  Probing the quantum geometry and topology in condensed matter systems has
relied heavily on static electronic transport experiments in magnetic fields.
Yet, contact-free optical measurements have rarely been explored. Magnetic
dichroism (MCD), the nonreciprocal absorption of circular polarized light, was
theoretically linked to the quantized anomalous Hall effect in magnetic
insulators and can identify the bands and momenta responsible for the
underlying Berry Curvature (BC). Detecting BC through MCD faces two challenges:
First, the relevant inter-band transitions usually generate MCD in the infrared
(IR) range, requiring large samples with high quality. Second, while most
magnetic materials are metallic, the relation between MCD and BC in metals
remains unclear. Here, we report the observation of MCD in the IR range along
with the anomalous Hall effect in thin film MnBi2Te4. Both phenomena emerge
with a field-driven phase transition from an antiferromagnet to a canted
ferromagnet. By theoretically relating the MCD to the anomalous Hall effect via
BC in a metal, we show that this transition accompanies an abrupt onset of BC,
signaling a topological phase transition from a topological insulator to a
doped Chern insulator. Our density functional theory calculation suggests the
MCD signal mainly originates from an optical transition at the Brillouin zone
edge, hinting at a potential new source of BC away from the commonly considered
{\Gamma} point. Our findings demonstrate a novel experimental approach for
detecting BC and identifying the responsible bands and momenta, generally
applicable to magnetic materials.
