Researchers from TU Delft and Radboud University have discovered that the two-dimensional ferroelectric material CuInP2S6 (CIPS) can be used to control the pathway and properties of blue and UV light. According to the researchers, the degree of control is unmatched by any other known material. Further, CIPS can be integrated onto chips, which can be used in integrated photonics. Blue and ultraviolet light play a critical role in chipmaking, high-resolution microscopy, and next-generation optical communication systems. As such, improving the on-chip control over these short wavelengths is vitally important for future optical and semiconductor technologies. Illustration of a 2D crystal inside a chip, where light (blue) couples to the electric field of the crystal (green). Courtesy of TU Delft. CIPS is an atomically layered ferroelectric material, which means it carries a built-in internal electric dipole due to the displacement of the copper ions that can also move inside the structure. What sets this material apart is that the motion of the copper ions is strongly dependent on the thickness of the two-dimensional crystal. The team found that this thickness-dependent ferroelectric behavior can be leverage to achieve a thickness-dependent refractive index. “Going from bulk material to a layer of only tens of nanometers thick, the refractive index of CIPS changed by almost 25% in an unexpected, ‘anomalous’ way,” said co-first author of the paper, Houssam El Mrabet Haje. The team also found that CIPS shows giant birefringence in the blue-UV range: light travelling out-of-plane through the crystal experiences a very different refractive index than light traveling in-plane. At wavelengths of around 340 nm, this difference reaches about 1.24, which is the largest intrinsic birefringence ever reported in this part of the spectrum. “This means that CIPS can act as an extremely powerful polarization and phase control element for short-wavelength light, without needing complicated nanostructuring,” said El Mrabet Haje. Although a full picture is still to be determined, the team proposed a new mechanism at work inside the CIPS crystal. “Light carries oscillating electric and magnetic fields; in CIPS, these fields couple not just to electrons, but also to the internal electric field created by the displaced copper ions,” El Mrabet Haje said. “What makes CIPS so special is that the copper ion configuration, and therefore the material’s coupling with light, changes with crystal thickness. This makes it possible to tune the optical response simply by choosing the right CIPS thickness.” According to the project's principal investigator, Mazhar Ali, CIPS isn't the only material with these properties. “Our discovery of a mechanism where ferroelectric polarization and mobile ions work together to shape light–matter interactions may extend to other ferroelectric materials,” he said. As such, the work points to a broader design principle, where materials are engineered to contain mobile ions that modulate internal fields, in order to gain new tools for sculpting light across a wide range of wavelengths. “With further work, CIPS-based structures could underpin tunable UV/blue components for integrated electro-optics — controlled not just by electrons, but by the motion of ions inside a crystal only billionths of a meter thick,” El Mrabet Haje said. The research was published in Advanced Optical Materials (www.doi.org/10.1002/adom.202502291).