True Collection Mode

In this mode, light illuminates the sample and is then collected through the NSOM probe and measured by an optical detector; thus there is far field excitation with near-field collection as seen in the schematic (Far-field  excitation from the top is also available).  Note that the optical microscope objective is not used for feedback; the feedback is instead through the tip-probe interaction.  The ability to scan the tip is important for collection mode so that the illumination point, either from above or below, is held fixed while the tip scans to collect the near-field signal.

NSOM transmission mode

In this mode, light is introduced through the optical fiber and collected by a detector underneath the sample as shown, allowing for NSOM imaging of transparent samples as shown in the schematic above.   This is suitable for transparent  or semi-transparent samples.  Thus you can perform experiments that require near-field excitation and far-field collection.

Reflection mode NSOM

In true reflection mode, light is introduced via the NSOM probe, and then collected by a detector above the probe as shown in the schematic below allowing for NSOM imaging of opaque samples.  True Reflection mode NSOM requires the use of cantilevered probes with an extended tip  such as the ones provided by Nanonics, as well as a free optical axis from above so that the probe does not obscure the optical path to the objective and then to the detector from above.   Furthermore, true reflection mode NSOM requires a separation of the excitation and collection paths so that they don’t interfere with one another; designs where the excitation and collection follow the same path – as is the case for instrumentation that uses apertured Si NSOM probes or when the reflected light is collected from the sides as  with straight fiber NSOM operated in shear force feedback - confound the NSOM measurement. 

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