Spin-Induced Optical Second Harmonic Generation in the Centrosymmetric Magnetic Semiconductors EuTe and EuSe

Abstract

Spectroscopy of the centrosymmetric magnetic semiconductors EuTe and EuSe reveals spininduced optical second harmonic generation (SHG) in the band gap vicinity at 2.1-2.4 eV. The magnetic field and temperature dependence demonstrates that the SHG arises from the bulk of the materials due to a novel type of nonlinear optical susceptibility caused by the magnetic dipole contribution combined with spontaneous or induced magnetization. This spin-induced susceptibility opens access to a wide class of centrosymmetric systems by harmonics generation spectroscopy.PACS numbers: 75.50. Pp, 42.65.Ky, 78.20.Ls Nonlinear optics is a highly active field of basic and applied research with optical harmonics generation playing a particularly important role [1,2]. Harmonics generation is associated with higher order optical susceptibilities, and opens access to unique information about the crystallographic, electronic and magnetic structure of solids that is inaccessible by linear optics [1,2,3]. Second harmonic generation (SHG) has attracted most interest, because of its exceptional sensitivity to space and time symmetry violations [3] and its importance for technological applications. Spectroscopy of semiconductors using SHG has been, however, mostly limited to narrow spectral ranges [4,5]. Recently, SHG was studied in detail for the noncentrosymmetric semiconductors GaAs, CdTe and (Cd,Mn)Te [6,7], where SHG is allowed in electric-dipole (ED) approximation. Two mechanisms of magnetic-field-induced SHG have been disclosed, based on changing ED contributions by mixing with magnetically-induced terms. In centrosymmetric materials with inversion symmetry SHG is forbidden in ED approximation, which imposes severe restrictions on the crystalline solids and artificial structures that can be explored by SHG.This restriction can be overcome by processes based on magnetic-dipole (MD) or electric-quadrupole (EQ) nonlinear susceptibilities. Other opportunities may be opened up by external or internal perturbations that break either space-inversion or time-reversal symmetry. For example, an applied electric field breaks the spaceinversion symmetry in centrosymmetric materials so that ED-SHG becomes allowed [8]. It would be highly attractive to find the counterpart SHG related exclusively to MD contributions triggered by applied magnetic fields or magnetic ordering. Evidently, the search for such mechanisms is facilitated in centrosymmetric materials, where crystallographic ED and EQ contributions to SHG vanish.In this Letter we report on spin-induced SHG in the centrosymmetric magnetic semiconductors EuTe and EuSe. No SHG was detected in the antiferromagnetic and paramagnetic phases. However, when a magnetic field is applied, SHG arises due to breaking of the antiferromagnetic order or by polarization of the paramagnetic phase, both resulting in appearance of a net magnetization. The observed spin-related nonlinearities arise due to a novel type of nonlinear optical susceptibility caused by the MD contribution in combination with spontan...

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