Biology Reference
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passive scatterer, or tip-enhanced NSOM when the metallic tip is excited to
enhance the electromagnetic ield at the end of the tip apex.
In both cases,
the sample is illuminated in the far ield and a metal probe is placed in the
tight focus of the illumination beam. The local interaction with the sample
surface is subsequently detected as a modulation in the scattered far ield.
Extreme sensitivity is required to observe the weakly scattered light from
the nanometre-sized tip in the presence of the light scattered by the sample.
When combined with luorescence, and the tip is properly excited with radial
ields along the tip axis, optical resolutions in the order to 30 nm can be
achieved.
31
This method is however accompanied by a large luorescence
background generated from far-ield illumination of the sample, therefore
requiring modulation techniques to recover the high-resolution signal.
32-34
On the positive side of the balance, this method is free from the associated
practical dificulties of fabricating circular apertures.
35
9.2.2 Fabricaon of NSOM Probes
The most crucial component of aperture-type NSOM is the fabrication
of the actual probe. Many different concepts for aperture probes were
explored during the past 15 years, each of them with distinct advantages.
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Commonly, a ibre is pulled to an apex of nanometre dimensions and coated
with aluminium to conine the light inside the tapered region. Aluminium
is commonly preferred to other opaque materials because of its very small
penetration depth, which implies a high relectivity. However, probes that
combine all necessary demands for NSOM have only scarcely been produced.
Generally, the evaporated aluminium coating has a grainy structure, resulting
in pinholes and an irregularly shaped aperture with asymmetric polarization
behaviour. Moreover, the grains increase the distance between aperture and
sample, causing reduction of resolution and loss of local excitation intensity.
Also, the damage threshold of the coating generally limits the probe brightness
to <10 nW in the far ield. In a reined approach, we, and others, have
fabricated high-deinition aperture probes, combining superior polarization
characteristics and high throughput, by making use of the focused ion beam
(FIB) technique, which is capable of polishing on a nanometre scale.
In the
FIB apparatus, a beam of Ga ions, collimated to 7 nm, is used to remove a very
thin slice of material from the aluminium-coated probe end. The resulting
“FIB probe” has a lat-end face with a roughness below 7 nm and a well-
deined circular aperture.
Figure 9.3
shows a series of apertures probe after
FIB milling, as imaged in the FIB apparatus at low beam dose. We managed
to fabricate apertures as small as 20 nm. The polarization extinction ratio
exceeds 100:1 for all polarization directions, with brightness up to 1 mW for
70 to 90 nm aperture probes.
37
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