Optical third-harmonic spectroscopy of the magnetic semiconductors EuSe and EuTe

EuX (X=O, S, Se, and Te) are magnetic semiconductors possessing the centrosymmetric crystal structure m3m of rock-salt type in which the second-harmonic generation is forbidden in the electric dipole approximation but the third-harmonic generation (THG) is allowed. We studied the THG spectra of EuSe and EuTe near the band gap at 2.1-2.6 eV and at higher energies up to 4 eV. The observed signals are attributed to four-photon THG processes involving specific combinations of electronic transitions between the 4f7 ground state at the top of the valence band and the excited 4f65d1 states of the Eu2+-ions forming the lowest conduction bands. Strong modifications of the THG intensity were observed in applied magnetic fields up to 10 T revealing the magnetic phases to play the leading role. Temperature, magnetic field, and rotational anisotropy measurements allow us to unambiguously separate the crystallographic and magnetic-field-induced contributions. In addition, we developed a microscopic quantum-mechanical model that is in good agreement with the experimental results.