Near-field imaging of surface plasmon polaritons in ultra-thin altermagnetic candidate RuO2 thin films

Metallic altermagnets promise new paradigms for spin-charge conversion, but candidate materials remain unproven or under-explored. Epitaxial strain engineering of metallic RuO2 thin films on TiO2(110) allows for controlled modulation of electronic anisotropy through anisotropic strain relaxation as film thickness is increased [1]. Motivated by its potential to stabilize altermagnetic order [2], we characterize the induced electronic anisotropy of ultra-thin RuO2 with a nano-optic probe. Using near-field optical microscopy, we probe surface plasmon polaritons in epitaxially strained RuO2 at film thicknesses from <2 nm to 17 nm. By resolving polaritons propagating differentially along the [001] and [1-10] directions we detect anisotropy in the polariton wavelength that cannot be explained solely by dielectric anisotropy of the substrate. Further, we find that this anisotropy varies deterministically with film thickness. By relating the measured polariton dispersion to optical conductivity of free carriers in epitaxial RuO2, we will establish a comprehensive view of the strain-induced anisotropic charge response of this candidate metallic altermagnet.

1. Jeong, S. G.; Choi, I. H.; Lee, S.; Oh, J. Y.; Nair, S.; Lee, J. H.; Kim, C.; Seo, A.; Choi, W. S.; Low, T.; Lee, J. S.; Jalan, B. Sci Adv 2025, 11 (26)

2. Jeong, S. G.; Lee, S.; Lin, B.; Yang, Z.; Choi, I. H.; Oh, J. Y.; Song, S.; Lee, S. W.; Nair, S.; Choudhary, R.; Parikh, J.; Park, S.; Choi, W. S.; Lee, J. S.; LeBeau, J. M.; Low, T.; Jalan, B. PNAS 2025, 122 (24)