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1917
Albert Einstein, Zur Quantentheorie der Strahlung (On the
Quantum Theory of Radiation), laid the foundation for the
invention of the laser by rederiving Planck’s law of radiation using
the concepts of probability coefficients ( Einstein coefficients )
for the absorption, spontaneous, and stimulated emission.

1928
Rudolph W. Landenburg confirmed the existence of stimulated
emission and negative absorption.

1939
Valentin A. Fabrikant predicted the use of stimulated emission
to amplify "short" waves.

1947
Willis E. Lamb and R. C. Retherford found apparent stimulated
emission in hydrogen spectra and made the first demonstration of
stimulated emission.

1950
Alfred Kastler proposed the method of optical pumping, which
was experimentally confirmed by Brossel, Kastler and Winter two
years later.
Development of the Idea of the Laser
1953
Charles Townes and graduate students James P. Gordon and
Herbert J. Zeiger produced the first microwave amplifier, a device
operating on similar principles to the laser, but amplifying microwave
rather than optical radiation. Townes s maser was incapable of
continuous output.

1955
Nikolay Basov and Aleksandr Prokhorov worked independently on
the quantum oscillator and solved the problem of continuous output
systems by using more than two energy levels and produced the first
maser. They suggested an optical pumping of multilevel system as a
method for obtaining the population inversion, which later became
one of the main methods of laser pumping.

1964
Townes, Basov, and Prokhorov shared the Nobel Prize
in physics "For fundamental work in the field of quantum
electronics, which has led to the construction of oscillators
and amplifiers based on the maser-laser principle".

1957
Charles Hard Townes and Arthur Leonard Shawlow published their
theoretical calculations on infrared maser. [Physical Review, Volume
112, Issue 6]. As ideas were developed, infrared frequencies were
abandoned with focus on visible light instead. The concept was
originally known as an "optical maser". Bell Labs filed a patent
application for their proposed optical maser a year later.

1957
After graduating from Brooklyn PolytechnicUniversity, Gordon
Gould, a graduate student at Columbia University, was working on a
doctoral thesis under supervision of Townes. Gould and Townes had
conversations on the general subject of radiation emission.
Afterwards Gould made notes about his ideas for a "laser", including
suggesting using an open resonator, which became an important
ingredient of future lasers.

1958
Prokhorov independently proposed using an open resonator, the
first published appearance of this idea. Schawlow and Townes also
settled on an open resonator design, apparently unaware of both the
published work of Prokhorov and the unpublished work of Gould.
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1959-60
The term "laser" was first introduced to the public in
Gould s 1959 conference paper "The LASER, Light Amplification by
Stimulated Emission of Radiation". Gould intended "-aser" to be a
suffix, to be used with an appropriate prefix for the spectra of light
emitted by the device (x-ray laser = xaser, ultraviolet laser = uvaser,
etc.). None of the other terms became popular, although "raser" was
used for a short time to describe radio-frequency emitting devices.
He continued working on his idea and filed a patent application in
April 1959. The U. S. Patent Office denied his application and
awarded a patent to Bell Labs in 1960. This sparked a legal battle
that ran 28 years, with scientific prestige and much money at stake.
Gould won his first minor patent in 1977, but it was not until 1987
that he could claim his first significant patent victory when a
federal judge ordered the government to issue patents to him for
the optically . pumped laser and the gas discharge laser.
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1960 The first working laser was made by Theodore Maiman at
Hughes Research Laboratories, beating several research teams
including those of Townes at Columbia University, Arthur L.
Schawlow at Bell Labs, and Gould at a company called TRG (Technical
Research Group). Maiman used a solid-state flashlamp-pumped
synthetic ruby crystal to produce red laser light at 694 nm.
Maiman s laser, however, was only capable of pulsed operation due to
its three energy level pumping scheme.

1960 Ali Javan, working with William Bennet and Donald Herriot, made
the first gas laser using helium and neon.
1962 The concept of the semiconductor laser diode was proposed by
Basov and Javan. The first laser diode was demonstrated by Robert
N. Hall in 1962. Hall s device was made of gallium arsenide and
emitted at 850 nm. The first semiconductor laser with visible
emission was demonstrated later the same year by Nick Holonyak, Jr.

1970 Zhores Alferov and Izuo Hayashi and Morton Panish
independently developed laser diodes continuously operating at room
temperature, using the heterojunction structure. 18

Laser Types
Gas Lasers
The helium-neon (HeNe) emits at a variety of wavelengths and units
operating at 633 nm are very common in education because of its low
cost.

Carbon dioxide lasers can emit hundreds of kilowatts[11] at 9.6 ?m and
10.6 ?m, and are often used in industry for cutting and welding. The
efficiency of a CO2 laser is over 10%.

Argon-ion lasers emit at 458 nm, 488 nm or 514.5 nm.
A nitrogen transverse electrical discharge in gas at atmospheric
pressure (TEA) laser is an inexpensive gas laser producing UV Light at
337.1 nm.[12]
Metal ion lasers are gas lasers that generate deep ultraviolet
wavelengths. Helium-silver (HeAg) 224 nm and neon-copper (NeCu) 248
nm are two examples. These lasers have particularly narrow oscillation
linewidths of less than 3 GHz (0.5 picometers),[13] making them
candidates for use in fluorescence suppressed Raman spectroscopy.

Chemical Lasers
Chemical lasers are powered by a chemical reaction, and can achieve
high powers in continuous operation. For example, in the Hydrogen
fluoride laser (2700-2900 nm) and the Deuterium fluoride laser
(3800 nm) the reaction is the combination of hydrogen or deuterium
gas with combustion products of ethylene in nitrogen trifluoride.
They were invented by George C. Pimentel.

Excimer Lasers
Excimer lasers are powered by a chemical reaction involving an
excited dimer, or excimer, which is a short-lived dimeric or
heterodimeric molecule formed from two species (atoms), at least
one of which is in an excited electronic state. They typically produce
ultraviolet light, and are used in semiconductor photolithography and
in LASIK eye surgery. Commonly used excimer molecules include F2
(fluorine, emitting at 157 nm), and noble gas compounds (ArF (193
nm), KrCl (222 nm), KrF (248 nm), XeCl (308 nm), and XeF (351 nm)).
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Solid State Lasers
Solid state laser materials are commonly made by doping a crystalline
solid host with ions that provide the required energy states. For
example, the first working laser was a ruby laser, made from ruby
(chromium-doped corundum).
Neodymium is a common dopant in various solid state laser crystals,
including yttrium orthovanadate (Nd:YVO4), yttrium lithium fluoride
(Nd:YLF) and yttrium aluminium garnet (Nd:YAG). All these lasers can
produce high powers in the infrared spectrum at 1064nm. They are
used for cutting, welding and marking of metals and other materials,
and also in spectroscopy and for pumping dye lasers. These lasers are
also commonly frequency doubled, tripled or quadrupled to produce
532nm (green, visible), 355nm (UV) and 266nm (UV) light when those
wavelengths are needed.

Ytterbium, holmium, thulium, and erbium are other common dopants in
solid state lasers. Ytterbium is used in crystals such as Yb:YAG,
Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF2, typically operating
around 1020-1050 nm. They are potentially very efficient and high
powered due to a small quantum defect. Extremely high powers in
ultrashort pulses can be achieved with Yb:YAG