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The first laser
Charles H. Townes
from A Century of Nature: Twenty-One Discoveries that Changed Science and the World
Laura Garwin and Tim Lincoln, editors
When the first working laser was reported in 1960, it was
described as "a solution looking for a problem." But before long the
laser's distinctive qualities—its ability to generate an intense, very
narrow beam of light of a single wavelength—were being harnessed for
science, technology and medicine. Today, lasers are everywhere: from
research laboratories at the cutting edge of quantum physics to medical
clinics, supermarket checkouts and the telephone network.
Theodore Maiman made the first laser operate on 16 May 1960 at
the Hughes Research Laboratory in California, by shining a high-power
flash lamp on a ruby rod with silver-coated surfaces. He promptly
submitted a short report of the work to the journal Physical Review Letters, but the editors turned it down. Some have thought this was because the Physical Review
had announced that it was receiving too many papers on masers—the
longer-wavelength predecessors of the laser—and had announced that any
further papers would be turned down. But Simon Pasternack, who was an
editor of Physical Review Letters at the time, has said that he
turned down this historic paper because Maiman had just published, in
June 1960, an article on the excitation of ruby with light, with an
examination of the relaxation times between quantum states, and that
the new work seemed to be simply more of the same. Pasternack's
reaction perhaps reflects the limited understanding at the time of the
nature of lasers and their significance. Eager to get his work quickly
into publication, Maiman then turned to Nature, usually even more selective than Physical Review Letters, where the paper was better received and published on 6 August.
With official publication of Maiman's first laser under way, the
Hughes Research Laboratory made the first public announcement to the
news media on 7 July 1960. This created quite a stir, with front-page
newspaper discussions of possible death rays, but also some skepticism
among scientists, who were not yet able to see the careful and
logically complete Nature paper. Another source of doubt came
from the fact that Maiman did not report having seen a bright beam of
light, which was the expected characteristic of a laser. I myself asked
several of the Hughes group whether they had seen a bright beam, which
surprisingly they had not. Maiman's experiment was not set up to allow
a simple beam to come out of it, but he analyzed the spectrum of light
emitted and found a marked narrowing of the range of frequencies that
it contained. This was just what had been predicted by the theoretical
paper on optical masers (or lasers) by Art Schawlow and myself, and had
been seen in the masers that produced the longer-wavelength microwave
radiation. This evidence, presented in figure 2 of Maiman's Nature
paper, was definite proof of laser action. Shortly afterward, both in
Maiman's laboratory at Hughes and in Schawlow's at Bell Laboratories in
New Jersey, bright red spots from ruby laser beams hitting the
laboratory wall were seen and admired.
Maiman's laser had several aspects not considered in our theoretical
paper, nor discussed by others before the ruby demonstration. First,
Maiman used a pulsed light source, lasting only a few milliseconds, to
excite (or "pump") the ruby. The laser thus produced only a short flash
of light rather than a continuous wave, but because substantial energy
was released during a short time, it provided much more power than had
been envisaged in most of the earlier discussions. Before long, a
technique known as "Q switching" was introduced at the Hughes
Laboratory, shortening the pulse of laser light still further and
increasing the instantaneous power to millions of watts and beyond.
Lasers now have powers as high as a million billion (1015)
watts! The high intensity of pulsed laser light allowed a wide range of
new types of experiment, and launched the now-burgeoning field of
nonlinear optics. Nonlinear interactions between light and matter allow
the frequency of light to be doubled or tripled, so for example an
intense red laser can be used to produce green light.
I had a busy job in Washington at the time when various groups were
trying to make the earliest lasers. But I was also supervising graduate
students at Columbia University who were trying to make continuously
pumped infrared lasers. Shortly after the ruby laser came out I advised
them to stop this work and instead capitalize on the power of the new
ruby laser to do an experiment on two-photon excitation of atoms. This
was one of the early experiments in nonlinear optics, and two-photon
excitation is now widely used to study atoms and molecules.
Lasers work by adding energy to atoms or molecules, so that there
are more in a high-energy ("excited") state than in some lower-energy
state; this is known as a "population inversion." When this occurs,
light waves passing through the material stimulate more radiation from
the excited states than they lose by absorption due to atoms or
molecules in the lower state. This "stimulated emission" is the basis
of masers (whose name stands for "microwave amplification by stimulated
emission of radiation") and lasers (the same, but for light instead of
microwaves).
Before Maiman's paper, ruby had been widely used for masers, which
produce waves at microwave frequencies, and had also been considered
for lasers producing infrared or visible light waves. But the second
surprising feature of Maiman's laser, in addition to the pulsed source,
was that he was able to empty the lowest-energy ("ground") state of
ruby enough so that stimulated emission could occur from an excited to
the ground state. This was unexpected. In fact, Schawlow, who had
worked on ruby, had publicly commented that transitions involving the
ground state of ruby would not be suitable for lasers because it would
be difficult to empty adequately. He recommended a different transition
in ruby, which was indeed made to work, but only after Maiman's
success. Maiman, who had been carefully studying the relaxation times
of excited states of ruby, came to the conclusion that the ground state
might be sufficiently emptied by a flash lamp to provide laser
action—and it worked.
The ruby laser was used in many early spectacular experiments. One
amusing one, in 1969, sent a light beam to the Moon, where it was
reflected back from a retro-reflector placed on the Moon's surface by
astronauts in the U.S. Apollo program. The round-trip travel time of
the pulse provided a measurement of the distance to the Moon. Later,
ruby laser beams sent out and received by telescopes measured distances
to the Moon with a precision of about three centimeters—a great use of
the ruby laser's short pulses.
When the first laser appeared, scientists and engineers were not
really prepared for it. Many people said to me—partly as a joke but
also as a challenge—that the laser was "a solution looking for a
problem." But by bringing together optics and electronics, lasers
opened up vast new fields of science and technology. And many different
laser types and applications came along quite soon. At IBM's research
laboratories in Yorktown Heights, New York, Peter Sorokin and Mirek
Stevenson demonstrated two lasers that used techniques similar to
Maiman's but with calcium fluoride, instead of ruby, as the lasing
substance. Following that—and still in 1960—was the very important
helium-neon laser of Ali Javan, William Bennett, and Donald Herriott at
Bell Laboratories. This produced continuous radiation at low power but
with a very pure frequency and the narrowest possible beam. Then came
semiconductor lasers, first made to operate in 1962 by Robert Hall and
his associates at the General Electric laboratories in Schenectady, New
York. Semiconductor lasers now involve many different materials and
forms, can be quite small and inexpensive, and are by far the most
common type of laser. They are used, for example, in supermarket
bar-code readers, in optical-fiber communications, and in laser
pointers.
By now, lasers come in countless varieties. They include the
"edible" laser, made as a joke by Schawlow out of flavored gelatin (but
not in fact eaten because of the dye that was used to color it), and
its companion the "drinkable" laser, made of an alcoholic mixture at
Eastman Kodak's laboratories in Rochester, New York. Natural lasers
have now been found in astronomical objects; for example, infrared
light is amplified by carbon dioxide in the atmospheres of Mars and
Venus, excited by solar radiation, and intense radiation from stars
stimulates laser action in hydrogen atoms in circumstellar gas clouds.
This raises the question: why weren't lasers invented long ago, perhaps
by 1930 when all the necessary physics was already understood, at least
by some people? What other important phenomena are we blindly missing
today? |
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