Unveiling Hidden Magnon Modes in van der Waals Magnets via Gain-Assisted Strong Photon–Magnon Coupling

Magnonic systems based on two-dimensional van der Waals (vdW) magnets offer a versatile platform for hybridizing magnons with photons, phonons, and electrons, promising advancements in information processing. A key challenge, however, is the detection and manipulation of magnons due to their ultra-weak signals in atomically thin samples. To overcome this, we integrate a van der Waals antiferromagnet, CrCl₃, with a high-quality-factor active cavity operating at cryogenic temperatures. Utilizing the cavity’s gain and self-sustained oscillation, we uncover multiple magnon modes previously inaccessible in conventional measurements. Specifically, we observe two low-damping magnon modes near the acoustic mode of CrCl₃, with damping rates three orders of magnitude lower. This exceptionally low damping enables strong cavity photon–magnon coupling, yielding two distinct bistable regions upon magnetic field sweep. Additionally, we also observe a ferromagnetic magnon mode located significantly below the Kittel frequency, twice the antiferromagnetic–ferromagnetic transition field. Our work establishes a gain-assisted strong-coupling approach as a powerful tool for probing magnon dynamics in two-dimensional vdW magnets, opening new avenues for engineering magnonic states for future information technologies.