ABSTRACT
Although Albert Einstein did not invent the laser, his work laid the foundation for its development. In
1917, Einstein was the first to explain how radiation could induce, or stimulate, more radiation when it
interacts with an atom or a molecule [14]. A few years later, Richard Tolman discussed stimulated
emission and absorption in his paper [65], realizing the important fact that stimulated emission is
coherent with the incoming radiation. In other words, the electric dipoles in the atoms oscillate with the
incoming photons, which in turn, re-radiate photons that have a fixed phase relationship with incoming
photons. If the re-radiated photons are in-phase with the incoming ones, they add constructively to
amplify the incoming photons. Thus, the general idea of coherent amplification via stimulated emission
was understood since the 1920s. However, it was not until the 1950s when the concept of the ‘maser’,
which is an acronym for microwave amplification by simulated emission of radiation, was developed
and demonstrated by Charles Townes and his coworkers at Columbia University [22, 23]. They directed
excited ammonia molecules into a cavity whose resonance frequency is tuned to the 24GHz transition
frequency of ammonia [66]. A sufficient number of these excited molecules will initiate an oscillating
microwave field in the cavity, part of which will be coupled out of the cavity (see figure A1.6.1). It is
interesting to note that maser operation was first demonstrated in the microwave region. Since the
spontaneous radiative lifetime is inversely proportional to the third power of the transition frequency, at
microwave transition frequencies, the radiative lifetime of the ammonia molecules is about 1 £ 10
longer than it would be at optical frequencies, which allows the system to achieve population inversion
easily with a reasonable amount of pump power.
