Two-Plasmon Intersubband Absorption in Quantum Wells

Linear light absorption of 2D electrons confined within a biased quantum well is studied theoretically. We demonstrate that for light polarization perpendicular to the 2D plane, in addition to conventional absorption peak at frequency h-bar ω ∼ Δ, where Δ is the intersubband energy distance, there exists a peak around a double frequency h-bar ω ∼ 2Δ. This additional peak is entirely due to electron-electron interactions, and corresponds to excitation of two electrons by one photon. The magnitude of two-electron absorption is proportional to U2, where U is the applied bias