What are lasers?
Ohhh, this is a fun one! And surprisingly complicated. I am not sorry for how complicated this got.
Fun Fact
Laser is actually an acronym! It stands for
Light
Amplification by
Stimulated
Emission of
Radiation
But that doesn't tell you much, so what is a laser?
The simple story
If you have something that makes light (a gain medium) and you bounce light back and forth through it a bunch, it makes extra-special "coherent" light which can be focused more intensely than normal light. This is because light gets emitted from the gain medium in sync with the light which passes through it.
But that leaves out a lot of details. Like what being 'in sync' means here, why it matters, and why passing light through a gain medium makes the light the gain medium emits be in sync with it! And that's just to start!
Why do we want lasers in the first place?
So light has this thing called "phase". You can think of "phase" as a point on a circle which rotates as the light moves. (If you've learned about complex numbers, it's a rotating complex number with a magnitude of 1.) If two bits of light with the opposite phase overlap, they stop existing! This is called "destructive interference". If two bits of light with the same phase overlap, they double in power! This is called "constructive interference". (The gif shows you how phase is related to the distance light has traveled, the left image shows constructive interference, and the right shows destructive interference.)
Now in practice light isn't a particle with a single phase, it's a big messy cloud with lots of differently rotated phases based on how far it's traveled from its origin. So two photons with "opposite" phase at one point might actually have different phase relationships at other points, destructively interfering in some places and constructively interfering in others. This is how light from a light bulb works. It's "incoherent", every photon has its own phase and they overlap in messy and complicated ways. (see figure below)
And when you try to focus incoherent light down to a point, you can only focus it down so much! The focus point is this big mess of variously interfering photons and it puts a harsh limit on how focused it can all get. So your maximum intensity ends up being proportional to the square root of the number of photons you have! This means you could quadruple the amount of light you're emitting, and only get twice the intensity! You could multiply your amount of light by 100 and only get 10x the intensity! That sucks!
But coherent light is much easier to focus down, and can be focused to an intensity proportional to the number of photons! If you multiple your amount of light by 100, you actually do get 100x the intensity when you focus it down to a point. There's also a bunch of neat and tricky ways to measure the phase change of coherent light, called interferometry, in ways which let you measure really small distances. It's also easier to make it go straight for longer. So we really want coherent light! (like in the figure below)
But ... how do we get it? Actually, why isn't all light coherent in the first place, huh?
Oscillating Electron States
So let's take an atom, and look at one of its valence (outermost) electrons. We poke this electron (maybe with a photon, maybe by running electricity through the atom) until it jumps up to a higher energy level, like this!
Well eventually that electron will fall back down to its ground state and emit a photon of equivalent energy in the process. But ... why? Why does the electron want to be in a lower energy level?
Well, the thing is that electrons don't really exist in one state. They exist as a back-and-forth oscillation between pairs of states. Like a tide coming and going back out. We can talk about high tide vs. low tide, and whether the tide is in or out at any given moment, but tides don't really do static. Nor do electrons. An electron in its excited state is actually oscillating between: a) this excited state, b) the ground state + a photon to keep the energy balanced, c) actually every other excited state +/- a photon too. But what we care about is the oscillation between a) and b), which is typically strongest.
Spontaneous Emission
However while this is technically an oscillation, the moment the state looks like b) the ground state + a photon, well, photons move. At the speed of light. So it'll just blast off into the sunset (it is the sunset) never to be seen again, and unavailable to help lift the electron back into its excited state. So the "oscillation" really just looks like the excited photon slowly falling back down to its ground state, by going from a quantum superposition of: 100% excited state -> 75% chance of excited + 25% chance of ground state -> 50% chance of excited + 50% chance of ground state -> 25% chance of excited + 75% chance of ground state -> 100% chance of ground state (wait photon come back!) -> 100% chance of ground state -> 100% chance of ground state forever.
(Yes lasers are inherently quantum, but then again so are lightbulbs and everything else in existence.)
And when electrons emit photons like this, which is called 'spontaneous emission', the photons have a random phase and move in a random direction. This is because they're 'coupling' to the possibility of a photon contained in the vacuum, which doesn't have a notion of phase or direction. (Technically they actually emit as an impure quantum superposition of all phases and directions, but that's off-topic.)
Stimulated Emission
But what if there was already a photon in the background? Let's say we keep a steady stream of photons moving through and around the atom. Well now we get an actual oscillation going! When the excited electron reaches its ground state, it just picks up a new photon to complete its oscillation back to its excited state.
The oscillation is also fundamentally different. First off, it's stronger. Stimulated emission to a preexisting photon state happens much more quickly. But also, an oscillation to a preexisting photon state produces a photon in the same state! The emitted photon has the same phase and direction as the stream of photons in the background!!! Do this with a random stream of photons and they'll eventually all synchronize together!
Huzzah! Now we have a laser, right?
We do not have a laser yet
HAHHAHAHAHAHAHAHAHAHAHAHAHA
NO!
WE DO NOT HAVE A LASER YET!
Well now we get an actual oscillation going! When the excited electron reaches its ground state, it just picks up a new photon to complete its oscillation back to its excited state.
This! This is what dooms us! For every synced up photon the atom emits, it also eats one! We have a filter now, something we could repeatedly pass photons through to sync up their phase and direction, but frankly it would be very weak. It would take so many passes through a bunch of these atoms to sync up a bunch of photons, and imperfections and spontaneous emission (that's still happening sometimes, even if it's weaker than stimulated emission) would eat the photons up before that happens.
We would need something that emits more photons than it eats to make light, to make a proper laser.
But electrons oscillate between states, so they spend just as much time in the "eat a photon" stage as they do in the "emit a photon" stage. And for quantum math reasons, no single oscillation can produce a superposition with more excited states than ground states. So it's impossible, right?
Hah! We are physicists. The science isn't done until we've dragged the laws of nature into a dark alley and mugged them for all they're worth.
Mugging the laws of nature for fun and profit
So first we're going to need an atom whose valence electron has a ground state and three excited states which don't interact much with the electron's other states. (You can do this with three states as well, but it's trickier.) We're going to call a big group of these atoms a "gain medium". Now we're going to excite the gain medium, with a flashbulb or an electrical charge or a chemical reaction or whatever. This is called 'gain medium excitation' and it puts all of its valence electrons into an even oscillation between the ground state (the 1st state) and the most excited state of the excited states (the 4th state), like this.
(I made these following graphics and it shows đ )
Now, we need the 4th and 3rd states, and the 2nd and 1st states, to have very strong spontaneous emission oscillations between them. (And for good measure, that needs to be stronger between the 2nd and 1st states.)
This means we end up with a crap-ton of electrons in the 3rd state, and any electrons which end up in the 2nd state immediately get slurped into the 1st state. This is called a 'population inversion', where there's waayyyyyy more electrons in an excited state than in a lower state, which isn't possible with only one pair of states involved.
Now we insert one little bitty photon with the energy difference between those electron states and-
BWAAAAAAAHHHHHHHHHHHHHHHHH
WE HAVE FIRED THE LASER!
(ish)
We have created a gain medium which outputs more photons than you put in, all synced up in phase and direction! You still need to feed that gain medium with gain medium excitation, but that's fine, even if we need to feed it with light. Because it's pretty easy to make tons of incoherent light, and what we get out is sweet sweet coherent light.
A Laser!!!
So there's a few more parts. We need a pair of mirrors to bounce light back and forth through the gain cavity. And we need one of those mirrors to be not quite perfectly reflective, so it'll let some of the light out. And we need to shape these mirrors very carefully to satisfy certain conditions, and there's also forms of coherence I haven't even mentioned here, like, there's a lot that goes into making lasers.
But this is it! This is a laser!!!
You drag physics into a back alley and mug it for a material with the right four (or three) electron energy levels, excite those electrons, then put it between two mirrors (one very slightly transmissive) and let it go BWAAAAHHHHHHHHH. And then you get an intense beam of light which can be focused better than ordinary light.
