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.