A team from Harvard University’s John A. Paulson School of Engineering and Applied Sciences (SEAS) has developed rewritable optical components for surface lightwaves. When a wave of light is confined on a 2D plane by certain materials, it becomes something known as a polariton — a particle that blurs the line between light and matter. Polaritons enable light to be tightly confined to the nanoscale, even potentially to the thickness of a few atoms. That capability poses interesting implications for the future of optical circuits. Unlike electronic integrated circuits, integrated optics is difficult to miniaturize with commonly used materials: All of the current methods of controlling light are 3D. A 2D prism. Courtesy of Harvard SEAS. “The ability to control and confine light with fully reprogrammable optical circuits is vital for future highly integrated nanophotonic devices,” said Michele Tamagnone, a postdoctoral fellow in applied physics at SEAS. In previous research, the team demonstrated a technique to create and control polaritons by trapping light in a flake of hexagonal boron nitride. In this study, the researchers put those flakes on the surface of a material known as GeSbTe (GST) — the same material used on the surface of rewriteable CDs and Blu-ray discs. “The rewriteable property of GST using simple laser pulses allows for the recording, erasing, and rewriting of information bits. Using that principle, we created lenses, prisms, and waveguides by directly writing them into the material layer,” said Xinghui Yin, a postdoctoral fellow at SEAS and co-first author of the study published in Nature Communications. The lenses and prisms written into the material are 2D. Instead of having a semispherical lens, the polaritons on the material pass through a flat semicircle of refracting material that act as a lens. Instead of traveling through a prism, they travel through a triangle, and instead of optical fibers, the polaritons move through a simple line, which guides the waves along a predefined path. Using a technique known as near-field microscopy, which allows for the imaging of features much smaller than the wavelength of light, the researchers were able to see these components at work. They also demonstrated for the first time that it is possible to erase and rewrite the optical components they created. “This research could lead to new chips for applications such as single-molecule chemical sensing, since the polaritons in our rewriteable devices correspond to frequencies in the region of spectrum where molecules have their telltale absorption fingerprints,” said Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering. The research was published in Nature Communications (www.dx.doi.org/10.1038/s41467-019-12439-4).