Negative index of refraction?

Question

heard something on radio about the new nano materials that bend light. guy mentioned n<0! what kind of n's have they obtained? -1<n<0 bothers me, n<-1 seems less annoying. what's this do exactly? D field opposes E field? what's that mean? are these materials also birefringent?

Answer 1

Phillip J is wrong.  And that&#039;s because he forgets that there are many ways to define velocity of light, and he is thinking of phase velocity, while group velocity is what is more relevant.  By carefully examining the phase and group velocities you will see that a negative index of refraction is possible and does not indicate time reversal.

These materials are not nano materials, but are metamaterails.  They are actually macroscopic, man made, devices.  They are made by patterning electric wires in a repeating pattern on sheets of material.  Then these sheets are fit together to make a grid, much like the collapsible cardboard dividers that keep bottles from knocking together in a box (like when you buy a case of wine or lightbulbs).  These are macroscopic devices that only work at very specific wavelengths, and since they are macroscopic, the wavelengths have to be pretty big (radio waves, microwaves, etc).  In theory, if you could make something like that on the order of visible light wavelengths (100&#039;s of nm&#039;s), it would work in the visible spectrum, but it would be very difficult if not impossible to manufacture something that small.

The index of refraction depends on the frequency ans is negative for only a very small range since the wire pattern has to be specially tailored for the given wavelength.  The range of index of refractions or the experiment that I am aware of went continuously from +3 to -3, crossing 0, as they the frequency was swept over a small range.  This implies the material is dispersive, but I do not know about birefringent.  I would assume, since almost anything is at least mildly birefringent, this would also be.  Certainly the material has a directionality since it is too difficult to make a 3D repeating structure that repeats in all three dimensions.  As such, there should be a polarization dependence and birefringence.

And yes, this gives rise to lots of funny stuff like light bending the wrong way and Cherenkov radiation pointing the other way and even doppler shifting reversing.  If you wnat to learn more, I would just google metamaterial.

Answer 2

Index of refraction for a material is the the speed of light in vacuum divided by the speed of light in the material. It is always greater than 1. A negative index would indicate time reversal; the light would have to exit the material before it enters.