  Brillouin oscillation appears in picosecond ultrasonics for a transparent
specimen because of backward light scattering by moving strain pulse. Its
amplitude is comparable with those of other responses, such as pulse-echo
signals and through-thickness resonance, obscuring these
non-Brillouin-oscillation responses. We here find that Brillouin oscillation
can be suppressed in a transparent free-standing film by coating both sides
with metallic thin film of appropriate thickness and that this peculiar
behavior is caused by strain pulses generated on both sides with a slight phase
difference. This phenomenon allowed us to fabricate a
Brillouin-oscillation-free diamond free-standing film, which showed high
capability for sensor applications.
