Fluctuation properties of phonons generated by ultrafast optical excitation of a quantum dot

摘要

In this paper, the fluctuation properties of phonons after the optical excitation of a quantum dot with an ultrashort laser pulse are studied theoretically. When a single pulse with pulse area p creates an exciton in the system, a coherent phonon state builds up. Fluctuations of coherent states are time independent and equal to the vacuum fluctuations. By an excitation with a π/2 pulse a superposition of ground and exciton state is created. In this case the phonon system becomes entangled with the electronic system. In the phonon subsystem this leads to the formation of a statistical mixture of the vacuum and the coherent state. The fluctuations of this mixture oscillate in time with both the single and the double phonon frequency, but never exhibit squeezing. The discrepancies with the predictions made in a Raman tensor model are briefly discussed. All scenarios are illustrated using the Wigner function.