28 - Evanescent coupling

Summary

Evanescent electromagnetic waves abound in the vicinity of luminous objects. These waves, which consist of oscillating electric and magnetic fields in regions of space immediately surrounding an object, do not transfer their stored energy to other regions and, therefore, remain localized in space. Like all electromagnetic waves, the behavior of evanescent waves is governed by Maxwell's equations, and their presence in the vicinity of an object helps to satisfy the requirements of field continuity at the object's boundaries. Evanescent fields decay exponentially with distance away from the object's surface, making them exceedingly difficult to detect at distances much greater than a wavelength.

When a beam of light shines on a diffraction grating, for example, various diffracted orders partake of the energy of the incident beam and carry it away in different directions. At the same time, evanescent waves are created around the grating, which ensure the continuity of the field at the grating's corrugated surface. Similarly, a beam of light shining on an aperture or on a small particle sets up evanescent fields around the boundaries of these objects. Perhaps the best-known example of evanescence, however, is provided by total internal reflection (TIR) from an internal facet of a prism (see Figure 28.1). Here the evanescent field is formed in the free-space region behind the prism, and remains distinct and isolated from the propagating (i.e., incident and reflected) beams; this phenomenon was discussed briefly in Chapter 27.