Being transparent or creating an illusion? I may propose a device to transfer the images from one location to display at another location using the optical fibers. Furthermore, invisibility may be re-defined over different IR-visible and UV spectral ranges. What optical materials, nanostructures or hybrid mechanisms can be used to enhance the invisibility process?
I believe the answer to your question is: no.
The reason is that when you try to do cloaking (eg. using metamaterials) all the techniques are narrowband. To get true invisibility you need your technique to work over a wide band. I there are no good wideband techniques.
Metamaterials are considered intensely for aforementioned applications. Negative refractive index should be run. Definite geometries are needed to be designed on the materials. This would result in invisibility in Visible region.
What are metamaterials?
What are the optical materials with negative refractive indices?
What geometries should be designed?
In fact , the absorption coefficient of the material at a certain wavelength exhibits to act against invisibility.
I'd like to assign you with the following paper:
http://www.ingentaconnect.com/content/asp/jctn/2006/00000003/00000002/art00002
U can also find many recent beauties on the net
The way widely disccused is "cloaking" an object by a shell made from meta-materals with negative refraction. but it works only in a narrow spectral range
What "object"?, what "fully invisible"?
Silly, take 1 liter clear water (= object) and throw ii into a clear rapid river: immediately completely "invisible", for ever.
Dear Anton
Funny interpretation that is practically useless. I think the question is quite lucid. Your magic could not make 1 lit water invisible, conversely you made a whole river visible!!
I believe the answer to your question is: no.
The reason is that when you try to do cloaking (eg. using metamaterials) all the techniques are narrowband. To get true invisibility you need your technique to work over a wide band. I there are no good wideband techniques.
Dear Parviz,
it is your business to ask a question, and I realize you dislike my answer. There is none interpretation, there is only canonical logic and correctness. Then, how comes your FURTHER demand of " practical use"?
The answer to your question is trivial: 1 liter? in your country sometime full rivers become "invisible".
"Invisibility" of objects arrives in addition below and above a certain level of luminosity.
The (real) sun is invisible on Earth, we see pictures,
go to Pluto to have a real look at it. Beyond: invisible.
So what!
The effect of Anderson localization of photons by disordered media was discovered
in 80-ties. There was something about applications for making airplanes invisible.
I attach a paper.
Stealth is invisible against radar. Quite practical example of making an airplane invisible in RF electromagnetic waves. According to Physics analogy, How can we hide the airplane (or any desired object) in shorter wavelengths (visible spectral region)?! Maybe covering special optical materials to absorb photons or using skin microtexturing or nano- structures to trap photons,etc??
In the paper I attached there is reference on the paper by Kaveh. As far as I remember this was the case You are interested in.
Anderson localization is not a good candidate because it doesn't occur for all the photons which illuminate the highly scattering medium, but only for those photons that penetrate in sufficiently deep areas; other photons that scattered from the surface scatterers have chance to scape from the media. Enhanced back-scattering technique employs such photons to measure the effect of coherent interference of back-scattered waves.This is a Reliable technique to characterize the Anderson localization regime. Notice that we can see for example very high concentrations of TiO2 nano-particles dispersed in dye solutions.
Even if all the photons become localized, object seems to be black and not invisible!
Easily we are searching the possible ways of optical camouflage similar to what animals can do by elusive techniques or by means of the advanced optical methods and meta-materials.
Being invisible, should not be a coherent reflections. As a stealth aircraft, the bottom surface of the oval shape makes the reflected waves on the radar screen looks like a clouds or a plume.
Another way is to use an external body that has all the colors of light scattered
Like silver, it makes a target, rather than several goals seen on the radar screen.
The primary property that TiO2 contributes to a coating is hiding power. Hiding power describes the ability of a coating to obscure a background of contrasting color. Hiding occurs when the penetration of incident light through a film is reduced either by light scattering or by light absorption. TiO2 contributes to hiding power by light scattering. Colored materials, whether they are colored impurities in the coating or intentionally added colored pigments, contribute to hiding power by light absorption.
http://www.specialchem4coatings.com/common/shared/ctl/displayfile.aspx?id=911
However, there is another way: when the object is visible, we cannot see the space behind the object. So if it is possible to record the landscape ahead, and display it behind the object, the object may be invisible!
Derek
The superposition of a number of narrow band structures may give us a wideband solution?!
The materials with magnetic property such as Ferrite absorb the 3-63 micro eV photons and reduce the reflection at 8-18 GHz, U= miu.B :
Other materials for reduction of reflection:photonic crystals, ferromagnetics, nanotubes, polymeric nano composites and micro cavities and the stacks of coated optical layers.
Dear Parviz Parvin,
You should clarify what you mean by "feasibility"
1. Theory tells as, that it is possible to create or recreate any wavefront of electromagnetic wave. So, theoretically we can create perfect illusions, including invisibility
2. Recent advancement in technology of metamaterials and photonic crystals enabled fabrication of "invisibility cloaks" for in narrow bandwidth, meaning that today we can make objects invisible for one "color" of light .
3. For different applications invisibility can mean different things. E.g. for some military application it is enough to reduce reflections from an object, so that it disappears from the radar. Glass balls will disappear in a liquid matching balls' refractive index. Mirrors do the tricks for magicians. In a digital world objects disappear with help of a green screen.
Dear Sergiusz Patela
Feasibility to create invisibility arises from the technical barriers in wide spectral range and becomes more complicated from microwave toward IR, Visible, UV and even X rays. The transparency and opacity and reflection of materials strongly depend on the incident light wavelength.
Glass BK7 is transparent in visible range and may be opaque at FIR and VUV.
Ge is a good window at IR while it looks like black for visible light.
If by feasibility you mean ability to fabricate economically justifiable, application-independent, broad-spectrum invisibility - the answer is: no, today it is not feasible to create full invisibility. What we can do today, is laboratory-scale proof of concept. Technologies that are used do not scale for inexpensive, large scale production.
Thank you for your response. Feasibity may ignore costs for technical innovations .
Perfect cloaking is impossible based on simple causality arguments if we are talking about waves in ordinary free space. If you shine a pulse at a volume but prevent the pulse from propagating through the volume because you want to cloak it, the information to reconstruct the pulse on the far side of the volume cannot get there fast enough. So the pulse cannot be reconstructed properly just on the far side of the volume. So perfect invisibility to pulses is impossible for light in free space because we could always detect the attempt by interrogating with pulses. Perfect cloaking for acoustic waves is possible in principle because you could send the information faster through the volume on electromagnetic waves. Generally perfect cloaking requires nonlocal response, however, so no material with the usual local response (the response of the material depends only on the wave field at that point) can actually offer perfect cloaking. Schemes with arrays of microphones and loudspeakers offer perfect acoustic cloaking in principle, based on the output from each loudspeaker being calculated from the input from all microphones, hence giving the required nonlocal response. See Opt. Express 14, 12457-12466 (2006).
Loudspeaker contains an acoustic spectrum and the the spectral analyzer is needed for the sound detection. On the other hand, the optical pulse contains spectral bandwidth, intesity profile and temporal duration. If you can reconstruct the same pulse, then cloaking happens.For simplicity ,let assume a point object, with a receiver and an emitter in aopposite direction, then the optical pulse received by the detector can be reconstructed by the emitter as if the object is being cloacked. This concept can be extended to other points accordingly.
Have a look at the reference I cite. This explains the causality argument as to why perfect cloaking is impossible in more detail. (Opt. Express 14, 12457-12466 (2006)). This article is open access.
hi David,
your argument, comparing acustics and optics is only "almost" tru,
for your causality argument the speed of light is still too slow,
you need infinity..
Hi Anton
To do "perfect" and causal cloaking in acoustics, you don't need infinite propagation velocity anywhere, but you do need the sum of the calculation time (to set the loudspeaker signals from the microphone signals) and the propagation time (e.g., on electromagnetic waves used to send the signals internally through the volume) to be less than the acoustic wave propagation time would have been through the volume.
In principle, we could also do such "perfect" and causal cloaking in electromagnetism also (for everything except static magnetic fields), but only if we were in a medium where light was propagating sufficiently much slower that we still had time inside our volume to calculate and to propagate our information sufficiently much faster (e.g., near free space light velocity).
Almost certainly, though, all these cloaking attempts would be discoverable quantum mechanically because they would involve measurement with the associated collapse of the wavefunction, which would make them discoverable in the same way that we could detect the presence of eavesdroppers in a quantum communication channel. Again, see Opt. Express 14, 12457-12466 (2006).
hi, David
consider:
6.2. Propagation and calculation time
As noted, our approximate “true” cloaking simulation has assumed no additional calculation delay, .....
I really like your causality argument, you need "at the instant of time"
and:
6.3 Quantum detection of cloaking
in principle, whether waterwaves- or quantum wave equations, who cares.
on the other hand: photons: on the lightcone there is "nonlocalizability",
here your simple
"... quantum mechanical collapse onto eigenstates..."
and so on does'nt apply.
Of course it is possible. A collegue of mine, wrote a peper long time ago abour it.
Time domain simulation of electromagnetic cloaking structures with TLM method
Cédric Blanchard, Jorge Portí, Bae-Ian Wu, Juan Antonio Morente, Alfonso Salinas, and Jin Au Kong
Optics Express, Vol. 16, Issue 9, pp. 6461-6470 (2008)
http://dx.doi.org/10.1364/OE.16.006461
To risk repeating myself, any attempt at cloaking a volume in free space in which you exclude the electromagnetic field from inside the volume is discoverable by looking at a "transmitted" pulse. Presuming you as the "cloaker" don't know the pulse is coming, then the information to reconstruct the pulse just on the far side of the volume can't get there fast enough because it has to make its way round the outside of the volume, which takes a longer time. So perfect invisibility to the "transmitted" pulsed light is not possible even classically.
And in the spirit of how one can always discover any eavesdropper who measures the information (as in the BB84 quantum encryption protocol), we can similarly quantum mechanically discover any attempt at cloaking that relies on measurement of the field.
Again, see Opt. Express 14, 12457-12466 (2006)
http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-25-12457
The interaction of electromgnetic waves with the target is inevitable. You can conduct the radiation or simulate the radiation or camuflage. In fact camuflage is an effective natural technique of cloaking where the visible emission of target approaches the background radiation.
So far as reflection phenomenon prevails, it is not possible to fully make objects invisible, unless some sort of illusions with some tools like optical fibers, that now being used in invisible outfits.
A transparent object when placed in a liquid whose refractive index is matched with that of the object will be invisible.Long back Christiansen developed to measure refractive index of unknown material using this technique. It is also called Christiansen's filter.
@Mehta: That is right, but there is no perfect transparent object as yet in nature or in fabricated form. For instance, material like glass is categorized as transparent, but still is visible. 100% transparency means 100% transmission through a medium. On the other hand if absorption of an electromagnetic wave in a medium is 100%, we can have no reflection back to the observer, hence invisibility of the medium. But yet, absorption could neither be 100%.
Faramarz, a perfect absorber (absorption coefficient -> + infinity) is a perfect reflector!! (think of Fresnel formula in the limit of very large imaginary part of the refractive index)
Aside from this, we don't think of something black in normal environment as invisible!
Rasbindu, the matching refractive index is a good point. Bit for something to be invisible, the match would have to be fulfilled over an extended spectral range! That brings you back to identical materials sooner or later. And yes, I can't see a drop of water in the swimming pool....(unless it's empty otherwise)
H don't agree that matching refractive index can guarantee 100% percent transmission and zero reflection. What does it mean, a perfect absorber is a perfect reflector? That is in which space?
Kai, I see your point. But it is possible to tune RI over certain range using appropriate medium. For example we have prepared a magnetic colloid containing large(micron sized ) magnetite spheres and nanomagnetic (single domain) particles. It was possible to tune RI over band a wavelength range between red and green so that scattering becomes zero. ( JOSA,,27,873,2010 and references therein). When we placed a thin aluminum wire then the object becomes invisible within this range(this result is yet to be published).
Faramarz, if I understand correctly, Kirchhoff''s law of radiation states that a perfect absorber is also a perfect reflector. Please correct if I am wrong.
What I understand from Kirchhoff's law of radiation is that an absorber after absorption has to radiate to be at equilibrium. This radiation may not indicates a reflection. If at all reflection is perceived, the object will be visible.
Faramarz, our normal conception of visibility (e.g. in the presence of sunlight, or artificial lighting) is not a situation of equilibrium. The light sources are much hotter than the objects we're talking about.
According to Fresnels law, we have for the amplitude coefficient of reflectivity:
r=(N1-N2)/(N1+N2) where I understand N1 to be the complex index of refraction of "the object" and N2 the one of the "surrounding". For simplicity I consider normal light incidence on a plane surface.
In either case we can write N = n + ik, where n and k are the rea, and imaginary components of the complex index of refraction. Therefore we may rewrite
r= (n1-n2 + i(k1-k2))/(n1+n2 + i(k1+k2))
At any given but finite N2=n2+ik2, this goes to unity as k1 -> infinity (limit of arbitrarily strong absorption).
In normal language we could state that as absorption is very strong, the light cannot significantly penetrate the interface significantly and therefore is constrained to being reflected. A nice example of this is observing the almost metallic mirror-like look of semiconductors with band gaps in the infrared (e.g. silicon).
@Rasbindu, thanks for the quotation, that sounds pretty interesting. It may take a while, though, to find the time really reading it. Allow me an intermittent question: does the magnetism of the micro- and nanoparticles matter for the camouflage effect? And is the camouflage specific to aluminum and its shape (rod) or even more general?
Anyway, interesting stuff!
@ Kai: Thanks for your explanations. Fresnel reflection occurs when there is an abrupt change in the refractive index of the medium of transmission. Very little as compared with the transmitted energy at the incident surface. I can't relate your reasoning with the invisibility phenomenon. Fresnel reflection can not be directly related to Kirchhoff's law of radiation. Please refer to 'Physical Principles of Remote Sensing" By Gareth Rees, W. G. Rees, Chap 3, Cambridge University Press. Copyright,
@ Faramarz, maybe we should settle for a clarification of what concept of "invisibility" we are talking about. (It would also be good if the OP would clarify his perspective in the original question.)
I can envision several kinds:
(i) object "illuminated" by external source (beyond my control, such as the sun). What can I do to limit detectability/visibility of the object.
(ii) I have a source of illumination (RADAR, Laser, whatever) which I control. How to achieve that despite my source I cannot detect the object (in some given spectral range)
(iii) No (significant) source of illumination, just detection of radiation.
Kirchhoffs laws, as you already stated, relate to thermal equilibrium. In situations (i) and (ii) for example, thermal equilibrium is not given and I do not understand the relevance of Kirchhoff's laws for discussing invisibility there. So again, I think it should be useful to clarify what we are talking about - and discuss separate cases separately if necessary.
Edit: I just revisited the original post/question, isn't it about (creating the illusion of) transparency? That would somehow fit into the context of Rasbindu's contribution, i.e. absence (or camouflage) of scattering of radiation in some spectral range.
Am I getting it wrong?
My dear Kai: Rasbindu Mehta referred to Kirchhoff’s law of radiation in response to my statement regarding perfect absorption … and.so on. I did not agree to it because in that law talks of thermal equilibrium in relation with absorption is stated. But when we talk about visibility, we are fully concerned with REFLECTION. In addition I said if at all that law is taken into account does not guarantee reflection, it maybe scattering either, which can not cause invisibility/visibility. You also agreed with Mehta and in addition put up Fresnel Law etc. for more clarification, that law shows how much reflection is there when there is a discontinuity in transmission medium in terms of refractive index.
Please read all my statements, I said there is no relation between my statement, Kirchhoff’s law of radiation, and invisibility/visibility. Whatever you noted in your latest contribution, I fully agree to it.
No doubt, but an emitter is not necessarily a reflector. All the effort in the invisibility of an object is that to suppress reflection, whether by means of a natural material or a fabricated process.
Let summarize as the current discussions as below:
1- Assume an object in atmosphere ---> Drastic change of refrction index at interface --> Significant reflection according to Fresnel eqs. ---> Visibility.
If we would like to make it invisible, we need to reduce this abrupt refractive change as much as possible by means of various techniques on the object surface such as implementing interference layers, nanocavities, nanostructures, using novel materials, etc. or perform camuflege as an illusion i.e, to decorate the object surface that looks like the surroundigs.
2- There is another way for cloaking the target, reducing this abrupt refractive change by altering the atmosphere around the object using graded refractive indices or crating plume, cloud etc . around to hide the object.
3- What about employing pixel by pixel monitoring of the object behind and diplaying that image in front ( an illusive technique) ? What about using hybrid methods for cloaking ?
4- The perfect absorber may be a perfect reflector, however at a definite spectral range! On the other hand, the stealth may not be identified on radar screen due to an absorber covering layer( at a definite spectral range) and its well designed surfaces. Here, a perfect absorber may not be a perfect reflector!!
5- It is essential to re-define the following terms based on Physics principles:
Invisibility, cloaking, camufledge, illusion and hiding optically at a definite spectral range.
Cloaking or camouflage( both are synonyms of each other) is device or material (say metamaterials) which covers an object so that it becomes invisible to an eye or to an optical detector. Illusion ,particularly optical illusion is a perception that is misleading to eye from reality. Hiding optically may be due placing an obstruction in path of the object to be hidden.
The spectral range is a key parameter in definition.
Camouflage is a natural technique that even animals get used to look like the atmospher surroundings. Successful camouflage needs the crowded background and the congestion of many things around the object. For instance, camouflage may be much easier in jungle than in desert.
Hiding does not necessarily mean optical diappearance behind a passive barrier, It may be actively done too . In fact, the scene behind the object may transfer in front face, bypassing the object.
Optical Ilusion is what the magician performs by very fast manipulation such that the eye fails to transfer correct data to the brain recognition unit. Fast displacement may be a dynamic illusion.
Invisibility, cloaking, camouflage with respect to human eyes and optical detectors have the same meaning unless one gives them some special meaning in a special application.
Let us forget to say a perfect absorber is (may be) a perfect reflector". There are many instances that prove otherwise, like Radar systems. It may be valid in case of thermal radiation. But illusion is not necessarily invisibility. We can say it is more a misconception of the observer visualization of real object.
I think if we want to tackle such scientific topic my sum-up is to suggest to follow this motto:
Let us think as PHYSICIST and act as ENGINEER.
Think as a physicist and act as engineer, I agree with you and always follow this rule.
however how can we apply this rule in the cotext of invisibility?
According to engineering view, no matter what are the definitions! we intend to acheive the goal (invisibility) by all means and possible ways.
Dear Prof. Parvin, THANKS for support of this motto. Actually, this is not merely motto but is my notion of doing research work. If we want realize this motto in real sense, we have to consider the demarcation between theory and practice. To some extent these two should have overlapping capacity so as to solve some phenomenal issues.
Now, let’s turn back to the topic of discussion. Why I stated such a motto is that after going through the opinions of the contributors, I have found some sort of prejudice in forwarding ideas against the present scientific issue, resulting in narrowing the overlaps amongst different experts. I am afraid my talk may look like more philosophical than realistic.
Anyway, if the niche amongst experts prevails, there won't be any conclusion in any discussion. That what I have come across with in this very valuable forum, that can bring all idea-makers together world wide.
Regarding invisibility, we should first make it clear for what purpose we want it. Based on the application, one should think of solution. As of today, there are several well-known approaches to hide objects.
I am thinking over a little idea, which needs to be cooked properly, i.e., creating a "Multiple Visions" such that an observer (Human eye or Instrumental detectors) can not be able to distinguish between received image pixels or bit of messages. Something like quantum teleportation!
My idea is based on Double Vision or Diplopia occurring in human eyes. Diplopia is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, or diagonally (i.e., both vertically and horizontally) in relation to each other.
For accessible objects, I think optical fibers can be used for creating double or multiple vision. Let's work on that. VIVA.
Dear Dr Seraji
In conjunction with your proposal, The lens aberrations may be used to enhance the multiple vision too.
A nanocomposite inclding carbon nanotube (CNT) embedded in epoxy resin may damp electromagnetic waves!
Yes, that is a good suggestion. I am thinking some mechanisms and technique not yet cooked to discuss about. Hoping for later time. VIVA!
I'm just curious. But wont this method work only for a certain angle only??
Refer the image attached.
Lets say you want to camouflage a box. Consider the point 'p' at the center of the box. If a person is looking at the box directly from front (v1), point 'p' should represent the color directly behind. If the color is green p should be green. If another person looks at the point p from a different angle(v2) his 'should be background' color would be different, lets say blue. If so the point p need to have to colors simultaneously. (To be more theoretical, since there are infinite possible angles number of possible colors could be infinite) But it is a contradiction. So my point is you cannot make an object invisible to all the angles.Can anyone please clarify this point ?
Thanks
Hi Gayan,
there are many ways to make an objekt "invisible":
put it on a wheel and rotate this with - say - 50Hz.
or, cover the object with a - perfect - black surface and put it into a "black box".
this works for a wide range of angles.
I agree with Anton. Any object similarity with background may look like to be hidden in the background.
I also agree with Anton and Parviz..But, the main question is physics or science behind all these observation. In meta-physics one refers to 'Appearance and Reality'.I In examples of Gayan and Anton the object is' invisible' due to either selective reflectance of the object or due to perception of vision. The recent works by Pendry and others deals with 'cloaking' of an object. Here ,object is 'invisible' due to specially fabricated meta-material which acts as a 'cloak'. Physics behind such '"invisibility'' is interesting as well as intriguing and the material will serve useful for novel applications.
Dear Rasbindu
Please kindly link the new article of cloaking if it is accessible.
Please refer the attached one. You may get others from cross references.
Regards.
R.V.Mehta
I believe that the answer of this question will be yes by keeping research on cloaking.
However, at present all of cloaking techniques are narrow band.
Hi everyone,
Sorry for the late reply (We celebrated our new year in Sri Lanka and was super busy). I think there are two ways to look at the problem. I agree with Anton to a certain extent. We have to understand what 'invisibility' means in this context. Is it been un noticeable to human perception ? Or is it that you cant see/ detect it with anything. For example with a high speed camera 50Hz rotation would become visible
The word "invisible" it self clarifies that it is related with human visibility-perception. For example certain creatures and birds can "see" object in dark which are " invisible" to human. For other physical detectors some other word say 'indetectable" may be appropriate!
It is important.The SUN is fully invisible onto the freguencies of Fraungofer lines !!!!!
Dear Rasbindu, that is indeed a significant constraint. We have had quite a few instances in the thread where e.g. RADAR was mentioned ofr discussed. Now it would be good to have the opinion/viewpoint (:-) of the OP (Parviz Parvin), since any fo our contributions should serve his question, shouldn't they...
Any failure in spectral detection can not be attributed to invisibility. An invisible body should not be detected over a certain spectral range using any type of detector at the same spectral range.
Dear, Parviz and Fauth, I do not know any technique or material which has covered entire visible spectrum yet it is claimed that objects will be "invisible" by using this technique or material!
Solution may be implementation of mutilayer nanocomposite arrays and fibers to conduct the front image to the opposite side as a cloacking technique. A couple of 2D array detector and emitters may be another approch to hide the desired object.
Unfortunately, a question put up in this forum is not discussed in a time frame. There is no solid conclusion exerted by the questioners. The contributions turn back to the same square again and again with boring answers. I would suggest some end-up time slot to be framed by the questioners for a relative conclusion in a given time. Otherwise, the path of the discussions turns back to the same starting place.
Let us respect the time. VIVA!
No matter how long it takes to acheive a resonable and solid resolution. The scientific achievements may be obtained from a sluggish but continuous process.
سالک ان است که اهسته و پیوسته رود
In the world of Metaphysics, where there is not formulation and modelling, and the debate is based on oral gestures, your reasoning could be applicable. BUT in Physics and Natural Sciences we have modelling of phenomena which rendered experimented laws that can help us in winding up such discussions. I wonder why the contributors repeatedly say same thing along the time! In any scientific project time is detrimental.
You can not find any rapid end for cloaking technology because it is in its primitive stages.
In this forum, all the contributions are meant rather for exchange of views not to prove any scientific truth. That is right, the cloaking technology is its infant stage. BUT What makes the discussions futile is the repetitive answers presented by contributors. VIVA!
Repetitive answers of experts and their multiple feedbacks exchanging among them may generate new ideas as it is. I have learned much using this new discipline created by RG.
Good luck Dr Seraji
What about the application of optical fiber or fiber laser in invisibility conetxt ? Something new ?!
Actually, in optical fiber communication systems, we are trying our best to make signals to be "seen" by detectors, i.e., as far as possible to be "visible". On the distal end of the matter, we desire also the objects to be invisible for "un-communication" purposes. Anyway, this very discussion deviated me to think of using optical fibers for invisibility purposes, too. Who knows one day or so it may come to work for a new technology termed as "Stealth Optical Fiber Technology (SOFT) employed in Communication Engineering for data transmission, which could be ultra-secured message exchange. VIVA!
I am just thinking if I could assign a wave spectrum a "FACE" and at the end of the transmission path (optical fiber) disguise the FACE, the detector may not recognize it, as good as invisibility. Looks absurd! VIVA!
Cloaking increases the overall scattering, so the answer is no:
http://arxiv.org/abs/1307.3996
Invisibility in a single spectral band may be feasible; wide spectral invisibility is not. Nothing can hide from LADAR with a 150 dB dynamic range.
Dear Russell
1-If invisibility is successful at a single spectral band, then according to superposition principle it can be extended to wider spectral range.
2-Which ladar or lidar do you address??
Answers to detailed questions may cross into an area that cannot be discussed in an open forum. Suffice to say that analyses were done and conclusion was that a sufficiently robust LADAR system can detect anything -- basically because even metamaterials collect dust in the real world. Evan a small amount of scattering is enough for detection. It's the difference between hypothetical, perfect environment and the real world.
Dear Russell
Interesting. Depolarization LIDAR may detect dust particles collected by methamaterials, but it is a secondary effect and notice that the spatial resolution of such LIDARS are not better than meters(due to limited pulse duration and FOV. So I think that LIDAR can only detect the average volume density of dust rather than IDENTIFICATION of an object.
Spatial resolution is limited only by the array size. Range resolution is limited by pulse duration and signal processing. APD arrays are much more accurate than your estimates.
Also recall that dust may be secondary but it becomes primary in the real world. The saying in laser world is everything reflects 10% and there is no exception.
In Lidars, the range resolution is important i.e, C * t /2 - for instance regarding 10 ns laser duration- it is around 1.5 m. On the other hand, the array resolution is based on pixel spacing and array size useful in passive imaging.
The AR layers significantly reduce the Fresnel reflectance. The invisibility may be achieved as a combination of multilayer structures of metamaterials to vanish light scattering and reflectance at a certain spectral range. However, Invisibility may be categorized according to type of detetors and relevant systems:
1- Human eye
2- Radar
3- Lidar (Ladar)
4- Hypersensitive spectometers
In the context of Invisibilty, even we can minimize the reflection and scattering at a certain wavelength, we need to reduce the contrast with the background too.
Let's go back to the terminologies used in different techniques for which limitations are defined in terms of "Invisibility". Cloaking, Invisibility, Camouflage, Undetectable, Stealth, etc., verbally may have the same meaning, but once they used in conjunction with some applications may differ in technical limitations. For instance, in case of Radar, Lidar etc., usually the word Stealth is employed while processing the received signals. I suppose the initial question was on INVISIBILITY of environmental objects as far as human eye is concerned, but in the course of discussions, the theme has changed to similar topics related to different techniques that altogether seem to be the context of INVISIBILiTY in general sense. But to be specific, we should confine the discussion of the question to a certain application, then follow up the analysis of the presented ideas form the contributors. Let us assign the afore-said words to particular application as follows:
Invisibility: for Human eyes; Camouflage: for hiding the equipment (in military), Undetectable: used for detection of optical signals (by photodetectors); Stealth: used in Radar and Lidar systems. In these applications, different techniques are used because the mission in each case is different. We can not generalise all the applications and further can not expect a unique means of "sensing"; if it is not realized, conclude it as "invisible". VIVA!
Thank you for your comments.
In the case of Radar, the nanowires can decrease the relative permittivity as low as 0.2 and even lower index is expected.
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