Page 190 -

180 5 Near Field
the scattered light to propagate it to the PMT placed at the other end, leading
to the prevention of background light noise. The metal-coated fiber has a cutoff
frequency for light transmission, i.e., an aperture minimum of 48 nm due to
skin depth for aluminum coating.
We use a cantilever or a metallic needle as an apertureless probe. The probe
is ideal in terms of resolution and field intensity. Its spatial resolution is higher
than that of the aperture probe because of the smaller radius of the apex. The
near-field intensity is much stronger than that of the aperture probe because of
the field enhancement effect [5.7,5.20]. The probe is easy to fabricate and has
a wide spectrum range owing to no waveguide, but the surrounding stray light
must be fully removed. There are two scanningmethods, the constant height
mode and the constant distance mode. We need the same control technology as
that used for scanningtunnel microscopy (STM) and atomic force microscopy
(AFM).
As a small metallic probe, we use an optically trapped gold particle [5.8,
5.21]. The metal particle scatters the surface plasmon excited dependingon
the sample surface property and the scattered light is gathered by an objective
lens guiding it to a PMT. The scattered light includes information not only
on the optical, physical, chemical, mechanical properties but also the profile.
We need to discriminate between these effects [5.12]. Trappingforce strength
is very weak (on the pN order), thereby not destroyingthe sample. So we need
not control the distance (gap) between the probe particle and the sample. The
metal particle probe is considered to have the followingadvantages (1) It has
a high experimental reproducibility depending on the shape and size of the
particles beingmade, (2) It does not require control of above mentioned gap,
(3) It not only has the ability to obtain a surface property but also obtains the
spectroscopic data of the sample. Table 5.2 shows the performance comparison
between the three.

5.3.2 Photocantilever Probe

To increase SNR for an apertureless SNOM, it is necessary to distinguish
the scattered light (signal) produced by the probe from the background light
(noise). One method is to place the detector close to the probe apex. A pho-
tocantilever is a photosensitive Si-based microfabricated cantilever with a PD

Table 5.2. Comparison of probes for detection of near field. The symbol ◦ indicates
excellent, good, and × poor
probe optical fiber metallic needle metal particle
reproducibility ◦
space resolution ◦ ◦
SNR ◦ ◦
gap control ◦
optical recording ◦ ×