COHERENT ACOUSTIC PHONONS IN A SEMICONDUCTOR QUANTUM-DOT

Coherent acoustic phonons in PbS quantum dots are observed using femto second optical techniques. This is the first observation of coherent a coustic phonons in a semiconductor quantum dot; the phonons are genera ted through the deformation-potential coupling to the quantum-dot exci ton. The acoustic modes are weakly damped, and we also find extremely weak coupling (S similar to 0.01) to the optical. modes. These conclus ions have important consequences for the vibronic nature of the excito n transition in the quantum dot and its dephasing.