Laser: UNIZOR.COM - Physics4Teens - Waves
Channel: Zor Shekhtman on UNIZOR Education
Date: 15/12/2022

UNIZOR.COM - Physics through a Looking Glass of Mathematician
Notes to a video lecture on http://www.unizor.com

Laser

We all are familiar with laser scanners used in stores to scan the product code.
Many of us know about laser pointers that direct a small spot of light on objects quite far away.
Compact discs technology is based on lasers.
Laser printers are nowadays the dominant printing device used in computers.
There are many other applications of lasers, so, let's examine the laser's principles of working.

First of all the term laser is an abbreviation of "Light Amplification by the Stimulated Emission of Radiation". This name fully characterizes the process at the base laser's functionality, and here is why.

The Idea

Generally speaking, all electrons of an atom of any element in a stable state occupy certain shells called ground level shells.
The atom of hydrogen with 1 electron has one ground energy level shell. The atom of sodium has 11 electrons positioned in 3 ground energy level shells: (2+8+1).

Let's assume that we supply some constant external energy to atoms of some element that causes some of its electrons on the outer ground energy level shells to get excited and move to a higher energy level shells.
By supplying energy in quanta that exactly equal to a difference in the levels of energy between a ground energy level shell (a stable state of electrons), characterized by the energy level Eground, and a higher energy shell (excited state of electrons), characterized by the energy level Ehigh, we force many electrons to absorb this energy, get excited and move from the ground level shell to the higher energy level shell (energy must be conserved!), eventually overpopulating that higher energy level (not that stable) shell.

The trick is to force these electrons, that overpopulate the higher energy level shell, to synchronously relax and jump back to a ground level shell, returning to a stable state.
If this is achieved, they synchronously emit a photon of radiation of the same frequency and phase producing a coherent (synchronized in frequency and phase) light.


Theoretical Solution

Assume, we have an overpopulated higher energy shell of energy level Ehigh.

Experiments showed that, when a single photon of frequency f carrying an energy
E = h·f = Ehigh−Eground
falls onto one of the excited electrons in the overpopulated higher energy shell, it is not absorbed by an already excited electron, but stimulates this electron to relax by going down to a ground level shell of the energy level Eground, emitting a photon of corresponding frequency
f = (Ehigh−Eground)/h
(energy must be conserved!)
which is the same as the frequency of the incident photon and is also synchronized with it in phase.

The above is a description of Stimulated Emission of Radiation, the second part of the full name that produced an abbreviation laser.

Now we have two identical in frequency and phase photons, the incident one and the emitted by an electron that switched its position from the high energy level shell to the ground shell.

If we can redirect these two identical photons back to electrons on high energy level shell, they will cause two new electrons to relax, emitting two more photons fully synchronized with incident ones, thus enhancing the light, while maintaining the same frequency and phase for all photons.

We can repeat this redirection of new photons back to excited electrons, and on each repetition the number of emitted photons would, in theory, double.
Eventually, when this light is strong enough, we can release it and use for some purpose.

If there is an external source of energy that constantly excites electrons, which, in turn, forces them to jump to a high energy level shell and, at the same time, we can stimulate these electrons to relax, emitting extra energy in a form of electromagnetic waves of certain frequency and phase (the same for all relaxing electrons in all atoms of an element), we will obtain a flow of coherent (all of the same frequency and phase) electromagnetic oscillations.
If the frequency of these oscillations corresponds to a visible spectrum, we will get a coherent light of some color.