Strong coupling of antiferromagnetic resonance in YFeO₃and Fabry-Perot type cavity modes at above 0.5 THz

We investigate the strong coupling of antiferromagnetic resonance and THz electromagnetic cavity modes. Since polariton modes are hybrid light-matter states, they have properties that can be used in quantum devices, as shown in many physical systems where such coupling was achieved. Recently, magnon-polaritons are intensively researched in ferromagnetic materials in the microwave range. There is an interest in obtaining THz magnon-polaritons with antiferromagnetic materials that would allow for spin quantum effects to be observed at much higher frequencies than with ferromagnets. However, there are only a few reports on strong magnon-photon coupling at THz frequencies, which is mostly due to difficulty in manufacturing high quality factor cavities. Here, we report an investigation of a room temperature system with yttrium ferrite (YFeO₃) single crystal of a 0.5 mm thickness placed on a copper mirror. We measured reflection from this system using a monochromatic continuous wave spectrometer operating in the range of 0.5-0.75 THz that is based on frequency extenders to a vector network analyser. Measured reflection spectra show modes of a Fabry-Perot type cavity created by the mirror and the sample-air boundary. With rising temperature, these modes are interacting with the quasi-antiferromagnetic mode of YFeO₃, that is softening with rising temperature. This strong interaction is manifested by avoided crossings in frequency-temperature dependence, with the splitting between the polariton modes of about 20 GHz. Obtained results show clearly that Farby-Perot type cavities can be used to obtain strong magnon-photon coupling, which is an important improvement that can allow for further investigations of THz strong coupling in antiferromagnetic materials.