I admit this failing: I am an optics geek, and all things light-related tend to get me... um... excited. But a recent paper in Nature Photonics presented a result that most people will find very startling: researchers have created a material which prevents light from exhibiting diffraction.

Some of you with experience in optics may be thinking that this is nothing special, because you have heard of solitons. For those of you who haven't heard of solitons, let me introduce you to them and then explain why this is not a typical soliton.

The sort of spatial soliton that most people refer to involves a wave that compensates for its own diffraction. The way this works is that the laser light has an intensity profile that is most intense near the center and least intense in the wings. If you send it through a material that is nonlinear, it will face a refractive index that will depend on the light intensity. The result is that the light creates a lens as it moves. The lens focuses the light while the natural diffraction of the light beam causes it to expand. At the right intensity, the two balance out and the light beam continues to move through the material without expanding.

But the size of the light beam that you put into the material is not the same as the light beam that you get out, since it will contract to some diameter that allows the focusing effect to balance diffraction. This size depends on how bright the light field is as well. A bright light field will contract to a smaller diameter than a weaker light field.

How does this differ from the research in the Nature Photonics paper? In the new material that the researchers made, there is no mechanism by which the diffraction is compensated. Instead, the material simply doesn't allow diffraction to occur—we will get to how this occurs later—meaning that the light field can't expand or contract. Indeed, what you put in is exactly what you get out, independent of the light intensity, making this very different from a normal soliton.

Now we get to the difficult part: how does this come about? The material consists of potassium, tantalum, and niobate, along with some impurities of copper and lithium. As the material cools after it is mixed, it creates a bunch of different regions that have slightly different arrangements in their crystalline structure, resulting in nanometer sized regions that have slightly different linear and nonlinear optical responses. The size and order of these regions depends on how fast the mix is cooled, so with some experimentation, the researchers can control the nonlinear optical properties of the material.

The end result is that, for some cooling rates, the researchers create a material that doesn't have any of the expected self focusing because the nonlinear properties vary on too fine a scale. Instead, the way the refractive index changes depends on how fast the brightness of the laser beam changes as we move out from the center of the laser beam—a property referred to as the gradient. Usually, this is very weak, but the random and very fine structure of the ceramic allows this gradient effect to dominate.

The critical thing about the gradient is that, for all normal laser beams, it is independent of the maximum intensity of the laser beam. So, it doesn't matter how intense the laser beam is, it will always form a spatial soliton that is exactly like the input beam.

The question then becomes: what do we use it for? I am not sure that the authors of the paper have a clear idea about this either. They mention possibilities for imaging, because if you focus the light at the entrance of the material, it is still focused on exiting, but I'm not quite sure how this would help. Nevertheless, it is a very cool development, and I suspect that applications will follow.

Nature Photonics, 2011, DOI: 10.1038/NPHOTON.2010.285

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