Enhanced response of current-driven coupled quantum wells

We have investigated the conditions necessary to achieve stronger Cherenkov-like instability of plasma waves leading to emission in the terahertz (THz) regime for semiconductor quantum wells (QWs). The surface response function is calculated for a bilayer two-dimensional electron gas (2DEG) system in the presence of a periodic spatial modulation of the equilibrium electron density. The 2DEG layers are coupled to surface plasmons arising from excitations of free carriers in the bulk region between the layers. A current is passed through one of the layers and is characterized by a drift velocity for the driven electric charge. By means of a surface response function formalism, the plasmon dispersion equation is obtained as a function of angular frequency, the in-plane wave vector and reciprocal lattice vector of the density modulation. The dispersion equation,is solved numerically in the complex frequency plane for real wave vector. It is ascertained that the imaginary part of the angular frequency is enhanced with decreasing period of modulation, and with increasing the doping density of the free carriers in the bulk medium for fixed period of the spatial modulation.