Optical spectroscopy is a highly sensitive tool to investigate the elementary excitations in correlated systems with coupled spin, lattice and orbital degrees of freedom. For example, low-lying electronic excitations of electric and magnetic origin in the THz range can provide information on the electronic level splitting due to orbital ordering. The coupling of spin and orbital degrees of freedom to the lattice can be traced by infrared spectroscopy. The infrared optical phonons reflect the influence of magnetic exchange interactions and magnetic ordering via spin-phonon coupling which often is the decisive interaction to release magnetic frustration. In addition, the magnetic excitation spectrum of antiferromagnetically coupled spin systems can be directly accessed via cooperative exciton-magnon transitions involving a pair of magnetic ions. As exciton-magnon transitions can occur already when short-range order develops, these excitations are particularly promising probes of the spin-wave spectrum in frustrated magnets where finite spin correlations persist up to the Curie-Weiss temperature.
As a first part basic concepts of THz and FIR spectroscopy will be discussed and an overview of magneto-optical excitations accessible by these techniques will be given.
In the second part the emphasis will be on recent work in specific archetypical systems e.g. transition-metal monoxides, highly frustrated magnets such as ZnCr2O4, and low-dimensional spin systems.
