Light-induced Faraday effect from dynamical breakdown of Kleinman symmetry

Light-induced breaking of time-reversal symmetry in insulating crystals is usually interpreted in terms of the so-called inverse Faraday effect (IFE), namely a weak magnetization induced in a material by circularly polarized light [1]. In the presence of dipolar lattice modes, circular light can also induce a phonon magnetization, which contributes to the IFE through the mechanism of dynamical multiferroicity [2]. Following this idea, a recent ultrafast THz pump-visible probe experiment in insulating SrTiO3 [3] attempted to measure the strength of the induced phonon magnetic field via the polarization rotation of the probe, reporting fields four orders of magnitude larger than those predicted by ab-initio calculations.

In this talk I will discuss a different and more general mechanism: a light-induced Faraday effect arising from the antisymmetric component of the nonlinear optical susceptibility in driven quantum materials [4]. I will discuss how such a response emerges already at the purely electronic level as a consequence of non-adiabatic breaking of Kleinman symmetry, and how resonance with phonons can significantly enhance the response. The emergence of this response is associated with a dynamical breaking of time-reversal symmetry, where the finite-frequency circularly-polarized light plays the role of a fictitious static magnetic field to induce the probe polarization rotation. An estimate of this effective field yields magnitudes on the order of 30mT, in agreement with the experimental results. Possible applications to superconductors are also discussed, where the antisymmetric response to circular light can be used to distinguish between quasiparticle and Higgs response [5].

[1] P.S. Pershan, Phys. Rev. 130, 919 (1963)
[2] D.M. Juraschek, M. Fechner, A.V. Balatsky, and N.A. Spaldin, Phys. Rev. Mat. 1, 014401 (2017)
[3] M. Basini, M. Pancaldi, B. Wehinger, M. Udina, V. Unikandanunni, T. Tadano, M.C. Hoffmann, A.V. Balatsky, and S. Bonetti, Nature 628, 534 (2024)
[4] N. Sellati, J. Fiore, and L. Benfatto, arXiv preprint, arXiv:2605.27127 (2026)
[5] I. Zanotti, N. Sellati, M. Ursini, and L. Benfatto, in preparation (2026)

Niccolo will be at MPSD on Tuesday, Wednesday and Thursday; to schedule a meeting, please reach out to Michael Fechner.