Materials displaying optical properties that can be tuned dynamically by controlling the spin of their electrons are appealing for active photonics. While spin properties are often controlled with an external magnetic field, they can also be tuned with light. This opens the way to all-optical spin-photonic devices, presenting possible advantages such as a smaller footprint and a faster response than their magnetic-driven counterparts. In the quest for candidate materials for such devices, Hong Ma et al. report the response of lead iodide, a widely used precursor for perovskite solar cells, in all-optical ultrafast pump-probe experiments. After shining a circularly polarized femtosecond pump beam onto the material, the polarization state of the reflected probe beam changes. The authors attribute this change to Kerr rotation, the direction of which depends on the handedness of the pump polarization. They propose that these features result from the excitation by the circularly polarized pump of spin-polarized electron populations. Such populations would generate an effective internal magnetic field, which could enable Kerr rotation without applying any external magnetic field. The reported Kerr rotation lasts nearly 1 picosecond after the pump, with its maximum angle being almost 1°. Therefore, the authors propose the material for applications in ultrafast photonic devices and optical isolators.

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