Excited state —–— —•—
Absorption ~~→
Relaxed state —•— —–—
Excited state —•— —–—
Spontaneous emission
Relaxed state —–— —•— ~~→
Excited state —•— —–—
Stimulated emission ~~→
Relaxed state —–— —•— ~~→
~~→
Suddenly the answer came to him.
Normally more of the molecules in any substance are in low-energy states
than in high ones. He would change the natural balance and create a
situation with an abnormally large number of high-energy molecules. Then
he would stimulate them to emit their energy by nudging them with
microwaves. Here was amplification.
He could even take some of the emitted radiation and feed it back into
the device to stimulate additional molecules, thereby creating an
oscillator. This _feedback_ arrangement, he knew, could be carried out
in a cavity, which would resonate (just like an organ pipe) at the
proper frequency. The resonator would be a box whose dimensions were
comparable with the wavelength of the radiation, that is, a few
centimeters on a side.
On the back of an envelope he figured out some of the basic
requirements. Three years, and many experiments, later the maser
(_m_icrowave _a_mplification by _s_timulated _e_mission of _r_adiation)
was a reality. The original maser was a small metal box into which
excited ammonia molecules were added. When microwaves were beamed into
the excited ammonia the box emitted a pure, strong beam of high
frequency microwaves, far more temporally coherent than any that had
ever been achieved before. The output of an ammonia maser is stable to
one part in 100 billion, making it an extremely accurate atomic
“clock”.[12] Moreover, the amplifying properties of masers have been
found to be very useful for magnifying faint radio signals from space,
and for satellite communications.
Ammonia gas was chosen for the first maser because molecules of ammonia
have two individual energy states that are separated by a gap
corresponding in frequency to 23,870 megacycles (23,870 million cycles)
per second. Ammonia molecules also react to a nonuniform electric field
in ways that depend on their energy level: low-level molecules can be
attracted and high-level ones repelled by the same field. Thus it is
possible to separate the low-energy molecules from the high, and to get
the excited molecules into the cavity without too much trouble.
This procedure for getting the majority of atoms or molecules in a
container into a higher energy state, is called _population inversion_
and is basic to the operation of both masers and lasers.
It should be noted that two Russians, N. G. Basov and A. M. Prokhorov,
were working along similar lines independently of Townes. In 1952 they
presented a paper at an All-Union (U.S.S.R.) Conference, in which they
discussed the possibility of constructing a “molecular generator”, that
is, a maser. Their proposal, first published in 1954, was in many
respects similar to Townes’s. In 1955, Basov and Prokhorov discussed, in
a short note, a new way to obtain the active atomic systems for a maser,
a method that turned out to be of great importance.
Thus on October 29, 1964, the Nobel Prize in Physics was awarded, not
only to Townes, but to Basov and Prokhorov as well. The award was for
fundamental work in the field of quantum electronics, which has led to
the construction of oscillators and amplifiers based on the “aser”
principle.