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. Incorporating lanthanide ions like Er³⁺ extends functionality into the hard infrared for telecom purposes although the localized nature of the excitations is anticipated to weaken magnetoelectric coupling and limit the size of any nonreciprocal effect. In this work, we reveal nonreciprocity in the f-manifold crystal field excitations of h-Lu_0.9Er_0.1MnO₃ demonstrating that, contrary to conventional expectations, localized features in the form of rare earth excitations can host strong dichroic effects. 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 S/A-type altermagnetism in this system. These findings provide a significant conceptual advance in understanding localized excitations in rare earth-containing systems and pave the way for entirely new types of telecom applications.