Einstein's 1917 idea of stimulated emission led to the laser

Person: Albert Einstein
Lived: 1879–1955
Field: Physics
From: German-American

The story

By 1917 Albert Einstein had completed his general theory of relativity, but he continued to think about the nature of light. In 1905 he had proposed that light comes in packets of energy, or quanta, and Max Planck had earlier worked out a law describing the radiation given off by hot bodies. Einstein wanted to understand, at the level of individual atoms, how matter and light exchange energy in a way that would produce Planck's law.

In a 1917 paper, "On the Quantum Theory of Radiation", published in Physikalische Zeitschrift, he described three processes. An atom can absorb light and jump to a higher energy state. An excited atom can release light on its own, which he called spontaneous emission. And an incoming particle of light can prompt an excited atom to release a second, identical particle travelling in the same direction, a process he called stimulated emission. He showed that Planck's law only made sense if all three were included, and he introduced the probability coefficients that now bear his name.

The paper was enormously influential in the development of quantum mechanics because it was the first to show that atomic transitions follow simple statistical laws. Stimulated emission itself seemed to have no practical use, but it contained the essential physics needed to amplify light. In 1928 Rudolf Ladenburg confirmed that it existed, and in 1947 Willis Lamb and Robert Retherford made the first demonstration of it.

Engineers then put the idea to work. In 1953 Charles Townes and his graduate students built the first maser, which amplified microwaves. On 16 May 1960 Theodore Maiman operated the first working laser at Hughes Research Laboratories in Malibu, California, using a synthetic ruby crystal to produce red light. The word laser, first published by Gordon Gould in 1959, stands for light amplification by stimulated emission of radiation, a direct echo of Einstein's 1917 idea.

Einstein's theory of radiation was ahead of its time and also helped lay groundwork for later fields such as quantum electrodynamics and quantum optics. It is a striking example of pure theory, pursued for its own sake, turning out decades later to be enormously practical.

Why it matters

Lasers built on Einstein's insight are used in fibre-optic communications, laser surgery, barcode scanners, chip manufacturing, cutting and welding, and scientific research.