A novel multi-plasmon concept of a light absorption and laser gain of low-dimensional structures are comprehensively discussed. A Generalized Semiconductor Bloch Equations are derived with account of multi-plasmon optical transitions in direct gap quantum wells, using the cumulant expansion method and fluctuation-dissipation theorem. We present results of computer simulations concerning gain spectra of In0.05Ga0.95As quantum wells with account of multi- plasmon optical transitions in two- dimensional systems. Multi-quantum LO-phonon-plasmon optical transitions are investigated with account of coherent memory effects in quantum wells. It is shown that a red shift of the absorption edge can be caused, not only by known mechanism of band gap shrinkage, but also by multi-plasmon transitions. The electron-hole plasma properties in the active region of the laser device and its interaction with the optical field are studied on a microscopic level using obtained Generalized Semiconductor Bloch Equations. The comparison with other theories and experimental data measured in In0.05Ga0.95As quantum wells is performed. The gain value g=50 cm-1 in 8 nm In 0.05Ga 0.95As quantum wells is obtained at a surface density of electrons nd0=1.64 10-12 cm-2 .