Circularly polarized light — that is, light with a spin angular momentum — can probe the magnetism of condensed-matter systems through an effect called circular dichroism. Light that carries an orbital angular momentum in addition to a spin angular momentum forms so-called vortex beams. Sirenko and colleagues now use such vortex beams at terahertz frequencies to detect magnetic excitations in a ferrimagnetic rare-earth iron garnet and observe an orbital-momentum-dependent dichroism in addition to the expected circular dichroism.
The researchers shine vortex beams of opposite orbital angular momentum onto the transparent crystal and measure the transmittance. They observe collective excitations of ligand field modes, which are linked to the precession of the rare earth and iron spins, and detect a dependence of the signal intensity on the orbital angular momentum of the vortex beams. Interestingly, the vortex beam dichroism is more pronounced than the circular dichroism also measured by the researchers. These experiments show the potential of vortex beams for the exploration of complex magnetic materials and for the distinction between spin and orbital contributions to the magnetic texture.