  Controlling acoustic phonons, the carriers of sound and heat, has been
attracting great attention toward the manipulation of sonic and thermal
properties in nanometric devices. In particular, the photo-acoustic effect
using ultrafast optical pulses has a promising potential to optically
manipulate phonons in picoseconds time regime. However, its mechanism has been
so far mostly based on the commonplace thermoelastic expansion in isotropic
media, limiting the spectrum of potential applications. We investigate a
conceptually new mechanism of photo-acoustic effect involving the structural
instability, by utilizing a transition-metal dichalcogenide VTe$_2$ with the
ribbon-type charge-density-wave (CDW). Ultrafast electron microscope imaging
and diffraction measurements reveal the generation and propagation of unusual
acoustic waves in the nanometric thin plate associated with the optically
induced instantaneous charge-density-wave dissolution. Our results highlight
the capability of photo-induced structural instability as a source of coherent
acoustic waves.
