Acoustic phonon assisted damping of Rabi oscillations in InAs quantum dots

Full text: A fundamental effect, the Rabi oscillation, is highly regarded as the most viable mechanism to control quantum mechanical states in order to establish quantum computation. Under strong optical excitation a two-level system tends to a periodic oscillation of the population in a sinusoidal manner over time. In this work we present measurements of the damping of Rabi oscillations in self- assembled InAs quantum dots in the temperature range from T = 5 K to 90 K. Femtosecond spectral hole burning spectroscopy revealed time domain as well as frequency domain resolved data, from which we concluded, that acoustic phonons are responsible for the damping. Initial measurements with a 350 μeV (FWHM) pump pulse and a broadband probe pulse, approximately 50 ps after the pump pulse, revealed spectral holes at the energy of the excitonic transition (0-X) due to bleaching of the ground state, accompanied by spectral anti-holes corresponding to biexcitonic transitions (X-XX) only a few meV apart (reduced by the biexciton binding energy of 4 meV) from the excitonic transitions, due to enhanced absorption. At low temperatures (5 K) the line shape of these transitions is entirely determined by the spectral shape of the pump pulse and shows only transitions not assisted by phonons (zero phonon line, ZPL), whereas at increasing temperatures a broad pedestal appears with the ZPL. The broad pedestal accounts for a fast initial temporal polarization decay, while the narrow ZPL corresponds to a very slow decay due to the excitons lifetime. This pedestal basically resembles phonon assisted excitonic transitions that become more and more significant with rising temperature until at T = 90 K the ZPL is completely covered by the acoustic phonon sidebands. In a first order approximation a single exponential polarization decay has been accounted for the emergence of this spectrum, eventually leading to a dephasing time T2 0.5 ps. In a second step, a stronger pump pulse intensity induces Rabi oscillations between the crystal ground state (0) and the excitonic state (X), that are indirectly detected by the observation of the biexcitonic transition (X-XX). The temporal evolution of the oscillation is usually described as a function of the pulse area, that depends on the dipole moment of the two-level system and the strength of the electrical field component of the optical excitation. At 5 K the oscillations can be seen with only a little damping. As the temperature is increased, the influence of acoustic phonons on the overall polarization has to be taken into account, leading to a stronger damping of the Rabi oscillations, until eventually at 90 K the dephasing time becomes smaller than the pump pulse duration, leading to the total loss of coherence between the polarization and the excitation, thus no Rabi oscillations occur. (author)