Tuesday, 4 October 2011

Graphene Dream

Graphene has experienced a rise to fame faster than the lolcat takeover of the internet. Last year two researchers from the University of Manchester; Andre Geim and Kostya Novoselov received the Nobel Prize in Physics for groundbreaking experiments regarding the two-dimensional material graphene. The ‘upzipped carbon nanotube’ became a hot topic, producing over 3000 journal articles and slowly starting a new science band wagon rolling.  This was well and truly jumped upon by the UK government early this week with an announcement that they would pledge £50million towards developing spin-off technologies from the material.

This amount of funding will drive graphene development to new levels of omnipresence, with rumours that it will supersede our dependence on silicon; the other superstar Mousketeer of material science.  So how good is graphene and what exactly can we use it for?

Graphene is essentially a thin slither of graphite. Graphite is naturally occurring, known as the Class 1 grade of coal, and consists of carbon atoms. P.R. Wallace first theorised the existence of graphene in 1947 when he was studying the band structure of graphite. The graphene sheet is 1 atom thick and was not isolated and studied until the 2000’s, a prevalent trend in experimental research where with each decade you go a little bit more quantum.  This 2D sheet has broken many records by being the strongest as well as most thermally and electrically conductive material measured. These properties are due to an extremely high current density, meaning it can carry a large amount of electricity throughout its volume. Graphene also has the longest mean free path of any material, which is similar to comparing the traffic in Central London to the middle of the Sahara in terms of electrons being able to travel through a material undisturbed. This leads to low resistance and more conductivity. Some other properties which have dubbed graphene as the most useful thing since plastic are its transparency and elasticity.

The extreme properties of graphene have made it a material of interest in industry. There has been research conducted by IBM to make a touchscreen out of graphene, possibly paving the way for flexible screens, roll-up mobiles and batteries that feel like rubber. On the flip side, the strength of graphene will make it lucrative for composite fabrication and tyre strengthening. It has been claimed that a hammock sized sheet which would weigh less than 30 grains of salt would carry the averaged sized cat, a measurement standard which certainly impressed the cat lovers amongst us.

Cat hammock patents aside, there are some major hurdles that need addressing before we go graphene mad. The material has no band gap which means there isn’t a space between the valence band where electrons sit firmly within the structure, and the conduction band, where electrons move into and  gain some freedom to conduct electricity. This means graphene is constantly ‘on’ and conductive, a feature that will hold back electronic component development.

The main barrier will be large scale manufacture of graphene. The Nobel prize winners used a ‘mechanical exfoliation’ method which is basically using sellotape to tear flakes off a lump of graphite. The sellotape was then dissolved in acetone to leave a residue of graphene flakes onto a silicon wafer. It’s then pot luck if you find a nice patch where a single sheet has formed. Chemical vapour deposition growth and growth from solid carbon in a furnace are techniques which could change this fabrication process, but exactly how much change will this leave the £50million fund?

There is a history of ‘fad’ scientific developments which undergo a slow burn lifecycle despite early excitement. Does anyone remember carbon nanotubes? And how many of us can afford to have solar cells on our rooftops? Seen any nano-bots in your cornflakes recently? These are all research topics that genuinely will bring about great changes to the way we live. Graphene can now be added onto the same list. But the LASER is one of the few examples of something we have incorporated into our everyday lives, and that took a good 40 years. So whenever you hear of reports about the next big thing in science, please add a mental question mark to the end of it. And then wait and see.

Tuesday, 20 September 2011

Stop Asking for a Lightsaber!

Taken from cassiroll.com
I like to play with lasers. And when I tell people my research involves building them, 9 times out of 10 the response is ‘Can you make me a lightsaber?'. Perhaps this is an indication that I need to find new friends; ones who understand that when I say ‘No, I can't' it doesn’t give them credence to respond with ‘But if they managed to do it a long time ago, in a galaxy far far away, why can't we?!'. So, to put an end to all the late night pub debates and endless neeeeeewaw noises let me explain exactly why I can't make your lightsaber.

Before we embark on our quest for truth, it is imperative that I explain what a laser is. Light Amplification by Stimulated Emission of Radiation describes how gain media (technically any material, even jelly babies) are excited to emit synchronised packets of light called photons. This synchronisation is the fundamental characteristic of a laser that differentiates it from light from a household bulb. The gain medium can be thought of as a series of tiny ladders that electrons sit on. If a photon, let's call him Luke, meets this electron it causes the electron to jump to the next rung of the ladder. Universal law says things can't stay high; they have to come back down and in order to ‘relax' back to its original position the electron pops out a clone of Luke to balance the books. You've now doubled your photon count. Adding a feedback mechanism like mirrors to bounce the Lukes back into the medium will amplify your original signal and before you can say Millenium Falcon you've got yourself a high power laser.

The two main reasons why a laser can't be crafted into a lightsaber are:
Ridiculously high powers needed
Lasers can't be shaped in air

Back of the envelope calculations are what experimental scientists do best. And I've done one for the power needed for an infrared (non-visible) laser to shear through the average arm. The equation I used is straightforward; the amount of power is related to the energy spent raising the temperature and then vaporising the arm, the velocity of the laser going through the arm and the spot size of the beam. These factors are limited by how deep the laser can penetrate before being absorbed and the thickness of the arm. With a few assumptions which I'll declare straight away (lightsaber speed was taken to be an average baseball batting speed of 70 MPH, thickness of arm 10 cm and beam area of 5x5 cm) this gives a required power output of over 160 kilowatts, the equivalent of 250 toasters running simultaneously for one minute! So you better strap on a 4 tonne generator on your back if you want to start fencing with your lightsaber.

The second reason is a purely superficial one. It explains why you won't be able to craft a rod shaped laser, fine-tuned to stop mid-air one meter from the source. Laser radiation is not a discrete entity like rope but a flowing wave that propagates and is subject to attenuation. Attenuation causes the intensity to decrease over distance, similar to sound getting quieter as you move away from the source. The intensity is reduced because the air will absorb and scatter photons. Some photons will interact with the atoms in the air and excite them (absorption) and others will bounce off the air particles, deviating from their original course (scattering). The most detrimental to high power laser radiation is absorption that leads to thermal blooming, an effect caused by intense heating of the surrounding air. The heating effect changes the refractive index which determines how much a material bends light. As the air increases in refractive index, it begins to act like a lens and defocuses the beam to break up into separate spots like a Jackson Pollock painting.

Despite these drawbacks, the destructive power of lasers is still fascinating to the public and heavily featured in blockbuster movies or fashioned onto cute cats captioned with ‘pew pew pew'. Perhaps it's not a coincidence that Ronald Reagan proposed the Strategic Defence Initiative, fondly known as 'Star Wars' two days before the release of Return of the Jedi. Although Reagan's idea was panned at the time for many of the reasons I've stated above, technological advances such as the use of adaptive optics to counteract beam wander and the ability to string together multiple kilowatt lasers have rekindled interest. Simple laser guns set to dazzle (as in causing glare, not razzle) are already used in non-deadly operations, and the US have supposedly reached 100 kilowatts by incoherent combining of seven laser amplifier chains. So I'm sorry I can't build you a glorified glow stick, would you like a giant laser cannon instead?

Wednesday, 3 August 2011

Colder Than Ice

Beautiful levitating microspheres
from James Millen's website
ucloptomechanics.wordpress.com
LASERs are pretty great. When they were first invented they were called the solution looking for a problem, translation: 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. When your fancy cd players stop working I’m taking over and putting BON JOVI on all night long.)

What is a Laser?
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. 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 these photons come in flavours like ice cream. You can think of the photon as one discrete scoop of 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.

The main 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; long and shaft-like beam shape 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).  

However, Lasers are also used to cool things down.

What makes a LASER cold?

First, what is cold? Well, cold is actually described as temperatures under 1K, and ultracold below 1mK. What is a 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 also has an important definition attached to it - it's the temperature at which all thermal (mechanical) motion ceases, not 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.

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.
Let’s revive the ice cream analogy where a photon is one scoop. And atom absorbing a photon is like hurling the scoop into a catapult, causing the elastic band to start to stretch. This is ‘excitation’ of the atom after photon absorption. You make the atom want to move in the same direction as it takes in the photon momentum. Now, if this catapult stretches too much (too much excitement) it will want to spring back and throw back the photon in a random direction. This is a recoil reaction. However, because recoiling occurs in all directions, the average direction turns out to be in no direction at all. This is like the dodgeball effect if you imagine balls are being thrown at you in all directions; there’s no where to escape so you stay still. 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.  The atom you want to cool will initially have a certain amount of velocity, hence trying to minimise it. The atom will experience 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. Therefore to stay in tune you send a LASER already red-shifted to counteract the Doppler effect.

You effectively trick the moving atoms into submission.

Once the atom is cooled you can conduct some experiments on it, for example using another set of LASERs to levitate it and move single atoms around. This is optical tweezering, and is responsible for micro-graffiti, where scientists will tag small components with really boring words. The atom can also be used as a very accurate clock using the transition levels, and even manipulated to display some quantum behaviours. In summary, the coolest thing about cooling is not the temperature reached, but the power of control it enables experimental physicists to have over the building blocks of the universe.

Sunday, 3 July 2011

Sound Bullets

The concept of high directional sound; or to grossly exaggerate 'SOUND BULLETS!' is one which has enjoyed a sporadic lifestyle, gaining interest and subsequent abandonment from various industries. It is the bell bottom jeans of technology; everyone thinks it’s a good look but have you seen anyone under 6ft wear them?

'Sound from ultrasound' would be the tagline that sums up my idea. The concept has been around since the 60's when hearing things from thin air was probably a regular occurrence. It gathered momentum from the defence industry to be used for transmission of covert messages disguised in a highly directional inaudible ultrasound wave but in recent years there has been an interest from niche entertainment venues to exploit this technology to a new level.

The audible sound is 'hidden' in a ultrasound wave whose frequencies cannot be heard by human ears and are revealed only when they hit surfaces or a listener is stood directly in its path. Imagine walking into a club and hearing sound burst from the walls and your own body. For those business types out there - projecting sound to individuals as they are walking through the mall, targeting them with information about opening times, discount brands and distances to the nearest McDonalds in a 'sonic bullet'.

To explain how this works lets go into a trance back to Yr 8 and that physics lesson you fell asleep in... the basics of sound waves are:

- A sound wave is basically like a punch to the face. The face of air that is. Unlike light it is a mechanical process which requires a medium to propagate through. Light and sound are both different types of energy and the energy carried by a sound wave is able to travel by doing a Mexican wave through air molecules.

- The shape of a sound wave is called sinusoidal. This basically means there are peaks and troughs that occur periodically, just like a tidal wave in the ocean.

- The frequency of a sound wave depends on how many peaks and troughs you can squeeze together, so a short wave will look bunched up and represents a 'high pitch' in sound. Ultrasound waves are extremely high frequency (classed as over 20kHz) and can't be heard because the middle ear acts as a low pass filter, basically like a bouncer who only lets in low frequencies.

The main features behind the effect are derived from properties
intrinsic to ultrasonic frequencies:

- Directivity: This is dependent on the ratio of the source size to the
frequency of the wave emitted by it. A normal loudspeaker will blast out
music in all directions because audible sound has a small frequency.
Ultrasound therefore is emitted in a narrow cone or beam

- Nonlinear properties: Nonlinear means the response of the wave, in this case, as it interacts with air molecules, does not follow a straight correlation i.e. doubling the strength equals double the effect. The nonlinear behaviour of ultrasound with air causes some parts of the sound wave to slow down, so that parts of the beam change
frequency. This is referred to as the Doppler effect where the change in velocity of a wave will also affect the frequency. Because parts of the ultrasound wave slow down, the peaks and troughs begin to spread out leading to a decrease in frequency (it goes from curly fries to a limp piece of wet spaghetti). You are left with bits of the wave at one frequency f1 and another frequency f2 which then undergo heterodyning (fancy word for mixing), to create sum and difference (f1+f2 and f1-f2) frequency components. Sum and difference literally does what it says on the tin; two slightly different high frequencies will crash together to produce both an even higher frequency wave (sum) and a tiny tiny small frequency wave (difference) but because the original waves were ultrasonic they still maintain the high directivity. The frequency difference component is important because this is where an audible sound
can be produced.

- The audible sound is only revealed after the ultrasound wave has propagated a certain distance and nonlinear-ed itself (great scientific lingo coming out today) or, an accelerated version of this effect is seen when the sound wave hits a surface. This is because the object will vibrate at the difference frequency like a loudspeaker.

And there you have it, the equivalent to a sound ninja. Unfortunately, despite how cool all of this appears to be, the sound ninja won't be kung foo chopping your ear drums anytime soon. The nonlinear property that allows ultrasound to be manipulated this way also creates a lot of energy dispersion, resulting in very short propagation distances and a sonic bullet that more resembles a sonic Frisbee - a bit feeble and lacking in force.

Friday, 3 June 2011

Scattering

Rayleigh Scattering making sun sets
The Electromagnetic spectrum is a beautiful thing; every classroom needs one, it’s as important to scientists as the alphabet was to Shakespeare. The EM spectrum describes the range of frequencies (in cm-1 for the moccasin wearing Chemists) or wavelengths (in nm for us hip converse wearing physicists) that electromagnetic radiation can take. The most notorious of these is ‘light’ although ‘light’ isn’t one entity as such as it still hogs a large portion of wavelengths (380 to 780 nm); it’s like the Kevin Smith of aeroplanes. Using the whole spectrum has allowed us to see things both in artificial and natural luminance, make chips that allow us to play Angry Birds sat on the loo, and provide us with ways to make LASERs which surely is the best way to use and abuse EM radiation.

One thing I have always managed to get confused about during my 4 years studying Physics, apart from trying to remember the Taylor Expansion are all the different types of scattering that affect EM radiation.

So, firstly, a note about EM radiation – it’s not just a squiggle that seeps out from sources like the sun but actually composed of quanta called photons. This is the major difference between the classical and quantum model and leads onto wave-particle duality.

Scattering involves light ‘waves’ essentially bouncing off ‘particles’ or volumes of stuff but in the quantum world this is described as the interaction of the photons with the matter it encounters. It is responsible for a lot of attractive things for example the sheen or lustre of objects is caused by scattering so you can thank physics for the rise of bling culture. Although the colour of objects is mostly down to absorption, the colour of the sky is actually due to Rayleigh scattering.

We can categorise scattering into 2 distinct boxes; elastic and inelastic. Elastic scattering causes negliable energy transfer whereas inelastic scattering causes shifts in energy so that the radiation leaving the interaction has a different wavelength/frequency.

Here’s a handy table about elastic and inelastic scattering:
Under the category of elastic scattering:
Rayleigh scattering – This occurs when light meets a small spherical particle with a refractive index. The limit for Rayleigh scattering to occur is that the particle size is smaller than the wavelength of the light.
Mie scattering – Similar to Rayleigh scattering except the particle size is around the same size as the wavelength.
Geometric scattering – Size of the particle is larger than wavelength.

These 3 types of scattering constitute atmospheric scattering and you can model the propagation of radiation through any type of weather/environment using a combination of these 3 scattering regimes. It this no small feat, and involves lots of equations and something called MODTRAN. This was my life for 5 months, having to simulate the propagation of a laser through Maine. Doesn’t do so well in rain.
 Stimulated Brillouin and Raman scattering is often used for amplification in fibres by choosing a lower frequency signal photon to inelastically scatter off a higher frequency pump photon to produce another low frequency signal photon. The aim is to always ‘reproduce’ something, in this case using the effects of scattering to reproduce a low frequency photon. It may sound boring but it is this process which allows us to have the broadband speeds we have, and research continues to find new ways to use processes like this to develop more amplification techniques.
Thomson scattering – this is the lil bro of Compton scattering (which is an inelastic process), he’s like totally on the lowdown, like..the low energy limit, that’s LOW. Anyway, it happens when EM radiation meets a free charged particle – like an electron. The E and B (electric and magnetic or as I’m now calling them, the ‘Bert and Ernie’) parts of the radiation cause a Lorentz force onto the charged particle causing it to accelerate. It then emits radiation at the same frequency as the perturbing radiation, hence conserving energy. The Lorentz force is a complex way of saying that a bit of the energy of the perturbing radiation gets transferred to the particle, causing it to move, so the particle emits radiation to stop moving. Thomson scattering is a main reason behind the linear polarization of the cosmic microwave background.

Inelastic Scattering

Since I’ve given a shout-out to Compton scattering already – this just involves the X ray and gamma ray part of the spectrum interacting with matter. The X/gamma-ray photon imparts energy to the matter atoms causing ionisation (excites the electrons in the atoms causing them to ping off like little fleas) therefore the photons decrease in energy after the interaction. This was one of the fundamental experiments to show wave-particle duality because in order for an electron to leave a quantised amount of energy must be transferred to it. Previously light was thought of as a wave, with an energy flux equal to the intensity (i.e. more light). So surely the higher the intensity the more electrons will ping off the metal surface? Well no. The photoelectric effect was still seen to happen even at low intensity. The classical description of light as a wave started to wane, allowing the young fresh faced quantum theories to take shape. Quantum mechanics describes light as consisting of photons which carry quantised amounts of energy. Basically you split the light wave into neat little balls, with the size of the ball proportional to the frequency. If the ball size matches the transition energy needed for the electron to ping off, ionisation is seen to occur. Tuning the intensity only changes the number of photons being released, however, tuning the frequency changes the ball size which will either provide a better or worse ‘fit’ that affects the ionisation rate.

Brillouin scattering occurs when light travelling in a medium interacts with time independent (fluctuating) optical density variations caused by fancy things like phonons, magnons or the less impressive sounding temperature gradient. It is seen as a quantum mechanical treatment of photons interacting with quasi-particles like the magnon (acoustic modes of the material) but is intrinsictly different from certain types of Rayleigh scattering (which involves random incoherent thermal fluctuations wheras Brillouin is a correlated periodic fluctuation) and Raman scattering (where the photons are scattered by an interaction with the vibrational and rotational transitions which are non propagating whereas Brillouin scattering involves propagating low frequency modes)