Researchers from TU Delft and Radboud University (The Netherlands) have discovered that the two-dimensional ferroelectric material CuInP₂S₆ (‘CIPS’) can be used to control the pathway and properties of blue and ultraviolet light like no other material can. With ultraviolet light being the workhorse of advanced chipmaking, high-resolution microscopy and next-generation optical communication technologies, improving the on-chip control over such light is vital. As the researchers describe in the journal Advanced Optical Materials , CIPS can be integrated onto chips, opening exciting new avenues for integrated photonics.
A special kind of ferroelectric
CIPS is an atomically layered ferroelectric material, which means that it carries a built-in internal electric dipole due to the displacement of the copper ions that can also move inside the structure. CIPS stands out because this motion of the copper ions is strongly dependent on the thickness of the two-dimensional crystal. The team from Delft and Nijmegen discovered that such thickness-dependent ferroelectric behaviour can be used to achieve a thickness-dependent refractive index, which is a measure of how much the crystal slows and bends light. First author of the paper, Houssam El Mrabet Haje: “Going from bulk material to a layer of only tens of nanometres thick, the refractive index of CIPS changed by almost 25% in an unexpected, ‘anomalous’ way.”
Potential game-changer
Most strikingly, the team also found that CIPS shows giant birefringence in the blue–UV range: light traveling out-of-plane through the crystal experiences a very different refractive index than light traveling in-plane. At wavelengths of around 340 nanometres (near-UV), this difference reaches about 1.24 – the largest intrinsic birefringence ever reported in this part of the spectrum. Houssam: “This means that CIPS can act as an extremely powerful polarisation and phase control element for short-wavelength light, without needing complicated nanostructuring. It confirms CIPS as a potential game-changer for many photonics applications.”
Choosing the right thickness
Although a full picture is still to be determined, the team propose a new mechanism at work inside the CIPS crystal. Houssam: “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. 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.”
New tools for sculpting light
Mazhar N. Ali, principal investigator for the project: “CIPS is not the only material with such 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.” As such, the work suggests 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.
Tunable UV/blue components
Houssam concludes, “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 metre thick.”
Paper:
“Anomalous Refractive Index Modulation and Giant Birefringence in 2D Ferrielectric CuInP 2 S 6 ”, Houssam El Mrabet Haje, Roald J.H. van der Kolk, Trent M. Kyrk, Sergii Grytsiuk, Malte Rösner, and Mazhar N. Ali. Adv. Optical Mater 2025 , e02291. DOI: 10,1002/adom.202502291
Advanced Optical Materials
Experimental study
Not applicable
Anomalous Refractive Index Modulation and Giant Birefringence in 2D Ferrielectric CuInP2S6
17-Nov-2025