DEPARTMENT OF
PHYSICAL CHEMISTRY
SFG Microscopy

Many surface science problems are intrinsically inhomogeneous on various length scales. It would thus be highly beneficial to have experimental tools at hand that are not only surface-specific but additionally also offer high spatial resolution. The Lattice Dynamics group has recently developed a new microscope that combines surface-specific vibrational spectroscopy with subdiffractional spatial resolution by means of infrared-visible sum-frequency generation. We employ the FHI infrared free-electron laser (FEL) to resonantly drive surface vibrations, and then use a time-synchronized visible laser for up-conversion. Conveniently, the resulting sum-frequency signal lies in the visible, and can thus be imaged with conventional optics, providing an image resolution well below the diffraction limit at the infrared resonant wavelength. By employing a wide-field imaging approach, we can harvest the full power of the FEL without damaging delicate surfaces.

First experiments with a normal incidence configuration of the visible laser provide a glimpse at the vast future opportunities of our approach. There, we observed localized surface phonon polariton resonances in subdiffractional SiC nanostructures with a spatial resolution of < 1 μm imaged at 11 μm infrared wavelength. The group is developing the SFG microscope into a universal surface science microscopy end station at the FHI FEL beamline.