Comments on: Reversing optical “shockwaves” using metamaterials (updated)

By: Shib Shankar

Shib Shankar — Fri, 17 Sep 2010 10:02:52 +0000

It is hard to have any particle gain velocity greater than light in air.
It is possible only with the help of the most powerful particle accilerators of
the day and that to in a very short range. It will require energy of the
order of GeVs at least if not in TeV’s. Such fast moving particles never occur
in nature. Also most part of the universe, just outside the lower atmospheric strata
is as good as vacuum. And you cannot just reach velocity > c.

By: daniel

daniel — Sat, 24 Jul 2010 14:22:58 +0000

what would happen when a object travling light speed…came near earth…my specific question is.”what kind of shock wave would impact us”…my follow up question is could there be a defense against it

By: skullsinthestars

skullsinthestars — Sat, 21 Nov 2009 03:08:46 +0000

Ah, I figured out the problem with my original construction: I drew the phase fronts in the negative material wrong, and phase and energy flow are opposite one another in a negative material! I’ve updated the post with the proper discussion, which is a lot more subtle.

By: IronMonkey

IronMonkey — Fri, 20 Nov 2009 20:10:53 +0000

I think the comparison here between an optical “shock wave” and the Cerenkov effect is a very intuitive approach to describe a very counter-intuitive phenomenon inside a negative refractive index medium.
What I did to understand the reversed direction of Cerenkov radiation is:
1. I use the figure with the z-shifted concentric circles which describes the flow of energy.
2. In the case of a positive index material: I draw one arrow in each circle starting from the center to the perimeter touching the upper line of the shock wave. The subtended angle from the arrows to z-axis of propagation is the standard direction of Cerenkov radiation.
3. In the case of a negative material: I draw one arrow in each circle starting from the perimeter touching the lower line of shock wave, and ending in the center. The subtended angle from these arrows with the z-axis is the reversed direction of Cerenkov radiation.

So the descriptions above make sense to me!

By: skullsinthestars

skullsinthestars — Fri, 20 Nov 2009 19:04:10 +0000

P.S. The more I look at it, the more I’m uncertain that my “point source” explanation of the reverse Cerenkov effect is technically correct. It has the right idea, but I’m not sure I’ve properly taken into account the difference between energy flow and wave direction in negative materials.

By: skullsinthestars

skullsinthestars — Fri, 20 Nov 2009 18:41:08 +0000

IM: Very nice observations! In my rush to complete my post, I hadn’t even thought of the separation and number of slots being used. I had a vague feeling that this result was not as conclusive as one would like, but your comment illustrates very nicely why that is.

As you said, though, credit must be given for getting an experiment like this to run at all!

By: IronMonkey

IronMonkey — Fri, 20 Nov 2009 18:38:09 +0000

Very interesting phenomenon, and again nice drawings to illustrate it. You said: “…I’m guessing that there will be some who will argue against it…” and of course you were right!!

I concur with the impression that the PRL article reports an indirect method to measure reversed Cherenkov radiation since no actual relativistic particle is involved. But one of the first thing I do when reading papers about metamaterial research is I check the dimensions involved in the system. As stated in the post, for a structured material to be considered effectively homogeneous, its constitutive elements must be much smaller than the wavelength of light. As a rule of thumb, the condition a < lambda/10 provides a good start but ideally a < lambda/20 is preferable (where "a" is the largest element's dimension and "lambda" is the wavelength). In the PRL article, for 8.5GHz source we have lambda=35mm, and a=3mm for the arrangements of rods and split-rings: thus the former condition is respected.

But things complicate further along as we note that the open slots are periodically spaced by a=10mm which is roughly a third of a lambda. While the latter might not be a serious concern since each slot can still be effectively considered as a point source; we must note that there are only 14 slots in the device for a total interaction length of 140mm, which is exactly 4 wavelengths in length. These previous observations mean that the slot arrays have dimensions comparable to the wavelength of light, and as such they are highly scattering. The large diffraction effects associated with such a small device could significantly affect the results (from my initial understanding). Of course, it is already very complicated to fabricate this type of device, minimize losses and set up this experiment, so I must give my two thumbs up for the team that did it successfully!
It would be interesting to see if one can reproduce these results using a much longer interaction length: that would make the demonstration even more convincing.