Optical second order magneto-electric susceptibility in the boracite Cu3B7O13Br

In the cubic phase of the boracite Cu3B7O13Br the anisotropy and dispersion of non-reciprocal linear dichroism (ND) and non-reciprocal linear birefringence (NB) have been studied. These two magnetic field induced effects have been observed near to the two strong absorption bands at the photon energies E-01 = 1.5 eV and E-02 = 1.2 eV. Anisotropy of NB and ND in wide spectral range shows that these phenomena are originated in a second order magneto-electric susceptibility for optical frequencies. The analysis of dispersion of ND and NB based on crystal field theory shows that an adequate description is obtainable only if the crystal field symmety at Cu2+ ions is lower than tetragonal. The reduction of symmetry may be caused by a structural disorder of the positions of halogen and/or metal ions in the boracite crystal lattice or by dynamic Jahn-Teller effect. Two absorption bands are assigned to electronic transitions from the singlet ground state B-2(2) to states of doublet E-2 which is split by low symmetry crystal field.