Lasers

The Principle

In his 1916 work on how atoms absorb and emit light, Einstein showed that an excited atom can be prompted by a passing light quantum to release an identical one. He needed the process only to make the bookkeeping balance. See Stimulated Emission.

One quantum becomes two, in step with each other. If those two meet more excited atoms, they become four. The word laser stands for light amplification by stimulated emission of radiation.

The Device

It took nearly forty years for anyone to see how to exploit this. The difficulty is that, normally, more atoms sit in the lower energy level than the upper, so light is absorbed more than it is amplified.

• In 1954 Charles Townes and his colleagues at Columbia University built the maser, which amplified microwaves.
• In 1958 Townes and Arthur Schawlow showed how the same idea could work for visible light.
• On 16 May 1960 Theodore Maiman, at Hughes Research Laboratories in California, switched on the first laser, a rod of synthetic ruby that emitted pulses of deep red light.

Townes shared the 1964 Nobel Prize with the Soviet physicists Nikolai Basov and Alexander Prokhorov, who had developed the maser principle independently.

What Lasers Do

At first the laser was described as a solution looking for a problem. It found a few.

• Communication. Laser light pulsed through glass fibers carries nearly all long-distance internet and telephone traffic.
• Medicine. Eye surgery, the removal of tumors, and countless diagnostic instruments.
• Manufacturing. Cutting, welding, and engraving with a precision no blade can match.
• Everyday life. Barcode scanners, laser printers, and optical discs.
• Measurement. Surveying, the mapping technique lidar, and the detectors that sense gravitational waves.

How Direct Is the Link?

The physics is Einstein's. The invention is not. He described the process and never suggested a device; the leap from a term in an equation to a working machine belongs to Townes, Schawlow, Maiman, and their contemporaries. It is fair to say that without the 1916 paper there would be no laser, and equally fair to say that he had no idea what he had started.