tag:theconversation.com,2011:/us/topics/optical-tweezers-60468/articles
Optical tweezers – The Conversation
2019-05-16T10:41:25Z
tag:theconversation.com,2011:article/116127
2019-05-16T10:41:25Z
2019-05-16T10:41:25Z
A new type of laser uses sound waves to help to detect weak forces
<figure><img src="https://images.theconversation.com/files/272882/original/file-20190506-103057-1kmqtej.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The crests (bright) and troughs (dark) of waves spread out after they were produced. The picture applies to both light and sound waves.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-vector/sound-waves-oscillating-dark-blue-light-527580616?src=5KJGauMtTQyucb5grE7WxQ-2-21">Titima Ongkantong</a></span></figcaption></figure><p>Most people are familiar with optical lasers through their experience with laser pointers. But what about a laser made from sound waves?</p>
<p>What makes optical laser light different from a light bulb or the sun is that all the light waves emerging from it are moving in the same direction and are pretty much in perfect step with each other. This is why the beam coming out of the laser pointer does not spread out in all directions.</p>
<p>In contrast, rays from the sun and light from a light bulb go in every direction. This is a good thing because otherwise it would be difficult to illuminate a room; or worse still, the Earth might not receive any sunlight. But keeping the light waves in step – physicists call it coherence – is what makes a laser special. Sound is also made of waves. </p>
<p>Recently there has been considerable scientific interest in creating phonon lasers in which the oscillations of light waves are replaced by the vibrations of a tiny solid particle. By generating sound waves that are perfectly synchronized, we figured out how to make a <a href="https://doi.org/10.1038/nphys1367">phonon laser</a> – or a “laser for sound.”</p>
<p>In work we recently published in the journal <a href="https://doi.org/10.1038/s41566-019-0395-5">Nature Photonics</a>, we have constructed our phonon laser using the oscillations of a particle – about a hundred nanometers in diameter – levitated using an optical tweezer. </p>
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<img alt="" src="https://images.theconversation.com/files/274399/original/file-20190514-60567-4oett6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/274399/original/file-20190514-60567-4oett6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=434&fit=crop&dpr=1 600w, https://images.theconversation.com/files/274399/original/file-20190514-60567-4oett6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=434&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/274399/original/file-20190514-60567-4oett6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=434&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/274399/original/file-20190514-60567-4oett6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=545&fit=crop&dpr=1 754w, https://images.theconversation.com/files/274399/original/file-20190514-60567-4oett6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=545&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/274399/original/file-20190514-60567-4oett6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=545&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A red laser beam from a high-power lab laser.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/laser-beam-lab-warning-notice-on-492024616">Doug McLean/Shutterstock.com</a></span>
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<h2>Waves in sync</h2>
<p>An <a href="https://blocklab.stanford.edu/optical_tweezers.html">optical tweezer</a> is simply a laser beam which goes through a lens and <a href="https://www.nobelprize.org/prizes/physics/2018/press-release/">traps a nanoparticle in midair</a>, like the tractor beam in “Star Wars.” The nanoparticle does not stay still. It swings back and forth like a pendulum, along the direction of the trapping beam. </p>
<p>Since the nanoparticle is not clamped to a mechanical support or tethered to a substrate, <a href="https://theconversation.com/experiments-with-optical-tweezers-race-to-test-the-laws-of-quantum-mechanics-105563">it is very well isolated from its surrounding environment</a>. This enables physicists like us to use it for sensing weak electric, magnetic and gravitational forces whose effects would be otherwise obscured. </p>
<p>To improve the sensing capability, we slow or “cool” the nanoparticle motion. This is done by measuring the position of the particle as it changes with time. We then feed that information back into a computer that controls the power in the trapping beam. Varying the trapping power allows us to constrain the particle so that it slows down. This setup has been used by several groups around the world in applications that have nothing to do with sound lasers. We then took a crucial step that makes our device unique and is essential for building a phonon laser.</p>
<p>This involved modulating the trapping beam to make the nanoparticle oscillate faster, yielding laser-like behavior: The mechanical vibrations of the nanoparticle produced synchronized sound waves, or a phonon laser. </p>
<p>The phonon laser is a series of synchronized sound waves. A detector can monitor the phonon laser and identify changes in the pattern of these sound waves that reveal the presence of a gravitational or magnetic force.</p>
<p>It might appear that the particle becomes less sensitive because it is oscillating faster, but the effect of having all the oscillations in sync actually overcomes that effect and makes it a more sensitive instrument. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/274393/original/file-20190514-60567-xq4cnx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/274393/original/file-20190514-60567-xq4cnx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/274393/original/file-20190514-60567-xq4cnx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/274393/original/file-20190514-60567-xq4cnx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/274393/original/file-20190514-60567-xq4cnx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/274393/original/file-20190514-60567-xq4cnx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/274393/original/file-20190514-60567-xq4cnx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/274393/original/file-20190514-60567-xq4cnx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">An artist’s depiction of optical tweezers (pink) holding the nanoparticle in midair, while allowing it to move back and forth and create sound waves.</span>
<span class="attribution"><span class="source">A. Nick Vamivakas and Michael Osadciw, University of Rochester illustration</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
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<h2>Possible applications</h2>
<p>It is clear that optical lasers are very useful. They carry information over optical fiber cables, read bar codes in supermarkets and run the atomic clocks which are essential for GPS. </p>
<p>We originally developed the phonon laser as a tool for detecting weak electric, magnetic and gravitational fields, which affect the sound waves in a way we can detect. But we hope that others will find new uses for this technology in communication and sensing, such as the mass of very small molecules. </p>
<p>On the fundamental side, our work leverages current interest in testing quantum physics theories about the behavior of collections of billion atoms – roughly the number contained in our nanoparticle. Lasers are also the starting point for creating exotic quantum states like the famous <a href="https://www.youtube.com/watch?v=OkVpMAbNOAo">Schrodinger cat state</a>, which allows an object to be in two places at the same time. Of course the most exciting uses of the optical tweezer phonon laser may well be ones we cannot currently foresee.</p><img src="https://counter.theconversation.com/content/116127/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Mishkat Bhattacharya receives funding from the Office of Naval Research, United States</span></em></p><p class="fine-print"><em><span>Nick Vamivakas receives funding from the Office of Naval Research, United States.</span></em></p>
Most people are familiar with lasers. But what about a laser made with sound rather than light? A couple of physicists have now created one that they plan to use for measuring imperceivable forces.
Mishkat Bhattacharya, Associate Professor of Physics and Astronomy, Rochester Institute of Technology
Nick Vamivakas, Associate Professor of Quantum Optics & Quantum Physics, University of Rochester
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/105563
2018-11-07T11:38:30Z
2018-11-07T11:38:30Z
Experiments with optical tweezers race to test the laws of quantum mechanics
<figure><img src="https://images.theconversation.com/files/244026/original/file-20181106-74766-1gb598v.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A silica sphere with a radius of 50 nanometers is trapped levitating in a beam of light.</span> <span class="attribution"><span class="source"> J. Adam Fenster, University of Rochester</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>One might think that the <a href="https://doi.org/10.1364/OL.11.000288">optical tweezer</a> – a focused laser beam that can trap small particles – is old hat by now. After all, the tweezer was invented by <a href="https://www.nobelprize.org/prizes/physics/2018/ashkin/facts/">Arthur Ashkin</a> in <a href="https://doi.org/10.1103/PhysRevLett.24.156">1970</a>. And he received the <a href="https://www.nobelprize.org/prizes/physics/2018/press-release/">Nobel Prize</a> for it this year - presumably after its main implications had been realized during the last half-century.</p>
<p>Amazingly, this is far from true. The optical tweezer is revealing new capabilities while helping scientists understand quantum mechanics, the theory that explains nature in terms of subatomic particles.</p>
<p>This theory has led to some weird and counterintuitive conclusions. One of them is that quantum mechanics allows for a single object to exist in two different states of reality at the same time. For example, quantum physics allows a body to be at two different locations in space simultaneously – or both dead and alive, as in the famous thought experiment of <a href="https://www.youtube.com/watch?v=OkVpMAbNOAo">Schrödinger’s cat.</a></p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/243270/original/file-20181031-122168-1tmls06.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/243270/original/file-20181031-122168-1tmls06.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/243270/original/file-20181031-122168-1tmls06.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/243270/original/file-20181031-122168-1tmls06.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/243270/original/file-20181031-122168-1tmls06.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/243270/original/file-20181031-122168-1tmls06.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/243270/original/file-20181031-122168-1tmls06.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The two states of Schrodinger’s cat: dead (on the left) and alive (on the right). Quantum physics says the cat can exist in both states simultaneously.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-vector/schrodingers-cat-vector-illustration-life-death-337646978?src=YHf1aSsGcOXqhh19qap9MA-1-7">Rhoeo / Shutterstock.com</a></span>
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<p>The technical name for this phenomenon is superposition. <a href="https://en.wikipedia.org/wiki/Quantum_superposition">Superpositions</a> have been <a href="https://www.nature.com/articles/nphys2863">observed</a> for tiny objects like single atoms. But clearly, we never see a superposition in our everyday lives. For example, we do not see a cup of coffee in two locations at the same time. </p>
<p>To explain this observation, theoretical physicists have suggested that for large objects – even for nanoparticles containing about a billion atoms –superpositions collapse quickly to one or the other of the two possibilities, due to a <a href="https://journals.aps.org/pra/abstract/10.1103/PhysRevA.84.052121">breakdown</a> of standard quantum mechanics. For larger objects the rate of collapse is faster. For Schrodinger’s cat, this collapse – to “alive” or “dead” – would be practically instantaneous, explaining why we never see the superposition of a cat being in two states at once.</p>
<p>Until recently, these “<a href="https://en.wikipedia.org/wiki/Objective-collapse_theory">collapse theories,</a>” which would require modifications of textbook quantum mechanics, could not be tested, as it is difficult to prepare a large object in a superposition. This is because larger objects interact more with their surroundings than atoms or subatomic particles – which leads to leaks in <a href="https://en.wikipedia.org/wiki/Quantum_decoherence">heat</a> that destroys quantum states.</p>
<p>As physicists, we are interested in collapse theories because we would like to understand quantum physics better, and specifically because there are theoretical indications that the collapse could be due to <a href="http://iopscience.iop.org/article/10.1088/1367-2630/16/10/105006">gravitational effects</a>. A connection between quantum physics and gravity would be exciting to find, since all of physics rests on these two theories, and their unified description – the so-called <a href="https://www.pbs.org/faithandreason/intro/purpotoe-frame.html">Theory of Everything</a> – is one of the grand goals of modern science.</p>
<h2>Enter the optical tweezer</h2>
<p>Optical tweezers exploit the fact that light can exert pressure on matter. Although the radiation pressure from even an intense laser beam is quite small, Ashkin was the first person to show that it was large enough to support a nanoparticle, countering gravity, effectively levitating it. </p>
<p>In <a href="http://www.pnas.org/content/107/3/1005">2010</a> a group of researchers realized that such a nanoparticle held by an optic tweezer was well-isolated from its environment, since it was not in contact with any material support. Following these ideas, several groups suggested <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.107.020405">ways</a> to create and observe superpositions of a nanoparticle at two distinct spatial locations.</p>
<p>An intriguing scheme proposed by the groups of <a href="http://www.physics.purdue.edu/people/faculty/tcli.php">Tongcang Li</a> and <a href="http://www-personal.umich.edu/%7Elmduan/">Lu Ming Duan</a> in <a href="https://doi.org/10.1103/PhysRevA.88.033614">2013</a> involved a nanodiamond crystal in a tweezer. The nanoparticle does not sit still within the tweezer. Rather, it oscillates like a pendulum between two locations, with the restoring force coming from the radiation pressure due to the laser. Further, this diamond nanocrystal contains a contaminating nitrogen atom, which can be thought of as a tiny magnet, with a north (N) pole and a south (S) pole. </p>
<p>The Li-Duan strategy consisted of three steps. First, they proposed cooling the motion of the nanoparticle to its quantum ground state. This is the lowest energy state this type of particle can have. We might expect that in this state the particle stops moving around and does not oscillate at all. However, if that happened, we would know where the particle was (at the center of the tweezer), as well how fast it was moving (not at all). But simultaneous perfect knowledge of both position and speed is not allowed by the famous <a href="https://www.aps.org/publications/apsnews/200802/physicshistory.cfm">Heisenberg uncertainty principle</a> of quantum physics. Thus, even in its lowest energy state, the particle moves around a little bit, just enough to satisfy the laws of quantum mechanics. </p>
<p>Second, the Li and Duan scheme required the magnetic nitrogen atom to be prepared in a superposition of its north pole pointing up as well as down. </p>
<p>Finally, a magnetic field was needed to link the nitrogen atom to the motion of the levitated diamond crystal. This would transfer the magnetic superposition of the atom to the location superposition of the nanocrystal. This transfer is enabled by the fact that the atom and the nanoparticle are <a href="https://www.quantamagazine.org/real-life-schrodingers-cats-probe-the-boundary-of-the-quantum-world-20180625/">entangled</a> by the magnetic field. It occurs in the same way that the superposition of the decayed and not-decayed radioactive sample is converted to the superposition of <a href="https://www.youtube.com/watch?v=OkVpMAbNOAo">Schrodinger’s cat</a> in dead and alive states. </p>
<h2>Proving the collapse theory</h2>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/243951/original/file-20181105-83654-zxblrz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/243951/original/file-20181105-83654-zxblrz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=404&fit=crop&dpr=1 600w, https://images.theconversation.com/files/243951/original/file-20181105-83654-zxblrz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=404&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/243951/original/file-20181105-83654-zxblrz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=404&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/243951/original/file-20181105-83654-zxblrz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=507&fit=crop&dpr=1 754w, https://images.theconversation.com/files/243951/original/file-20181105-83654-zxblrz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=507&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/243951/original/file-20181105-83654-zxblrz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=507&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Collapse of the superposition into a single location.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/hypnotising-watch-on-chain-swinging-above-397393039?src=hYdMjeVlMqihc9BE2LxKeQ-1-97">DreamcatcherDiana / Shutterstock.com</a></span>
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<p>What gave this theoretical work teeth were two exciting experimental developments. Already in <a href="https://doi.org/10.1103/PhysRevLett.109.103603">2012</a> the groups of <a href="https://www.photonics.ethz.ch/en/no_cache/general-information/people/professor.html">Lukas Novotny</a> and <a href="https://www.icfo.eu/lang/research/groups/member-details?gid=27&people_id=33">Romain Quidant</a> showed that it was possible to cool an optically levitated nanoparticle to a hundredth of a degree above absolute zero by modulating the intensity of the optical tweezer. The effect was the same as that of slowing a child on a swing by pushing at the right times. </p>
<p>In <a href="https://doi.org/10.1103/PhysRevLett.116.243601">2016</a> the same researchers were able to cool to a ten-thousandth of a degree above absolute zero. Around this time <a href="https://www.rit.edu/science/people/mishkat-bhattacharya">our</a> groups <a href="https://doi.org/10.1364/OPTICA.3.000318">published</a> a paper establishing that the temperature required for reaching the quantum ground state of a tweezed nanoparticle was around a millionth of a degree above absolute zero. This requirement is challenging, but within reach of ongoing experiments. </p>
<p>The second exciting development was the experimental levitation of a nitrogen-defect-carrying nanodiamond in <a href="https://doi.org/10.1038/nphoton.2015.162">2014</a> in <a href="http://www.hajim.rochester.edu/optics/people/faculty/vamivakas_nick/index.html">Nick Vamivakas’s</a> group. Using a magnetic field, they were also able to achieve the physical coupling of the nitrogen atom and the crystal motion required by the third step of the Li-Duan scheme.</p>
<p>The race is now on to reach the ground state so that – according to the Li-Duan plan – an object at two locations can be observed collapsing into a single entity. If the superpositions are destroyed at the rate predicted by the collapse theories, quantum mechanics as we know it will have to be revised.</p><img src="https://counter.theconversation.com/content/105563/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Mishkat Bhattacharya receives funding from the Office of Naval Research, United States. </span></em></p><p class="fine-print"><em><span>Nick Vamivakas receives funding from the Office of Naval Research, United States. </span></em></p>
The discovery and development of optical tweezers won the 2018 Nobel Prize in physics. Now physicists are using this tool to crack some of the fundamental questions behind how the universe works.
Mishkat Bhattacharya, Associate Professor of Physics and Astronomy, Rochester Institute of Technology
Nick Vamivakas, Associate Professor of Quantum Optics & Quantum Physics, University of Rochester
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/104282
2018-10-03T05:14:24Z
2018-10-03T05:14:24Z
Arthur Ashkin’s optical tweezers: the Nobel Prize-winning technology that changed biology
<figure><img src="https://images.theconversation.com/files/238983/original/file-20181002-85608-q4o4py.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-vector/abstract-red-laser-beam-isolated-on-631003721?src=ynKx9mYkRVMRbxz64yP0sg-1-5">Maryna Stamatova/Shutterstock</a></span></figcaption></figure><p>The 2018 Nobel Prize in Physics has been awarded to three pioneers of the laser technology that has made a big impact on the world. Gérard Mourou and Donna Strickland were recognised for their method of generating high-intensity, ultra-short optical pulses, which today is used in laser eye surgery. The other recipient was Arthur Ashkin for his groundbreaking work on optical tweezers. This method of using light to capture and manipulate tiny objects has changed the way we’re able to study microscopic life. </p>
<p>But how can light be used to move matter? The energy carried by light is fundamental to life on our planet. But as well as energy, light beams also have a momentum, which is called <a href="https://phys.org/news/2018-08-momentum-year-mystery.html">radiation pressure</a>. This means that if I shine a laser pointer at you, in addition to making you ever so slightly hotter, it will push you away with a very small force.</p>
<p>To use this force to lift something as big as, say, an apple would be almost impossible. The laser power required would run to many megawatts, probably enough to vaporise the apple before it got off the ground. But when an object gets ten times smaller in each direction it also gets 1,000 times lighter. So moving from something the size of an apple to a single cell means that the laser power needed to lift it falls from megawatts to milliwatts, a similar power to that of a laser pointer.</p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/238998/original/file-20181002-85620-rx6l89.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/238998/original/file-20181002-85620-rx6l89.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=900&fit=crop&dpr=1 600w, https://images.theconversation.com/files/238998/original/file-20181002-85620-rx6l89.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=900&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/238998/original/file-20181002-85620-rx6l89.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=900&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/238998/original/file-20181002-85620-rx6l89.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1131&fit=crop&dpr=1 754w, https://images.theconversation.com/files/238998/original/file-20181002-85620-rx6l89.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1131&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/238998/original/file-20181002-85620-rx6l89.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1131&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Arthur Ashkin.</span>
<span class="attribution"><span class="source">Nobel Foundation</span></span>
</figcaption>
</figure>
<p>As long ago as 1970, Ashkin (working at the world famous Bell Telephone Laboratories) began studying how you could use radiation pressure to <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.24.156">accelerate and trap</a> individual particles. Over the next 15 years he refined his ideas, brilliantly making the laser systems involved ever less complicated as time went on.</p>
<p>In 1986, working with Steven Chu (who later won his own Nobel Prize in Physics in 1997 for work on trapping atoms and ultimately became US secretary for energy) he published his <a href="https://www.osapublishing.org/ol/abstract.cfm?uri=ol-11-5-288">seminal paper</a> on what we now call optical tweezers. In this paper, Ashkin showed that if the laser beam was focused very tightly using a microscope then, rather than pushing objects away with radiation pressure, it would counter-intuitively attract particles towards it. When the laser beam was then moved, the particles would follow it, held in the focus of the beam at all times. </p>
<p>Since then, optical tweezers have been used by many physicists and engineers, who have extended the technique so that it can <a href="https://www.sciencedirect.com/science/article/abs/pii/S0030401807008784">trap many particles at once</a> and even transform the tweezers into <a href="https://link.springer.com/article/10.1023/A:1006911428303">optical spanners</a> that cause the objects to spin. This later area is one of my own research interests and I remember, as a young researcher, the thrill of Ashkin asking me for a copy of my talk at a conference.</p>
<h2>Impact in biology</h2>
<p>Perhaps the greatest impact of optical tweezers has been in biophysics. Optical tweezers can be used to <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2408388/">sort healthy cells</a> from infected ones, or identify those that <a href="https://www.nature.com/articles/s41598-017-13205-6">might be cancerous</a>. It is also possible to use optical tweezers to measure both the <a href="https://arxiv.org/abs/1507.05321">minute movements</a> of a trapped object (equivalent to a few atoms in diameter) and <a href="https://link.springer.com/chapter/10.1007/978-3-642-02525-9_32">similarly tiny forces</a>. </p>
<p>Turning optical tweezers from a manipulation tool into a measurement device has allowed biologists to study the workings of the <a href="https://pubs.acs.org/doi/full/10.1021/acs.chemrev.6b00638">individual molecular motors</a> which are responsible for movement in the biological world. Such motors transport chemicals within molecules, allow cells to swim and, when acting collectively, allow whole creatures to move.</p>
<p>Ashkin showed us all just what can be done by having an idea and then seeing it through to completion. For years he worked in a minority field, pioneering and then refining his ideas inventing techniques that scientists now use as as essential tools of their trade - thank you Arthur.</p><img src="https://counter.theconversation.com/content/104282/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Miles Padgett receives funding from the Engineering and Physical Sciences Research Council and the European Union
Miles Padgett is employed by the University of Glasgow</span></em></p>
Using lasers to trap and move particles changed the way we’re able to study microscopic life.
Miles Padgett, Kelvin Chair of Natural Philosophy (Physics and Astronomy), University of Glasgow
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/104274
2018-10-02T20:56:33Z
2018-10-02T20:56:33Z
2018 Nobel Prize for physics goes to tools made from light beams – a particle physicist explains
<figure><img src="https://images.theconversation.com/files/239067/original/file-20181003-695-1082hzo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The 2018 Nobel Prize for physics recognized discoveries that can make more powerful lasers.</span> </figcaption></figure><figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/239027/original/file-20181002-101582-dsaoih.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/239027/original/file-20181002-101582-dsaoih.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=900&fit=crop&dpr=1 600w, https://images.theconversation.com/files/239027/original/file-20181002-101582-dsaoih.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=900&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/239027/original/file-20181002-101582-dsaoih.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=900&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/239027/original/file-20181002-101582-dsaoih.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1131&fit=crop&dpr=1 754w, https://images.theconversation.com/files/239027/original/file-20181002-101582-dsaoih.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1131&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/239027/original/file-20181002-101582-dsaoih.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1131&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Arthur Ashkin.</span>
<span class="attribution"><a class="source" href="http://www.Nobelprize.org">Niklas Elmehed. © Nobel Media</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/239029/original/file-20181002-101558-4607n7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/239029/original/file-20181002-101558-4607n7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=900&fit=crop&dpr=1 600w, https://images.theconversation.com/files/239029/original/file-20181002-101558-4607n7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=900&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/239029/original/file-20181002-101558-4607n7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=900&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/239029/original/file-20181002-101558-4607n7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1131&fit=crop&dpr=1 754w, https://images.theconversation.com/files/239029/original/file-20181002-101558-4607n7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1131&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/239029/original/file-20181002-101558-4607n7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1131&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Gerard Mourou.</span>
<span class="attribution"><a class="source" href="http://www.Nobelprize.org">Niklas Elmehed. © Nobel Media</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Our world is full of light, and we depend upon it to power life on our planet. So it is appropriate to honor three scientists who invented new ways of using light rays to explore our world.</p>
<p><a href="https://www.nobelprize.org/prizes/physics/2018/summary/">The 2018 Nobel Prize in physics was awarded to Arthur Ashkin, Gérard Mourou and Donna Strickland</a> for developing tools made from light beams. <a href="https://history.aip.org/phn/11409018.html">Ashkin</a> won half of the prize for his work on optical tweezers, which are beams of light that can actually manipulate tiny objects like cells or atoms, while <a href="https://www.polytechnique.edu/annuaire/en/users/gerard.mourou">Mourou</a> and <a href="https://uwaterloo.ca/physics-astronomy/people-profiles/donna-strickland">Strickland</a> won the other half for creating technology that generates high-intensity, ultra-short laser pulses, which are used for eye surgeries, material sciences, studies of very fast processes and plasma physics, among others. </p>
<p>Alfred Nobel specified in his will that the physics prize should be awarded for <a href="https://www.nobelprize.org/prizes/physics/">“the most important discovery or invention within the field of physics,”</a> so as a physicist I think he’d be pleased that this year’s award recognizes inventions made in the 1970s and 1980s that have led to practical applications that benefit mankind. </p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/239031/original/file-20181002-101585-1oz9yex.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/239031/original/file-20181002-101585-1oz9yex.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=900&fit=crop&dpr=1 600w, https://images.theconversation.com/files/239031/original/file-20181002-101585-1oz9yex.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=900&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/239031/original/file-20181002-101585-1oz9yex.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=900&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/239031/original/file-20181002-101585-1oz9yex.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1131&fit=crop&dpr=1 754w, https://images.theconversation.com/files/239031/original/file-20181002-101585-1oz9yex.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1131&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/239031/original/file-20181002-101585-1oz9yex.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1131&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Donna Strickland.</span>
<span class="attribution"><a class="source" href="http://www.Nobelprize.org">Niklas Elmehed. © Nobel Media</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Donna Strickland is only the third woman to win the Nobel Prize in physics, out of 210 recipients, and the first since 1963. <a href="https://www.nobelprize.org/prizes/physics/1903/summary/">Marie Curie was the first, in 1903</a>; she won another one in <a href="https://www.nobelprize.org/prizes/chemistry/1911/summary/">1911 for chemistry</a>. <a href="https://www.nobelprize.org/prizes/physics/1963/summary/">Maria Goeppert-Mayer was the second</a>. Hopefully in the future the Nobel Prize committee can lower the average of 60 years between women laureates being named. </p>
<h2>What are optical tweezers?</h2>
<p>Using light to manipulate our world has become very important in science and medicine over the past several decades. This year’s physics Nobel recognizes the invention of tools that have facilitated advances in many fields. Optical tweezers use light to hold tiny objects in place or measure their movement. It may seem odd that light can actually hold something in place, but it has been well-known for more than a century that <a href="https://en.wikipedia.org/wiki/Radiation_pressure">light can apply a force on physical objects through what is known as radiation pressure</a>. In 1969, Arthur Ashkin used lasers <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.24.156">to trap and accelerate micron sized objects</a> such as tiny spheres and water droplets. This led to the invention of optical tweezers that use two or more focused laser beams aimed in opposite directions to attract a target particle or cell toward the center of the beams and hold it in place. Each time the particle moves away from the center, it encounters a force pushing it back toward the center.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/239010/original/file-20181002-85608-1k4vl5k.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/239010/original/file-20181002-85608-1k4vl5k.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=427&fit=crop&dpr=1 600w, https://images.theconversation.com/files/239010/original/file-20181002-85608-1k4vl5k.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=427&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/239010/original/file-20181002-85608-1k4vl5k.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=427&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/239010/original/file-20181002-85608-1k4vl5k.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=537&fit=crop&dpr=1 754w, https://images.theconversation.com/files/239010/original/file-20181002-85608-1k4vl5k.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=537&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/239010/original/file-20181002-85608-1k4vl5k.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=537&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The Optical Cell Rotator uses laser beams from optical fibers to hold living cells in place. The beams can be used to rotate the cells for detailed imaging.</span>
</figcaption>
</figure>
<p><a href="https://www.nobelprize.org/prizes/physics/1997/summary/">Steven Chu, Claude Cohen-Tannoudji and William D. Phillips won the 1997 Nobel Prize in physics</a> for development of laser cooling traps, known as optical traps, that hold atoms within a confined space. <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.57.314">Askhin and Chu worked together at Bell Laboratories in the 1980s</a> laying the foundation for work on optical traps. While Chu continued work with neutral atoms, Ashkin pursued larger, biological targets. <a href="http://science.sciencemag.org/content/235/4795/1517">In 1987, Ashkin used optical tweezers to examine an individual bacterium</a> – without harming the microbe. Now optical tweezers are routinely used in studies of molecules and cells.</p>
<p>Ashkin earned his bachelor’s degree from Columbia University and his Ph.D. from Cornell. He started at Bell Laboratories in 1952 and retired in 1992. But he assembled a home laboratory to continue his scientific investigations. He has been awarded more than 45 patents.</p>
<h2>Why are fast laser pulses important?</h2>
<p>Gerard Mourou and Donna Strickland worked together at the University of Rochester, where they developed the technique called <a href="https://www.sciencedirect.com/science/article/abs/pii/0030401885901208">chirped pulse amplification for laser light</a>. Strickland was a graduate student and Mourou was her thesis advisor in the mid-1980s. At the time, progress on creating brighter lasers had slowed. Stronger lasers tended to damage themselves. Strickland and Mourou invented a way to create more intense light, but in short pulses. </p>
<p>You are probably most familiar with laser pointers or barcode scanners, which are just some of the ways we use lasers in everyday life. But these are relatively low-intensity lasers. Many scientific applications need much stronger ones. </p>
<p>To solve this problem, Mourou and Strickland used lasers with very short (ultrashort) pulses – quick bursts of light separated in time. With chirped pulse amplification, the pulses are stretched in time, making them longer and less intense, and then the pulses are amplified up to a million times. When these pulses are compressed again (through reversing the process used to stretch), the pulses are much more intense than can be created without the chirped pulse amplification technique. As an analogy, consider a thick rubber band. When the band is stretched, the rubber becomes thinner. When it is released, it returns to its original thickness. Now imagine that there is a way to make the stretched rubber band thicker. When the band is released, it will end up thicker than than the original band. This is essentially what happens with the laser pulse.</p>
<p>There are a variety of ways the stretching and amplification can be done, but nearly all of the highest-power lasers in the world use some variation of this technique. Since the invention of chirped pulse amplification, the maximum intensity of new lasers has continued a dramatic rise.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/239023/original/file-20181002-101585-jit8r9.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/239023/original/file-20181002-101585-jit8r9.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/239023/original/file-20181002-101585-jit8r9.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=296&fit=crop&dpr=1 600w, https://images.theconversation.com/files/239023/original/file-20181002-101585-jit8r9.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=296&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/239023/original/file-20181002-101585-jit8r9.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=296&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/239023/original/file-20181002-101585-jit8r9.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=372&fit=crop&dpr=1 754w, https://images.theconversation.com/files/239023/original/file-20181002-101585-jit8r9.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=372&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/239023/original/file-20181002-101585-jit8r9.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=372&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The chirped pulse amplification technique creates extremely intense pulses of light by stretching in time short pulses of light before amplifying them up to a million times. When the pulse is compressed again, it results in pulses that are a million times more intense than the original light.</span>
<span class="attribution"><a class="source" href="https://www.nobelprize.org/uploads/2018/10/popular-physicsprize2018.pdf">NobelPrize.org</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>In my own field of particle physics, chirped pulse amplification-based lasers are used <a href="http://science.sciencemag.org/content/312/5772/374.full">to accelerate beams of particles</a>, possibly providing a path to greater acceleration in a shorter distance. This could lead to lower-cost, high-energy accelerators that can push the bounds of particle physics – enabling us to detect evermore elusive particles and gain a better understanding of the universe. </p>
<p>But not all particle accelerators are behemoths like the Large Hadron Collider, which has a circumference of 17 miles. There are some 30,000 industrial particle accelerators worldwide that are used closer to home for material preparation, cancer treatment and medical research. Mourou and Strickland’s work may be used to shrink the size of these accelerators making them smaller and cheaper. </p>
<p>Ultrafast, high-intensity lasers are also now being <a href="http://spie.org/newsroom/2509-ultrashort-pulse-laser-eye-surgery-uses-fiber-technology-at-16-microns">used in eye surgery</a>. It can be used to treat the cornea (surface of the eye) to improve vision in some patients. The chirped pulse amplification invention is also used in attosecond science for studying ultrafast processes. An attosecond is one million trillionth of a second. By having lasers that produce pulses every attosecond, we can get a snapshots of extremely fast processes such as atoms losing an electron (ionizing) and then recapturing it.</p>
<p>The Nobel Prize-winning work was the basis for Strickland’s Ph.D. thesis from the University of Rochester. Dr. Strickland is now an associate professor at the University of Waterloo in Canada. Mourou became the founding director of the Center for Ultrafast Optical Science at the University of Michigan in 1990. He later became director of the Laboratorie d’Optique de Applique in France.</p>
<p>The 2018 Nobel Prize in physics shines a light on the pioneering work of these three scientists. Over the past three decades, their inventions have created avenues of science and medical treatments that were previously unattainable. It is certain that we will continue to benefit from their work for a long time.</p><img src="https://counter.theconversation.com/content/104274/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Todd Adams receives funding from the U.S. Department of Energy. </span></em></p>
The Nobel Prize for physics was awarded to three scientists for the inventions of optical tweezers – in which two laser beams can hold a tiny object – and a method for creating powerful lasers.
Todd Adams, Professor of Physics, Florida State University
Licensed as Creative Commons – attribution, no derivatives.