Rami Arieli: "The Laser Adventure"

Rami Arieli: "The Laser Adventure" Chapter 3.1 page 1

3.1 The laser active medium:

The active medium is a collection of atoms or molecules, which can be excited into a population inversion situation, and can have electromagnetic radiation extracted out of it by stimulated emission.
The active medium can be in any state of matter: solid, liquid, gas or plasma.
The active medium determines the possible wavelengths that can be emitted from the laser. These wavelengths are determined by the specific transitions between the laser energy levels in this material.
The list of materials that lase under certain laboratory conditions include hundreds of substances, and the number increases with time.

The basic physics of the laser is similar for all types of lasers, and we will use the term "Active Medium" and assume that it is composed of "atoms". In reality, the active medium can be atoms, molecules, ions, or semiconductors, according to the laser type.