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) over bar omega approximate to Delta, where Delta is the intersubband energy distance, there exists a peak around a double frequency (h) over bar omega approximate to 2 Delta. 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 U-2, where U is the applied bias.