  The ability to manipulate phonon waveforms in continuous media has attracted
significant research interest and is crucial for practical applications ranging
from biological imaging to material characterization. Although several spatial
focusing techniques have been developed, these systems require sophisticated
artificial structures, which limit their practical applications. This is
because the spatial control of acoustic phonon waves is not as straightforward
as photonics so there is a strong demand for an alternative approach. Here we
demonstrate a phononic time lens in a dispersive one-dimensional phononic
crystal waveguide, which enables the temporal control of phonon wave
propagation. Pulse focusing is realized at a desired time and position with
chirped input pulses that agree perfectly with the theoretical prediction. This
technique can be applied to arbitrary systems and will offer both an
improvement in time and spatial sensing resolution and allow the creation of a
highly intense strain field, enabling the investigation of novel nonlinear
phononic phenomena such as phononic solitons and rogue waves.
