Dopant

A chemical element (impurity) added in small amounts to an otherwise pure semiconductor material to modify the electrical properties of the material. An n-dopant introduces more electrons. A p-dopant creates electron vacancies (holes).

An impurity added to an optical medium to change its optical properties. Note: Dopants are used in optical fibers to control the refractive index profile and other refractive properties of the fiber. [FAA]