If a micron-sized sphere is coated half in gold and half in platinum, then placed in a solution of hydrogen peroxide, it will start to “swim” with its gold side pointing forwards. Dubbed “Janus spheres” for the two-faced Roman god, the motions of these self-propelling particles have long captivated physicists. Now, a team of UK researchers led by Daan Frenkel at the University of Cambridge has shown that their movements can be largely explained by momentum boosts imparted on them by chemical reactions taking place on their platinum sides.
Key to the motion of a Janus sphere is the asymmetry of the chemical activity that occurs at its surface. While gold is unreactive, platinum acts as a catalyst that significantly hastens chemical reactions. Hydrogen peroxide in contact with platinum will decompose into hydrogen and oxygen, releasing energy in the process. In 2012, researchers in the US showed that this reaction produces bubbles on the platinum surface that initially grow, then burst suddenly. While the bursting does contribute to the propulsion of a Janus sphere, it cannot fully explain the motion.
Rocket fuel
In their study, Frenkel’s team re-visited the problem after discussing how hydrogen peroxide is a common ingredient in rocket fuel and contributes significantly to a rocket’s propulsion through its decomposition. For this to occur with a Janus sphere, chemical reactions near the platinum surface should transfer momentum to the sphere. The concept was relatively simple, but had nonetheless gone largely unexplored in previous studies.
To test the idea, Frenkel and colleagues created a computer simulation of a Janus sphere, in which each decomposition of a hydrogen peroxide molecule on its platinum side imparted a small amount of momentum to the sphere. When they accounted for the fluid properties of the solution, Frenkel and colleagues found that the resulting speed of their virtual sphere showed a clear relationship with the energy released during these reactions.
The team says its results agree reasonably well with observations from real experiments, and therefore provide the first clear evidence that chemical reactions must be considered as an important Janus sphere propulsion mechanism, along with bubbles. The team now plans to study Janus particles with non-spherical shapes and also look at spheres with more complex surface patterns of catalysts. Their research could lead to new types of micro- and nanoscale motors, as well as provide new insights into biological process such as the self-propulsion of living cells.
“Particle of doubt” is the latest musical offering from David Ibbett, who is guest composer at Fermi National Accelerator Laboratory outside Chicago. It is about the neutrinos and is sung in the above video by the soprano Beth Sterling. My favourite line is “You should be changeless. But the change gives us hope we’ll know where we came from”. The video also features Ibbett talking about the process of creating physics inspired compositions.
Engineers at the University of Sheffield in the UK have made the website Flashy Science free to use by UK schools during the coronavirus pandemic. Aimed at GCSE and A-Level physics students, the website contains virtual experiments and is normally a paid-for resource.
Low rider: NASA astronauts will travel in style to the launch pad. (Courtesy: NASA)
Later this month the first crewed test flight of the Crew Dragon spacecraft will blast off from the Kennedy Space Center in Florida. Manufactured by Elon Musk’s SpaceX, the reusable spacecraft will be the first crewed orbital spacecraft launched from the US since 2011 – when NASA’s Space Shuttle programme ended.
The test mission is called DEMO-2 and will be piloted by two astronauts, Douglas Hurley and Robert Behnken. The NASA administrator Jim Bridenstine has just tweeted about how they will be transported to the launch pad – no surprises that they will arrive in style in a Tesla Model X complete with NASA livery. Perhaps for the next launch, Musk’s The Boring Company can excavate a tunnel under Cape Canaveral for the car to drive through.
1. In the first laser – demonstrated on 16 May 1960 by Ted Maiman – the lasing medium was a crystal of pink ruby. Which of the following substances can also be made to lase?
A. Gin and tonic B. Flavoured gelatin (Jello) C. Carrots D. All of these
2. Before the name “laser” became widely adopted, what were Maiman’s device and other subsequent prototypes known as?
A. Optical masers B. Optical phasers C. Optical tasers D. Optical xasers
3. The output power of early lasers was measured in “Gillettes”. But who or what does this unit refer to?
A. A Bell Labs researcher called Gillette who sustained minor burns in the first documented laser-safety incident B. An American mispronunciation of gilet, the item of clothing set on fire during the first documented laser-safety incident C. A New Jersey suburb where several of the early laser pioneers at Bell Labs lived D. The number of razor blades the laser could cut through
4. Although the laser was initially described as “a solution looking for a problem”, lasers were soon incorporated into many devices, including something called a Bolt-117. What was it?
A. The first lidar prototype, developed in 1961 B. The first surgical laser, developed in 1962 C. The first laser-guided bomb, developed in 1967 D. The first laser pointer, developed in 1981
5. In the 1964 James Bond film Goldfinger, the villain threatens to slice Bond in half with a laser. In Ian Fleming’s original novel – published in 1959, before the laser was invented – what object takes its place?
A. A samurai sword suspended on a thread B. A circular saw C. An industrial meat grinder D. There is no such scene in the novel
6. Within what accuracy did astronauts on the Apollo 11 mission use a laser to measure the distance between the Moon and the Earth?
A. 1 kilometre B. 1 metre C. 1 centimetre D. 1 millimetre
7. What was the first toy to incorporate a real laser?
A. Star Wars lightsaber (1977) B. Star Trek phaser (1979) C. Glowing E.T. finger (1982) D. Ghostbusters proton pack (1984)
8. In the 1997 film Austin Powers: International Man of Mystery, the character Dr Evil wants to stick “giant frickin’ lasers” to the heads of which animal?
A. Beavers B. Dolphins C. Sharks D. Meerkats
9. Which facility holds the current world record for laser power, at 10 PW?
A. The Extreme Light Infrastructure in Romania B. The Laser for Fast Ignition Experiments in Japan C. The National Ignition Facility in the US D. The Station of Extreme Light in China
10. Plans for the Breakthrough Starshot project call for 1000 spacecraft to be propelled through space using lasers. What are these (so far hypothetical) spacecraft intended to explore?
A. The moons of Saturn B. The Oort Cloud C. Proxima Centauri D. The outer limits of Elon Musk’s ego
Stumped? Scroll below our sponsor’s message to reveal the answers.
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Gravitational waves from neutron star mergers could provide vital information for testing theories of the quark-gluon plasma — a hot and dense state of matter that is thought to have existed in the very early universe. That is the conclusion of physicists in Germany, who have done computer simulations of how a quark-gluon plasma could be formed when such mergers occur.
In August 2017, the LIGO-Virgo observatories made the first-ever detection of gravitational waves from two merging neutron stars. Another merger has been since observed and more are expected — so physicists are very keen to study these gravitational-wave signals to gain insights into the violent processes that occur at the point of merger.
Quantum chromodynamics was developed in the 1960s by Murray Gell-Mann and others. It explains how, under normal conditions, the strong nuclear force causes quarks to be confined by gluons to create discrete particles called hadrons – which include protons and neutrons. At sufficiently high temperatures and pressures, however, the hadrons dissolve into a soup of free quarks and gluons known as quark-gluon plasma or quark matter. The first convincing observations of the quark-gluon plasma were made about 20 years ago by smashing lead ions together at CERN. Since then, minute quantities of the stuff have been made at other accelerators and studied in greater detail.
Quark epoch
The quark-gluon plasma is also of great interest to physicists who study the early universe, which some models describe as have a “quark epoch” from about 10−12-10−6 s after the Big Bang. Too hot and dense for quarks to be bound into hadrons, the universe in the quark epoch is thought to have comprised a quark-gluon plasma. Understanding the detailed properties of this state of matter therefore helps cosmologists model the universe’s evolution further back towards the Big Bang.
An open question is whether stable quark-gluon plasma exists naturally in the observable universe today. “In 1984, Edward Witten proposed that quark matter is the fundamental state of matter,” explains theoretical astrophysicist Luciano Rezzolla of the Institute for Theoretical Physics in Frankfurt. Many astrophysicists had once believed that entire stars can be made of quark matter, but more recent observations have made this a minority view: “If you asked a roomful of scientists how many of them think pure quark stars exist, you would find only a few hands raised,” says Rezzolla.
More plausible, explains Rezzolla’s PhD student Lukas Weih, are hybrid stars with “degenerate quarks in the centre where the pressure is highest, and then around this a normal neutron star”. The neutron stars with the highest central pressure would be the heaviest ones, which raises the intriguing possibility of two lighter neutron stars in a binary orbit merging and undergoing a phase transition to a such a heavy hybrid star. However, there is little agreement on under what circumstances, or exactly how, this would occur.
Gradual phase transition
In the new research, Rezzolla, Weih and Matthias Hanauske compared computer models of mergers between binary neutron stars with equal masses either 2.64 or 2.68 times that of the Sun. The models predicted the gravitational wave signals produced in such mergers. The signals produced if no phase transition occurs, or if an immediate phase transition occurs on merger, had been previously calculated by other research groups. The Frankfurt researchers simulated a phase transition-triggered core-collapse leading to black hole formation; and a gradual post-merger phase transition to a hybrid neutron star with a quark-gluon plasma in its centre.
If no phase transition occurs, for example, the frequency of the gravitational waves after the merger should remain constant over time. If the star undergoes a gradual phase transition, however, their frequency should climb over the course of a few milliseconds: “Also, if you do some signal processing, you will find that the purely hadronic case gives you one peak in the frequency spectrum,” explains Weih, , “whereas if there is a [gradual] phase transition to quark-gluon plasma, you also get a second peak.”
The research is described in a paper in Physical Review Letters. “This is a very interesting paper,” says astrophysicist David Radice at Pennsylvania State University in the US: “It’s not the first paper to look at the impact of phase transitions in neutron star binaries, but people in the past have taken one particular model for how matter should behave at high densities and performed simulations with that. But there are many different models all predicting different things, and here they’ve parameterized this uncertainty and considered all possibilities.” He cautions, however, that interpretation of gravitational wave data is an embryonic art: “Many things are uncertain,” he says, “For example, there may be reasons why two characteristic frequencies appear in the post-merger signal that have nothing to do with a phase transition.”
Cosmonaut Borisenko Andrey Ivanovich with the SPRUT-2 bioimpedance analyser. (Courtesy: Roscosmos)
Microgravity is an unhealthy environment for the human body. Long-term exposure causes a decrease in bone density, loss of muscle mass and a shift of body fluids into the top half of the body, which can impact cardiovascular system function. From long stays aboard the International Space Station, to the future possibility of long-range space missions, it’s vital to address the health of astronauts and cosmonauts spending extended periods in space.
Early studies on rats showed that providing artificial gravity during a space flight reduced adverse health effects. This finding reinforced the idea that creating artificial gravity on board a space station, by use of rotation, could prevent health problems during space missions. As such, space agencies are developing and testing short-radius centrifuges (SRC) to create an optimal centrifuge model for use on space stations.
For research on Earth, an SRC can create similar effects on the human body to those generated by a centrifuge in orbit. A Russian research collaboration has now demonstrated the use of bioimpedance monitoring to detect and predict blood circulation changes during rotation on an SRC. The investigation is part of a large study at the Institute of Biomedical Problems of the Russian Academy of Sciences.
“Over several years, we have been developing portable bioimpedance analysers with a wide range of software intended both for independent work by cosmonauts aboard a spacecraft and for experimental studies simulating the effect of various space flight factors on Earth,” explains Svetlana Pavlovna Shchelykalina from Pirogov Russian National Research Medical University.
Blood redistribution
The study included nine healthy male volunteers, who underwent rotations in an SRC. The subjects lay on their backs in the centrifuge, with their heads 60 cm from the axis of rotation and their feet further away, thus creating acceleration along the body’s vertical axis in the direction of the legs.
During rotations, the researchers used their SPRUT-2 bioimpedance analyser to monitor the redistribution of participants’ body fluids, primarily blood. This technique works by evaluating the change in electrical resistance at a probing current frequency of 5 kHz, using electrodes placed on the subject’s head, arms and ankles. The setup enabled the team to measure resistance in various body regions: head and neck; thoracic (upper chest); abdominal (lower chest and abdomen); left and right legs; and left and right hands.
Shchelykalina and collaborators monitored each of the subjects during three different SRC rotation modes. Each mode included a 15 min acceleration phase, a constant plateau phase and a 15 min stop-down phase. For the first mode, the plateau phase comprised 30 min rotation at an acceleration of 2.05 standard Earth gravity (g). For the second and third modes, the plateau phases involved 30 min rotation at 2.47 g and 15 min rotation at 2.98 g, respectively.
The team recorded 21 completed tests, with four tests halted early as the subjects felt unwell. Measuring the electrical resistance of various body regions during rotation revealed that resistance of upper body regions – the head and neck, chest and abdomen – increased during the rotation, indicating a decrease in blood filling. Conversely, electrical resistance of the legs decreased, indicating increased blood filling.
This blood redistribution was independent of the rotation mode of during the first 30 minutes, and varied on average by 10% and -15% in head and legs regions, respectively. As the SRC slowed down, these changes in resistance reversed, although they did not have time to fully recover by the end of the rotation.
Cyclogram graphs showing the SRC rotation (dashed lines) and active resistance at 5 kHz in the head region and left leg of the nine subjects. (Courtesy: Physiol. Meas. 10.1088/1361-6579/ab840b)
The maximum changes in resistance occurred at the end of the plateau phases. In the head region, the maximum increase ranged from 15.4% in the first rotation mode to 10.2% in the third mode. In the legs, the maximum decreases were -16.5% and -15.2% in the first mode, and -13.4% and -13.6% in the third.
Predicting fainting
The researchers separately analysed the four experiments halted for medical reasons. In one case, the subject lost consciousness. Here, they observed a sharp drop in resistance of the legs compared with the average, while resistance of the head region was similar to that seen in other subjects. The three subjects who felt ill but remained conscious did not exhibit any significant changes in resistance compared with successful test cases.
The team suggest that it may therefore be possible to use bioimpedance monitoring to detect and predict blood circulation changes associated with syncopal states – in which loss of blood to the brain causes fainting.
“Syncopal states occur with a decrease in blood flow and blood filling in the vascular pools of the head and lungs, and an increase in blood filling of the legs,” explains study leader Galina Yur’evna Vassilieva from the Institute of Biomedical Problems. “Therefore, even a decrease in resistance in the legs can be a sufficiently reliable indirect predictor of the oncoming fainting.”
The researchers note that the high sensitivity demonstrated by their bioimpedance technology, with relatively high changes in resistance, shows potential for developing automated analysis algorithms similar to contour analysis.
“We are currently starting to test a new computer program designed specifically for bioimpedance observation during training on a short radius centrifuge,” Shchelykalina tells Physics World. “We hope to train the program to also recognize predictors of syncopal states.”
In October 1959, Theodore “Ted” Maiman, a relatively unknown 32-year-old physicist, set out to make what was then known as “optical maser” out of a crystal of pink ruby. The project didn’t have the most auspicious of starts. Maiman’s employers at Hughes Research Laboratory were sceptical that it would lead to anything useful. Worse still, in September that year a far more prominent scientist, Art Schawlow, declared that it was impossible.
Maiman proved him wrong. On 16 May 1960, he and his assistant Irnee d’Haenens observed a large decrease in the ruby’s fluorescence lifetime (as seen in the device’s spectral output), once the voltage of the flashlamp shining onto it exceeded a certain level. The change signalled the onset of stimulated emission, in which an incoming photon from the flashlamp caused an excited electron in the ruby to drop to a lower energy level and, in the process, emit a photon with the same energy as the incident wave. And with that, the laser – light amplification by stimulated emission of radiation – was born.
That wasn’t the end of the story, though. Other scientists weren’t far behind Maiman, and several of them – Schawlow included – went on to make their own seminal contributions to laser technology. These advances, in turn, spurred others to get involved. Although d’Haenens jokingly called the laser “a solution looking for a problem”, it wasn’t long before this supposed novelty was making itself indispensable in surgery and industrial processing. By 1964, the laser had risen sufficiently far in the public eye for it to appear in the James Bond film Goldfinger, in a famous scene where the villain threatens to use a laser to cut the dashing super-spy in half.
Since its invention, the laser has gone from strength to strength, as new designs and wavelengths opened up an ever wider range of applications. Indeed, the laser is now in such widespread use that we at Physics World struggled to decide which of its many successes to highlight in our collection of articles celebrating the 60th anniversary of Maiman’s breakthrough. In the end, we narrowed the list down to 10, as discussed on this week’s podcast.
Pauline Rigby’s wonderful feature on the laser’s early history (originally published for the laser’s 50th anniversary in 2010, but now available on our website for the first time)
A classic essay from Lucian Hand, explaining why the laser industry needs physicists
A pair of articles from Richard Stevenson on recent advances in blue and green semiconductor diode lasers
An interview with the founders of an award-winning start-up firm, Opsydia, that uses lasers to etch microscopic marks inside diamonds
We hope you enjoy this celebration of laser technology and applications.
Physics World’s Laser at 60 coverage is supported by HÜBNER Photonics, a leading supplier of high performance laser products which meet the ever increasing opportunities for lasers in science and industry. Visit hubner-photonics.com to find out more.
Your company, Opsydia, started out as a research project at the University of Oxford. What did that project involve?
Martin Booth, co-founder and director: Our fundamental interests were in the development of adaptive optical systems. We’d originally used this technology on microscopes to correct for aberrations, but the same techniques also have applications in laser fabrication, because when you focus a laser beam inside a material you get aberrations there as well. We’d been doing a lot of work on transparent materials such as glass and crystals, and we realized that diamond was another crystal we could work in.
After that, we found that when you use short-pulse lasers to focus into diamond, you can convert some of that diamond into a graphite-like substance. Initially, we used this to demonstrate why we needed aberration corrections – we created graphite-like structures with and without corrections to illustrate the benefits of correcting for aberration. But once we’d done that, we went looking for other applications, and we found that if you can create graphite-like structures inside diamond, which is an insulator, you can make electrical circuits. We also realized that you could use this same technology to create colour centres inside the diamond, which could have applications in sensing or be used as light sources or single-photon emitters.
The third thing we realized is that if we could write very small visible features inside industrially produced diamond crystals, we could also do it inside gemstones. That led us to talk to diamond producers, and we discovered that we had a unique and valuable capability: we could make marks inside a diamond, not just on the surface. Using lasers to mark the surface of diamonds is quite commonplace, but these marks can be polished off and changed relatively easily, whereas if you can create marks deep inside the diamond, it becomes nearly impossible to remove them. So this work we’d been doing at the fundamental end of optics eventually led to a practical application in industry, and that’s why it’s attracted attention.
Patrick Salter, co-founder and director: I was the one in the lab twisting the knobs, trying to make it all work, and from my perspective the project was all about taking a technology that was being used reasonably widely for bioimaging in optical microscopes and transferring it into laser processing. Initially, as Martin said, it was just an interesting demo, but as we developed our adaptive optics and the associated algorithms, we started collaborating with people at CERN and Diamond Light Source who were interested in building radiation detectors inside synthetic diamond wafers. We also collaborated with quantum physicists at the University of Oxford and the University of Warwick who were trying to make scalable sources of quantum bits embedded inside diamond.
Bright start (left to right) Opsydia chair Alastair Smith, Martin Booth, Andrew Rimmer, Patrick Salter, and systems engineer Lewis Fish. (Courtesy: Opsydia)
At what point did these ideas coalesce into a company?
MB: The turning point was really when the diamond firm De Beers, which is now a major customer, asked us to do demonstrations for them. That made it apparent that there was a commercial opportunity. Universities are very good at doing research, but they’re not well equipped for commercial operations, so for the project to continue, we knew we had to set up a company.
Andrew Rimmer, chief executive: After those initial engagements with De Beers, we started taking a process that had been developed and refined in a university lab and translating it into a process that would work in a high-volume industrial manufacturing environment. From there, we put together a package that we could pitch to investors and gain the seed investment we needed.
Going to pitch was a bit like Dragon’s Den, but without the cameras or theatricals. The two investors that agreed to put in money very quickly were Oxford Sciences Innovation, which has a fund that supports many Oxford spin-outs, and Parkwalk Advisors, which is a London-based fund. Between them, they provided us with £1.9m to set up the company, recruit a team and undertake the initial development projects.
What was the biggest hurdle you had to overcome?
PS: On the technical side, it was the massive speed-up needed to make the laser-marking process commercially viable. When we started the company, we had a process that worked in a university lab and produced a mark that the customer was happy with, but it took 10 or 20 minutes to mark each individual stone. By bringing together a team of skilled engineers with different backgrounds and different expertise, we were able to reduce that by an order of magnitude.
AR: Commercially, the biggest challenge was getting that speed up accomplished quickly enough. In contrast to a lot of start-ups, we had our first customer – a De Beers spin-out called Lightbox Jewelry – lined up right away. That was great, but it also meant we had to develop our product very quickly.
Within a year of starting the company in 2017, we had a prototype that could mark a stone in about one minute, with all the safety and other features needed for a semi-skilled operator to use it. Since then, we’ve shipped machines that can mark 100 000 stones per year, and we’ve been talking to customers about other applications that might require more than double that. To go from a university lab to a fully industrialized machine that can be shipped overseas, commissioned and made operational, all within about 24 months, needed a lot of focus from the team.
Innovation honours Members of the Opsydia team with Oxford West MP Layla Moran (centre) and Institute of Physics (IOP) vice-president for business James McKenzie (right) at a ceremony held in the UK House of Commons in October 2019 to honour winners of the IOP Business Innovation Awards. Opsydia won a 2019 IOP Business Start-up Award for creating a novel security solution for diamond gemstones based on developments in laser processing. (Courtesy: Opsydia)
What’s next for Opsydia?
AR: The first work we were doing in the diamond gemstone industry was about being able to produce the right mark – a hallmark, if you like – for Lightbox Jewelry inside their laboratory-grown diamonds. It provides a visual identification of the stones as being laboratory grown, while at the same time assuring consumers that this product has been manufactured to high standards by a world leader in this field. But the diamond industry is made up of many different operations, including the miners and producers, manufacturers who cut and polish, grading laboratories, and of course the jewellers who produce the final piece of jewellery.
Around 10 million stones per year receive a grading certificate, and the current verification process relies on linking that certificate to a serial number that’s been laser etched on the edge of each stone. But surface etching is not secure: it can be counterfeited, it can be removed. And in the diamond industry, that matters. People want to know that the stone they’re buying is what it says it is, and increasingly they also care about its provenance – where it was produced, where it was mined, the whole journey. So if we can be the glue that links a stone’s identity to the story behind it, we think demand for our product is going to increase. With that in mind, we have done trials on gem materials other than diamond – rubies, emeralds, sapphires, jadite.
But if we step away from the gemstone industry, one of the other patents we’ve licensed from the university and Martin and Patrick’s team is about using adaptive optics to control a laser at very high speed to simultaneously write multiple features into plastics and polymers. This enables us to write security features and holograms inside plastic as opposed to on the surface. Finally, as Martin and Patrick alluded, there are applications in sensing. Being able to write circuits inside diamond opens up the possibility of using diamond rather than traditional silicon semiconductors as a substrate for electronics. That work is very much still in the research stage, but we certainly see future applications that could be very important.
High precision Opsydia’s technology makes it possible to etch microscopic features like this array of dots inside a diamond gemstone. (Courtesy: Opsydia)
What do you know now that you wish you’d known when you started?
PS: From an academic perspective, I didn’t necessarily know quite how much work it would be on an ongoing basis. I knew there would be a big flurry of activity when the company formed, but I thought it might drop off a bit after that, whereas in fact it’s been fairly consistent. Also, there’s sometimes a feeling within academia that industrial work will get a bit dull relative to academic research, particularly after the company’s been running for a short period. But I don’t think that’s the case. There are always interesting new challenges arising and difficult problems to solve.
MB: Often, when people ask questions like this, what they’re actually saying is, “What did you do wrong, and what should I do better?” But I can’t answer the question in that way. I have previous experience of spin-outs, and I was surprised at how smoothly this one went. I think the reason is that we already had a big customer ready and waiting to go. That makes a huge difference. So I would say that my experience with Opsydia confirmed what I’d already suspected, which is that the optimum time to set up a company is when you’re pretty certain there are customers out there who want your product.
Pay attention to your intellectual property and think about it in a commercial sense rather than a scientific sense
PS: Another important thing was getting commercial expertise early on. You see a lot of spin-out companies where a PhD student or postdoc tries to run the company despite not having any previous commercial experience, but we got someone with a lot of commercial experience – Andrew – right at the beginning. That made everything much, much easier.
MB: We also had help from our chair, Alastair Smith. So before we even went to speak to investors, we had two people with significant commercial experience on board. That was incredibly valuable.
AR: I hadn’t started a company before, but I’d been involved in early-stage companies in engineering, so I’d seen the challenges and rewards. But the other thing I’d like to mention is the set-up around the University of Oxford. They have an organization called Oxford University Innovation that helps with licensing and spinning out technologies into new companies, but they’ve also got connections to investors who are used to investing in those spin-outs, as well as lawyers and accountants who deal with the legal processes and agreements many, many times a year. It was hugely beneficial to have that sort of support.
Any advice for people thinking of starting their own company in optics or photonics?
AR: Be very sure about the market demand for your product. Can you make it at a price where it’s going to be accepted? What are the potential barriers you’re going to have to overcome to get into that market? Because you can have the best idea, and you can have big demand for it, but if there’s some fundamental barrier – regulatory hurdles or something like that – it’s so, so important that you know about it. Those barriers are going to drive your behaviour, the priorities for your team, and the timescales for getting to market – and therefore how much money you need to raise.
PS: Pay attention to your intellectual property and think about it in a commercial sense rather than a scientific sense. You often look at a patent and think, scientifically, that’s not very good. But commercially, it can still be incredibly valuable, so you shouldn’t discount it.
MB: Following on from that, don’t assume that your best scientific idea is going to be your best commercial idea. In fact, it’s highly unlikely that those two things correlate.
Physics World’s Laser at 60 coverage is supported by HÜBNER Photonics, a leading supplier of high performance laser products which meet the ever increasing opportunities for lasers in science and industry. Visit hubner-photonics.com to find out more.
If you want to curb your carbon-dioxide emissions, buying an electric vehicle is an increasingly attractive way of doing it. Purchasing incentives, tax breaks and scrappage schemes – not to mention the chance to drive to areas off-limits to gas guzzlers – are all persuading more people to switch. Alongside these tempting “carrots”, there is also a major “stick” on the horizon. Governments in more than a dozen countries have set dates for when they will ban the sales of new petrol and diesel cars, and the UK has gone one step further, prohibiting the sale of hybrid electric vehicles from 2035.
Given these measures, sales of electric vehicles are expected to climb sharply. Makers of batteries and electric motors are sure to benefit, along with companies that make tools that assist their production. But there are some hurdles to overcome. For battery manufacturers, in particular, the process of joining copper foils to create an anode with a large surface area has long been a rate-limiting step. Soldering these foils together is too slow, so instead they are welded with a directed heat source that causes the copper to melt. As the parts cool, they unite.
The standard way of welding copper is to use an ultrasonic heat source, but for batteries, this has three significant drawbacks. First, it ejects particulates of copper from the foil, and aluminium from the tool used, that can compromise the battery’s efficiency. Second, it means introducing a new ultrasonic weld head whenever the geometry of a specific weld changes. Finally, it tends to leave gaps between the foils in the welded area, producing a device with inadequate electrical current and poor mechanical strength.
A more attractive alternative – and one that addresses all these weaknesses – is to weld the foils with a high-power laser. Laser welding is also versatile, as it can be used to join copper during the production of electric motors and make electrical connections to copper busbars – that is, the metallic strips used to distribute power locally at high currents.
Unfortunately, the infrared sources that dominate the multibillion-dollar high-power laser industry are far from ideal for welding copper. One issue is that copper absorbs only about 2–5% of light at infrared wavelengths (700 nm – 1 mm). Welding it with a standard 1 μm laser therefore requires a great deal of energy – a problem exacerbated by copper’s high thermal conductivity. But that’s not the biggest barrier. A far greater concern is the poor quality of the welds. Once copper starts to melt, it absorbs infrared light much more strongly. As a result, it boils, creating low-pressure and high-pressure bubbles within the melt pool. Low-pressure bubbles fail to break free, and are frozen in place, creating voids; meanwhile, high-pressure bubbles eject material from the weld, creating spatter. In both cases, the result is weak welds with a high resistivity that increases electrical loss within the battery.
Benefitting from the blues
Neither problem is insurmountable. At wavelengths of 500 nm (visible blue light) or below, copper’s absorption rockets to around 50%, leading to superior welds and a heat conduction process that causes the metal to melt without evaporating. This reduces the need for re-working and streamlines production. The catch, of course, is that it requires a high-power laser designed specifically for welding copper.
At first glance, it might seem like a frequency-doubled infrared laser would be enough to solve the problem. In practice, though, the wavelength conversion process is inefficient, leading to high power loss. Conversion also requires complex cooling arrangements and a sophisticated optical set-up. Instead, leading companies – including Nuburu, a US start-up that launched in 2015, and Laserline, a Germany-based firm with a pedigree in high-power infrared laser manufacturing – are combining the output of tens or even hundreds of blue laser diodes into a single high-power source.
At first glance, it might seem like a frequency-doubled infrared laser would be enough to solve the problem. In practice, though, the wavelength conversion process is inefficient
The diodes in these new sources are based on gallium nitride (GaN). In some sense, they are descended from the ultraviolet GaN lasers invented in 1996 by Shuji Nakamura, who shared the 2014 Nobel Prize for Physics for inventing a related device, the blue LED. These earlier lasers, which Nakamura developed during his time at Nichia in Japan, found their first application in Blu-ray players, where outputs in the tens of milliwatts made it possible to read information off discs.
In theory, it would be possible to combine the outputs of thousands of these low-power ultraviolet laser diodes to produce a source with enough power to weld copper. However, the result would be bulky as well as very tricky (and costly) to put together. Instead, Nuburu and Laserline have drawn on recent advances in the performance of GaN laser diodes. Over the past decade or so, chipmakers have focused on making powerful blue lasers for colour projectors. Substantial gains in output power have followed, along with a shift in emission from ultraviolet to blue.
The increase in output power stems, in part, from reducing the material defects within GaN that tend to quench its emission. An increase in the volume of the laser’s active region also played a role. Normally, another way of increasing the power of a semiconductor laser is to make its cavity longer – that is, you increase the distance between two mirrors that are at either end of the cavity. However, this cannot be done with a GaN laser – at some point the level of absorption becomes too high, and too much light is lost before it reaches the mirror. On the other hand, you can increase the “ridge width”. The ridge is used in a compound semiconductor laser to control the light coming out from the laser chip – the wider it is, the more light comes out and therefore the more powerful the laser. The ridge width of GaN lasers has gone from just 2 μm or so in lasers used in Blu-ray players to 40 or even 50 μm for the most powerful diodes.
Power of blue Copper absorbs around 50% of incident light at blue wavelengths, compared to 2–5% of infrared light, making powerful blue lasers a better choice for welding this industrially important metal. (Courtesy: Laserline)
Today, Nichia’s most powerful blue laser diodes have an output of several watts. However, they are not ideal for building sources of several hundred watts or more, because they are housed in bulky metallic cans. A better solution is the bare laser chips produced by the world’s other leading source of blue lasers, Osram Opto Semiconductors. Based in Regensburg, Germany, Osram OS is a wholly owned subsidiary of Osram, and over the past decade it has significantly improved the performance of its blue laser diodes. Back in 2008, the output powers of these diodes were limited to 60 mW, with an efficiency of just 15%. By 2017 these figures had leapt to 3.5 W and 44%, while in 2019 company officials reported even higher efficiencies, of 46%, for a chip emitting 2.2 W.
In early 2016, Osram OS started working with Laserline on a three-year project aimed at producing a kilowatt blue laser. This effort, known as Blaulas – a portmanteau of blau, the German word for “blue”, and “laser” – had the backing of the German Federal Ministry for Education and Research and a budget of €5.8m. It also drew support from other teams within academia and industry, including laser-characterization specialists at the Max Born Institute in Berlin.
Working with bars…
The Blaulas project attempted to replicate the established approach for producing powerful infrared sources. Like their blue counterparts, individual infrared laser diodes emit no more than a few watts. To propel their output to several kilowatts, manufacturers produce a strip of lasers known as a laser bar. These bars can then be stacked on top of one another to further increase the device’s output. This is an elegant way to scale output power, since rather than using lenses to individually align the output from each laser, two lenses can control the emission from an entire bar.
At the outset, though, many researchers in the GaN laser-diode community did not think it would be possible to produce a high-power bar at blue wavelengths. Due to the high level of defects in the material, they reasoned that some lasers in the bar would fail to emit. These failed lasers would have a lower forward voltage – meaning that all the current would be routed through them, and the bar would be destroyed.
Blue streak The blue laser developed at Laserline as part of the Blaulas project proved powerful enough to weld etched, oxidized and polished areas of a 0.5 mm-thick copper sheet, demonstrating its versatility for welding copper parts. (Courtesy: Laserline)
To avoid this, Harald König and colleagues on the Blaulas project at Osram worked to improve the GaN growth process, increasing the material’s homogeneity and reducing defects. They also refined the design and processing steps. The result – made public at the Photonics West meeting in February 2018 – was the world’s first high-power GaN laser bar, with 23 emitters delivering a 98 W output when driven at a current of 60 A, and an efficiency of 46% when operated at the intended output of 50 W. More recently, König and the Blaulas team have improved the bars’ performance still further. At the 2019 Photonics West meeting, they reported a 107 W output from a bar with 23 emitters. “I’m really proud that we proved them wrong,” König says.
To push output power beyond the 100 W range, the engineers at Laserline stack together many bars, mounting them on heat sinks to prevent overheating. During the Blaulas project, they made two stacks of 10 bars, and used polarization coupling to combine their output into a fibre and create a 730 W source. The fibre-coupled blue lasers in Laserline’s portfolio now have output powers ranging from 300 W to 1.5 kW and “a very nice ‘top-hat’ intensity profile coming from the fibre”, according to Laserline technology and product manager Simon Britten. When used to weld copper, Britten explains, a regular profile produces a very even temperature distribution across the weld, improving the weld’s quality.
…and single emitters
At Nuburu, laser experts have taken a markedly different approach to making high-power blue lasers. Rather than using bars, the Colorado-based start-up is relying on single emitters. According to Jean-Michel Pelaprat, who co-founded Nuburu with fellow laser diode industry veteran Mark Zediker, one advantage of this approach is that single emitters have a longer lifetime than bars. Work at Osram OS indicates that the average single emitter lasts about 65,000 hours, which is five times longer than a bar – although the shorter lifetime of bars is nevertheless considered long enough for the industrial-lasers market.
The real reason for using a collection of single emitters, though, is their higher beam quality. A common metric for judging the quality of the beam is the beam-parameter product, which is equal to the smallest radius of the laser beam, multiplied by its divergence angle. Taking the power of the laser and dividing it by the beam-parameter product gives the brightness, and squaring this gives the power density – the key figure to consider when welding. Working through the maths, this means that halving the beam-parameter product quadruples the beam’s power density, allowing the laser to complete a weld four times faster – or penetrate 60% deeper into the metal, as Pelaprat points out.
Going for gold Like copper, gold has weak absorption in the infrared and strong absorption in the blue, making a GaN laser an ideal source for the welding of this precious metal. (Courtesy: Nuburu)
To ensure a high beam-parameter product, Nuburu’s design positions a lens next to each emitter to correct any difference between them. A total of 20 laser diodes are placed together to create a 50 W source, while four of them are combined with interleaving mirrors to form a 200 W “engine”.
These engines form the heart of Nuburu’s first generation of products, known as the “AO” series after the Japanese word for “blue”. The AO-150, which was released in 2017, has a single 200 W engine, offering a substantial margin for the specified output of 150 W, and a beam-parameter product below 15. The following year, Nuburu launched the AO-500, which it formed by combining four of its 200 W light engines. To reduce energy loss, this design avoids traditional beam splitters in favour of a series of mirrors and a polarizing cube that focus the beams of 320 lasers into a fibre. This ensures a coupling efficiency of more than 90%. Although combining the output of four engines compromises the beam-parameter product, it is still less than 30.
Market traction
Both Nuburu and Laserline report a great response to the launch of their blue lasers. “I thought at the beginning that we would have to do a lot of evangelization, but we didn’t,” Pelaprat says. In February 2020 Nuburu launched a source that breaks new ground for the beam-parameter product. The new line, dubbed the “AI” (Japanese for “deep blue”), is based on a light engine that is five times smaller than its predecessor, produces 500 W, and has a beam-parameter product below five. The first product, the AI 1500, combines four of these new light engines to deliver an output of at least 1.5 kW, with a beam-parameter product below 11.
According to Pelaprat, the AI 1500 is the first source that is powerful and bright enough to be paired with a scanner for welding busbars. By combining several high-power sources, Pelaprat thinks they could achieve still higher powers. “It could go way beyond 10 or 20 kW,” he says. “There are no limits.”
For Laserline, the introduction of the 1 kW laser has spurred sales growth thanks to a greater processing speed than its 500 W cousin and an ability to process thicker copper. The desire to expand processing capacities still further has also driven Laserline to investigate the benefits of overlapping beams of blue and infrared lasers. According to Britten, welds in copper are limited to about a millimetre’s depth when using a blue laser, but can be far deeper when a powerful infrared laser is added to the mix. “There is a synergy between both wavelengths,” he says, adding that while the blue laser creates the initial melt pool in the copper, infrared emission can help keep the copper molten.
As Nuburu and Laserline develop higher power sources, they are in an envious position of seeing their existing products improve through gains in the efficiency of laser diodes. This efficiency is currently climbing at around one or two percentage points per year, steadily making its way from current values above 35% towards a likely upper limit of 70%. While the higher figure is a goal that can only be dreamt of today, there is no doubt that the future is very bright for the high-power blue laser, and it’s only going to get brighter still.
Article updated 24 June 2020 to match print edition.
Physics World’s Laser at 60 coverage is supported by HÜBNER Photonics, a leading supplier of high performance laser products which meet the ever increasing opportunities for lasers in science and industry. Visit hubner-photonics.com to find out more.
In this episode of the Physics World Weekly podcast Margaret Harris and Hamish Johnston celebrate 60 years of the laser by delving into the history of its invention and rise to ubiquity. We also chat about some fascinating applications of the laser including tracking African mosquitoes during a solar eclipse.
James Dacey joins us from Madrid to talk about his experiences at the recent Sharing Geoscience Online conference, which was organized by the European Geosciences Union. He explains how geoscientists are contributing to the battle against COVID-19 and also doing their bit to protect a much larger African animal.
We also hear from Matin Durrani, who explains how Physics World magazine made the decision to soon scrap its plastic wrapper in favour of another material. Finding a more environmentally friendly way of mailing the magazine to our readers was more complicated than you may imagine, as Durrani explains.
Laser light could turn a normally insulating material into a conductor and vice-versa, according to calculations done by physicists in the US and UK. This feat relies on quantum control, the idea that a system’s output can be tailored at will by applying a suitable time-varying field at its input. In future, they say, their scheme could allow everyday materials to take on the properties of more exotic systems such as superconductors.
When at equilibrium, a system’s behavior is largely dictated by its inherent characteristics. The very different compositions of gold and iron, for example, confer quite different chemical, electrical and magnetic properties. But as pointed out by Gerard McCaul of Tulane University in New Orleans, those properties can be altered by applying time-varying fields to the substances – typically an electromagnetic field like a laser. “There is a sort of ‘universality’ to driven systems,” he says, “where they can exhibit almost any observable behaviour you choose, provided you can find the appropriate driving field”.
In the latest work, McCaul and Tulane’s Denys Bondar, along with colleagues at King’s College London and the US Army Research Laboratory in Maryland, have devised a new theoretical framework for controlling a system’s properties using laser beams whose electromagnetic fields vary with time in a very well-defined way. This coherent control is already exploited in many applications, such as the use of radio-frequency pulses to prepare the states of quantum bits used in nuclear magnetic resonance experiments.
Hydrogen mimics argon
Three years ago, Herschel Rabitz, Bondar and colleagues at Princeton University showed theoretically that quantum control could be used to render the output of two distinct physical systems identical – so that one in effect mimics the other. The systems in question were atoms of argon and hydrogen, with the Princeton researchers calculating how a suitably shaped laser pulse fired at the hydrogen atom would force that atom to emit light with the same spectrum as argon.
Now, McCaul and colleagues have extended this idea to many-body systems, and in particular to solid-state materials. To do so they use the one-dimensional Fermi-Hubbard model, a stripped-down representation of a solid consisting of a lattice of interacting electrons. The model can predict a wide variety of material properties, including whether or not a system is a conductor, simply by comparing electrons’ kinetic and potential energies.
When exposed to a laser, a material absorbs energy from the beam, setting its electrons in motion. Conversely, electron motion can generate light. But the types of current and emission spectra generated in this way depend on whether the material is in a conducting or insulating phase. The Anglo-American group has shown that it is possible to vary the incoming fields in such a way to convert a conductor’s emission spectrum to that of an insulator, and vice-versa. As such, they predict, it should be possible to flip a solid between its insulating and conducting phases by careful design of the driving laser field.
Very non-linear relationship
Being able to shape a system’s properties in this way relies on a very non-linear relationship between the material’s output and the laser field at the input. That makes controlling the input far from straightforward. To do that, the researchers have adapted a technique known as tracking control to handle complex, everyday matter modeled with a lattice. Rather than calculating the optimal value for the laser field at each point in time, they instead use a non-linear equation of motion that continuously recalculates the field using the system’s output – something they found that they can do efficiently while avoiding non-physical singularities.
McCaul points out that this laser-based scheme is not able to dictate the value of every observable in a system. Changing the mass of a proton, for example, would be off limits. But nonetheless, he says, most material properties studied experimentally are electromagnetic in nature, and can therefore be influenced.
“If it quacks like a duck”
He also argues that the difference between a system’s appearance and behavior is a semantic issue. “We can only define states of matter by the behaviour we observe,” he says, “so if we can mimic that, our system is by definition in that state of matter. If it quacks like a duck…”
One potential application of the work, says McCaul, is “high harmonic generation” – producing light with a frequency several times that of the field driving the system. One way of probing very small or short-lived systems, it suffers from the fact that the harmonics are orders of magnitude less intense than the driving radiation. But the new scheme shows how it might be possible to boost the intensity of one of the harmonics produced, given that the spectrum is a parameter of the tracking model. Although the actual enhancement would depend on the type of laser and other hardware available, McCaul believes the new approach is “nevertheless a promising avenue” to generate high harmonics.
Looking further ahead, the researchers are also aiming to show how their scheme might create new types of superconductor or else raise the transition temperature of materials that already superconduct. George Booth of King’s College London, who also worked on the research, points out that superconductivity is not entirely defined by a system’s optical properties. Still, he says, it should be possible to design laser pulses that enable one system to mimic the superconducting properties of another.
Physics World’s Laser at 60 coverage is supported by HÜBNER Photonics, a leading supplier of high performance laser products which meet the ever increasing opportunities for lasers in science and industry. Visit hubner-photonics.com to find out more.
