Meta-Materials

A subject that has held some interest for me these past few years is the area of meta-materials. These are man-made materials that have special properties that do not exist in nature. Among the many unique properties that these materials have, the one that I find especially fascinating is their negative refractive Index (RI).

FIGURE 1

First, a short optics lesson. The reason we are able to see is due to light striking an object, bouncing off said object, and then entering our eyes. The way the light hits our eyes is determined in part by the RI of the object that we are looking at, and in part by the RI of the medium the light is traveling through (e.g. water, air, etc.) If you want to understand the effect that RI has on our vision, take for example a pencil in a glass of water. The portion below the surface appears to jut off at an angle due to the difference between the indices of refraction of air and water (FIGURE 1).

FIGURE 2

Now, even though different materials and media have different RIs, all of them have a POSITIVE RI. In other words, they all cause light to be bent in the same direction, if at different angles. This was true for every object under the sun until the advent of meta-materials. Meta-materials have a NEGATIVE RI. They cause light to be bent in the opposite direction (FIGURE 2).

This characteristic has many interesting effects, but the one I want to focus on is the ability to make things invisible. You read that correctly; invisible. Like Harry Potter’s invisibility cloak.


The math describing how such a thing is possible is perhaps beyond the ken of the average person, but conceptually, it’s actually quite easy to understand. Instead of having a positive RI and causing the light to reflect off of the object and into our eyes, a meta-material object, with its negative RI, causes light to be bent completely around the object, rendering it invisible. To better help you understand, visualize a boulder in a stream. When the water of the stream (light) encounters the boulder (the meta-material object), it parts to go around the boulder and reforms on the other side. If one were to look at the water just a few feet beyond the boulder there would be no evidence that the boulder is there.


But before you rush out to Macy’s to buy yourself some cream that will make those varicose veins disappear, keep in mind that a few big obstacles remain before we have true invisibility. A meta-material can only affect light waves if it has structural features smaller than the wavelength of the light wave it's trying to affect. In addition, scientists have been finding it difficult to make objects invisible across more than a few frequencies of light at a time. Thus far, scientists have had some success making objects invisible to the microwave portion of the electromagnetic spectrum, which has a wavelength of a millimeter or more. But causing objects to become invisible to the visible portion of the spectrum, which has a wavelength of 400-750 nanometers -or billionths of a meter- is considerably harder to accomplish. 


While it might be a while before invisibility via meta-materials is perfected and made available, it’s certainly exciting to think of all the ways it will be harnessed once it does get here.
Aside from the obvious military applications, there would be many practical consumer applications. For years, there has been opposition to building a wind farm off of Nantucket Sound in Massachusetts. One of the primary concerns has been residents’ fears that large wind turbines will spoil the view. If these wind turbines were built out of meta-materials that rendered them invisible, this would not be a problem. Just erect fences around the bottom of each turbine to prevent ships from running into them and voilà! Problem solved. The same solution could be applied to unsightly cell towers. Now you see it, now you don’t.
What if you buy a ticket to a baseball game and your seat is behind a support pillar? Does it mean you won’t be able to see the game? Not if the portion of the pillar at eye level has been made invisible by applying a meta-material coating to its surface.


These are clearly just a few of the many applications such a wondrous invention will make possible. What other ways will we be able to utilize this technology? I guess we’ll just have to wait and see. Or maybe we won’t see; after all, they will be invisible.