Coherent Magnons Driven by Photomodulated Anisotropy in Altermagnetic MnTe
Manganese telluride ($\text{MnTe}$) has recently emerged as a prototypical $g$-wave altermagnet, providing an ideal platform to investigate the non-equilibrium excitations of altermagnetic order. Here, we report simultaneous spatial mapping of the local equilibrium orientation of the N\'eel vector, $\varphi_L(\mathbf{r})$, alongside the amplitude, $\Delta\varphi(\mathbf{r},t)$, and frequency, $\Omega(\mathbf{r})$, of photoexcited spin waves. Based on these measurements, we place a remarkably low upper bound of $\approx 60~\mu\text{eV}$ (0.7 K) on the spin-wave gap arising from intrinsic anisotropy. This exceptionally weak hexagonal anisotropy ($K_6$) renders the altermagnetic order highly susceptible to optical tuning, allowing coherent spin waves to be driven by a photoinduced enhancement of $K_6$. Above a threshold pump fluence, our spatial maps reveal that this photomodulation manifests as a six-fold symmetric sawtooth dependence of $\Delta\varphi$ on $\varphi_L$ and a cycloid-like modulation of $\Omega$. Ultimately, the near-isotropy of the N\'eel vector in $\text{MnTe}$ enables optical and mechanical control over the orientation of spin-splitting in the electronic band structure, offering new pathways for altermagnetic spintronics.