Probing Quantum Magnets via Non-Linear Spectroscopy

The response of many-body systems to weak perturbations can be cast within the general framework of linear response, in which the response of the system is linearly proportional to the applied perturbation. This framework forms the foundation for many of the most popular tools in use today in condensed matter physics, but still provides only a limited set of information. Pushing beyond the conventional boundaries of this linear framework and into the realm of non-linear responses holds enormous potential in providing new insights and phenomena. In the field of quantum magnetism, the recent application of multi-dimensional coherent spectroscopy, the backbone of non-linear optics, to the THz range of magnetic excitations, has made the study of dynamical non-linear magnetic response now possible. In this talk, I will present a brief overview of some of the relevant challenges and opportunities, and discuss concrete examples in which non-linear spectroscopy provides new information not accessible within conventional linear response.