Lasers Visually Explained

The physics of a laser - how it works. How the atom interacts with light. I’ll use this knowledge to simulate a working laser. We will learn how Lasers relies on Stimulated absorption, Spontaneous emission, and most importantly: Stimulated Emission- This last type interacts with an excited atom, causing it to relax / de-excite. It will send out a photon with the same phase, frequency, polarization and direction. We exploit this principle in a optical cavity, with the atom as a gain medium.

It turns out the pump will cause stimulated emission for the wrong excited state and we need to use a 3 level atom in order to obtain population inversion.

I'll also cover meta-stable state, phosphorescence, radiation-less transitions and photo-luminescence.

Help me produce more physics /science content here

Longer simulation video:

0:00 - Introduction
0:18 - 1.1: Atom and light interaction
1:53 - 1.2: Phosphorescence
2:40 - 1.3: Stimulated emission
4:10 - 2.1: The Optical cavity
5:19 - 2.2: Overall plan for LASER
6:20 - 2.3: Population inversion problem
7:43 - 3.1: The 3 level atom
8:18 - 3.2: Photoluminescence
9:03 - 3.3 Radiationless transitions
10:25 - 4.1: A working LASER
11:44 - 4.2: Coherent monochromatic photons

[Higgsinophysics]

Physics Teacher would probably be mad I didn't cover Laser Resonator Modes and the nice property of using 4 level atoms. But nevertheless I hope you learned something new :)
- Special thanks to "Pretty Much Physics", "Zap Physics", "Ancient Accounts" and "everything science" for correcting this video.
- Thank you to the nice folks over at p5py for making an amazing python animation package.
- Thank you to the wonderful patreons <3

[noelomaolchraoibhe]

FWIW, you are the first subject matter expert I have found that was able to explain stimulated emission in a way that made it comprehensible! I have watched many videos that purported to explain the phenomenon and all failed to get it across to me. Your script & visuals are just right.

[companyjoe]

Thank you! I did have to pause and rewind constantly but after all this video explained lasers better than any other video I've seen.

[zapphysics]

I can't get over how great the animation at 10:50 is. Excellent job showing this in action! Great job, man!

[ParthGChannel]

Ahh so clear, so crisp. Loving it my dude!

[AncientAccounts]

"most important tool for teasing cats" lmao

[rajivk]

Thanks, If only I had these resources when I was a kid... Our teachers never explained it to us like this.

[minerscale]

What an awesome video! Beautiful looking, clear and consice, incredibly educational. Thanks for making top quality videos!

[aaron]

Thank you for an informative video! I don't fully understand why the light coming out of a laser is coherent.

"Stimulated emission" explains why an initial photon bouncing back-and-forth can accumulate a large number of coherent photons.

But it seems like there would be countless "initial photons" all over the cavity, each accumulating their own, independent collections of photons.

Why would the phase of any one group match other groups?

[xavierlebeuf]

I've always wondered how laser worked. And you explained it very well, thank you!

[NoOne-mxnx]

Thank you Sebastian Vettel for this amazing video.

[quahntasy]

*Such an amazing simulation. I loved it.*

[alexwang]

Would you consider diving into other laser topics such as modes, mode locking, and q-switching?

[alfredfeldt]

cool, random guy on the internet tells me what my physics teacher could not

[false]

I learnt something cool today thanks to you! Great content.

[pranitabaruah]

Why when the optical cavity gets pumped by the optical pump only half of the atoms get excited? I mean yes, when half of the atoms are excited and optical pump lights up again then the already excited atoms undergo stimulated emission and get de-excited and the relaxed atoms get excited and so again only half of the atoms are excited each time and the cycle continues (this is true only if the atom has two energy levels). But, why only half of the atoms are excited in the first place? Why when the optical cavity is first blasted by light from the optical pump only half of the atoms are excited? Why don't all of them get excited in the first place?

[harshvardhan]

2:55 I want to ask if I have understood it correctly.

Lets take a hill, with a rounded top, and there's a ball about to roll down. By the top being rounded, I meant that there is a very narrow spot, where if the ball is placed very carefully (with zero velocity), it stays, or else it rolls down, or in other words, an unstable equilibrium. So for spontaneous emission, it is like- when downhill, we gave the ball just enough velocity for it to reach the top and then just a teensy bit further. So, it will eventually fall off on the other side, but it will be taking a very long time. By analogy, we made the electron to jump from ground state to a excited state, by giving it a precise frequency of radiation, which it will lose after some time, giving out the same frequency of light.

Now for Stimulated Emission- it is the same for making it go uphill, but then, instead of waiting for it to slide off the other side, we give it a little nudge, to make it happen faster. And since it was just a small nudge, it doesn't have much of an effect on its velocity when it reaches the bottom, but it does happen earlier than without the nudge. So in the analogy, the frequency of light emitted back is still the same, but it happens earlier than spontaneous emission.

Now I have some questions, which relate to both, the actual laser and the analogy used here- 1. Why does the radiation which stimulates the emission (the nudge on the ball) have to be of the same frequency? 2. However small, how does the nudge not change the final velocity of the ball, or analogously, why does the electron not absorb the ray used for stimulation too, get even more excited, and then emit an even higher frequency? Or is it that this *does* happen and I just described Second Harmonic Generation in Non-Linear Optics?

[burada]

Thanks. In the next step, how are these laser emissions are taken out from the optical cavity?

[ashutoshsethi]

Why does the new photon has same phase and frequency and polarisation as the incodent light, why does it not undergo randomness like the phosphorescence atom?

[ItsMe-rprf]

Hey ..plz tell me the background music...
10:15 to end