Multiferroics, materials that exhibit both ferroelectric and magnetic order, have emerged as key platforms for studying strong magnetoelectric interactions, where magnetic and electric properties influence each other. In type-II multiferroics, spontaneous electric polarization arises directly from specific magnetic spin arrangements. When exposed to external oscillating electric and magnetic fields, this spin-driven ferroelectricity generates unique terahertz-range excitations known as electromagnons, which are collective spin waves carrying an electric dipole moment.

A recent review published in the Journal of the Physical Society of Japan provides a comprehensive overview of electromagnons and their role in nonreciprocal optical phenomena in spin-spiral multiferroics.

The study explains the origin of spin-driven ferroelectricity and describes how electromagnons emerge through spin current and exchange striction mechanisms in perovskite manganites, a representative family of spin-spiral multiferroics. It then details how these excitations resonantly enhance four distinct nonreciprocal terahertz optical effects: directional dichroism, gyrotropic birefringence, the magnetochiral effect, and natural optical activity. These nonreciprocal phenomena challenge conventional knowledge of the magnetoelectric phenomena and open pathways for realizing even more gigantic nonreciprocal optical phenomena.

Importantly, nonreciprocal terahertz phenomena provide a way to control the polarization and propagation direction of terahertz radiation using external electric and magnetic fields. This capability helps overcome longstanding challenges in harnessing the terahertz region of the electromagnetic spectrum—often referred to as the “terahertz gap”—and could pave the way for applications in next-generation wireless communication and sensing technologies.

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