A dopant, also called doping agent and dope, is an impurity element added to a crystal or semiconductor lattice in low concentrations in order to alter the optical/electrical properties of the semiconductor. Also, doping with Yb, Er, Nd, Tm, Cr of optical crystals, ceramics and glasses is used to make the solid-state lasers; the elements mentioned are the most common dopants. In many optically-transparent ghosts, such active centers may keep their excitation for a time on the order of millisecon… (More on Dopant)