Unique NSOM Reflection Mode
Imaging opaque specimens by the NSOM method requires the application of a reflection configuration, such as the oblique collection or oblique illumination mode. This tutorial presents a unique (and more difficult) configuration for NSOM imaging of opaque specimens, which can produce exceptional results. The technique requires passing an optical fiber probe through holes drilled in the center of the objective lens elements, for delivery of near-field illumination. Currently, this type of objective is not available commercially and must be fabricated by the end user.
Oblique collection mode relies upon illuminating the specimen obliquely from the far field, and collecting the reflected light through the NSOM probe situated in the near field. In oblique illumination mode, the specimen is illuminated through the probe in the near field, with collection occurring either through a far-field objective or through the same fiber probe.
Presented in the tutorial window is a cutaway illustration of the objective configuration described previously, in which an optical fiber NSOM probe is inserted through small holes that have been drilled through the center of the lens system. Light coupled into the optical fiber exits at the probe tip, which is positioned in the specimen near-field, and is partially reflected from the surface of the specimen. The reflected component can then be collected in the far-field by the objective. The center-most of the three sliders located beneath the specimen in the tutorial window adjusts the Tip-Specimen Separation within a range of 5 to 543 nanometers. Note that as the fiber is lowered closer to the specimen, more light (illustrated by the more intense color) is collected by the objective.
The Wavelength slider can be utilized to vary the illumination wavelength, which is reflected in the tutorial by a color change. In practice, the utilization of shorter wavelengths produces a slight increase in resolution, although a much larger resolution improvement would result from positioning the tip closer to the specimen. The right-hand slider, labeled Applet Speed, can be employed to alter the speed of the light passage through the virtual system.