Ultrafast polarization dynamics in optically excited biased quantum wells

We observe ultrafast polarization dynamics in strongly internally biased InGaN/GaN multiple quantum wells during intense femtosecond optical excitation by means of time-resolved detection of THz emission, correlated with time-integrated photoluminescence measurements. We demonstrate that in the case of strong enough excitation the built-in bias field (on the order of MV/cm) can be completely screened by the carriers excited into spatially separated states. This ultrafast screening of the initial bias field across the quantum well leads to dynamical modification of the band structure of the sample, and consequently to dynamical modification of the optical absorption coefficient within the duration of the excitation pulse. We show that such an optically induced dynamical screening of the biased quantum well can be described in terms of discharging of a nano-scale capacitor with a femtosecond laser pulse. The electrostatic energy stored in the capacitor is released via THz emission. A realistic quantum-mechanical model of the temporal evolution of the polarization inside the quantum wells shows that due to its nonlinearity such a process may lead to emission of a THz pulse with bandwidth significantly exceeding that of the excitation pulse.