The magnetic europium chalcogenide semiconductors EuTe and EuSe are investigated by the spectroscopy of second harmonic generation ͑SHG͒ in the vicinity of the optical band gap formed by transitions involving the 4f and 5d electronic orbitals of the magnetic Eu 2+ ions. In these materials with centrosymmetric crystal lattice the electric-dipole SHG process is symmetry forbidden so that no signal is observed in zero magnetic field. Signal appears, however, in applied magnetic field with the SHG intensity being proportional to the square of magnetization. The magnetic field and temperature dependencies of the induced SHG allow us to introduce a type of nonlinear optical susceptibility determined by the magnetic-dipole contribution in combination with a spontaneous or induced magnetization. The experimental results can be described qualitatively by a phenomenological model based on a symmetry analysis and are in good quantitative agreement with microscopic model calculations accounting for details of the electronic energy and spin structure.

No abstract

The magneto-Stark effect of excitons is demonstrated to be an efficient source of optical nonlinearity in hexagonal ZnO. Strong resonant second harmonic generation signals induced by an external magnetic field are observed in the spectral range of 2s and 2p excitons. The microscopic theoretical analysis shows that for excitons with a finite wave vector, exciton states of opposite parity are mixed by an effective odd parity electric field induced by the magnetic field despite its even parity. The field, spectral, and polarization dependencies of the second harmonic generation intensity validate the proposed mechanism. The observed phenomenon is not limited to a certain symmetry class and therefore must be effective in other semiconductors.

EuTe possesses the centrosymmetric crystal structure m3m of rocksalt type in which the second-harmonic generation is forbidden in electric dipole approximation but the third-harmonic generation ͑THG͒ is allowed. We studied the THG spectra of this material and observed several resonances in the vicinity of the band gap at 2.2-2.5 eV and at higher energies up to 4 eV, which are related to four-photon THG processes. The observed resonances are assigned to specific combinations of electronic transitions between the ground 4f 7 state at the top of the valence band and excited 4f 6 5d 1 states of Eu 2+ ions, which form the lowest energy conduction band. Temperature, magnetic field, and rotational anisotropy studies allowed us to distinguish crystallographic and magnetic-field-induced contributions to the THG. A strong modification of THG intensity for the 2.4 eV band and suppression of the THG for the 3.15 eV band was observed in applied magnetic field. Two main features of the THG spectra were assigned to 5d͑t 2g ͒ and 5d͑e g ͒ subbands at 2.4 eV and 3.15 eV, respectively. A microscopic quantum-mechanical model of the THG response was developed and its conclusions are in qualitative agreement with the experimental results.

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...