How lasers work – II
We’ve seen in How lasers work – I how atoms give off spontaneous emissions of light when an electron drops to the ground state from an excited state. But what happens if an electron that is already excited is hit by another ray of light? You might think it jumps to a yet higher level of excitement. But it doesn’t. Instead what happens is that it drops to a the ground state just like before but gives off two photons. And these two photons are said to be coherent or “in step”. This means that the crests and troughs of their waves are all neatly lined up with one another. This is called a stimulated emission.
This may happen sporadically, but it isn’t a self-sustaining process, because at any given moment, only a minority of the atoms of a substance are in an excited state. But what happens if you pump up a substance so that more than half of its atoms are in an excited state? Then one electron in an atom drops to the ground state and gives off photon which may then hit another excited electron, causing it to give off two coherent photons which may then hit two other excited electrons which then give off four coherent electrons and so on.
But how do you keep this going in practice?
Well one way is to create what is sometimes called an optical cavity with curved mirrors at either end and in the middle the substance that you are going to pump up into an excited state. This could be a gas and you could pump it up with heat or with an electrical charge. This creates something called a population inversion in which at least half the atoms in the substance are in an excited state. The process of electrons dropping and giving off stimulated emissions of coherent photons that in turn stimulate other emissions gets underway. The light produced is reflected back and forth between the two mirrors, preventing it from merely being absorbed by the non-excited wall material and being lost.
By creating an opening in the centre of one of the mirrors that is either there continuously or opened and closed, enables light to be emitted from the laser onto a target. That target can be a DVD a bar code or a plate or object used in holography.
So when you carry a bank card with a hologram or wear an i balance bracelet on your wrist, you are carrying on your person the results of an invention that was once thought of as having no practical application. As Einstein said when asked what use his theory of relativity had: “Of what use is a new born baby?”