Generally, the active medium of a solid-state laser consists of a
glass or
crystalline "host" material, to which is added a "
dopant" such as
neodymium,
chromium,
erbium,
thulium, or
ytterbium. Many of the common dopants are
rare-earth elements, because the excited states of such ions are not strongly coupled with the thermal vibrations of their crystal lattices (
phonons), and their
operational thresholds can be reached at relatively low intensities of
laser pumping. There are many hundreds of solid-state media in which laser action has been achieved, but relatively few types are in widespread use. Of these, probably the most common is
neodymium-doped yttrium aluminum garnet (Nd:YAG). Neodymium-doped glass (Nd:glass) and ytterbium-doped glasses or
ceramics are used at very high power levels (
terawatts) and high energies (
megajoules), for multiple-beam
inertial confinement fusion. The first material used for lasers was
synthetic ruby crystals.
Ruby lasers are still used for a few applications, but they are no longer common because of their low power efficiencies. At room temperature, ruby lasers emit only short pulses of light, but at
cryogenic temperatures they can be made to emit a continuous train of pulses. The second solid-state gain medium was
uranium-
doped calcium fluoride. Peter Sorokin and Mirek Stevenson at
IBM's laboratories in
Yorktown Heights (US) experimented with this material in the 1960s and achieved lasing at 2.5 μm shortly after
Maiman's
ruby laser. Some solid-state lasers can be made
tunable by using intracavity
etalons,
prisms,
gratings, or a combination of these.
Titanium-doped sapphire is widely used for its broad tuning range, 660 to 1080
nanometers.
Alexandrite lasers are tunable from 700 to 820 nm and yield higher-energy pulses than titanium-
sapphire lasers because of the gain medium's longer energy storage time and higher
damage threshold. ==Pumping==