Fast decoherence of slowly relaxing polarons in semiconductor InAs quantum dots

We report on polaron dynamic measurements in InAs/GaAs semiconductor quantum dots. The polaron dynamics is monitored by transient nonlinear spectroscopy by recording the transmission variation as a function of the pulse area of an infrared picosecond excitation. A non-monotonous transmission is observed for pulse areas larger than a π pulse. We attribute this transmission dependence with pulse area to a signature of damped optical Rabi oscillation of polarons. The damping of the Rabi oscillation is modeled by two level optical Bloch equations. A short dephasing time T2 around 5 ps is deduced for the resonantly excited polaron, one order of magnitude smaller than the relaxation time. We show that the decoherence of the polaron state is not limited by its slow relaxation but rather by the fast dephasing of its phonon component.