  The way nuclear motion affects electronic responses has become a very hot
topic in materials science. Coherent acoustic phonons can dynamically modify
optical, magnetic and mechanical properties at ultrasonic frequencies, with
promising applications as sensors and transducers. Here, by means of ultrafast
broadband deep-ultraviolet spectroscopy, we demonstrate that coherent acoustic
phonons confined in anatase TiO$_2$ nanoparticles can selectively modulate the
oscillator strength of the two-dimensional bound excitons supported by the
material. We use many-body perturbation-theory calculations to reveal that the
deformation potential is the mechanism behind the generation of the observed
coherent acoustic wavepackets. Our results offer a route to manipulate and
dynamically tune the properties of excitons in the deep-ultraviolet at room
temperature.
