City College of New York (CCNY) researchers have demonstrated the ability to combine topological photons with lattice vibrations, or phonons, to manipulate their propagation in a robust and controllable way. The researchers made the photon-phonon combination in hexagonal boron nitride (hBN), which demonstrated a platform to control and guide hybrid states of light and lattice vibrations. The topological edge states of the produced phonon-polaritons carried nonzero angular momentum that was locked to their propagation direction. According to the research team, this physical quality enabled them to be easily transported. As a result, the topological quasiparticles enabled the funneling of infrared phonons mediated by helical infrared photons along arbitrary pathways and across sharp bends. The topological photonics study supports future advancements in Raman and vibrational, or infrared spectroscopy, as well as in the implementation of directional radiative heat transfer. Conserved quantities in the mathematical field of topology — topological invariants — remain constant when altering parts of a geometric object under continuous deformations. One example of such invariants is the number of holes, which makes something like a doughnut equivalent to a mug from a topological point of view. Topological properties endow photons with helicity, when photons spin as they propagate, which can lead to unexpected characteristics such as robustness to defects and unidirectional propagation along interfaces between topologically distinct materials. From interactions with vibrations in crystals, these helical photons can then be used to channel infrared light along with vibrations. Topologically distinct photonic crystals (orange and blue) with a layer of hexagonal boron nitride on top. Researchers at CCNY demonstrated the ability to combine topological photons with phonons to manipulate their propagation in a robust and controllable way. Courtesy of Filipp Komissarenko and Sriram Guddala. According to Alexander Khanikaev of CCNY’s Grove School of Engineering, the new state of matter is half light, half vibrations. “Since infrared light and lattice vibrations are associated with heat, we created new channels for propagation of light and heat together,” he said. “Typically, lattice vibrations are very hard to control, and guiding them around defects and sharp corners was impossible before.” “We can create channels of arbitrary shape for this form of hybrid light and matter excitations to be guided along within a two-dimensional material we created,” said Sriram Guddala, a postdoctoral researcher in Khanikaev’s group and the first author of the manuscript. “This method also allows us to switch the direction of propagation of vibrations along these channels, forward or backward, simply by switching polarizations handedness of the incident laser beam. Interestingly, as the phonon-polaritons propagate, the vibrations also rotate along with the electric field. This is an entirely novel way of guiding and rotating lattice vibrations, which also makes them helical.” The research was published in Science (www.doi.org/10.1126/science.abj5488).