1. A light detection system, comprising: an active region between a back contact layer and a front contact layer, said active region comprising a quantum well structure having a quantum well between quantum barriers, said quantum barriers being defined by an energy height above which there is a continuum of electronic states; wherein said quantum well comprises foreign atoms inducing an excited bound state at an energy level which is above said energy height of said quantum barriers; wherein said foreign atoms are other than atoms that form a semiconductor compound forming said quantum well, and other than donor atoms of a doping agent that is inserted into said semiconductor compound.