On December 14, 1900, German physicist Max Planck delivered a lecture before the German Physical Society (Deutsche Physikalische Gesellschaft) in which he theorized that light consisted of discrete and indivisible packets of radiant energy that he named quanta. He described what eventually became known as the elemental unit of energy (E), as E = hv, where h is a constant of nature with the dimension of action (= energy × time, with a value of 6.626 × 10⁻³⁴ joule-second), subsequently called Planck’s constant, and v is the frequency of radiation. Planck’s theory was published late in 1900.^14–16 Eleven years later, British physicist Ernest Rutherford contributed to quantum theory when he postulated a planetary model of the atom based on his experimental observations of the scattering of alpha particles by atoms. In his view an atom comprises a central charge surrounded by a distribution of electrons orbiting within a sphere.¹⁷

Danish physicist Niels Bohr synthesized Rutherford’s atom model with Planck’s quantum hypothesis (Figure 1-4). In a series of papers published in 1913, Bohr proposed a theory in which electrons revolve in specific orbits around a nucleus without emitting radiant energy. He described the stable, “ground state” of an atom, when all of its electrons are at their lowest energy level. Bohr also theorized that an electron may suddenly jump from one specific orbital level to a higher level; to do so, an electron must gain energy. Conversely, an electron must lose energy to move from a higher energy level to a lower energy level. Thus an electron can move from one energy level to another by either absorbing or emitting radiant energy or light.^18,19

It was in this burgeoning milieu of nascent quantum theory that Albert Einstein made three significant contributions. First, in 1905, Einstein developed his light quantum theory: “In the propagation of a light ray emitted from a point source, the energy is not distributed continuously over ever-increasing volumes of space, but consists of a finite number of energy quanta localized at points of space that move without dividing, and can be absorbed or generated as complete units.”²⁰ Singh²¹ points out that this paper on photoelectric effect was the first that Einstein published during his annus mirabilis (“extraordinary year”), in the scientific journal Annalen der Physik in 1905; his other papers that year treated Brownian motion, special theory of relativity, and matter and energy equivalence (E = mc²). Notably, Einstein himself regarded his light quantum paper as the “most revolutionary” of those that he had published in 1905. He was awarded the 1921 Nobel Prize in physics for this paper. Hallmark and Horn²² stated that Einstein’s light quantum theory was so radical in comparison with other contemporary theories of light that it was not generally accepted until American physicist Robert A. Millikan performed additional experiments in 1916 to support the theory.

Einstein’s 1905 paper made the case for the particle nature of light. In 1909, Einstein made his second significant contribution to laser theory by publishing the first reference in physics to the wave-particle duality of light radiation, using Planck’s radiation law. Einstein stated: “It is my opinion that the next phase in the development of theoretical physics will bring us a theory of light which can be interpreted as a kind of fusion of the wave and emission theory. … Wave structure and quantum structure … are not to be considered as mutually incompatible. … We will have to modify our current theories, not to abandon them completely.”^21,23 British mathematician and physicist Banesh Hoffmann fancifully characterized the quandary for many early twentieth-century physicists regarding the apparent wave-particle duality of light: “They could but make the best of it, and went around with woebegone faces sadly complaining that on Mondays, Wednesdays, and Fridays they must look on light as a wave; on Tuesdays, Thursdays, and Saturdays, as a particle. On Sundays they simply prayed.” ²⁴

In 1916-1917, Einstein made his third important contribution to laser theory by providing a new derivation of Planck’s radiation law,^25–27 with vast implications. As he wrote to his friend Michele Angelo Besso in 1916, “A splendid light has dawned on me about the absorption and emission of radiation.”²¹ Indeed, his new idea provided the basis for subsequent laser development.

Based on quantum theory, two fundamental radiation processes associated with light and matter were known before Einstein’s new derivation: (1) stimulated absorption, a process in which an atom can be excited to a higher energy state through such means as heating, light interaction, or particle interaction; and (2) spontaneous emission, the process of an excited atom decaying to a lower energy state spontaneously, by itself. Einstein’s breakthrough was the addition of a third alternative: stimulated emission, the reverse of the stimulated absorption process. In the presence of other incoming radiation of the same frequency, excited atoms are stimulated to make a transition to the lower energy state—more quickly than in spontaneous emission—and in the process release light energy identical to the incoming form of light. The emitted light has the same frequency and is in phase (i.e., coherent) with the stimulating radiation wave. Stimulated emission occurs when there are more excited atoms than atoms that are not excited (i.e., more atoms in upper of two energy levels than in lower level), a condition called population inversion. Einstein also showed that the process of stimulated emission occurs with the same probability as for absorption from the lower state.^28–31 Hey et al.³² summarized the significance of Einstein’s insight as follows:

For over 35 years this stimulated emission process gained hardly more than a cursory comment in quantum mechanics textbooks, since it seemed to have no practical application. What had been overlooked, however, was the special nature of the light that is emitted in this way. The photons that are emitted have exactly the same phase as the photons that induce the transition. This is because the varying electric fields of the applied light wave cause the charge distribution of the excited atom to oscillate in phase with this radiation. The emitted photons are all in phase—they are coherent—and, furthermore, they travel in the same direction as the inducing photon.

At this point, it should be clarified that the term photon was not used by Planck, Bohr, or Einstein up to the time of Einstein’s 1916-1917 papers. American chemist Gilbert Lewis³³ apparently was the first to use the term when he argued, in a letter to the editor of Nature magazine in 1926, for the need for new nomenclature to describe discrete units of radiant energy:

It would seem inappropriate to speak of one of these hypothetical entities as a particle of light, a corpuscle of light, a light quantum, or a light quant, if we are to assume that it spends only a minute fraction of its existence as a carrier of radiant energy, while the rest of the time it remains as an important structural element within the atom. It would also cause confusion to call it merely a quantum, for later it will be necessary to distinguish between the number of these entities present in an atom and the so-called quantum number. I therefore take the liberty of proposing for this hypothetical new atom, which is not light, but plays an essential part in every process of radiation, the name photon.

The following accepted definition appears in the American Heritage Dictionary³⁴:

photon n. Physics. The quantum of electromagnetic energy, regarded as a discrete particle having zero mass, no electric charge, and an indefinitely long lifetime.

Decades followed Einstein’s 1916-1917 articles on stimulated emission before significant progress was made in laser development, both theoretically and practically, in the 1950s and 1960s, partly because of the outlook and training of physicists at that time, as suggested by American physicist Arthur L. Schawlow and later observers. Schooled in the idea that “thermodynamic equilibrium,” a state of energy balance, was the normal condition of matter throughout the universe, these scientists tended to believe that population inversion was merely an unusual event or brief permutation, not something particularly significant.^35,36

However, the 1920s and 1930s were not entirely bereft of discovery and insight. In 1928, German physicist Rudolf Ladenburg indirectly observed stimulated emission while studying the optical properties of neon gas at wavelengths near a transition where the gas absorbed and emitted light. This was the first evidence that stimulated emission existed.^35,37 In his 1939 doctoral dissertation, Soviet physicist Valentin A. Fabrikant had envisioned a way to produce a population inversion, writing that “such a ratio of populations is in principle attainable. … Under such conditions we would obtain a radiation output greater than the incident radiation.”^35,36,38

Nevertheless, the works of Ladenburg and that of Fabrikant were isolated incidents. Another impediment to laser development after Einstein was two world wars, although World War II then actually accelerated research toward laser development. Efforts of physicists were diverted from performing fundamental research to helping propel technology that would help win the war. Afterward, the sophisticated equipment developed for the war effort became military surplus, and physicists accustomed to low budgets received some of this equipment as they resumed their research.

In 1951, at the spring meeting of the American Physical Society in Washington, D.C., Townes proposed the concept of a maser, an acronym he and his students coined for microwave amplification by stimulated emission of radiation. He indicated that the “primary object of the work that led to the maser was to get shorter wavelengths so we could do better spectroscopy in a new spectral region.”³⁹ Townes elaborated on April 26, 1951: “I sketched out and calculated requirements for a molecular-beam system to separate high-energy molecules from lower [-energy] ones and send them through a cavity which would contain the electromagnetic radiation [photons] to stimulate further emission from the molecules, thus providing feedback and continuous oscillation.”⁴⁰ On May 11, Townes sketched the idea in his laboratory notebook, dated it, and signed it “Chas. H. Townes.” In February 1952, his colleague and brother-in-law Arthur L. Schawlow also signed the page.^35,36

On his return to Columbia University after the April 1951 conference, Townes and postdoctoral fellow Herbert J. Zeiger and doctoral student James P. Gordon commenced work on building a maser. They began to experiment with a beam of ammonia molecules, a compound familiar to Townes from his work on the 24-GHz radar system. It was known that ammonia molecules absorb microwaves at a frequency of 24 GHz, causing the nitrogen atom of that molecule to vibrate. Initial success was achieved in late 1953, when Gordon saw evidence of stimulated emission and amplification from their device; then, in early April 1954, they achieved the desired oscillation.³⁵ They reported their success in a late paper presented at a meeting of the American Physical Society on May 1 and then in a short paper published in the journal Physical Review.⁴¹

While on sabbatical from Columbia University in 1955, Townes worked with French physicist Alfred Kastler at the École Normale Supérieure in Paris. Kastler developed the technique of “optical pumping,” a process by which light is used to raise (or pump) electrons from a lower to a higher energy level, as a new way to excite materials for microwave spectroscopy.³⁵ Townes recognized that optical pumping might excite the optical energy levels necessary for an optical maser. In fall 1957, Townes and Schawlow, a postdoctoral fellow under Townes at Columbia until he joined Bell Labs in 1951, proposed extending maser principles to the infrared and visible regions of the electromagnetic spectrum.^36,39 They subsequently published their influential paper in Physical Review in 1958.⁴²

Meanwhile, another American physicist, Gordon Gould, a Columbia graduate student in 1957, asked whether optical pumping could excite light emission. He recorded his ideas in nine handwritten pages of a laboratory notebook, with the first page titled “Some rough calculations on the feasibility of a LASER: Light Amplification by Stimulated Emission of Radiation”—the first time the term laser was used. Gould had his notes notarized on November 13, 1957, which he saw as a necessary step in applying for a patent. His patent defense efforts were finally recognized after 30 years of delays, challenges, and litigation.^35,39,43

The Schawlow and Townes paper stirred a number of organizations to conduct additional research into optical masers as follows³⁵:

Numerous other companies also had joined the quest for building the first laser, including aerospace company Hughes Research Laboratories in California, which was under a maser development contract with the U.S. Army Signal Corps. The Corps became interested in developing a more practical version of a previously developed ruby solid-state microwave maser, one that could serve as a low-noise microwave amplifier aboard an airplane. American physicist Theodore H. Maiman, who joined Hughes in 1956, and his assistant, Irnee D’Haenens, were assigned to the project. Their task was daunting; the existing desk-size device weighed 2.5 tons. They succeeded in developing a 4-pound version, but the continuing need to incorporate cryogenic cooling of the device limited its practicality.

Nevertheless, Maiman used this experience with ruby in his later work on the laser. Some investigators, including Wieder at Westinghouse as well as Schawlow and others at Bell Labs, had dismissed ruby as an unsuitably inefficient laser material, but their calculations were based on inadequate data. Maiman conducted his own investigation and found that ruby could indeed be suitable, provided that it could be optically pumped with an intensely bright light source. His calculations showed that a pulsed flashlamp would provide enough light to excite a ruby laser. His experimental laser design ultimately was elegant, incorporated in a device that could fit in the palm of the hand: a ruby rod 1 cm in diameter and 2 cm long placed within the coils of a small flashlamp, and an aluminum cylinder with reflective interior surface that slipped around the lamp to reflect light toward the ruby rod. The ends of the rod were polished flat, perpendicular to the length of the rod and parallel to each other. Maiman applied a reflective silver coating to both ends and then removed the silver from the center of one end, to allow a transparent opening for the laser beam to escape and subsequently be detected. The apparatus was connected to a separate power supply.³⁵

On May 16, 1960, Maiman and D’Haenens aimed the laser cylinder toward a white poster board. They started firing the flashlamp with pulses of 500 volts (V), gradually increasing the voltage to produce progressively more intense light flashes, and measured the laser’s output tracing on an oscilloscope. Finally, with the power supply set above 950 V, the oscilloscope’s trace surged, a red glow filled the room, and a brilliant red spot appeared on the poster board. After 9 months of intense effort, Maiman accomplished his goal, and the laser was born. In so doing, he beat out Bell Labs, TRG, Westinghouse, IBM, Siemens, RCA Labs, Massachusetts Institute of Technology’s Lincoln Laboratory, General Electric, and all others in contention.^35,36,39,44 Maiman submitted a paper reporting his evidence for a ruby laser to Physical Review Letters, the leading U.S. journal for publishing new physics research. Its editor, Samuel Goudsmit, rejected the manuscript, apparently not appreciating the breakthrough Maiman had achieved, perhaps mistakenly believing it was just a follow-up to previously published work on masers. Maiman then submitted his report to the British weekly journal Nature, which accepted it immediately and published it on August 6, 1960.^45,46

Other laser types followed^36,39,46: