Near-Field Microscopy with Small Apertures

The optical apertures used in near-field microscopy are usually prepared by pulling a heated optical fibre until it breaks [25]. The sides of the tips are coated with aluminum. The typical diameters of the apertures produced by this technique are 60 10 nm, which is about one tenth of the optical wavelength. The transmission of such a tip ranges from 10 to 10 . The near-field tip is mounted on a xyz-piezo-electric (PZT) tube scanner to control the fine approach (2) to the surface and the lateral dithering (x,y) of the tip. The coarse z positioning was achieved by a coupled spring and steel plate comparable to the setup described in [30]. The sample was connected to a small glass hemisphere to minimize losses due to internal reflections and mounted in the focus of a paraboloid mirror with a numerical aperture of NA = 0.98. The whole setup, paraboloid mirror, sample, and PZT tube with the fibre tip, was then mounted inside a cryostat and immersed in superfluid Helium at 1.8 K. 

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