Material With Negative Refractive Index Created 210
holy_calamity writes "The race to build a material with a negative index of refraction for visible light has been won by researchers in Germany. The advance could lead to super-lenses able to see details finer then the wavelength of visible light, or the previously predicted invisibility cloak for visible light." From the article: "[The researcher] determined the refractive index of the material by measuring the 'phase velocity' of light as it passed through. His measurements show the structure has a negative refractive index of -0.6 for light with a wavelength of 780 nm [the far red end of the visible light spectrum]. This value drops to zero at 760 nm and 800 nm, and becomes positive at longer and shorter wavelengths."
does this mean? (Score:3, Interesting)
--josh
Re:obligatory (Score:2, Interesting)
Re:Invisibility cloak? (Score:5, Interesting)
What happens is that left-handed (aka negative refractive index) materials will bend light away from the surface of the material instead of towards it. So making an "invisibility cloak" is not that hard. First off, to solve the problem of knowing where the eye is, you simply make the surface of the material symmetric. So for a three-dimensional object, the left-handed material needs to be spherically symmetric. They have produced an example in the microwave region for a cylindrically symmetric configuration. But the cylindrical symmetry means that the shroud will only work for certain polarizations of light.
So what happens is that when light hits the curved surface, instead of being bent in towards the center, it is bent outwards. If the refractive properties of the medium are properly tuned, what you end up doing is bending the light around the obstacle such that it leaves the medium in the same path that it would have without the obstacle. So the "invisibility cloak" works by bending light around and emitting it so that the light behaves as if there was no object. Since the medium is symmetrical, it does not matter where the source and receivers are.
For a true cloak to work will require a really neat feat of engineering because the refractive properties of the material must be constantly adjusting with the movement of the cloak.
Re:Negative or less than one? (Score:2, Interesting)
Hang in there. (Score:2, Interesting)
Re:Negative or less than one? (Score:1, Interesting)
I think you'll find that C is the speed of light in a vacuum. In most media (i.e. everything we've found so far) light is actually slower than C. In fact, it's actually possible for particles to travel faster than the speed of light in a particular medium (see Cherenkov radiation [wikipedia.org]) -- though so far, not faster than C.
Re:Group vs. Phase Velocity (Score:3, Interesting)
And it's important to point out that the material they're talking about has a negative phase velocity.
If you had a material with a negative group velocity, it would violate causality, because the information would get to its destination before it was transmitted. (In fact, any material with a group velocity n<1 would also violate causality, because according to special relativity, there would be a frame of reference in which the reception came after the emission.)
A few years ago, when the first news articles started appearing about n<0 microwave media, a music prof I know e-mailed me excitedly about whether I'd heard about the new technology for time travel. He'd have been right, if it was group velocity.
Camera lenses (Score:5, Interesting)
cloak of invisibility -- maybe not (Score:3, Interesting)
Tetrachromats need not apply (Score:3, Interesting)
Re:Negative or less than one? (Score:3, Interesting)
fractional index? (Score:2, Interesting)
This value drops to zero at 760 nm and 800 nm, and becomes positive at longer and shorter wavelengths. Previously, the shortest wavelength at which a negative refractive index had been demonstrated was 1400 nm. "
how is this possible? fractional indices would imply that the light is going faster than light in a vacuum. i would expect negative index materials to have indices of less than -1 and no material to be able to have anything between -1 and 1.
The emperor's new clothes (Score:2, Interesting)
There are several weak points in this whole business of "Harry Potter cloaks" where physicists with little experience in electromagnetics (and even less in radar cross section reduction) go astray. To list but a few points:
Irrelevance of group velocity
It has long been known that effects like anomalous dispersion in resonant media can render classical group velocity concepts irrelevant. Several authors seem to lack an understanding of the inherent assumptions when equaling the group velocity with a power or information transfer speed. Thus an interpretation leading to an "equivalent" negative index of refraction can be misleading.
Bandwidth
The bandwidth of these materials is inherently small. There is also often a significant loss as well.
Misuse of models
The assumption of monochromatic and plane waves interacting with an infinite structure will be like pressing a square peg into a round hole when dealing with some cases. For example, it is well-known that a simplistic plane-wave model is invalid when dealing with lossy materials (apart from normal incidence).
Publications in out-of-field journals
It is clear that a lot of the metamaterial material has been published in journals that are outside the typical antenna or microwave area, such as Nature, Science, and Phys. Rev. This could potentially lead to deficient papers slipping through, due to lack of a proper review. An example of something that hopefully would have been curbed in an IEEE journal is a Phys. Rev. paper [*] that showed a transmission vs. frequency plot with a dynamic range of 1600 dB! (The range of scale of the size of the universe compared to the Planck length is dwarfed by this...). There are numerous examples of publications without even the most basic sanity checks performed by the authors and the reviewers. The situation has been bad enough for the microwave field, now it is unfortunately spreading to optical frequencies.
[*] R.W. Ziolkowski and C.-Y. Cheng, "Existence and design of trans-vacuum-speed metamaterials", Phys. Rev. E, 68, 026612, 2003.
Peer review endangered
The field of metamaterials has now grown to such a volume that a wholly separate sub-science or "sect" with its own special issues and conferences, etc. has formed. There is an inherent problem with this, since the peer review process will be endangered. The people most knowledgeable within the subject are by definition those that are active within the subject, and fewer outside reviewers will be used after a while.
"Publication by news releases"
Several of the groups within this field are heavy on marketing their results as revolutionary. In the present "publish or perish" environment it is very important to secure funding, and gullible grant-givers are abundant...
Human vision can extend a bit further (Score:3, Interesting)
Also if you make the source REALLY bright then apparently human vision can extend a very short distance into the near infrared as a very bright near infrared source will excite the rods or cones (not sure which it is) a tiny amount.
Re:yes, but RTFA, they were not first. (Score:3, Interesting)
If the phenomenon can only be dealt with in narrow band, the 'invisibility' aspects are strictly BS. Other facets from the 'magic' of this could produce some significant benefits. Possibly higher powered microscopes, perhaps a method of semiconductor fabrication capable of small detail without having to use hard UV or x-rays or nonoptical stamping.
It does make for some interesting physics and perhaps opens up a bit more insight into nature. And, who knows, maybe you too can have a cloak of invisibility so that anyone wearing 10nm filters of the appropriate wavelength won't be able to see you.