Saturday, June 25, 2011

Invisibility Cloaks – when light images nothing

Fig. 1. The blue stuff is the polymer
and golden stuff is gold used to SEM
image the structures. The upper one as
they point out is for reference while the
polymer in the lower rectangle contains the
3D features that give invisibility.
Ref: Joachim Fischer, Tolga Ergin, and Martin
Wegener, "Three-dimensional polarization
-independent visible-frequency carpet
invisibility cloak,"
Opt. Lett. 36, 2059-2061 (2011)

Invisibility cloaks are always interesting; no matter how they do it. Some people use ideas for camouflage like imaging and then displaying the background behind the invisible person on the cloak, effectively rendering the individual see-through.  Others use optical metamaterials.

Metamaterials are artificial materials that could be made to demonstrate un-natural properties like a negative refractive index, unusual optical frequency tuning capabilities, non-linear properties, etc. Optical metamaterials used to obtain invisibility are made up of sub-wavelength structures that bend light away from the object so as to render it “invisible”.

Earlier work on metamaterials started off at microwave frequencies. For optical wavelengths much finer nano-structure was needed. Soon the technology progressed to optical frequencies, but provided invisibility only along 2 dimensions. So if you looked along the 3rd dimension, the person wouldn’t be invisible. Then last year someone managed to do it for 3D. But it was still in a restricted range of wavelengths of 1000s of nanometers.

But I recently read articles here and here, that report a group at Karlsruhe Institute of Technology's Center for Functional Nanostructures (CFN) that have managed to write small enough features in their cloak that they can now guide light around it. Their team uses a combination of Stimulated Emission Depletion (STED) with direct laser writing to produce the polymer based, fine 3-D nano-structure. Their cloak works in non-polarized red light.

I realize one gets a lot of coverage for research like this. But we rarely hear about the effort. There is a lot of detail involved in work like this.. math, derivations, calculations, fabrication, chemistry, SEMs, microscopy.. Here is their paper in Optics Letters (finally out) for more detail. They show angle dependence, wavelength dependence, comparisons with ray-tracing simulations...

Fig. 2. This figure taken from their paper demonstrates the visibility
and invisibility of the vertical bars in the center of each rectangular
block for different wavelengths. The bars are visible in the reference
(upper rectangles) and invisible in the cloaked rectangles (lower ones).
Ref: Joachim Fischer, Tolga Ergin, and Martin Wegener, "Three
-dimensional polarization-independent visible-frequency
carpet invisibility cloak," Opt. Lett. 36, 2059-2061 (2011)

All that effort condensed into a 3 page easy-to-read optics-letter! But this is really the kind of work engineers and scientists do.. and love to do! So it's way cool to see coverage of the kind of work we love! :) Check it out! (If you can.. I wish every research paper were open and unlocked.)

Three cheers for all the good news coverage scientists and engineers get! And some more cheers for absolutely every individual involved in the effort!

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