This image shows theoretical (right) and experimental (left) iso-frequency contours of photonic crystal slabs superimposed on each other. Image courtesy of the researchers.
This image shows theoretical (right) and experimental (left) iso-frequency contours of photonic crystal slabs superimposed on each other. Image courtesy of the researchers.

Researchers at Massachusetts Institute of Technology (MIT) have developed a new technique for revealing the inner details of photonic crystals, synthetic materials whose exotic optical properties are the subject of widespread research.

Photonic crystals are generally made using microchip fabrication methods to drill millions of closely-spaced, minuscule holes in a slab of transparent material. Depending on the exact orientation, size and spacing of these holes, photonic crystals can exhibit a variety of peculiar optical properties. These include ‘superlensing’, which allows for magnification that pushes beyond the normal theoretical limits, and ‘negative refraction’, in which light is bent in an opposite direction to its normal path through transparent materials.

But understanding exactly how light of various colors from various directions moves through photonic crystals requires extremely complex calculations. Because of this, researchers often use highly simplified approaches; for example, they may only calculate the behavior of light along a single direction or for a single color.

The new technique developed by the MIT researchers makes the full range of information directly visible in the form of a pattern of so-called ‘iso-frequency contours’ that can be photographed and examined. In many cases, these contours eliminate the need for any calculations. The technique is detailed in a paper in Science Advances by MIT postdoc Bo Zhen, recent Wellesley College graduate and MIT affiliate Emma Regan, MIT professors of physics Marin Soljacic and John Joannopoulos, and four others.

The discovery of this new technique, Zhen explains, came about by looking closely at a phenomenon that the researchers had noticed and even made use of for years, but whose origins they hadn't previously understood. Patterns of scattered light seemed to fan out from samples of photonic materials when the samples were illuminated by laser light. The scattering was a surprise, since the underlying crystalline structure of these materials was fabricated to be almost perfect.

"When we would try to do a lasing measurement, we would always see this pattern," Zhen recalls. "We saw this shape, but we didn't know what was happening." The pattern did at least prove useful in helping to get their experimental setup properly aligned, because the scattered light pattern would appear as soon as the laser beam was properly lined up with the crystal. Upon careful analysis, however, the researchers realized the scattering patterns were generated by tiny defects in the crystal – holes that were not perfectly round in shape or that were slightly tapered from one end to the other.

"There is fabrication disorder even in the best samples that can be made," Regan explains. "People think that the scattering would be very weak, because the sample is nearly perfect." At certain angles and frequencies, however, the light scatters very strongly: as much as 50% of the incoming light can be scattered. By illuminating the sample in turn with a sequence of different colors, it becomes possible to build up a full display of the relative paths taken by the light beams, all across the visible spectrum. The scattered light produces a direct view of the iso-frequency contours – a sort of topographic map of the way light beams of different colors bend as they pass through the photonic crystal.

"This is a very beautiful, very direct way to observe the iso-frequency contours," Soljacic says. "You just shine light at the sample, with the right direction and frequency," and what comes out is a direct image of the needed information, he says.

This finding could potentially prove useful in a number of different applications, the team says. For example, it could lead to a way of making large, transparent display screens where most light would pass straight through, as if through a window, but light at specific frequencies would be scattered to produce a clear image on the screen. Or the method could be used to make private displays that would only be visible to the person directly in front of the screen.

Because it relies on imperfections in the fabrication of the crystal, this method could also be used as a quality-control measure for the manufacture of such materials. The images not only provide an indication of the total amount of imperfections, but also of their specific nature – that is, whether the dominant disorder in the sample comes from noncircular holes or etches that aren't straight – allowing the manufacturing process to be tuned and improved.

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.