Vibrational spectroscopy is an established method for the analysis of biological samples.
It provides chemical information and requires no staining. Biological questions in
biomembranes require spatial resolution, that can not be reached by conventional microscopes
due to the diffraction limit. The structures are smaller than the wavelength of visible
light and therefore can not be resolved by classic Raman or Infrared spectroscopy.
Near-field optical microscopy allows this limit to be overcome while retaining the
possibilities of classical light microscopy - combining atomic force microscopy with the possibilities
of spectrometers. The lateral resolution of apertureless near-field optical microscopy
like tip-enhanced Raman spectroscopy (TERS) and scattering scanning near-field optical microscopy,
s-SNOM) is only limited by the radius of the metallic tip of the atomic force microscope
(diameters typically: 10 - 40 nm).
By systematically scanning of the sample surface topological information from the atomic force
microscope and in addition chemical information about the structure of the molecules by vibrational
spectroscopy can be obtained point-by-point. In s-SNOM the metallized tip acts as an antenna, focussing
the incident light in the optical near-field of the tip. The emitted light is modulated by an interferometer
and phase-sensitively detected using lock-in amplification.