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‘Twistronics for photons’ brings tunable diffraction-free light rays

Light diffracts as it travels around objects. If it didn’t, designing quantum optics devices and getting optical microscopy to resolve nanoscale images would be heaps easier. Now an international collaboration of researchers has shown that dispersion- and diffraction-free propagation is possible, with a resolution that beats the diffraction limit by more than an order of magnitude, in twisted layers of 2D molybdenum trioxide. These photonic effects mirror the behaviour of electrons in twisted bilayer graphene, where reports of electrons travelling with no resistance kicked off a rich new field of 2D materials research known as “twistronics”.

In 2018 Pablo Jarillo-Herrero and colleagues observed that electrons in pristine bilayer graphene  – layers of a honeycomb-shaped carbon atom lattice – could be made to superconduct, or exist in Mott insulator states (depending on whether an electric field is applied or not) when one layer is twisted by a “magic angle” with respect to the other. Following this announcement, reports of other twisted 2D systems exploded in number. But while scientists with a penchant for curious electron antics pondered how electrons racing through magic-angle graphene could couple strongly enough to form the Cooper pairs behind superconductivity, this so-called “flat band” behaviour was also stimulating new ideas for researchers in photonics.

Photonic flat bands

Andrea Alù, a researcher at City University of New York and senior author of the latest report (which is published in Nature) notes that in magic-angle twisted bilayer graphene, the term “flat band” refers to a state in which electron energies flatten as their momentum increases, rather than increasing linearly with momentum (as they would in a single layer of graphene). This behaviour arises because when the two carbon-lattice layers are twisted ever so slightly out of sync with each other, electrons tunnelling between them experience a potential field with a new periodic variation, like the beating of two musical tones.

Photonics, of course, is concerned with light rather than electron transport. However, Alù and his collaborators realized that similar flat-band behaviour can also occur in photonic “metasurfaces” – that is, materials with a composition and structure engineered to support unusual photonic effects at their surface.

Normally, light emanates from a point source in circular wavefronts, like the rings round a pebble dropped in a pond. In metasurfaces engineered to have anisotropic photonic responses, however, these rings become squashed into ellipses. At their extreme, these ellipses can even take on a hyperbolic shape, like the trajectory of a rocket reaching escape velocity. The photonic flat bands Alù and colleagues observed appeared at the transition between elliptical and hyperbolic responses to light, which occurs at the resonance frequency of a metasurface made from a grid of graphene nanoribbons.

Inspired by developments in twistronics, the researchers decided to investigate whether the photonic flat-band behaviour in two layered metasurfaces would change if the metasurfaces were twisted relative to each other. They calculated that they could shift the flat band away from the resonance frequency simply by controlling the relative angle between the two surfaces. This would be a huge bonus, because the metasurface strongly absorbs light at its resonant frequency, which would make it hard to exploit the flat-band effect. However, for this effect to work without suffering from large nonlocalities, they found that they needed to make the nanoribbons in their metasurface very densely spaced, with features that are prohibitively small for even the most sophisticated nanofabrication techniques.

Natural flat bands

These stringent nanofabrication requirements posed a serious challenge to the researchers’ effort to experimentally validate their results. The breakthrough came with reports that a natural (that is, non-structured) 2D material known as α-molybdenum trioxide (α-MoO3) exhibits unusual dispersion behaviour in quasiparticles known as phonon-polaritons, which arise when incident photons and vibrations of the material’s lattice oscillate in unison. “Interestingly, MoO3 naturally supports hyperbolic polariton propagation in plane, which is exactly what we needed for our concept to work, without any complicated fabrication requirements,” Alù tells Physics World.

He and his collaborators took two layers of 2D α-MoO3 rotated with respect to each other and used a nanoscale metallic tip to excite the phonon-polaritons. They then used the same tip as a scanning near-field optical microscopy (SNOM) probe, which measures the non-propagating near-field, to achieve non-diffraction-limited polariton images.

These experiments revealed the existence of an all-important flat band at magic-angle twists of the two α-MoO3 layers, where phonon-polaritons propagate as rays with no diffraction. While diffraction limits the resolution of propagating light to around half its wavelength, the researchers clocked the full-width half maximum of the ray near the defect at 1/40th of its free-space wavelength. The ray’s decay length was also almost eight times the phonon-polariton decay length at the flat band resonance in a single layer of α-MoO3. The effects were observed even when the thicknesses of the layer varied, indicating they were not easily perturbed – a robustness typical of topological phenomena.

“To me, the most exciting part is the beauty of how you can predict this phenomenon to arise from purely geometric formulae,” explains Alu. “Overlaying the hyperbolic shapes associated with each of the isolated layers and gradually increasing the twist between the layers will lead to crossing points where the hyperbolic dispersion curves intersect. At first there will be just two intersections, but when the other sides of the hyperbolae intersect as well, there will be four, and it is at this ‘magic angle’ for which the number of intersections changes that the flat band and associated effects occur. Because the shape the hyperbola makes with respect to the lattice for a given frequency is known, it is possible to predict the magic angle just by counting the intersections. You can then tune the flat band to any frequency of interest within the hyperbolic band of α-MoO3 by twisting.”

Frank Koppens, a researcher at the Institute for Photonic Science in Spain who was not involved in the paper, notes that twisted two-dimensional materials “have opened a completely new era in material science as a completely new way to design correlated matter in a controlled fashion.” Koppens, who works at the frontiers of both 2D materials and nanophotonics, adds that this latest work “has taken twistronics into the realm of twist-nanophotonics, and demonstrated completely new ways of working with optical fields at the nanoscale.”

The researchers expect their effects to have important implications for nano-imaging, unusual resonant features, quantum optics, low-energy optical signal processing and computing.

Ask me anything: Sarah Cruddas

Sarah Cruddas

What skills do you use every day in your job?

My degree was in physics with astrophysics, and it has been hugely valuable to me throughout my career. Although I have never worked as a professional scientist, the ability to understand complex mathematics and scientific concepts has made me better at my job. In order to communicate a subject, you have to understand what you are communicating. By having a scientific background, I am equipped with the skills needed to ask the right questions of scientists and understand what they are talking about. My degree has also added to my credibility when communicating science.

But studying physics has also provided me with many other skills, which I still use to this day. These include team work – developed through years of group work and doing labs at university – as well as the ability to communicate complex scientific topics and ideas to others, a skill I developed thanks to group work during my undergraduate studies.

What do you like best and least about your job?

What I love most about my job is the opportunity I have to inspire as many people as possible about space. Be that through social media, partnership with organizations and companies, hosting events, television shows, radio shows or books. My audience is a mix of people who are interested in space and want to learn more, and those who don’t necessarily know a huge amount, but who I get to inspire about how space is shaping their lives. It is a privilege to help explain to as many people as possible the importance of space exploration.

The second thing I love about what I do is the travel and the opportunity to work with incredible people. I have travelled the world filming and giving talks about space exploration and I have worked with the likes of Buzz Aldrin, Jeff Bezos, Chris Hadfield to name but a few. I have appeared on screen in Times Square, New York, hosted events in countless countries and been fortunate enough to see several space launches.

However, what I do involves long hours and extended periods of time away from home. It also involves being a self-starter and an element of hustle, developing new ideas and projects, which may not be for everyone. Added to that is rejection; it’s a natural part of working in the media and you need to have thick skin in order to be able to deal with this element. Overall, though, I think that I have the best job in the world, and am always exited to be playing my small part in helping humanity continue to explore space.

What do you know today, that you wish you knew when you were starting out in your career?

I wish I had known to aim higher. One of the phrases that frustrates me the most is “manage your expectations” – I heard it a lot in my early career. My advice to anyone starting out in their careers is to follow their dreams. However, don’t do so blindly – think about what you want to achieve and the skills you need, and work towards developing them. Have a plan in place, but don’t stick to it rigidly. Early in my career I also didn’t really know of anyone who did the type of job I wanted to do, but it is really important to find mentors and look at what other people are doing. I wish I had known more about how to find a mentor. It is so important to research who is doing the type of work you want to do and then reach out to them and ask questions. The worst that can happen is that they will say no, but most people are keen to help.

The other thing I wish I had known when I was starting out is that time is your friend. It can sometimes be frustrating; you don’t think you are getting where you hoped you would be. Nothing happens overnight. You have to be patient and keep working consistently hard, that is the key to success. And sometimes you will fail, but failing or getting rejected isn’t always a bad thing. Instead, the most important thing to do is to embrace failure and learn from it. Failure doesn’t define us, but how we react to things not going as planned does.

Physicists come out in support of today’s Strike for Black Lives

Graphic showing cartoons of protesters carrying "Black Lives Matter" and "No justice no peace" signs over the slogan "#ShutDownAcademia" and the date June 10th, 2020
A day of action: The logo of the #ShutDownAcademia and #ShutDownSTEM campaign. (Courtesy: #ShutDownAcademia)

Today – Wednesday 10 June – will not be a normal day in the physics calendar. That’s because, at the time of writing, more than 3000 physicists around the world have agreed to take part in a “Strike for Black Lives”.

Organized in the wake of the death of George Floyd – part of a wider pattern of police killings of black Americans that has now been condemned by several US scientific societies – the strike is not intended to be a “day off” for non-black people. Instead, participating physicists have pledged to use the time they would have spent teaching classes or doing research to explore what practical actions they can take to support black physicists.

The strike has come about following discussions between the cosmologists Chanda Prescod-Weinstein from the University of New Hampshire and Brian Nord from Fermilab and the University of Chicago, working together with members of the #ShutDownSTEM initiative and the Particles for Justice Group.

If you’re wondering what you can do, the Strike for Black Lives organizers have compiled a list of suggested steps and actions. Staff at the arXiv pre-print server have already agreed not to announce new papers on the day of the strike, while staff at the Institute of Physics (Physics World‘s parent organization) plan to suspend normal work in favour of “reflect[ing] on the role that we can take as individuals and as an organization”. If you wish, you can pledge your personal support too.

Here at Physics World, we’re highlighting – on our home page and on social media – relevant stories we’ve published by or about black physicists and the issues they face.

Some physicists, I am sure, will question the need for this initiative or feel it has no relevance to them. But we all harbour unconscious biases, myself included – as I discovered while writing this feature on diversity in physics.

If there’s only one thing you do, read this powerful open letter by Nord and Prescod-Weinstein, who explain far better than I can precisely why the Strike for Black Lives is so vital.

Posted on behalf of the Physics World editorial team

Physics in the pandemic: mailing lab kits to students enhances learning at home

What is the point of a first-year physics lab? As lab instructors, we had to grapple with that question when we learned that the summer term at McMaster University would be done online because of the COVID-19 pandemic. We had to consider not just what we could accomplish with the lab assignments we gave, but also, what we wanted our students to accomplish

Traditionally, our course (1A03) included eight hours of in-lab work, done in groups of 2-3 under the close supervision of teaching assistants. Students also did one major at-home experiment of their choice. When it was announced that the summer term would be online, me and my colleagues Sara Cormier and Kari Dalnoki-Veress decided to develop a kit that students could use to do all the experiments for the course at home.

For our cohort, who are mainly life science students (and in many cases not excited to be taking physics), we thought that a more pragmatic approach focusing on building experiments, collecting good data, and developing a physical intuition for the concepts in the course would better suit their needs. However, the COVID-19 shelter-in-place order was going to make obtaining materials and performing experiments difficult for students. Our solution was to mail each student a tiny “At-Home Lab Kit”.

The contents of the kits were simple by design (see figure). Using household items, we wanted to reinforce the connection between the real world and what they learn in class. Additionally, the labs focused on determining what they can measure with limited tools, and how those measurements can be turned into reproducible data that can be used to test a theory. Indeed, the skills developed by doing these experiments go well beyond what we usually teach in this first year course.

Exceptional accuracy

Students were surprised by the kind of results that can be obtained with such simple items. In the first lab session, students collected data that allowed them to measure the acceleration due to gravity with exceptional accuracy. Another lab used a clear plastic box, protractor, and some pins to measure the index of refraction of water. Further experiments with diffraction were done and students investigated total internal reflection, which is crucial to the operation of optical fibres for telecommunication.

With a bouncy ball, students measured the time between successive bounces to investigate energy transfer, energy loss, and the validity of concepts like conservation of energy and momentum. And of course, no first-year course would be complete without dealing with frictional forces. In pre-COVID times, students would use a digital force-transducer to make precise measurements. This year, they were given a spring (a coiled-plastic keychain). An example of how students had to calibrate their spring and make measurements is shown in the video below.

Early action and planning were crucial to the success of this project. Despite the simplicity of the kits, we first had to obtain approval from the university’s Crisis Management Group, convincing them the kits were safe to be sent to students. Perhaps misunderstanding our intentions, we were asked to provide Workplace Hazardous Materials Information System assessments for each object to be shipped. In the end, we all agreed students already understood how to safely handle and store rulers and elastic bands.

More difficult were dealing practical issues like shipping over 200 kits across the world. For students who were unable to receive packages (including several undergoing a 21-day hotel-based quarantine), we offered one-on-one advice on scavenging for materials — a skill that, as an experimental physicist, I take pride in. In only two cases could students not perform the labs themselves. For them, we provided videos that they could make measurements from.

At the time of writing, we are releasing the final lab exercise for the course, and early feedback from students has been very positive. Though we wish we could be back in our labs in September, we are thrilled to share these labs with 900 more students in the fall term. We are confident that we are not simply making the best of a bad situation; we are providing a unique experience for our students that will hopefully compliment whatever path they take at McMaster.

Graphene electrodes enable functional MRI during deep brain stimulation

Echo-planar images
Echo-planar images from rat brains implanted with GF (top row) and platinum–iridium (PtIr) electrodes; and (right) magnetic field distortions observed in rats with GF and PtIr electrodes. Red and blue arrows point to GF and PtIr implants, respectively. (Courtesy: CC BY 4.0/Nat. Commun. 10.1038/s41467-020-15570-9)

The ability to perform functional MRI (fMRI) during deep brain stimulation (DBS) is important for understanding the effects of DBS therapies. DBS, which involves electrical stimulation of neural tissues via implanted electrodes, is used for treatments of movement disorders such as Parkinson’s disease and essential tremor. But the therapeutic mechanisms and neuromodulatory effects of DBS are not well understood.

While fMRI provides a powerful tool for mapping brain activity, strong magnetic field interference from conventional metal DBS electrodes creates artefacts in the MR images. Such artefacts preclude functional and structural mapping of large volumes of brain tissues surrounding the electrodes and impede visualization of local responses at the stimulation site during simultaneous DBS and fMRI.

A team in China has now shown that use of novel graphene fibre (GF) DBS microelectrodes can enable full activation pattern mapping during DBS–fMRI in a rat model of Parkinson’s disease. The GF electrodes caused minimal interference with the magnetic field of a 9.4 T MRI scanner, enabling significantly better, artefact-free or near artefact-free, images than those created when using tungsten wire or platinum–iridium electrodes.

The researchers, at Peking University and the Institute of Neuroscience, Chinese Academy of Sciences, fabricated GF electrodes with a charge-injection-capacity 70 times greater than DBS electrodes made of platinum–iridium, the material most commonly used in clinical neural stimulation devices. This high charge-injection-capacity is desirable to decrease MRI artefact size and improve stimulation resolution.

Xiaojie Duan and Zhifeng Liang
Xiaojie Duan (left) and Zhifeng Liang.

Co-principal investigators Xiaojie Duan and Zhifeng Liang created the GFs from aqueous graphite oxide suspensions injected into a glass pipeline. They baked the pipeline at 230°C to produce a GF matching the pipe geometry, with a fibre diameter of about 75 µm.

To fabricate a GF-stimulating microelectrode, they pasted together two parallel-aligned insulated GFs, with one end soldered to a custom MRI-compatible copper connector that interfaces with the stimulation pulse generator. The GFs were then mechanically cut to expose their cross sections as electrically active sites. These GF electrodes showed a higher charge-injection limit than most available electrode materials.

Locomotor activity
Activity of a Parkinsonian rat before, during and after DBS with a GF electrode. (Courtesy: CC BY 4.0/Nat. Commun.)

The researchers tested the DBS capability of the GF electrodes by using them to stimulate the subthalamic nucleus in Parkinsonian rats, with stimulation parameters replicating a clinical DBS setting. They observed significant improvement in the rats’ mobility, confirming the therapeutic efficacy of subthalamic nucleus-DBS with GF electrodes.

Comparing MRI artefacts generated by the GF microelectrodes with those created by platinum-iridium microelectrodes of the same diameter showed that the GF electrodes exhibited much smaller artefacts in both anatomical images and functional (echo planar imaging) scans. The GF electrodes also showed high stability under continuous overcurrent pulsing.

“The GF electrodes caused minimal interference to the magnetic field, and their presence would not cause significant attenuation in fMRI signals, thus enabling a full and unbiased mapping of the activation pattern under DBS–fMRI studies,” write the researchers. “Such advantage is critical for exploring the neuromodulatory effects and mechanisms of DBS therapies.”

“With the unique capability for full and unbiased mapping of the entire circuit and network connectivity without obstructing brain nuclei, future DBS–studies with the GF electrodes at different targets and with varied stimulation frequency and strength could provide important insights into brain circuitries and network connections, as well as the therapeutic mechanisms underlying various DBS therapies,” they write.

The team now hopes to use the GF electrodes for DBS–fMRI studies on other neurological disorders, such as treatment-resistant depression.

The study details are reported in Nature Communications.

 

Ultraporous metal-organic frameworks could make clean energy carriers

A new metal-organic framework material based on aluminium can store large amounts of hydrogen and methane at relatively low pressures. The material might be used to carry clean energy in fuel cell-powered vehicles according to the researchers at Northwestern University in the US who developed it.

In 2017, transport vehicles (including cars, trucks, planes, trains and boats) overtook power plants as the largest source of greenhouse gas emissions in the US. The share of transport-based emissions increased still further in 2018, and the trend is expected to continue – making the search for transport-friendly alternative energy sources ever more important.

Methane and hydrogen are often touted as potential replacements for diesel and petrol fuels in vehicles. While methane is considered a “transition” fuel, since its combustion still emits carbon dioxide (albeit less than that of petrol), hydrogen has been hailed as “the fuel of the future” since burning it produces neither carbon dioxide nor particulate-based pollution.

The problem is that because both hydrogen and methane are gases at ambient temperatures, they need to be compressed and kept at high pressures (of 700 bar and 250 bar respectively) whenever they are transported and stored. Since such high pressures can create hazards for drivers and others involved in handling stored fuel, the maximum storage pressure limit for real-world vehicles has been set to 100 bar – significantly reducing the amount of gas that can be stored in a given space.

Storing increased amounts of gas without increasing the pressure

In recent years, researchers have investigated high-surface-area porous adsorbent materials as a means of increasing the amounts of gas that can be stored in a given volume without increasing the pressure. Metal-organic frameworks (MOFs), which have surface areas of 2000m2/g or more, are considered promising candidates. These highly crystalline materials are made up of organic molecules and metal ions or clusters that self-assemble into multidimensional structures, and are easy to design thanks to their tailorable pore chemistry and shape.

The Northwestern team, led by Omar Farha, used molecular simulations to inform the design of ultraporous MOFs based on trinuclear clusters, called NU-1501-M (where M is Al or Fe).

The researchers found that NU-1501-Al boasted high gravimetric (mass) and volumetric (size) storage performances for hydrogen and methane. Indeed, the material proved capable of storing 0.66 g of methane per gram of material at 100 bar and 270K – a value that exceeds the 0.5 g/g target set by the US Department of Energy (DOE) for developing the next generation of clean-energy automobiles. The material also has a high deliverable storage capacity of 14% by weight for hydrogen, which means it can store 14% of its own mass of hydrogen. While this figure seems low compared to its ability to store methane (66% by weight), it again surpasses the DOE target for 2020 of 4.5% by weight.

Nanosized pores mean a high surface area for gas adsorption

These high values are possible thanks to the material’s tiny pores, which measure less than 2.5nm across and thus offer a very high surface area for gas adsorption. As Farha notes, a one-gram sample of the material, with a volume equivalent to six M&M candies, has enough surface area to cover 1.3 American football fields.

This extensive surface area means that the team’s MOFs can store “tremendous” amounts of hydrogen and methane within their pores, Farha says, adding that the materials could deliver either gas to a car engine at lower pressures than are needed for current fuel-cell vehicles.

The new MOFs are detailed in Science.

Safe training: the linac simulator

The linear accelerator – or linac – that’s used to deliver radiation treatments to cancer patients is one of the most complicated devices that exists within healthcare. Training medical physicists on how to use such machines requires instruction in the fundamental physics of beam acceleration, and how these principles apply to the clinical device. Trainees also need access to the linac itself, but such systems are only available when not in use for patient treatments. The devices can be easily damaged if set up incorrectly, and risk unwanted radiation exposure if used in the wrong way.

Some years ago, these concerns inspired Marco Carlone, then a medical physicist at the Princess Margaret Cancer Centre in Canada, to develop a linac simulator, aptly named the SIMAC. The simulator is an online teaching tool that’s designed to train people to use medical linacs. The idea is to provide a real-time simulated radiation beam that’s accessible round the clock from any PC, and enables trainees to safely operate a virtual linac without any of the related risks. In October 2019, Carlone founded Linax Technologies to commercialize this simulator. 

What was your motivation for developing SIMAC?

Today, linac training is mainly performed by hands-on learning. One of the reasons that I created an online simulator was to allow people to have access to a linac without having to actually be at one physically. Linacs are large, complicated machines and, in many ways, also highly dangerous. If used incorrectly they could expose the trainee to radiation. Another challenge when teaching people how to operate a linac is that if the device isn’t left in the correct state, it could harm a patient afterwards.

An online tool takes these problems away, allowing unsupervised access to linacs. The student doesn’t need to have someone with them to ensure that they are using the device correctly. They can also feel free to experiment with the device and learn how it works by changing parameters that you wouldn’t want to change on a machine that’s used in the clinic.

What made you decide to start a company?

I have a background in linear accelerator engineering, but about 20 years ago, I changed fields into medical physics. It was always apparent to me that there was a gap in approach between the people who design and build linacs, and the people who ultimately use them. The first time that I built a simulated linac environment was about six years ago, with grants from my institution and other sources. That was helpful in getting some people together to work on this, but as soon as the money ran out, the project slowed down quite a bit. At the same time, a lot of people had given me feedback that this approach was very useful. So, I thought it would be helpful to have a more continuous way of keeping the project going and keeping it sustainable through a commercial model.

There was a gap in approach between the people who design and build linacs, and the people who use them

Who are the target users for SIMAC?

At present, the linac simulator is designed for young medical physicists who would like to understand the functioning of the device, while learning how the physics of the accelerator relates to the clinical properties that they would likely see in the clinic. For example, users can see how adjusting the energy of the machine affects its clinical properties, which would be important for things like treatment planning or quality assurance. 

One of my main drivers is making linac training and learning available to anyone who wants it. In developing countries, one of the challenges with these highly advanced machines is training people to use them when there isn’t anyone locally who can teach this and access to resources is sometimes difficult. The ability to teach people anywhere in the world is something I’ve been trying to do since this idea dawned on me. 

I’m hoping that an online community, through a company like Linax Technologies, will allow people to have access to the materials they need in order to operate very complicated equipment, to create plans for their patients that are safe, and to communicate with anybody they’d like to in order to improve that. 

Do you have any advice for someone wanting to commercialize their own research?

The most important thing that I have learned is to focus on the people who are going to use your product – it sounds like a simple thing to say but it’s sometimes difficult to understand. As an example, when I built the simulator, I thought that all medical physicists would be interested in this because they all recognize the importance of linacs and will want to understand how they work. And while that was true, I found that not many people were willing to spend money on it. 

So one of the reasons why I’m focusing this on a specific segment – early-career medical physicists – is because those are the people who really need technologies like this, not just for their own knowledge, but also to help them with their career and to be more effective in what they do. It’s important to understand how people are using your product and what they are using it for. That will drive how you make decisions around how you develop it and what to focus on. The other problem with a start-up company is that you have very limited resources, and you have to make sure that you put your energy into things that will be valuable for the company moving forward.

How are you planning to develop SIMAC further?

Ultimately, the goal of Linax Technologies is to simplify radiotherapy. The name of my company is a hybrid of Linux and linac. Linux is an open-source operating system, and in my mind, radiotherapy is really an open-source community. We don’t do anything in radiotherapy unless the whole community endorses it, whether that be through the literature or via conferences. So what I’d like to do with Linax Technologies is provide a means to do that, starting with the linear accelerator, but hopefully expanding to other areas of radiotherapy and making the necessary information available to anyone who wants it. 

It’s quite an ambitious goal. I’m starting very small with one core problem – how to make your linac work – but I’m hoping we’ll be able to expand that out into other areas that would be helpful for people, in developing countries or anywhere else.

Electric fields control spintronics devices

A new low-power technique for detecting the spin of electrons in a non-magnetic system could aid the development of spintronics devices that work using ferroelectricity rather than ferromagnetism. Such devices may eventually form the backbone of a new generation of efficient, low-energy computer processors, and thus help maintain progress in high-speed information processing.

For more than half a century, computing power has increased exponentially. Recently, however, this “Moore’s law” growth – named for Intel co-founder Gordon Moore, who predicted in 1965 that the number of transistors per square inch on integrated circuits would double every year for at least a decade – has slowed, as it is becoming increasingly difficult to make conventional transistors smaller than they already are. Researchers are therefore seeking ways of allowing computing power to continue to grow, even as traditional sized-based scaling runs up against fundamental limits.

Among the many solutions being investigated are those that seek to reduce power consumption in the field-effect transistors (FETs) that form the basis of modern silicon computer chips. One way to do this would be to replace conventional transistors with an alternative version that does not require a continuous power supply to maintain its ON or OFF state.

Spintronics circuits are smaller and more efficient

Spintronics technologies, which use electrons’ quantum spin (or intrinsic angular momentum) rather than their charge to store and process information, may offer a way to reach this goal. Since electronic spins can point “up” or “down”, this binary property can be used to perform logical operations in spintronic circuits in much the same way as electric charge is used in electronic circuits. The key advantage is that when an electron’s spin switches direction, its new state is stored permanently (that is, it is “non-volatile”). Hence, spintronics circuits do not require any additional input power for their states to remain stable.

Spintronic circuits do, however, have a major drawback for efficiency advocates. In spintronics, information is carried or manipulated via spin currents, which consists of electrons with opposite spins moving in opposite directions. These currents are usually generated using ferromagnetic materials. That’s a problem, because the magnetization of such materials cannot be switched except by applying very strong magnetic fields or currents. Hence, in any practical, switchable device, the energy advantage associated with non-volatile storage would quickly be wiped out.

A new way to control spins

Researchers at the Spintec Laboratory (CNRS/CEA/Université Grenoble Alpes) and the CNRS/Thales Laboratory led by Jean-Philippe Attané and Manuel Bibes have now developed a new, lower-power way to control spin currents. Their method uses the ultrathin layer of conducting electrons – known technically as a two-dimensional (2D) electron gas – that develops at the interface between strontium titanate (an electrical insulator in its pure state) and a covering layer of aluminium.

The researchers began by injecting a spin current from a ferromagnetic nickel-iron alloy into the strontium titanate (SrTiO3). Once the electrons are confined in the 2D electron gas, their spin couples to their momentum thanks to a phenomenon known as spin-orbit interaction. “This effect converts the spin current into conventional charge current, which allows us to detect the injected spins,” Attané and Bibes explain.

SrTiO3 behaves like a ferroelectric material

Next, the team applied a voltage across the insulating SrTiO3 beneath the electron gas to tune the spin-orbit coupling, and thus the direction of the charge current. At this point, they discovered that the insulating SrTiO3 was behaving like a ferroelectric material – that is, it had a permanent electric dipole moment, just as a ferromagnetic material has a permanent magnetic dipole moment. This is a significant benefit, since electric dipole moments can be oriented using electric fields, which are much easier to control than the magnetic fields required to switch the magnetization of ferromagnetic materials.

Attané and Bibes and their colleagues also confirmed that the direction of the SrTiO3’s overall electrical polarization depends on the polarity of the applied voltage. This electrical polarization remains even when the electric field is switched off, meaning that they could permanently control the spin-orbit coupling and thus the direction of the charge current produced from the spin current.

The research is detailed in Nature.

Biochemical quantitative phase imaging goes photothermal

Optical imaging is widely used to image biological cells thanks to its non-destructive, label-free nature. A team of researchers in Japan has now combined two common optical techniques – quantitative phase microscopy and molecular vibrational imaging – to produce detailed images of live biological cells. The new approach could be used to observe how fundamental types of molecules are distributed in single cells, with potential applications in biology and medicine.

Quantitative phase imaging (QPI) is a technique that measures the 3D distribution of the refractive index in a transparent sample by calculating how light waves shift as they pass through the sample. It can thus be used to visualize the black-and-white outline of a biological cell as well as the outlines of the major structures inside. This information can then be used to reconstruct a 3D image of the cell.

Molecular vibrational imaging (MVI), for its part, provides information on biomolecular bonds based on Raman scattering or mid-infrared (MIR) light absorption. When molecules are excited with MIR light from a laser, they vibrate at a certain frequency and heat up their surroundings in a process known as the photothermal effect. By applying different wavelengths of MIR light to biological samples, researchers can selectively increase the temperature of specific types of chemical bonds, and so identify biostructures such as intracellular proteins, lipids or nucleic acids.

Both techniques combined

In their experiments, a team led by Takuro Ideguchi of The University of Tokyo combined QPI and MVI into a single, streamlined process. After taking a quantitative phase microscopy image of a live cell with their MIR light source turned off, they then repeat the measurement with the source turned on. The difference between the two images reveals both the outline of the major structures inside the cell and the exact locations of the type of molecule that was excited by the MIR light.

In their work, Ideguchi and colleagues say they were impressed when they first observed the molecular vibration signature characteristics of proteins, and then when this protein-specific signal appeared in the same location as the nucleolus — a structure within the cell’s nucleus in which high amounts of proteins would be expected.

Although it currently takes around 50 seconds or more to capture one complete image, the Tokyo team are confident that they can speed up the process by incorporating a higher-power MIR light source and a more sensitive camera into their experimental set-up.

The approach, which is detailed in Optica, could be used to study complex and fragile biological processes involved in cellular diseases and stem cell development, and to monitor drug delivery, the researchers say. In such applications, the cells need to be observed over long periods without disturbing them.

‘Photon crystals’ could be made using Rydberg atoms

Physicists in the US have come up with a way of making photons repel each other by sending them through an ultracold atomic gas. This astonishing feat could lead to the creation of “photon crystals” and exotic quantum states such as a Mott insulator.

Photons have zero rest mass and zero charge, and as they zip past each other at the speed of light, they barely feel each other’s effects. Physicists, however, have devised various means to amplify this tiny photon-photon interaction.

In 2013, a team led by Vladan Vuletić at the Massachusetts Institute of Technology and Mikhail Lukin of Harvard University created an attractive interaction by firing photons into an ultracold atomic gas, putting some atoms into an energetic “Rydberg state”. While travelling through the gas, the photons tended to stick together more often than they would have had the gas not been there.

Slow-moving quasiparticles

“Classically speaking, the photon comes in, gets absorbed by one atom and propels it to a Rydberg state,” explains Vuletić. “After a while, the photon gets released and absorbed by another atom, which stays in the Rydberg state a while and so on. Quantum mechanically, the gas is in a superposition state of all these possibilities.” These photon-atom couplings can be described as slow-moving quasiparticles called polaritons that interact with each other much more strongly than photons.

Using this technique to create repulsive interactions, however, is trickier. Polaritons interact by changing the local refractive index of the medium. To switch this interaction from attractive to repulsive, requires polaritons that shift the local refractive index in the opposite direction. While this can be achieved by changing how the incident light is detuned, this shift also gives the polaritons negative effective mass. “For one sign of detuning, the particles attract each other” explains Vuletić: “The other side of the detuning, you change both the interaction and the mass term – and the particles still attract each other.”

In 2014 Lukin’s group teamed-up with theoretical physicist Hans-Peter Buchler of the University of Stuttgart in Germany to develop a mathematical description of slow-light polaritons. Under certain specific conditions, the team proposed, repulsive interactions between the polaritons should dominate. Unfortunately, when Lukin and Vuletić’s groups tried to realize these conditions, they found it impossible: “We don’t think this proposal was wrong,” says Vuletić, “It just wasn’t practical in our system and required very, very high laser power that we were not able to reach.”

Two atomic states

After exploring several possibilities, the two groups have now devised and demonstrated a successful alternative scheme in which the photons are coupled simultaneously to two atomic states in the same atom. “Coupling the photons to matter not just once but twice gives us another free parameter where we were able to alter the mass of the particles independently from the index of refraction,” explains Vuletić. Using this, the researchers showed that, by tuning the wavelength of the incident light, they could alter whether or not photons exiting the gas were more or less likely to do so together than would have been expected by chance. This showed that they could control whether polaritons in the gas attracted or repelled each other. The researchers went on to demonstrate three-body repulsion between polaritons, which matched their theoretical predictions.

The researchers are now exploring several possible extensions of the work: “If you have repulsion at short distances and attraction at long distances you would create a molecular-like structure,” explains Lukin’s student Aditya Venkatramani, who along with Vuletić’s student Sergio Cantu were joint first authors of a paper in Nature Physics that describes the work.

This could create “crystalline order” in photon structures, potentially opening up experiments impossible in traditional matter. “If you have an atom interacting, it always has the same mass no matter how it travels around,” explains Cantu, “but here you could have a mass of one sign and magnitude in one direction and another sign and magnitude in another direction. It’s possible to change a lot of parameters that, in regular matter, are just given by your material.”

“It’s really cool,” says Mohammed Hafezi of the University of Maryland, College Park. He suspects the work will unlock many doors in photon many-body physics that were hitherto closed by the fact that all attractions between polaritons were attractive. He is excited, for example, by the potential of producing an exotic quantum state called a Mott insulator with photons: “You need repulsive interactions to have a traffic jam of photons,” he explains, “They’ve now achieved a repulsive bond, and if they manage to have a finite number of modes and to populate them, then they should get a Mott insulator.”

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