Optical third-harmonic spectroscopy of the magnetic semiconductor EuTe

EuTe possesses the centrosymmetric crystal structure m3m of rock-salt type in which the second harmonic generation (SHG) is forbidden in electric-dipole approximation, but the third harmonic generation (THG) 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 4f7 state at the top of valence band and excited 4f65d1 states of Eu2+ 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 the THG intensity was observed in applied magnetic fields at particular resonances due to interference of crystallographic and magnetic field induced contributions. A microscopic quantum mechanical model of the THG response was developed and its conclusions are in good agreement with the experimental results.