Reconstruction of Acoustic Echoes and Their Effect on Brillouin Oscillations in Nanoscale Characterization of Supported Thin Films by Ultrafast Photoacoustics

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.