[cdn-nucl-l] Article in APS News: Einstein Predicts Stimulated Emission

["Jerry Cuttler" <jerrycuttler@rogers.com>]

Interesting article ...
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APS News Aug/Sep 2005, Vol. 14, No. 8

Einstein Predicts Stimulated Emission

The laser's invention launched a multi-billion dollar industry. Lasers are
used to remove unwanted tattoos; to correct vision defects in laser eye
surgery; to cut through steel and other materials in industrial assembly
lines; to scan prices in supermarkets and department stores; for optical
communications and optical data storage; and in electronic devices like CD
and DVD players. The root of all this technological innovation lies in
fundamental physics research, specifically, a 1917 paper by Albert Einstein
on the quantum theory of radiation.

"Laser" is an acronym for Light Amplification by Stimulated Emission of
Radiation. It describes any device that creates and amplifies a narrow,
focused beam of light whose photons are coherent. In a laser, the atoms or
molecules of the lasing medium-either a crystal like ruby or garnet, or a
gas or liquid-are "pumped," so that more of them are at higher energy levels
than at the ground state.

The end result is a sudden burst of coherent light as the atoms discharge in
a rapid chain reaction. This process is called "stimulated emission." Albert
Einstein first broached the possibility of stimulated emission in a 1917
paper, having turned his attention the year before from general relativity
to the interplay of matter and radiation, and how the two could achieve
thermal equilibrium. Einstein devised an improved fundamental statistical
theory of heat, embracing the quantum of energy.

First, Einstein proposed that an excited atom in isolation can return to a
lower energy state by emitting photons, a process he dubbed spontaneous
emission. Spontaneous emission sets the scale for all radiative
interactions, such as absorption and stimulated emission. Atoms will only
absorb photons of the correct wavelength: the photon disappears and the atom
goes to a higher energy state, setting the stage for spontaneous emission.
Second, his theory predicted that as light passes through a substance, it
could stimulate the emission of more light.

Einstein postulated that photons prefer to travel together in the same
state. If one has a large collection of atoms containing a great deal of
excess energy, they will be ready to emit a photon randomly. However, if a
stray photon of the correct wavelength passes by (or, in the case of a
laser, is fired at an atom already in an excited state), its presence will
stimulate the atoms to release their photons early-and those photons will
travel in the same direction with the identical frequency and phase as the
original stray photon. A cascading effect ensues: as the crowd of identical
photons moves through the rest of the atoms, ever more photons will be
emitted from their atoms to join them.

It wasn't until the 1940s and 1950s that physicists found a use for the
concept, even though all that was required to invent a laser was finding the
right kind of atom, and adding reflecting mirrors to fortify the stimulated
emission process by producing a chain reaction. Charles Townes had worked on
radar systems during World War II. After the war ended, he turned his
attention to molecular spectroscopy, a technique that studies the absorption
of light by molecules. Just like radar, molecular spectroscopy bombards the
surface of molecules with light and analyzes the scattered radiation to
determine the molecule's structure.

But the technique was limited by the wavelength of the light produced: in
this case, the microwave regime of the electromagnetic spectrum. Townes
noticed that as the wavelength of the microwaves shortened, the more
strongly the light interacted with the molecules, and the more one could
learn about them. He thought it might be possible to develop a device that
produced light at much shorter wavelengths. The best way to do this, he
thought, would be to use molecules to generate the desired frequencies
through stimulated emission.

Townes mentioned the idea to a colleague (later his brother-in-law), Arthur
Schawlow, who proposed that the prototype laser be fitted with a pair of
mirrors, one at each end of the lasing cavity. Photons of specific
wavelengths would then reflect off the mirrors and travel back and forth
through the lasing medium. By doing so, they would in turn cause other
electrons to relax back into their ground states, emitting even more photons
in the same wavelength. So only photons in the selected wavelength and
frequency range would be amplified.

The two men wrote a paper detailing their concept, published in the December
1958 issue of the Physical Review, although they had yet to build a working
prototype. They received a patent for their design two years later-the same
year that the first working laser was built by Theodore Maiman at Hughes
Aircraft Company.
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