New IR birefringent material could improve optical systems, including microscopes

The barium titanium sulfide crystal has 50 to 100 times greater optical birefringence in the mid-IR than any other crystal.

Birefringent optical crystals have uses in devices such as lasers, filters, microscopes, and tabletop optical-system components.

Now, a team of scientists and engineers led by the University of Wisconsin–Madison and the University of Southern California (Los Angeles, CA) have created a crystal that has a higher degree of optical anisotropy than all other solid substances on earth, at least for infrared light.1

"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-IR transparency window, an especially important range of wavelengths that penetrate Earth's atmosphere with little absprption.

The new crystal has roughly 50 to 100 times greater optical birefringence for mid-IR light than has ever been measured before. This spectacular light-splitting ability comes from a molecular structure consisting of long chains of atoms arranged in parallel rows.

Made of barium titanium sulfide (BaTiS3), the crystal has a quasi-one-dimensional structure and maximizes its birefringence via a specific selection of constituent ions. Its birefringence of 0.76 is broadband, covering the mid- and long-wave IR regions.

The scientists are filing a patent on the material through USC and the Wisconsin Alumni Research Foundation at UW–Madison. Other collaborators included scientists at the University of Missouri and the Air Force Research Laboratory at Wright-Patterson Air Force Base.



1. Shanyuan Niu et al., Nature Photonics (2018); doi: 10.1038/s41566-018-0189-1.

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