Editing Active laser medium (section)

{{Short description|Source of optical gain in a laser}}
[[File:Laser rods 13.jpg|thumb|upright=1.3|Laser rods (from left to right): [[Ruby laser|Ruby]], Alexandrite, [[Er:YAG laser|Er:YAG]], [[Nd:YAG laser|Nd:YAG]]]]
The '''active laser medium''' (also called a '''gain medium''' or '''lasing medium''') is the source of optical [[gain (laser)|gain]] within a [[laser]].  The gain results from the [[stimulated emission]] of photons through electronic or molecular transitions to a lower energy state from a higher energy state previously populated by a [[laser pumping|pump source]].

Examples of active laser media include:
* Certain [[crystal]]s, typically doped with [[rare-earth element|rare-earth]] [[ion]]s (e.g. [[neodymium]], [[ytterbium]], or [[erbium]]) or [[transition metal]] ions ([[titanium]] or [[chromium]]); most often [[yttrium aluminium garnet]] ([[yttrium|Y]]<sub>3</sub>[[aluminium|Al]]<sub>5</sub>[[Oxygen|O]]<sub>12</sub>), [[yttrium orthovanadate]] (YVO<sub>4</sub>), or [[sapphire]] (Al<sub>2</sub>O<sub>3</sub>);<ref>Hecht, Jeff. ''The Laser Guidebook: Second Edition.'' McGraw-Hill, 1992.  (Chapter 22)</ref> and not often [[caesium cadmium bromide]] ([[caesium|Cs]][[Cadmium|Cd]][[bromine|Br]]<sub>3</sub>) ([[solid-state laser]]s)
* [[Glass]]es, e.g. silicate or phosphate glasses, doped with laser-active ions;<ref>Hecht, Chapter 22</ref>
* [[Gas]]es, e.g. mixtures of [[helium]] and [[neon]] (HeNe), [[nitrogen]], [[argon]], [[krypton]], [[carbon monoxide]], [[carbon dioxide]], or metal vapors;<ref>Hecht, Chapters 7-15</ref> ([[gas laser]]s)
* [[Semiconductor]]s, e.g. [[gallium arsenide]] (GaAs), [[indium gallium arsenide]] (InGaAs), or [[gallium nitride]] (GaN).<ref>Hecht, Chapters 18–21</ref>
* Liquids, in the form of dye solutions as used in [[dye lasers]].<ref>[[F. J. Duarte]] and L. W. Hillman (Eds.), ''Dye Laser Principles'' (Academic, New York, 1990).</ref><ref>[[F. P. Schäfer]] (Ed.), ''Dye Lasers'', 2nd Edition (Springer-Verlag, Berlin, 1990).</ref>

In order to fire a laser, the active gain medium must be changed into a state in which [[population inversion]] occurs. The preparation of this state requires an external energy source and is known as [[laser pumping]].  Pumping may be achieved with electrical currents (e.g. semiconductors, or gases via [[glow discharge|high-voltage discharges]]) or with light, generated by [[discharge lamp]]s or by other lasers ([[semiconductor laser]]s).  More exotic gain media can be pumped by [[chemical reactions]], [[nuclear fission]],<ref>{{cite journal |last1=McArthur |first1=D. A. |last2=Tollefsrud |first2=P. B. |title=Observation of laser action in CO gas excited only by fission fragments |journal=Applied Physics Letters |date=15 February 1975 |volume=26 |issue=4 |pages=187–190 |doi=10.1063/1.88110 |bibcode=1975ApPhL..26..187M |doi-access=}}</ref> or with high-energy [[electron beam]]s.<ref name="rp">[http://www.rp-photonics.com/gain_media.html Encyclopedia of laser physics and technology]</ref>