Fifty years ago, a handy little device was invented that would go on to change the world. I'm talking about the laser: without it, there would be no DVDs or BluRays, no laser light shows, no handy laser pointers to highlight the relevant portion of one's PowerPoint slide, no fiber optic communications systems, no corrective eye surgery, no supermarket scanners, tattoo removal, and so on. The laser is ubiquitous to modern technology, which is why the physics community is celebrating Laser Fest all this year, kicking off this past weekend in Berkeley with a special exhibit at the Lawrence Hall of Science.
“Laser,” for those who aren't inclined to Google it, is an acronym for Light Amplification by Stimulated Emission of Radiation -- quite the mouthful, so you can see why everyone just decided to call it a laser. It describes any device that creates and amplifies a narrow, tightly focused beam of light whose photons are all traveling in the same direction, rather than emitting every which way at once, like the light emitted from, say, a household flashlight.
How often do you stop and ask yourself, "Where does light even come from?" The answer is that it comes from atoms as their energy levels rise and fall back down to ground state. Anything that produces light – the heating element in toasters, gas lanterns, incandescent bulbs – does so by raising the temperature of the atoms that make up said object. This causes the electrons orbiting the atomic nuclei to jump to higher energy levels; when they relax back into their ground state, the excess energy is released as photons. The wavelength of the emitted light depends on how many levels the electron had to drop. Laser light is unique because it is "tuned" -- that is, it contains only one specific color, or wavelength.
There are many different types of laser, but at heart, they exploit the same fundamental physics. You start with an empty cavity with two mirrors -- one of which is half-silvered so it reflects some light and lets some light through -- on each end. Inside that cavity is a crystal like ruby or garnet, or a gas or liquid. This is the "lasing medium." Now it's time to pump things up by applying intense flashes of light or electricity. You're literally pumping in extra energy, which is absorbed by the atoms or molecules of the lasing medium. This boosts them above their ground state to a higher energy level. Atoms thus boosted are said to be "excited," and who wouldn't be when subjected to a sudden electric shock?
Then a photon enters the laser cavity, just minding its own business, but the space is pretty crowded, so chances are, it will bump into an excited atom. If that happens, the atom drops back down to its ground state and emits a second photon of the same frequency, in the same direction as the bombarding photon.
This produces a kind of domino effect: each of these may in turn strike other energized atoms, prompting the release of still more photons in the same frequency, all traveling in the same direction. The end result is a sudden burst of coherent” light as all the atoms discharge in a rapid chain reaction. This process is called “stimulated emission.”
Okay, stop giggling. That's what it's called! Check Wikipedia if you don't believe me. Anyway, Albert Einstein first broached the possibility of stimulated emission (oh, stop it!) in a 1917 paper. (Einstein did a lot of things in addition to special and general relativity.) He was a bit too far ahead of his time; or rather, it was an interesting effect, but nobody could figure out what it might be good for.
It wasn’t until the 1940s and 1950s that physicists found a novel use for the concept. It all started with radar, invented in 1935 by Sir Robert Watson-Watt. Yes, radar. Laser pioneer Charles Townes worked on radar systems during World War II. After the war ended, Townes switched his focus to molecular spectroscopy, a technique that studies the absorption of light by molecules (different types of molecules absorb different wavelengths of light). Just like radar, molecular spectroscopy bombards the surface of molecules with light and analyzes the ones that bounce back to determine the molecule’s structure. By 1953 he had patented a device he called a maser, for “microwave amplification by stimulated emission of radiation.”
Initially the technique was limited by the microwave wavelength. Townes noticed that as the wavelength of the microwaves shortened, the light interacted more strongly with the molecules, which meant one could learn more about them. He thought it might be possible to gain even more information by developing a device that produced light at much shorter wavelengths, essentially extending the maser concept to the optical range of the electromagnetic spectrum. The best way to do this, he thought, would be to use molecules to generate the desired frequencies through stimulated emission.
Townes mentioned the idea to a colleague (later his brother-in-law), Arthur Schawlow, who came up with the idea of outfitting the lasing cavity with mirrors on either end in such a way that only photons in the selected wavelength and frequency range would be amplified.
The two men wrote a paper detailing their concept for what they called a laser, and published it in the December 1958 issue of the scientific journal the Physical Review, although they had yet to build a working prototype. They received a patent for their design two years later – the same year that the first working laser was built by Theodore Maiman at Hughes Aircraft Company.
You can learn about all the gritty details at the LaserFest Website, which includes video interviews with Townes, Maiman, and a special documentary on laser applications. And for anyone who's in the Berkeley area tomorrow afternoon, Townes and a couple of other physicists will be giving a public talk at the Lawrence Hall of Science at 4:30 PM.
Interesting historical side note: as is often the case with revolutionary inventions, there was some question of the patent rights for the laser. Gordon Gould, a scientist at Columbia University and later with Technical Research Group (TRG), sued to earn patent rights based on his research notebook, which contained an entry dated and notarized in November 1957, describing his own design for a laser. Here's what the Washington Post had to say about it, in Gould's 2005 obituary:
While studying physics at Columbia University, Mr. Gould said, he conceived the idea for the laser Nov. 9, 1957, in the middle of the night. After jumping from bed, he chain-smoked his way through the weekend and that Wednesday raced his notebook from his Bronx, N.Y., apartment to a candy store to have it stamped by a notary public.
Confused by a lawyer's advice, he thought he first needed a working model and neglected to apply for a patent. This was a costly oversight, allowing other scientists to win credit for the laser, which Mr. Gould believed could cut, weld, measure distances and create heat that would trigger nuclear fission.
Gould fought for decades, and in 1973 the U.S. Court of Customs and Patent Appeals ruled that the original patent awarded to Schawlow and Townes was too general, and did not supply enough information to create certain key components of a working laser. Gould was entitled to patent rights -- which by then probably added up to a tidy sum of money.
There are several different types of lasers. Solid-state lasers use crystals whose atoms are arranged in a solid matrix, such as ruby. CO2 lasers emit energy in the far-infrared and microwave regions of the spectrum. This type produces intense heat, and is capable of melting through objects. Dr. Evil coveted such a laser when he demanded “sharks with frickin’ laser beams” on their heads to torture Austin Powers to death – only to be foiled because sharks are an endangered species. Imagine his disappointment if, in addition to having to make do with cranky mutated sea bass, they were equipped not with CO2 lasers, but with conventional diode (semiconductor) lasers. These are the type used in pocket laser pointers and CD and DVD players. They are not even remotely lethal.
The laser revolution continues, with ultrafast lasers, quantum cascade lasers, and other innovations, each enabling a brand new host of applications. So the next time you whip out that power pointer or fire up your DVD player, take a moment to reflect on what decades of scientific research has wrought, and maybe even say a quiet thank you.