Scientists from the Fritz Haber Institute of the Max Planck Society, Vanderbilt University, City University of New York, University of Nebraska, and University of Iowa have published new results on asymmetric light-matter waves in ‘Nature’. They have uncovered that certain types of crystals support special types of light propagation, enabled by the ‘shear forces’ in the crystal that open new possibilities for controlling the properties of light. These discoveries could enable new types of compact optical technologies, which could be used to detect trace quantities of chemical markers, enable new ways to guide light like in optical fibers, and even store information optically.

Currently different combinations of materials are used to make optical components to achieve different functionalities, including anti-reflection coatings, optical fibers and focusing. Leveraging crystals (such as quartz and sapphire) with asymmetric structures allows the creation of novel optical components as the light propagates in unusual ways. For instance, crystalline anisotropy is the basis of optical waveplates that are used to control the polarization of light.

For many advanced applications in the infrared, it is desired to confine the light to nanoscale dimensions, however, standard refractive dielectric materials this is not possible. Yet by combining light with charged particles can provide a means to achieve this through the formation of a polariton. Much as light matter interactions are strongly modified in highly anisotropic materials, so too are these polaritons, but not all types of crystals have been explored for optical research and technology in this regard. The research team led by the FHI in collaboration with the Advanced Science Research Center at the City University of New York, Vanderbilt University in Nashville, University of Nebraska and the University of Iowa, explored such light propagation via polariton excitation within the monoclinic crystal beta-gallium oxide. This ‘monoclinic’ crystal class designates a unit cell that has low symmetry, that is, the crystal axes along all three directions are different in length, and one of them is not orthogonal to the other two. As such, this gives rise to more complicated light matter interactions, which had been previously been unnoticed in the context of polaritons. Here, the team uncovered that these crystals exert shear forces on light propagating along its surface.

“Using the infrared radiation of our institute’s free-electron laser our experiments are able to access spectral ranges that are otherwise very challenging”, explains Dr. Alex Paarmann of the Department of Physical Chemistry of the FHI. “The structure of the “monoclinic” crystals used in our studies looks like a distorted cuboid, where 4 of 6 sides are rectangular but 2 are tilted parallelograms.” Paarmann explains. “Because of this distortion, the new shear waves not only run along well-defined directions across the crystal surface but are also no longer mirror-symmetric. Thanks to the “hyperbolic” dependence of their wavelength on the propagation direction, we can squeeze these waves into tiny volumes. These so-called “hyperbolic shear polaritons” emerge from the coupling of infrared light to polar lattice vibrations called “phonons” in these crystals. In contrast to previous observations of hyperbolic phonon-polaritons using crystals with a symmetric structure, the team discovered new properties of the shear polaritons, where their propagation direction depends on the infrared wavelength and their wave fronts are distorted. Optical shear phenomena were identified to be responsible for these new features, which exclusively arise because of the lower crystal symmetry and the associated alignment of the lattice vibrations. Therefore, the crystal symmetry is the fundamental reason for these discoveries, which are entirely general to any crystal with the same structure.

“We expect our results to open up new avenues for polariton physics in materials with low symmetry that includes many geological minerals and organic crystals,” says FHI scientist Paarmann. This will provide a much greater choice of materials for technological development, which will substantially enhance design options for compact photonic components. This work therefore represents a big step forward for miniaturization of optical circuitry and components, as well as novel opportunities in active modulation of the light propagation in future nanophotonic technology.