Subharmonic generation in a driven asymmetric quantum well

A realistic model of a doped quantum well heterostructure exhibits a subharmonic signal in the presence of intense far-infrared radiation. Nonlinearity enters the model through the effective potential due to the density of the electrons in the well. Driven at twice the resonant frequency of a “bare” two-level system the electron states undergo a subcritical pitchfork bifurcation. The signature of this nonlinear bifurcation is an emitted subharmonic radiation at half of the frequency of the drive. This nonperturbative response is found by applying the technique of averaging from nonlinear dynamical systems theory to the density matrix dynamical equations, for a many-body two-level system of electrons confined in the asymmetric square well, of Galdrikian and Birnir [Phys. Rev. Lett. 29, 3308 (1996)]. The averaged equations determine the parameter ranges of the subharmonic response and they turn out to be well within the capabilities of present quantum well technology and occur at moderate values of field strength of the drive. Potential technological applications of subharmonic generation include frequency conversion and the production of “squeezed” states of THz radiation.