Evanescent field

In electromagnetics, an evanescent field, or evanescent wave, is an oscillating electric and/or magnetic field that does not propagate as an electromagnetic wave but whose energy is spatially concentrated in the vicinity of the source (oscillating charges and currents).For instance, in the illustration at the top of the article, energy is indeed carried in the horizontal direction.However, in the vertical direction, the field strength drops off exponentially with increasing distance above the surface.This leaves most of the field concentrated in a thin boundary layer very close to the interface; for that reason, it is referred to as a surface wave.Everyday electronic devices and electrical appliances are surrounded by large fields which are evanescent; their operation involves alternating voltages (producing an electric field between them) and alternating currents (producing a magnetic field around them) which are expected to only carry power along internal wires, but not to the outsides of the devices.Even though the term "evanescent" is not mentioned in this ordinary context, the appliances' designers still may be concerned with maintaining evanescence, in order to prevent or limit production of a propagating electromagnetic wave, which would lead to radiation loss, since a propagating wave "steals" its power from the circuitry or donates unwanted interference.The term "evanescent field" does arise in various contexts where a propagating electromagnetic wave is involved (even if confined).In other, similar cases, where a propagating electromagnetic wave would normally be expected (such as light refracted at the interface between glass and air), the term is invoked to describe that part of the field where the wave is suppressed (such as light traveling through glass, impinging on a glass-to-air interface but beyond the critical angle).For instance, the propagation constant of a hollow metal waveguide is a strong function of frequency (a dispersion relation).The formal solution to the wave equation can describe modes having an identical form, but the change of the propagation constant from real to imaginary as the frequency drops below the cut-off frequency totally changes the physical nature of the result.Although this article concentrates on electromagnetics, the term evanescent is used similarly in fields such as acoustics and quantum mechanics, where the wave equation arises from the physics involved.In these cases, solutions to the wave equation resulting in imaginary propagation constants are likewise called "evanescent", and have the essential property that no net energy is transferred, even though there is a non-zero field.In optics and acoustics, evanescent waves are formed when waves traveling in a medium undergo total internal reflection at its boundary because they strike it at an angle greater than the critical angle.Electromagnetic evanescent waves have been used to exert optical radiation pressure on small particles to trap them for experimentation, or to cool them to very low temperatures, and to illuminate very small objects such as biological cells or single protein and DNA molecules for microscopy (as in the total internal reflection fluorescence microscope).In electrical engineering, evanescent waves are found in the near-field region within one third of a wavelength of any radio antenna.Recently, a graphene-based Bragg grating (one-dimensional photonic crystal) has been fabricated and demonstrated its competence for excitation of surface electromagnetic waves in the periodic structure using a prism coupling technique.[6] In quantum mechanics, the evanescent-wave solutions of the Schrödinger equation give rise to the phenomenon of wave-mechanical tunneling.In microscopy, systems that capture the information contained in evanescent waves can be used to create super-resolution images.Conventional optical systems capture only the information in the propagating waves and hence are subject to the diffraction limit.For example, consider total internal reflection in two dimensions, with the interface between the media lying on the x axis, the normal along y, and the polarization along z.Maxwell's equations in a dielectric medium impose a boundary condition of continuity for the components of the fields E||, H||, Dy, and By.Their Hx components, however, superimpose constructively, so there can be no solution without a non-vanishing transmitted wave.direction), then the electric field of any of the waves (incident, reflected, or transmitted) can be expressed as whereis ignored since it does not physically make sense (the wave amplification along y the direction in this case).[9] One classical example is frustrated total internal reflection (FTIR) in which the evanescent field very close (see graph) to the surface of a dense medium at which a wave normally undergoes total internal reflection overlaps another dense medium in the vicinity.The device is usually called a power divider in the case of microwave transmission and modulation.Depending on the nature of the source element, the evanescent field involved is either predominantly electric (capacitive) or magnetic (inductive), unlike (propagating) waves in the far field where these components are connected (identical phase, in the ratio of the impedance of free space).The evanescent wave coupling takes place in the non-radiative field near each medium and as such is always associated with matter; i.e., with the induced currents and charges within a partially reflecting surface.[10] Evanescent wave coupling is used to excite, for example, dielectric microsphere resonators.Evanescent wave coupling plays a major role in the theoretical explanation of extraordinary optical transmission.