Associate Professor, Department of Physics, Faculty of Science, Kyushu University
The interaction between light and magnetism is considered a promising route to the development of energy-efficient data storage technologies. The Faraday effect is a well-known magneto-optical effect. Linearly polarized light passing in the direction parallel to the magnetization is subject to Faraday rotation. In this effect, we can say that magnetism acts on light. There is also an inverse effect; namely, where light acts on magnetism. The inverse Faraday effect is where circularly polarized light generates an effective magnetic field. Conversely, linearly polarized light passing in the direction perpendicular to the magnetization is subject to magnetic birefringence. This is called the Cotton-Mouton effect, which is a second-order magneto-optical effect. The inverse Cotton-Mouton effect is where linearly polarized light generates an effective magnetic field. These inverse magneto-optical effects have been used for non-thermal optical excitation of spin oscillations in (weak)ferromagnets.
The resonance frequency of spin oscillation in antiferromagnets is extremely high due to the exchange interaction between adjacent spins, and faster magnetization control had been expected. However, it was believed that the inverse Faraday effect does not act on pure antiferromagnets with zero net magnetization. Optical control of antiferromagnets had not been reported. We succeeded in excitation of spin oscillation in NiO by illuminating circularly polarized light pulses and in optical control of antiferromagnets We also demonstrated, for the first time, time and phase resolved imaging of spin wave propagation in a ferrimagnet induced via the inverse Faraday effect. It was shown that the wave number distribution of the excited spin wave is proportional to the frequency component of spatial spot of the excitation beam. This fact led to the directional control of spin wave propagation, thus demonstrating spin wave manipulation by using spatially shaped optical pulses.
Finally, we have demonstrated the one-to-one transfer of the polarization eigenstates of a fully polarized pump light wave onto the magnetic eigenmodes of a three-sublattice antiferromagnet. We converted the magnetic information back into the optical polarization eigenstate of a probe light wave in an equally one-to-one process.
These results will pave the way for new fields of “terahertz-spintronics” and “optomagnonics” for generating and controlling magnetic excitations by polarized light.
Fig.1) Schematic for propagation of spin waves excited by femtosecond laser pulses.