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.
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