Ultrafast polarization dynamics in biased quantum wells under strong femtosecond optical excitation

We observe ultrafast polarization dynamics in strongly internally biased InGaN/GaN multiple quantum wells during intense femtosecond optical excitation. In the case of strong enough excitation we demonstrate that the built-in bias field (on the order of MV/cm) can be completely screened by the carriers excited in spatially separated states. The removal of the initial strong bias leads to dynamical modification of the band structure, the electron and hole wave functions, and the 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 nanoscale capacitor driven by a femtosecond laser pulse. The electrostatic energy stored in the capacitor is released via THz emission. Due to its nonlinearity such a process may lead to emission of a THz pulse with bandwidth significantly exceeding that of the excitation pulse.