Optical rectification of circularly polarized light generates a static magnetization through the inverse Faraday effect. Recent ultrafast experiments have unveiled a substantial, orders-of-magnitude gap between the measured magnetic fields and theoretical predictions [1]. In this talk, we show that the discrepancy arises due to a missing factor on the order of α^{-2} ≈ 2 × 10^4 , where α ≈ 1/137 is the fine structure constant.

We demonstrate that circular polarization generally creates large non-Maxwellian fields that disrupt time-reversal symmetry, effectively mimicking authentic magnetic fields within the material while eluding detection externally [2]. These unconventional fields, reaching effective magnitudes as high as 100 T, lead to phenomena akin to Faraday rotation and robustly interact with magnons in magnetically ordered materials. The connection between the non-Maxwellian fields and the Autler-Townes and AC Stark effects of atomic physics will be discussed.

These considerations are particularly relevant to the direct, resonant excitation of polar phonons. Contrary to common perception, the origin of phonon-induced magnetic activity does not stem from the motion of ions themselves; instead, it arises from the effect their motion exerts on the electron subsystem via the electron-phonon interaction [3]. Because the light-induced non-reciprocal fields depend on the square of the phonon displacements, the chirality the photons transfer to the ions plays no role in magnetophononics.

1. M. Basini, [..], A. V. Balatsky, and S. Bonetti, “Terahertz electric-field driven dynamical multiferroicity in SrTiO3,” Nature 628, 534 (2024).
2. R. Merlin, “Unraveling the effect of circularly polarized light on reciprocal media: Breaking time reversal symmetry with non-Maxwellian magnetic-esque fields,” Phys. Rev. B 110, 094312 (2024).
3. R. Merlin, “Magnetophononics and the chiral phonon misnomer,” PNAS Nexus 4, pgaf002 (2025).