Wednesday, July 4, 2007

Some Knurdly Background about Lasers I

A lot of people link Einstein to the Atom Bomb because of the E=MC2 thing. This is a pretty fundamental misunderstanding, because mass/energy equivalence is a general property of nature. Every chemical reaction also has E=MC2 going on; it’s just that the change in mass from a chemical reaction is too small to measure, while for nuclear reactions the change is measurable.

No, the go-to guy for the Atom Bomb was Enrico Fermi; Einstein’s primary contribution to the deal was that he wrote that famous letter to Roosevelt, ironically, a political contribution that drew on Einstein’s celebrity status, but had little to do with Einstein’s contributions to science.

Lasers, however, are a different thing entirely.

In 1916, Einstein wrote a letter to Michael Angelo Besso that included the poetic line, "A splendid light has dawned on me about the absorption and emission of radiation." He documented his insight in a paper that was published the following year, describing the absorption and emission of photons by atoms in a gas, but which went further and described a third process, stimulated emission.

This insight of his was an “Einstein Special.” From the large-scale macroscopic properties of gases, Einstein deduced a phenomenon that occurs at very small, quantum mechanical scales. This is not something that just leaps out at you from staring at some equations for a bit. It’s one of those things that makes some people think that Einstein had God’s phone number (although, realistically, God sometimes made prank calls, like that “God does not play dice with the universe” thing. It turns out that, not only does God play dice, He uses dice with so many sides to them that they might as well be ball-bearings. Also, He uses a lot of dice).

The stimulated emission phenomenon says that, if you have an atom or molecule in an excited state of a particular energy, if you hit it with a photon of exactly that same energy, you will stimulate the emission of energy by the atom or molecule. So now you have two photons, of exactly the same energy. Moreover, they are identical in all other respects, same phase, polarization, everything. They are coherent.

The closest thing I can come up with by way of a large, mechanical analogy is, suppose you have a ledge on a pool with some ball on it, and a wave that is exactly the same height as one of the balls slaps it off the ledge and it falls into the water. And only a wave of the correct height will do that, and when it does, the ball makes another wave, exactly the same as the first one.

Yeah, I know. Crummy analogy. But it was the best I could do.

So anyway, suppose you had a huge number of these excited atoms, and you sent a single photon into the mass of them. Bingo! Now you’ve got a chain reaction, with each photon setting off two more, and so on and on. That’s the “Amplification” part of “Light Amplification by Stimulated Emission of Radiation (LASER).” All very cool. The only problem is, “how do you get all those excited atoms in a mass?”

Suppose you try to do it by shining a light on them. At first, you’re creating excited atoms, but as time goes on, more and more the atoms you are hitting are already excited, and whoops, you’ve just stimulated them into giving photons. In fact, you can never get to the point where you have more atoms amplifying the photons than absorbing them. I think that’s actually part of the thermodynamic argument that Einstein was using in the first place.

Well, it nevertheless turns out that there are ways to do it. But it took almost 40 years after Einstein’s “splendid light” for someone to do it. And man, what a Rube Goldberg device it was.

It was called a Maser Amplifier, and the “M” stands for “Microwave.” The gas in it was ammonia, which has a nice emission line in the microwave spectrum. Also, and this is very important, ammonia has a large dipole moment that is different for its excited state as compared to its “ground state.”

The guys who did it were J. P. Gordon, H. J. Zeiger, and C. H. Townes, with Townes going on to later invent the Laser as well.

They started out with a “molecular beam” of ammonia. When you release a gas into a vacuum, eventually the atoms in it stop banging into each other because the gas pretty much stops being a gas; the atoms in it are too diffuse to bump into each other. So they follow “ballistic trajectories,” and if you’ve put the whole thing through slots and holes, all the atoms are moving in more or less the same direction. You get a beam of atoms.

Some of those atoms had been excited by some prior microwave treatments, as it were, and the difference between the dipole moments of the excited vs. ground state ammonia allowed them to be separated by a series of magnets and electrodes. The excited molecules went into a “resonant cavity,” the sort of things that radar guys had gotten really good at building in WWII.

But Rube Goldberg is only warming up. While the ammonia molecules weren’t bumping into each other very often, them being so rarified and all, they were banging off the walls like bikers on a meth bender. And there was a big problem. If the excited ammonia interacted with the walls in anything other than a purely elastic collision, then there was a high probability that it would lose its energy and revert to the ground state.

So the walls had to be chemically inert. I mean really inert. Fortunately, Teflon had been invented. Unfortunately, that wasn’t quite enough. Commercial Teflon still has a few chemical bonds remaining in it; it’s hard to get a 100% chemical reaction product formation, so some of the Teflon ingredients were still unreacted, and still reactive (just don’t ask me about the story of the first experimental attempts at making Teflon condoms, where it turned out that there was just a little smidge of unreacted fluorine remaining, and God, do I hope that story is apocryphal).

So what one of the intrepid threesome did (I haven’t been able to learn which one), was to put fluorine gas into the resonating chamber and shine some light on it. Fluorine gas photolyzes into fluorine atoms and if there is something in the universe more reactive than fluorine atoms, I do not know what it is. So those super-reactive F atoms located every remaining chemical bond in the Teflon and glommed onto it with a vicey grip. Then, (I expect) after multiple flushings and other treatments to get rid of the last bits of fluorine, the walls of the resonating chamber were passivated.

[Brief aside: smog chambers, the experimental device of choice for much of my career in science also had Teflon wall, and my colleague Gary Whitten tried a few times to get the smog chamber guys to do a similar passivation on them, as wall effects were a known problem. No one ever did it. I’m of mixed feelings about it. It would have made the experiments more reliable. On the other hand, it’s fluorine!]

So anyway, after all that, the microwave excitation, the molecular beam, the dipole separation into a passivated resonant cavity, and I hesitate to think how many other minor kludges along the way, in 1954 James P. Gordon rushed into the class that Townes was teaching and announced that they were getting microwave amplification from the damn thing, a mere 38 years after Einstein had pointed out the basic theory.

Oh, and the thing about having enough atoms or molecules in an excited state to create amplification through stimulated emission? That’s called an “inverted population.” It’s a Boltzmann thing.

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