A research team led by Professor Wie Jeong-jae of the Department of Organic and Nano Engineering at Hanyang University recently developed a high-performance infrared polarizer with an extinction ratio that is 19 times higher than that of previous studies by synthesizing and nanostructurizing polymers with sulfur as the main chain.

Although elemental sulfur, a by-product generated as a result of the desulfurization process during the petroleum refining process, has the advantage of being produced with high purity unlike general waste, 7 million tons of surplus sulfur is generated every year due to its insufficient use.

In order to solve this problem, research has recently been actively conducted to find a practical application by synthesizing a polymer having a high content of sulfur through a reverse vulcanization reaction in which a crosslinking agent having two or more vinyl groups is added to elemental sulfur. In particular, sulfur polymers with a high content of sulfur, an inorganic material, can be used as an infrared optical material because they have high transmittance in the infrared region unlike conventional carbon-based polymers.

When these infrared optical materials are used in polarizers, they can obtain clearer infrared images due to the characteristics of polarizers that can selectively transmit only desired light components, which can improve performance militarily such as night vision and industrially as in remote gas detection.

In general, infrared polarizers are mostly made of expensive semiconductor materials or ceramic-based materials such as chalcogenide. In order to make nanostructures with those materials, complex optical lithography processes is required. In addition, they are expensive but very vulnerable to external scratches and are burdensome to use in harsh conditions. However, when inexpensive elemental sulfurs are used as a polymer with good thermal processability, both the material and the process have great economic advantages. Thus, even if they are scratched, they can be replaced without burden.

The performance of the polarizer is expressed as an extinction ratio, which is the value obtained when the value of transmission of transverse magnetic field is divided by the value of transmission of transverse electric field. An ideal polarizer has a transmission of transverse magnetic field value of 100% and transmission of transverse electric field value of 0% resulting in infinite extinction ratio. 

In addition, excellent polarizers not only require high extinction ratio, but also an overall high transmission of transverse magnetic value in the target wavelength region. Professor Wie Jeong-jae's research team collaborated with Dr. Koo Ja-hyun of the U.S. Air Force Research Institute to design the dimensions and shape of a double-layer polarizer with high extinction ratio and transmission of transverse magnetic field value for a wide mid-infrared region through optical simulation.

The designed polarizer consists of a double layer of gold with a nanolattice structure and a sulfur polymer-based spacer (see Figure) and is manufactured through a total of 3 processes. (1) A sulfur polymer solution is spin-coated on a silicon substrate to make a sulfur polymer thin film; (2) a formation of nanolattice structure through nanoimprinting; (3) a gold deposition process on a formed nanolattice.


[Figure 1] Figure showing how to manufacture a sulfur polymer based polarizer.

The sulfur polymer can form a sulfur polymer film in which thickness is adjusted by controlling the concentration and spin speed of the sulfur polymer solution during the spin coating process, and the depth of spacer layer can be freely adjusted at level of tens of nanometers by nanoimprinting over the film. When incident ray passes through the polarizer, numerous reflections occur in the spacer layer between the gold layer and the silicon substrate.

When the phase of these reflected rays become identical, constructive interference occurs due to a phenomenon called Pabry-Férot resonance, resulting in maximum transmittance of the transmitted transverse magnetic field value. Fine adjustment of the spacer layer thickness is essential because the thickness of the spacer layer depends on the satisfaction of the Pabry-Férot resonance required to maximize polarization performance in the broadband infrared region.

In addition, if the nano-lattice structure designed through simulation is implemented with low quality in the manufacturing process, the nano-lattice shape is manufactured in a round curved shape rather than a right angle, and polarization performance is rapidly reduced, which requires optimal nanoimprinting conditions. In the actual manufacturing process, the thickness of the spacer layer of the sulfur polymer-based polarizer could be widely adjusted in the range of 90-5100nm. The thermal nanoimprinting conditions such as temperature, pressure, and time were systematically studied, in consideration of rheological and thermal properties of the sulfur polymer.

Accordingly, it showed up to 19 times improvement over the extinction ratio shown by the previously reported sulfur polymer-based polarizers. Professor Wie said, "We expect to contribute to the formation of a new value chain by securing technology for manufacturing high-performance polymer-based infrared polarizers using waste."

The study was conducted by the support from the Ministry of Trade, Industry and Energy (No. 20011153), The US Air Force Asian Office of Aerospace Research & Development (AOARD); FA2386-22-1-4086) It was supported by the Korea Research Foundation (NRF-2022R1A2C2002911) and was internationally conducted by Dr. Koo Ja-hyun of the Air Force Research Laboratory, Dr. Kim Joon-oh of the Korea Research Institute of Standards and Science, Professor Lee Ji-hwan of Purdue University, and Professor Kang Sang-woo of the Korea Research Institute of Standards and Science.

The paper (Title: Highly Sensitive and Cost-Effective Polymeric-Sulfur-Based Mid-Wave Length Infrared Polarizers with Tailored Fabry-Pérot Resonance) was published by Hanyang University's Cho Woong-bi (who currently is taking the MS-Ph.D. integrated course at the Department of Organic and Nano Engineering) as 1st author along with Professor Hwang Jae-hwan (Purdue University’s Biomedical Engineering department), and Alumni Lee Sang-yun(Inha University’s Department of Polymer Science and Engineering) on international journal Advanced Materials (IF=32.086). 


[Figure 2] Photographs and scanning electron microscope images of sulfur polymer-based polarizers. The yellow area of the scanning electron microscope represents a spacer layer. During the solution process, the thickness of the spacer layer can be adjusted on a nanometer scale according to the concentration used.

[Picture 3] (from the left) Professor Wie Jeong-jae of the Department of Organic and Nano Engineering at Hanyang University, and Cho Woong-bi, a student of the integrated master's and doctorate course