This image shows theoretical (right) and experimental (left) iso-frequency contours of a photonic crystal slabs superimposed on each other. (Courtesy of the researchers)
The internal characteristics of photonic crystals have been revealed with a new technique developed by MIT researchers. These photonic crystals are synthetic materials whose exotic optical characteristics are the subject of extensive research.
Generally, photonic crystals are developed by drilling millions of closely spaced, tiny holes in a slab of transparent material, using variations of microchip-fabrication techniques. These materials exhibit a wide range of peculiar optical properties, including “superlensing,” based on the exact orientation, spacing and size of these holes.
Superlensing enables magnification that goes beyond the normal theoretical limits, and “negative refraction,” in which light is bent in a direction opposite to its path through standard transparent materials.
Extremely complex calculations will help to understand how light of different colors and from various directions passes through photonic crystals. Highly simplified approaches have often been used by researchers. For instance, the researchers can only calculate the behavior of light for a single color or along a single direction.
The new method makes the full range of information directly visible. A straightforward laboratory setup can be used by researchers to display the information, a pattern of so-called “iso-frequency contours”, in a graphical form that can be just photographed and analyzed, in several cases eliminating the need for calculations.
The latest issue of the journal Science Advances describes this technique in a paper presented by MIT postdoc Bo Zhen, recent Wellesley College graduate and MIT affiliate Emma Regan, MIT professors of physics Marin Soljačić and John Joannopoulos, and four others.
Zhen explains that the discovery of this new method was a result of looking closely at a phenomenon that the researchers had noticed and then used for years. However, the origins this phenomenon was not earlier understood by the researchers. Illuminating the samples of photonic materials by laser light resulted in generating patterns of scattered light from the samples.
The scattering was surprising as the underlying crystalline structure was fabricated to be almost perfect in these materials.
When we would try to do a lasing measurement, we would always see this pattern. We saw this shape, but we didn’t know what was happening.
Bo Zhen, Postdoc, MIT
But it helped them to get their experimental setup aligned correctly, as the scattered light pattern would appear instantly after the laser beam was accurately lined up with the crystal. Following a thorough analysis, the researchers understood that the scattering patterns were generated by small defects in the crystal, referring to holes that were somewhat tapered from one end to the other or not perfectly round in shape.
“There is fabrication disorder even in the best samples that can be made,” Regan says. “People think that the scattering would be very weak, because the sample is nearly perfect,” but it seems that at specific frequencies and angles, the light scatters extremely strongly; as much as 50% of the incoming light can be scattered.
Illuminating the sample in turn with a variety of colors allows building up a complete display of the relative paths taken by the light beams all across the visible spectrum. A direct view of the iso-frequency contours is produced by the scattered light. These contours are a type of topographic map of the way light beams of varied colors bend as they travel through the photonic crystal.
“This is a very beautiful, very direct way to observe the iso-frequency contours,” Soljačić says. “You just shine light at the sample, with the right direction and frequency,” and what comes out is a direct image of the required information, he says.
The team suggests that the finding can be potentially used for a wide range of applications. For instance, it could develop a way of making display screens that are transparent and large and in which most of the light passes through as if through a window, but light of particular frequencies would be scattered to develop a clear image on the screen. The method could also be used to develop private displays that are visible only to an individual directly in front of the screen.
This method relies on imperfections in the fabrication of the crystal and it could also be used as a quality-control measure to develop such materials; the images offer an indication of not only the overall amount of imperfections, but also their particular nature, that is, whether the dominant disorder in the sample generates from noncircular holes or etches that are not straight, such that the process can be tuned and enhanced.
Using a clever trick, the Soljačić group turned what is ordinarily a nuisance (i.e., unavoidable disorder in nanofabrication) to their advantage. The random scattering caused by the disorder allowed them to directly image the iso-frequency contours of the photonic crystal slab structure. Since any nanofabricated structure always has some degree of disorder, and since disorder is invariably difficult to model a priori in simulations, their method provides an extremely convenient characterization tool for photonic crystal resonant mode band structures. This could become an essential tool in the hunt for high-power single-mode semiconductor lasers (in particular, photonic crystal surface emitting lasers), with wide-ranging applications including telecommunications and manufacturing.
Mikael Rechtsman, Assistant Professor, Pennsylvania State University
The team also included researchers at MIT Research Laboratory of Electronics, including Yuichi Igarashi (now at NEC Corporation in Japan), Ido Kaminer, Chia Wei Hsu (now at Yale University), and Yichen Shen. The research was supported by the Army Research Office through the Institute for Soldier Nanotechnologies at MIT, and by the U.S. Department of Energy through S3TEC, an Energy Frontier Center.