Exploiting magnetic materials’ response to light could potentially lead to all sorts of new technology, from magnetic lasers to novel memory devices. But magneto-optic effects in most naturally occurring materials are small and require either high-power lasers or sensitive optical detectors to be discerned.
Now scientists have shown that the magnetic response of an antiferromagnetic semiconductor just a few atoms thick can be tuned across a broad spectral range (Nature, doi: 10.1038/s41586-023-06275-2). According to the researchers, this is due to the formation of what are known as exciton–polaritons, quasiparticles that are part matter and part light.
The quasiparticles are typically realized by placing an exceptionally thin piece of semiconductor in the center of a micrometer-sized optical cavity. Resonant light waves liberate electrons in the material, creating electron–hole pairs known as excitons. If merging pairs emit radiation with a frequency very similar to that of the light in the cavity, the photons and excitons come to form a distinct entity: an exciton–polariton.
In the latest work, Vinod Menon at the City College of New York, USA, and colleagues have studied this light–matter coupling in crystals consisting of several layers of a semiconductor made from chromium, sulfide and bromine, with each layer just a few hundred nanometers thick. They were able to trap light inside the crystals both with mirrors on each end of the samples and without—in the latter instance, exploiting the material’s unusually large dielectric constant compared with its surroundings.
Rather than a single resonance—as would be expected in the case of an exciton only—the researchers instead observed optical signals at multiple frequencies and therefore energies.
Menon and colleagues first demonstrated the purely optical characteristics of the crystals by shining green laser light at them and measuring the photoluminescence. Rather than a single resonance—as would be expected in the case of an exciton only—the researchers instead observed optical signals at multiple frequencies and therefore energies. Combining these experimental results with theoretical models, the team concluded that the emissions must be the result of dispersion by exciton–polaritons.
Magnetic fields and polariton dispersion
With that result in the bag, the researchers went on to investigate the influence of magnetic fields on this dispersion. As they point out in the paper, an antiferromagnet consists of small regions of oppositely aligned atomic or molecular magnetic moments with no net magnetization. But when exposed to a magnetic field, the material becomes a ferromagnet in which all of the magnetic moments line up in the same direction. The effect is not black and white; intermediate fields cause the magnetic moments from neighboring regions to become partially aligned.
Menon and colleagues examined the effect of an external magnetic field on the different branches of the polariton dispersion. The highest-energy branch corresponds to a pure exciton (which they simulated rather than measured), with progressively lower energy branches becoming more photon-like. The researchers found that increasing the strength of the magnetic field reduced the energy of all branches but diminished that of the exciton-like branches the most.
Whereas crystals with very few layers are transparent at energies significantly below the exciton resonance, the team’s material—which has more layers—instead experiences major changes in optical reflectance when subject to magnetic fields, according to the researchers.
However, the same was not true of reflectance. As with its energy, a polariton’s reflectance could be changed by the external field. But in this case, the researchers saw the biggest effect with more-photon-like polaritons. For pure excitons, on the other hand, the modulation was minimal. In other words, whereas crystals with very few layers are transparent at energies significantly below the exciton resonance, the team’s material—which has more layers—instead experiences major changes in optical reflectance when subject to magnetic fields, according to the researchers.
Potential practical uses
Finally, the researchers investigated the effect of time-dependent fields in the form of magnetic disturbances known as magnons, which should also modulate the alignment between the two magnetizations in an antiferromagnet. Using pump–probe measurements, the researchers found that the relative reflectance of two different exciton-like branches varied as the magnon oscillations swept through their crystals. “Like in the response to a static field,” they write, “polaritons substantially increase the spectral bandwidth of this magneto-optic effect.”
To exploit the research for practical ends, Menon and colleagues are looking to develop nanophotonic chips that use electric currents to excite magnons and polaritons. Such devices, they say, could find use in quantum transduction (converting microwaves into near-infrared light), memory devices and novel light emitters.