Antiferromagnetic (AFM) materials are considered as promising building blocks of novel data storage devices, however, detecting and manipulating AFM domains remains challenging. Here, we demonstrate that the two antiphase domains of the magnetoelectric antiferromagnet ${\mathrm{LiCoPO}}_4$ can be distinguished by their light absorption difference. Using visible and infrared spectroscopy, we observed spontaneous nonreciprocal absorption, also referred to as directional dichroism, at the crystal-field excitations of ${\mathrm{Co}}^{2+}$ ions coordinated by distorted oxygen octahedra. This absorption contrast is particularly pronounced near the telecommunication wavelength of 1550 nm. These findings allowed us to image the AFM domains in ${\mathrm{LiCoPO}}_4$ using a simple transmission light microscopy setup. Our findings suggest that optical magnetoelectric effects offer promising routes for probing the AFM order parameter in noncentrosymmetric transition metal compounds.

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• Antiferromagnetism
• Magnetic domains
• Magneto-optical effect