Optical excitation of nonreciprocal antiferromagnetic magnons at sub-terahertz frequencies.

High-frequency and fast-propagating antiferromagnetic magnons hold significant potential for ultrafast spintronic technologies, particularly at terahertz frequencies. While conventional electrical methods for exciting antiferromagnetic magnons are limited in coherence, frequency range, and wavevector control, optical approaches offer significant opportunities to overcome these constraints. Here, we report the first observation of nonreciprocal magnon propagation in a canted antiferromagnet α-Fe2O3 at sub-terahertz frequencies. Using ultrafast optical excitation and Bragg-selective magneto-optical Kerr detection, we reveal distinct propagation dynamics for quasi-ferromagnetic and quasi-antiferromagnetic magnons at room temperature. Notably, the quasi-ferromagnetic magnon exhibits pronounced nonreciprocity and a record-high group velocity (33 km s-1). The amplitude nonreciprocity arises primarily from the asymmetric dipolar interaction enhanced by the Dzyaloshinskii-Moriya interaction, as supported by theoretical calculations. These findings establish fundamental principles for ultrafast and directional control of antiferromagnetic magnons, paving the way for high-frequency spintronic applications.