Scanning Near-Field Optical Microscopy
Near-field optical imaging using laser-illuminated metal tips



 Farfield vs Near-Field Imaging

The wave-like nature of light causes it to diffract, which limits the spatial resolution of a microscope. Under certain assumptions, the minimum detectable separation of two light scatterers for a given optical system is the Rayleigh Criterion. This limits traditional light-microscopy to a resolution of 200-300 nm, at best, with the exception of cutting edge photolithographic systems whose 100 nm resolution is achieved by using vacuum ultraviolet light. There are other techniques available to extend resolution, but by and large, this is what is possible.

Aperture scanning near-field micro-scopy is a a technique that allows for arbitrarily small details to be resolved. It works by scanning a small aperture over the object. Light can only pass through the apperture, and so this size determines the resolution of the system.This technique is typically implemented by tapering a fiber optic to a narrow point and coating all but the tip with metal. By this method, images with resolution far beyond what is possible with traditional microscopy can be recorded.

However, the amount of light that can be transmitted by a small aperture poses a limit on how small it can be made before nothing gets though. To a degree this can be lived with, as more optical power can be generated, but the cutoff is so severe that it cannot be made smaller. As the figures illustrate, this is not a subtle extinction.

When the aperture is 100 nm, the cutoff is down four orders of magnitude, and when it reaches 50 nm, only one part in 10^8 makes it through. Furthermore, the input power cannot be increased arbitrarily because 1/3 of the power is absorbed in the coating. Increasing the input power above approximately 10mW will destroy the coating. This severly limits the signal-to-noise ratio of small apertures, and is the reason our group uses another approach.

Instead of using a small aperture, we use a metal tip to provide a local excitation. If a sharp metal tip is placed in the focus of a laser beam, an effect called local field enhancement will cause the electric field to become roughly 1000 times stronger. This enhancement is localized to the tip, which has a typical diameter of 10 nm. As this tip is scanned over the surface, an image can be formed with a resolution as fine as the tip.

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