used as a sample carrier in optical microscopy.
Cantilever based back-illuminated full body glass tips with thin metal layers can be used as apertureless optical near-field probes with single molecule sensitivity and optical resolutions down to 15 nm exceeding by far the classical diffraction limit of Abbe.
Single molecule Raman spectroscopy of molecules on metal surfaces was achieved by means of optical field-enhancement from a scanning probe tip. This was made possible by the optical antenna configuration formed by optical coupling between metallic tip and the substrate.