Reconstruction of Acoustic Echoes and Their Effect on Brillouin Oscillations in Nanoscale Characterization of Supported Thin Films by Ultrafast Photoacoustics
Year: 2026
Type: article
Abstract: The paper presents an experimental and theoretical analysis of the effects accompanying the non-contact, non-destructive nanoscale characterization of thin-film metal–dielectric structures through the substrate, using high-amplitude picosecond acoustic pulses generated by an ultrafast laser. Time-resolved reflectivity changes ΔR(t)/R0 at 793 nm are measured in the picosecond time domain using an all-optical pump-probe setup with balanced, phase-sensitive detection tuned to 0.5 kHz. For a glass-supported nanoscale nickel film, the measurements at the glass–film interface are carried out at absorbed laser fluences ranging from 1.7 to 10.3 mJ/cm2, generating peak pressures of up to several gigapascals. The ΔR(t)/R0 signal is dominated by the resulting Brillouin oscillations at a frequency of fBO = 1/TBO = 21.5 ± 0.1 GHz within the substrate, which undergo amplitude-phase modulation due to the superposition of the individual Brillouin oscillations from the acoustic pulses leaking into the substrate after reverberating in the film. Through post-processing, the obscured ΔR(t)/R0 signal of the acoustic pulses reverberating in the film at fecho = 1/Techo = 39.1 ± 0.2 GHz is reconstructed, showing excellent agreement with the reference signal directly measured at the air–film interface. It is shown that, unlike sufficiently thick submicron films, which exhibit no amplitude-phase modulation by reverberating acoustic pulses during the first several hundred picoseconds (Techo >> TBO), nanoscale films—where Techo ≈ TBO/2—cause a significant reduction in the resulting amplitude of the Brillouin oscillations in the substrate, which reaches its minimum due to destructive interference.
Source: Nanomaterials
DOI: 10.3390/nano16201307
Author: Sergey A. Romashevskiy
Institution: Joint Institute for High Temperatures
Language: English
Subfield: Mechanics of Materials
Field: Engineering
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $2,896
OpenAlex ID: W7222647798