Modeling EUV time domain Brillouin scattering in VO₂ using rigorous coupled wave analysis

Vanadium dioxide (VO₂) exhibits an ultrafast (<1 ps) transition from an insulating to a metallic phase around 340 K [1], making it a compelling candidate for high-speed optical and electronic switching technologies [2]. However, VO₂ is opaque to visible light-based spectroscopies. Here, we implement extreme ultraviolet (EUV) time-domain Brillouin scattering (TDBS) to nondestructively probe the subsurface acoustic dynamics of a ~100 nm VO₂ thin film with significant surface roughness [3, 4]. We use an infrared laser pump pulse to excite an acoustic pulse in the VO₂ film and then detect a chirped reflectivity oscillation arising from the interference between EUV light reflected at the VO₂–substrate interface and that reflected by the propagating acoustic pulse. The oscillation frequency shows a frequency upshift as the EUV photon energy rises. We simulate the change in reflectivity using rigorous coupled wave analysis (RCWA) with a supporting software RETICOLO [5]. To match the experimental data, we model a periodic VO₂ structure, which resembles the high surface roughness of our sample, and are able to reproduce the experimentally observed frequency shift of the reflectivity oscillation as a function of EUV photon energy.

[1] Jager et al., PNAS 114 (36), 9558 (2017)

[2] Lysenko et al., Phys, Rev. B 82 (20), 205425 (2010)

[3] Gusev and Ruello, Appl. Phys. Rev. 5 (3), 031101 (2018)

[4] Johnsen et al., Rev. Sci. Instrum. 94 (3), 033001 (2023)

[5] Hugonin and Lalanne, arXiv:2101.00901