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Some like it cold: cryogenic innovation forges quantum opportunities

Photo of Keeping it cool: ICEoxford managing director Chris Busby (left) and technology director Paul Kelly from ICEoxford — Keeping it cool: ICEoxford managing director Chris Busby (left) and technology director Paul Kelly foster a customer-first mindset across the company’s engineering and commercial teams. (Courtesy: ICEoxford)

It’s been a busy 12 months at ICEoxford, the UK-based designer and manufacturer of specialist cryogenic systems for scientific research and industry. While continuing to develop ultralow-temperature solutions for its established customers – among them neutron and X-ray beamline scientists and advanced materials R&D laboratories – ICEoxford has been delivering new product innovations and significant new revenues in the burgeoning field of quantum computing.

Beyond the financial upside, ICEoxford’s diversification strategy also yielded high-profile national and international recognition last month when the company was among this year’s winners of a Queen’s Award for Enterprise (Innovation). The award specifically acknowledges development and commercialization of the DRY ICE 1.0K, a high-cooling-power cryostat that can reach temperatures as low as 0.8 K to support optical quantum-computing applications.

Here Chris Busby, ICEoxford managing director and co-founder, talks to Physics World about the secrets of success at ultralow temperatures.

Specialist cryogenics is a competitive marketplace. What’s the ICEoxford value proposition?

ICEoxford’s core competency is design and development of leading-edge cryogenic products, though what really sets us apart is the relentless focus on customer service and collaborative innovation. Put another way: our product design team works directly with scientists to understand their requirements at a granular level, giving them confidence that we can deliver the optimum system versus their budget and technical specifications.

We’re very good at what we do, but high-end cryogenics is not a straightforward business. Problems are inevitable in ultralow-temperature systems, so the way you handle post-sale issues arising really matters. As ICEoxford managing director, I take it personally – and will get involved directly – if a customer is unable to conduct their research using one of our machines. That mindset informs the customer-first approach across our engineering and commercial teams. We are not, and never will be, a business that takes the money and walks away.

How will the Queen’s Award benefit ICEoxford in terms of growth and business development?

The Queen’s Award caps a fantastic year for the ICEoxford team and gives us confidence that we can compete on a level playing field with the bigger vendors in the specialist cryogenics market. For me, as one of the co-founders, it really feels like ICEoxford has found its identity and come of age over the past 18 months. At the top line, we set ourselves a target of £3m in orders in the last financial year (ending March 2020). We actually exceeded £5m, while growing margins, profitability and increasing our sales into industry to around 40% of new business. On the personnel side, we’ve scaled up to 30 employees and now have critical mass across all customer-facing functions – product design, engineering, sales and marketing, and technical support.

Photo of iceOxford's DRY ICE 1.0K cryostat — ICE station: the DRY ICE 1.0K cryostat provides high cooling powers (>250 mW in the sample area) to enable R&D on next-generation quantum technologies. (Courtesy: ICEoxford)

The innovations underpinning the DRY ICE 1.0K cryostat are formally recognized in the Queen’s Award. What are the unique features of this product?

The DRY ICE 1.0K was developed in response to a request from scientists at Massachusetts Institute of Technology (MIT) in the US. In short, the MIT team needed to optimize the performance of single-photon detectors in its quantum computing set-up while also managing a large experimental heating load. Our solution is the DRY ICE 1.0K cryostat, which can reach temperatures as low as 0.8 K while delivering cooling powers in excess of 250 mW in the sample area – five times greater than competing commercial products. On the back of the MIT project, we subsequently sold five Dry ICE 1.0K systems into a single US industry customer in 2019. We’re bound by a non-disclosure agreement, but all told that was a $2 million contract to kit out their R&D lab for work on next-generation quantum technologies.

Have other customers subsequently shown interest in the DRY ICE 1.0K?

Absolutely. We’ve already customized the cryostat for a Silicon Valley quantum computing start-up venture – though again we’re unable to name them at this point. As well as high cooling power, this customer requested an integrated probing station to carry out real-time electronic and optical characterization of their photonic ICs within the cryostat’s cold-head. Elsewhere, we’re working up a design study with Shanghai University scientists for a possible electron-beam application of the DRY ICE 1.0K. The latter represents a significant engineering challenge that will push the cryostat performance along several coordinates – low temperature, high cooling power and automation.

ICEoxford is clearly in growth mode. How do you manage specialist recruitment into the product development and engineering functions?

We recruit largely with the team in mind, rather than focusing on individuals. In other words, how will this candidate and his/her expertise support our wider cross-disciplinary requirements in product development and engineering. Generally, that means bringing in new graduates – physicists, mechanical engineers, recently a couple of biotechnologists – and training them in-house to become cryogenic specialists.

For context, Oxfordshire is home to lots of technology companies and big science facilities – including the Rutherford Appleton Laboratory and Culham Centre for Fusion Energy – which means there’s intense regional competition for science and engineering talent. As a small company, that means we have to work doubly hard to attract and retain the best people, with structured training and development among our big selling points for prospective employees.

How has ICEoxford been affected by the Coronavirus pandemic?

We’re a specialized technology business that, in normal times, relies on sales staff meeting face-to-face with scientists to initiate the requirements-gathering process, whether that’s in their laboratories or at scientific conferences and exhibitions. Like everyone else, though, we’re making the best of the workarounds. Our engineering team is currently doing lots of Zoom calls to design and iterate cryogenic systems on-screen with our customers. We’re trying to be as flexible as possible while maintaining rapid turnaround of designs and 3D drawings to customers.

What’s next for ICEoxford?

Despite the uncertainty with the global health crisis, we have started our new financial year with a full order book. The timing of the Queen’s Award is another huge boost, providing international recognition as well as renewed confidence in our competitive positioning versus other cryogenics vendors. Growth and investment are very much on the agenda and we are evaluating further development and diversification of our DRY dilution refrigerator range up to the 1.0 K regime. The task, as always, will be to combine sustainable growth with personalized customer support.

MRI monitoring of brain glucose levels may help detect Alzheimer’s disease

Alzheimer’s disease is the most common form of dementia, but early diagnosis is challenging as many of the disease biomarkers also occur as part of normal aging. Once patients start to exhibit symptoms, such as cognitive decline, they are likely already in the mid to late stages of the disease.

A collaboration headed up at City University of Hong Kong (CityU) and Johns Hopkins University has now developed an MRI-based technique that dynamically monitors changes in cerebral glucose levels. Abnormal glucose uptake and clearance in the brain’s lymphatic system are a hallmark of early Alzheimer’s disease. As such, the team aims to apply this approach to diagnose Alzheimer’s at an early stage, enabling early intervention to halt or slow down disease progression.

Dynamic imaging of brain glucose levels, using a technique called dynamic glucose-enhanced (DGE) MRI, can provide information on glucose delivery, tissue transport and metabolism. To ease translation into clinical diagnosis, the method must be compatible with clinical MRI scanners with 3T field strength. To achieve this, the team designed an adjusted on-resonance variable delay multiple pulse (onVDMP) MRI approach, which can simultaneously measure dynamic glucose response curves for both brain parenchyma and cerebrospinal fluid (CSF) on a 3T MRI animal scanner.

Using the 3T MRI animal scanner at CityU, the researchers tested their technique on six-month-old and 16-month-old transgenic mice with Alzheimer’s disease (AD), and healthy age-matched wild-type mice. Following a single injection of glucose, they used DGE MRI to monitor glucose uptake and clearance in the animals’ brain parenchyma (neurons and glial cells) and CSF.

In the younger mice, the team observed substantially higher glucose uptake in brain parenchyma in AD mice than in healthy mice. In the 16-month-old mice, the MRI results revealed notably lower glucose uptakes in both brain parenchyma and CSF of AD mice compared with age-matched healthy animals. In this older group, four typical brain regions studied (cerebral cortex, hippocampus, thalamus and entorhinal cortex) also had significantly lower maximum glucose uptake values after glucose injection.

Looking at glucose clearance, the team found that both young and old AD mice had significantly slower CSF clearance rates than age-matched healthy mice. This finding is consistent with recent reports of hampered CSF clearance, which leads to protein accumulation in the brain, and is attributed to abnormalities in the brain’s drainage system.

The team suggests that this reduced CSF clearance detected by non-invasive DGE could serve as an imaging biomarker to reveal the early pathology of Alzheimer’s disease, especially as its features are distinct from normal aging.

Eye scan can detect signs of Alzheimer’s disease

“By using glucose as a ‘tracer’, our imaging method can sensitively detect the distinctive changes of glymphatic system function at the molecular level at an early stage of the disease, helping us to differentiate [Alzheimer’s disease] from normal ageing,” says corresponding author Kannie Chan from CityU. “Besides, glucose is natural, biodegradable and commonly used in hospitals, such as the glucose tolerance test. Using it as a contrast agent for MRI is non-invasive and safe.”

The team published the results in Science Advances.

Lidar tracks mosquito behaviour by monitoring wingbeats

Mosquito lidar — Wingbeat scanner: the lidar system in operation. The bright line in the upper right of the beam reveals the motion of a mosquito. (Courtesy: Mikkel Brydegaard)

Lidar technology has been used to monitor how mosquito activity varies throughout the day in a natural African landscape. The measurements were made by an international team led by Mikkel Brydegaard at Lund University, and could lead to important advances in malaria prevention measures in many impoverished communities.

Malaria is a mosquito-borne disease that causes close to half a million deaths worldwide each year. Over 90% of these deaths are in Africa, where the problem is excerbated by a combination of widespread poverty, and several endemic mosquito species that only feed on humans. Global efforts to fight the disease have now made significant progress through approaches including pesticides, vaccines, and bed nets. However, the rapid evolution of both mosquitoes, and the Plasmodium parasites that cause malaria, means that resistance to some of these measures is developing fast.

The problem calls for a more universal approach to understanding the ecology of mosquito populations, particularly in natural outdoor landscapes where most of the insects reside. Brydegaard’s team achieved this using a high-resolution, near-infrared lidar system – which sends out laser beams that reflect off objects. Variations in the wavelengths and return times of the light is measured and this allows researchers to build up accurate, time-varying pictures of dynamic 3D systems.

Solar eclipse

The team set up their lidar next to a village in Tanzania. Over five days and four nights, the device scanned 3-5 m above the ground over a wedge of land at distances of up to 596 m. They observed over 300,000 insects during this time, allowing them to build up a clear picture of how mosquito activity varied throughout the day. To determine whether these behaviour patterns were caused by light levels or the circadian rhythms of mosquitos, the researchers also made measurements during a solar eclipse in September 2016.

To quantify mosquito activity, Brydegaard and colleagues used previously measured values for the characteristic wingbeat frequencies of mosquitos, which vary between male and female mosquitoes and between different species. Determining the sex of an insect is important because only female mosquitos bite.

By searching for these wingbeat frequencies in their lidar date, they found that mosquitos were most active during two precisely timed “rush hour” periods in the morning and evening. The insects also became far more active during the eclipse, which suggested that light levels, not circadian rhythms, are responsible for these bursts in activity.

Brydegaard’s team believes the results prove that lidar is a ground-breaking tool for studying insect ecologies over large geographical areas; and in turn, for informing measures to reduce the risk of malaria infection. The researchers will now focus on improving their techniques to incorporate rapid advances in lidar technology, and ultimately hope their findings could bring about techniques which will help to ease the suffering of many millions of people.

The research is described in Science Advances.

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.

Tunable light speeds up the search for the perfect qubit

Physicists are still looking for the perfect quantum qubit: a two-level quantum system that can be precisely measured and controlled, while remaining unaffected by its environment. One of the most promising candidates are defect centres in solid-state materials, also known as colour centres, which have been found to emit a single photon per excitation event when excited by laser light of a particular frequency.

Most early attention has focused on nitrogen vacancy centres in diamond − which offer single-photon emission at room temperature − but they are not ideal for all applications because their asymmetric charge distribution makes them sensitive to local fluctuations in the electric field. Researchers are therefore investigating the properties of different colour centres in diamond, including silicon and germanium vacancies, as well as other material systems such as 2D hexagonal boron nitride (hBN).

But it can be difficult and time consuming to map out the energy levels in these delicate quantum systems. One important technique is photoluminescence excitation (PLE) spectroscopy, which measures the tiny optical signals produced by single-photon emitters when they are excited by continuous-wave (CW) laser light.

Wide frequency range

“Researchers typically want to measure the response from the sample over a wide frequency range,” explains Jaroslaw Sperling, a chemical physicist at Hübner Photonics. “You need a light source that generates light of a very well defined frequency, and that can easily be tuned across a wide range of frequencies.”

Sperling believes that CW light sources based on optical parametric oscillators (CW OPOs) offer the most effective solution. Instead of the gain medium inside a conventional laser, an OPO generates coherent light from a nonlinear optical crystal pumped by a high-performance laser. “OPOs first emerged about half a century ago,” says Sperling. “But commercial devices have until recently only operated in the infrared or in pulsed mode, since high peak powers are needed to drive the nonlinear process.”

Over the last few years, however, improved design techniques have yielded more sophisticated and more efficient nonlinear materials, such as periodically poled lithium niobate, that can be phase-matched to the pump laser. A new generation of continuous-wave lasers also provides higher pump powers across the frequency spectrum, making it possible for the first time to produce widely tunable CW OPOs in a turnkey system that delivers narrow-linewidth output at power levels of a few hundred milliwatts.

Big advantage

One of the big advantages of OPO technology, compared to a conventional CW tunable laser, is that it provides more convenient control of the output wavelength. “There is no need to change the lasing medium or the laser cavity optics to achieve the desired wavelength, since in principle you can generate almost any colour in the visible range with a carefully chosen and suitably operated nonlinear crystal,” explains Sperling.

OPO technology also makes it easier to measure the spectra from different samples with the same experimental set-up. “There is no need to change or realign any of the optical components, because the position and direction of the beam always stays the same,” says Niklas Waasem, a laser physicist at Hübner who specializes in quantum applications. “In contrast, other tunable lasers typically need re-alignment of the beam path or even different optics inside the laser after larger changes in the wavelength. With an OPO, researchers can scan through the full frequency range from a computer, which makes it much quicker to characterize different colour centres in the same sample, as well as different samples or material systems.”

The latest commercial systems – such as Hübner Photonics’ C-WAVE – combine this wide tunability with the narrow linewidth needed to resolve specific spectral resonances. OPOs also offer a higher spectral purity than equivalent tunable lasers, which ensures that the weak optical signals generated by atomic-scale defects are not obscured by amplified spontaneous light emission that lasers can generate at unwanted wavelengths. “The spectral purity of an OPO is a huge advantage for PLE experiments,” says Waasem.

Quantum researchers have been quick to recognize the benefits of continuous-wave OPOs for assessing the potential of different single-photon emitters. For example, scientists from the USA, Germany, China, Singapore, and Japan have exploited the C-WAVE platform to measure the photoluminescence spectra from silicon, germanium and tin vacancy centres in diamond, none of which suffer from the same sensitivity to electric field as nitrogen defects.

In room-temperature experiments carried out on an ensemble of germanium vacancy centres, a broad sweep through the visible frequency range revealed a strong peak in the PLE spectrum at around 602 nm. More detailed experiments at 5 K explored the excitation dynamics of this specific single-photon emitter, showing that resonant fluorescence is only seen when the vacancy centre is also excited by a separate laser that can effectively act as an on/off switch.

The C-WAVE system has also been used by researchers in Germany to study individual defect centres in hBN, which are difficult to characterize because they have a wide range of transition energies across the entire visible spectrum. Measuring the photoluminescence intensity at more than 50 different excitation frequencies revealed the effect of phonon coupling on light emission, which offers a route to identifying the most efficient single-photon emitters in this 2D material.

Such early successes suggest that continuous-wave OPOs will become a popular choice for characterizing a range of different quantum systems. “The OPO concept can be used across different tuning ranges, which will allow for continuous adaption to new experimental requirements,” says Sperling. “Overall, we expect continuous-wave OPO technology to mature into a recognized choice among laser light sources that advance the rapidly evolving field of quantum research.”

Intensity modulation takes the next step

Once an ensemble of colour centres has been characterized using photoluminescence spectroscopy using a widely tunable laser light source, researchers often turn to other optical techniques to probe the dynamics of the most promising candidates at a specific frequency. These experiments typically require a light source operating at a single well-defined wavelength, and which also enables the intensity of the light to be changed extremely rapidly.

Until recently the most common approach would be to combine a laser with an acousto-optical modulator. Now, however, the same performance can be achieved with modulated diode lasers, which makes the set-up more compact, more robust and easier to align. “Modulated diode lasers from Hübner Photonics are popular for initializing and reading out spin-states because they offer fast modulation capabilities, a high on/off extinction ratio, a high level of spectral purity, as well as an excellent Gaussian beam profile,” comments Waasem.

Lasers making a difference in patient alignment: CT and LINAC solutions from LAP

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Different workflows and solutions will be elaborated on while questioning if lasers still make the difference in patient alignment. The need for a fluent process in treatment units will be discussed and how this requirement can be implemented.

The presenters guide through solutions of the leading manufacturer of laser systems for patient alignment and explain supported workflows for an optimal use and integration in clinical routine.

This webinar presented by Raphael Schmidt and Michael Uhr will discuss:

The advantages of using lasers for patient positioning in RT and how precise laser systems enhance safety for patient alignment, and therefore gain more time for treatment.
An overview of the current laser system portfolio from LAP including integrated workflow solutions and installation options.

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Raphael Schmidt (left) is responsible for the product management of laser systems for CT and MRI at LAP. During his studies at KIT (Karlsruhe Institute of Technology) he gained broad experience in different workflows in radiation therapy while analysing them. The topic of his final thesis dealt with the improvement of workflows in RT through new information and assistance systems. Raphael holds a degree in industrial engineering and management. Michael Uhr (right) is the product manager responsible for the APOLLO and ASTOR room lasers at LAP. Before switching to product management, Michael gained many years’ extensive experience at LAP’s customer sites worldwide.

Tunable lasers in quantum research: novel lasers for novel colour centres

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In the experimental quantum research community, widely tunable continuous-wave optical parametric oscillators (CW OPOs) are gaining recognition as novel sources of tunable laser light with great potential – not least due to their unprecedented wavelength coverage. Yet, the overall experimental requirements remain often challenging for the performance of turnkey OPO devices.

In this webinar HÜBNER Photonics specialists will present the characteristics of state-of-the-art tunable CW OPO designs and will describe several tuning schemes tailored to meet specific experimental needs. To illustrate the performance in the real-world laboratory, we will showcase several experiments recently published by research groups studying nitrogen-vacancy colour centres and other novel single-photon emitters alike.

Every knowledgeable experimentalist, and in particular those who aim at manipulation of novel quantum systems, should keep up to date with the latest developments in instrumentation available on the market. In this webinar you will get a review of state-of-the-art CW OPO technology comprehensible even for the non-specialist, illustrated with recently published real-world experiments.

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Korbinian Hens is a product manager for HÜBNER Photonics. He joined the company in 2015, specializing in terahertz spectroscopy and tunable laser systems based on OPO technology. With a background in laser-induced fluorescence spectroscopy for atmospheric research, he carried out his PhD thesis at the Max Planck Institute for Chemistry in Mainz, Germany.

Jaroslaw Sperling is global sales manager for HÜBNER Photonics. He joined the company in 2016, bringing his passion and experience gained in several sales and marketing related roles across photonics research and industry. With a background in laser spectroscopy, he holds a PhD in physical chemistry from the University of Vienna, Austria.

Niklas Waasem is regional sales manager and applications specialist for HÜBNER Photonics. He joined the company in 2014, bringing in profound photonics knowledge gained during his PhD thesis at the Fraunhofer Institute for Physical Measurement Techniques in co-operation with the University of Freiburg, Germany.

Mechanical oscillations cause iron to become transparent to gamma rays

Iron nuclei can be made transparent to gamma rays that they would normally absorb using a new technique called “acoustically induced transparency” (AIT). This feat was achieved by physicists in the US and Russia, who vibrated an iron Mössbauer absorber using a piezoelectric transducer. The researchers believe the effect could help to control the emission of radiation from nuclei, allowing more accurate atomic clocks and other quantum optical devices to be created. The technique could even be used to slow the passage of gamma rays through a material.

The new effect is reminiscent of electromagnetically induced transparency (EIT), which involves light at much lower optical frequencies than gamma rays. In general, EIT is achieved by using light at one frequency to manipulate the electronic energy levels in an atom in a way that affects its ability to interact with light at another frequency. EIT has been used to produce spectrally pure light in lasers and frequency standards in atomic clocks. But perhaps the most spectacular use of EIT is to slow light to a temporary halt in a medium before letting it loose again.

Physicists would like push EIT to higher photon frequencies but this has proven very difficult because it involves using highly energetic electronic states that decay quickly and produce light at poorly defined photon frequencies. “It’s very difficult for an electron to stay in a highly excited state,” explains Olga Kocharovskaya of Texas A&M University. “It wants to go to the ground state, so there are many mechanisms for relaxation.”

Extremely sharp transitions

One potential solution lies in the use of the gamma rays emitted by atomic nuclei. “The nuclear transitions can be extremely sharp,” says Kocharovskaya, “Nuclei in the excited state may live for hundreds of years in principle – and these very narrow linewidth transitions are available even at room temperature.” However, she says there are also considerable challenges, “Nowadays there are no spectrally bright sources of gamma-ray photons, and there are no optical elements such as delay lines, switches, mirrors or lenses to manipulate and control these gamma-ray photons”.

Multiple research groups have tried to develop variations of EIT that work in the hard X-ray and gamma-ray regions of the spectrum. In 2012, for example, Ralf Röhlsberger and colleagues at DESY in Hamburg, Germany, placed two 2 nm thick layers of iron-57 inside a cavity supporting an X-ray standing wave. They were able to vary the iron’s absorption of incident 14.4 keV photons by a factor of four depending on whether they placed the first or second layer at the node of the standing wave. Nobody, however, has achieved near-complete transmission through an otherwise absorbent medium.

Now, the Texas A&M scientists and colleagues at the Institute of Applied Physics of the Russian Academy of Sciences in Nizhny Novgorod and Kazan Federal University have created AIT, which relies on conservation of momentum. Iron-57 has a nuclear transition at 14.4 keV, which means that the nucleus can absorb a photon at this energy and then re-emit it. However, when a nucleus absorbs a gamma-ray photon it must recoil slightly. Energy from the photon is required to drive this recoil, which means that the absorption must involve a photon with a slightly higher energy (frequency) than the nuclear excitation alone. The opposite occurs when a photon is emitted – the photon has a slightly lower energy than the excitation energy.

Mössbauer effect

If the nuclei are free to move, this energy mismatch means that a photon emitted from one iron-57 nucleus cannot be absorbed by another iron-57 nucleus. However, when the nuclei are bound in a solid lattice, the recoil is negligible and so absorption can occur. This phenomenon is called the Mössbauer effect and is widely used to study the properties of solids.

When nuclei in a solid oscillate back and forth – because the sample is being mechanically vibrated, or by sound wave or even thermal motion – the Doppler effect causes the absorption to be shifted to higher and lower frequencies. The result is a comb-like structure of discrete absorption lines centred on the absorption frequency. Intriguingly, however, under certain conditions, Kocharovskaya and colleagues calculated that central peak should vanish entirely, leading the material to be completely transparent to gamma rays at the absorption frequency.

This was tested experimentally by Farit Vagizov both at Texas A&M University and the Kazan Federal University. He used 14.4 keV gamma-ray photons from the decay of excited iron-57 nuclei to the ground state. They used the same material to absorb the photons, mounting a film of iron on a vibrating piezoelectric transducer. They found that when they vibrated the absorber at the appropriate frequency, its absorption of 14.4 keV photons was suppressed by a factor of 148.

The iron-57 excited state has a lifetime of only 100 ns, but the researchers now intend to study longer-lived, less-studied excited states such as scandium-45, which has a lifetime of about 1 s. “During this time, the photon is stored in the excited state,” explains Kocharovskaya. This could potentially find applications in quantum memories and quantum communication.

Röhlsberger suspects the research will be “quite eyebrow raising”. “There are people in this field working with more sophisticated schemes. and this is remarkably simple and could have been discovered a long time ago,” he says. “It doesn’t require sophisticated instrumentation to set up, so – especially at modern synchrotron sources and X-ray lasers – I can imagine this could have impact, as you can effectively modulate the transmission from one to zero in thick samples.”

The research is described in Physical Review Letters.

Graphene joins the fight against COVID-19

Could a graphene coating make surgical masks easier to sterilize and re-use? According to a team of researchers at The Hong Kong Polytechnic University (PolyU), the answer is yes. Led by Guijun Li of PolyU’s industrial and systems engineering department, the team developed a laser manufacturing process that deposits a few layers of the carbon sheet onto commercially-available non-woven masks. The coating makes the masks superhydrophobic, which reduces the chances of infectious drops adhering to them, while graphene’s strong light-absorption properties makes it possible to sterilize them with exposure to sunlight.

To curb the COVID-19 pandemic, health officials are recommending that citizens wear masks in public, especially in situations where physical distancing is difficult or impossible. While any mask is better than no mask at all, the consensus is that surgical masks made from nonwoven polymer fibres are among the best types available for widespread use outside hospital settings.

Such masks do, however, have limitations. While they help prevent viruses (including SARS-CoV-2, the cause of the current pandemic) from entering the wearer’s nose and mouth via droplets generated when an infected person sneezes, coughs or talks, virus-laden droplets tend to remain on the mask. This means that masks must either be discarded after each use or sterilized before re-use. Neither option is attractive, since polymer-based masks are difficult to sterilize even with steam, while discarding them poses an environmental challenge. The estimated 40 million pieces of protective gear produced worldwide every day amount to a daily 15,000-tonne mountain of waste – much of which must be incinerated, adding to carbon emissions.

Superhydrophobic surfaces

One possible solution lies in making masks that are superhydrophobic, meaning that they strongly repel aqueous liquids. Researchers have previously made advanced superhydrophobic nanostructured surfaces out of materials such as fluorinated polymers, metallic nanowires and, most recently, graphene. Some of these materials could have medical applications, but according to Li, their potential has not been fully explored. “To the best of our knowledge, such materials have never been used on surgical masks before,” he says.

Li and colleagues began by synthesizing graphene using a low-cost laser technique that heats up precursors such as polyimide, SPEEK and Bakelite. They made the carbon sheet superhydrophobic by controlling the parameters during the laser processing.

Next, they used a new technique, developed in their lab, to deposit a few layers of this graphene onto commercial surgical masks. This new process is known as dual-mode laser-induced forward transfer (LIFT), and it uses a pulsed laser beam with a pulse duration of 10 ns. This short pulse length means that the momentum of the photons is high enough to transfer the graphene without significantly increasing the mask’s temperature – an important point, Li explains, because the melting point of the polymer fibres in the mask is only 130 °C. The LIFT method is also compatible with a roll-to-roll system, meaning that it can easily be integrated with existing automated mask manufacturing production lines, he adds.

Water droplets roll off

Since the graphene is superhydrophobic, it is self-cleaning, like a lotus leaf, Li tells Physics World. Indeed, water droplets freely roll off the surface of the mask before they have time to adhere to it.

Another benefit, the researchers say, is that they can sterilize their graphene-coated masks simply by exposing them to sunlight for 40–100 seconds. This is possible because graphene absorbs more than 95% of light across the solar spectrum from 300–2500 nm, so the coated masks quickly increase in temperature, reaching 70°C after 40 seconds of solar illumination and more than 80°C after 100 seconds. That is high enough to inactivate most types of viruses, meaning that the mask can then be reused or (if damaged) safely recycled. In contrast, masks without a graphene coating do not show this photothermal effect, since they absorb sunlight only weakly. Even after 5 minutes of solar illumination, their temperature does not exceed 50°C, Li explains.

Testing for antiviral capacity

Valentina Palmieri, a researcher at Italy’s Institute for Complex Systems who was not involved in the PolyU study, commends Li’s team for developing a way to transfer graphene onto surgical masks, and says that using the light absorbance properties of graphene to recycle and sterilize them is “a feasible approach”. As the author of a recent paper entitled “Can graphene take part in the fight against COVID-19?”, Palmieri points out that a similar light-based process could also be used to sterilize graphene-coated environmental air filters. She also notes that large surface area graphene has already been used to construct sensors embedded in textiles, as well as platforms that use antibody conjugation to diagnose disease. In some cases, the nanomaterial itself showed antiviral capacity.

The PolyU team, who report their work in ACS Nano, say they have successfully tested the LIFT graphene against E. coli bacteria. They now plan to test their graphene-coated masks against viruses, including SARS-CoV-2.

Tracking elephant rumbles without breaking the bank

Although more famous for their trademark trumpet sounds, elephants also make loud low frequency vocalizations known as “rumbles”, which extend below the range of human hearing. In a new study, a team of geoscientists has tracked the acoustic and seismic signals of rumbles among a family of elephants at a reserve in South Africa. They did this using a relatively low-cost sensor kit – showing promise as an affordable tool for wildlife studies and conservation projects.

In ecology, there is a growing understanding that monitoring animal sounds can reveal a lot about animal behaviour. At the same time, the increasing availability of cost-effective and scalable acoustic sensors is leading to a growing number of animal studies that make use of this technology. Tracking animals in this way also removes the need to physically tag them, which can be logistically difficult and stressful for the animals.

Elephants are big. Given the vast size of their vocal tracts, elephants can produce sounds that are loud and low – with frequencies below the 20 Hz limit of human hearing. As well as travelling through the air, this infrasound can also couple with the ground and propagate through the Earth as seismic waves. Earlier research, reported in Physics World, observed that the seismic component of rumbles could play an important role in long-range communication among elephant herds.

“There is evidence from previous studies that rumbles are used to coordinate movement and spacing of social groups, help individuals find each other, as well as triggering defensive or exploratory behaviour,” said Oliver Lamb of University of North Carolina at Chapel Hill in the US, who led the latest study and presented it last week at EGU2020: Sharing Geoscience Online.

Equipment costs – the elephant in the room

Lamb’s group set out to see if they could get meaningful results using an off-the-shelf device known as a Raspberry Shake and Boom (RS&B). Resulting from a Kickstarter campaign in 2016, the RS&B – based on the Raspberry Pi computer – is popular among educators and amateur seismologists due to its accessibility and cost. While conventional geophysical sensor systems tend to cost thousands of dollars, the RS&B seismo-acoustic is currently sold to researchers for approximately $865.

Lamb and his team took five RS&B units to study a family of seven African elephants at Adventures with Elephants – a 300 hectare area reserve in the north-east of South Africa. During a four-day period in October 2019, the group focussed on reunion events, which tend to involve a significant amount of vocalization. To calibrate readings, they also deployed a collection of more sensitive – but more costly – monitors.

To the researchers’ surprise, the acoustic component of rumbles below 50 Hz was more clearly recorded using the low-cost device than with the more sensitive microphone. This extra information would be useful in ecology studies for distinguishing individual animals by age and size. On the down side, the acoustic range was limited to roughly 400 m, which would only be of practical use in locations where elephants are known to congregate, such as waterholes and food sources.

Unfortunately, the seismic range of the device was even smaller. Individual rumbles were detected to within 100 m, while elephant footsteps were limited to just 50 m. Describing their results in a paper submitted to Bioacoustics, the researchers suggest they could extend the range up to 1 km by anchoring sensors to rocks rather than burying them in the ground.

Stomping towards a monitoring system

In fact, the clarity and range of seismic signals is likely to be affected by a number of factors, according to Tarje Nissen-Meyer, a geophysicist at the University of Oxford in the UK, who was involved in a previous seismic study of elephants in Kenya. “We showed that the distances up to which seismic cues may be detectable were strongly dependent on many factors, including local soil and geomorphology, geological structures, noise environment and the type of elephant motion or vocalization,” he said.

Nissen-Meyer describes the new study as promising and believes the range could be extended with just a few alterations to the experiment set up. “I personally maintain that seismic monitoring of wildlife in the savanna can indeed be a viable path forward since seismic signals are, as seen in the Lamb study, detectable with little effort compared to approaches such as tagging after tranquilizing.”

But even with range limitations, the study could still represent an important step towards developing a practical system for elephant monitoring. One of the goals of tracking wild animal vibrations in Africa is to develop early warning systems that could alert park wardens of distressed animals or even the presence of poachers.

“Aside from the noises generated by the elephants, the Raspberry Shake and Boom can also pick up noises generated by humans either from the vehicles they use and, if the conditions are right, the footsteps of the humans,” said Lamb. “We are exploring the potential of integrating the Raspberry Shake and Boom sensor into some kind of package that can be deployed in game farms, game reserves and national parks, especially those that cannot afford the expensive technologies found at Kruger National Park and some private reserves.”

Measuring molecules with nanometre precision

A new motion-correction method for single-molecule localization microscopy (SMLM) lets researchers measure the position of individual molecules with unprecedented accuracy. By stabilizing SMLM images in real time, a team at the University of New South Wales (UNSW) determined the distance between individual proteins on the surface of human immune cells with single-nanometre precision. This improvement in SMLM resolution could lead to greater understanding of cell signalling, but the technique might also be applied to other high-precision instruments such as DNA sequencers and atomic force microscopes.

Observing cellular processes at the level of individual molecules requires images with a resolution that’s impossible to achieve using conventional microscopy: at optical wavelengths, the best resolution permitted by the diffraction limit is far too coarse. In recent years, however, researchers have found a way to get around this limit using a family of techniques known collectively as SMLM.

In SMLM, target molecules are labelled with fluorescent markers that emit for brief periods at random intervals. An image captured at any given moment might record just a few individual excitation events, but by stacking thousands of frames acquired over hours or days, a comprehensive molecular map can be built. As long as the emission events in each frame are spatially separate, the accumulated signals can be statistically fitted to locate the source of each molecule to the nearest nanometre.

That, at least, is how it works under ideal conditions. In reality, minuscule movements of the camera over the course of the process mean that, until now, researchers have struggled to pin down molecules’ positions with a precision better than 20–30 nm.

“There are a number of things that cause drift in the instrument,” says team leader Katharina Gaus. “The biggest cause is probably vibrations – from people walking in adjacent rooms and corridors, for example. All buildings also have a natural frequency, and vibrate when cars (or, in our case, the tram) go past outside.”

University of New South Wales researchers — Katharina Gaus (second from left) and colleagues at the University of New South Wales. (Courtesy: Single Molecule Science, UNSW Sydney)

Gaus and her colleagues developed a way to compensate for this motion using three separate techniques. First, they placed 3 µm polystyrene beads on the imaging stage (out of the camera’s field-of-view) as fiducial markers. When illuminated with an infrared laser, the beads produced diffraction rings that revealed any relative motion between the sample and the rest of the instrument. These measurements were passed into a feedback loop that corrected the sample’s position 12 times per second, limiting motion to less than 1 nm over a 20 hour period.

The team set up another feedback loop by integrating a white LED into the microscope body and focusing it on the corner of the camera’s sensor – an electron-multiplying charge-coupled device (EMCCD). This produced an optical fiducial whose intensity peak could be located with a precision of 0.05 nm. Any change in this position represented a drift in the fluorescence signal, which the researchers corrected automatically with a piezoelectric mirror.

The third correction addressed potential discrepancies in how the camera’s EMCCD registered the positions of green or red emitters. To detect and compensate for such discrepancies, the researchers measured the sensor’s responses to a nanohole array filled with green and red dye.

Gaus and colleagues tested their technique – which they call Feedback SMLM – by measuring the positions of signalling proteins on human T cells. They found that the process of T cell activation is determined by the distances between specific proteins, and that a difference in separation of just 4–7 nm distinguishes one cellular response from another.

Although discerning such tiny distances would be impossible using conventional SMLM, it is within the scope of an existing method called fluorescence resonance energy transfer (FRET). In this technique, the varying proximity of two fluorophores is reflected in changes to their emission intensity. However, Gaus thinks that Feedback SMLM could replace FRET, as the latter technique is blind to inter-molecule distances larger than 10 nm, and is also affected by other factors including the fluorophores’ dipole alignment and the pH of the environment.

Full details of the research are reported in Science Advances.

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