Spin torque antiferromagnetic resonance (ST-AFMR) is fundamental to high-frequency spintronic devices, such as ultrafast magnetic storage and terahertz spin nano-oscillators. However, limited by generating terahertz spin torques, it has been confined to the gigahertz in-plane linearly polarized low-frequency mode of easy-plane antiferromagnets. Here we employ optical terahertz pulses as high-frequency excitation sources and time-resolved Faraday effect as detection to investigate ST-AFMR of the out-of-plane linearly polarized high-frequency mode in $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3/\mathrm{Pt}$ bilayers. We observe ST-AFMR at a sub-terahertz frequency of 280 GHz, with fieldlike spin-orbit torque as the dominant mechanism. The net magnetization revealed by Faraday detection exhibits only in-plane oscillation, consistent with the high-frequency mode of easy-plane antiferromagnets. Moreover, the film thickness-dependent frequency indicates an inherent origin of the high frequency rather than standing waves. The ST-AFMR frequency also demonstrates high stability against external magnetic fields, which can be attributed to the absence of precession in the mode. Our findings open new avenues for ultrafast applications of antiferromagnetic-based devices.

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• Spin dynamics
• Spin torque
• Spintronics
• Ultrafast magnetic effects