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5

Near Field

The near field represents an electromagnetic evanescent field that has its
intensity maximum at the surface and an exponentially decayingfield per-
pendicular to it. It is localized within a hundred nanometers. The near field
cannot be probed by conventional light detectors because it does not propa-
gate. However, if we could access and control it, we would be able to observe or
read/write or fabricate beyond the wavelength resolution. These science and
technology fields, which include near-field scanning microscopy, optical data
storage, and the processing industry, are also concerned with micromechanical
photonics.
In this chapter we describe the near field’s features, theoretical analyses,
experimental analyses, and applications mainly related to optical storage.

5.1 Background

The evanescent field is widely known as a solution to Maxwell’s equations
[5.1–5.4]. It is localized near the surface and decays exponentially within a
hundred nanometers. To excite the near field, it is necessary to have the
following nonpropagating conditions, as shown in Fig. 5.1:
1. Attenuated total reflection (ATR): the use of a highly refractive prism is
the simplest method of producinga near field. When a light is incident at
the interface from a high-refraction medium to a low-refraction medium
at an angle greater than that of total reflection, the light is reflected
totally to the high-refraction medium and a near field is produced at the
low-refraction medium surface, as shown in Fig. 5.2.
2. An aperture smaller than the wavelength of light: if the aperture size is
smaller than the wavelength, the light does not propagate through the
aperture but produces evanescent light. An optical fiber with a sharpened
end is often used for this purpose.