 1994 Quantum cascade laser

A quantum cascade laser is a sliver of semiconductor material about the size of a tick.Inside, electrons are constrained within layers of gallium and aluminum compounds, called quantum wells are nanometers thick, much smaller than the thickness of a hair.Electrons jump from one energy level to another, rather than moving smoothly between levels and tunnel from one layer to the next going "through" rather than "over" energy barriers separating the wells.