In the growing field of quantum materials, researchers keep finding things that defy expectations. Case in point – a metal capable of conducting light is detailed in a new paper from Columbia University.
Since most metallic materials are reflective at visible light wavelengths, any light that strikes them will bounce back. Metals are great at moving heat and electricity, but people don’t usually think of them as a way to move light.
Yinming Shao, a postdoc at Columbia who led the work, has been investigating the optical properties of a semimetal material called ZrSiSe. In a study published in 2020, Shao and his colleagues showed that ZrSiSe and graphene, the first “Dirac material” found in 2004, have similar electronic properties. Enhanced electronic correlations, which are uncommon for Dirac semimetals, are present in ZrSiSe.
Graphene is a single, atom-thin layer of carbon, whereas ZrSiSe is a three-dimensional metallic crystal made up of layers that exhibit anisotropy, or different behaviour in the in-plane and out-of-plane directions.
“It’s sort of like a sandwich: One layer acts like a metal while the next layer acts like an insulator. When that happens, light starts to interact unusually with the metal at certain frequencies. Instead of just bouncing off, it can travel inside the material in a zigzag pattern, which we call hyperbolic propagation,”
Hyperbolic Waveguide Modes
Shao and his colleagues from Columbia and the University of California, San Diego saw this zigzag movement of light when they used ZrSiSe samples of different thicknesses. This movement is also called hyperbolic waveguide modes.
These waveguides, which here are plasmons, are produced when light photons combine with electron oscillations to form hybrid quasiparticles that can direct light through a material.
Although many kinds of layered metals can produce plasmons that can spread hyperbolically, it was ZrSiSe’s distinct range of electron energy levels, or electronic band structure, that enabled the team to observe plasmons in this substance.
Plasmons have the ability to “magnify” features in a sample, enabling researchers to see past the diffraction limit of optical microscopes, which otherwise are unable to resolve details smaller than the wavelength of light they use.
“Using hyperbolic plasmons, we could resolve features less than 100 nanometers using infrared light that’s hundreds of times longer,”
According to Shao, ZrSiSe is an intriguing choice for nano-optics research that favours ultra-thin materials because it can be peeled to different thicknesses. However, it’s unlikely to be the only material that has value.
From here, the team wants to investigate other materials that are comparable to ZrSiSe but may have even better waveguiding properties. This could make it easier to make better nano-optics techniques and better optical chips that can be used to solve fundamental problems with quantum materials.
“We want to use optical waveguide modes, like we’ve found in this material and hope to find in others, as reporters of interesting new physics,”
said Dmitri Basov, Higgins Professor of Physics at Columbia University.
Research in the physics of Dirac electrons at Columbia is supported by a grant from the U.S. Department of Energy, Office of Science, Basic Energy Sciences.
Yinming Shao et al. Infrared plasmons propagate through a hyperbolic nodal metal. Science Advances (2022) Vol. 8, No. 43