Generation of high harmonics of light fields had led to numerous scientific and technological advancements since its discovery 30 years ago, including attosecond spectroscopy of atoms, molecules and condensed matter and tabletop high-resolution imaging of nanostructures. However, the polarization state of high harmonics was essentially limited to linear, which prevented experimental access to the role of photon-spin in harmonic generation, and hindered the use of high harmonic radiation for chiro-optical investigation with magnetic and other chiral matter.
My talk presents the first demonstration of high harmonic generation with circular polarization and emerging applications. In the first part, I describe the dynamical symmetries that impose polarization constraints on the interaction of light and matter, both at the level of a single atom and of an ensemble. The second part discusses application of this novel polarization controllability for magneto-optical experiments. Most importantly, I show that quantitative mapping of magnetic domains can be done using high harmonics, overcoming the notoriously weak magneto-optical scattering coefficients. In the future, the unique coherence and femtosecond nature of high harmonics will facilitate quantitative, element-specific and spatially-resolved studies of ultrafast magnetization dynamics, advancing both fundamental and applied aspects of nanoscale magnetism.
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