A new study has revealed the unexpected optical properties of pencil lead.
Optical materials are the building blocks of the modern world. They are essential components of a wide range of devices, from smartphones to medical imaging equipment. Optical materials, such as glass, plastics, and crystals, interact with light in specific ways, allowing us to capture and manipulate light for various applications. For example, glass can be used to create lenses that focus or disperse light, making it essential in devices such as eyeglasses, cameras, and microscopes. Crystals can modify the wavelength and the phase of light, enabling technologies such as holography and optical computing.
However, optical materials often come at a high cost. This is because their production involves intricate processes, such as precise cutting, polishing, and coating, followed by strict quality inspections to guarantee their reliability. Further, the sustainability of these materials is a concern, as they are frequently made from rare or nonrenewable resources, such as rare-earth elements (e.g., lanthanum, neodymium, yttrium).
With these issues in mind, scientists Tomoya Kamine, Yusuke Okada, Shouhei Koyama and Hiroshi Morawaki from Shinshu University in Japan have sought to develop an alternative to traditional optical materials. In their recent study, they explored the possibility of turning pencil lead, a common everyday material, into a functional optical material using plasma.
In a previous study, the scientists have discovered that irradiating pencil lead with plasma causes it to develop various structural colors. They attributed this phenomenon to the interference of light within the thin layers of clay and graphite present in the pencil lead. Building on this insight, they conducted a more in-depth experiment in their recent study. They prepared pencil lead samples and placed them in a plasma chamber for different durations, ranging from ten seconds to over three minutes. Then, they measured the changes in the reflectance spectra of the samples.
The results revealed that prolonged plasma exposure can transform pencil lead into an optical material capable of producing interference in the near-infrared and mid-infrared ranges of the electromagnetic spectrum. The scientists ascribed this change to the plasma’s ability to remove the graphite layer from pencil lead and expose the underlying thick layer of clay. This clay layer acts as an optical interference filter for infrared light. That is why when the scientists etched letters onto the surface of a pencil lead sample, the letters could be seen only using an infrared camera.
The study has demonstrated how pencil lead, a common and environmentally friendly material, can be utilized as an optical material. Hopefully, it could pave the way for the development of innovative and sustainable optical technologies.