Antiferromagnetic materials attract much attention due to their inherently ultrafast spin dynamics and potential application to the magnetic memory devices. Among them, two-dimensional easy-plane type antiferromagnets are of particular interest because they show anomalous magnetic behaviors such as BKT transition and long-distance spin transport via spin superfluidity. Non-thermal excitation of spin waves is crucial to investigate the exotic spin phenomena because one can avoid thermal excitations and detect pure spin contributions.

 We demonstrate all-optical generation and detection of the antiferromagnetic magnon using a pump-probe spectroscopy in a prototype of 2D XY spin system BaNi₂P₂O₈. This material has a honeycomb lattice structure and Ni²⁺ magnetic moments show antiferromagnetic ordering below 24 K. The magnon was excited with linearly polarized pump pulses of the photon energy below the band gap where the optical absorption is negligibly small. The excited spin dynamics was detected with the polarization rotation of probe pulses. A clear magnetic resonance was observed around 13 GHz below the magnetic ordering temperature, which corresponds to the in-plane antiferromagnetic magnon mode where the Néel vector rotates in the xy-plane. The 90-degree rotation of the pump polarization reverses the sign of the signal, clearly indicating the non-thermal nature of the excitation.