Thermoelastic dynamics of a nanoscale metallic film: Role of bilayer coupling revealed by extreme ultraviolet transient gratings

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Abstract
We investigate the thermoelastic dynamics of an 80 nm titanium thin film supported by a Si3N4 membrane by extreme-ultraviolet transient-grating (EUV-TG) spectroscopy across varying excitation fluences. A dominant oscillatory response at 143.8–150.6 GHz from time-domain fits is identified as a guided Lamb wave of the Ti/Si3N4 bilayer membrane with an in-plane phase velocity of approximately 6.3 km s−1. The near-coincidence of the Ti longitudinal and Si3N4 transverse acoustic velocities (∼6.1 km s−1) means that the dominant guided mode phase velocity simultaneously approximates both—a direct fingerprint of strong mechanical coupling between the layers. With increasing fluence, a systematic phase velocity increase of 4.7% is observed. A lower-frequency feature near 20–25 GHz is also detected, consistent with a breathing-type thickness response driven by transient uniform stress. Thermal relaxation analysis yields effective in-plane thermal diffusivities of (1.3–2.7) × 10−6 m2 s−1 and thermal conductivities of approximately 3–7 W m−1 K−1, significantly reduced relative to bulk Ti and consistent with boundary-limited heat transport in the confined membrane geometry. By combining nanometer-scale spatial selectivity with picosecond temporal resolution, EUV-TG enables simultaneous access to guided elastic wave propagation and nanoscale in-plane thermal transport in metallic membranes under non-equilibrium excitation. These results demonstrate how nanoscale confinement and multilayer coupling modify thermoelastic dynamics relative to bulk materials and highlight EUV-TG as a powerful tool for probing ultrafast thermoelastic dynamics in thin metallic films.

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