Optical diode effect at telecom wavelengths in a polar magnet

Magnetoelectric multiferroics such as rare earth manganites host nonreciprocal behavior driven by low symmetry, spin-orbit coupling, and toroidal moments, although less has been done to explore whether lanthanides like Er3+ might extend functionality into the hard infrared for optical communications purposes. In this work, we reveal nonreciprocity in the f-manifold crystal field excitations of h-Lu0.9Er0.1MnO3. In addition to contrast in the highest fields, we demonstrate nonreciprocity at technologically-relevant energy scales--specifically in the E-, S-, and C-bands of the telecom wavelength range--and at low magnetic fields and room temperature. In fact, the low field behavior is consistent with possible altermagnetism. These findings advance the overall understanding of localized excitations in rare earth-containing systems and pave the way for entirely new types of telecom applications.