A light exercise to explain lasers
Last week, the Nobel Prize in physics was awarded to Arthur Ashkin for inventing optical tweezers and to Donna Strickland and Gerard Mourou for developing the shortest, most powerful laser pulses presently available. To explain their work, we need to discuss lasers and how they work. Not surprisingly, these devices are now ubiquitous in our lives and used in everything from grocery scanners to eye surgery.
I will try to explain how they work with an analogy, so here it goes:
Imagine you are standing below a box, made of cardboard and suspended from the ceiling some distance above your head – let’s say about two metres.
The top of the box is open so you are able to throw tennis balls up and into the box if you want. Being provided with a large basket of tennis balls and with nothing better to do, that is exactly what you decide to do.
Now, if all the tennis balls were to land in the box and not return to ground level, this would be a very short game. However, someone has very kindly cut a hole in the bottom of the box just big enough for a tennis ball to fall through.
Of course, you are standing to one side so the balls don’t fall on your head and you can shoot for the top.
On average, the number of balls and speed with which you throw tennis balls into the box is the same as the rate at which the tennis balls fall out of the hole in the bottom and return to the ground. If I may indulge your patience, you have achieved what scientists refer to as a state of equilibrium. The rate of the forward reaction – the balls going into the box – exactly equals the reverse reaction – the rate they fall out. Nothing much would change and you could keep throwing tennis balls until your arm got too tired or you got bored.
Someone, though, has attached a second box to the side of the first – a little bit lower and with two special properties. The first is a tennis ball can roll from the first box to the second but because the second is a bit lower it can’t roll back again.
The hole connecting the two boxes is exactly the same size as the tennis ball so not every ball will get trapped in the second box but some will. Let’s say it is 10 per cent or one out of every ten balls landing in the first box will find its way into the second.
If you start with 100 tennis balls, after the first pass 10 will be in the second box and you will have 90 to throw. After the second pass, you will be down to 81 and 19 will be trapped in the second box. Eventually, you will have a “population inversion” where there will be more balls trapped in the second box than you have available to throw. This will produce an unstable state.
This is where the other special feature comes in. The second box is fitted with a trap door and the only way to open it is to hit it exactly right with a tennis ball moving exactly the right speed and height.
This is not an easy task. And it distracts from trying to get balls into the first box.
So, you keep lobbing tennis balls into the first box and while most of them fall back down around you, some find their way into the second box waiting for the perfect strike by a tennis ball.
And occasionally, you take a shot at the trap door on the second box, trying to get it to dump its load of tennis balls.
All of a sudden, you get it just right, the trap door flies open and box empties releasing a large number of tennis balls.
If the tennis balls are electrons and the boxes are energy states within either an atom or a molecule, then this the process which produces laser light. LASER is an acronym for Light Amplification by the Stimulated Emission of Radiation. The one tennis ball which hits the trap door is stimulating the simultaneous release of all of the tennis balls and amplifying the effect in much the same way as a photon induces electrons to return to the ground state and amplify the light.
Because all of the photons in a laser beam have exactly the same energy, their waves are in synchronization with all of the troughs and peaks lined up.
Ashkin was able to catch atoms and small molecules in one of these troughs, holding them in place. He was then able to use a second laser beam to manipulate the atoms.
Mourou and Strickland were able to increase the power of each laser pulse while cutting the duration to attoseconds or 0.000000000000000001 seconds. By doing so, they were able to create laser pulses which could probe the structure of electrons in atoms or remove cells from a cornea without damaging the tissue.
All done with lasers.