Sulfur Copolymers Boost IR Optics

A new polymer made from sulfur demonstrates high transmission of IR light that could greatly enhance thermal imaging systems.

While crystalline IR materials such as silicon, germanium, zinc selenide and halide salts are all frequently used for IR optics, they can be difficult to form into the complex shapes that are often needed for high-performance optics.

A team from the University of Arizona conducted a study on sulfur because it is a heavy atom present at small concentrations in many organic compounds, can be easily molded and has high transmission properties from 600 nm to 6 µm.

A mid-infrared thermograph. The eyeglasses are dark due to IR absorption in the conventional optical plastic. The round sulfur copolymer window is transparent in the MIR. Courtesy of SPIE.

Using a new technique called inverse vulcanization, the team developed sulfur-rich copolymers that provided improved transparency at near-infrared and mid-infrared wavelengths, as compared to conventional organic polymers.

In the study, the researchers melted and heated elemental sulfur (S8) to temperatures higher than 159 °C. The S8 rings opened and bonded together to create polymeric sulfur. However, pure polymeric sulfur is not stable at ambient temperatures, the researchers said, so when the liquid cools, the polymer chains may break apart and revert back to elemental sulfur. The team found this could be avoided by directly reacting the molten sulfur with diisopropylbenzene (DIB).

The polymeric sulfur’s refractive index and transmission spectrum depend on the ratio of DIB to sulfur. Both are transparent from the NIR up to 5 or 6 µm with roughly a 0.5-µm-wide absorption peak around 3 µm. Such polymers could replace semiconductor optics and chalcogenide glasses used in existing IR systems.

The researchers will next study the mechanical and optical properties of this copolymer at different temperatures.

The new research was published online by SPIE Newsroom (doi: 10.1117/2.1201407.005508).

For more information, visit www.arizona.edu.

Related: Earlier this year, the same team conducted a separate study in which they used waste sulfur to create thin, inexpensive plastic lenses for IR imaging devices.