It is well known that surface plasmon existing on a metal surface and a
metal / dielectric interface causes strong field enhancement at the interface.
According to the theoretical analysis, surface refractive index distribution
mapping with a high contrast is predicted to be possible. A metal particle
probe is considered to have the advantages of high experimental reproducibility,
not requiring gap control, and not only the ability to obtain the surface
image, but also to obtain the spectroscopic data of the sample. The scattering
efficiency of a silver particle is higher than that of a gold particle,
but the latter is more chemically stable. Therefore a gold particle is
frequently used as a SNOM probe.
Sugiura et
al. observed a dip on a cover glass and a gold colloidal particle adhering
to the cover glass. However, these images were thought to have been an
artifact problem due to the vertical displacement of the gold probe. On
the other hand, the following are observed for a refractive index grating
on a flat surface, which was made on a planar light waveguide circuit (PLC), by scanning an optically trapped 100-nm-diameter gold particle.
The scattered Ar+ laser light from the gold particle has a high
intensity due to the high refractive index of the grating with periods of 1.06 mm and 0.53 mm, both by s- and p-polarized
illuminations.
Moreover, the surface profile of an optical
disk tracking groove is also observed with and without the gold particle and
the results compared to discuss the artificial effect due to the vertical displacement
of the particle caused by the surface topology.
Experimental setup
Figure 1 shows an experimental setup to trap a gold particle
with an upward-directed Nd:YAG laser beam (l = 1.06 mm) and to scan it on
the sample surface two-dimensionally using an XY stage. The upward-directed laser beam has a higher trapping efficiency than the
downward one. Figure 2(a) shows a photograph of the setup. The gold particle at the
focal point of the objective lens is in the medium of a coverslip-shield
chamber and is pushed onto the sample surface and scanned as shown in Fig.
2(b).
Fig. 1. Experimental setup of SNOM using an optically trapped gold particle.
A Nd:YAG laser is used for trapping, and an Ar+ laser is used for
illuminating the gold particle. All the optical elements except mirrors to
guide the lasers are installed inside the small optical box (white box in Fig.
5.22(a))for easy operation
Fig. 2. Photograph of experimental setup of SNOM (a), and enlarged view of
sample chamber (b)
An Ar+ laser (l = 488 nm) is
focused through the same objective to illuminate the particle. The scattered
light from the gold particle is collected through the objective and imaged on
the pinhole (5 mm in diameter) in front of a PMT. The scattered light variation due
to the interaction between the gold particle and the sample surface is recorded
on a personal computer (PC). A CCD camera observes the operation of the gold
particle in the medium. All the optical elements, except the mirrors to guide
the Nd:YAG laser and Ar+ laser, are installed inside the optical box
for easy operation.