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{jIAPS} Journal of the International Association of Physics Students

Issue 2 | 2010

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Cover photo: “Formules mathématiques au CERN”, http://www.flickr.com/photos/1suisse Issue 2 | 2010


In This Issue A few words from

...the Editors

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...the IAPS Executive Committee

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IAPS Activities 2009-2010: CERN Trip Balaton Summer School

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CERN: Black Holes and the Higgs Particle, by Mark Eaton

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The Loneliness of the Long-Distance Archivist, by Jim Grozier

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XKCD: Centrifugal Force

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Semiconductor Lasers, by Jessica Stanley and Milan Vrućinić

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Terrestrial Gamma-Ray Flashes, by Ragnhild Schrøder Hansen

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The Back Page

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Editors: Jessica Stanley Contributors: Mark Eaton Anne Pawsey Jim Grozier Ragnhild Schrøder Hansen Ragnhild Schrøder Hansen Jessica Stanley Milan Vrućinić Design: Jessica Stanley Contact: jiaps@iaps.info www.iaps.info/jiaps Issue Issue 22 || 2010 2010

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A few words from the Editors... IAPS celebrates its 23rd birthday this year, and in this edition of jIAPS Jim Grozier tells us about his experiences in managing, and trying to organise, the archives of our association. The International Association of Physics Students was founded in 1987 as a result of the success of the first International Conference of Physics Students (ICPS), which was first held in Budapest in 1986, and has survived against all odds, in the words of Jim’s article in Euro Physics News earlier this year. That’s 24 years’ worth of volunteers, passing the torch of IAPS around Europe, and growing to the IAPS we know today. jIAPS is mentioned in the archives as far back as the early nineties, and the first printed issue that Jim has found is from November 1996. In this issue we continue the tradition of publishing articles from IAPS members on various topics in physics, as well as reporting IAPS news. The topics covered this time are semiconductor lasers and terrestrial gamma ray flashes, along with an article on the Large Hadron Collider by one of the participants of the CERN trip that took place in March this year. As always we are looking for new articles and ideas, so don’t hesitate to contact us at jiaps@iaps.info. Jessica Stanley Jessica Stanley is a master student of experimental physics (specialising in neurophysics of visual perception) at Utrecht University in the Netherlands, and is from a small village called Enniskerry just south of Dublin, Ireland. She has a B.A. in Theoretical Physics from Trinity College Dublin, where she was an active member of the Mathematical Society and the Ireland branch of the Institute of Physics. Jessica was also a student representative for Nexus, the student branch of the Institute of Physics, was IAPS secretary in 2008/2009, and has attended ICPS since 2006

Anne Pawsey is from Sheffield in the UK, and is a PhD student of ‘squishy stuff’, otherwise known as soft condensed matter, at Edinburgh University. She has an MSci from Bristol University, where for her master research project she had to make the tough choice between studying cake, chocolate or icecream. Anne has attended ICPS four times since 2006, was jIAPS editor previously in 2006/2007, IAPS secretary in 2007/2008, and was on the ICPS 2007 organising committee. She was a student representative for Nexus, the student branch of the Institute of Physics, for many years.

Ragnhild Schrøder Hansen is from Bergen, Norway. She has just finished her masters in space physics, researching Terrestrial Gamma-Ray Flashes, at the University of Bergen, where she also did her bachelor degree. Ragnhild is the former president of the Norwegian Association of Physics Students, and is an active member of Fysikkshow Bergen, doing demonstrations of experiments to teach kids about physics. She was a general member of the IAPS Executive Committee in 2008/2009, and has attended ICPS since 2007.

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...and the IAPS Executive Committee Here we go again. You see before you the second issue of jIAPS that the current team of editors has produced. This will be the last issue before the ICPS in Graz. We rub our eyes and notice that a year has passed: the next ICPS is getting closer and our term on the EC is also coming to an end. This year saw the highly successful CERN trip, and another Balaton Summer School has just taken place. If you want to be informed about future IAPS events, you should join our Google group, IAPS Agora, or check www.iaps.info from time to time. For us it is time to look back. The last year also saw new activity in IAPS’ links with the International Association for the Exchange of Students for Technical Experience (IAESTE) and we may work more closely with the European Physical Society (EPS) in the future. We were happy to welcome new member committees to IAPS, one of whom introduces itself in the next issue, a large part of which will be devoted to information about a number of local and national committees of IAPS. Maybe this will encourage you to seek direct contact with your neighbours and plan joint events? IAPS always depends on people who are motivated to become engaged in it. A new Executive Committee will be elected in Graz, we are still taking bids for ICPS 2012, and there are many other areas where you can become involved. Interested in getting to know the fun of working with physics students from different countries? What about organising an excursion to your region and showing other students from abroad what it has to offer? Initiate a regional IAPS meeting or set up a virtual problem solving group. Or maybe you have other plans? Contact us if you think we canhelp you and do not hesitate to tell us about your ideas. Looking forward to seeing you in Graz, your EC Contact the EC: ec@iaps.info Back: Jelmer Renema, Konrad Schwenke, Marko Banušić Middle: Sérgio Domingos, Milan Vrućinić, Istvan Szecszeni Front: Sahra Haji, Camelia Florica, Agnieszka Leyko

Issue 2 | 2010

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IAPS Activities 2009-2010:

CERN Trip

Organised by the IAPS EC

Photos by Sahra Haji

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Issue 2 | 2010


IAPS Activities 2009-2010:

Balaton Summer School

Organised by Mafihe, NC Hungary

Photos courtesy of Mafihe

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CERN: Black Holes and the Higgs Particle by Mark Eaton Considering that the Large Hadron Collider (LHC) at CERN in Geneva is looking at some of the smallest particles in the universe, the numbers associated with it are truly ‘astronomical’. It has never failed to amaze me how sub-atomic particles can teach us so much about the biggest issues in astronomy and cosmology such as dark matter and the Big Bang. As a mature student of astrophysics I was able to visit CERN on the IAPS trip in March 2010, and we were able to access some of the latest thinking at the world’s biggest experimental site as well as visiting some of the facilities surrounding the LHC. One immediate thing that hit me was the age of the site. CERN has been hitting the news for over 50 years and whilst most people in the world know about the search for the Higgs particle, CERN has already generated a number of Nobel prize winning discoveries and such things as World Wide Web. The site is home to around 10,000 people, including some 2,500 staff and researchers as well as students and researchers from the nations who support the facility. It is fully equipped with its own travel agency, post office and shops, and the roads are all named after famous scientists. The Large Hadron Collider itself is obviously the centrepiece of the facility, although you would

be hard pressed to spot it even at 27km long, as it is located 100m underground, with the majority of it residing in France (where it is easier to buy land). The LHC itself consists of over 9,000 magnets designed to steer two proton ‘beams’ around the 27km circuit. The beams are then converged at four experimental points called ATLAS, ALICE, CMS and LHCb, or they can be sent toward a fixed target. Each of the experiments is huge. One, the CMS (Compact Muon Solenoid) contains more metal than the Eiffel Tower and weighs 12,000 tonnes. In addition, each of the four experiments has its own control room. Protons collided in the LHC are travelling at 99.9999% of the speed of light and they impact with a combined power of 14TeV (Tera Electron Volts) (this is 14,000,000,000,000 Electron Volts where 1 Electron Volt is the kinetic energy gained by an electron when it is accelerated by 1 Volt battery). For reference, the power given to an electron in the cathode ray tubes that used to be popular in TVs is only 20,000 electron volts, so interactions within the LHC are some 700 million times more powerful. The particles in the LHC are referred to as a beam but in reality they travel around the system in ‘packets’, each containing 1011 protons. When the LHC is running it has a total of 2808 ‘pack-

ets’ of protons, each located 7.5m apart as they travel around the 27km ring. These packets are brought to four collision points within each of the four experiments producing 6.5x108 protonproton collisions every second during a run. The amount of data collected by the LHC experiments is truly vast. The CMS alone has a camera with a surface area of 220m2 and a resolution of 75 megapixels. More than that, it can take 40 million pictures per second of proton impacts when it is running, and this is more data than could ever be stored, so they have to quickly screen the image and decide whether to keep it or discard it. This reduces the number of frames taken when the machine is running to around 100 per second. Much has been written about the possibility of the LHC producing a ‘micro black hole’. They told us that this is a distinct possibility, but given that the black hole created will evaporate in one million million million millionth of a second (10-24) through ‘Hawking radiation’ it is unlikely to be a big issue. The LHC is only the end of the proton’s journey, as it first has to be accelerated by three smaller systems, raising its speed from a few metres per second to 87% of the speed of light (261,000 km/s) before injection into the LHC.

Image: Adapted from a one loop Feynman diagram of the first order correction to the Higgs mass, from Wikipedia.

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CERN: Black Holes and the Higgs Particle

The CMS Control Room

What’s more is that the collisions going on at the LHC have been occurring naturally in the upper atmosphere at the rate of 100,000 per second for 4.5 billion years, and therefore if a serious black hole were to be created it would probably have already occurred. So no need to worry in the next few weeks! One thing that they do find regularly at CERN is antimatter. Indeed, for every million protons produced they identify one anti-proton. The team at CERN provided advice to the producers of the film ‘Angels & Demons’ (based on the Dan Brown book of the same name). Ron Howard, the director of the film, was keen to reduce many of the scientific errors that were in the book and wanted some real scientific advice. However, not all of the inaccuracies could be eliminated and in the film the Vatican is threatened by a bomb containing half a kilo of a ‘highly combustible’ material called antimatter. First of all, antimatter does not combust, it just converts normal matter into energy and in addition the time required

Issue 2 | 2010

to produce half a kilo of antimatter would need the LHC (working at full pelt) to be running continuously for around 10 billion years. Apart from this the CERN physicists also destroyed my faith in the availability of matter transporters and warp drives from Star Trek anytime soon (or ever!) It is a shame that we could not go down into the tunnels to see the LHC, but given that they are cooled by liquid Helium at a temperature of 1.9 Kelvin (-269 oC), and that if it warms up by only 1 or 2 degrees it will expand 800 times (meaning that anyone in the tunnels will speak with a high pitched voice for a few seconds and then drop dead), it was probably for the best. What is even more impressive is the temperatures that they can achieve in the impacts (1015 K – that’s 1,000,000,000,000,000 degrees), along with the fact that they have a vacuum that contains even fewer particles than outer space, and the ability of the LHC to recreate temperatures and energy levels not seen since one billionth of a second after the Big Bang.

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So, is all this money being used wisely? If we forget the fact that understanding how atoms work will help us to understand the universe more completely, the applications of particle physics have found their way into medical diagnostics such as PET (Positron Emission Tomography) and MRI scanners while the need to analyse data has given us the world wide web and will, they believe, in the future give us an even better system called the ‘Grid’. In terms of understanding the universe more effectively one key development they hope will come from the work at CERN is a better understanding of dark matter and dark energy. The surprising thing about all of the objects we can see in the sky is that they account for only 4% of the total mass of the universe. 26% of the rest of the mass is believed to be dark matter, which is material that does not emit electromagnetic radiation (including visible light) but that does interact gravitationally with ordinary matter. It is the halo of dark matter that is thought to enable spiral galaxies to maintain their ‘arms’ for extended periods rather than simply falling apart. The remaining 70% of the mass is in the form of dark energy, which at its simplest level is thought to be the force driving the expansion of the universe. Returning to the visible 4% of matter, what CERN is trying to understand is why the universe favours matter over anti-matter. At the Big Bang matter and antimatter particles were converted into energy as they annihilated each other, but for every one billion antimatter particles there appear to have been one billion and

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CERN: Black Holes and the Higgs Particle one particles of normal matter. This one particle per billion that was ‘left over’ actually constitutes everything we are and everything we can see. The most interesting matter to us as humans is what is termed baryonic matter. A hadron (as in Large Hadron Collider) is the generic name given to any particle made of ‘quarks’ (of which there are six types). Baryon is the name given to any particle made of three quarks, with protons being made of two up and

as muons and gluons. Of course, discovering the Higgs Boson and the Higgs field is the priority for CERN at present. This will help us to understand more about the fundamental forces in the Universe and in particular to understand how matter has mass. To understand more about the Higgs particle and field in simple terms the physicists provided us with the briefing paper that was given to Margaret Thatcher when she was asked to help fund the development of it (yes,

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to discuss the rumour: this is the creation of a real particle (the Higgs Particle). Of course, the Higgs may not be real and even at best they only expect to see it once in every million million collisions, but as Rolf Landau (a leading figure in the search for the Higgs particle) said, “Not finding the Higgs will be even more exciting than finding it as it will prove that our existing model of how atoms work is wrong.”

Left: The dipole test facility; the large blue objects are dipoles, of which the there are over 2000 in the LHC, and another 9000 other magnets. Right: part of the linear accelerator, where Hydrogen atoms are ionised, and the resulting proton is passed through a further four acceleratiors before entering the LHC.

one down quark and neutrons being made of two down and one up quark. To get a feel for the size of things for those of us who had a classical education that stopped at the proton, trying to determine the size of these particles is amazing. The nucleus of an atom is actually only 1/10,000th the size of the atom and quarks are only 1/10,000th the size of a proton or neutron. The amazing thing is that quarks really don’t want to be separated from each other, which is why they need to be involved in collisions with such high energies, and in the process they give off a blast of other particles with strange names such

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the LHC is that old) and it goes like this: 1. Imagine there is a cocktail party with a room full of people: this is the Higgs Field. 2. When someone famous enters the room people flock toward them: this is the creation of a new particle. 3. The famous person tries to cross the room but is ‘slowed down’ by people wanting autographs: this is the Higgs field giving the particle ‘inertia’. 4. A rumour starts that the personality will make a speech: this is the Higgs field becoming ‘excited’ and receiving energy. 5. Many guests clump together

Mark Eaton is based in the UK and is a mature physics student. Originally trained as an engineer he also has an MSc and MBA but has had a lifelong interest in astronomy and in 2004 decided to study a part time degree in this area split between the Open University and University of Manchester. Mark’s physics related interests are Astrobiology and Astrophysics, with a particular focus on variable stars. He is married with two children and runs a management consultancy focused on the health sector when he is not studying.

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The Loneliness of the Long-Distance Archivist by Jim Grozier In early 2007, as a member of the IAPS Charter Committee, I attended a meeting at the European Physical Society headquarters in Mulhouse, France. While I was there, somebody gave me a CD containing the IAPS electronic archive. I found it fascinating; but it clearly needed some work done on it – the documents it contained had obviously been hurriedly scanned and did not have meaningful titles, being catalogued simply under the default numbers allocated to them in the scanning process; and the folders into which they were divided seemed, in some cases, quite arbitrary. I offered to take on the role of IAPS archivist and go through the whole lot (about 130 megabytes), sorting the documents and giving them meaningful names so that the archive could be more easily navigated in future. As I read through the archive, the story of how a small group of physics students organised their own international conference back in 1986 and then went on to found an international, student-led organisation, in Hungary, under a communist regime which discouraged that sort of “grass roots” activity, unfolded before my eyes. As someone with a keen interest in the history of physics, including the sort of history that is quite recent and might not therefore be thought to qualify as history by some people, I wanted to re-write the story in a more readable form so that it could be discovered by others; so I started to write a historical article, and, while I continued to process the archive, the article came to take up more and more of my time, leaving less time for cataloguing. ICPS that year was in London – the first time in the UK – and I was one of the organisers. So there was a publishing opportunity for the article, which appeared, in its earliest form, a mere two and a half pages long, in the conference booklet, to mark IAPS’ 20th birthday. After ICPS, I continued to extend it as I gathered more and more material, and in 2009 I was able to submit a longer version to Europhysics News; it was published in the autumn edition of that year. However, EPN’s word limit of 1500 words did not allow me to include anything of great interest; it was merely a skeleton. I was keen to publish a longer version, although I realised that this would make it too long for most magazines, but too short for a book. There was another problem too; the article as it stood in late 2009 was mainly a collection of anecdotes – all of them interesting, but giving an inevitably “bitty” feel to the whole thing. It needed structure – a theme around which to build the story. That meant only one thing – I would have to personalise the story, by getting it first-hand from the five founders: Patroklosz Budai, Tamás Fülöp, Ákos Horváth, Péter Lévai and Péter Ván. An attempt to do this by e-mail proved fruitless – I was nominally in contact with four of them, but, while all seemed keen to help, only two had actually answered my questions. I would have to travel to Budapest and interview them face-to-face.

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The Loneliness of the Long-Distance Archivist

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My visit took place in February 2010, and I was warmly received. We had a “group chat” over an extended lunch, joined by Marton Major, the 10th president of IAPS, and – probably more to the point for ICPS-goers – the inventor of the National Party. Then over the next few days I interviewed them all individually. After my return to the UK, the article grew, and a plot emerged – that IAPS had in fact resulted from a unique combination of time and place, for it became clear that the need for such an organisation was felt most strongly in the then Iron Curtain countries where budding physicists felt isolated and needed to add an international dimension to their studies, and of those countries, only Hungary, with its uniquely liberal regime born out of the tragedy of the failed 1956 revolution, could foster such an initiative, and even there, only in the mid-1980s. Because of this I added a section outlining the political and historical background. The article, or “booklet”, as it now was, stretched to over 11,000 words, and took up 40 pages by the time a few photos had been added. I am hoping to get it published, preferably in time for ICPS’s 25th anniversary in 2011, with copies being made available to ICPS attendees, although I appreciate that it is never going to be a best-seller. Meanwhile, the archiving itself has had to take a back seat. This was not just because it was less interesting than writing the article; in fact, I have found archiving to be a far from trivial task. For a start, what I found in the archive was not the sort of material one would think of as obviously worth preserving. Much of it consisted of copies of e-mails, and of the telex messages and letters that preceded them. Of conference handbooks and AGM minutes, which would be a bit more “meaty”, there was a woeful scarcity. This can be explained perhaps by the fact that the archive led a nomadic existence for the first 10 years of its life, being taken to wherever the Central Office was based each year; only after the cementing of relations with EPS in the late 1990s did it find a permanent home, and no doubt much of it got lost on the way. Then again, there have undoubtedly been years when the archive was neglected and nothing much added, just as there are years when almost everything seems to have been preserved.

The IAPS founders, the author and Marton Major, pictured in Budapest in 2010. Left to right: Péter Lévai, Péter Ván, Jim Grozier, Patroklosz Benatos, Ákos Horváth, Tamás Fülöp, Marton Major.

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The Loneliness of the Long-Distance Archivist

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I actually ended up being quite grateful for this quasi-natural selection process, for, had it not taken place, the archive today would be absolutely huge, and my job would then be to cut it down somehow. This too would be far from trivial. For it is at the “microscopic” level that the archive becomes interesting; an exchange of e-mails over a particular topic, interspersed with some chatty, personal remarks, gives one a flavour of the personalities concerned, whereas minutes, while informative, are usually fairly dry and impersonal. An archivist’s job, however, is to look to the present and the future as well as the past; and here it does become seriously tricky. How does one decide which, of the huge number of documents now being produced, should go in? To say nothing of the hundreds of digital photos! (A dearth of photos from the early days is one of the disappointments I have had to face, but now, in the digital age, I have the opposite problem). I have even considered doing an archiving course, so that I can do it properly. Of course, if anyone out there gets really fired up by such things, I would willingly hand the job over… Jim Grozier is a postdoc at University College London. Besides working on the archives he was on the ICPS 2007 organising committee, the IAPS charter committee, and was elected to honorary membership of IAPS by the 2008 AGM in Krakow. His article on IAPS that is mentioned in this article was published in Europhysics News Volume 40 Issue 5 (www.europhysicsnews.org). Jim is sometimes mistaken for a certain Time Lord, but claims to be from planet Earth, not Gallifrey.

A webcomic of romance, sarcasm, math and language

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www.xkcd.com

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Semiconductor Lasers by Jessica Stanley & Milan Vrućinić Image: :Laser Play by Jeff Keyzer, from Wikimedia Commons

There can be few physicists who are unaware that 2010 is the 50th anniversary of the laser, one of the most influential technological inventions of the last century. Much of the technology we now take for granted relies on laser technology, from phones and computers to medical procedures like laser eye surgery. In scientific research as well, the laser, in its various forms, is an essential tool in a wide variety of fields. In 2009 the laser market amassed to six billion US dollars, with semiconductor diode lasers taking a 55% share of this. This class of lasers, which are tiny, efficient, and cheap to make, are by far the most common type, and are used in phones, computers, CD and DVD players, laser printing, and telecommunications, to name just a few applications. In this article the principles of these lasers will be explained, and some examples of the different types will be given. In 1962, a mere two years after the invention of the very first laser, the first semiconductor diode laser was produced by Robert N. Hall at General Electric in the USA. In 1978 the first commercial laser disc player was introduced, and in 1996 the first blue laser diode was created, eventually resulting in blu-ray technology which became available in 2006. Another notable event is the awarding of the 2000 Nobel Prize to Zhores Alferov and Herbert Kroemer, pioneers in semiconductor laser technology, for their invention of the double heterojunction laser diode 2 4. Figure 1 shows the basic principles of a simple diode laser. In solids the electronic structure is quite different to that a single atom: instead of discrete energy levels the available states for electrons can be considered as bands of allowed energies. The number of electrons within each band determines the properties of the material; the more electrons in the (higher energy) conduction band, the more conductive the material. In metals these two bands overlap, and electrons are easily able to occupy the empty conduction band states. In insulators and semiconductors there is a region of forbidden energies between the two bands, called the bandgap, and valence band electrons must Fig 1: Process of electron excitation and decay gain enough energy to overcome this barrier in order between conduction and valence bands. to take part in conduction. In an insulator the bandgap Image from www.rp-photonics.com is so large that the high currents needed to overcome it cause damage to the material, whereas semiconductors are somewhere in between, having bandgaps that are small enough to allow conduction to take place.

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Semiconductor Lasers

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Electrons excited into the conduction band can decay back to the valence band by emitting a photon with energy equal to the bandgap energy. This is the basis of light emission using semiconductors. For lasing the minimum of the conduction band must be at the same point (in momentum space) as the maximum of the valence band. These materials are said to have a direct bandgap. In some materials, such as silicon, this is not the case, and although very useful for other semiconductor applications they are not useful for lasing. Much more success has been had with direct bandgap materials like Gallium Arsenide, Indium Phosphide, and more recently Gallium Nitride 3. The key to choosing the wavelength of the laser light is the bandgap of the material. Gallium Arsenide, for example, has a bandgap of 1.42 eV, and making alloys with small quantities of materials with slightly higher or lower bandgaps allows different wavelengths to be produced, covering a range of 635-870nm, i.e. red and near infrared light. Similarly, adding Indium to GaN, which on its own emits violet light, increases the bandgap, allowing blue, and, very recently, green, wavelengths to be obtained.

Figure 2: LED vs. Laser: low currents result in an LED, with light emitted in many directions with low power and relatively large wavelength range compared to the high current laser situation. Image: www.rp-photonics.com

The simplest form of semiconductor laser, the homojunction laser, consistis of a p-n junction. A pdoped material, with an excess of holes, is brought into contact with an n-doped sample, with an excess of electrons, of the same material. The excess electrons in the n-doped material travel across the junction to recombine with the holes in the p-doped material, and light is emitted. Passing a current through the material forces more electrons across the junction, so that recombination continues steadily. This is called pumping. Figure 2 shows the dependence on the current: small currents result in light emitted in random directions, with low optical power and a relatively large wavelength range. This is the kind of situation that occurs in a light emitting diode (LED). Once the current is increased past a point called the threshold current, stimulated emission replaces the spontaneous emission that takes place in the LED case, the wavelength range is reduced, and the optical power increases dramatically. Including mirrors on each side of the junction further increases the emission, and that is all that is needed to form a basic semiconductor laser. The homojunction is not a practical device, because it typically has a threshold current of several thousand amps, and the considerable heating caused by this current requires the device to be cooled to temperatures of around 77 Kelvin. A solution to this problem is to restrict the recombination to a small region of the device, and this was implemented in the form of the heterostructure laser, the name meaning that the device is made of several different materials (compared to the homojunction, which is made from a single material). The heterojunction laser consists of a p-type active layer, typically 0.1 microns thick, sandwiched by two thicker layers of a different material, one p-type and one n-type, which have a slightly larger bandgap.

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Semiconductor Lasers

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A common example of this would have an active layer of Gallium Arsenide (GaAs), surrounded by thicker (up to 1 micron) confining layers of Aluminium Gallium Arsenide (AlGaAs). These confining layers force the electrons and holes into the thin active layer increasing the chances of them meeting and recombining there, and reducing the threshold current and the area in which heating is produced. The heterojunction laser is the basis for almost all modern and practical semiconductor lasers. Strange things begin to happen, however, when the active region is reduced to less than 10 nm: the conduction band, instead of being a continuous range of energies, becomes quantised, meaning that the electrons can only inhabit a discrete number of energy levels, like in a single atom. This situation, familiar from any quantum mechanics course, is called a quantum well, and the benefits are as follows. First, the smaller active layer means less heat is generated. Second, the restriction of the electron energies means that the number of electrons per energy level increases, compared to normal heterojunction lasers. This decreases the threshold current. Quantum well lasers also offer another parameter for wavelength tuning, as the electron energy depends on the width of the quantum well, and thus the wavelength of emitted light can be tuned by changing the size of the active region.

Fig 3: the VCSEL, left, and the VECSEL, right (images from www.rp-photonics.com)

Finally we will discuss a few of the newer types of semiconductor laser. One example is the Vertical Cavity Surface Emitting Laser (VCSEL). This consists of an active region of stacked quantum wells, sandwiched by Bragg reflecting layers which act as mirrors. This device is typically a few microns in size, and is often made from GaAs/AlGaAs. The beam quality is high due to the quantum well structure, but the output power is quite low, typically 0.5-5 mW. A typical use for VCSELs is in computer mice. The Vertical External Cavity Surface Emitting Laser (VECSEL) solves the problem of low output power by replacing one of the Bragg reflecting stacks of the VCSEL with an external mirror. Using optical pumping, i.e. creating stimulated emission by shining light from another source on the device, a large area of the device can contribute to lasing and a larger total beam is produced, compared to other types of semiconductor laser which use electrical pumping (a ring electrode provides current that drives the recombination). Though this device requires extra cooling because of the increased currents, the high output power of the VECSEL rivals that of traditional high power solid state lasers like Nd:YAG, and may replace them in the future. One application is in spectroscopy.

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Semiconductor Lasers

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Fig 4: The Quantum Cascade Laser: successive quantum wells have lower energy, allowing the electron to travel through them successively. Image: Wikipedia

Laser diode material (active region/substrate) InGaN / GaN

Typical emission wavelengths 380 - 470 nm

data storage

AsGaAs / GaAs InGaAs / GaAs InGaAsP / InP

720 - 850 nm 900 - 1100 nm 1000 - 1650 nm

CD players, laser printers pumping other lasers, high-power VECSELS optical fiber communications

AlGaInP / GaAs

635 - 670 nm

Applications

laser pointers, DVD players

Fig 5: Overview of semiconductor laser materials and examples of applications 2 3

Our final example, the Quantum Cascade Laser, also uses quantum wells but in a novel way: up to 75 quantum wells are stacked in succession 5, with an electric field applied to create a decreasing energy profile (see Figure 4). An electron is injected into the first well, and decays to a lower level of the quantum well conduction band, emitting a photon. It then relaxes to the bottom of the well, tunnels out, and is collected and injected into the next well. This process continues all the way along the series of wells, so that one electron creates a ‘cascade’ of photons. The active region in this case is made of an n-type material, to provide extra electrons, as electrons are the only carriers involved in lasing in the QCL. These lasers have applications in medical diagnostics. Figure 5 shows typical applications and wavelengths for different laser materials and types. Semiconductor lasers have many and varied applications, and are efficient and cheap to manufacture, making them a popular choice for practical applications, and with continuing developments in high power semiconductor lasers, they are likely to even further dominate the laser market in years to come. References: Optoelectronics, S. O. Kasap, 1999 (Prentice Hall) Encyclopedia of Laser Physics and Technology, R. Paschotta (www.rp-photonics.com) 3 Laser Fundamentals, 2nd Ed., W.T. Silfvast, 2004 (Cambridge University Press) 4 Physics World, May 2010 issue (IoP Publishing) 5 www.ru.nl/tracegasfacility/trace_gas_research/laser_spectroscopy/quantum_cascade/ 1 2

Jessica Stanley is a jIAPS editor. Milan Vrućinić is a student in the Nanomaterials master program at Utrecht University in the Netherlands, and he previously studied at the University of Banja Luka, Republic of Srpska, Bosnia and Herzegovina. He is a member of the IAPS executive committee, as well as being involved in LC Banja Luka, and was national secretary of the International Association for the Exchange of Students for Technical Experience (IAESTE) in Bosnia and Herzegovina.

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Terrestrial Gamma-Ray Flashes by Ragnhild Schrøder Hansen In the early 90s a completely new phenomenon in the Earth’s atmosphere was discovered by coincidence. For the first time, natural gamma rays coming from the Earth were detected. The phenomenon was named Terrestrial Gamma-ray Flashes (TGF). The first predictions of TGFs were made by C. T. R. Wilson in 1924. He described the possibility of high-altitude discharges and acceleration of electrons to high energies in the regions above thunderclouds. However, TGFs were first discovered 70 years later by Fishman et al. [1994]. The Burst and Transient Source Experiment (BATSE) on board Compton Gamma Ray Observatory (CGRO) was originally designed to detect gamma ray bursts from distant galaxies, but also discovered gamma rays from the Earth’s atmosphere. Some years later the satellite Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) also made observations of this radiation. RHESSI was designed for detection of solar X-rays, and the first observations of TGFs from this instrument were reported by Smith et al. [2005]. In 2010 came the first reports of TGF observations from the satellite AGILE (Astrorivelatore Gamma and Immagini LEggero). This satellite was

also created to detect cosmic gamma rays. The observed TGFs are bursts of gamma rays lasting for around 1 ms that are seen to originate from the Earth. BATSE observed 76 TGFs over around 7 years of observation; this is somewhat less than 1 TGF/month. RHESSI has so far observed around 820 TGFs (February 2008) and is still observing. RHESSI observed around 10-12 TGF/ month for its first years, with the number decreasing as the instrument became damaged by radiation. AGILE observed 34 high confidence events in 9 months and is also still in operation. Analysis from BATSE showed that the TGFs consist of many photons with energy >315 keV. As this high-energy channel was an integral channel no further information about the high-energy photons could be obtained from BATSE. RHESSI is observing in the energy range of 50 keV to 20 MeV and has a high energy resolution. From the RHESSI measurements, photons up to the maximum energy limit of 20 MeV have been found. The AGILE satellite made observations of TGFs with an instrument with a 300 keV to 100 MeV energy range and has reported photon energies up to 43 MeV in one TGF.

Photo: Toronto Thunderstorm by John R. Southern, from Wikipedia.

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Terrestrial Gamma-Ray Flashes

Fig 1: The dynamic friction force and electrical threshold forces for air at ground pressure. Figure taken from Moss et al. [2006]

The Gamma-ray Burst Monitor (GBM) on the Fermi Gamma-ray Space Telescope Observatory (Fermi) has also observed TGFs. The GBM is still in operation and has an energy range of 8 keV too 45 MeV with high energy resolution. Between June 2008 and June 2009 the GBM observed 12 TGFs, some containing photons with energies above 30 MeV. The first analysis of the TGF spectrum indicated that TGFs were bremsstrahlung from MeV electrons. This was due to the hard spectrum that would be consistent with bremsstrahlung and the high energies of the photons requiring high-energy electrons. As the photons propagate through the atmosphere they experience attenuation and the energy spectrum and direction of motion of the photons will be somewhat changed.

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TGF production requires electrons with energies >2040 MeV. This motivated the search for mechanisms that could give acceleration of electrons into this energy range. A new acceleration process for high-energy electrons named Relativistic Runaway Breakdown (RRB) was presented by Gurevich et al. [1992]. Further development of the theory has produced the theory of Relativistic Runaway Electron Avalanche (RREA). This theory was first proposed by Wilson [1924] and is based on the fact that the friction force for electrons changes with energy. The friction force in air at ground pressure is shown in Figure 1. For low electron energies the frictional force increases with increasing electron energy and will usually stop the acceleration of the electrons when the electrons reach a

{jIAPS} given energy <100 keV. For electron energies >100 keV the frictional force will decrease and have a local minimum at around 1 MeV. For higher electron energies >1 MeV the friction force will increase due to relativistic effects. If we have an electron with energy >1 MeV and an electrical force larger than the frictional force for that specific electron energy the electron can be accelerated up to a relativistic energy in a runaway process. This is because as the electron energy increases the friction force is decreasing and the electric force will continue to be larger than the friction force (up to 1 MeV). The electron is then a Relativistic Runaway Electron. The process was named Relativistic Runaway Breakdown (RRB). The critical energy Ec is the energy where the friction force is smaller than the electric force, for a given electric force. This energy is then dependent on the electric force as seen in Figure [insert number later]. For example if the electric field is Ek the critical electron energy is Ec = 10 keV. If a relativistic runaway electron is accelerated to more than two times the critical energy and collides with another electron it can give the new electron an energy larger than Ec in the collision. There will then be two relativistic runaway electrons. Further acceleration and collisions can give an avalanche effect and create a Relativistic Runaway Electron Avalanche (RREA). This process needs an electric field larger than a given threshold, and a duration long enough to accelerate the electrons to

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Terrestrial Gamma-Ray Flashes energies >40 MeV (which is the maximum observed photon energy). The RREA also needs at least one high-energy seed electron, and to get the observed TGFs several seed electrons are needed. The necessary energy of this electron is given by the magnitude of the local electric field. There are at least 4 regions in connection to a thunder system where a large enough electric fields can be found. First of all inside the thundercloud itself, second around a lightning discharge, third the quasi static electric field after a lightning discharge (QES) and finally the radiated electromagnetic pulse from a lightning discharge (EMP). There are two proposed theories of the origin of the seed electrons. One is the secondary electrons from cosmic rays, and the second is from production in the electric field in the tip of a lightning bolt. From the analysis of measurements so far TGFs are believed to be produced at altitudes of 15-20 km. The theory of EMP is most important at altitudes of 40-60 km and the QES theory only gives TGF production at 20-70 km and hence both are considered to be improbable.

The theory that most scientists now work with is the production of TGFs inside lightning discharges. The TGF discoveries motivated the Atmosphere-Space Interaction Monitor (ASIM) project that was accepted for a phase A study in 2004. With long experience in building Xray instruments for space, the space physics group at University of Bergen (UoB) was asked to develop large parts of the Xray detection system (Modular X-ray and Gamma ray Sensor, MXGS) of ASIM. At UoB, both the task of developing this instrument and the scientific work on this phenomenon started around 2004. Since then the knowledge of the phenomenon has developed much due to an increased number of observations. We now assume that the phenomenon is much closer to the Earth’s surface than earlier expected. The phenomenon is seen to be in close connection to parts of standard atmospheric physics. The ASIM instrument is to be deployed at the International Space Station (ISS) and will be qualified for space. As a part of the instrument development a prototype with exactly the same layout and design, but with com-

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Fig 2: The MXGS gamma ray detector to be mounted on ASIM and The International Space Station.

ponents not qualified for space, will be made for testing. This prototype, or a copy of it, can be flown on an aircraft or balloon. As the phenomenon is believed to exist lower in the atmosphere than earlier assumed the question of observations closer to the Earth’s surface arises. As X-rays are efficiently absorbed when the density of air increases, TGFs have not been seen from the ground. But where in the atmosphere it will be possible to get good observations is not known. To learn more about this topic and for further discussions of the questions raised in this article the talk “Constraints for observing Terrestrial Gamma-ray Flashes by a high-altitude aircraft” at ICPS 2010 in Graz, Austria is highly recommended.

References:

Fishman, G. J., et al., Discovery of intense gamma-ray ashes of atmospheric origin, Science, 264 (5163), 1313 1316, 1994. Fishman, G. J., et al., Fermi GBM observations of TGFs, American Geophysical Union Fall meeting, San Francisco, 2009. Grefenstette, B. W., D. M. Smith, B. J. Hazelton, and L. I. Lopez, First RHESSI terrestrial gamma ray flash catalog, Journal of geophysical research, 114 (A02314), doi:10.1029/2008JA013721, 2009. Gurevich, A. V., G. M. Milikh, and R. Roussel-Dupre, Runaway electron mechanism of air breakdown and preconditioning during a thunderstorm, Physics letters A, 165, 463 468, 1992. Marisaldi, M., et al., Detection of terrestrial gamma-ray ashes up to 40 MeV by the AGILE satellite, Journal of geophysical research, 115 (A00E13), doi: 10.1029/2009JA014502, 2010. Moss, G. D., V. P. Pasko, N. Liu, and G. Veronis, Monte Carlo model for analysis of thermal runaway electrons in streamer tips in transient luminous events and streamer zones of lightning leaders, Journal of geophysical research, 111 (A02307), doi:10.1029/2005JA011350, 2006. Smith, D. M., L. I. Lopez, R. P. Lin, and C. P. Barrington-Leigh, Terrestrial gamma-ray flashes observed up to 20 MeV, Science, 307 (1085), doi: 10.1126/science.1107466, 2005. Tavani, M., et al., The AGILE mission, Astronomy and astrophysics, 502, 995-1013, doi:10.1051/0004-6361/200810527, 2009. Wilson, C. T. R., The electric field of a thundercloud and some of its effects, Proc. Roy. Soc. London, pp. 32D 37D, 1924.

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The Back Page

From the Archives

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Jim Grozier, IAPS’ hardworking archivist, has dug into the archives once again to retrieve this amusing email exchange from October 2003: “AMOROUS PORTUGUESE STUDENT to NANNA NICOLAISEN: I want to greet the new IAPS president, which I ashamedly didn’t do in Bodrum [venue of ICPS 1993]. Yes, shame on me, but I had a very stupid reason which is that as I’d never spoken to you before you might think I’d do it just because you’re the new IAPS president. Yes, you’d be right. Anyway, my congratulations: I bet you succeeded to be the most beautiful of all IAPS presidents. I mean, yes, you’re pretty good-looking, but anyway, all previous IAPS presidents were men … NANNA NICOLAISEN to A.P.S.: I’m glad you think the president is pretty, and I guess that Bente is too, because it is Bente and not me who is the president. I am the secretary …. A.P.S. to N.N.: Well, I sure did what in Portugal we call “to put the feet in the big hole”, I mean, I blew it. I was really thinking you were the IAPS president, but then, I must not repeat the previous compliment. I mean, now I withdraw it and restate: Not the president, but the actual secretary is quite beautiful” In the hole, and digging furiously! Moral: Always check your data, like a good physicist …

Mathematics: the answer to the Ultimate Question? M =13 A = 1 T = 20 H=8

M + A + T + H = 42

42 is the answer to the Ultimate Question*, so it has been suggested (on the internet, of course) that this means that mathematics is the answer to life, the universe and everything. But does this mean that the pan-dimensional beings who organised the calculation of the Ultimate Answer spoke American rather than British English (in which case mathematics is abbreviated to ‘maths’, not ‘math’)? A matter that requires some deep thought, perhaps. *If you are confused at this point you should be ashamed for not having read Douglas Adams’ Hitchhiker’s Guide to the Galaxy and thus not being a proper nerd.

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jIAPS Issue 2 2010