A new study published in Science Advances by researchers from the National Graphene Institute at University of Manchester and the University of Oviedo, has revealed a previously unseen behaviour of light in gypsum, a mineral better known for its use in building plaster and chalk.
The team uncovered a rare type of wave, known as a shear phonon polariton, in a two-dimensional form of the material. Phonon polaritons are light-matter hybrid waves that emerge when light interacts with atomic vibrations in certain crystals. They can travel through materials in unusual ways and concentrate light into extremely small volumes.
In this study, the researchers found that in thin films of gypsum, these waves undergo a topological transition, shifting from hyperbolic to elliptical behaviour, passing through a unique canalized state.
This transition allows scientists to tune how light propagates through the material.
"The studies of shear phonon polaritons in previous studies were limited to bulk crystals in the hyperbolic regime. In our study we aimed to complement those initial findings with shear polaritons in a 2-dimentional material," said Dr Pablo Díaz Núñez, who co-led the study. "And remarkably, we discovered that shear phonon polaritons in gypsum support a topological transition from hyperbolic to elliptical propagation, with canalization in between."
Dr Díaz Núñez added, "Moreover, we were able to confine light to a space twenty-five times smaller than its wavelength and slow it down to just a fraction of its speed in vacuum, this opens up new possibilities for manipulating light at the nanoscale."
The research also highlights the role of crystal symmetry. Gypsum belongs to a class of materials with low symmetry, specifically to the monoclinic crystal system, which gives rise to asymmetric light propagation and energy loss, the central characteristic of shear polaritons.
These findings extend beyond fundamental research of phonon polariton propagation and could support future developments in areas that rely on precise control of light, such as thermal management, sensing, and imaging beyond the limits of conventional optics. Moreover, the study introduces gypsum as a new platform for exploring advanced photonic concepts in emerging areas like non-Hermitian photonics.
This research was published in the journal Science Advances.
Full title: Visualization of topological shear polaritons in gypsum thin films
The National Graphene Institute (NGI) is a world-leading graphene and 2D material centre, focussed on fundamental research. Based at The University of Manchester, where graphene was first isolated in 2004 by Professors Sir Andre Geim and Sir Kostya Novoselov, it is home to leaders in their field - a community of research specialists delivering transformative discovery. This expertise is matched by £13m leading-edge facilities, such as the largest class 5 and 6 cleanrooms in global academia, which gives the NGI the capabilities to advance underpinning industrial applications in key areas including: composites, functional membranes, energy, membranes for green hydrogen, ultra-high vacuum 2D materials, nanomedicine, 2D based printed electronics, and characterisation.
