Physicists see light waves moving through metal

When we encounter metals in our daily life, we perceive them as shiny. This is because common metallic materials are reflective at visible light wavelengths and reflect back any light that strikes them. While metals are well suited for conducting electricity and heat, they are not generally considered a medium for conducting light.

But in the burgeoning field of quantum materials, researchers are increasingly finding examples that challenge expectations of how things should behave. In new research published in Scientists progress, a team led by Dmitri Basov, Higgins Professor of Physics at Columbia University, describes a metal capable of conducting light. “These findings challenge our everyday experiences and common understandings,” Basov said.

The work was led by Yinming Shao, now a Columbia postdoc who transferred as a Ph.D. student when Basov moved his lab from the University of California, San Diego to New York in 2016. While working with the Basov group, Shao explored the optical properties of a semi-metallic material known as ZrSiSe. In 2020 in Natural PhysicsShao and colleagues showed that ZrSiSe shares electronic similarities with graphene, the first so-called Dirac material discovered in 2004. ZrSiSe, however, has enhanced electronic correlations which are rare for Dirac semimetals.

While graphene is a single, atom-thin layer of carbon, ZrSiSe is a three-dimensional metallic crystal composed of layers that behave differently in the in-plane and out-of-plane directions, a property known as anisotropy. “It’s kind of like a sandwich: one layer acts like a metal while the next layer acts like an insulator,” Shao explained. “When this happens, light begins to interact with the metal in unusual ways at certain frequencies. Instead of just bouncing around, it can travel inside the material in a zigzag pattern, which we call hyperbolic propagation.”

In their current work, Shao and collaborators at Columbia and the University of California, San Diego observed such zigzag motion of light, called hyperbolic waveguide modes, through ZrSiSe samples of thicknesses variables. Such waveguides can guide light through a material and here result from photons of light mixing with oscillations of electrons to create hybrid quasi-particles called plasmons.

Although the conditions for generating plasmons capable of hyperbolically propagating are present in many layered metals, it is the unique range of electronic energy levels, called electronic band structure, of ZrSiSe that has allowed the team to observe them in this material. Theoretical support to help explain these experimental results came from Andrey Rikhter in the group of Michael Fogler at UC San Diego, Umberto De Giovannini and Angel Rubio at the Max Planck Institute for the Structure and Dynamics of Matter , and Raquel Queiroz and Andrew Millis at Columbia. (Rubio and Millis are also affiliated with the Flatiron Institute of the Simons Foundation)

Plasmons can “magnify” features in a sample, allowing researchers to see beyond the diffraction limit of optical microscopes, which otherwise cannot resolve details smaller than the wavelength of light. they use. “Using hyperbolic plasmons, we could resolve features smaller than 100 nanometers using infrared light that is hundreds of times longer,” Shao said.

ZrSiSe can be peeled to different thicknesses, making it an attractive option for nano-optics research that favors ultra-thin materials, Shao said. But it’s probably not the only material of value – from there, the group wants to explore others that share similarities with ZrSiSe but might have even more favorable waveguiding properties. This could help researchers develop more efficient optical chips and better nano-optical approaches to exploring fundamental questions about quantum materials.

“We want to use optical waveguide modes, as we found in this material and hope to find in others, as reporters of interesting new physics,” Basov said.