Terahertz surprises in condensed matter: nutating magnets and light-induced coherent atomic motions

The interaction between light and matter is and it has been at the heart of our understanding of condensed matter physics, and of physics at large. Historically, the development of previously unavailable light sources, extending both the achievable wavelength and brightness ranges, have greatly impacted fundamental research and eventually technology, such as for the case of the laser. In this colloquium, I will present two of the newest kids on the light block, namely coherent THz radiation and X-rays, the latter produced at free electron lasers. Thanks to these novel light sources, we have recently been able to experimentally discover two novel and elusive phenomena in condensed matter: the evidence of magnetic "nutation", predicted over 10 years ago, and the observation of coherent control of phonons in the nonlinear regime. I will also offer some perspectives of the new opportunities ahead for the study condensed matter physics using light.