Evanescent wave coupling

In electromagnetics, an evanescent field, or evanescent wave, is an oscillating electric and/or magnetic field which does not propagate as an electromagnetic wave but whose energy is spatially concentrated in the vicinity of the source (oscillating charges and currents). Even when there in fact is an electromagnetic wave produced (e.g., by a transmitting antenna) one can still identify as an evanescent field the component of the electric or magnetic field that cannot be attributed to the propagating wave observed at a distance of many wavelengths (such as the far field of a transmitting antenna).

A hallmark of an evanescent field is that there is no net energy flow in that region. Since the net flow of electromagnetic energy is given by the average Poynting vector, that means that the Poynting vector in these regions, as averaged over a complete oscillation cycle, is zero.

In many cases one cannot simply say that a field is or is not evanescent. For instance, in the above illustration energy is indeed transmitted in the horizontal direction. The field strength drops off exponentially away from the surface, leaving it concentrated in a region very close to the interface, for which reason this is referred to as a surface wave. However there is no propagation of energy away from (or toward) the surface (in the z direction), so that one could properly describe the field as being "evanescent in the z direction." This is one illustration of the inexactness of the term. In most cases where they exist, evanescent fields are simply thought of and referred to as electric or magnetic fields, without the evanescent property (zero average Poynting vector in one or all directions) ever being pointed out. The term is especially applied to differentiate a field or solution from cases where one normally expects a propagating wave.

Everyday electronic devices and electrical appliances are surrounded by large fields which have this property. Their operation involves alternating voltages (producing an electric field between them) and alternating currents (producing a magnetic field around them). The term "evanescent" is never heard in this ordinary context. Rather, there may be concern with inadvertent production of a propagating electromagnetic wave and thus discussion of reducing radiation losses (since the propagating wave steals power from the circuitry) or interference. On the other hand "evanescent field" is used in various contexts where there is a propagating (even if confined) electromagnetic wave involved, to describe accompanying electromagnetic components which do not have that property. Or in some cases where there would normally be an electromagnetic wave (such as light refracted at the interface between glass and air) the term is invoked to describe the field when that wave is suppressed (such as with light in glass incident on an air interface beyond the critical angle).

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Wikipedia