Stimulated Optical Radiation in Ruby
Pump a ruby hard enough and its atoms shine as one — the first laser light.
In May 1960 a coil of flashlamp lit up a fingertip of ruby, and a thin, pure-red beam crossed the lab bench — light had a new kind.
The idea
The light from a bulb or the Sun is a jumble: many colours mixed together, spilling out in every direction, the waves all out of step. Laser light is the opposite — one single colour, one direction, every wave marching in lockstep. Physicists call that last quality coherent.
The trick that makes it is stimulated emission. An atom holding extra energy, when a passing light wave of just the right colour brushes by, lets go of a photon that is an exact copy of the one that triggered it — same colour, same direction, in step. One photon becomes two, two become four, and an avalanche of identical light builds. Maiman's ruby was the first machine to make that avalanche.
How it happened
The theory came first. Einstein predicted stimulated emission back in 1917; in 1958 Arthur Schawlow and Charles Townes worked out how one might build an "optical maser." A quiet race followed. Theodore Maiman, at Hughes Research Laboratories in California, bet on a material others had written off — a small rod of synthetic ruby, wrapped inside a helical photographic flashlamp. On 16 May 1960 it fired. His write-up was so terse that a leading physics journal had already rejected it; it appeared instead as a half-page note in Nature.
Why it mattered
A laser is a tap of pure, controllable light. Because the beam is one colour and one direction, you can focus it to a pinpoint hot enough to cut steel, send it for thousands of miles down a glass fibre thinner than a hair, or use its waves as a ruler accurate to a fraction of their own length. At first almost nobody knew what it was good for — it was joked about as "a solution looking for a problem." Within a generation the problems came flooding in.
An analogy
Think of pushing a child on a swing. A shove at a random moment does little, and can even fight the motion. But push exactly in time with the swing — again, and again, and again — and the arc grows huge from small efforts. Stimulated emission is light pushing atoms in time: each photon nudges an excited atom to release another photon in perfect step, so the wave reinforces itself and the beam swells.
Where it sits
The laser grew straight out of quantum physics — the quantized energy rungs of Planck (planck-1900) and Bohr (bohr-1913), and Einstein's 1917 insight about stimulated emission. It is the optical cousin of the microwave maser that came a few years earlier. And it turned around to serve the rest of science: the exquisitely stable lasers that steady the mirrors of LIGO, listening for gravitational waves (ligo-2016), are direct descendants of Maiman's red flash.
Schawlow and Townes have proposed a technique for the generation of very monochromatic radiation in the infra-red optical region of the spectrum using an alkali vapour as the active medium.
an optical pumping technique has been successfully applied to a fluorescent solid resulting in the attainment of negative temperatures and stimulated optical emission at a wave-length of 6943 Å.; the active material used was ruby (chromium in corundum).