One-way transparency of four-coloured spin-wave excitations in multiferroic materials.
The coupling between spins and electric dipoles governs magnetoelectric phenomena in multiferroics. The dynamical magnetoelectric effect, which is an inherent attribute of the spin excitations in multiferroics, drastically changes the optical properties of these compounds compared with conventional materials where light-matter interaction is expressed only by the dielectric permittivity or magnetic permeability. Here we show via polarized terahertz spectroscopy studies on multiferroic Ca2CoSi2O7, Sr2CoSi2O7 and Ba2CoGe2O7 that such magnetoeletric spin excitations exhibit quadrochroism, that is, they have different colours for all the four combinations of the two propagation directions (forward or backward) and the two orthogonal polarizations of a light beam. We demonstrate that one-way transparency can be realized for spin-wave excitations with sufficiently strong optical magnetoelectric effect. Furthermore, the transparent and absorbing directions of light propagation can be reversed by external magnetic fields. This magnetically controlled optical-diode function of magnetoelectric multiferroics may open a new horizon in photonics.
1] Department of Physics, Budapest University of Technology and Economics, Budapest 1111, Hungary [2] Condensed Matter Research Group of the Hungarian Academy of Sciences, Budapest 1111, Hungary.
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Kézsmárki
Department of Physics, Budapest University of Technology and Economics, Budapest 1111, Hungary.
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Szaller
Quantum-Phase Electronics Center and Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan.
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Bordács
1] Department of Physics, Budapest University of Technology and Economics, Budapest 1111, Hungary [2] RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan.
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Kocsis
RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan.
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Tokunaga
RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan.
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Taguchi
RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan.
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Murakawa
1] Quantum-Phase Electronics Center and Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan [2] RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan.
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Tokura
High Field Magnet Laboratory, Institute for Molecules and Materials, Radboud University, Nijmegen 6525 ED, The Netherlands.
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Engelkamp
National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, Tallinn 12618, Estonia.
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Rõõm
National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, Tallinn 12618, Estonia.
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Nagel