Two weeks ago, my sixteen year-old son asked me for some money. He wanted to work on a “project,” to build a laser pointer capable of emitting blue light. I’m sure very few of you have seen such thing, as most laser pointers are red or green. Blue laser pointers are quite expensive. “Dad, I can sell it much cheaper than the current market value. And there is nothing more beautiful than a blue laser!” Duly convinced and proud (I had no clue how to build a laser when I was 16), I gave him the money (“As an investment!”). Ten days later, he had his laser—after ordering parts from Korea, China, and the US—and proceeded to pop party balloons from a distance, to the amazement of his little brother: science turned magic. Blue lasers pack quite a punch, even at the commercial level.
I got to think about the long trajectory from Einstein’s original 1916 idea to my son’s blue laser; of how basic science — Einstein’s interest was in the interaction between light and matter — can become technology, reshape society, and turn into a household item which smart kids (yes, my son is smart) can emulate without much trouble.
In 1916, Einstein wrote to his friend Michele Besso: “a splendid light illuminated my mind in relation to the emission and absorption of radiation.”
Everything starts with Bohr’s simple model for the atom (at least it helps visualizing what goes on), where electrons orbit nuclei in distinct, step-like orbits. They can “jump” either outwards or inwards, toward the nucleus. Each orbit has a fixed energy. If incoming light (or, more precisely, electromagnetic radiation) with the correct energy hits the electron in a given orbit, it can induce it to jump to a higher orbit. Imagining light as little packets, called photons, you can think that the electrons “eat” the photons, absorb their energy and move up: this is “spontaneous absorption.” On the other hand, an electron on a higher orbit can be coaxed to come down by being hit by a photon. As it does so, it emits both the photon that hit it plus the one with the extra energy between its initial and final orbit. This is “stimulated emission”: you get two photons for the price of one. This is the key process behind the laser, short for “Light Amplification by Stimulated Emission of Radiation.”
The history of the laser is an example of the bitter wars that can develop between scientists working in academia and industry when major patents are involved. Even if the original idea came from Charles Townes when he worked at Columbia University, the competition and implementation came from physicists working at Hughes Research and Bell Labs.
In 1954, Townes (and, independently, Nikolai Basov and Aleksandr Prokhorov; the trio would later share the Nobel) invented the maser, a laser in the microwave region of the electromagnetic spectrum. To make a maser into a laser, excited atoms had to be coaxed into emitting visible light, something many physicists thought impossible. The trick was to use a system of mirrors that allowed for light to pass several times across a gas sample, keeping the population of excited atoms (with electrons in elevated orbits) at high levels. The electrons in the excited states would decay, emitting two photons with the same frequency (read “ same color” for visible light); these photons would hit more excited electrons making them decay, and soon enough you had a cascading effect: myriad photons (i.e. light) of the same frequency marching forward.
In 1960, Theodore Maiman, from Hughes, used a ruby crystal to build the first laser. That same year (hence the 50th B-day celebration), Ali Javan and collaborators from Bell Labs built a different laser using a mixture of helium and neon gases. The “death ray” of sci-fi movies was finally at hand. Even H. G. Wells, in his War of the Worlds (1898), had dreamed up such a weapon. And yet, although high-powered lasers can cut through metal, that’s not what they are mostly known for. They record CDs and DVDs at hundreds of millions of households across the world, they travel along fiber optics cables as the blood stuff of telecommunications, they are used in supermarkets checkout stations to read off prices in bar codes, they’re used in surgery, and, yes, in fundamental research. For example, lunar ranging lasers are used to test Einstein’s very own theory of relativity. I’m sure he would be pleased with this turn of events. And would get a kick out of playing with my son’s blue laser pointer.