Subharmonic generation in a driven asymmetric quantum well

A realistic model of a doped quantum well heterostructure exhibits a subhar monic signal in the presence of intense far-infrared radiation. Nonlinearit y enters the model through the effective potential due to the density of th e electrons in the well. Driven at twice the resonant frequency of a "bare" two-level system the electron states undergo a subcritical pitchfork bifur cation. The signature of this nonlinear bifurcation is an emitted subharmon ic radiation at half of the frequency of the drive. This nonperturbative re sponse is found by applying the technique of averaging from nonlinear dynam ical systems theory to the density matrix dynamical equations, for a many-b ody two-level system of electrons confined in the asymmetric square well, o f Galdrikian and Birnir [Phys. Rev. Lett. 29, 3308 (1996)]. The averaged eq uations determine the parameter ranges of the subharmonic response and they turn out to be well within the capabilities of present quantum well techno logy and occur at moderate values of field strength of the drive. Potential technological applications of subharmonic generation include frequency con version and the production of "squeezed" states of THz radiation.