NSOM Probe Aperture Throughput

Although minimizing the size of the NSOM probe tip aperture is a primary factor in achieving high image resolution, a sufficient diameter to provide the desired optical signal output level must be maintained. The aperture diameter can be controlled by modifying the tip physical characteristics, for example, by changing the heating and pulling parameters, or the etching variables, or by varying the angle of the fiber during metal evaporation.

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A more precise method of fabricating the probe aperture is through the use of focused ion beam (FIB) milling. FIB milling enables not only accurate control of the aperture size, but also provides a means of producing specific aperture shapes with nanometer-scale precision.

The probe tip optical throughput is proportional to the diameter of the aperture, d, divided by the wavelength of light, λ, raised to the fourth power:

Optical Throughput ~ (d/λ)4

In the tutorial, both the wavelength of the light passing through the aperture and the aperture diameter can be manipulated by adjusting the value of the Wavelength and Aperture Diameter slider bars. Changes made in these two variables are reflected in the graphical plot of optical throughput as a function of aperture diameter. Note that as the aperture diameter is increased and decreased, the optical throughput (indicated by the red dot) changes dramatically, as would be predicted from inspection of the exponential equation given above. In contrast, the wavelength of light passing through the aperture has a much smaller effect on the optical throughput, producing only about one order of magnitude variation over the entire visible spectrum, for a given aperture diameter.

As previously stated, the size of the probe aperture determines (to a large extent) the resolution of the NSOM image, with smaller apertures providing higher resolution. However, reducing the aperture diameter decreases the number of photons that can pass through the probe tip. This in turn lowers the signal levels at the detector and decreases the signal-to-noise ratio, which imposes a conflicting limit on the resolution improvement that can be achieved by further reduction of aperture diameter.

Contributing Authors

Jeremy R. Cummings, Matthew Parry-Hill, Thomas J. Fellers, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.