Extreme Ultraviolet Time-Domain Brillouin Light Scattering for Characterizing Opaque Thin Films

Vanadium dioxide (VO₂) is a phase change material that undergoes an ultrafast, insulator-to-metal phase transition near room temperature, making it a promising material for next-generation computing [1,2]. Visible-light-based time-domain Brillouin scattering (TDBS) can be used to nondestructively characterize sub-surface dynamics in transparent materials [3]. Here, we implement extreme ultraviolet (EUV) TDBS experiment [4] to study an opaque, ~100 nm thick VO₂ thin film with high surface roughness. A 40 fs, ~785 nm laser pump pulse impulsively heats the VO₂ film, launching a longitudinal acoustic pulse. Then, a ~20 fs EUV comb based on high harmonic generation is used to stroboscopically probe the change in reflectivity due to the acoustic pulse [5]. The reflected EUV pulses are spectrally separated using a diffraction grating, allowing simultaneous detection of the change in reflectivity of each harmonic order. The resulting reflectivity signal exhibits chirped oscillations due to interference between the EUV light reflected from the surface and reflected from the acoustic pulse within the film. The frequencies of this oscillation can then be used to fit elastic and thermal properties of the VO₂ film.

1. PNAS 114 (36), 9558 (2017)

2. Phys. Rev. B 82 (20), 205425 (2010)

3. Appl. Phys. Rev. 5 (3), 031101 (2018)

4. Rev. Sci. Instrum. 94 (3), 033001 (2023)

5. Science 280 (5368), 1412 (1998)

6. arXiv: 2101.00901

​