It turns out light can not only be twisted, but at different speeds.
- An unsuspected property of light, called "self-torque," had just been discovered.
- The discovery will allow scientists to control the behavior of light in a new way.
- The potential applications are still being worked out, but look very exciting.
First, the history of orbital angular momentum<img type="lazy-image" data-runner-src="https://assets.rebelmouse.io/eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpbWFnZSI6Imh0dHBzOi8vYXNzZXRzLnJibC5tcy8xOTY0MjUwOC9vcmlnaW4uanBnIiwiZXhwaXJlc19hdCI6MTYyMjE3OTQzOH0.sysab1kwPtdLP-AqRdWw__tg-I_Dy9i3U9QStZk9OE8/img.jpg?width=980" id="30a93" class="rm-shortcode" data-rm-shortcode-id="5300d6a0ce7ca74e78e21cafe4c20455" data-rm-shortcode-name="rebelmouse-image" />
Orbital angular momentum in a light beam and a particle within it. Image source: E-karimi / Wikimedia Commons<p>Twisted light beams have to do with a property called "orbital angular momentum" (OAM). It's a subset of angular momentum. Imagine an object attached to a string swinging round and around a pole to which the string is connected — the force with which it goes around the pole is its angular momentum. Technically, it's calculated in the other direction, if you will: It's the measurement of the amount of force it would take to stop the object from circling the pole.</p><p>In 1932, scientists realized that a perpendicular cross-section of a light wave revealed oscillating mini-waves within it. While typically these mini-waves oscillate together, that's not always the case. In some light beams, researchers found mini-waves out of phase with each other and rotating around the larger beam's center. A particle hit by such a beam of light will orbit that center like a planet orbiting a star. Hence "orbital angle momentum." At the time, these weird light waves were considered to be organically produced by oddly behaving electrons spinning around nuclei.</p><p>In the 1970s, lasers allowed the creation of "vortex beams," with "vortex" here meaning a hole in the middle of a light beam. Now we know that it's not really a hole, but rather an area where out-of-phase mini-waves overlap and cancel each other out as they spin around the center of a beam. Though it wasn't realized at the time, what the scientists were seeing was a manifestation of OAM.</p><p>In 1991, physicist Robert Spreeuw in Han Woerdman's lab at Leiden University in the Netherlands began dreaming up ways to deliberately create light beams with OAM. He presented his ideas to his team during a coffee break. "The first reactions were a bit skeptical," Spreeuw <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5984548/" target="_blank">says</a>. "But we kept thinking about it and, bit by bit, it started to look more realistic."</p><p>In 1992, Woerdman, working with colleague Les Allen, successfully twisted light and demonstrated how a photon within it would share the beam's OAM. In 1993, they published their technique of sending a light beam through a lens shaped like a seashell to produce twisted light.</p><p>In such a beam, mini-waves rotate around the center of the beam as a <a href="https://www.merriam-webster.com/dictionary/helix" target="_blank">helix</a>. If you shine the beam onto a table, or make a perpendicular cross-section, it looks like a donut: Light around a seemingly empty center.</p><p>Since then, twisted light beams have proven extremely useful as optical tweezers with which microscopic particles can be captured and manipulated. In the area of communications, they've enabled higher data rates by allowing the manipulation of light characteristics such as color, intensity, and polarization. They also may make possible finer-grained medical diagnostic tools, the stimulation of atoms and molecules into exotic states, and controllers for micro- and non-scale machinery.</p>
Enter self-torque<iframe width="560" height="315" src="https://www.youtube.com/embed/3PGoEI5go5c" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe><p>The researchers behind the new discovery had been combining pairs of waves with the same OAM by firing them into a cloud of argon gas, from whence they emerged as a single twisted beam, having overlapped and merged within the cloud. The scientists started wondering what would happen if they tried the same thing with two donut beams that had different OAMs and that were out of sync with each other by a few quadrillionths of a second.</p><p>The resulting beam was something surprising and unpredicted. It corkscrewed around its center, more tightly — and so, faster — at one end than the other. A photon at the front of the beam would actually be traveling slower than one at the back. The conclusion was that not only did the light beams have OAM that allowed them to twist, but that the application of one to another in the right way produced a force that could affect the speed of the waves' twisting — they named that force "self-torque," as a previously unsuspected type of push that can alter the speed at which light waves twist.</p><p>Cross-sectioned or shined on a flat surface, a beam with self-torque looks like a French croissant instead of a donut. One of the scientists, Kevin Dorney, muses to <a href="https://www.nationalgeographic.com/science/2019/06/physicists-discover-croissant-shaped-twists-of-light-new-property-optics/" target="_blank"><em>National Geographic</em></a>, "You wouldn't expect from adding donuts that you would get a croissant."</p><p>Twisted light, already so useful in so many ways, just gained a new level of malleability.</p>
Lasers could cut lifespan of nuclear waste from "a million years to 30 minutes," says Nobel laureate
Physicist plans to karate-chop them with super-fast blasts of light.
- Gérard Mourou has already won a Nobel for his work with fast laser pulses.
- If he gets pulses 10,000 times faster, he says he can modify waste on an atomic level.
- If no solution is found, we're already stuck with some 22,000 cubic meters of long-lasting hazardous waste.
Who is Gérard Mourou?<iframe width="560" height="315" src="https://www.youtube.com/embed/W5Fz_BsWCjU" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe><p>Mourou was the co-recipient of his Nobel with <a href="https://www.nobelprize.org/prizes/physics/2018/strickland/facts/" target="_blank">Donna Strickland</a> for their development of <a href="https://physics.stackexchange.com/questions/432137/what-is-chirped-pulse-amplification-and-why-is-it-important-enough-to-warrant-a" target="_blank">Chirped Pulse Amplification</a> (CPA) at the University of Rochester. In his speech, he referred to his "passion for extreme light."</p><p>CPA produces high-intensity, super-short optical pulses that pack a tremendous amount of power. Mourou's and Strickland's goal was to develop a means of making highly accurate cuts useful in medical and industrial settings.</p><p>It turns out CPA has another benefit, too, that's just as important. Its <a href="https://simple.m.wikipedia.org/wiki/Attosecond" target="_blank">attosecond</a> pulses are so quick that they shine a light on otherwise non-observable, ultra-fast events such as those inside individual atoms and in chemical reactions. This capability is what Mourou hopes give CPA a chance of neutralizing nuclear waste, and he's actively working out a way to make this happen in conjunction with <a href="https://www.physics.uci.edu/people/toshiki-tajima" target="_blank">Toshiki Tajima</a> of UC Irvine. As Mourou explains to <em>The Conversation</em>:</p><p style="margin-left: 20px;"><em>"Take the nucleus of an atom. It is made up of protons and neutrons. If we add or take away a neutron, it changes absolutely everything. It is no longer the same atom, and its properties will completely change. The lifespan of nuclear waste is fundamentally changed, and we could cut this from a million years to 30 minutes!</em></p><p style="margin-left: 20px;"><em>We are already able to irradiate large quantities of material in one go with a high-power laser, so the technique is perfectly applicable and, in theory, nothing prevents us from scaling it up to an industrial level. This is the project that I am launching in partnership with the </em><a href="http://www.cea.fr" target="_blank"><em>Alternative Energies and Atomic Energy Commission</em></a><em>, or CEA, in France. We think that in 10 or 15 years' time we will have something we can demonstrate. This is what really allows me to dream, thinking of all the future applications of our invention."</em></p><p>While 15 years may seem a long time, when you're dealing with the half-life of nuclear waste, it's a blink of an eye.</p>
Nuclear waste in Europe<p>Although nuclear energy struggles for acceptance as an energy source in the U.S. after a series of disturbing incidents and the emergence of alternative sources such as solar and wind energy, many European nations have embraced it. France is chief among them, relying on nuclear energy for 71% of its energy needs. Ukraine is the next most dependent on it, for 56% of its power, followed closely by Slovakia, then Belgium, Hungary, Sweden, Slovenia, and the Czech Republic, according to <a href="https://www.bloomberg.com/graphics/2019-nuclear-waste-storage-france/" target="_blank">Bloomberg</a>. None of them have a good plan for nuclear waste, other than storing it somewhere in hopes of an eventual solution or thousands of trouble-fee years during which it stays put and doesn't escape into water supplies or the air.</p><p>And there's a lot of this stuff. Greenpeace <a href="http://www.nuclear-transparency-watch.eu/documentation/relevant-studies/new-report-by-greenpeace-the-global-crisis-of-nuclear-waste.html" target="_blank">estimates</a> there are roughly 250,000 tons of it in 14 countries across the world. Of that, about 22,000 cube meters is hazardous. The cost of storing it all, according to <a href="https://nuclear.gepower.com/company-info/nuclear-power-basics" target="_blank">GE-Hitachi,</a> is more than $100 billion, (discounting China, Russia, and India).</p>
Transmuting the nuclear waste problem<img type="lazy-image" data-runner-src="https://assets.rebelmouse.io/eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpbWFnZSI6Imh0dHBzOi8vYXNzZXRzLnJibC5tcy8xOTMzOTg2My9vcmlnaW4uanBnIiwiZXhwaXJlc19hdCI6MTY0NzA0MDQ4MX0.KJsmsT0yqGsA23AXEf4Xt8xQwgICcsba4wS4-vZ2JnI/img.jpg?width=980" id="45911" class="rm-shortcode" data-rm-shortcode-id="3ec4bd44e92c57eeca7e8eb61eb1af10" data-rm-shortcode-name="rebelmouse-image" />
([general-fmv]/Shutterstock)<p>The process Mourou is investigating is called "transmutation." "Nuclear energy is maybe the best candidate for the future," he told the Nobel audience, "but we are still left with a lot of dangerous junk. The idea is to transmute this nuclear waste into new forms of atoms which don't have the problem of radioactivity. What you have to do is to change the makeup of the nucleus." After his speech he phrase his plans for lasers and waste more plainly: "It's like karate — you deliver a very strong force in a very, very brief moment."</p><p>The idea of transmutation's not new. It's been under investigation for 30 years in the U.K., Belgium, Germany, Japan, and the U.S. Some of these efforts are ongoing. Others have been given up. Rodney C. Ewing of Stanford tells Bloomberg, "I can imagine that the physics might work, but the transmutation of high-level nuclear waste requires a number of challenging steps, such as the separation of individual radionuclides, the fabrication of targets on a large scale, and finally, their irradiation and disposal."</p><p>Mourou and Tajima hope to be able to shrink the distance a light beam has to travel to transmute atoms by a further 10,000 times. "I think about what it could mean all the time," Mourou says at Ecole Polytechnique, where he teaches. "I don't overlook the difficulties that lie ahead. I dream of the idea, but we will have to wait and see what happens in the years to come."</p>