Magnetic order parameters in altermagnets can couple to quantized lattice vibration via both piezomagnetic and magnetoelastic effects, leading to the renormalization of phonon dispersion. Here, we demonstrate photo-induced dynamic frequency modulation of THz phonons excited in anisotropically-strained epitaxial RuO2 thin films using ultrafast coherent phonon spectroscopy and time-resolved magneto-optic Kerr effect measurement. A coherent oscillation of a transverse acoustic phonon appears in the sub-THz range with increasing film thickness above 4 nm due to local dislocation arising from the anisotropic strain relaxation, which hosts large non-zero shear strain. Interestingly, this phonon mode exhibits a time-varying mode hardening below ~ 500 K. Furthermore, an optical phonon oscillation emerges in magnetization dynamics of the photo-induced non-equilibrium state, and it becomes significantly softened near the critical temperature, while there is no observable magneto-optic signal in fully-strain-relaxed films. Such notable dynamic frequency modulations in acoustic and optical phonons offer an opportunity to manipulate phonons in the THz range through the spin-phonon coupling controlled by epitaxial design, which can inspire the new class of altermagnetic applications in the ultrafast quantum opto-spintronics.
@article{arxiv.2509.23969,
title = {Strain-induced Dynamic Spin-Phonon Coupling in Epitaxial RuO2 Films},
author = {In Hyeok Choi and Seung Gyo Jeong2 and Jae Hyuck Lee and San Kang and Sreejith Nair and Changyoung Kim and Dirk Wulferding and Bharat Jalan and Jong Seok Lee},
journal= {arXiv preprint arXiv:2509.23969},
year = {2025}
}