  In two-dimensional antiferromagnets, we identify the mixed Berry curvature as
the geometrical origin of the nonreciprocal directional dichroism (NDD), which
refers to the difference in light absorption with the propagation direction
flipped. Such a Berry curvature is strongly tied to the uniaxial strain in
accordance with the symmetry constraint, leading to a highly tunable NDD, whose
sign and magnitude can be manipulated via the strain direction. As a concrete
example, we demonstrate such a phenomenon in a lattice model of MnBi2Te4. The
coupling between the mixed Berry curvature and strain also suggests the
magnetic quadrupole of the Bloch wave packet as the macroscopic order parameter
probed by the NDD in two dimensions, distinct from the multiferroic order P
times M or the spin toroidal and quadrupole order within a unit cell in
previous studies. Our work paves the way of the Berry-curvature engineering for
optical nonreciprocity in two-dimensional antiferromagnets.
