Crystals with Giant Optical Anisotropy

Artist’s rendering of a new material that splits light more dramatically than any other substance on Earth. (Source: T. Spencer)

Place a chunk of the clear mineral Iceland spar on top of an image and suddenly you’ll see double, thanks to double refrac­tion caused by optical aniso­tropy. Beyond just a nifty trick, materials with optical aniso­tropy are vital for a variety of devices such as lasers, liquid-crystal displays, lens filters and micro­scopes. Now, a team of scientists and engineers led by the Univer­sity of Wisconsin-Madison and Univer­sity of Southern Cali­fornia have created a crystal that has a higher degree of optical aniso­tropy than all other solid substances on earth especially for infrared light.

“The optical aniso­tropy is enormous, making the material promising for a range of optics applications,” says Mikhail Kats, a professor of electrical and computer engi­neering at UW–Madison. One especially promising use for the new crystal could be imaging and other types of remote sensing using the mid-infrared trans­parency window, an especially important range of wave­lengths that pene­trate Earth’s atmo­sphere with little distor­tion. “This class of materials and this approach has a lot of potential,” says Jayakanth Ravi­chandran, a professor of chemical engineering and materials science and elec­trical engineering-electro­physics at USC. “We designed the material, made it, and saw a huge effect.”

The new crystal has roughly 10 times greater optical bire­fringence for mid-infrared light than has ever been measured before. That spectacular light-splitting ability comes from a unique molecular structure consisting of long chains of atoms arranged in parallel rows. Using advanced compu­tational methods, the researchers carefully selected rows of atoms, precisely grew them in the lab, and meti­culously studied them.

Light waves in the same beam traveling through a material with optical aniso­tropy will slow down more or less depending on polarization, a measure of the direction in which waves vibrate. Human eyes cannot see polari­zation on their own, but the ability to alter the vibra­tional orien­tation of light is essential for LCD screens, 3D movies, lasers and lens filters. Most devices that change light’s polari­zation rely on materials with optical aniso­tropy. The new material might also be useful in energy-harvesting photo­voltaic cells or light-emitting diodes. In the future, the researchers plan to explore other properties of the new material as they also work to develop stra­tegies to synthesize it in large quan­tities.

The project was a team effort involving researchers at multiple insti­tutions with varied expertise. “This is a big success for colla­borative science,” says Kats, who led the optical measure­ments, while Ravi­chandran and USC electrical engi­neering Professor Han Wang syn­thesized the material. “The wide array of knowledge and capa­bilities across our team enabled this break­through.” (Source: UW Madison)

Reference: S. Niu et al.: Giant optical anisotropy in a quasi-one-dimensional crystal, Nat. Phot., online 18 June 2018; DOI: 10.1038/s41566-018-0189-1

Link: Kats Laboratory of Applied Physics, Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, USA

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