Scientific Method / Science & Exploration

Superfluid light flows past obstacles without scattering

Scientists show that, under the right conditions, light that can flow past …

It is a truism that first impressions really count. So you would think that a paper entitled "Superfluid motion of light" that had me running off to discuss a potential experimental demonstration with colleagues would have me raving. Unfortunately, on second, third, and now fourth reading, I find myself slightly at a loss. It seems very interesting, but what is it that we are actually interested in?

The paper, published in Physical Review Letters, plays on the idea that the mathematics of some very different physical phenomena are often very similar. This means that if you find a property in one system, then you can try and figure out what the analogous property would be in the other from the mathematics. In this case, scientists have compared superfluidity with light propagation to try and figure out what superfluid motion in light might look like.

Let's start with describing what a superfluid is. A superfluid is generally a liquid that flows without dissipation. That means that when it encounters an obstacle in its path, it will flow around the obstacle without slowing or experiencing turbulence—turbulence being a source of dissipation. The explanation for this behavior lies in the quantum nature of matter, but we can boil it down to one essential ingredient: sound waves. As the atoms move and encounter obstacles, their motions set up sound waves that propagate through the liquid. In a sense, these sound waves tell the atoms where the obstacles are, so the atoms don't simply collide with them.

This tells us that a superfluid can't flow faster than the speed of sound in said fluid. And, indeed, experiments bear this out, with superfluidity shown to appear and vanish as flow speed drops below and rises above the speed of sound. So what has this got to do with light?

The interesting thing about superfluids is that the mathematics used to describe their flow are very similar to those that describe the flow of light in particular types of media, specifically, those that are nonlinear—the brightness of the light modifies the optical properties of the media. If the mathematics are the same, then it should be possible to work backwards and figure out what superfluid light would look like and how that superfluidity could be observed.

This is pretty much what Patricio Leboeuf and Simon Moulieras of France have attempted to do. So, the first question would be what would constitute a superfluid light flow. The key property of superfluid light is that it shouldn't exhibit the equivalent of dissipation. In the case of light, dissipation can be thought of as scattering. 

Imagine you have a fiber optic cable, where light pulses flow down the cable. Even when the cable is bent, light remains in the cable. However, if I put a small air bubble in the center of the cable, when a light pulse encounters that, some of the light will be reflected back down the cable, but still more will scatter out of the cable entirely. Although the total amount of light remains the same, the light propagating in the direction we intended is reduced. This is dissipation and, if it's a superfluid, light must be immune to it. In the superfluid case, light would flow around the air bubble and continue down the cable.

But wait, there's more. Superfluids have a critical flow speed as well. This is where things will get a little complicated later, so suffice it to say that such a speed does exist, but it isn't the speed of light in the medium.

Now that we've got these predicted behaviors, how do we make it happen? The idea presented by the authors is to create an array of closely spaced waveguides made from material that is nonlinear in its response to light. 

When light is confined in a waveguide, part of it hangs outside the waveguide. If two waveguides are closely spaced, then the part of the light outside of the first waveguide will be inside the second waveguide. This allows light to tunnel from one waveguide to another. So, in this array of waveguides, light hops back and forth across the different waveguides as it propagates along. Furthermore, because the brightness of the light modifies the optical properties of the waveguides, the rate of hopping depends on how much light is hanging around in a particular waveguide.

The end result is that light zig-zags its way along the waveguide array with its forward speed reduced. There is now a transverse speed as well—how fast the most intense part of the light pulse jumps from waveguide to waveguide. This transverse speed is determined by how intense the light pulse is: brighter pulses hop faster.

Leboeuf and Moulieras show that if an obstacle is placed in the array, then, provided the transverse speed of the light pulse is less than about one-hundredth of its speed along the waveguide, the light pulse is not scattered and continues along the array. As the light intensity increases, the transverse speed increases and the zig-zag motion becomes disrupted as light flies off in all directions. Well, actually, since this is a 2D simulation, it can only fly in two directions, but you get the picture, I hope.

This paper has me quite excited, because it probably isn't that difficult an experiment to perform. We just have to decide if we want to do it. As for what it might be good for, or mean for light transport, I really have no idea yet. It just seems interesting.

Physical Review Letters, 2010, DOI: 10.1103/PhysRevLett.105.163904

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19 Reader Comments

  1. What is the relation to solitons, if any?
    1198 posts | registered
  2. Might it be good for fiber-optics over shorter, lower-quality cabling? The distance is low, so attenuation from non-linear media would be less problematic, but the cabling itself could have imperfections line your hypothetical bubble.
    3773 posts | registered
  3. Well, if dissipation is not an issue, a low intensity light would continue in this medium forever. So, even for miles long cables, what you put is what you get.

    It's just weird that there is no mention of how big the bubble can be. It seems to me that there must be a relationship between the size of the bubble and the speed ratio (longitudinal / transversal). The "one hundred-ish" seems too "engineery" for my taste. ;)
    3 posts | registered
  4. What's it good for? How about a long-distance optical cable, such as a trans-oceanic cable without the need of boosters at regular intervals?
    387 posts | registered
  5. Does this "superfluid" effect cancel out absorption as well? Flowing around a bubble or other dissipative flaw without loss is one thing, but if all the nonlinear waveguides had even a small uniformly distributed absorption (which would seem to be the real-life situation), I don't see how the light could continue without loss. Unless the waveguides had gain.
    376 posts | registered
  6. Yes, the critical speed depends on the size of the obstacle as well, but that is relative to the size of the waveguide and the refractive index contrast of the waveguide... When all of that is taken into account, then for most "largish" obstacles it flattens out to about 1/100th of the speed of light.

    It isn't really useful for communications purposes because the speed of pulse propagation is low, and, no, it certainly can't overcome absorption unless there is some kind of gain (I suppose you could have some kind of four wave mixing gain, since it is nonlinear).

    Finally, yes, this is very related to solitons. In fact, when the pulse is traveling at a speed above the critical velocity and it runs into the obstacle, part of the scattered light becomes a soliton. So I would guess that you go from superfluidity -> solitons in terms of increasing intensity, even if there is no scatterer in the waveguide
    1472 posts | registered
  7. I agree, on the face of it looks fairly straightforward to demonstrate. Couple some ultrashort pulses into nonlinear waveguide arrays with varying separation between array elements and thus varying coupling between waveguides. Introduce a defect some distance along the array and vary the power to see if the effect breaks down resulting in a reduced output due to the scattering.

    The substrate material would need to have a negative n2 so that higher powers reduce mode confinement and increase evanescent coupling right?

    Its an interesting phenomenon for sure but I also can't see a practical use!
    1 post | registered
  8. When money and effort are spent on research it's tempting to ask what the immediate practical use and value of the outcome is and normal to feel disappointed when the researchers don't have a practical use ready to wow the world with.

    I like to take a fairly romantic view of scientific research (because I'm in the fortunate position of being the researcher, not the person/institution handing out the funding) in that it's all about the pursuit of knowledge anyway.
    If a practical use comes of the research immediately, great!
    If a practical use becomes apparent somewhere further on (when someone else is looking through existing research for an answer to an engineering conundrum), great!
    If a practical use never shows up, well... we still learned something, even if it's something 'useless'. It probably still enhances what we know about a related topic, or as the article says, it might give insights into a phenomenon in a loosely related field.

    I'm admittedly biased in favour of blue sky science but to me that's what science and research are all about, so the fact that it's interesting is enough for me!

    (The funding bodies may not agree.. )
    25 posts | registered
  9. @Zurechial - Well answered. Interesting to see the the actual experiment follows the theory correctly.
    1321 posts | registered
  10. I agree wholeheartedly with @Zurechial. But because this tension exists in the real world, the media is apparently REQUIRED to speculate on how this will help society.

    And of course, the ol' tried-and-true default answer is: "Drug delivery systems and life-extending procedures." I'm convinced that if you ran to a reporter to demonstrate your perpetual motion machine he'd just yawn and tell you "Meh, how does that make me live longer?"

    People are pigs. They are life pigs.
    330 posts | registered
  11. I always am reminded of the Laser when the question of the usefulness of basic research comes up. Until the 1950s, nobody knew if stimulated radiation could be made to work at all, and if asked for applications, none of the researchers would have answered "watch movies at home" or "track the movements of an input device for a personal computer".
    1028 posts | registered
  12. My first thought after reading through was the mention of the "critical speed" being equivalent to the speed of sound in a fluid ... so the speed of sound propagating in a light beam is approx 1% speed of light in the media being traversed :D

    On a more serious note:

    Nested waveguides being used to demonstrate the effect suggests a practical application. Build these waveguides into nanotech as part of the next generation of optical circuitry. Another use would be multi-material optical fiber designed so that the innermost core is one waveguide and the surrounding middle layer is the second waveguide, wrapped by an outer layer that protects the twinned waveguide core. Not sure if this is possible, but you can be sure it is being looked into now that this research is published :)
    3413 posts | registered
  13. What medium for the light are they suggesting, exactly?
    1329 posts | registered
  14. Very cool article! Using the comparison to fiber, the multiple waveguides sound like graded-index or multimodal geometries to me. I would think that the size at which the obstacle (or impurity) becomes dissipative (or diffractive) to be a function of the half-wavelength of the light propagating through the structure. Evanescent coupling occurs in fibers as well, with that energy being dissipated in the cladding; being able to capture some of this energy and redirect it into the direction of propagation was one of the appealing aspects of graded-index fibers. Electro-optics have extended that into additional practical applications using lithium niobate and other substrates.

    I suppose the caveat here is not to grasp at a superconductive equivalent just yet. There are other noise components besides scattering.

    Love this stuff...!
    7 posts | registered
  15. 1% of the speed of light in any transparent material is still pretty fast. And as for possible uses, Just coat any object with this and make the incident light become "superfluidic". Presto; another kind of invisibility cloak!
    11529 posts | registered
  16. any chance that there might be an explanation for or against the wave particle duality of light here?
    2 posts | registered
  17. I am sooooo tired of the experience of hearing or seeing otherwise seemingly literate people say or write "try and". This author does it more than once.

    NO!!! The correct phrase is "try to". The phrase "try and" makes no linguistic sense at all, and makes you appear ignorant.

    This is not a nitpick. It is a basic rule of English. Please get it straight.
    253 posts | registered
  18. Oh sh*t, the grammar-police is here. Let's try and avoid them :)
    1308 posts | registered
  19. Such a stark contrast between this thread and the one not long ago about Houston's muni wifi research project. Here, the practical use can only be imagined, so it's kind of okay... But when dealing with something that arguably has immediate practical use, and something lots of people seem to think they understand well like wifi, there's so much outrage, cries of socialism, and wasted government money. It's pretty sad.

    The laser mouse example above is terrific. Understanding more about our world and universe will never be truly useless. We'll use it someday, in ways we just can't presently imagine.
    2708 posts | registered

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