  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.
