I wrapped up my previous post, which gave Planck’s solution for the blackbody radiation problem, wondering whether or not one could find the same equation using some other model, not involving the assumption that atomic oscillators have discrete energy levels.
I still don’t have an answer to that question but, sure enough, Feynman introduces another model a few pages further in his Lectures. It’s a model developed by Einstein, in 1916, and it’s much ‘richer’ in the sense that it takes into account what we know to be true: unlike matter-particles (fermions), photons like to crowd together. In more advanced quantum-mechanical parlance, their wave functions obey Bose-Einstein statistics. Now, Bose-Einstein statistics are what allows a laser to focus so much energy in one beam, and so I am writing this post for two reasons–one serious and the other not-so-serious:
- To present Einstein’s 1916 model for blackbody radiation.
- For my kids, so they understand how a laser works.
Let’s start with Einstein’s model first because, if I’d start with the laser, my kids would only read about that and nothing else. [That being said, I am sure my kids will go straight to the second part and, hence, skip Einstein anyway. :-)]
Einstein’s model of blackbody radiation
Einstein’s model is based on Planck’s and, hence, also assumes that the energy of atomic oscillators can also only take on one value of a set of permitted energy levels. However, unlike Planck, he assumes two types of emission. The first is spontaneous, and that’s basically just Planck’s model. The second is induced emission: that’s emission when light is alrady present, and Einstein’s hypothesis was that an atomic oscillator is more likely to emit a photon when there’s light of the same frequency is shining on it.
The basics of the model are shown above, and the two new variables are the following:
- Amn is the probability for the oscillator to have its energy drop from energy level m to energy level n, independent of whether light is shining on the atom or not. So that’s the probability of spontaneous emission and it only depends on m and n.
- Bmn is not a probability but a proportionality constant that, together with the intensity of the light shining on the oscillator–denoted by I(ω), co-determines the probability of of induced emission.
Now, as mentioned above, in this post, I basically want to explain how a laser works, and so let me be as brief as possibly by just copying Feynman here, who says it all:
Of course, this result must match Planck’s equation for blackbody radiation, because Planck’s equation matched experiment:
To get the eħω/kT –1, Bmn must be equal to Bnm, and you should not think that’s an obvious result, because it isn’t: this equality says that the induced emission probability and the absorption probability must be equal. Good to know: this keeps the numbers of atoms in the various levels constant through what is referred to as detailed balancing: in thermal equilibrium, every process is balanced by its exact opposite. While that’s nice, and the way it actually works, it’s not obvious. It shows that the process is fully time-reversible. That’s not obvious in a situation involving statistical mechanics, which is what we’re talking about there. In any case, that’s a different topic.
As for Amn, taking into account that Bmn = Bnm, we find that Amn/Bmn =ħω3/π2c2. So we have a ratio here. What about calculating the individual values for Amn and Bmn? Can we calculate the absolute spontaneous and induced emission rates? Feynman says: No. Not with what Einstein had at the time. That was possible only a decade or so later, it seems, when Werner Heisenberg, Max Born, Pascual Jordan, Erwin Schrödinger, Paul Dirac and John von Neumann developed a fully complete theory, in the space of just five years (1925-1930), but that’s the subject of the history of science.
The point is: we have got everything here now to sort of understand how lasers work, so let’s try that to do that now.
Laser is an acronym which stands for Light Amplification by Stimulated Emission of Radiation. It’s based on the mechanism described above, which I am sure you’ve studied in very much detail. 🙂
The trick is to find a method to get a gas in a state in which the number of atomic oscillators with energy level m is much and much greater than the number with energy level n. So we’re talking a situation that is not in equilibrium. On the contrary: it’s far out of equilibrium. And then, suddenly, we induce emission from this upper state, which creates a sort of chain reaction that makes “the whole lot of them dump down together”, as Feynman puts it.
The diagram below is taken from the Wikipedia article on lasers. It shows a so-called Nd:YAG laser. Huh? Yes. Nd:YAG stands for neodymium-doped yttrium aluminium garnet, and an Nd:YAG laser is a pretty common type of laser. A garnet is a precious stone: a crystal composed of a silicate mineral. And that’s what it is here, and why the laser is so-called solid-state laser, because the so-called laser medium (see the diagram) may also be a gas or even a liquid, as in dye lasers). I could also take a ruby laser, which uses ruby as the laser medium. But let’s go along with this one as for now.
In the set-up as shown above, a simple xenon flash lamp (yes, that’s a ‘neon’ lamp) provides the energy exciting the atomic oscillators in the crystal. It’s important that the so-called pumping source emits light of a higher frequency than the laser light, as shown below. In fact, the light from xenon gas, or any source, will be a spectrum but so it should (also) have light in the blue or violet range (as shown below). The important thing is that it should not have the red laser frequency, because that’s what would trigger the laser, of course.
The diagram above shows how it actually works.The trick is to get the atoms to a higher state (that’s h in the diagram above, but it’s got nothing to do with the Planck constant) from where they trickle down (and, yes, they do emit other photons while doing that), until they all get stuck in the state m, which is referred to as metastable but which is, in effect, unstable. And so then they are all dumped down together by induced emissions. So the source ‘pumps’ the crystal indeed, leading to that ‘metastable’ state which is referred to as population inversion in statistical mechanics: a lot of atoms (i.e. the members of the ‘population’) are in an excited state, rather than in a lower energy state.
And then we have a so-called optical resonator (aka as a cavity) which, in its simplest form, consists of just two mirrors around the gain medium (i.e. the crystal): these mirrors reflect the light so, once the dump starts, the induced effect is enhanced: the light which is emitted gets a chance to induce more emission, and then another chance, and another, and so on. However, although the mirrors are almost one hundred percent reflecting, light does get out because one of the mirrors is only a partial reflector, which is referred to as the output coupler, and which produces the laser’s output beam.
So… That’s all there is to it.
Really? Is it that simple? Yep. I googled a few questions to increase my understanding but so that’s basically it. Perhaps they’ll help you too and so I copied them hereunder. Before you go through that, however, have a look at how they really look like. The image below (from Wikipedia again) shows a disassembled (and assembled) ruby laser head. You can clearly see the crystal rod in the middle, and the two flashlamps that are used for pumping. I am just inserting it here because, in engineering, I found that a diagram of something and the actual thing often have not all that much in common. 🙂 As you can see, it’s not the case here: it looks amazingly simple, doesn’t it?
Q: We have crystal here. What’s the atomic oscillator in the crystal? A: It is the neodymium ion which provides the lasing activity in the crystal, in the same fashion as red chromium ion in ruby lasers.
Q: But how does it work exactly? A: Well… The diagram is a bit misleading. The distance between h and m should not be too big of course, because otherwise half of the energy goes into these photons that are being emitted as the oscillators ‘trickle down’. Also, if these ‘in-between’ emissions would have the same frequency as the laser light, they would induce the emission, which is not what we want. So the actual distances should look more like this:
For an actual Nd:YAG laser, we have absorption mostly in the bands between 730–760 nm and 790–820 nm, and emitted light with a wavelength with a wavelength of 1064 nm. Huh? Yes. Remember: shorter wavelength (λ) is higher frequency (ν = c/λ) and, hence, higher energy (E = hν = hc/λ). So that’s what’s shown below.
Q: But… You’re talking bullsh**. Wavelengths in the 700–800 nm range are infrared (IR) and, hence, not even visible. And light of 1064 nm even less. A: Now you are a smart-ass! You’re right. What actually happens is a bit more complicated, as you might expect. There’s something else going on as well, a process referred to as frequency doubling or second harmonic generation (SHG). It’s a process in which photons with the same frequency (1064 nm) interact with some material to effectively ‘combine’ into new photons with twice the energy, twice the frequency and, therefore, half the wavelength of the initial photons. And so that’s light with a wavelength of 532 nm. We actuall also have so-called higher harmonics, with wavelengths at 355 and 266 nm.
Q: But… That’s green? A: Sure. A Nd:YAG laser produces a green laser beam, as shown below. If you want the red color, buy a ruby laser, which produces pulses of light with a wavelength of 694.3 nm: that’s the deep red color you’d associate with lasers. In fact, the first operational laser, produced by Hughes Research Laboratories back in 1960 (the research arm of Hughes Aircraft, now part of the Raytheon), was a ruby laser.
Q: Pulses? That reminds me of something: lasers pulsate indeed, don’t they? How does that work? A: They do. Lasers have a so-called continuous wave output mode. However, there’s a technique called Q-switching. Here, an optical switch is added to the system. It’s inserted into laser cavity, and it waits for a maximum population inversion before it opens. Then the light wave runs through the cavity, depopulating the excited laser medium at maximum population inversion. It allows to produce light pulses with extremely high peak power, much higher than would be produced by the same laser if it were operating in constant output mode.
Q: What’s the use of lasers? A: Because of their ability to focus, they’re used a surgical knives, in eye surgery, or to remove tumors in the brain and treat skin cancer. Lasers are also widely used for engraving, etching, and marking of metals and plastics. When they pack more power, they can also be used to cut or weld steel. Their ability to focus is why these tiny pocket lasers can damage your eye: it’s not like a flashlight. It’s a really focused beam and so it can really blind you–not for a while but permanently.
Q: Lasers can also be used as weapons, can’t they? A: Yes. As mentioned above, techniques like Q-switching allow to produce pulses packing enormous amounts of energy into one single pulse, and you hear a lot about lasers being used as directed-energy weapons (DEWs). However, they won’t replace explosives anytime soon. Lasers were already widely used for sighting, ranging and targeting for guns, but so they’re not the source of the weapon’s firepower. That being said, the pulse of a megajoule laser would deliver the same energy as 200 grams of high explosive, but all focused on a tiny little spot. Now that’s firepower obviously, and such lasers are now possible. However, their power is more likely to be used for more benign purposes, notably igniting a nuclear fusion reaction. There’s nice stuff out there if you’d want to read more.
Q: No. I think I’ve had it. But what are those pocket lasers? A: They are what they are: handheld lasers. It just shows how technology keeps evolving. The Nano costs a hundred dollars only. I wonder if Einstein would ever have imagined that what he wrote back in 1916 would, ultimately, lead to us manipulating light with little handheld tools. We live in amazing times. 🙂