Stimulated Emission

If we had a light bulb, it would give photons in all direction as illustrated in figure 1. below.

Fig 1: Light bulb radiating photons.

If we can get all the photons in one direction then it’s called a laser.

Fig 2: Laser radiating photons.

We create this one directional flow of photons using stimulated emissions.

Fig 3: Electron jumping into higher state (a) Hit by photon (b) Stimulated absorption

If we have an atom at low energy level, it can be stimulated(Here, stimulated means, atom can be hit with a photon).

If we stimulate the atom, it can take energy of the photon and move to a higher energy level. We call that stimulated absorption.

If it falls down again, it will give out the same exact photon. We call that spontaneous emission. Because nothing happens it just happens spontaneously.

Fig 4: Probability of atom moving from one state to another.

The probability of an atom to move higher energy level without a photon is very small whereas moving in a lower energy level is more probable.

What if the atom is already in an exited state and a photon hit. Atom despite going on a higher state will come into a lower energy level and two identical photon will be emitted.

Fig 5: Stimulated emission.

So, an identical photon is created using the energy. We call it stimulated emission because it was hit by another photon.

What if we had a number of different atoms.

Fig 6: Stimulated emission in an atom.

Let’s say, one atom is stimulated, gets hit by a photon. It is going to kick of two photons. Then, these two is going to hit another two and create four and so on.

If we set mirrors on each side, we will be able to build a laser.