Properties of an electron in a quantum double well driven by a strong laser: Localization, low-frequency, and even-harmonic generation

We study how a strong semi-infinite laser pulse affects an electron confined by a potential whose parameters mimic an AlAs-GaAs-AlAs double quantum well. Interesting phenomena take place for special values of laser frequency, intensity, and pulse rise time. There are values of these parameters for which the dipole moment of the system has a low-frequency Fourier component whose magnitude is higher than that of the fundamental (i.e., the component having the same frequency as the laser). For other parameter values, the low-frequency component disappears and the Fourier transform of the dipole moment has a large zero-frequency component and intense even-harmonic components (i.e., with frequency 2nω, where n is an integer and ω is the laser frequency). The presence of the even harmonics is intriguing: The system has inversion symmetry and even harmonics are forbidden by symmetry rules valid to all orders in perturbation theory. Finally, a laser pulse with well-chosen parameters can drive an electron that was initially in a delocalized eigenstate, to a state in which it is almost completely localized in one well. These processes are systematically investigated by numerical calculations and are rationalized with the help of a simple model which predicts the qualitative behavior observed numerically. The model suggests that these phenomena occur at those values of the parameters for which two Floquet states having different generalized parities become degenerate or nearly degenerate. This condition is rather general and we see no reason why it will not be fulfilled in systems other than double quantum wells (e.g., atoms or molecules).