The Enhancement Effect at Local Reflectance and Emission Back to Apertureless SNOM Tips in the Shear-Force Gap

ABSTRACT

Physical basics, technique, equipment, proper experimental conditions, and performance of apertureless reflection back-to-the-fiber SNOM (Rayleigh, Raman, fluorescence) under shear-force control are reported. This technique achieves optical resolutions of < 10 nm on flat surfaces and < 20 nm in the presence of topographies with very sharp tips (< 10 - 20 nm end radius). It works due to the large enhancement of the reflection to very sharp tapered uncoated tips in the shear-force gap. The dependence of preset vibration damping is linear with variations between different chemical entities. Thus, the optical contrast appears as chemical contrast in reflection or emission. Such enhancement is however not obtained with blunt tips (> 30 nm end radius or broken). Typical images for all thinkable situations show that topographical artifacts are excluded and that, unlike conventional aperture SNOM, precise site coincidence of optical contrast and chemical variation (often traceable in the topography) is obtained on planes, hills, slopes, depressions, and hidden geodes, even on very rough surfaces that give, however, no optical contrast if they are chemically uniform. Furthermore, positive and negative contrast on hills, slopes, and depressions is featured within the same SNOM image each. Object sizes range from 30 nm in small scans to 25 µm in 40 µm wide scans. The versatile technique is therefore qualified for applications on real world samples without severe manipulations just as they occur in science, technique, and medicine. The principal differences of apertureless SNOM with collection in the shear-force gap and conventional SNOM are worked out.