Direction Dependent Luminescence in Multiferroic CuB₂O₄

In materials where both space-inversion and time-reversal symmetries are simultaneously broken, luminescence intensities can be different between the opposite emission directions. However, only small directional asymmetry has been reported so far. One possible reason is that the effect has been investigated only in the paramagnetic materials where time-reversal symmetry is violated by an external magnetic field. In this presentation, I will report that the effect is robust in a noncentrosymmetric magnet CuB₂O₄ with spontaneous breaking of space-inversion and time-reversal symmetries. The sample was excited by a He-Ne laser of wavelength 633 nm, and the photoluminescence spectra in near infrared region were measured by using a grating-type optical spectrometer. We have found that the luminescence intensity changes by 70 % between the opposite directions of the emission, which is about 100 times stronger than the previously reported values. We also demonstrate that such a gigantic nonreciprocal emission of light can be applied to the imaging technique of magnetic domain structures.