10.1117/2.1201407.005508 Sulfur copolymers for IR optics Soha Namnabat, Jared Griebel, Jeffrey Pyun, Robert Norwood, and Eustace Dereniak A novel polymer made from sulfur is easily molded and has superior transmission properties in the IR from 600nm up to 6m with a 1m stop band. Optical imaging and optical materials have developed hand-in- hand since antiquity. Glass science has matured due to the de- mands placed by the disciplines of optics and astronomy, with a tremendous range of glass compositions developed over the past 300 years. More recently, advances in chemistry allowed amor- phous polymers with low light scattering to emerge as an al- ternative to inorganic glasses because they are inexpensive and easily molded. Polymers are now widely used in visible imaging systems. However, the materials commonly used for these appli- cations, such as polymethylmethacrylate and polycarbonate, are not transparent in the mid-wave IR (3–5m) or the long-wave IR (8–12m). Indeed, when IR imagers developed, new optical materials were needed. Over the past decade, the demand for compact, low-cost IR instruments has grown, in turn creating a demand for inex- pensive optical materials that are transparent in several key IR spectral regions. Neither conventional oxide glasses nor hydrocarbon polymers can fill this need because their absorp- tion increases significantly for wavelengths longer than 2–3m. Crystalline IR materials such as silicon, germanium, zinc se- lenide, and halide salts are frequently used for IR optics, but are difficult to form into the complex shapes often needed for high-performance optics. Chalcogenide glasses 1–3 have excellent transmission properties in the mid- to long-IR, but they incorpo- rate either arsenic, sulfur, selenium, or tellurium. All except sul- fur are toxic elements, and therefore difficult to fabricate safely. An IR-transmitting polymer could overcome many difficulties associated with the currently available materials. While considering how to create nontoxic polymers for use in this part of the spectrum, we must understand why materials absorb light. IR absorption is generally due to molecular vibra- tions. More specifically, when chemically bonded atoms vibrate, they absorb IR light related to their vibrational frequencies. By modeling bonds as mass-spring oscillators, we concluded that Figure 1. Transmission spectrum for a freestanding 200m-thick poly- mer made from 80% sulfur and 20% 1,3-diisopropylbenzene (DIB) shows unusually high transmission for a polymer in most of the mid- IR. The inset shows the molecular structure for the polymer. higher vibration frequencies occur when the mass of at least one of the atoms is much smaller than the other. Standard optical polymers have an abundance of carbon, hydrogen, and oxygen, which are very light elements compared to the atoms in typical semiconductors, such as silicon and germanium, or the sulfur, selenium, or tellurium used in chalcogenides. The heavy atoms in chalcogenides shift their IR absorption to longer wave- lengths. However, as we noted earlier, typical polymers have limited IR optical transparency at wavelengths longer than approximately 2m. To make a polymer that is transparent in the mid-IR, we needed to reduce the hydrogen content and use predominantly heavy atoms. We chose to concentrate on sulfur because it is a heavy atom present at small concentrations in many organic compounds. Using a new technique called inverse vulcanization, 4 we created sulfur-rich copolymers that provide greatly improved transparency at near- and mid-IR wavelengths compared to conventional organic polymers. Continued on next page