Second harmonic generation spectroscopy in magnetic and multiferroic materials

Abstract

Spectroscopic studies of magnetically ordered materials, multiferroic insulators, and magnetic semiconductors reveal several previously unexplored mechanisms of optical second harmonic generation (SHG). In contrast to non-magnetic materials, SHG in such systems is determined by arrangements of both charges and spins. In general, the nonlinear optical susceptibility arises from electric-dipole contributions in the case of noncentrosymmetric crystal structures and from magnetic-dipole contributions in the case of centrosymmetric crystal structures. We show that both these mechanisms are efficient in magnetic materials. Obeying different selections rules, SHG spectroscopy may provide new information in comparison to linear optics. The observation of resonance SHG at d–d transitions in antiferromagnetic oxides Cr2O3, NiO, CuB2O4 is discussed. The SHG spectroscopy was successfully applied for distinguishing subtle variations of magnetic structures in hexagonal manganites RMnO3. Giant SHG in the region of charge-transfer transitions was observed in multiferroic manganite TbMn2O5. Spin-induced SHG was observed near the band gap in centrosymmetric magnetic semiconductors EuTe and EuSe. One of the most striking potentials of SHG is visualization of antiferromagnetic 180° domain structures which are indistinguishable in linear optics.