Negative Refractivity for Optical Computing 125
zero_offset writes "This article in EE Times details Purdue's efforts to create a material with negative refractivity. One of the important results would be the ability to create optical computers due to the effect's tendency to amplify and focus light at wavelengths larger than the thickness of the nanowires used in the transmission system. Purdue's School of Electrical and Computer Engineering's Vladimir Shalaev says, "Using these plasmonic nanomaterials, we hope to directly manipulate light, guide it around corners with no losses and basically do all the fundamental operations we do with electronic circuits today, but with photons instead." Nanowires, surface plasmon polaritons, optical computers, nanoscale metamaterials, unnatural refractivity -- what's not to like?" We did a story on the first material known to have a negative index of refraction last year.
Some Claims are absurd (Score:3, Informative)
the ways one can escape these limits in a semantic sense is that you can change the index of refration of the media so the wavelength is shorter than in vaccum, but that's not really accomnlishing the goal. Alternatively, near field or or ther diffraction effects can confine a light field to a region smaller the wavelength, but it cant propagate in vacuum/air that way.
likewise the claim you could make a perfecly flat focusing lens by combining poistive and negative materials is pretty hilarious too. You can do that right now with conventional positive only materials. (example take two plano confave lenses of high index material, and fill the space between them with water. voila!).
on the other hand you could do a lot of really interesting stuff with negative index materials that is harder to put in laymans terms. one example, the speed of light might be faster than in vacuum.
Re:Remember your physics (Score:2, Informative)
So for dense thingies refractive index greater than 1. Eg glass. Now the refractive index of material A with respect to material B is Vel in B / Vel in A. So light travelling from Inside a glass slab to outside would think it encountered refractive index less than 1.
Now negative refractive index mean negative velocity ?? I dunno. Refractive index can also be calculated from Sine (incidence angle) / Sine (refracted angle). The only way to get negative refractive index is if Refracted angle greater than 180. (Remember high school trigonometry. Sine is negative only in the third and fourth quadrant). Now refracted angle greater than 180 would mean that light has suffered total internal reflection. So a negative refractive index material would behave like a mirror and not a lens. (hence giving negative velocity - velocity is a vector, has magnitude as well as direction). I smell a rat in the article.
-Dracken
Re:Meaning? (Score:3, Informative)
n = (speed of light in vacuum)/(speed of light in medium),
another definition, IIRC, is c/sqrt(mu*epsilon)
mu = permeability
epsilon = permittivity
both are coeeficients of the linear response of meterials to the EM field.
now, if the linear response of a material to EM fields is complex, I guess you can have negative (or imaginary) n.
imaginary means exponential decay or growth, BTW, but of course in the case of growth the material stops responding linearly at some point, thus changing the dependance.
IIAC, negative n does not really mean the speed of light reverses
Now, convenctional wisdom and all modern science says c is always the bigger value, so n is always >= 1
AFAIK you're right in saying c is always the bigger value, however there exist superluminal photons [cerncourier.com] , which have phase velocity higher than c.
This is not, again AFAIK, related to the response medium but to other quantum phenomenas.
The universe can do some weird, convoluted vodoo
Re:Meaning? (Score:2, Informative)
Your definitions are correct, but incomplete. The speed of light, c, (and cmed for that matter) are defined as c^2 = e*u (electric permittivity of the substance times the magnetic permeability of the substance). In other words, the speed of light is determined by how well the substance it is travelling through can be influenced by electric and magnetic fields.
To complete the definition:
n^2 = c^2/cmed^2 = (e0 * u0)/(e * u).
(The zeroes indicate that they are in free space.)
Now to the questions: Negative refractivity should be impossible. Both e and u are positive quantities, and if they weren't, the square would make them at the minimum postive imaginary numbers. The problem is that e and u are only scalars if you are working with the prefered direction of the substance. Otherwise, they are 2nd order tensors (3x3 matrices). (e0 and u0 are always scalars.) I am not sure how this would influence the outcome w/o doing the math. But, it may allow for this type of effect.
Who said anything about FTL travel? (Score:2, Informative)
but in regards to the article, the final comment was sheer speculation. THe existence of a negative index suggests that it might be possible to create a composte substance with an index less than one yielding an electomagnetic propagation media with a speed faster than vacuum.
What negative refraction means (Score:3, Informative)
03/2001 photonics.com article [photonics.com]