One-way transparency of four-coloured spin-wave excitations in multiferroic materials

Abstract

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 Ca 2 CoSi 2 O 7 , Sr 2 CoSi 2 O 7 and Ba 2 CoGe 2 O 7 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.

Magnetochiral dichroism resonant with electromagnons in a helimagnet

“…In fact, the magnitude of the MCh dichroism (Fig. 2c,e), Dk(o)B0.3 or Dk(o)/k 0 (o)B1, are considerably high among the optical ME effects observed so far 17,18,22,23,[37][38][39] . (Note that the normal term…”

“…near-infrared region 39 and local spin excitation in a terahertz region 17,18 , where the magnetism assists the presence of the chirality in cooperation with the crystal symmetry. So far, the candidate materials for the MCh dichroism were restricted by the crystalline symmetry.…”

“…One important outcome of the electromagnon excitation is to mediate the dynamical (optical) ME effect; the resonance is ME in nature, that is, responding to both a.c. (optical) electric and magnetic fields [14][15][16][17][18] . This cross-coupled response to light leads to the novel optical property termed directional dichroism, which differentiates the optical responses for two counter-propagating light beams 19,20 .…”

Spin-induced polarizations and nonreciprocal directional dichroism of the room-temperature multiferroicBiFeO3

“…is the complex refractive index for a linearly polarized beam, χ ee , χ mm and χ me are the dielectric, magnetic, and magnetoelectric susceptibility tensors describing the dynamical response of the spin system [31][32][33][34] and ǫ ∞ is the background dielectric constant tensor associated with charge excitations at higher energies. Subscripts i and j refer to the electric and magnetic polarization directions, respectively.…”

“…Both the magnetic and electric components of THz radiation can excite SWs in multiferroic materials. The NDD exhibited by simultaneously electric-and magneticdipole active excitations has been extensively studied in Ba 2 CoGe 2 O 7 [31][32][33][34] [36].…”