New optical phenomena in multiferroic materials

Abstract: Simultaneous breaking of time-reversal and spatial inversion symmetries can lead to the emergence of new optical phenomena in multiferroics. As the most peculiar example, we have recently found that multiferroics can exhibit strong directional dichroism [1,2], i.e. they absorb counter-propagating light beams with different strength, where the absorbing and transparent directions of propagation can be reversed by the application of either magnetic or electric fields. In the THz spectral range directional dichroism is the consequence of the optical magnetoelectric effect, i.e. the coupled dynamics of the local magnetization and electric polarization. Collective spin modes in multiferroics can be resonantly excited by both the electric and magnetic components of light, hence, they are the “elementary excitations” of such hybrid magnetoelectric response. Our recent studies on several multiferroic compounds, including Ba₂CoGe₂O₇, Ca₂CoSi₂O₇, Sr₂CoSi₂O₇ and BiFeO₃, show that such magnetoelectric spin excitations efficiently generate directional optical anisotropy leading to one-way transparency at these resonances [3].
These unique optical functionalities, present in a large variety of multiferroic materials, can open new horizon in photonics [4,5]. Since directional optical anisotropy is also related to the static magnetoelectric phenomena, hence, these optical studies can provide guidelines for the systematic synthesis of new materials with large dc magnetoelectric effect.

[1] I. Kezsmarki et al. Enhanced Directional Dichroism of Terahertz Light in Resonance with Magnetic Excitations of the Multiferroic Ba₂CoGe₂O₇ Oxide Compound, Phys. Rev. Lett. 106, 057403 (2011).
[2] S. Bordacs et al. Chirality of matter shows up via spin excitations, Nat. Phys. 8, 734 (2012).
[3] I. Kezsmarki et al. Four-coloured spin-wave excitations in multiferroic materials, arXiv:1310.0789 (2013).
[4] K. Penc et al. Spin-Stretching Modes in Anisotropic Magnets: Spin-Wave Excitations in the Multiferroic Ba2CoGe2O7, Phys. Rev. Lett. 108, 257203 (2012).
[5] D. Szaller et al. Symmetry conditions for nonreciprocal light propagation in magnetic crystals, Phys. Rev. B 87, 014421 (2013).