An optomechanical device that adjusts – or “squeezes” – the uncertainties in the quantum properties of laser light has been developed by Nancy Aggarwal at the Massachusetts Institute of Technology and colleagues. The team created their source of squeezed light using a mirror that oscillates under radiation pressure, but displays little thermal fluctuation, even at room temperature. Their approach could soon be used to boost the performance of gravitational wave detectors.
When researchers make measurements on a laser signal, uncertainties arise from quantum fluctuations in the numbers of photons detected and the times at which the photons arrive at the detector. The relationship between this pair of uncertainties is described by the uncertainty principle, which dictates that a decrease in uncertainty in photon number must be accompanied by an increase in uncertainty in timing and vice versa. Reducing the uncertainty of one measurement at the expense of increasing the other can be advantageous in some experiments — and is called quantum squeezing.
Currently, squeezing light must be carried out at cryogenic temperatures to minimize thermal fluctuations – and this requires bulky experimental equipment. Now Aggarwal’s team has created a room temperature system involving two opposite-facing mirrors within a spherical cavity, housed in a vacuum chamber. One of the mirrors has a radius of 1 cm and is permanently fixed in place, while the other is just 70 micron across and is supported by a moveable cantilever of about the same size.
Radiation pressure
When a laser is fired into the cavity, radiation pressure imparted by its photons forces the smaller mirror to oscillate. This movement creates correlations between the number of photons hitting the mirror and the timings of those photons. By fine-tuning their setup, therefore, Aggarwal and colleagues could cause the cavity to squeeze light by adjusting the uncertainties in numbers and timing.
To avoid the need for cold temperatures, Aggarwal’s team constructed their oscillating mirror from alternating layers of gallium arsenide and aluminium gallium arsenide – both of which contain pure, highly ordered atomic structures. Within this composite material, any heat-driven collisions between electrons were suppressed. This meant that instead of jittering due to thermal fluctuations, the mirror’s motion was dominated by light-driven radiation pressure. For the first time, this allowed the team to produce squeezed light at room temperatures, and at a broad range of frequencies; demonstrating a reduction in quantum noise of 15% compared with previous techniques.
Perhaps the most exciting potential application for the team’s device will be improvements to the LIGO and Virgo gravitational wave detectors, which require compact and stable setups, as well as constant operation at room temperatures. Through future research, Aggarwal and her colleagues now hope to adapt their setup to work with all possible wavelengths of incoming laser light.
While protons remain stable for at least 1034 years outside the nucleus, a free neutron survives for just under 15 minutes before it decays. The neutron’s precise lifetime is, however, a matter of some debate, as the two techniques commonly used to measure it have produced conflicting results of 880 and 888 seconds.
A team of researchers at the Johns Hopkins University Applied Physics Laboratory in the US led by Jack Wilson has now put forward a third, radically different technique that involves measuring the number of neutrons near a planet. Using data acquired by the neutron spectrometer on NASA’s MESSENGER spacecraft during flybys of Venus and Mercury in 2007 and 2008, they calculated the neutron lifetime to be 780 +/- 90 seconds. While this measurement has a large uncertainty, the researchers note that the MESSENGER instrument was never designed to perform studies of this type – meaning that a dedicated instrument on a future mission could produce a measurement with much higher precision.
Bottle and beam methods
The established techniques for measuring neutron lifetime are both laboratory-based. In the first, known as the “bottle” method, researchers use magnetic fields or mechanical forces to confine low-energy neutrons in a trap. They then count the number of particles remaining after a fixed period of time. In the “beam” method, they count the number of decay products – protons, electrons and antineutrinos – from a beam of neutrons.
The eight-second discrepancy between the neutron lifetimes measured using the bottle and beam methods has important implications for fundamental physics. For example, the neutron lifetime is a key parameter in studies of nucleosynthesis (element formation) in the Big Bang. Resolving this discrepancy (which has been blamed on unknown experimental errors) is therefore key to understanding how our universe formed 13.8 billion years ago.
The MESSENGER spacecraft’s neutron spectrometer was designed to measure Mercury’s surface composition, and to determine whether the planet’s poles might contain water ice. The instrument consisted of a 103 cube of borated plastic scintillator sandwiched between two 4-mm thick 100cm2 lithium-glass plates that are sensitive to neutrons created via the neutron capture reaction 6Li +n → 3H + 4He.
This reaction occurs when cosmic rays strike Mercury’s atmosphere or surface, so in effect, MESSENGER’s spectrometer measured the rates at which neutrons “leak out” from the planet. The number of neutrons detected depends on how long it takes the particles to “fly up” and reach the spacecraft’s neutron spectrometer. Hence, the shorter the neutron’s lifetime, the fewer the neutrons that survive long enough to reach the detector.
Venus flybys
Before MESSENGER entered Mercury’s orbit, it went through a series of flybys of Earth, Venus and Mercury. During the second Venus flyby, its neutron spectrometer was switched on to check that the instrument was working properly. The data used in this study, which is published in Physical Review Research, were taken during this Venus encounter and during MESSENGER’s first flyby of Mercury.
Venus’ atmosphere is both simple and relatively uniform, containing mainly carbon dioxide (96% by volume) and nitrogen (most of the remainder). Because MESSENGER made observations over a large range of heights above the planet’s surface, the researchers were able to measure how the neutron flux changes with distance.
“The basis of our measurements is a set of models of the neutron production, propagation and detection during these flybys that are modelled with different lifetimes,” Wilson explains. “The shorter the neutron lifetime, the more rapidly the neutron counts decrease with altitude and the model that best fits the data gives us the lifetime.”
A “giant bottle experiment”
The researchers describe their spacecraft-based technique as being conceptually like a giant bottle experiment, one that uses Venus’ gravity to confine neutrons for periods comparable to their lifetime. However, they emphasize that its systematic uncertainties are completely different from those of previous measurements. This, they argue, is potentially a big advantage, as the most likely cause of the disagreement between the beam and (conventional) bottle techniques is that one or both of them underestimated or missed a systematic error.
The work is a proof-of-principle demonstration that such an approach is at least possible, Wilson tells Physics World. More progress might be made using other planetary neutron spectrometer data sets, but he suggests that the best path forward would be to design a dedicated instrument on a future mission optimized to measure neutron lifetime from orbit. Venus, he adds, is a good candidate in this respect thanks to its thick atmosphere and large mass that effectively traps neutrons.
“The main thing that held our measurement back was the short time MESSENGER spent at Venus (roughly 45 minutes),” he says. “If we could spend a longer time there, we could improve the statistics of the measurement and avoid introducing the systematics associated with using the data from this spacecraft.”
Tiny fragments of plutonium may have been carried more than 200 km by caesium particles released following the meltdown at the Fukushima Daiichi nuclear power plant in Japan in 2011. So says an international group of scientists that has made detailed studies of soil samples at sites close to the damaged reactors. The researchers say the findings shed new light on conditions inside the sealed-off reactors and should aid the plant’s decommissioning.
The disaster at Fukushima occurred after a magnitude-9 earthquake struck off the north-east coast of Japan and sent a 14 m-high tsunami crashing over the plant’s seawalls. With low-lying back-up generators knocked out, the site’s three operating reactors overheated and melted down. At the same time, hot steam reacted with the zirconium cladding of the nuclear fuel, generating hydrogen gas that exploded when it escaped from containment.
Caesium is a volatile fission product created in nuclear fuel. During the Fukushima meltdown, it combined with silica gas created when melting fuel and other reactor materials interacted with the concrete below the damaged reactor vessel. The resulting glass particles, known as caesium-rich microparticles (CsMPs), measure a few microns or tens of microns across.
Satoshi Utsunomiya and Eitaro Kurihara at Kyushu University and colleagues in Japan, Europe and the US analysed three such particles obtained from soil samples dug up at two sites within a few kilometres of the Fukushima plant. They used a range of techniques to study the physical and chemical composition of these CsMPs, with the aim of establishing whether they contained any plutonium.
Mapping plutonium spread
To date, plutonium from the accident has been detected as far as 50 km from the damaged reactors. Researchers had previously thought that this plutonium, like the caesium, was released after evaporating from the fuel. But the new analysis instead points to some of it having escaped from the stricken plant in particulate form within fragments of fuel “captured” by the CsMPs.
Utsunomiya and colleagues used electron microscopy and synchrotron X-ray fluorescence to look inside the CsMPs. Based on these data, they were able to map the distribution of various elements coming from materials within the damaged reactors – including iron from stainless steel, zirconium and tin from the fuel cladding and zinc from cooling water. They also found uranium within one of the CsMPs, in the form of discrete uranium oxide particles less than 10 nm across.
However, the researchers were unable to find any traces of plutonium using these methods – probably due to interference from strontium, another fission product. Instead, they turned to X-ray absorption. To compensate for high levels of noise, they carried out the measurement at two different synchrotrons, transporting their roughly 20 µm diameter particle from Japan to be blasted with X-rays at the Diamond facility in the UK and the Swiss Light Source in Switzerland.
The researchers focused their attention on the three areas of the particle that generated the most fluorescence from uranium. They failed to detect plutonium at two of these locations, but succeeded at the third, with absorption spectra produced at both synchrotrons indicating the element’s presence. The low signal-to-noise ratio meant they couldn’t identify exactly which plutonium species were present, but the shape of the spectra told them that it probably existed as an oxide, rather than as a pure metal.
Utsunomiya and co-workers also used mass spectrometry to measure the relative abundance of different plutonium and uranium isotopes within the microparticles. They found that three ratios – uranium-235 to uranium-238, as well as plutonium-239 compared to both plutonium-240 and -242 – all agreed with calculations of the proportions that would have been present in the fuel at the time of the disaster. This agreement, coupled with the fact that the measured amount of uranium-238 was nearly two orders of magnitude greater than would be the case if it had simply evaporated from the melted fuel, led them to conclude that the uranium and plutonium existed as discrete fuel particles within the CsMPs.
Implications for decommissioning
The researchers note that previous studies have shown that plutonium and caesium are distributed differently in the extended area around Fukushima, which suggests that not all CsMPs contain plutonium. However, they say that the fact plutonium is found in some of these particles implies that it could have been transported as far afield as the caesium – up to 230 km from the Fukushima plant.
As regards any threat to health, they note that radioactivity levels of the emitted plutonium are comparable with global counts from nuclear weapons tests. Such low concentrations, they say, “may not have significant health effects”, but they add that if the plutonium were ingested, the isotopes that make it up could yield quite high effective doses.
With radiation levels still too high for humans to enter the damaged reactors, the researchers argue that the fuel fragments they have uncovered provide precious direct information on what happened during the meltdown and the current state of the fuel debris. In particular, Utsunomiya points out that the composition of the debris, just like that of normal nuclear fuel, varies on the very smallest scales. This information, he says, will be vital when it comes to decommissioning the reactors safely, given the potential risk of inhaling dust particles containing uranium or plutonium.
The United Arab Emirates (UAE) has launched a spacecraft to Mars, making it the first Arab country to do so. The uncrewed probe – called the Emirates Mars Mission or Hope – launched from Japan’s Tanegashima Space Center at 6:58 a.m. local time and has now begun a seven-month journey to the red planet. Hope will spend at least two years in orbit around Mars with a possible further two-year extension.
Estimated to cost around $200m, Hope will orbit Mars between 20,000 and 43,000 km from the surface. From there it will investigate the Martian atmosphere, studying daily and seasonal changes in the climate as well as analysing hydrogen and oxygen in Mars’ atmosphere and why the gases are being lost into space. The mission carries three main instruments: two spectrometers – one operating in the infrared and the other in ultraviolet – and an imager that will study the lower atmosphere at visible and ultraviolet wavelengths.
[Hope] is a showcase for how space exploration has become an increasingly international endeavour
Daniel Baker
A UAE-led Mars mission was first mooted in 2014 with the aim of arriving at Mars in 2021 – timed to coincide with the 50th anniversary of the UAE’s independence from the UK. Funded by the UAE Space Agency, Hope is a collaboration between the Mohammed Bin Rashid Space Centre in Dubai, which will oversee the mission, and the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado, Boulder, which built and tested most of the craft. “Hope will capture the ebbs and flows of weather on Mars to a degree that wasn’t possible before,” says LASP director Daniel Baker. “It’s a showcase for how space exploration has become an increasingly international endeavour.”
Indeed, Hope is the first of three Mars craft that will be taking off this month. China is planning to launch Tianwen-1 on 23 July. This mission will consist of an orbiter, lander and rover and will, among other things, measure the water and ice content on Mars. Then on 30 July, NASA is scheduled to launch Perseverance – a robotic rover to find signs of ancient, extinct life. Perseverance will also release a small experimental helicopter called Ingenuity, which will attempt a short flight in the Martian atmosphere.
Virtual conference: Speaker Paul Lui watches his own Best-in-Physics presentation on the online meeting platform. (Courtesy: Paul Liu)
The “Best-in-Physics” poster session at the American Association of Physicists in Medicine (AAPM) Annual Meeting highlights the top five abstracts in the imaging, therapy and multi-disciplinary science tracks. In 2020, however, as with many events this year, things were different.
This year’s annual meeting, held jointly with the Canadian Organization of Medical Physicists (COMP), went virtual. Instead of crowds gathering around the posters, presenters shared their research via talks presented on the meeting’s online portal. Here is part one of my selection from this year’s top scoring studies at the 2020 Joint AAPM|COMP Virtual Meeting. Look out for a second report later this week.
MRI-Linac enables simultaneous MLC tracking of two moving targets
Paul Liu, from the ACRF Image X Institute at the University of Sydney, described the use of an MRI-Linac to simultaneously track the motion of two treatment targets. “There are many radiotherapy cases that involve simultaneous treatment of multiple targets,” he said, citing examples such as locally advanced prostate or lung tumours, and oligometastases.
The challenge here is that both targets can move independently of each other. Currently, this is addressed by using large treatment margins, but this then confers extra dose to healthy tissue. Liu explained that the improved imaging capabilities of an MRI-Linac can be used to localize multiple targets simultaneously.
Liu and collaborators used the Australian MRI-Linac, which has a 1T magnet and a 120-leaf multileaf collimator (MLC), to track two spherical targets irradiated with a 6 MV conformal field. They used a motion platform to test three sinusoidal motion traces, plus lung and prostate motion traces recorded from patients.
Multi-target tracking is achieved via an extra pre-treatment aperture segmentation step, Liu explained. An MLC tracking algorithm uses the target volumes, geometries and MLC positions from the treatment plan to divide the MLC aperture into two segments specific to each target.
Multi-target tracking: the motion phantom driving two targets, MR images of the targets, the MLC leaf positions adjusted in response to the target motion, and EPID images acquired during treatment for analysis. (Courtesy: Paul Liu/ACRF Image X Institute)
During treatment, the team recorded 4 Hz cine MR images and used a template matching algorithm to calculate the motion of each target. When motion was seen, rather than shift the entire MLC aperture, the leaf positions of each segment were calculated independently and then recombined into a single MLC aperture encompassing the motion of both targets.
For sinusoidal motion, the team recorded a tracking latency between the targets and their corresponding apertures of 328 ms, comparable to single-target MLC tracking systems. Liu noted that two-thirds of this time is related to the MRI step, potentially enabling latency reduction by using faster imaging and reconstruction techniques
For lung motion traces, “multi-target tracking does a good job overall in tracking both targets simultaneously,” said Liu. The geometric uncertainty – defined as the RMS error between the target and aperture positions during treatment – was reduced from 5.5 mm without tracking to 2.7 mm. After correcting for latency, this error reduced further to 1.2 mm. For prostate traces, the RMS error was reduced from 4.2 mm with no tracking to 1.4 mm with multi-target tracking.
Putting these findings into clinical context, simulations showed that in both cases, large margins of up to 7 mm were needed to maintain target coverage without tracking. With multi-target tracking, 3 mm margins provided over 99% coverage.
“This is the first experimental demonstration of tracking two independent moving targets on an MRI-Linac,” Liu concluded. “We showed that this technology can help reduce margin sizes in cases of differential motion.”
Carotid plaques within arteries are at risk of rupture, which can cause adverse health effects such as cognitive deficits or stroke. But assessing plaque vulnerability – how likely it is to break off, travel to the brain and cause hypoxia – is difficult, as the mechanisms of plaque rupture are unknown and each plaque is unique in size, shape and composition.
“Our research tries to improve the ways that we can noninvasively and inexpensively see the carotid plaque that can cause stroke or other harmful heath events,” explained Catherine Steffel from the University of Wisconsin-Madison. “We want to give clinicians an inside look at which plaques are vulnerable.”
Steffel and colleagues are investigating the use of quantitative ultrasound imaging to noninvasively identify a plaque prone to rupture via its scattering structures. Their goal was to define quantitative ultrasound parameters that can identify plaque composition and help assess its risk.
Steffel examined the effective scatterer diameter (ESD) – a parameter that describes the frequency dependence of acoustic backscatter and has been used previously to examine other tissues. She notes that her team’s previous studies on excised plaque showed that small ESD corresponded to calcified regions, while larger ESD was seen from lipid particles in plaque.
Parametric map showing the variation in ESD throughout plaque. This example shows a 70% stenosis in the right internal carotid artery of an asymptomatic patient. (Courtesy: Catherine Steffel)
The researchers performed in vivo ultrasound imaging of carotid arteries in 52 patients scheduled for clinical removal of plaque. Just over half of the participants displayed symptoms of stroke or transient ischemic attack and they had a median stenosis (artery narrowing) of about 74%. After the plaque was surgically removed, it was scored by a surgeon and assessed by a pathologist. The team then compared the in vivo images with the surgical scores and histopathology assessment.
Analysis revealed weak correlations between ESD and surgical calcification rating, and between ESD and histopathology cholesterol/fibrinoid level. Stronger correlations were seen between ESD and histopathology hemosiderin score. Steffel explained that the hemosiderin scores are important for evaluating plaque composition as hemosiderin deposits are key markers of plaque vulnerability.
“Our results for calcium and cholesterol/fibrinoid do not agree with previous literature, perhaps because our parameters were computed for the entire plaque rather than a region-of-interest,” said Steffel. “The hemosiderin findings intrigued us, as to our knowledge, this has not been studied previously.”
In particular, the team found that two parameters – the average ESD and standard deviation ESD – could differentiate between hemosiderin scores of 0 and 2. “Overall, this work demonstrates that ESD parameters may help identify plaque composition and microstructure, providing a future clinical tool for assessing plaque vulnerability with noninvasive, relatively inexpensive ultrasound imaging,” Steffel concluded.
Have you ever thought about how much human hair is cut and thrown away every day around the world, and wondered if something useful could be done with this renewable resource? Beyond making wigs – which is a major industry in itself – Megan Murray and colleagues at the University of Technology Sydney have found that human hair and dog fur are extremely good at soaking up oil spills on roads and other hard surfaces.
“Dog fur in particular was surprisingly good at oil-spill clean-up, and felted mats from human hair and fur were very easy to apply and remove from the spills,” says Murray. Although synthetic materials are readily available for soaking up oil, they are not biodegradable. As a result, the Australian researchers have tested several natural materials including peat moss – which was not as good as human hair. They report their results in Environments.
The shortlist for the Insight Investment Astronomy Photographer of the Year 2020 competition has been announced and includes some fantastic images of the sky. My favourite is a photo of polar stratospheric clouds above a snow landscape in Finland that was taken by Thomas Kast. He says the clouds “are something I have been searching for for many years and had seen only in photographs until that day”.
The winners will be announced on 10 September and the shortlisted works will be exhibited at the National Maritime Museum in London starting in October.
This week marks the 75th anniversary of the Trinity test and on his blog Restricted Data, Alex Wellerstein asks what would have happened if that first test of a nuclear weapon had failed? He considers three different types of failure and looks at some of the military and political implications.
Planet Nine is a hypothetical world in the far reaches of our solar system. Proposed in 2016 by Caltech astronomers Mike Brown and Konstantin Batygin, its existence would explain the unusual orbits of certain Kuiper belt objects (KBOs). But are we completely sure that Planet Nine in fact a planet?
A paper in September 2019 suggested the gravitational effects could instead be explained by the presence of a primordial black hole smaller than your fist. To get to the bottom of this mystery, there have been recent proposals to send fleets of tiny probes to the general region of this mysterious object.
In the July episode of Physics World Stories Andrew Glester gets the latest on the mystery of Planet Nine. Appearing in the podcast are Mike Brown and the University of Maryland’s Zeeve Rogoszinski, co-author of one of the mission proposals.
A new X-ray technique has revealed hitherto unknown structures in human bone. The technique, which uses a synchrotron beam to map the 3D orientation of nanocrystals and nanostructures within a material, advances our understanding of bone structure and how it relates to bone mechanics and bone disease. According to its developers, the method could also shed light on other natural and synthetic structures in which crystal orientation plays a role in determining the material’s properties.
Bone is an impressive natural material. It is both stiff, able to endure load without deforming much, and tough, meaning that it doesn’t break easily when bent or placed under load. Stiffness and toughness are rarely found together in a single material, and indeed bone gets its unusual properties from two main constituents: protein-based collagen fibrils, reinforced by nanocrystals of a calcium phosphate mineral called hydroxylapatite. These building blocks appear in various twisted patterns, and incorporate thin layers known as lamella that stack together like plywood. However, the precise way they fit together has proved difficult to understand because their organization is complex at every length scale.
Researchers from Aarhus University in Denmark, the European Synchrotron (ESRF) in France, Chalmers University in Sweden and the Paul Scherrer Institute in Switzerland have now used X-ray scattering data to show that the small-scale structure of bone is not uniform, as previously assumed, but contains deviations in the orientation of the hydroxylapatite nanocrystals with respect to the bone’s nanostructure. “To the best of our knowledge, this is the first time that that the ‘twisted plywood’ pattern of human lamellar bone has been mapped in 3D at such high resolution,” says Marianne Liebi, a materials physicist at Chalmers and the study’s co-first author.
Tensor tomography
The researchers focused on a type of human bone known as trabecular bone, which is porous and found at the ends of long bones like the femur. The pores in the bone are connected by thin rods and plates of bone tissue.
In their experiments, members of the team scanned a 40x40x40 micron cross-section of trabecular bone across a beam of synchrotron radiation while rotating the sample around its x and y axes. By combining small-angle X-ray scattering and wide-angle X-ray scattering, they were able to probe the bone’s nanoscale structures and the atomic structure of its biomineral crystal lattice at the same time.
To extract information on the bone’s 3D structure from their x-y scan, they used a technique called tensor tomography to analyse the intensity distribution of the scattered radiation in reciprocal space (an imaginary space in which planes of atoms are represented by reciprocal points and all lengths are the inverse of their length in real space). If, for example, the crystals in the material are not randomly oriented, the intensity of the X-ray scattering signals will be large for some orientations and small for others, explains co-team leader Henrik Birkedal, who is in the Department of Chemistry and iNANO at Aarhus. “This can be likened to the presence of landmasses on a globe,” he says. “By mapping the variation of the scattering signals with sample orientation (corresponding to making a topographic map in the globe analogy), we can determine how the crystals or nanostructures are oriented in 3D.”
The scattered intensity “projection” of the x-y scan revealed a clear lamellar structure of oriented material, interlaced with disordered regions. According to Tilman Grunewald, the study’s other first author and a former ESRF scientist, “it’s a bit too early” to explain why the orientation of the hydroxylapatite nanocrystals is sometimes disordered, but the data nevertheless provides a focus for later research. “Essentially, our approach provides a new way of looking into the underlying structure of bone,” he says.
Synchrotron advances
Manfred Burghammer, the study’s co-leader and the scientist in charge of the ESRF’s ID13 beamline where the measurements were made, says that this new approach was made possible by “drastic progress” that is currently being made in X-ray synchrotrons. “We are expecting that the current upgrades of synchrotron sources will increase our power immensely in the next years,” he adds.
The team now plan to use their method to analyse bone material from patients with diseases such as osteoporosis and other disorders associated with age-related changes in bone structure, which can impair the bone’s mechanical properties. They also hope to study types of bone microstructure other than the one analysed in this work.
“Finally, we wish to understand the origin of the localized differences in orientation between nanostructure and crystals that we have observed,” Birkedal tells Physics World. “This will likely require improving the resolution of our technique by using smaller X-ray beams.”
This time last year I had just returned from the AAPM Annual Meeting in San Antonio, Texas, full of enthusiasm for the great talks I’d heard, old friends I’d caught up with, and all the new medical physics I’d learned. I was also somewhat exhausted by the scorching temperatures in Texas and the long-haul flight home. In 2020, however, things are inevitably very different.
This year’s American Association for Physicists in Medicine (AAPM) Annual Meeting, held jointly with the Canadian Organization of Medical Physicists (COMP), was meant to take place this week in Vancouver. But due to the coronavirus pandemic, the organizers decided to turn the entire event virtual. Despite missing out on a trip to Canada, I nevertheless “attended” this year’s meeting. As well as hearing about some of the latest scientific developments, I was also keen to find out how a virtual conference could actually work.
The 2020 Joint AAPM|COMP Virtual Meeting incorporated the presentations that would have been given in Vancouver into five days of online sessions. Speakers had recorded their scientific presentations in advance, and these were then played during the scheduled session over a live Zoom call. The presenters were also present at their respective sessions, to answer any questions from the audience after their recordings finished. The first talk that I listened to had a few teething troubles, with some erroneous ghostly chat playing out over the recorded presentation. But overall, and considering the immense technical task involved, all ran pretty smoothly on the technology side.
One major advantage of a virtual conference is that there’s no tie to any particular time zone. After their scheduled slots, all of the scientific sessions are available to watch on-demand for six weeks after the meeting. Indeed, being in Central European Summer Time, some six hours ahead of the conference’s Eastern Time-scheduled agenda, I watched all of the talks on catch-up the next day. While this felt a little disconnected from the live event, it was great to be able to choose exactly which sessions I wanted to hear, and when to listen – with none of those disappointing timetable clashes that are inevitable at this type of large event.
Alongside, this virtual format opens up meeting attendance to many attendees who, for geographical, financial or other constraints, may not otherwise been able to attend. Looking at the map of delegates that the organizers had created revealed that this truly was a global event. And also perhaps more of a family-friendly event than in previous years. Judging by the photos that I saw on Twitter, of delegates watching the conference talks accompanied by their children, babies, flatmates and cats (lots of cats), the ability to take part whenever, from wherever, is certainly a big bonus to many. As one attendee Tweeted: “I miss seeing colleagues in person, but as a new parent I definitely appreciate the benefits of a virtual #AAPMCOMP2020!”
Of course, online attendance does come with issues of its own. “Overall, I’ve found it harder to focus during the virtual meeting relative to an in-person meeting,” Catherine Steffel, a PhD student at the University of Wisconsin-Madison, told me. “Someone on Twitter mentioned that the amount of screen time you’re exposed to during a virtual conference makes it a challenge, while another person said that what makes a virtual conference tiring is sitting in the same place for long periods. I definitely agree with both comments.”
Another obvious downside to the meeting going virtual is the lack of social contact. Whether reconnecting with old friends and colleagues, or random meetings with strangers who may turn into new collaborators or contacts for future research – without the evening social events, the crowded coffee breaks, and the opportunity to simply bump into people wandering around the show, such interactions are beyond reach in this new regime.
The organizers definitely tried to address these shortfalls, for example by including “social hour discussions” in the agenda, running various Twitter-based competitions and introducing an app-based fitness challenge to replace the traditional 5K race that’s run each year. A personal favourite of mine was the “Medical Physics Family Storytime” held on the first evening, in which “fellow medical physicists share STEM-centric books for children of all ages”.
Overall, as a delegate, I experienced both benefits and downsides to the virtual conference. As a journalist, there are pros and cons too. While it’s great to be able to pause a recorded presentation to take notes, it’s also impossible to catch a speaker after their talk to ask a quick question or take an on-site photo to use in a future article.
While the conference has now finished, there are still many presentations that I’d like to listen to. And with six weeks to catch up on the content, my main problem may be knowing when to stop.
Hear more of my thoughts on the 2020 Joint AAPM|COMP Virtual Meeting in this week’s Physics World Weekly podcast.
A new type of metasurface lens developed by US researchers is far less susceptible to optical dispersion than previous designs – which means that it can focus a wider range of wavelengths more efficiently. Boubacar Kanté of the University of California, Berkeley and colleagues believe their new lens could find a range applications including hi-tech medical imaging, energy harvesting and virtual reality.
Optical dispersion occurs when the refractive index of a medium varies with the light’s wavelength. One of its most visible manifestations is chromatic aberration in a lens: the lens has a shorter focal length for wavelengths for which its refractive index is higher, resulting in a blurred image. Since dispersion was first explained by Isaac Newton in 1660, people have developed ways to mitigate it. One involves achromatic doublet lenses, comprising two lenses with different refractive indices stuck together such that the dispersion of one roughly compensates the dispersion of the other.
Today, however, optical technology is moving away from traditional lenses and towards flat optical metasurfaces. These are thinner and lighter and offer functionalities such as reconfigurability and variation of refractive index with polarization that are impossible to achieve with traditional lenses. However, there are difficulties associated with metasurfaces – and among the most prominent are increased chromatic aberration and other performance variations with wavelength.
Wavelength independence needed
The principal cause of these problems is that, whereas bulk lenses control light through cumulative effects on the waves passing through, metasurfaces use optical elements to interact with the wavefronts directly and tailor their shape and phase. To achieve this interaction, these optical elements comprise structures on the scale of the wavelength of the light they control. Variations in the wavelength therefore significantly affect this interaction. Moreover, many of the most efficient metasurface designs are based on resonators, which have a sharp peak at one wavelength: “If you want to make a laser, you want one colour, so a resonant cavity is perfect,” explains Kanté, “But if you want to make a lens, one colour is not good enough!”
In the new work, Kanté and colleagues took a different approach. Instead of using resonators, they used a “fishnet” structure – a lattice of interconnected 350 nm titanium dioxide grids spaced 370 nm apart. Each element behaves as a waveguide: “You need to make those little waveguides as broadband as possible,” explains Kanté. Waveguide-based metasurfaces have been designed before, but they have invariably had efficiencies well below those of the current design. However, the researchers had another, subtler trick up their sleeves.
The phase of each waveguide must be carefully selected to produce a surface with maximum possible efficiency. Previously, researchers have selected the phase of each waveguide independently, by assuming it to be part of a lattice of identical waveguides. “When I make my lens, the waveguide next to it is different,” explains Kanté, “but I assume that it’s not that different, so the error that I get by assuming that it’s the same is not that big.” In 2017, however, Kanté and colleagues showed that this is simply inadequate. They also presented a method for calculating the correct phase. In their latest research, they used this to select the orientation of each pillar in the lattice. The resulting lens focused at least 70% of incident light in the wavelength range 640-1200 nm to the same point.
Fluorescence imaging
The researchers believe that, if a scalable production process can be developed, their design could perform a range of tasks, such as producing virtual reality images and focusing light in solar cells – a process that helps harvest energy more efficiently. “If you are now comparable in efficiency [with traditional lenses] the next question becomes ‘how large scale can you make these things and how much do they cost?’” says Kanté. Even without large-scale production, however, Kanté believes the lenses could contribute to techniques such as fluorescence imaging, which is a Nobel-prize winning super-resolution microscopy technique now widely used in research and medicine: “You pump the molecules with one colour and they emit at another,” explains Kanté, “Now that our lenses are broadband, you could actually pump and collect with the same ultrathin lens.”
Francesco Monticone of Cornell University is impressed “While several recent papers have proposed a large variety of design methods to realize broadband achromatic metasurfaces – with some impressive results…the experimental demonstration and measurements presented [by Kanté’s team] are outstanding.” Monticone’s group recently published fundamental limits on the bandwidth of optical metalenses: “The fabricated metalenses [made by Kanté’s team] represent an important step toward reaching these physical bounds, extending the current state of the art of metalenses and their potential for many applications requiring broad bandwidth and high efficiency,” he says.
