Color center

A point defect in a crystal that absorbs visible light because electrons or holes are trapped at the defect. Its absorption gives the crystal a color not present in the ideal lattice.

F-center: An electron trapped at an anion vacancy in an ionic crystal. It is the classic color center: its trapped electron absorbs visible light.
Crystal lattice: A repeating three-dimensional arrangement of atoms, ions, or molecules in a crystalline solid. A color center is defined by how a localized defect disrupts this periodic structure.
Friedrich Seitz: An American physicist who helped establish the modern theory of defects in ionic crystals. His theoretical work explained how lattice defects produce optical absorption in crystals.
F-center laser: A tunable solid-state laser that uses optical transitions of electrons trapped at anion vacancies. It turns the absorption and emission of color centers into coherent, tunable light.
Coloration by impurities: Visible coloration caused by foreign atoms or ions incorporated into a material. Impurities can color crystals too, but a color center specifically involves localized defect-bound charge.
Point defect: A localized disruption of a crystal lattice involving one or a few atomic sites. Color centers are point defects whose electronic states interact with light.
Vacancy defect: A crystal defect formed when an atom or ion is missing from its lattice site. An empty lattice site can trap charge and become a color center.
Robert W. Pohl: A German physicist whose experiments on alkali halide crystals advanced solid-state physics. His crystal irradiation studies helped establish color centers as identifiable defects.
Nitrogen-vacancy center: A point defect in diamond consisting of a nitrogen atom next to a carbon vacancy. This well-studied color center provides optically readable spin states.
Band-to-band transition: An electronic transition between the valence band and conduction band of a solid. Unlike a localized center transition, it involves extended states across the crystal.

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