Not to be needlessly Kirkian, but I know how a laser works - an material is pumped with energy, putting into a metastable state, and then as the molecules return to a lower energy state, they emit radiation. This radiation then stimulates other molecules to also fall to the lower-energy state, resulting in a cascade of radiation, all in phase.
But - why?
Why does a photon of the correct wavelength stimulate a molecule to emit a like photon?
Ultimately, I think, because photons are bosons - particles that have the oroperty that the existence of one or more in a particular state increases the likelihood of another taking on that state. Beyond that it’s all math.
The way that I remember it being explained in my high school chemistry class is that atoms “want” to be in their lowest-energy state that’s stable. So if you pump energy into them, they’re going to get rid of it at their first opportunity.
I misread this as “because photons are bosses” and now I think it makes intuitive sense that way too.
There is a discussion beginning on Page 163 in Itzykson and Zuber in which they come up with an expression for the scattering matrix for a quantized electromagnetic field interacting with a classical source.
When you turn on your source, the average number of radiated photons does not change(!), but the probabilities for emission increases.
In their notation (J(k) is the Fourier transform of some classical current j(x) and S=\exp\left[-i\int d^4x A_{\text{out}}(x)\cdot j(x)\right] and
\left| \bar n_{\text{inc}}\;\text{in} \right> =
\frac{1}{\sqrt{\bar n_{\text{inc}}!}} {\left[ \int d\tilde k\sum_{\lambda=1,\,2}f_\lambda(k)a^{(\lambda)\dagger}(k) \right]}^{\bar n_{\text{inc}}}
\left| 0\;\text{in} \right>
), the probability of getting 1 photon is approximately
p_{0\to 1} = \int d\tilde k\,\left(\bigl|J_1(k)\bigr|^2+\bigl|J_2(k)\bigr|^2\right)
while
p_{\bar n_{\text{inc}}\to\bar n_{\text{inc}}+1} =
\int d\tilde k\sum_{\lambda=1,\,2}\bigl|J_\lambda(k)\bigr|^2 +
\bar n_{\text{inc}} {\left| \int d\tilde k\sum_{\lambda=1,\,2}f_\lambda^*(k)J_\lambda(k)\right|}^2
so the presence of photons in the initial state increases the probability of emission, even though the average number of radiated photons does not change.
That makes sense.
I don’t have time to explain it now, but I will just note that stimulated emission is a classical effect. Spontaneous emission is a quantum mechanical effect.
From what I remember, the emitted photons don’t always stimulate other molecules to emit the same photon. The stimulation has to come from the external source that is driving the material to lase. When the material absorbs the incoming energy, it has to do something with it. That action is a transfer of electrons to a specific higher, more energetic state. However, that orbit is unstable, so the electron falls back to its ground, or “normal” orbit. The energy released when the electron falls is a quantum of energy with a specific amount of energy and wavelength. The physical contruction of the laser (here I’m thinking of gas HeNe or solid ruby lasers) forces the photons to align by wavelength, and when they have enough collective energy, they escape the material as a single wavelength, coherent light beam.
Laser’s DON’T work. If you believe they do, you’ve been duped by the Conspiracy. This is why that whole ‘the wildfires were started by space lasers’ thing was so laughable.
I’m not a laser guy, but I think that this might be the root of the misunderstanding: What kind (i.e., wavelength, because there no other “kind” for a photon) is emitted is governed by the material, not by the stimulating photon. If you take a ruby or whatever, and stimulate it with a blue photon, you won’t get stimulated emission of more blue photons; you’ll just get no emission at all.