Magneto-optical detection of terahertz cavity magnon-polaritons in antiferromagnetic HoFeO₃

An intense THz pulse excites a high-Q magnetic resonance mode in the antiferromagnetic insulator HoFeO3 by the THz Zeeman torque. By using magneto-optical detection and sweeping the temperature, we observed an anomalous beating in the magnon dynamics for certain temperatures. The beating originates from the formation of cavity magnon-polaritons upon the intersection of the antiferromagnetic resonance frequency with the frequencies of the Fabry–Pérot modes inside the etalon formed by the sample cavity in the weak coupling limit. The validity of this idea is demonstrated by simulations using Maxwell's equations. Furthermore, the observed beating pattern depends on the polarization of the probe pulse. This dependence can be reproduced in the simulations by considering an imaginary Verdet constant, which could be a result of an interplay between the magneto-optical Faraday effect and static linear birefringence.