Scanning near-field optical microscope improves FEL spectroscopy

Scanning near-field optical microscope improves FEL spectroscopy

Infrared spectroscopy with a free-electron laser (FEL) can serve as an excellent probe of semiconductor interfaces--the semiconductor energy barriers and discontinuities fall within the FEL spectral domain, while the FEL radiation penetrates deeply and can analyze buried interfaces. Unfortunately, however, properties of these interfaces can change from one location to another, and the FEL lacks lateral resolution. Now, a group of researchers working with the FEL at Vanderbilt University (Nashville, TN) has solved that problem by using a scanning near-field optical microscope with the FEL. An optical fiber with its tip stretched to create an aperture a few tens of nanometers wide collected the reflected radiation and achieved a spatial resolution of better than l/10, well below the classical diffraction limit.

The group scanned a platinum silicon system at wavelengths of 1.2 and 2.4 µm and at different depths and measured variations in reflectivity, revealing localized changes in the bulk properties of the sample. Additional shear-force measurements showed surface topology of the interface and allowed the group to distinguish topological features from true lateral variations of the optical properties of the sample.