Total angular momentum dichroism of the THz vortex beams at the antiferromagnetic resonances

Circularly polarized light with spin angular momentum (SAM) is one of the most valuable probes of magnetism. Recently we demonstrated that light beams with orbital angular momentum (OAM), or vortex beams, can also couple to magnetism exhibiting dichroisms in a magnetized medium. Terahertz (THz) vortex beams with various combinations of the orbital angular momentum $l=+$/-1, 2, 3, and 4 and spin angular momentum $\sigma \quad = \quad +$/-1, or conventional circular polarization, were used for studies of the magnon spectra at the antiferromagnetic resonance conditions in TbFe$_{\mathrm{3}}$(BO$_{\mathrm{3}})_{\mathrm{4}}$ and Ni$_{\mathrm{3}}$TeO$_{\mathrm{6}}$ single crystals. In both materials we observed strong vortex beam dichroism for the magnon doublet, which is split in an external magnetic field applied along the spin ordering direction. The absorption conditions at the magnon frequencies depend on the total angular momentum of light $j$ that is determined by the combination of the spin and orbital angular momenta: $j=\sigma +l$. For the higher orders of $l, $the selection rules for AFM resonances dictated by $l$ completely dominate over that for conventional circular polarization. Our results demonstrate the high potential of the vortex beams with OAM as a new spectroscopic probe of magnetism in matter. https://arxiv.org/abs/2008.08670