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Thermal diodes bridge the gap

Electronic devices are hard to keep cool. Mechanical fans suck up energy. Heat sinks add unwanted bulk and mass. And thermal diodes – devices that allow heat to flow more easily in one direction than the other – are fragile, inefficient and require gravity to operate. Now, however, a new type of thermal diode looks set to overcome some of these drawbacks, with developers at Virginia Tech in the US suggesting that a future commercialized version could help manage heat in computer chips and aircraft components.

The directional thermal effect was first observed in the 1930s, when physicist Chauncey Starr of the Rensselaer Polytechnic Institute in New York developed a thermal diode based on a copper-cuprous oxide interface. The new device, dubbed a “bridging-droplet” diode, consists of two opposing copper plates separated by an insulating micron-thick gasket.

The surface of the first plate has a wick-like structure consisting of micropillars that hold and conduct water. The second plate, meanwhile, is coated with a water-repelling layer. In the diode’s forward operation mode, water on the wicking plate absorbs heat from its surroundings and evaporates into steam. The steam propagates across the narrow gap between the plate and condenses into dew-like droplets on the hydrophobic plate. When the droplets grow large enough to bridge the gap, they get sucked back into the wick structure, starting the process anew.

A “diodicity” as high as 85

During the device’s reverse mode of operation, the situation is different. In this case, the heat source is now on the hydrophobic plate, but no steam can be produced because the water remains in the wicking structure on the other plate. This transfer of water, explains team leader Jonathan Boreyko, is what allows the device to conduct heat unidirectionally. The new device can also be used upright, sideways or even upside-down and would thus work just as well in space.

The researchers measured the ratio of heat transferred from the wick side to the hydrophobic side – a quantity known as “diodicity” – to be as high as 85. While assymmetric heat pipes offer an even larger diodicity of around 100, their 1D heat transfer is ineffective for large 3D systems, Boreyko says. Placing an array of directional heat pipes into a wall panel solves this problem, but this is both complex and decreases the effective diodicity by more than a factor of 10, he explains.

Boreyko notes that the water-repellent coating he and his colleagues used to create their test device (a mixture of 1-hexadecanethiol in ethanol) is not suitable for practical applications. However, he says this coating could be replaced with more durable alternatives such as graphene or grafted polymers. This gives the Virginia Tech device a potential advantage over a conceptually similar device known as a “jumping-droplet” thermal diode, which requires a fragile superhydrophobic nanostructure to operate.

Members of the team, who report their work in Advanced Functional Materials, have filed a provisional patent for their diode and are now looking to collaborate with industry partners to continue the research. One of the items on their to-do list is to boost the diodicity of their device to 100. This could be achieved, for example, by decreasing the height of the micropillars so that smaller droplets could bridge the insulating gap between the two copper plates. The researchers would also like to test more durable hydrophobic coatings.

CERN’s emissions equal to a large cruise liner, says report

Greenhouse-gas emissions emitted by the CERN particle-physics lab near Geneva in 2018 were 223 800 tonnes of carbon-dioxide equivalent – similar to the emissions from a large cruise liner. That is according to the lab’s first public Environment Report that details the status of CERN’s environmental footprint and outlines some objectives to reduce it in the coming years. The report finds that three quarters of these emissions came from the fluorinated gases used for particle detection and cooling of the particle detectors.

Covering the years 2017 and 2018, the report underlines the scale of the challenge that CERN faces to reduce its emissions. “It has provoked debate and increased the environmental awareness of all the people who work here as well as our user community, and made us think hard about what we do now and how we design the next generation of accelerators,” says Frédérick Bordry, CERN director for accelerators and technology.

It’s really positive that CERN staff are being transparent about their impacts and that they have set themselves an absolute reduction in emissions target, as opposed to a greenwash-style intensity metric

John Barrett

The report covers everything from noise and biodiversity to water use and radiation. Comparing all these aspects, reducing the use of fluorinated gases will have the greatest positive impact and the report sets out a path to do this such as repairing gas leaks in the LHC, optimizing gas re-circulation systems. The ultimate goal is to replace fluorinated gases in the detector cooling systems with carbon dioxide, whose global warming potential is a few thousand times lower. “When we built the Large Hadron Collider we didn’t appreciate the global-warming potential of these gases; our main concern was the ozone hole,” says Bordry. The facility has set itself an objective of reducing its direct greenhouse gas emissions by 28% by the end of 2024.

The report also sets out plans for tackling CERN’s indirect greenhouse emissions – those due to its small-city-sized appetite for electricity. “We are implementing energy-recovery systems at the LHC, and pioneering the use of superconductivity on a large scale, which could improve the efficiency of electricity distribution networks,” says Bordry. But until the LHC’s successor arrives – maybe in the 2040s or 2050s – there is a limit to how much CERN can reduce its environmental footprint and some will question whether probing the mysteries of the universe can justify such significant greenhouse emissions.

“It’s really positive that CERN staff are being transparent about their impacts and that they have set themselves an absolute reduction in emissions target, as opposed to a greenwash-style intensity metric,” says John Barrett, from the Sustainability Research Institute at the University of Leeds. “The advancement of scientific understanding is clearly important and you don’t get much bigger than CERN. Personally, I would prefer to spend our carbon budget on CERN than short high-impact flights for a drunken weekend in Prague.”

Curved toes point to sore feet, the chaos of the knuckleball

Most conventional running shoes have a “toe spring” – a gentle upward curve of the sole towards the tip of the shoe. While this makes stepping more comfortable and easier, Harvard University’s Daniel Lieberman, Oliver Hansen,  Freddy Sichting and Nicholas Holowka have found that a toe spring can weaken the foot’s ability to push off the ground. This, they say, could be associated with a range of foot problems including plantar fasciitis – a painful condition affecting the tissue that connects the heel to the toes.

“We think that what happens is that people are relying on their plantar fascia to do what muscles normally do,” explains Lieberman. “When you get weak muscles and the plantar fascia has to do more work, it’s not really evolved for that, and so it gets inflamed.”

In their experiment, 13 participants walked barefoot and in four pairs of custom-made sandals on a treadmill equipped with force plates and an infrared camera system. You can read more about the team’s research in “Your shoes were made for walking. And that may be the problem”.

Enigmatic pitch

“The knuckleball is perhaps the most enigmatic pitch in baseball,” is the opening line of the abstract of a paper by Nicholas Nelson and Eric Strauss at California State University in Chico. The knuckleball pitch involves the slow rotation of the ball as it speeds towards the batter. Because a baseball has prominent seams that disrupt airflow around the ball, the aerodynamics of the ball changes as it rotates, which causes its trajectory to change repeatedly during flight. This makes it very difficult for the batter to anticipate the motion of the ball and hit it effectively. Conversely, it is also a very difficult pitch to throw because tiny changes in technique can have huge effects on outcome. Indeed, some pitchers say that identically thrown knuckleballs can take very different trajectories.

In their paper, Nelson and Strauss develop a model of the knuckleball and show the motion of the ball is chaotic. The duo’s model predicts that the position of the ball when it reaches the batter can vary by as much as 1.2 m for typical initial conditions used by knuckleball pitchers. This variation can take the ball well outside the strike zone (where the pitch must be), which illustrates why the knuckleball can be a dangerous choice for a pitcher.

You can read more in their paper “Dynamical chaos in a simple model of a knuckleball”.

 

Astronomers plan huge neutrino observatory in the Pacific Ocean

Astrophysicists in Germany and North America have published plans to build the world’s larg­est neutrino telescope on the sea floor off the coast of Canada. The Pacific Ocean Neutrino Experiment (P-ONE) is designed to snare very-high-energy neutrinos generated by extreme events from beyond our galaxy.

Neutrino telescopes observe the Cerenkov radiation that is emitted when neutrinos passing through the Earth interact very occasionally with atomic nuclei resulting in the production of fast-moving secondary particles. Being uncharged and exceptionally inert, neutrinos can penetrate gas and dust as they travel through the universe, allowing astronomers in principle to identify the exceptionally energetic phenomena that generate them. Photons from such events, in contrast, are absorbed on their journey.

We are now on the verge of opening up neutrino astronomy

Elisa Resconi

The world’s largest neutrino tele­scope, known as IceCube, consists of dozens of strings of photomultiplier tubes suspended in holes drilled deep into the ice at the South Pole. Covering a volume of 1 km3, Ice­Cube made history in 2013 when it reported intercepting the first extra­galactic neutrinos. Four years later it then recorded an event that could be tied to a very distant, bright galactic nucleus known as a blazar, thanks to concurrent gamma-ray observations.

According to P-ONE head, Elisa Resconi at the University of Munich, IceCube’s 2017 result strictly speak­ing only constitutes “evidence” for the blazar source. To really claim a discovery and pinpoint the origin of other cosmic neutrinos, she argues, requires the construction of addi­tional neutrino observatories as well as the extension of IceCube. “We are now on the verge of opening up neutrino astronomy,” she says, “but if we base this process on just one telescope it could take a really long time, perhaps decades.”

Heading underwater

P-ONE will consist of seven groups of 10 detector strings creat­ing an instrument volume of about 3 km3. Being larger than IceCube, it will detect rarer, higher-energy neutrinos, and will be most sensi­tive at a few tens rather than a hand­ful of teraelectronvolts. It will also observe a different part of the sky, mainly capturing neutrinos from the southern hemisphere rather than the north. But there will be some over­lap between the two, says Resconi, potentially allowing independent observations of the same event.

The new facility will be located at a depth of about 2.6 km in the Cas­cadia Basin, some 200 km from the coast of British Columbia. As such, it will take advantage of pre-existing infrastructure – an 800 km-long loop of fibre-optic cable operated by the University of Victoria’s Ocean Net­works Canada that supplies power and ferries data to and from existing sea-floor instruments.

Having already confirmed that this site has the necessary optical prop­erties by sending down two initial strings of light emitters and sensors in 2018, the P-ONE collaboration are now planning to deploy a steel cable with addi­tional detectors to investigate the site – including spectrometers, lidars and a muon detector. The plan then, says Resconi, is to install the first part of the observatory – a ring containing seven 1 km-long strings – around the end of 2023. And if that succeeds, the researchers will then apply for the bulk of the $50–100m needed to complete the project, with personnel costs adding roughly $100m more.

Resconi hopes that the full obser­vatory will be installed and taking data by the end of the decade. But she describes this timeline as “very ambitious”. In addition to delays caused by the ongoing COVID- 19 pandemic, she says it will be a challenge to ensure that the detec­tors work as planned – given the huge pressures and the presence of salt and sea creatures, which together make the seabed such a harsh environment.

Indeed, scientists had already planned on operating a cubic-kilome­tre scale neutrino telescope known as KM3NeT on the floor of the Mediter­ranean Sea back in 2014, which was delayed to 2020. According to col­laboration member Feifei Huang, just two of the 230 strings due to be installed off the coast of southern Italy are so far in place, while another site in French waters currently has six out of a planned 115 strings running – with completion not foreseen until 2026 and 2024 respectively.

Resconi says that part of the problem with that project is a lack of specialist personnel, with the physicists essentially doing everything themselves – for example, their self-built junction boxes, which connect cables on the sea floor, having failed. She hopes that the experience acquired by Ocean Networks Canada will mean a similar fate can be avoided for P-ONE. With 30 or 40 people dedicated to laying cables in the ocean, she says that her team “can focus on the physics”.

Floating oil droplet contains hundreds of degenerate optical modes

Microscopic oil droplets held aloft with optical tweezers can contain more than 200 resonant optical modes of similar energies, creating “hyperdegeneracy” for the first time. That is the claim of researchers in Israel, Spain and the US, who say that their breakthrough could ultimately find application in high-speed optical communications, sensing, quantum data processing and even the creation of dynamic optical circuits.

When optical materials with a high refractive index are formed into certain symmetrical shapes — such as rings, cylinders or spheres —light can be repeatedly reflected around the inside of the material, much in the same way that sound waves pass around the inside edge of St Paul’s Cathedral’s famous “whispering gallery”. The circulating light undergoes constructive interference, forming discrete resonant modes – or so-called degenerate states – with similar energies.

The number of modes is dependent on the ratio between the light’s wavelength and the circumference of the resonator — meaning that, in theory, a spherical object with a circumference tens of microns in size could support hundreds of modes of either visible or near-infrared light. In practice, however, achieving such hyperdegeneracy has proven impossible with conventional fabrication techniques. This is because even a single stem supporting the sphere will reduce the object’s symmetry and thereby reduce the extent of the potential degeneracy.

Clean and unscratched

In a new study, however, mechanical engineer Tal Carmon of the Technion-Israel Institute of Technology and his colleagues have circumvented this issue by supporting a 10 micron spherical droplet of silicone oil within an optical tweezer, thereby removing the need for a disruptive structural support. In fact, the radiation pressure from the laser-based tweezers acts to almost completely preserve the spherical symmetry of the oil droplet – along with the potential for hyperdegeneracy. In addition, the researchers explain, the levitation keeps the surface of the microresonator clean and unscratched.

Writing in the journal Physical Review X ,they say “unlike solids, the liquid droplet does not contain any dislocation, inclinations, and thermally induced stresses, which are typical for solid resonators and reduce their quality”.

To reveal the modes, the team placed a tapered fibre close to the surface of the oil microsphere and passed near-infrared laser light into and out of the droplet by means of an evanescent coupling. In the resulting transmission spectrum, the team observed signals of more than 200 modes – the largest-recorded set of degenerate states to ever be measured. The modes did exhibit slight differences in energy; this was a product of the droplet not being perfectly spherical, but distorted slightly as a result of the pressure from the optical tweezers, the presence of the coupled fibre, and the effect of gravity.

Simulating atomic optics

The technology could have several practical applications, write Peking University physicists Qi-Tao Cao and Yun-Feng Xiao in a commentary on the Physical Review X paper. “As a mesoscopic analogue to a single atom, levitated microresonators could serve as well-controlled platforms for simulating atomic optics,” they explain. Other potential applications could lie in using single-photon versions of the resonators as qubits for quantum processing, the creation of malleable circuits using multiple droplets, and high-capacity optical communications through the use of different modes to form densely packed information channels.

Cao and Xiao also point out that the microspheres could be used in existing sensing applications. “The frequency of hyperdegenerate modes is extremely sensitive to external perturbation, and even a tiny [such disturbance] – such as a biomolecule near the resonator surface — could lead to measurable modulation [of the light modes]”.

Dmitry Skryabin at the University of Bath adds, “The extremely high degeneracy and ability to manipulate it for fundamental studies of many coupled oscillators in linear, nonlinear and quantum regimes links these results to many cross-area ideas”. “Ultra-high finesses and near degeneracies in resonators also link to the cross-disciplinary concept of Arnold tongues and oscillator synchronization in the context of frequency comb research.”

Microswimmers benefit from thermoelectric guidance

Microscopic devices made from so-called Janus particles can be made to “swim” through liquid with the help of light-induced thermoelectric fields. The devices, which can travel 100μm along a straight course in 39 seconds, might find applications in biomedical sensing and the targeted, non-invasive delivery of drugs, according to developers at the University of Texas at Austin, US.

Janus particles – named for the famously two-faced Roman god of beginnings and transitions – are tiny spheres coated with different materials on each side. With the right choice of coatings, such particles will act as “microswimmers”, travelling in a specific direction when placed in a chemical solution and driven by light, magnetic, electric or ultrasonic fields.

Light-driven microswimmers are particularly promising for applications inside the body, as they can be controlled remotely with high spatial and temporal resolution. Their chief drawback is that their direction of travel becomes increasingly erratic over time thanks to rotational Brownian motion – the random motion of particles suspended in a medium.

Asymmetric photothermal response

In designing their microswimmers, Yuebing Zheng and colleagues found a way of overcoming this problem. The researchers made the microswimmers by covering a glass substrate with a single layer of pristine polystyrene beads using a technique called spin coating. They then used physical vapour deposition to cover one side of the beads with a gold film. The resulting Janus particles were freely dispersed in an aqueous solution containing a cation surfactant called CTAC. This surfactant makes the beads positively charged, while also introducing spherical fatty molecules, or micelles, of CTAC into the solution along with Cl– ions.

While the gold sides of the Janus particles heat up when illuminated with laser light, the uncoated sides do not. The temperature gradient thus produced redistributes the CTAC micelles and Cl– ions, causing an electric field to build up around the charged particles. According to Zhihan Chen, the study’s co-first author, this opto-thermoelectric force plays a key role in determining the particle’s behaviour.

Comparison with swimming microorganisms

When the researchers illuminated the particles with de-focused laser light, the particles swam in the direction of the optothermally-generated light fields. When they switched to a focused laser beam, however, the particles rotated in-plane. The combination of linear travel and rotations is similar to the “run-and-tumble” motion of swimming microorganisms such as E. Coli bacteria, and it can be maintained thanks to the balance between the opto-thermoelectric, optical and Stokes drag forces.

To keep their Janus particles moving in the right direction, Zheng and colleagues developed a feedback control algorithm to switch between the particles’ swimming and rotating states. By carefully observing the particles in real time, the researchers were able to adjust their control algorithm to make it automatically set the particles rotating whenever they deviate from the desired path. Once the particles realign, the algorithm re-activates their swimming state. Through repeated switching between states, the researchers showed that they could make the Janus particles travel in a straight line – behaviour that could be exploited for non-invasive drug delivery in the body, Chen says.

Improving navigation efficiency

The researchers, who report their work in Light: Science & Applications, now plan to improve the navigation efficiency of their microbots. “In our present study, we showed that 5-μm microswimmers can directionally transport over 110 μm in 39 seconds, but we would like to double this figure and deliver the particles over the same distance in just 18 seconds,” Chen says. “We could achieve this by further improving the response time of our imaging camera and laser shutters.”

The team also plan to further develop their control algorithm so it can steer multiple particles at the same time, while also adding non-collision and path optimization functions.

The half-gold, half-uncoated Janus particles studied in this work are a common type, but in the future, Zheng and colleagues hope to functionalize their polystyrene beads by loading macromolecules onto their uncoated surfaces. “This strategy would enable efficient and targeted cargo delivery totally driven by light,” Chen tells Physics World.

Build a bot: new book covers history and future of robotics

They grow up so fast. It seems like only yesterday that the cherubic little darling was gazing wide-eyed at the world, waving its arms incoherently as it figured out how to move. Now it reaches out with those pudgy little arms, purposefully picking up toys and stacking them on top of each other as it actively plays. However, this change didn’t happen over months, or weeks, or even since “only yesterday” – it’s been mere hours. The reason is that this is not a child, but iCub, a wide-eyed, one-metre tall robot built by researchers at the Italian Institute of Technology in Genova that is designed to resemble a small child, as well as learn like one.

The robotic youngster starts out only able to move its eyes, learning to focus on objects of interest. As time progresses, skills develop and motor restrictions are unlocked, to simulate muscle development, and iCub learns to point, play with toys and even use objects as crude tools to push buttons.

iCub was developed to explore the so-called “embodied cognition hypothesis”, the notion that the development of human-like cognition is dependent on learning to physically interact one’s environment – and that, by extension, the development of a truly human-like artificial intelligence is dependent on it having a physical body. The rationale for the hypothesis is at the heart of computer scientist Mark Lee of the University of Aberystwyth’s new book, How to Grow a Robot: Developing Human-Friendly, Social AI.

The first third of Lee’s work explores the history of and current developments in robotics and artificial intelligence (AI) – from pallet-carrying bots in warehouses to computer chess champions – and highlights the issues that arise from trying to derive generalized AI from the prevailing task-based approach to developing AI. The middle section moves on to how robots like iCub might be taught to grow and learn through developmental interactions with their surroundings.

The final section speculates on the future of robotics, considering where and how fast AI might develop and touching on topics including the risk of the singularity, a concern that Lee emphatically dismisses. These areas are rich enough that more could easily have been made out of them. It was also a pity to see trans-humanism defined reductively as being solely about “downloading the brain”. It strikes me that less extreme concepts, such as technological augmentation to expand the limits of the human body could have been explored in a book that examines how we are in many ways defined by the nature and extent of our embodiment.

warehouse robots
Back to the future Mark Lee’s new book spans pallet-carrying bots and the potential for transhumanism. (Courtesy: iStock/Ekkasit919)

Overall, How to Grow a Robot is a rich and comprehensive introduction to robotics and artificial intelligence, with a very clear message at its heart. However, it has one central flaw – in my opinion, it is sadly not a sufficiently engaging read.

Both of Lee’s previous books – on intelligent robotics and assembly systems, respectively – appear to have been intended for a specifically academic readership, rather than the popular audience that this book is marketed at. Perhaps Lee, like many scientists before him, found it a challenge to reshape his material for non-specialists. Yes, the explanations are largely there – and I appreciated the use of periodic jargon-busting fact boxes (although I still remain none-the-wiser as to Lee’s distinction between consciousness and self-awareness). However, the work has the feel of a “recommended class reading”, down to the end-chapter bullet lists repeating key “take-aways” for the reader. Personally, I prefer my casual non-fiction to not be presented as if there might be a pop quiz on the material later. More images of some of the commercial robots described in the opening chapter would also have been welcome for the general reader, and were conspicuous by their absence.

Personally, I prefer my casual non-fiction to not be presented as if there might be a pop quiz on the material later

Other structural aspects, meanwhile, serve to bring to mind a different academic format: that of a dissertation or thesis. On the positive side, the work has one well-explained central argument around which the whole book is constructed – a strength some of its popular-science peers could stand to learn from. Unfortunately, this is countered by repeated instances where the author sets up an interesting area of exploration before punting it to a reference text. The clichéd academic phrase “beyond the scope of the present work” is not used, but it might as well have been – variations on “we do not have the space” recur in its place, and create the looming impression of some word limit narrowly met and avenues curtailed. Taking time to direct the reader to extraneous material is admirable, but such links belong in footnotes, not as disruptions to the main text that leave the average reader feeling underserved – especially if they do not have access to the kind of academic library that would be needed to follow up on such references.

In short, How to Grow a Robot is a detailed and informative read – but one whose style and framing might better recommend it to a computer science syllabus rather than your coffee table.

  • 2020 MIT Press 384pp £22.50hb

Giant Magellan Telescope receives cash injection from the National Science Foundation

The National Science Foundation has awarded the GMTO Corporation — the organization overseeing construction and management of the $1bn Giant Magellan Telescope (GMT) – a grant of $17.5m over the next three years to accelerate the construction of the 25 m-wide telescope.

The GMT will be located at Las Campanas in Chile’s Atacama Desert and is on-track for first light in 2029. Hard rock excavation at the site is complete and in October 2019 the GMTO signed a $135m contract with German company MT Mechatronics and US-based Ingersoll Machine Tools in Illinois to design, build, and install the GMT’s telescope structure. This is set to be delivered to the Chilean site at the end of 2025.

[The NSF award] will enable us to accelerate our progress on critical components of the telescope

Robert Shelton

The GMT will have seven circular mirrors, each 8.4 m in diameter. When put together, they will create a telescope equivalent to one mirror 25.4 m wide. Two of the mirrors are complete and in storage in Arizona, while three are in various stages polishing. The final two haven’t been started yet, but the sixth mirror will be cast in early March 2021.

Acting as one

Each primary mirror is flexible, but they must remain in a precise shape for all seven to function together as one. “The mirror itself is supported, like on a bed of nails, where we have about 160 actuators behind each of these mirrors,” says GMTO project manager James Fanson. “We measure the shape of these mirrors and we adjust them every 30 seconds.” The NSF grant provides the funding to test a full-size primary mirror and actuator system, and adds Fanson, “to demonstrate we can control the primary mirrors the way we need to.”

Each GMT primary mirror reflects light to a corresponding 1 m-diameter secondary mirror with 675 actuators, which alter its shape every millisecond to counteract Earth’s atmospheric blurring effect. With the NSF grant, the GMTO will also build a portion of one of the secondary mirror systems. The grant also provides funding to build a laboratory-bench test to simulate the mirrors, actuators, disturbance sources, and that the seven primary mirrors can all phase together as one — technology not used before.

“One of the areas of great emphasis the team has had from the beginning is to tackle the riskiest, most difficult questions early on to make sure they can be surmounted,” says GMTO president Robert Shelton, who adds that the NSF award will “enable us to accelerate our progress on critical components of the telescope”.

Evidence for life is found on Venus, wider access to the best radiotherapy

In this episode the astronomy writer Keith Cooper is on hand to chat about the surprising discovery of phosphine in the atmosphere of Venus. He explains that here on Earth, microbial life is the only natural source of phosphine – which could mean that life exists in the clouds of Venus. Cooper also speculates about how future missions to the “habitable zone” of the Venusian atmosphere could search for life.

Today, there is a huge disparity in cancer care across the world with people living in low- and middle-income countries having limited access to the best radiotherapy treatments. This imbalance is the focus of the social enterprise company EmpowerRT, which was founded by the University of North Carolina medical physicist Sha Chang. In this episode, Chang and her colleague Cielle Collins talk to Physics World’s Tami Freeman about what can be done to provider greater access to the best treatments.

Industrial lasers generate attosecond light pulses

Studies of ultrafast processes could become more widely accessible thanks to researchers at the University of Central Florida (UCF) in the US, who have shown that commercially available, industrial-grade lasers can generate attosecond pulses of light. Until now, such pulses could only be created at large laboratories boasting complex laser systems.

Researchers make attosecond-scale measurements by passing an attosecond light pulse through a material. When this pulse interacts with electrons inside the material, it gets distorted. By monitoring these distortions, scientists can create 3D maps of the electrons and make movies of their motion. As an example, the classical Bohr model of hydrogen indicates that an electron takes roughly 150 attoseconds (10-18s) to orbit the hydrogen nucleus. Measurements with attosecond precision therefore enable researchers to study motion at a subatomic scale, which is vital for understanding fundamental physics phenomena such as interactions between light and matter.

Such measurements are, however, currently only possible in world-class laser facilities. While UCF houses such a facility, and another dozen or so exist worldwide, team leader Michael Chini explains that none of them truly operate as user facilities – that is, institutions that allow scientists from other fields to come in for a short period and use their equipment for research. This lack of access creates a barrier for chemists, biologists, materials scientists and others who could benefit from applying attosecond science techniques to their work, he says.

Obtaining few-cycle pulses from industrial-grade lasers

The extremely short light pulses employed in attosecond-scale experiments consist of a single oscillation cycle of an electromagnetic wave. Such cycles are typically generated by propagating femtosecond (10-15s) laser pulses through tubes filled with noble gases such as argon or neon. The interaction between the light pulses and the gas broadens their spectrum, making it possible to compress them further in time.

Chini and colleagues have now developed a way of obtaining such few-cycle pulses from industrial-grade lasers, which could previously only produce pulses of much longer duration. They achieved their feat by compressing approximately 100-cycle pulses in tubes that contained molecular gases instead of noble gases and varying the length of the pulses sent though the tubes. This procedure made it possible to compress the pulses by a factor of 45, squeezing them down to just 1.6 oscillation cycles. At that point, Chini says, they showed that they could use these compressed pulses to produce attosecond pulses by generating an extreme ultraviolet supercontinuum – something he describes as “a hallmark of attosecond pulse generation”.

Choice of gas and pulse duration are key

Study lead author John Beetar notes that the duration of the initial laser pulse is key. Filling the tube with a molecular gas – and especially a gas of linear molecules, such as the nitrous oxide used in this work – enhances the compression effect because the molecules tend to rotate into alignment with the laser field. However, this alignment-induced enhancement is only present if the pulse is long enough to rotationally align the molecules. The choice of gas is important, too, because the rotational alignment time depends on the molecule’s inertia. To maximise the enhancement, the researchers aim to make this inertia coincide with the duration of their light pulses.

The UCF researchers, who report their work in Science Advances, say that single-cycle pulses are within reach using their technique. With further refinements, Beetar adds that the reduction in complexity associated with commercial, industrial-grade lasers should make attosecond science more approachable and could enable more interdisciplinary applications.

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