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5.4 Future Applications 193
to the gradient of the surface profile, whereas the scattered light without a
gold particle had no split peaks.
From the result mentioned earlier, we confirm that an optically trapped
gold particle is effective in observing both the physical and topological prop-
erties of a sample. Further investigation will be required to clarify the exact
effect of the vertical displacement of the particle, i.e., to distinguish the probe
vertical displacement effects from the SNOM signal [5.12].
On the other hand, to improve the spatial resolution further, the robust-
ness of the optically trapped particle must be increased by an increase in
medium viscosity or by a follow up control of the particle position [5.30].

5.4 Future Applications

For future ultrahigh density optical storage, the so-called fourth generation
optical disk, many types of methods are beingproposed. These methods
are holography [5.31,5.32], superresolution near-field structure (super-RENS)
recording[5.33], near field recording[5.34], and a 3-D recording[5.35]. Holo-
graphic storage is expected to have the possibility of storing over 1 terabyte
of data on a 120 mm diameter disk for data archiving[5.32]. In this sec-
tion, near-field methods, particularly super-RENS, are introduced in detail.
Super-RENS has the merit that a recordingapparatus the same as that of a
conventional optical storage system can be used and is considered to have the
highest potential for on-line storage.

5.4.1 Conventional Superresolution

I would like to start to explain beyond the diffraction limit readout principle
(superresolution) [5.36], which is used actually for today’s digital versatile
disk (DVD). Fig. 5.33 shows the pit size comparison between a CD, a DVD,
a next-generation DVD and a near-future optical disk. The pit size decreases
owing to not only the short wavelength [5.37] and high objective NA but also
the superresolution scheme.
We can make a write mark infinitesimally small by choosingthe critical
thermal conditions so that only the peak area of the temperature distribution
corresponds to the writable temperature. This is called “brush tip record-
ing”. However, information bits cannot be detected when two marks are in-
cluded in a diffraction-limited spot (λ/NA) in conventional readout as shown
in Fig. 5.34a.
Nevertheless, bits can be detected by superresolution because its effective
aperture is restricted within a crescent-shaped region, as shown in Fig. 5.34b.
Fig. 5.34c shows a spot intensity profile and the temperature distribution in a
mask layer. The material is melted in the rear area of the light spot because
the absorbed heat dissipated in the movingdisk direction. The reflectance is
designed to be very low for the melted region so that it works as a mask.