Phys. Rev. B 96, 224301 (2017)

Michael Wismer and colleagues from Germany and the USA have now predicted an ultrafast optical Faraday effect in an achiral transparent dielectric. The phenomenon could be attractive for studying chiral dynamics with attosecond temporal resolution. In the pump–probe measurements performed by the researchers, a 5 fs circularly polarized few-cycle infrared (IR) pump pulse impinges at normal incidence along the optical axis of a sapphire crystal. A weak 2.5 fs linearly polarized ultraviolet pulse is used to probe the induced optical Faraday effect. Analysis shows that under a strong 1 V A–1 IR field, the induced chirality vanishes for probe delays longer than the duration of the IR pulse and the ellipticity of the probe pulse has an oscillatory dependence on the pump–probe delay. The team attributed the ultrafast Faraday effect to the relaxation of time-reversal symmetry due to a transient transfer of angular momentum from light to matter. The findings may be useful for developing ultrafast all-optical circular-polarization modulators, isolators and circulators.