Lenses can bend light and sound in almost any direction

Lenses can bend light and sound in almost any direction
3D view of the electric field for a lens that bends light 90 degrees. Image credit: T. M. Chang, et al. ©2012 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
(PhysOrg.com) -- When an optical fiber is bent by 90° or more, the light begins to leak away, posing a problem for fiber optics communications. But by using special lenses that can bend light by not only 90°, but also 180° (i.e., a U-turn) or 360° (i.e., a full loop), scientists may limit light leakage in optical fibers and overcome this problem, not to mention provide a useful material for many other applications. Recently, a team of scientists has theoretically investigated materials for achieving this kind of advanced light control, which could work equally well for sound waves.

The scientists, Sebastien Guenneau and coauthors from Institut Fresnel, CNRS, University of Aix-Marseille in Marseille, France, have published their study on the focusing and bending of and sound waves in a recent issue of the .

As Guenneau explained, both light and sound control techniques use gradient index (GRIN) lenses, which can effectively be used to curve space for light and sound trajectories. By creating a change, or gradient, in the refractive index of a lens, the scientists could create anisotropies – that is, make the lens' refractive index directionally dependent.

“We explain how one can either focus or bend (at 90°, 180° and 360°) electromagnetic or pressure waves simply by creating some gradient of the refractive index, which in essence is associated with effective anisotropy,” Guenneau told PhysOrg.com. “In layman's terms, we have curved the metric of space in the electromagnetic and acoustic contexts. We used realistic physical parameters and stress that one can use analogies between optics and acoustics to further the understanding: sometimes, thinking in terms of sound waves allows us to better grasp the foundations of transformational optics, as light waves are more elusive (light has complex physical properties such as polarization and moreover, it can propagate in vacuum unlike sound waves, which is something that still puzzles me).”

These GRIN lenses, which can be considered metamaterials and metafluids, build upon the principles of the invisibility cloak, which was co-discovered in the spring of 2006 by Sir John Pendry of Imperial College London and Ulf Leonhardt of the University of St. Andrews in Scotland.

“GRIN lenses considered in the NJP paper are designed using structured solids (for light) or structured fluids (for sound) deduced from a homogenization approach: when the size of the structural elements in a solid (or a fluid) is small compared to the wavelength of light (or sound), one can replace the solid (or the fluid) by an effective anisotropic solid (or fluid),” Guenneau said. “The trick is that light or sound, depending upon the physical context, will then follow curved trajectories in the anisotropic effective medium (solid or fluid): waves propagate in the direction of highest anisotropy, which is the principle of Einstein's relativity.”

Sebastien Guenneau of the University of Aix-Marseille explains more about optical illusions and light control in this video abstract. ©2012 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft

In the future, the ability to control wave trajectories could lead to solving problems such as the light leakage in optical fibers.

“One of the greatest challenges in optical fibers nowadays is that, as soon as you bend the waveguide by an angle of 90°, light leaks away and you lose the signal at the end of the fiber,” Guenneau said. “So one would like to find ways to limit this unwanted leakage. One way to do so is to use ideas from our paper: one can use the GRIN spherical lens in order to bend light at a sharp angle in optical fibers.”

The 180° bend (the U-turn), which is called a retro-reflector, can also be used to build a photonic or phononic micropolis, in which a variety of devices are assembled to resemble a tiny city. Another application of these involves mimicking black holes, which Leonhardt demonstrated both theoretically and experimentally in a previous study.

“A lens that mimics a black hole for light or sound offers a beautiful paradigm of a device that mimics the quantum effects of a black hole, which is a black body radiation near the event horizon known as Hawking radiation,” Guenneau said.

Invisibility cloaks, arguably the most well-known application of metamaterials, may also experience exciting advances in the future due to better light control. One area that scientists are working on is hiding bigger and bigger objects.

“One of the biggest challenges in the control of light is dealing with the very high absorption of metamaterials for visible wavelengths (due to the fact they use metals), which is the reason why invisibility cloaks are most of the time fabricated for microwaves (where metals have much less absorption), and nearly always hide only very small objects you would not see with the naked eye,” Guenneau said, noting that some recent experiments have achieved macroscale cloaking.

“For sound waves, one does not need to go into nanotechnologies, as frequencies of sound waves are much smaller than visible light,” he explained. “Also, there is no such thing as absorption for acoustic metamaterials, which is a good thing. Therefore, one can structure fluids with particles a few millimeters in size without having to resort to nanotechnology (to be compared with a few nanometers for light). So, in terms of fabricating nice paradigms, one should first think of waves, and if that works, then try to replicate for light.”

In the future, Guenneau said that he plans to fabricate the acoustic metamaterials studied in this paper, as well as investigate hydrodynamic and seismic metamaterials, which control water and geological waves, respectively.

Explore further

New 'thermal' approach to invisibility cloaking hides heat to enhance technology

More information: T. M. Chang, et al. “Enhanced control of light and sound trajectories with three-dimensional gradient index lenses.” New Journal of Physics 14 (2012) 035011. DOI: 10.1088/1367-2630/14/3/035011
Journal information: New Journal of Physics

Copyright 2012 PhysOrg.com.
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User comments

Apr 02, 2012
Such a lenses are called the Luneburg lens and they're known and used a long time, for example in some satellites. They're made of layers of polystyrene of variable density.
Nothing very new in this context, we want our money back.

Apr 02, 2012
I also thought that this was common practice for optical cables.

Apr 02, 2012
Really? You are going to criticize the work based on the blurb on Physorg, cite wikipedia and some company selling lenses, but not even bother to read the abstract linked at the bottom of the article?

Applications are sought in the design of Eaton and Luneburg lenses bending light at angles ranging from 90° to 360°

Go away AWT boy. This is progress in existing science and they don't claim it to be otherwise. Sorry if it is too tangible for your liking and not strictly intuitive enough.

Apr 02, 2012
The bending of light with Luneburg lenses is notoriously known, too.
The censorship of the opponents is everything, what the proponents of science can do, when facing the critique. Nothing very new or even surprising in this extent.

Apr 02, 2012
Nothing very new in this context, we want our money back.

May be what you want is something more interesting. Such as is the conventional interpretation which said that light wave could propagate by itself (via mutual creation between electric field and magnetic field), right or wrong? See below


Apr 02, 2012
This could lead to the most incredible sound reproduction. Imagine loudspeakers that made recordings sound more "live" than anything available, and didn't cost $50,000 for the pair, but a fraction of that.

Apr 03, 2012
Sub: Reflector Concepts- Cosmology Studies-Dimensional frame
Cosmology Vedas Interlinks- Source, Fields, Flows, Reflectors.
Search further - Cosmic Pot Energy of the Universe

Apr 03, 2012
This fancy lens is generating some very weird comments.

Apr 03, 2012
"sometimes, thinking in terms of sound waves allows us to better grasp the foundations of transformational optics, as light waves are more elusive (light has complex physical properties such as polarization and moreover, it can propagate in vacuum unlike sound waves, which is something that still puzzles me).

Well, Mr. Guenneau, let's start here: http://www.howstu...ight.htm

and end here: http://www.howstu...info.htm

Apr 07, 2012
He doesn't understand why sound waves do not propagate in vacuum. They can't because auditory vibrations affect the medium at the macro level. Optical light waves are the result of interaction at the atomic, electron cloud scale. That's why we can't see optical light waves from a black hole, because the electrons are stripped from their nuclei once matter crosses the event horizon. X-rays and gamma rays result from interaction at the nuclear level, sound waves at the macro-object level. Space does not provide a medium for the propagation of sound waves.

Apr 08, 2012
For those interested in the analogy between a variable refractive index and curved space-time in general relativity see http://en.wikiped...e_vacuum and the paper "Polarizable-Vacuum (PV) representation of general relativity" which is linked at the bottom of that Wikipedia page.

This allows a mathematically precise but intuitive euclidean-space alternative to the usual curved space-time view. (It is still just an useful analogy, however and does not account for frame-dragging effects (observed) or gravitational radiation (indirectly observed).)

Apr 09, 2012
In layman's terms, we have curved the metric of space in the electromagnetic and acoustic contexts

If they think those are 'layman's' terms, they vastly overestimate joe six-pack's intelligence!

Apr 10, 2012
Yes Tausch, vibrations. Trap a ruler so that most of it sticks out over the edge of a desk, then while holding it down under pressure, make it vibrate with the other hand by pushing down on the cantilever portion and letting go fast. Follow me so far? If you pull the ruler back slowly you are changing the frequency of the vibrations. You can hear them. Sound waves are caused by vibrations. So are light waves and so on. The ability to detect these vibrations by hearing them is a function of the wave's frequency. You can only hear vibrations if they are tuned to the range which is detectable by the apparatus in your ears, and then only if the medium is sufficiently dense with particulate matter to facilitate propagation of the audible waves.

I suspect that this explanation might be too complex for you, Tausch. Oh, well, keep the company that you can relate too.

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