Research progress of coherent control of terahertz spin waves and strong coupling in rare-earth orthoferrites

Antiferromagnets (AFM) are promising for future spintronic applications due to their advantageous properties. Antiferromagnets produce no stray fields and are insensitive to external magnetic field perturbations. Furthermore, antiferromagnets show intrinsic high terahertz (THz) frequency dynamics. The THz pulses are a direct and general probe of ultrafast spin dynamics in insulating antiferromagnets. In this review article, we discuss the excitation and control of the antiferromagnetic spin resonances in rare-earth orthoferrites (RFeO3, R indicates Y and rare-earth element) with the THz electromagnetic pulsetime-domain spectroscopy. We believe that this approach is general and can be applied to a broad range of materials with different AFM spin alignments, giving a novel non-contact approach to probing AFM order with ps temporal resolution. We summarize different quasi-ferromagnetic modes (qFM) and quasi-antiferromagnetic modes (qAFM), as well as the spin reorientation transition temperatures of RFeO3. Coherent control of spin waves at THz frequency promises fruitful applications in ultrafast magnetization control and has received increasing attention. It is demonstrated that not only the delay time between the excitation and control THz pulses arriving DyFeO3, but also the intrinsic dielectric anisotropy of YFeO3 in the THz range allow the coherent control of both the amplitude and the phase of the excited spin waves. Moreover, we outline the current observation of Dicke cooperativity in magnetic interaction of ErxY1－xFeO3, which presents a route to understanding, controlling, and predicting novel phases of condensed matter by using the concepts and tools available in quantum optics. Finally, magnon-polaritonsare demonstrated to play a key role in preparing the THz waves through TmFeO3.