Subverting magneto-optical technology? Scientists have succeeded in trapping light inside a magnet

Controlling quantum materials with light is of fundamental and technical importance. In particular, the strong optical response of magnets is important for the development of magnetic lasers and magneto-optical memory devices, as well as for emerging quantum conduction applications.

A new study led by Vinod M. Menon and his group at the City College of New York shows that trapping light inside magnetic materials may dramatically enhance their intrinsic properties. Strong optical responses of magnets are important for the development of magnetic lasers and magneto-optical memory devices, as well as for emerging quantum transduction applications.

“Magneto-optics in a van der Waals magnet tuned by self-hybridized polaritons”

In their new article in Nature, Menon and his team report the properties of a layered magnet that hosts strongly bound excitons—quasiparticles with particularly strong optical interactions. Because of that, the material is capable of trapping light—all by itself.

As their experiments show, the optical responses of this material to magnetic phenomena are orders of magnitude stronger than those in typical magnets. "Since the light bounces back and forth inside the magnet, interactions are genuinely enhanced," said Dr. Florian Dirnberger, the lead-author of the study.

"To give an example, when we apply an external magnetic field the near-infrared reflection of light is altered so much, the material basically changes its color. That's a pretty strong magneto-optic response."

"Ordinarily, light does not respond so strongly to magnetism," said Menon. "This is why technological applications based on magneto-optic effects often require the implementation of sensitive optical detection schemes."

The vdW crystals used in this study were chromium sulphide bromide (CrSBr), which consists of quasi-2D layers of CrSBr held together by weak van der Waals forces. At low temperatures, the material is in an antiferromagnetic state in which the electron spins of adjacent layers are oppositely aligned. However, it is possible to switch the crystals into a ferromagnetic state (all spins are aligned) by applying a moderate magnetic field. While this transition often results in a magneto-optic effect that changes the polarization or intensity of the light (effects that most existing magneto-optic devices rely on), in CrSBr it is the exciton energy – and therefore the materials’ optical spectrum – that is altered.

In this study, Florian Dirnberger, Jiamin Quan and colleagues studied two types of CrSBr cavities. The first resembled a traditional optical cavity in which external, highly reflective mirrors were deposited on either side of a CrSBr crystal. The second relied on the strong dielectric contrast between the crystal and its environment to confine the cavity photon mode within, forming a “mirrorless” cavity. Due to the extremely large exciton oscillator strength of CrSBr crystals, strong coupling between the photon mode and the magnetic excitons – and therefore the presence of exciton–polaritons – was observed.

Exciton-photon coupling in CrSBr cavity

Polaron-magnon coupling and coherent magnon effect

Magneto-optical response of CrSBr to incoherent magnetons

By applying an external magnetic field to the crystals, the researchers were able to reduce the angle between the oppositely aligned spins. This resulted in a decrease in exciton energy and switched the crystals from the antiferromagnetic to the ferromagnetic state. This energy change altered the relative exciton–photon fraction of the polaritons, shifting their energy levels and modifying the measured reflectivity spectrum.

In a weakly coupled CrSBr crystal, the magneto-optic response would only occur around the exciton energy. In this strongly coupled system, however, polariton states exist far below the band gap, giving a significantly increased bandwidth of the magneto-optical response.

The researchers also investigated the effect of magnons on the system. These are quantized oscillations in the angle between the oppositely aligned spins that also alter the exciton energy. Using ultrashort laser pulses to generate coherent magnons, they observed that the cavity reflectivity spectrum exhibited oscillations with a frequency matching that of coherent magnons in CrSBr. While this effect occurs in both cavities, it is greatly enhanced in the sample with external mirrors due to the reduced linewidth of the polaritons.

Surprisingly, the researchers also observed that incoherent magnons, which are generated thermally, can produce a pronounced magneto-optical response. Until this study, it was thought that coherence was necessary for such an effect. Using theoretical modelling, the researchers have now shown that, below a certain temperature, the temperature-dependence of the excitons in CrSBr is mainly affected by the population of incoherent magnons. This shows that optical spectroscopy of polaritons in such a system can be used as a new method for studying incoherent magnons in magnetic crystals.

On how the advances can benefit ordinary people, study co-author Jiamin Quan said, "Technological applications of magnetic materials today are mostly related to magneto-electric phenomena. Given such strong interactions between magnetism and light, we can now hope to one day create magnetic lasers and may reconsider old concepts of optically controlled magnetic memory." Rezlind Bushati, a graduate student in the Menon group, also contributed to the experimental work.

"Given the strong interaction of magnetism and light observed in our study, magnetic lasers and all-optically controlled magnetic storage devices could one day revolutionize magneto-optical technology."

This research has important implications for the development of magneto-optical devices: such as sensors and imaging devices that can directly determine and map magnetic domains in materials. The research could also lead to the creation of high-speed switches and fully optically controlled magnetic storage devices.

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