[An amazing image http://physics.illinois.edu/people/Lev/DSC00662.jpg from University of Illinois of a magneto-optical trap showing a cluster of Dysprosium at 100uK]
LASERs are pretty rad. When they were first invented they were called the solution looking for a problem, translation: pretty awesome but useless. Fast forward 40 years and LASERs are probably more widespread than rats. I bet you can't move 1 foot without coming across a LASER. They are used for entertainment, communication, surgery, defence and manufacturing - so much of our daily life revolves around LASERs we'd all morph back into monkey-state if they suddenly ceased to work. (Preceisely the reason why I've kept my VHS player and cassette tapes. When your fancy cd players stop working I’m taking over and putting on BON JOVI all night long.)
I'd like to write a series of blog posts big-pimpin' the LASER. I seriously think they are the bees knees. But first I have to do the obligatory ‘what is a LASER’ spiel.
LASER - WAT IS DAS?
A laser is ‘made’ out of light. I use quotation marks because its not like you can knit yourself a laser from the yarn of light, because a LASER is just a very specific version of light. So the light that the sun emits is made of the same stuff as in a LASER – light in the form of packets called photons*[technically not really true – the sun emits broadband radiation i.e. lets say ‘light’ is the same as ‘ice cream’ – comes in different flavours but its still ice cream. The sun emits lots of the flavours at once – it’s like neopolitan ice cream. A LASER is usually just one narrow range of flavour – i.e. plain vanilla. But my point is, it’s all still ice cream in the end. You can think of the photon as I discrete scoop of ice cream. Analogy over and out).
The difference between the Sun's emitted radiation and that from a LASER is just that LASERS have very synchronised photons which all have identical properties such as phase. The synchronisation makes the LASER a LASER - it gives it the characteristic beam shape; long and shaft-like and it also makes it very intense (the LASER set up also builds up a lot of energy too so you get a focussed beam of light). So if you shine a LASER at a piece of metal, the amount of power delivered per area is huge compared to using focussed sunlight or a lightbulb (our neopolitan ice creams) and cuts through the metal with great speed and ferocity (i.e. efficiency). Like a buff tiger made of LIGHT! For the first article I'm going to talk about something many people don't realise LASERs can be used for: cooling. Now, I'm talking cooler than ice cold. That’s like, cooler than that Outkast song, cooler than Ice T solving sexually motivated crimes, and cooler than wearing double D to a BBQ.
Laser cooling is counterintuitive.
LASERs = HOT?
You may think of lasers as hot things - you're right, they can be pretty damn hot, but that is due to the absorption properties of certain materials. For example; you feel the burn of the sun because the light transmitted by the sun is absorbed by your skin. It just so happens that the particles (mostly carbon based elements) which make up your skin have a transition level which matches the wavelength of UV light in the sun's spectrum. I’ll explain transition levels: think of a particle as having an empty hole - this is a METAPHORICAL hole - they are actually transition levels for electrons but you know, there's no room for being pedantic in analogies. Anyway, a photon can fill this hole if it the correct size. And by size I mean wavelength. This is pretty exciting. Literally exciting for the atom and it tries to remain calm by spluttering out extra energy in the form of photons or phonons. Ok. The emission of photons is easy to understand - what goes in, then comes out - it's called spontaneous emission and this process of photon in/out is known as scattering. But the phonon is a different story - a phonon is basically a quantity of mechanical energy - and is better known as heat. This is a non-radiative decay and is usually due to very small transition levels. It is also the main reason why people think of LASERs or light i.e. Sunlight as being 'HOT' because most of the time non-radiative emission occurs producing heat.
LASERs = Cool? WELL DUH.
So we've covered the hot bit of LASERs. Now to move onto the cold. First, what is cold? Well, cold is actually described as temperatures under 1K, and ultracold below 1mK. What is a bloody K? It's a Kelvin! It's an alternative temperature scale to the common Celsius one. It's the science version of the American Fahrenheit. To convert Celsius to Kelvin just add 273.14. So zero degrees Celsius becomes 273.14K. There's a good reason for this - it makes you realise just how cold 0K is! -273.14C. However 0K has an important definition attached to it - it's the temperature at which all thermal (mechanical) motion ceases...I'm not harping on about being stationary like sleeping-lions stationary, but stationary to the atomic scale. Till the particles stop moving. According to Wikipedia the lowest temperature recorded on the surface of the Earth is 184K. So you can imagine why scientists started racing towards achieving 0K in the labs - just to give mother nature a can of whoop-ass.
There are lots of methods nowadays to achieve cold and ultracold gases. But the main one is Doppler Cooling. This involves using LASERs to slow down the velocity of particles and then trap them using magnetic fields. You can do this because of radiative scattering. This is the photon in/photon out process I described above but now we’ll describe it in the form of momentum conservation.
When the atom absorbs a photon travelling towards it, the photon momentum gets absorbed as well so the atom is now wanting to move in the same direction. When the photon is then emitted by the atom, it is pinged off in a random direction and the atom loses momentum (you can think of it as a 'recoiling' action). If an atom is bombarded with photons of the correct transition energy, lots are absorbed and then emitted and this scattering leads to the atom slowing down as it is recoiled in all directions. Velocity is linked to kinetic energy because velocity involves something travelling and to do this you need fuel or energy. Energy is basically a free-form - it can be changed from kinetic to potential to thermal. So a measure of kinetic energy also gives you the thermal energy. So low temperature means lower velocity. This is how you achieve 0K.
However, there's a limitation to this. As the atoms are initially also travelling very fast they encounter a problem known as the Doppler shift. The Doppler shift is better known as the ambulance conjecture - i.e. the sound of the siren shifting in pitch as it flies past you. This happens to the frequency of light too. If we split the spectrum of visible light into a rainbow, you'll find there is a difference in wavelength for red and blue light. Red has a higher wavelength than blue. If you move towards a source of radiation, the light which you perceive will become blue-shifted - i.e. the light will seem to have a lower wavelength, and moving away from a source causes a redshift. As the scattering process is highly dependent on wavelength because this is what dictates the spacing between the transition levels, a LASER which is perfectly tuned to a transition level quickly becomes out of tune because of the atoms movement. Think of it like this - you've totally screwed your hearing over listening to too much dubstep, you massive hipster, and you can only hear the pitch of a violin playing an open A string. If a violinist continues to play this note but is on a bus travelling past you, you'll find that the pitch will go from a G to an A to a B, or in your ears, silence - A - silence.
Therefore to stay in tune you send a LASER already red-shifted to counteract the Doppler effect like a boss. So the atom is like 'that’s totally not my wavelength' but then as it travels towards it it goes 'holy moly, it is now. Crap'.
And that my friends, is what Doppler Cooling is. Conning a diffuse gas of atoms. Speed raping them into submission.
As a side note: Doppler cooling can only really be used for a handful of gases because elements actually have very complex transition energy levels, even virtual ones - I'll talk about those methods another time. There is also a minimum temperature limit called the Doppler temperature because when the atoms emit photons in random directions, the recoil momentum is also ‘emitted’ in random directions (usually the opposite direction to emission direction). The sum of all the random recoiling will produce a stationary atom but the fact that momentum is transferred in non-predicable discrete steps produces something called a random walk – foraging woodland critters undergo random walks so its a concept inherent in nature. Random walk in terms of the atomic scale creates heat, and this will counteract the cooling action of Doppler cooling. When the two opposing forces balance this sets the minimum temperature – the Doppler Temperature.