Isakson Prize (2022): Seeing is believing: nonliner optics on ferroic materials

For millennia, ferromagnetism was the only form of ferroic order known to humankind. Now, however, a large variety of magnetic, electrical and mechanical types of ferroic phenomena are being discussed. All of these all have one property in common: The ferroic ordering breaks the symmetry of the host material. Nonlinear, that is, frequency-converting coherent optical processes are very sensitive to these symmetry changes. Even the simplest nonlinear optical process, doubling of the frequency of the light, termed "second harmonic generation" (SHG), therefore couples to the ferroic order parameter and accesses important features of the ferroic state that are often inaccessible to non-optical techniques. Hidden ferroic structures like antiferromagnetic 180° domains can thus be imaged. Novel states like ferrotoroidicity as a spontaneous order of magnetic whirls can be probed. Ultrafast processes in the dynamics of the ferroic state can be resolved in terms of the question how fast a magnetic state can be switched. In particular, the coexistence of different types of ferroic order in a material can be imaged by SHG in the same experiment. SHG thus became an invaluable tool for resolving the magnetoelectric coupling of domains in multiferroics as materials uniting magnetic and ferroelectric order. In my talk I will give an overview of the most important milestones in the classification of (multi-)ferroic materials by nonlinear optics. On the one hand, I will discuss basic questions such as the search for yet unknown types of ferroic order and correlations. On the other hand, I will address highly application-relevant issues such as the use of SHG as a new in-situ characterization technique that tracks the emergence of ferroic order in thin films during the growth process. Presentation of a concept for "magnetoelectric teleportation" will be the not-too-serious conclusion of the lecture.