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A mirror with a peephole

MIT graduate student Yichen Shen designs a photonic crystal system that lets light pass through at a specific angle.
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Yichen Shen
Yichen Shen
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When light shines through air onto water, some of the light usually will be reflected back into the air. But at one specific angle, called the Brewster angle, all of the p-polarized light travels into the water with no reflection. MIT graduate student Yichen Shen found a way to manipulate that Brewster angle in a specially designed photonic crystal mirror, achieving control over the light by controlling its angle of travel into a material.

A graduate student in MIT physics professor Marin Soljacic's Photonics and Modern Electro-Magnetics Group, Shen was lead author of a March 28 Science article, "Optical Broadband Angular Selectivity," describing the behavior of a sample stack of about 100 photonic crystal layers with alternating dielectric constants, or refractive indexes. The stack acts as a mirror at most angles, but becomes transparent at its so-called Brewster angle.

"We knew for a while that we wanted such a system, but we didn't really have a good solution," Soljacic says. "Then my student, Yichen, had this great idea how we could make it work. We had some idea how to make it work in systems that are very, very difficult to fabricate, probably even impossible. One of the biggest challenges in nanotechnology in general, and hence also in nanophotonics, is fabrication. Yichen figured out how we can see the effect we want, this angular selectivity, even in a system that's quite amenable for fabrication. So that's what enabled us to make this huge leap."

Shen is also working on several other projects. One involves creating materials that change color in a dynamic way — for example, a material that turns from blue to red when it is stretched. Another project is studying butterfly wings to determine what creates their color and if he can develop materials that mimic that mechanism.

Hands-on work

Shen conducted the angular selectivity experiments with second author Dexin Ye of Zhejiang University in China. "When the light propagates at the Brewster angle in this material system, at each interface, there will be no reflection. So at that propagation direction, light just goes right through the whole material system," Shen explains.

In the experiment, the photonic crystal system was transparent at an angle of roughly 55 degrees, with about 97 percent of light (specifically, p-polarized light) transmitted across the sample. Nearly the full spectrum of visible light was transmitted. Shen says by increasing the number of layers in the stack, he can narrow the width of the angular window. "Because the Brewster angle depends only on the materials' refractive index, and the refractive index for many materials is not that sensitive to the wavelength of the light, we can keep the Brewster angle the same for a broad band of frequencies," Shen explains.

"In order to achieve angular selectivity, what I also need to do is to prevent light from propagating at all the other angles. In order to achieve that, I simply vary the thickness of each layer," he says. With a system of about 100 alternating layers, Shen produced many interfaces. By varying and controlling the thickness of each layer, he was able to reflect the light that propagates at all other angles except the Brewster angle.

Taking the next steps

In the initial angular selectivity work, the Brewster angle for the system was fixed based on the refractive indexes of the materials. Shen presented his latest work at the Conference on Electro Optics (CLEO), in San Jose, Calif. On June 9, he presented his study, "Metamaterial Broadband Angular Selectivity," which extends the angular selectivity work by showing that the Brewster angle can be varied by changing the mix of photonic crystal structures and the thickness of individual layers within the stack and the overall height of the stack. That work is pending publication.

Shen, Ye and colleagues showed the new feature of tunability in a microwave demonstration using a simple photonic crystal system composed of a commercially available laminate designed for microwave circuits and polypropylene plastic interacting in air. The principle could be applied to new applications in microwave and optical frequency regimes, including radar tracking and laser scanning. Shen also presented his Brewster angle work, "Optical Broadband Angular Selectivity," at CLEO, on June 12.

Shen is also writing a paper focused on applying angular selectivity to specific applications, such as increasing privacy protection for visual displays, improving signal-to-noise ratios for light detectors, including telescopes, and boosting solar thermal energy harvesting. The group has filed patent applications for its work.

The work, which is partially funded by the Department of Energy-funded Solid State Solar Thermal Energy Conversion Center, promises better solar energy generation. In a solar thermal system, as the system heats up, it begins to lose energy through black body radiation from the absorber. "The higher the temperature, the more severe the radiation loss is from the absorber," Shen explains, "but imagine if we have a filter that only lets light pass through at one angle and reflects the light from all other angles. Because sunlight only comes in at one angle, we let that angle face the sunlight, so all the sunlight comes in to our absorber, but most of the radiated light is at other angles and that radiated light will hit the filter and be reflected back to the absorber. So that portion of the energy will be saved and that could potentially increase the efficiency of solar thermal systems."

"Basically, the idea of achieving this angular selectivity is quite simple," Shen adds. "It's just depositing multi-layer films. I think if there are some people really interested in using this technology, there are no technical obstacles to achieving it."

Shen received joint bachelor's degrees in mathematics and physics, as well as a master's in physics at Johns Hopkins University in 2011. He attended Nanyang Technological University in Singapore before transferring to Johns Hopkins in 2009.

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