Highlight: Light-induced Magnetism in Materials

Posted 05/08/2019

Justin C. W. Song
Assistant Professor Justin C. W. Song of NTU Singapore.

Physicists from Nanyang Technological University, Singapore (NTU Singapore) and the Niels Bohr Institute in Denmark have devised a method to turn a non-magnetic metal into a magnet using laser light.

Magnetic materials generate their magnetic fields because of tiny circulating currents, similar to those found in electromagnetic coils. The direction of the magnetic field is determined by the ‘handedness’ of these coils — i.e., whether the currents circulate clockwise or anticlockwise. The new theoretical analysis, performed by Assistant Professor Justin Song from NTU’s School of Physical and Mathematical Sciences and Associate Professor Mark Rudner from the Niels Bohr Institute, predicts that when non-magnetic metallic disks are illuminated by linearly polarized light, circulating electric currents and hence magnetism can spontaneously emerge. The work was published in the scientific journal Nature Physics in July 2019.

In formulating their proposal, Song and Rudner developed a new way of thinking about the interaction between light and matter, using a combination of pencil-and-paper calculations and numerical simulations. If the present proposal is realised experimentally, it may open up a variety of applications involving the generation of magnetism “on-demand” in materials like graphene, which are normally non-magnetic.

The properties of materials are conventionally thought to be fixed by the arrangement of its constituent atoms. For example, the configuration of atoms affects how electrons flow between them, which dictates whether the material is an electrical conductor (a metal) or an insulator. In recent years, however, physicists have been exploring the possibility of customizing the properties of materials by illuminating them with laser light.

Song and Rudner made their discovery while investigating how plasmons — local oscillations of charge in metals that are typically accompanied by extremely intense electric fields — might alter the properties of the underlying materials. Normally, plasmons oscillate in the same direction as the electric fields producing them, such as the polarization direction of a laser beam shining on the material. While modeling this phenomenon, however, Song and Rudner found that when the driving light field is intense enough, plasmons can spontaneously rotate in either a left-handed or right-handed fashion, even if the original light field has no intrinsic handedness and the underlying metal has no magnetism of its own.

“This was a signature that the material’s intrinsic properties had been altered,” explains Song. “The plasmon’s strong internal fields transform the material’s electronic band structure, which sets up a feedback loop acting on the plasmon itself, enabling a handedness to develop.”

According to the two physicists, the key idea behind their theoretical analysis is any oscillating electric field in a material affects the motion of electrons within it. “From the point of view of an electron, an electric field is an electric field: it doesn’t matter whether the field originates within the material itself, or is produced by a laser shining on the material,” says Rudner. Using this insight, Song and Rudner theoretically derived the conditions under which feedback from the internal fields of the plasmons can trigger the spontaneous formation of magnetism. They found that the necessary conditions are met in certain high-quality materials, such as graphene.

Emergent behavior

Previous researchers have shown, both theoretically and experimentally, that material properties can be altered by illumination with intense laser light. In those examples, however, the imparted properties were carried by the light field itself. For example, when a material is illuminated by circularly-polarised light, which has a definite handedness, the motion of electrons within the material can acquire the same handedness.

The striking thing about Song and Rudner’s new finding is that handedness spontaneously appears despite not being present in the original light field, which is linearly polarized. “The plasmons acquire a kind of separate life, meaning that new ‘emergent’ properties appear that were not present in either the metal or the light driving it,” says Song.

Emergent behaviour, where the whole is more than the sum of its parts, arises when fundamental particles interact with each other to act in a collective way. It is responsible for a range of useful phenomena such as ferromagnetism and superconductivity. This new research extends the idea of emergence to plasmons, and, more generally, the context of materials driven by light fields.

“Perhaps the most meaningful take-home message of our work is that collective modes can exhibit distinct new phases of matter,” says Song. “If plasmonic magnetism is possible, what other phases of collective modes are waiting to be uncovered?”

M. S. Rudner and J. C. W. Song, Self-induced Berry flux and spontaneous non-equilibrium magnetism, Nature Physics (2019)