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5.3 Experimental Analysis 179
– Electromagnetic field around a near-field optical head 5,6,7
– Readout performance for ultrahigh density near-field recording 8,9,10

5.3Experimental Analysis

In order to observe the near field, the evanescent light scattering characteris-
tics of a tip probe should be understood. In this section, we first compare the
characteristics of different tip probes. Next we measure the evanescent field
intensity by detectingthe scattered light by a photocantilever, vibratingat
its mechanical resonant frequency, placed near the interface. Then we observe
the profile of a topological grating by scanning the photocantilever. At last we
observe the distribution of the refractive index grating and topological grat-
+
ing, by detecting the scattered light of an Ar laser, by scanninga nanogold
particle optically trapped by a YAG laser, two-dimensionally on the surface.

5.3.1 Comparison of Near-Field Probes

When a sample is illuminated by a light the evanescent field is locally excited
near the surface accordingto its surface property and structure. This evanes-
cent field is scattered by a tip probe and then can be detected by a photodiode
(PD) or a photomultiplier tube (PMT). We can observe the surface by scan-
ningthe probe two-dimensionally on the surface. The imagingmechanism of
the SNOM is different from that of conventional optical microscopy; the scat-
tered light intensity is detected as a result of the interaction between the tip
probe and the sample surface. With growing understanding of the underlying
probe-sample interaction mechanism, SNOM has found applications in many
scientific and industrial fields.
As a typical near-field probe, a small aperture [5.3], a metallic needle
[5.5, 5.6], and a small metallic sphere [5.7, 5.8] are well known. The most
popular probe is a metal-coated sharpened optical fiber with a subwavelength
aperture at the end. We use this aperture to illuminate the surface and collect

5
Tanaka K, Ohkubo T, Oumi M, Mitsuoka Y, Nakajima K, Hosaka H, Itao K
(2001)Numerical simulation on read-out characteristics of the planar aperture-
mounted head with a minute scatterer. Jpn J Appl Phys 40:1542–1547
6
Mansuripur M, Zakharian AR, Moloney JV (2003)Interaction of light with sub-
wavelength structures. Opt Photon News:56–61
7
Kataja K, Olkkonen J, Aikio J, Howe D (2004)Readout modeling of superreso-
lution disks. Jpn J Appl Phys 43:4718–4723
8
Liu J, Xu B, Chong: TC (2000)Three-dimensional finite-difference-time-domain
analysis of optical disk storage system. Jpn J Appl Phys 39:687–692
9
Nakano T, Yamakawa Y, Tominaga J, Atoda N (2001)Near-field optical simula-
tion of super-RENS disks. Jpn J Appl Phys 40:1531–1535
10
Chiu KP, Lin WC, Fu YH, Tsai DP (2004)Calculation of surface plasmon effect
on optical disks. Jpn J Appl Phys 43:4730–4735