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In 1964 the Nobel
Prize for physics was given to Nikolay Gennadiyevich Basov, Aleksandr Mikhailovich Prokhorov and Charles Hard Townes for the invention of the maser ("microwave amplification by
stimulated emission of radiation")
and the laser ("light amplification by
stimulated emission of radiation").
The key to the invention is the concept of stimulated emission which
was introduced by Einstein already in 1917. By a theoretical analysis
of the Planck radiation formula he found that the well-known process
of absorption must be accompanied by a complementary process implying
that received radiation can stimulate the atoms to emit the same kind
of radiation. In this process lies a potential means for
amplification. However, the stimulated emission was long regarded as a
purely theoretical concept which never could be put to work or even be
observed, because the absorption would be the completely dominating
process under all normal conditions. An amplification can occur only
if the stimulated emission is larger than the absorption, and this in
turn requires that there should be more atoms in a high energy state
than in a lower one. Such an unstable energy condition in matter is
called an inverted population. An essential moment in the invention of
the maser and the laser was, therefore, to create an inverted
population under such circumstances that the stimulated emission could
be used for amplification.
The first papers about the maser were published in 1950s as a
result of investigations carried out simultaneously and independently
by Townes and co-workers at Columbia University in New York and by
Basov and Prochorov at the Lebedev Institute in Moscow. In the
following years there were designed a number of masers of widely
different types, and many people made important contributions to this
development. In the type that is now being mostly used the maser
effect is obtained by means of the ions of certain metals imbedded in
a suitable crystal. These masers work as extremely sensitive receivers
for short radiowaves. They are of great importance in radio astronomy
and are being used in space research for recording the radio signals
from satellites.
The optical maser, that is, the laser, dates from 1958, when the
possibilities of applying the maser principle in the optical region
were analysed by Schawlow and Townes as well as in the Lebedev
Institute. Two years later the first laser was operating.
The step from the microwaves to visible light means a 100000-fold
increase in frequency and causes such changes in the operation
conditions that the laser may be regarded as an essentially new
invention. In order to achieve the high radiation density required for
the stimulated emission to become dominating, the radiating matter is
enclosed between two mirrors that force the light to traverse the
matter many times. During this process the stimulated radiation grows
like an avalanche until all the atoms have given up their energy to
the radiation. The fact that the stimulated and stimulating radiation
have exactly the same phase and frequency is essential for the result
of the process. By virtue of resonance all parts of the active medium
combine their forces to give one strong wave. The laser emits what is
called coherent light, and this is the decisive difference between the
laser and an ordinary light source where the atoms radiate quite
independent of each other.
Lasers have now been made in many different shapes. The first, and
still most frequently used, type consists of a ruby rod, a few inches
long, with the polished and silvered end faces serving as mirrors. The
radiation leaves eventually the crystal through one of the end faces
which is made slightly transparent. The ruby consists of aluminium
oxide with a small admixture of chromium. The chromium ions give to
the ruby its red colour, and they are also responsible for the laser
effect. The inverted population is produced by the light from a xenon
flash lamp. This is absorbed by the ions, putting them in such a
condition that they can be stimulated to emit a red light with a
welldefined wavelength.
Normally, a large number of successive pulses of laser light is
emitted during the time of one flash from the lamp, but by retarding
the release until the stored energy has reached a maximum all the
energy can be put into one big pulse. The power of the emitted light
can then reach more than a hundred million watts. Since, moreover, the
emerging ray bundle is strictly parallel, the whole energy can be
concentrated by means of a lens on a very small area, producing an
enormous power per unit area. From a scientific point of view it is
especially interesting that the electrical field strength produced in
the light wave may amount to some hundred million volts/cm and thus
surpass the forces that keep the electron shells of the atoms
together. The high photon density opens up quite new possibilities for
studying the interaction of radiation and matter.
Another type of laser, in which the light is emitted from a gas
excited by an electric discharge, produces continuously a radiation
with a very sharply defined wavelength. This radiation can be used for
measurements of lengths and velocities with a previously unattainable
precision.
The invention of the laser has provided us with a powerful new tool
for research in many fields, the exploitation of which has only just
started. Its potential technical applications have been much
publicised and are therefore well known. Regarding, especially, the
extreme power concentration obtainable with a laser, it should be
noted that this effect is limited to short time intervals and very
small volumes and therefore attains its main importance for
micro-scale operations. It should be emphasized, finally, that the use
of a laser beam for destructive purposes over large distances is
wholly unrealistic. |
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