Until now, it was widely thought that only hyperbolic crystals could support the light-matter waves needed to confine, steer, or manipulate light in precise ways. But a discovery made by a team at Nanyang Technological University (NTU) suggests otherwise. The team’s findings challenge long-held beliefs about the kinds of materials capable of supporting light-matter interactions at the atomic and nano scales. The researchers showed that the type of wave formed when light couples with hyperbolic phonon polaritons can also emerge in a crystal called yttrium vanadate (YVO4), and can be tuned by adjusting the temperature. Unlike hyperbolic crystals, YVO4 is readily accessible. The ability to use a common crystal to induce light-matter interactions could accelerate advancements in miniaturized, ultraprecise applications for medical imaging, semiconductor chip inspection, and other fields. A visual representation of hyperbolic dispersion, a phenomenon in which light can travel in unusual, tightly confined paths that resemble the shape of a hyperbola. Courtesy of Nanyang Technical University/Guangwei Hu. For a crystal to be hyperbolic, it must demonstrate both positive and negative permittivity. This creates hyperbolic dispersion, a condition that allows the light to travel in tightly confined paths that resemble the shape of a hyperbola. As the light travels along these paths, phenomena occur that create hot spots of light. These hot spots can interact with very small objects to allow imaging systems to detect an extremely fine level of detail. The NTU team showed that while the bulk of the YVO4 crystal is not hyperbolic, its surface could be. At the interface between the crystal and air, the material demonstrated the ability to support hyperbolic surface phonon polaritons without requiring the inside of the crystal to be hyperbolic. The researchers used a combination of theoretical modeling and scanning near-field optical microscopy (SNOM) nanoimaging experiments to demonstrate this phenomenon. They were able to visualize hyperbolic wavefronts of surface phonon polaritons on YVO4 crystal surfaces within the material’s non-hyperbolic frequency range, where the permittivity tensor components of the material have the same negative sign. By varying the temperature from room temperature to cryogenic levels, the researchers could manipulate polariton dispersions, enabling a topological transition from hyperbolic to canalization and eventually to the elliptic regime. This temperature-controlled dispersion engineering provided the researchers with precise control over polariton topology and allowed them to modulate the wavelength and group velocity. By precisely heating the crystal, they could fine-tune the permittivity enough to shift the frequency range of the surface waves, causing the surface to switch in and out of hyperbolic behavior. Using temperature control, the researchers confined light into regions as small as 20 nm, a size 10-20x smaller than can achieved using traditional optical methods. “We’ve essentially broken the physical rules people thought were fixed,” professor Guangwei Hu said. “We’ve shown that surface properties can both be dramatically different from a material’s bulk and also have its properties manipulated.” The discovery makes light-matter interaction, which is necessary to achieve resolution at the atomic scale, more accessible for a range of applications. “Imagine medical imaging that can see cellular structures with unprecedented clarity, or semiconductor chips that can be inspected at near-atomic scales,” professor Qijie Wang said. “That’s the potential of this research.” By removing the need to rely on hyperbolic crystals, this approach to initiating light-matter interactions could expand the scope of nano-optics and help drive device miniaturization using optical methods. “In electronics, we’ve seen how miniaturization leads to more powerful and cheaper devices,” Wang said. “We’re applying the same principle to optical technologies.” The research was published in Nature (www.doi.org/10.1038/s41586-025-09288-1).