Sulfur copolymers for IR optics

Optical imaging and optical materials have developed hand-in-hand since antiquity. Glass science has matured due to the demands 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 amorphous polymers with low light scattering to emerge as an alternative 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 applications, such as polymethylmethacrylate and polycarbonate, are not transparent in the mid-wave IR (3–5μm) or the long-wave IR (8–12μm). 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 inexpensive optical materials that are transparent in several key IR spectral regions. Neither conventional oxide glasses nor hydrocarbon polymers can fill this need because their absorption increases significantly for wavelengths longer than 2–3μm. Crystalline IR materials such as silicon, germanium, zinc selenide, 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 incorporate either arsenic, sulfur, selenium, or tellurium. All except sulfur 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 vibrations. 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 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 wavelengths. However, as we noted earlier, typical polymers have limited IR optical transparency at wavelengths longer than approximately 2μm. 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.

The synthesis begins by melting and heating pure sulfur (S₈) to temperatures higher than 159°C. The S₈ rings open and bond to one another to make polymeric sulfur. Pure polymeric sulfur is not stable at ambient temperature, so when the liquid cools, the polymer chains may break apart and revert to elemental sulfur. To make this process irreversible and maintain these high-molecular-weight sulfur polymer chains, we directly reacted the molten sulfur with 1,3-diisopropylbenzene (DIB), a divinylic monomer. By using DIB as the comonomer and controlling the ratio of DIB to sulfur, we were able to obtain different compositions for this copolymer. We made polymers with sulfur concentrations as high as 85%.

The polymers' optical properties,⁵ such as refractive index and transmission spectrum, depend on the ratio of DIB to sulfur. The materials all share an interesting and useful feature: they are transparent from the near-IR up to 5–6μm, except for a roughly 0.5μm-wide absorption peak around 3μm (see Figure 1). Therefore, this material could be used for some mid-IR optics and imaging applications. It retains the advantages of polymers, which can easily be molded into lenses or other optical elements.⁵ As Figure 2 shows, the sulfur copolymer window is transparent at mid-IR wavelengths, whereas the more conventional polymer used for vision correction in eyeglasses is opaque. Furthermore, the refractive indices of these polymers range from 1.78 to 1.85, which is much higher than the refractive index of standard optical polymers.⁶ Materials with higher refractive indices bend light more, which enables thinner lenses and thus lighter, smaller optical systems. Next, we will continue to study the mechanical and optical properties of the copolymer at different temperatures. Ideally, we want to partner with a company to test the materials in IR imaging or sensing devices.

Figure 1. Transmission spectrum for a freestanding 200μm-thick polymer 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.

Figure 2. A mid-IR thermograph of one author's face. Note that the eyeglasses are dark due to IR absorption in the conventional optical plastic. The round sulfur copolymer window, however, is transparent in the mid-IR.

New optical materials are desperately needed for IR optical technology. Moldable optics can be made at a lower cost, more easily, and with less concern about toxicity than current options. These polymers could replace semiconductor optics and chalcogenide glasses used in current IR systems, as well as opening the door to entirely new applications.