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.