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 [1]. A few years later, Richard Tolman discussed stimulated emission and absorption in his paper [2], 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, reradiate photons that have a fixed phase relationship with incoming photons. If the reradiated 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 [3, 4]. They directed excited ammonia molecules into a cavity whose resonance frequency is tuned to the 24 GHz transition frequency of ammonia [5]. 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 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 × 10¹² longer than it would be at optical frequencies, which allows the system to achieve population inversion easily with a reasonable amount of pump power.