Coherent Acoustic Phonons in a Semiconductor Quantum Dot

Coherent acoustic phonons in PbS quantum dots are observed using femtosecond optical techniques. This is the first observation of coherent acoustic phonons in a semiconductor quantum dot; the phonons are generated through the deformation-potential coupling to the quantum-dot exciton. The acoustic modes are weakly damped, and we also find extremely weak coupling ( $S\sim 0.01$) to the optical modes. These conclusions have important consequences for the vibronic nature of the exciton transition in the quantum dot and its dephasing.