Talk:Evanescent field

Total internal reflection
not clear from explanation why kz is larger than k when total internal reflection occurs.


 * It does gloss over that. Basically, if you solve the problem of matching the incident, reflected, and refracted waves at the boundaries, you find they all must have the form
 * $$\mathbf{E}(y=0) = \mathbf{E}_0 e^{- i k_z z}$$
 * But we also know from Snell's law that
 * $$n_1 \sin \theta_{incident} = n_2  \frac{k_{z}}{|\mathbf{k}_{refracted}|}$$
 * When that $$\sin \theta_{incident} $$ on the left equals $$n_2/n_1$$, $$k_z = |\mathbf{k}_{refracted}|$$. For $$n_1 > n_2$$, at larger incident angles, $$k_z$$ is bigger than $$|\mathbf{k}_{refracted}|.$$

Practical Applications
See this spectrum article.

direction of evanescence
Isn't this page lacking the information (or at least needing to put it more clearly) that the direction of the evanescent electric field is the same of the exponential decaying? I have the impression sometimes that people care much about the fact that the evanescent waves "evanesce" exponentially, but this is by no means its most notable characteristic. The fact that the field is in the same direction of this decay is the one worth noting. -- NIC1138 (talk) 05:18, 13 January 2009 (UTC)


 * Correct me if I'm wrong. My understanding of evanescent waves is that they are already by nature "directional." It would follow that the exponential decay is also directional. Pointing it out, seems to me to be akin to saying "The sky is blue, but it is not black." The last half of the sentence is unnecessary. Simple logic should lead the reader to infer the decay is in the same direction IMHO. --24.63.66.140 (talk) 16:53, 28 August 2009 (UTC)

Merger proposal
As the separate page on "evanescent wave coupling" is quite short, and concentrates on practical applications, it seems to me that it would be better to merge it into this page. Is anybody opposed to this idea ? (RGForbes (talk) 21:43, 11 March 2009 (UTC)).
 * A merged article would probably have a more consolidated content.-Thurth (talk) 06:54, 9 April 2009 (UTC)
 * I copied the comment below by User 60.240.156.100 from Talk:Evanescent wave coupling:


 * I vote for the suggestion that this page be merged with Evanescent Waves. As a non-expert of quantum physics, I have been dragged through these pages from the original article I selected, Quantum Tunnelling. Now I am following increasingly tedious links - it would be easier and more sensible surely to couple Evanscent Waves with their coupling? - JD (Biochemist not physicist!... trying to read about how protons tunnel in neurones... presumably with a quantum shovel?). —Preceding unsigned comment added by 60.240.156.100 (talk) 11:13, 16 July 2009 (UTC)


 * Since there is no opposing consensus against this merger, I will (forwith) execute the merge of "Evanescent wave coupling" into "Evanescent wave". Steve Quinn (formerly Ti-30X) (talk) 02:45, 6 May 2010 (UTC)

Possible addition/correction
I found this article while preparing an exam in photonics and I think a correction might be necessary. My understanding is that an evanescent wave appears when a wave crosses from a denser to a thinner material, in my case optically denser/thinner. This is one of the prerequisites for total reflection, which is connected to the appearance of evanescent waves. Is this true for any electromagnetic wave? Everything I've read seems to imply so, however I'm rather a layman in the field so I'd like to be sure :)

Longbowman3 (talk) 13:57, 16 April 2009 (UTC)

Exhibit
Can I take it that "with an intensity that exhibits exponential decay with distance" means "of an intensity which diminishes exponentially with distance"? Or is something subtler being implied here, something to do with "exhibit" or "decay"? Unfree (talk) 04:56, 7 March 2010 (UTC)

Phenomena
I find "Evanescent waves are a general property of wave-equations" confusing. Surely, an equation is something a human being can write down in pencil on paper. Surely, evanescent waves must be physical phenomena. Surely, "property" and "general property" must have quite well established meanings in science. Can it be said that evanescent waves are phenomena which can be seen to emerge naturally from the wave equations, are implicit in them, or can be accounted for by them? Unfree (talk) 05:19, 7 March 2010 (UTC)
 * How about "Evanescent waves are one general form of solution to wave equations"? It's referring to the fact that, depending on coordinate system, spatial functions of the forms exp(-az), exp(-j*az), and Bessel/Neumann/Hankel functions (among others) are all permissible solutions to the general wave equation, not specific to Maxwell's equations. 03:52, 16 March 2010 (UTC) —Preceding unsigned comment added by 66.57.254.204 (talk)

Incident angle
Considering the direction of movement and the surface of the medium, one would naturally expect the incident angle of the photon, phonon, or whatever, to refer to the angle between the two, rather than between the photon and the normal to the surface. So "they strike it at an angle greater than the so-called critical angle" would seem to imply less shallow rather than shallower. A rewording is called for, or at least a clarification. (Also, how is "so-called" called for?) Unfree (talk) 05:31, 7 March 2010 (UTC)

EM
"EM waves", I take it, is an abbreviation for electromagnetic waves? If so, let's insert "(EM)" after the first occurrence of "electromagnetic", or even better, spell it out each time. Unfree (talk) 05:38, 7 March 2010 (UTC)

The "nearfield" region
Just a hunch, but thinking about "in the nearfield region within one-third wavelength" makes me wonder whether this might be two ways of saying the same thing. Is it? If so, let's state so explicitly and eliminate a layer of logical abstraction. Unfree (talk) 05:44, 7 March 2010 (UTC). I feel it would be more precise to use the terms induction field or reactive near field and this would avoid the need for mentioning one-third wavelength.Beamtube (talk) 23:56, 24 January 2011 (UTC)

Quantum mechanics
The way "in quantum mechanics" enters the discussion suggests (considering quantum mechanics to be valid) that all that has gone before is phony. It's as if we weren't discussing the real world, but an erroneous, prehistoric notion of it. I'm sure this is a familiar situation to those who learn Maxwell's equations before Schroedinger's, and being unfamiliar with any of them, I'm out of my league here, but an introductory sentence or two ought to suffice to explain why quantum mechanics is unnecessary at the outset, but ought to crop up later. Unfree (talk) 06:08, 7 March 2010 (UTC)

Unit vector notation
Explaining "the unit vector in the z direction" crops up a little late, after we've been exposed to the unit vectors in the x and y directions without any explanation. Also, we're told that alpha is a real number, which, when multiplied by i, gives an imaginary result. That's clear. But it isn't clear at the outset (especially without understanding the unit vector notation) exactly what the "components" are, and what alpha and beta are. Unfree (talk) 06:19, 7 March 2010 (UTC)

Diffraction?
The caption of the pictures of the refracted versus evanescent wave in the "Total internal reflection of light" section incorrectly called the refracted wave the "diffracted wave." I fixed this and also removed a link in that caption to Diffraction limit because I didn't see how it was relevant (admittedly that page does talk about evanescent waves being used to beat the diffraction limit in imaging, but that didn't seem relevant to the picture). If I am wrong in these things, please revert. 165.124.205.144 (talk) 08:13, 27 May 2010 (UTC)
 * It appears to me that your edit is correct. The caption now reads: Representation of a refracted incident wave and an evanescent wave at an interface. I don't see where diffraction would be invloved in these two images. It seems obvious the wave is "refracting" in the top picture. Thanks for your correction. Steve Quinn (formerly Ti-30X) (talk) 18:00, 27 May 2010 (UTC)

History
The history of the discovery should be mentioned. I think two men discovered the wave at about the same time. — Preceding unsigned comment added by 92.27.109.117 (talk) 15:37, 26 February 2013 (UTC)

An evanescent field is not a standing wave.
In a standing wave, the nodes don't move. However, in the case of total internal reflection, the nodes move in a direction that is parallel to the reflecting surface.Constant314 (talk) 18:01, 8 April 2013 (UTC)


 * That's not exactly true. You're thinking of a fully standing wave which happens when a wave (on a transmission line or a plane wave in space) is fully reflected. When the reflection is partial there is incomplete cancellation and the field has a general "motion" in the direction of the stronger component. Also, even with perfect reflection of a plane wave at an angle, you get "motion" of the standing wave in the transverse direction. I'm not sure why this is important though. Interferometrist (talk) 18:01, 17 October 2015 (UTC)

An evanescent field is probably not a near field.
The near field article is entirely about antennas. That near field decreases with the cube of distance whereas the evanescent field decreases exponentially with distance.Constant314 (talk) 18:05, 8 April 2013 (UTC)
 * A plane wave in an absorptive media decreases exponentially with distance (typically specified as so many dB per km). So, exponential decrease alone is not sufficient to distinguish an evanescent wave from an ordinary plane wave.  The defining characteristic of an evanescent field is that it achieves its attenuation without absorption.  No energy is dissipated.  Also, typically, the field tends to damp out very rapidly, on the order of several dB per wavelength.Constant314 (talk) 18:23, 8 April 2013 (UTC)


 * Evanescent field and near field are synonymous in my book. You are thinking of one type of purely evanescent field due to a polarization with a spatial periodicity smaller than the wavelength in the medium where you call it that (such as with TIR). In other cases (such as the r^-2 and r^-3 field components from an antenna) that component of the field is evanescent because it doesn't propagate indefinitely and returns all of its energy to the source. Don't overgeneralize. Interferometrist (talk) 18:11, 17 October 2015 (UTC)


 * And for the reason I just stated, the first sentence in the article is wrong. A more general definition is needed. I'd also point out that the second sentence is also sort of wrong, certainly misleading. In electromagnetics, evanescent fields are solutions of Maxwell's equations, period. Insofar as the wave equation is a restatement of Maxwell's curl equations, yes you could argue for the second sentence being valid. Interferometrist (talk) 18:32, 17 October 2015 (UTC)


 * I found the following in my IEEE Dictonary
 * evanescent field (fiber optics). A time varying electromagnetic field whose amplitude decreases monotonically, but without accompanying phase shift, in a particular direction is said to be evanescent in that direction.
 * evanescent mode (cutoff mode) (waveguide). A field configuration in a waveguide such that the amplitude of the field diminishes along the waveguide, but the phase is unchanged.  The frequency of the mode is less than the critical frequency.
 * evanescent mode. See: cutoff mode
 * cutoff mode (evanescent mode) (waveguide) A non-propagating waveguide mode such that the variation of phase along the direction of the guide is negligible.
 * So, I guess the essence of an evanescent field is that the phase in independent of position. That is approximately the case with the near-field so I withdraw my objection.  On another note, in the case of total internal reflection, the field in the less dense media would be evanescent in a direction normal to the interface but not so in the direction parallel to the interface.  It might be appropriate to work that into the article.Constant314 (talk) 13:35, 18 October 2015 (UTC)

"Evanescent wave" a misnomer
Sorry to point this out and cause consternation. But an evanescent field is not a wave, for instance according to the definition of wave in Wikipedia. A wave travels in space (or a medium) unless and until it is attenuated or deflected, and carries energy away from the source. By definition an evanescent field does neither. Yes I know that "evanescent wave" is a common expression, and I hear it more than "field" where I work from people who understand what I am saying but don't care about semantics (I've even caught myself saying it!). There is already a redirect page from "evanescent field" to this one. My suggestion would be to reverse them!

Also, on a different subject: a standing wave is not a "wave" either. It is two waves. What is "standing" is the oscillation of the electric (or magnetic) field. Another misnomer but not one that causes confusion.

Evanescent wave is a confusing term because it sounds like something exotic. Actually evanescent fields are ubiquitous, and most electric and magnetic fields in everyday life fall in this category. In your house you are surrounded by 50 (or 60) Hz fields very little of which is radiated into space. It is only when you are very far from all currents (many wavelengths away) that fields are dominated by traveling waves (since, again by definition, any evanescent fields have greatly decayed, whether exponentially or otherwise). Interferometrist (talk) 18:39, 17 October 2015 (UTC)


 * I looked in an older IEEE Dictionary and found "evanescent field" and "evanescent mode" but not "evanescent wave". If you want to change the name of this article to "evanescent field", I would support the change.Constant314 (talk) 13:22, 18 October 2015 (UTC)


 * Alright I would be so inclined, but will wait a few days to see if anyone else wants to weigh in. Also, I'm not sure how to do this. There is a page for each name, one a redirect. Should I move the content from one to the other? Or delete the current redirect and MOVE this page to the new name? Interferometrist (talk) 10:21, 20 October 2015 (UTC)


 * Probably better ask for help. I'm sure that moving the content is not the way to go because it loses the history log.Constant314 (talk) 11:41, 20 October 2015 (UTC)


 * I submitted a request to move the page to Evanescent field. Constant314 (talk) 18:57, 24 October 2015 (UTC)


 * The article has been renamed. There are many instances of wave that need to be addressed. Constant314 (talk) 14:14, 25 October 2015 (UTC)


 * Hi, sorry I've been ill and not attending to this. But yes, the article is a mess now if you focus on the linguistics. However it's the content that is important. I expected that in the first sentence it would point out the common usage of the other term (whether identifying it as a misnomer or not) so that someone recognizes it as a synonym when read elsewhere as well as in the article if those references persist. Or you could do a quick search and replace throughout the article. If I have the energy I will edit the whole lede because I think it has additional errors. Thanks for taking care of the technical task. Interferometrist (talk) 13:58, 26 October 2015 (UTC)


 * Please edit boldly. My editing takes place mainly on the weekend so we are unlikely to interfere with each other.  I don't think that simply replacing wave with field everywhere is a good idea.  The connotations of the two words are different.  Also, I think clearly that some sources do use evanescent wave so, I think that we should replace wave with field for consistency within the article, but we should avoid depreciating that term evanescent wave.  I hope that makes sense. Constant314 (talk) 14:08, 26 October 2015 (UTC)

Evanescent-wave coupling, missing important detail
When the second object is introduced into the evanescent field of the first object the field stops being evanescent. The field reflects off the second object, re-reflects off the first object, etc. The field between the objects must become non-evanescent or there will no power flow. Evanescent-wave coupling means there is coupling where an evanescent field would exist if the object being coupled were not there.Constant314 (talk) 22:42, 27 October 2015 (UTC)


 * You are absolutely right, and if you want to be very exact with your terminology then this term is self-contradictory and shouldn't be used. But an important point is that "evanescent field/wave" is already a rather inexact term thrown around loosely in different contexts so this usage isn't surprising, and in fact is so common that the name can't be changed. In a similar context, I regularly hear about plane waves at an interface beyond the critical angle being "evanescent waves" even when they excite a nearby atom/conductor/surface such as in FTIR, also called "tunneling" of photons according to quantum concepts. Again, if they actually carry power they are no longer evanescent and when I point this out people agree, shrug their shoulders, and keep on talking that way the next day. I would boldly call these uses a "misnomer" in the article except that someone will (and should!) demand a RS saying so. Interferometrist (talk) 14:37, 28 October 2015 (UTC)

Revised lede
Nice job. I'd like to see the zero phase shift attribute brought back in. It is in the definitions and there is a nice tie-in with other knowledge. Zero phase shift means infinite phase velocity and zero group velocity which dovetails nicely with zero energy transport. Constant314 (talk) 23:27, 28 October 2015 (UTC)


 * Thank you. I intend to edit further and will address this issue. However what you say is an overgeneralization, and the zero phase shift property is not (or shouldn't be) in the definition. It is a feature of the evanescent solutions of the wave equation with plane wave symmetry (no dependence in two spatial dimensions) in a dielectric, which is also when you encounter an exponentially decaying amplitude. I will rework section 3 and move it up before most of section 2 as an important case and point out the zero phase aspect (already contained in the final equation but not stated in words). Interferometrist (talk) 13:42, 30 October 2015 (UTC)


 * Here are the IEEE definitions, Zero phase shift is featured in every definition.


 * Evanescent Field (1) (fiber optics) A time varying electromagnetic field whose amplitude decreases monotonically but without an accompanying phase shift in a particular direction is said to be evanescent in that direction. 812-1984  (2) (radio-wave propagation) An electromagnetic field for which the phase is everywhere the same (no spatial variation) and the amplitude decays exponentially as one moves away from the boundary.  An evanescent field is a special case of a non-homogenous plane wave.  211-1990


 * Evanescent mode (cutoff mode) (1) (wave-guides) A field configuration in a wave-guide such that the amplitude of the field diminishes along the waveguide, but the phase is unchanged. The frequency of this mode is less than the critical frequency.  See wave-guide. [35]  (2) See Cutoff mode.  146-1980w


 * Cutoff mode (evanescent mode) (waveguide) A non-propagating waveguide mode such that the variation of phase along the direction of the guide is negligible. 146-1980w


 * 812-1984 = IEEE Standard definitions of terms relating to fiber optics.
 * 211-1990 = IEEE Standard definitions of terms relating for Radio wave propagation.
 * [35] = IEEE Antenna and Propagation Society - Antennas and waveguides.
 * 146-1980 = IEEE Standard Definitions of Fundamental Waveguide Terms


 * Let me first say that I'm only interested in getting the science/math right and don't care much about terminology per se. If I am using the wrong definition (compared to everyone else) I will correct it. However I have always used "evanescent field" in a broader sense than admitted by these definitions. In fact (and maybe you've been looking too!) I don't find any fundamental definition of the term, that is as part of basic optics or E-M theory, and I don't think there ever was one. That is why I took out anything like a definition from the lede and titled the second section "Usage of the term", because that's exactly what it is: a term that got applied in an ad-hoc sense to differentiate non-propagating E-M fields from propagating waves. Unfortunately some poor sole at IEEE had the job of coming up with a set of definitions, so he scoured the literature and came up with what I believe are overgeneralizations on that basis.
 * As far as I'm concerned, "evanescent" can be used in any context where there is not a propagating wave. If you limit the definition (as above) then one would need to consider some fields that are neither propagating waves nor evanescent, and thus there would have to be a third term describing these! There isn't. So we are stuck with the normal usage: in situations where you might have been expecting a propagating wave but didn't get one, but you still have nonzero fields, you call it "evanescent". Thus another term for "evanescent modes" is non-existing modes or "no such solution".
 * That is my only explanation because there are so many many cases where people don't use the term simply because they weren't expecting wave propagation in the first place. They just use the terms electric or magnetic field, because after all, that's all we're talking about.
 * However I WILL edit further and I WILL give special attention to the section on TIR which I'll move up and treat mathematically since it's an important case of relevance (and, thankfully, one where the use of the term isn't controversial). But I will include the near field of an antenna as another example, even though it fails the above definition because there is a propagation (time shift) factor. And most of all, I will include some relevant figures but haven't produced them yet. Interferometrist (talk) 21:59, 5 November 2015 (UTC)
 * I realize that my post just below interfere with this topic, and others before. The right question is: can we qualify everything that is not propagating as evanescent ? Don't you think that this will help to amplify the already existing "all wave like" dogmatism ? Henri BONDAR (talk)

Evanescent meaning
I am, like many others before, not comfortable with the modern use of the term evanescence. The bad direction it takes is to replace well known phenomena by fuzzy descriptions allowing some researchers to pretend making new discoveries and to patent already known ideas. For instance it is suggested that the near-field of a quasi-static dipole is made of evanescent waves whereas it simply decreases as 1/r3. So why discard the Coulomb's field and says that all charges are surrounded by an evanescent wavelike field? Why not going further and say that magnets and electrostatic charges are attracted and repelled by evanescent wavelike forces? I think (as suggested by many others in various section of this talk), that evanescent fields or waves should be limited, as they were originally, to the exponential decay of the field (or its envelope); Evanescent field, if there is no visible envelope (when decay is much shorter than the wavelength if any) and evanescent wave if an envelope can be seen. Such situations arise when an infinite number (at least a large amount) of charges or small dipoles (optionally oscillating ones) interacts in some distributed manners. As a result, evanescent fields and waves will still concerns some near-field optics situations and wave propagation inside conductors as well as many other situations, but will not pervade the whole quasi-static frame.Henri BONDAR (talk) —Preceding undated comment added 13:19, 22 January 2016 (UTC)


 * Evanescent field, non-propagating field, standing wave, near field, reactive field have some overlap in meaning. The article should include, or at least not exclude, the uses of the term evanescent field or evanescent wave as they are used in reliable sources.  The IEEE dictionary is an excellent source, but it is not the only one.  The IEEE dictionary documents some uses of the term, but it is by no means exhaustive.


 * I’m not fond of referring to the near-field of an antenna as being evanescent. I have a have dozen antenna books including books by Balanis and Kraus.  Evanescent does not even appear in the index.  Nor am I fond of calling a standing wave as being evanescent.


 * For me, one of the most interesting cases is total internal reflection of a wave in a dense medium at the interface with a less dense medium. There is a field in the less dense medium that transports energy in the direction parallel to the interface and is evanescent in the direction that is normal to the interface.  This case should definitely not be excluded.  That is, if there is a definition in the article, it should allow this case.


 * I expect evanescent fields to attenuate rapidly in the evanescent direction and to exhibit zero phase shift (or equivalently that the zero crossings do not move or equivalently that the phase velocity is infinite) in the same direction. I further expect the attenuation to happen without dissipation.

Constant314 (talk) 20:48, 29 January 2016 (UTC)
 * Fully agreed with that. Evanescence should remain in well defined cases and avoided in others where it creates confusions. I do not understand why evanescent fields should be non-dissipative, but I am not competent in this domain, I guess it is related to the zero phase shift condition.--Henri BONDAR (talk) 07:38, 30 January 2016 (UTC)

Next step in dogmatism: the Coulomb's field is "evanescent"
The usage of evanescent field is pervading situations that were well described using classical near-fields a few years ago. To my knowledge the field for a single charge, the Coulomb field, is decreasing as 1/r2 and is not evanescent or the gravity field should also be considered as an evanescent one. If you admit that the dipole field in 1/r3 is directly obtained using the Coulomb one, it should not be considered as evanescent too. By extension you cannot consider that non-radiating dipole-dipole interactions used for WPT transfer as evanescent. If anything decreasing with distance is evanescent then the radiation of an antenna is also evanescent. Should we say that evanescence starts for a decreasing higher than 1/r, 1/r2, 1/r3?

As many contributors here I prefer to use the term "evanescent" to describe 'specific processes where the field is decreasing exponentially'. As in such situation the average Poynting vector is zero "evanescent wave" is then also an improper expression as previously said. I don't like the growing dogmatism introduced by Soljacik, Witricity and followers to defend that their non-radiative near-field device involves a kind of magic quantum focalisation process. These ideas pervade many wikipedia pages and articles whereas the device is only a modern declination of Tesla coupled coils associated to resonant circuits (resonances do not alter the link as falsely suggested but reduce the losses in the devices). They also renamed the coupling index kQ used since around 1930 to described capacitive and magnetically coupled resonant circuits into "factor of merit". It is now common to see in the literature as a first example of evanescent field the Witricity system of coils. The dogmatic pictures showing a kind of magic concentration of field lines coming with it are the first you see when you use the words "resonant circuits", "WPT" on Google search. Even worse in this page you can read that the interaction between the two plates of the capacitor is evanescent.

Is anyone ready to fight against obscurantism or do we have to say goodbye to Charles-Augustin de Coulomb and several centuries of science.Henri BONDAR (talk) 21:07, 11 January 2019 (UTC)
 * If you think some of the facts are just someone's opinion mark it as needing a citation. If no citation appears then remove it.Constant314 (talk) 04:24, 12 January 2019 (UTC)


 * I think the situation is worst than that, most recent citations are not reliable as the peer reviewing process is dying for most publishers (IEEE, Springer,....), if not the publishers themselves. You may easily find papers and even books that pretend that the resonant magnetic coupling is evanescent. Concerning the dogmatic pictures on top of the resonant inductive page, that are not sustained by anything except some believings; originating from wikipedia they are now pervading the web (just make a search on google with "resonant inductive"). This page is less contaminated than many other ones, however I attract your attention on the fact that considering the interaction between two coils in non-radiating near-field regime as evanescent is formally equivalent to consider that gravitation field, as well as electric and magnetic near fields are all evanescent. The only clear limit that can be set and originally served as the definition of the evanescence is for exponential decays related only to very specific processes such as wave propagation through interfaces (that is formally the same that for the skin effect as there is no propagation of energy in the evanescent field), cutoff frequencies in wave guides,....Henri BONDAR (talk) 07:27, 12 January 2019 (UTC)


 * I consider removing the sentence Evanescent wave coupling is used in powering devices wirelessly.[8][9][10] and the three associated link for "non neutral point of view" (WP:NPOV policy). Only one reference is academic and the two others can be considered as promotional. All three links are directly related to Witricity patent and associated commercial activities. The only academic article (8) introducing that magnetic coupling between coils could be called evanescent (more accurately they propose to call evanescent the dipole-dipole interaction field) is not sustained by any secondary independent sources. Any objections? Henri BONDAR (talk) 12:51, 12 January 2019 (UTC)


 * I’m not arguing either way; I just want clear up a point about evanescent coupling. The field is evanescent when there is nothing to couple to, like in the case of total internal reflection.  When there is something to couple to, like another slab of glass, the field is not evanescent, but it is still called evanescent coupling.  So, the question about wireless charging comes down to the question as to whether the field is evanescent when there is nothing in the field to be charged.Constant314 (talk) 23:14, 12 January 2019 (UTC)


 * I’ve been through my E&M books by Jackson (2nd and 3rd), Harrington and Kraus. The only examples of evanescent that they give are total internal reflection, waveguides below cutoff frequency and orifices (like the mesh on the inside of a microwave oven’s glass window).  In all cases, the evanescent wave arises when and incident wave meets an interface and cannot propagate through it.  In this case, an evanescent field may be necessary to account for field continuity.


 * Harrington equates non-propagating mode with evanescent mode and further equates reactive field with evanescent field. He also says that when the propagation constant is purely real, that the field is evanescent.  That would imply exponential attenuation with distance (it could be a very slow attenuation) and no phase shift with position.  But, he does not say that all evanescent fields must have this characteristic.


 * Jackson equates cutoff mode with evanescent mode. He further says that when the wave number is purely imaginary, that the mode is equates non-propagating mode with evanescent.  This also means that the field decays exponentially with distance and no  phase shift with distance.  But, he also does not say that all evanescent modes must have this characteristic.


 * I’ve also been through my antenna books by Balanis and Kraus. Evanescent is not in the index, but Balanis divides the field around the antennas into three regions: Reactive near-field, radiating near field and far field.  Given that Harrington equates reactive field with evanescent field, I think that there is some leeway to say that there is an evanescent field near an antenna.  However, the extent of this region is taken to be  0.62 x D x sqrt{D/λ}.  For example, if you had a 1 meter dipole operating at 30 meters wavelength, the reactive near field would extend out to about 11 cm.  So, if you had a second dipole less than 11 cm away, I think you can reasonably say that there is evanescent coupling. Constant314 (talk) 23:53, 12 January 2019 (UTC)


 * Come on, if you open such a Pandora's box, you will have to consider that all quasi-static fields including gravitation are evanescent fields. You will have to consider that the constant electric field in between the two electrodes of a capacitor is evanescent. You will finally have to equate "evanescent" = "non propagating", so that standing waves should also be considered as evanescent, and on and on....


 * All examples you give are for specific situations where the field is expected to decrease exponentially. In the Harrington case, the near-field is only considered as a localized contribution to a main propagative component that is the core subject of the study. I know no secondary sources that consider the Coulombs field, dipole fields and dipole-dipole discrete interactions to be explicitly of evanescent nature. Remind that an encyclopedic content should reflect the general consensus and be neutral (WP:NPOV policy). Henri BONDAR (talk) 15:07, 13 January 2019 (UTC)


 * The article already says that evanescent field is an oscillating electric and/or magnetic field so we can disregard gravity and static fields. Perhaps, equate was too strong of a word.  Harrington uses "evanescent mode" and "non propagating mode" as alternative names in the context of a wave guide.  He uses "evanescent field" and "reactive field” as alternative names in the context of total internal reflection.  Jackson uses "evanescent mode" and "cut off mode" as alternative names in the context of waveguides. These are simply examples of usage in reliable sources. In these examples, the field does decrease exponentially with distance and there is no phase shift with distance.


 * Harrington and Jackson go on to say that when certain conditions are met, the field is evanescent. They do not say that all evanescent fields must meet these conditions.  They both give equivalent conditions and that is when the propagation constant is purely real which means exponential decay and no phase shift.  None of this requires a standing wave on a transmission line to be called evanescent.


 * The IEEE Dictionary, under the entry for evanescent field, has a definition evanescent field in the context of radio-wave propagation.


 * Balanis has two books: Antennas and Advanced Engineering Electromagnetics. In the second book, in the context of waveguides he uses “evanescent (reactive) or non-propagating waves.”  He uses waves instead of modes.  He further says that “Evanescent fields are exponentially decaying fields that do not possess real power.”  In the Antenna book he refers to the field next to the dipole as a the reactive near-field.


 * I found this article ieeexplore: “Evanescent Reactive-Near-Field Measurement for ESPAR Antenna Characterization.” It is behind a paywall, but the abstract has the phrase “evanescent reactive-near-field.”


 * Given that both Harrington and Balanis uses evanescent and reactive as alternative names, that Balanis refers to the reactive near field of an antenna, that the IEEE Dictionary has an explicit entry for evanescent field in the context radio wave propagation, and there is an IEEE scholarly article that uses the phrase “evanescent reactive-near-field”, it is not an abuse of the term “evanescent field” to describe the field very close to an antenna. That is the only point that I am trying to make. Constant314 (talk) 22:51, 13 January 2019 (UTC)


 * I use the gravitation field as another example as it decreases in the same way that the Coulomb's field and then can be considered as a near-field too.


 * For the rest we basically agree, the field is said evanescent when it decrease exponentially (wave transition between two medium, cut-off frequency of wave guides, and to some extend when a very large amount of dipoles are implied leading to a field in 1/r^n where n is extremely large such in the case of antenna reflectors, moreover in such cases the near-field is aside the propagating wave, a kind of parasitic that do no carry energy.


 * Did you find any independent reliable source where the single dipole to dipole non-radiative near-field interaction (electric or magnetic) is called evanescent? If not, I suggest at least to tag the Witricity coupled coil example of near-field with "secondary independent sources required". Henri BONDAR (talk) 05:09, 14 January 2019 (UTC)


 * I did not find any example of electric dipole to electric dipole described as evanescent, but I wasn't looking for it. I think, I did see an electric dipole in a wave guide described as evanescent  coupling to an evanescent mode of the guide.  But that is obvious; if the guide has an evanescent mode and the dipole in the guide is operating at that frequency then it couples to the guide's evanescent  mode.


 * However, magnetic dipole to magnetic dipole (a loop antenna to a loop antenna) is used for wireless charging and at least some in that industry are using the term evanescent Constant314 (talk) 21:23, 15 January 2019 (UTC)


 * Some in the industry are according to me mostly Witricity proselytes and suggestible persons, they cannot be considered as secondary reliable sources. Henri BONDAR (talk) 08:35, 16 January 2019 (UTC)

Modes, fields and waves
There seems to be excessive hand wringing over whether an evanescent wave is a wave. The article is about evanescent fields. I propose that we change all instances in the article to evanescent field (except in direct quotes) an give the alternate names one time in the lede. Constant314 (talk) 22:15, 15 January 2019 (UTC)
 * I totally agree with that. Henri BONDAR (talk) 08:35, 16 January 2019 (UTC)

Introduction's Suggestion - Update to state-of-the-art information
In the introduction section, a definition of evanescent field is presented. In the literature, it is presented a novel methodology to analyse this type of waves, regarding its excitation and its propagation. I think it is important to add a simply description about this in this introduction. I suggested the following:

A novel methodology to analyse the excitation and propagation of this type of waves is proposed in, based on geometry.


 * Wikipedia doesn't publish novel methodology. It may be exciting, but for Wikipedia, we wait until it percolates up to reliable secondary sources, like text books. Constant314 (talk) 20:24, 10 May 2023 (UTC)