| Abstract: |
Scattering-type Scanning Near-field Optical Microscopy (s-SNOM) is a scanning probe approach for optical microscopy and spectroscopy bypassing the ubiquitous diffraction limit of light to realize a spatial resolution below 20 nanometers. s-SNOM exploits the strong wavelength-independent confinement of light at the apex of a laser-illuminated sharp metallic AFM tip to create a nanoscale optical hot-spot. Analyzing the scattered light from the tip apex enables the extraction of the optical properties (refractive index – that is reflectivity and absorption) of the sample directly below the tip and yields nanometer scale resolved optical images simultaneous to topography. Combining s-SNOM with Fourier-Transform Infrared spectroscopy (nano-FTIR) using broadband radiation from the far-infrared to the visible spectral range enables the realization of an ultra-sensitive tool for chemical identification on a nanometer scale which can be applied to a wide range of materials like polymers, biominerals, semiconductors and 2D materials. Adapted to the needs of the respective studies, hyperspectral imaging can be conducted by recording a full FTIR-spectrum in each scanned pixel of an image. It allows for instance the elucidation of clustering and localized chemical interactions of polymers in a three-component polymer blend. Furthermore, an in-situ nanoscale infrared analysis of individual keratin-embedded melanin granules in the cross section of a human hair can be shown.
The combination of near-field microscopy with ultrafast pump-probe experiments constitutes a new valuable approach to solid-state physics where intriguing phenomena like surface plasmons polaritons or charge carrier relaxation dynamics are observed with a combined <200fs temporal and <20nm spatial resolution. Extending s-SNOM to nanometer scale ultra-broadband terahertz time-domain spectroscopy enables tracing the time-dependent dielectric function at the surface of a single photoexcited InAs nanowire in all three spatial dimensions and reveals the ultrafast (<50fs) formation of a local carrier depletion layer. |