Current-driven plasma instabilities for a bilayer two-dimensional electron system

We have formulated a theory to help us investigate the conditions which are needed to achieve stronger plasmon instability 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 metal grating placed on the surface which modulates the 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 2DEG layers and is characterized by a drift velocity υD. With the use of the surface response function, the plasmon dispersion equation is obtained as a function of frequency ω, the in-plane wave vector qll = (qx, qy) and reciprocal lattice vector nG where n = 0,±1,±2, ... with G = 2π/d and d denoting the period of the grating. The dispersion equation, which yields the resonant frequencies, is solved in the complex ω-plane for real wave vector qll. It is ascertained that the imaginary part of ω is enhanced with decreasing d, and with increasing the doping density of the free carriers in the bulk medium for fixed grating period.

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