What do I actually do? Part 2: Lasers

In a previous post I introduced some of the basics of what I do as part of my DPhil (the Oxford term for a PhD: actually more English as the term PhD, I believe, originates in America.)  In this post I will cover a bit more about the lasers we use to perform these experiments.

I mentioned two concepts in the last post: Lasers and Plasmas.  We use lasers to create a plasma in the lab, but what exactly is a laser and why can it be used to make plasmas?

LASERs – “A solution looking for a problem”*

When they were first discovered no one could think of a use for them.  However lasers are now ubiquitous in our lives: from DVD or Blu-Ray players to barcode readers, medical applications (eye surgery, skin treatments) to science research, lasers have made a big impact.

Laser is actually an acronym which stands for Light Amplification by Stimulated Emission of Radiation.  Effectively a laser is a very special source of light that has specific properties:

• It is coherent – this allows the light to be focussed very tightly,
• Many can be made with only a single colour of light (wavelength),
• It is highly directional – unlike a normal light, you tend not to be able to see a laser if you are side on to the direction the light is moving,
• It can be amplified to contain a large amount of energy, in a short amount of time.

The light in a laser comes from atoms within the material.  Atoms are made up of a positive nucleus around which negative electrons move.  The electrons are arranged into certain positions (which we call orbits) around the nucleus, and they want to be as close to the nucleus as possible.

We can see the nucleus with the electron orbiting. The orbitals are shown in different colours increasing in distance from the nucleus. To the side we see these orbitals arranged in terms of energy. The electron is sitting in the lowest energy level.

An electron can absorb light and move further from the nucleus, gaining in energy.  Now the electron is in a higher energy level than it wants to be, so it can emit light and drop back down, we call this Spontaneous Emission.  In certain cases, the electron can be made to emit energy if light comes along that has the same amount of energy as the electron wants to lose.  In this case twice the amount of light comes out – the original light and the light from the electron – with the same energy and both are coherent.  This is called Stimulated Emission and is the basis for a laser working!  We call these individual pieces of light ‘photons’.

In Absorption, a photon of light comes in and is absorbed by the electron. The electron moves further from the nucleus, increasing in energy. In Spontaneous Emission, an excited electron decides it doesn’t want so much energy, so releases a photon and drops back down to a lower energy orbit. In Stimulated Emission, a photon with energy equal to the separation of the two energy orbits stimulates the electron to emit a photon and drop down.

*This appears to be what was said about lasers when they were first discovered, however I can’t find the original quote, or who even said it!

The Physical “Machine”

Now that we know how light and matter can interact, we can put together an actual laser.

We need:

1. A source of photons,
2. A way of making sure that once one photon of light is released, it goes on to stimulate more photons being released.

This, is turns out, is solved using special crystals and mirrors.

We place a crystal between two mirrors: we call this a cavity.  Around the crystal we place lamps, know as flash lamps.  These are just like standard bulbs, and they are designed to repeatedly flash on and off around the crystal.  As they emit light, the electrons in the crystal absorb this and raise their energy level.  Some electrons will then decide to drop down and release a photon of energy (the squiggly lines in the photo below.)  This light is released in random directions, however if one happens to travel along the direction that it hits one of the mirrors and is reflected back into the crystal, this can stimulate another electron to drop down, and now we get 2 photons: doubling the light!  These then reflect off the other mirror and return to the crystal where these two photons stimulate two electrons to emit and now we get 4 photons.  This carries on until we have a vary high number of photons!

Showing a schematic of a laser.  The squiggly red arrows are spontaneously emitted photons, the heavy red arrows are photons that have been emitted by stimulated emission.

One of the mirrors is not completely reflective, so it allows some light through.  As the amount of light in the cavity increases, so too does the output.  This output is the laser light!

A laser can be used to create a plasma

As I mentioned in my previous post, and earlier, we use lasers to create plasmas that we are interested in studying.  But how does this work?

An experiment showing the light emitted from a plasma that was formed when the laser was fired onto a carbon rod (much like a pencil lead).

A plasma is formed by separating electrons from their atoms.  This takes a lot of energy, and laser light contains a lot of energy.  When the laser hits onto a piece of material, the light is absorbed by the electrons in the atom.  However; whereas before when the flash lamp caused an electron to increase in energy but stay in an orbital of the atom, here the energy is high enough that the electron can be kicked out of the atom completely.  When an electron leaves its parent atom, this is known as ionisation.  As the laser is so intense (there is a lot of light in a short amount of time and area), many electrons absorb the energy, so many atoms can be ionised multiple times.

These electrons are now have a lot of energy, and are hot: they move around very fast.  Whereas the atoms before were neutral and happy, now the electrons (negatively charged) are moving separately from the remaining ions (positively charged).  As there is a separation of the charge, electric forces can act between the two.  In this way the ions can also gain energy from the electrons.  (This is a simplified view of how energy is given to the electrons and ions as there can be many other processes.  However it is sufficient to get a general idea of what is happening.)  Voilà: a plasma is formed!

What Next?

We now know how lasers work, and that we can produce plasmas with them.  But, how can we use lasers to recreate a star in a lab?  Tune in next time to find out!

Physics on Both Sides of the Pond

February saw my return to San Francisco (more specifically Livermore) for the NIF/JLF User Group meeting.  NIF stands for “National Ignition Facility” and is, currently, the most energetic laser system on Earth.  (The word choice of ‘energetic’ may seem odd here, but there is a good reason for this which I will cover in one of my future posts on Physics.)  JLF is the “Jupiter Laser Facility”, which is a smaller laser system, which is more ‘hands on’.

The purpose of this meeting is to discuss science that is happening at these facilities, of which there was significant interest in recent experiments at NIF.  Previously NIF was almost exclusively used for the National Ignition Campaign [NIC], which is a series of experiments aiming to create viable fusion energy power using lasers.  (This will also be a topic in a future Physics post.)  However, recently, some time has been given to experiments and so new, and interesting, data were shown.

There was also a poster competition for students, and I am happy to say that I won 2nd Prize!  Visiting Alcatraz, the warm weather and catching up with many colleagues also made this trip quite enjoyable.

The inside of one of our lasers. All those mirrors need to be precisely aligned, whilst also wearing goggles to block the hazardous emission (like that mass of green you can see on the left).

Back in Oxford I shifted scale somewhat and have been more involved with getting our laser to work in the lab.  Lasers can be temperamental things, and if they are not maintained (or even used!) frequently, then it can take a while to coax them back to being useful.  Thus much of the time in our lab has been spent ‘coaxing’, but it is slowly coming together.

We also recently had a productive meeting relating to the experiment that we carried out last year.  We are now approaching the point of polishing a paper on the results.  I have high hopes!

Be careful what you wish for… (Part 2)

I bet you thought this wasn’t coming!  Oh yee of little faith!  I have been characteristically busy since I last wrote, mainly due to Vulcan experiment  which will feature in a later post!), but now I shall update events from almost 2 months ago.

I left you with me heading to Rochester, NY, after exploring Chicago.  Now I have been to Rochester before for an experiment at the Omega laser facility, and I was returning for the next leg of this experiment.  Things progressed similarly to before, with target characterisation, preparation, and the long shot day.  Unfortunately this time I was rather cack-handed and managed to knock a very delicate target off its alignment.  When these targets can take a day or so to make, you feel rather bad for pushing one off in half a second!  Thankfully it wasn’t damaged and was returned to its proper place quickly – and we ended up not using these targets at all!  The shot day itself had a few problems, not least with me almost not waking up in time, but I left feeling optimistic for the next run.

Before (left) and after (right) shooting our targets with the massive laser!

One of my main complaints of Rochester last time was the lack of things to do.  Thankfully this time I not only had my American colleague but also someone at the University that another friend had put me in contact with.  I therefore had visits to the supermarket – to buy pop tarts which unfortunately disintergrated on the trip home – and I saw some of the night life that Rochester has to offer.  Along the way I think I taught a few people some British-isms, and I even saw a Praying Mantis!

Casually sitting on the boot of the car!

I left Rochester after not having slept very much and, as such, was not immediately impressed when my co-passenger caught wind of my British accent and wanted to talk.  However; he turned out to be a rather interesting guy and now, whenever I return to Rochester, I have an open invite to go shooting for deer if I so choose!  The flight back to the UK was a lot better than the flight out, though the film selection was poor, and I was very glad when I eventually got back.

When I got back I had to catch up on work that had been set aside before I went and, of course, complete the next part of my transfer of status: preparing my presentation to other members of the department.  However; first I did manage to have a fun time in Birmingham for a friend’s birthday.  Birmingham is not so bad as everyone says, and I had a whale of a time from both the foam party and the cafe for breakfast the next day.

The number of empty shot glasses here probably explains a lot of the evening…

After Birmingham the deadline for my presentation was rapidly approaching.  However, slightly annoyingly, I had been told to go to a meeting in Paris and give a presentation as part of a return leg of a collaboration.  “What’s so wrong with a trip to Paris?!” I hear you cry!  Well, I feared that 1) it would be so short I wouldn’t be able to actually visit Paris, 2) the meeting would be irrelevent to me and 3) it was a distraction from my transfer presentation.  Unfortunately, all three of these were true; though we did have a nice meal in the centre of Paris one evening.  Sadly I could not add much to the meeting, and my topic area was unrelated to the majority of other people there, though I did get to practice my French.

The conference was set in the Paris Observatory, which we had a rather cool tour around.

The return to England, and the weekend, just led to more work on the presentation.  At this point, going on previous year student experiences, we were also expecting our transfer vivas to be soon after, so I wasn’t just writing and practising speaking, but also trying to work out which plasma physics derivations were useful to memorise!  I was more worried about the viva being sprung upon straight after the talk that I concentrated more on that, but did manage to have a practice of the talk in the lecture theatre beforehand.  The presentation was completely fine, and also to the largest number of people I had ever presented too, so I was pretty happy!  It also calmed my nerves for the viva as the tone of the questions were somewhat different to what I was expecting, yet I was able to answer them.

My presentation practice was a sell out…

After the presentation we were finally given dates for our vivas, and I had two weeks before mine was due.  It was also at the start of our Vulcan experiment, but that, I feel, is a story for another post!