Markus B. Raschke

We study the properties of plasmonic metal nanostructures to transfer and concentrate light to highly confined regions. The spectral selectivity of the confinement sensitively depends on sample material and geometric details. The fundamental understanding of the near-field coupling between the radiation field and optical excitation of the medium thus provides the basis for the rational design of photonic devices for bioanalytic sensor applications, wave guides, optical switches, or nanoscopic light sources.

Taking advantage of the optical antenna properties of a nanoscopic metal tip allows for a new combination of optical spectroscopy with scanning probe techniques (scattering-type near-field microscopy). Here, our group has achieved scanning probe vibrational Raman microscopy with single molecule sensitivity. We have also advanced infrared microscopy for chemical analysis obtaining ultrahigh spatial resolution down to 10 nm. Applicable under ambient and in situ conditions this allows us to probe microscopic composition, thermodynamics phases, or chemical functionality of nanoscale materials that have been difficult to access so far.

The nonlinear optics on the nanoscale is a yet largely unexplored research area. Taking advantage of the higher symmetry selectivity of the nonlinear response compared to linear optics allows for the unique distinction of surface and bulk properties for certain nanostructures. Here, we study ferroelectric ordering, electron-vibration coupling and its ultrafast dynamics of intrinsic and adsorbate covered planar surfaces as well as of nanostructures using both far-field techniques and scanning probe near-field microscopies.