The idea Einstein supplied
In 1917 Albert Einstein, extending earlier work on how matter and light exchange energy, described three processes. An atom can absorb a photon and move to a higher energy level. An excited atom can drop to a lower level spontaneously and emit a photon. And, in a third process, an excited atom can be prompted by a passing photon to emit a second photon of the same wavelength, traveling in the same direction and in step with the first. This third process is stimulated emission.
Einstein did not propose a device. For decades the effect was of interest mainly to theorists, because in normal matter, more atoms sit in lower energy states than in higher ones, so absorption wins over stimulated emission and light is weakened rather than amplified.
Population inversion
Making stimulated emission dominate requires reversing the usual balance so that more atoms sit in an excited state than in a lower one. This condition is called population inversion. In the 1950s, physicists worked out ways to create it. Once present, a passing photon can trigger a cascade, with each new photon able to stimulate more. That is amplification.
A second ingredient is feedback. If the amplifying material sits between two mirrors, the light passes back and forth, growing each time and selecting a narrow set of wavelengths and directions. One mirror is partly transparent so that some light escapes as a beam. This arrangement is called an optical cavity or resonator.
The maser
The first devices to use stimulated emission worked with microwaves, not visible light. Charles Townes, with students James Gordon and Herbert Zeiger at Columbia University, built an ammonia maser around 1953 to 1954. Its name stands for microwave amplification by stimulated emission of radiation. In the Soviet Union, Nikolay Basov and Aleksandr Prokhorov independently developed the theory of the idea and related designs. Townes, Basov and Prokhorov shared the 1964 Nobel Prize in Physics for their work leading to the maser and laser principle.
Masers were important in their own right. They were extremely stable, so they supported precision frequency standards and the work that led to the atomic clock. Their low-noise amplification was useful in sensitive receivers for early communications satellites and in radio astronomy. Microwave engineering had also benefited from wartime radar detection programs, where devices like the radar magnetron had made high-power microwave sources a familiar tool, and physicists who learned that technology were well placed to try microwave experiments afterward.
The leap to light
An obvious next question was whether the same process could work at the much shorter wavelengths of light. In 1957 Townes discussed the idea with Arthur Schawlow, then at Bell Laboratories, and the two developed a design for an optical maser. In 1958 they published a paper describing how a long, narrow cavity formed by two mirrors could select a clean beam. The Schawlow-Townes paper is usually seen as the key theoretical step toward the laser.
Meanwhile, others worked in parallel. Gordon Gould, then a graduate student at Columbia, wrote notes in 1957 describing a similar device and is credited with the word laser. He took a long time to file for a patent, and a patent dispute with Townes and Schawlow ran for years. Gould eventually received patents in the late 1970s and 1980s covering some applications, which became commercially significant, but the dispute remains a standard example of how priority in science can be unclear. Different sources describe the outcome somewhat differently, so details are best checked in specialist accounts.
The first working laser
The race to build the first working optical device included well-funded groups at several companies and universities. Many researchers considered ruby an unpromising material, and the first successful device came from Theodore Maiman at Hughes in 1960. That story is covered separately. By the end of 1960 other lasers had followed, including the helium-neon gas laser built at Bell Laboratories by Ali Javan and colleagues.
Why credit is shared
The invention of the laser involved:
- Einstein’s theory of stimulated emission (1917)
- earlier optical studies by Rudolf Ladenburg and proposals by Valentin Fabrikant in the Soviet Union
- the maser work of Townes and of Basov and Prokhorov
- the optical maser proposal of Schawlow and Townes
- Gould’s independent notes and terminology
- Maiman’s experimental demonstration
Some of these contributions are well documented while others are debated, so it would be a mistake to name a single inventor. Even the Nobel Prize in 1964 did not go to Maiman or Gould. Schawlow shared a later Nobel Prize in 1981 for contributions to laser spectroscopy.
What the theory made possible
Seen through its theory, a laser is a controlled version of a process that happens in nature, such as in some astronomical gas clouds, where natural masers have been observed. The scientific achievement was to engineer the conditions deliberately. Once that was done, the next decades were spent finding materials and uses.