(a) Schematic diagram of the time-domain characteristics of fiber Rayleigh scattering restoration based on a deep learning model. (b) In the frequency domain, different network parameter settings result in differences in the restoration of fiber Rayleigh scattering. Credit: Science China Press
Scattering in media has unique inherent randomness. Fiber Rayleigh scattering is one of the typical scattering effects. The exploration and understanding of the characteristics of fiber Rayleigh scattering holds significant research value for optical fiber sensing, random fiber lasers, etc.
However, in practical noisy environments, accurately extracting the fiber Rayleigh scattering is a challenging issue. If deep learning models can express the general characteristics of fiber Rayleigh scattering and be trained by numerical simulation datasets only, it will have significant scientific and practical value.
The general characteristics of fiber Rayleigh scattering based on a deep learning model were successfully extracted for the first time by a research group from UESTC. Published in Science China Information Sciences, the research utilized a purely numerical simulation dataset to train a self-built deep learning model, to extract the general characteristics of fiber Rayleigh scattering.
The experimental verification was then carried out by a widely-used distributed acoustic sensing system known as the phase-sensitive optical time domain reflectometry (Φ-OTDR). The specific innovations and significance are as follows:
1. By training with a numerical simulation dataset, the deep learning model successfully captures the general characteristics of fiber Rayleigh scattering, and accurately recovers the broadband fiber Rayleigh scattering based on narrowband sensing signal in the experiment. This model has strong generalization ability, and will not overly rely on the data generated under specific experimental conditions.
Therefore, it is possible to use numerical simulation to construct large-scale and diverse training datasets for Φ-OTDR systems and train multi-functional artificial intelligence based on complex models.
2. With the fiber Rayleigh scattering obtained by the deep learning model, the optimal probe pulse can be estimated beforehand, and then the time-domain responses of the sensing signal can be optimized, achieving adaptive waveform modulation in the Φ-OTDR system. This opens up new ideas for other research directions based on Rayleigh scattering optical systems, such as random fiber laser, adaptive optics with wavefront correction, etc.
This work investigates the utilization of a deep learning model for extracting the characteristics of Rayleigh scattering in optical fibers. The discoveries provide a basis for developing a multi-functional artificial intelligence specifically designed for distributed optical fiber sensing and also offer valuable insights for other optical systems based on Rayleigh scattering.
The two rings in the image are microresonators. The bigger ring is the one where the microcomb is generated. The microcomb is formed by a pulse of light – here illustrated with a red spike and also known as a soliton – that recirculates in the cavity forever. The key aspect is that the smaller ring helps in coupling the light from the straight waveguide, illustrated by the straight orange line at the bottom, into the bigger ring. In other words, it behaves as impedance matching, and therefore the soliton is generated more efficiently. Credit: Óskar Helgason
Microcombs can help us discover planets outside our solar system and track new diseases in our bodies. But current microcombs are inefficient and unable to reach their full potential. Now, researchers at Chalmers University of Technology in Sweden have scored a world first with their solution to make microcombs 10 times more efficient. Their breakthrough opens the way to new discoveries in space and health care and paves the way for high-performance lasers in a range of other technologies.
Laser frequency combs can measure frequencies with revolutionary precision and are considered the most disruptive technological advance in the field since the birth of the laser.
Simply put, a microcomb is like a ruler made of light. The principle is based on a laser sending photons that circulate inside a small cavity, a so-called microresonator, where the light is divided into a wide range of frequencies. These frequencies are precisely positioned in relation to each other, like the markings on a ruler. Thus, a new kind of light source can be created consisting of hundreds—or even thousands—of frequencies, like lasers beaming in unison.
Since virtually all optical measurements are connected to light frequencies, the microcomb has a multitude of applications—from calibrating instruments that measure signals at light-year distances in space in the search for exoplanets, to identifying and keeping track of our health via the air we exhale.
Unprecedented efficiency
Up to now, a fundamental problem with microcombs has been that their efficiency was too weak to produce a wider technological impact on society. The conversion efficiency between the laser and the microcomb was too weak, meaning that only a fraction of the power contained in the laser beam was usable. But now, a Chalmers research team has successfully developed a method to increase the effect of the microcomb’s laser beams tenfold.
“We’ve developed a new method that breaks what was previously thought to be a fundamental limit for optical conversion efficiency. Our method increases the laser power of the soliton microcomb by ten times and raises its efficiency from around 1% to over 50%,” says Victor Torres Company, Professor of Photonics at Chalmers.
The new method uses two microresonators, as opposed to just one. They form a unique ensemble with properties that are greater than the sum of its parts. One of the resonators enables the light coming from the laser to couple with the other resonator; rather like impedance matching in electronics.
“The new microcombs have transformative potential because they make high-performance laser technology available to many more markets. For example, frequency combs could be used in lidar modules for autonomous driving, or in GPS satellites and environmental sensing drones, or in data centers to enable bandwidth-intensive AI apps,” says Torres Company.
The method opens up completely new areas of application for high-performance lasers. The technology was recently patented by the project’s researchers. They have founded Iloomina AB, a company which will launch the technology onto a wider market.
The study was conducted by researchers Óskar B. Helgason, Marcello Girardi, Zhichao Ye, Fuchuan Lei, Jochen Schröder and Victor Torres Company at the Department of Photonics at the Department of Microtechnology and Nanoscience at Chalmers University of Technology.
More information: Óskar B. Helgason et al, Surpassing the nonlinear conversion efficiency of soliton microcombs, Nature Photonics (2023). DOI: 10.1038/s41566-023-01280-3
Lighting up a blue organic LED with a single AA battery. Credit: Associate Professor Izawa and the member and authors of this research team.
Blue light is vital for light-emitting devices, lighting applications, as well as smartphone screens and large screen displays. However, it is challenging to develop efficient blue organic light-emitting diodes (OLEDs), owing to the high applied voltage required for their function. Conventional blue OLEDs typically require around 4 V for a luminance of 100 cd/m2. This is higher than the industrial target of 3.7 V—the voltage of lithium-ion batteries commonly used in smartphones. Therefore, there is an urgent need to develop novel blue OLEDs that can operate at lower voltages.
In this regard, Associate Professor Seiichiro Izawa from Tokyo Institute of Technology and Osaka University has collaborated with researchers from University of Toyama, Shizuoka University, and the Institute for Molecular Science. The team recently presented a novel OLED device with a remarkable ultralow turn-on voltage of 1.47 V for blue emission and a peak wavelength at 462 nm (2.68 eV). Their work is published in Nature Communications.
The choice of materials used in this OLED significantly influences its turn-on voltage. The device utilizes NDI-HF (2,7-di(9H-fluoren-2-yl)benzo[lmn][3,8]-phenanthroline-1,3,6,8(2H,7H)-tetraone) as the acceptor, 1,2-ADN (9-(naphthalen-1-yl)-10-(naphthalen-2-yl)anthracene) as the donor, and TbPe (2,5,8,11-tetra-tert-butylperylene) as the fluorescent dopant. This OLED operates via a mechanism called upconversion (UC). Herein, holes and electrons are injected into donor (emitter) and acceptor (electron transport) layers, respectively. They recombine at the donor/acceptor (D/A) interface to form a charge transfer (CT) state.
Dr. Izawa observes, “The intermolecular interactions at the D/A interface play a significant role in CT state formation, with stronger interactions yielding superior results.”
An upconversion organic light-emitting diode (OLED) based on a typical blue-fluorescence emitter achieves emission at an ultralow turn-on voltage of 1.47 V, as demonstrated by researchers from Tokyo Tech. Their technology circumvents the traditional high voltage requirement for blue OLEDs, leading to potential advancements in commercial smartphone and large screen displays. Credit: Associate Professor Seichiro Izawa
Subsequently, the energy of the CT state is selectively transferred to the low-energy first triplet excited states of the emitter, which results in blue light emission through the formation of a high-energy first singlet excited state by triplet-triplet annihilation (TTA).
“As the energy of the CT state is much lower than the emitter’s bandgap energy, the UC mechanism with TTA significantly decreases the applied voltage required for exciting the emitter. As a result, this UC-OLED reaches a luminance of 100 cd/m2, equivalent to that of a commercial display, at just 1.97 V,” explains Dr. Izawa.
In effect, this study efficiently produces a novel OLED, with blue light emission at an ultralow turn-on voltage, using a typical fluorescent emitter widely utilized in commercial displays, thus marking a significant step toward meeting the commercial requirements for blue OLEDs. It emphasizes the importance of optimizing the design of the D/A interface for controlling excitonic processes and holds promise not only for OLEDs, but also for organic photovoltaics and other organic electronic devices.
More information: Blue Organic Light-Emitting Diode with a Turn-on Voltage of 1.47 V, Nature Communications (2023). DOI: 10.1038/s41467-023-41208-7
Gravity-driven and magnetically-driven flowing layer of ferromagnetic Janus particles. Intensity average images of (a) a gravity driven flow in a granular heap of unactuated Janus particles and, in contrast, (b) an uphill flow of the Janus microrollers driven by magnetic actuation, including an illustration of the direction of particle rotation. Movies of uphill granular flow are available (see Supplementary Information in paper). The relative magnetic field strength is (β/β0)2 = 3.5 and the granular bed depth is Δ/2a = 26.0. The dotted white line is an approximate representation of the flowing layer. Credit: Nature Communications (2023). DOI: 10.1038/s41467-023-41327-1
Engineering researchers at Lehigh University have discovered that sand can actually flow uphill.
The team’s findings were published today in the journal Nature Communications. A corresponding video shows what happens when torque and an attractive force is applied to each grain—the grains flow uphill, up walls, and up and down stairs.
“After using equations that describe the flow of granular materials,” says James Gilchrist, the Ruth H. and Sam Madrid Professor of Chemical and Biomolecular Engineering in Lehigh’s P.C. Rossin College of Engineering and Applied Science and one of the authors of the paper, “we were able to conclusively show that these particles were indeed moving like a granular material, except they were flowing uphill.”
The researchers say the highly unusual discovery could unlock many more lines of inquiry that could lead to a vast range of applications, from health care to material transport and agriculture.
The paper’s lead author, Dr. Samuel Wilson-Whitford, a former postdoctoral research associate in Gilchrist’s Laboratory of Particle Mixing and Self-Organization, captured the movement entirely by serendipity in the course of his research into microencapsulation. When he rotated a magnet beneath a vial of iron oxide-coated polymer particles called microrollers, the grains began to heap uphill.
Wilson-Whitford and Gilchrist began studying how the material reacted to the magnet under different conditions. When they poured the microrollers without activating them with the magnet, they flowed downhill. But when they applied torque using the magnets, each particle began to rotate, creating temporary doublets that quickly formed and broke up. The result, says Gilchrist, is cohesion that generates a negative angle of repose due to a negative coefficient of friction.
“Up until now, no one would have used these terms,” he says. “They didn’t exist. But to understand how these grains are flowing uphill, we calculated what the stresses are that cause them to move in that direction. If you have a negative angle of repose, then you must have cohesion to give a negative coefficient of friction. These granular flow equations were never derived to consider these things, but after calculating it, what came out is an apparent coefficient of friction that is negative.”
Increasing the magnetic force increases the cohesion, which gives the grains more traction and the ability to move faster. The collective motion of all those grains, and their ability to stick to each other, allows a pile of sand particles to essentially work together to do counterintuitive things—like flow up walls, and climb stairs. The team is now using a laser cutter to build tiny staircases, and is taking videos of the material ascending one side and descending the other. A single microroller couldn’t overcome the height of each step, says Gilchrist. But working together, they can.
“This first paper just focuses on how the material flows uphill, but our next several papers will look at applications, and part of that exploration is answering the question, can these microrollers climb obstacles? And the answer is yes.”
Potential applications could be far ranging. The microrollers could be used to mix things, segregate materials, or move objects. Because these researchers have discovered a new way to think about how the particles essentially swarm and work collectively, future uses could be in microrobotics, which in turn could have applications in health care. Gilchrist recently submitted a paper exploring their use on soil as a means of delivering nutrients through a porous material.
“We’re studying these particles to death,” he says, “experimenting with different rotation rates, and different amounts of magnetic force to better understand their collective motion. I basically know the titles of the next 14 papers we’re going to publish.”
More information: Samuel R. Wilson-Whitford et al, Microrollers flow uphill as granular media, Nature Communications (2023). DOI: 10.1038/s41467-023-41327-1
Fig 1. a, The physical model and principle of the optical non-Hermitian metasurfaces operating at an EP based on the in-plane loss. b, The physical model and principle of the optical non-Hermitian metasurfaces operating at an EP based on the interlayer loss. Credit: Tao He, Zhanyi Zhang, Jingyuan Zhu, Yuzhi Shi, Zhipeng Li, Heng Wei, Zeyong Wei, Yong Li, Zhanshan Wang, Cheng-Wei Qiu, Xinbin Cheng
Non-Hermitian systems have numerous alluring optical properties at exceptional points (EPs) and have attracted extensive attention because of their great prospects in applications such as optical sensing, integrated optics, and other fields.
Recently, metasurface, a class of artificial materials that transcends natural materials through the orderly design of subwavelength structures, has become a new platform to realize complex optical EPs.
A common way to implement scattering EP in the acoustic/microwave range is regulating the loss of the gradient metasurfaces by introducing a specific loss in a unit cell. However, direct extensions of in-plane loss in a gradient metasurface from non-visible waveband to visible light remain a formidable challenge, since the adjustable in-plane loss and corresponding manufacturing process in the visible are lacking.
Furthermore, the complex and rebellious interplay between in-plane lossy structure and lightwave restricts optical efficiency. Therefore, achieving high-efficiency EP at optical non-Hermitian metasurface is still a challenging task in photonics. Notably, two-dimensional scattering systems operating at EPs in the visible are unexplored.
In a paper published in Light: Science & Applications, a team of scientists, led by Professor Cheng-Wei Qiu from Department of Electrical and Computer Engineering, National University of Singapore, Singapore and Xinbin Cheng from Institute of Precision Optical Engineering, School of Physics Science and Engineering, Tongji University, Shanghai, and co-workers have reported a universal paradigm for achieving a high-efficiency EP in the visible by leveraging interlayer loss to accurately control the interplay between the lossy structure and scattering lightwaves.
A bilayer framework is demonstrated to reflect back the incident light from the left side (|r-1| > 0.999) and absorb the incident light from the right side (|r+1| < 10-4).
“As a proof of concept, a bilayer metasurface composed of TiO2 metagrating and Si subwavelength grating is designed: the metagrating in the upper layer achieves the directional regulation of lightwave, and the lossy subwavelength grating in the lower layer achieves an adjustable absorption.”
“When the proper absorption of Si subwavelength grating is selected, a wave vector-dependent perfect retroreflector and absorber is realized (Fig. 2a). The eigenvalues and phase change all proves that we have reached the EP (Fig. 2b and 2c).”
Fig 2. a, The spectra and schematics of the high-efficiency optical non-Hermitian meta-system at EP. The |r₋₁| is larger than 0.999 and |r₊₁| is less than 10⁻⁴. b, Trajectories of eigenvalues with the evolution of absorption parameter. c, The amplitude of non-specular reflection coefficients r₋₁ and r₊₁ by varying absorption parameter. Credit: Tao He, Zhanyi Zhang, Jingyuan Zhu, Yuzhi Shi, Zhipeng Li, Heng Wei, Zeyong Wei, Yong Li, Zhanshan Wang, Cheng-Wei Qiu, Xinbin Cheng
“We fabricated the sample (Fig. 3a) and conducted two separate tests with incident angles of 30° and −30° as shown in the purple and red boxes of Fig. 3b. When the incident light impacted from the left, most of the light was reflected by the sample to the incident path. When the incident light came from the right, there was very little retroreflection light. The fabricated sample is experimentally demonstrated to reflect and absorb incident light with efficiencies of 88% and 85%, respectively, at 532 nm. (Fig. 3c).”
“Our work paves a new avenue toward the design of versatile optical metasurface platforms involving the EP or higher-order EP, which may inspire more functional photonic devices for wave manipulation,” the scientists say.
More information: Tao He et al, Scattering exceptional point in the visible, Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01282-4
a, Imaging results of single LC lens using a laser projector as the light source, b, imaging results of the proposed achromatic LC lens system using a laser projector as the light source, c, imaging results of single LC lens using an OLED display panel as the light source, d, imaging results of the proposed achromatic LC lens system using an OLED display panel as the light source. Credit: Zhenyi Luo, Yannanqi Li, John Semmen, Yi Rao and Shin-Tson Wu
As a promising candidate for the next-generation mobile platform, mixed reality (MR) such as Apple Vision Pro and Meta Quest Pro (both are passthrough virtual reality headsets) has the potential to revolutionize the way we perceive and interact with various digital information.
By providing more direct interactions with digital information, MR is one of the key enablers for metaverse, spatial computing, and digital twins that have found widespread applications in smart tourism, smart health care, smart manufacturing, and smart construction, just to name a few.
To further enhance the human factors and ergonomics of MR displays, it is essential to improve the overall user experience, particularly for long-term wearing comfort. To achieve this goal, an ultracompact formfactor and a lightweight device are key goals.
In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Shin-Tson Wu from College of Optics and Photonics, University of Central Florida, U.S., and co-workers, have demonstrated an achromatic diffractive liquid-crystal (LC) optics system with an ultrathin formfactor and light weight.
Unlike refractive optics that use optical path difference to produce phase patterns, diffractive LC optics produce desired phase patterns by satisfying the half-wave condition along the thickness direction, which is typically several microns for a visible light.
These LC optics offer several advantages, such as high diffraction efficiency (nearly 100%), easy fabrication, polarization selectivity, and dynamic switching, making them promising candidates for near-eye display applications. However, the diffraction angle of LC optics depends on the wavelength, which in turn leads to a severe chromatic aberration and cannot be used for imaging purposes .
To overcome this longstanding chromatic aberration issue while maintaining the ultrathin formfactor, Prof. Wu’s team proposed an achromatic LC optics system consisting of three components taking advantage of polarization selectivity. The transmission spectrum and phase pattern of each optical element is carefully designed to control the polarization states and correct the chromatic aberrations.
Specifically, for an achromatic LC lens system to eliminate the focal shift between blue and red light, the first component would be a broadband lens that shows high efficiency in the visible spectral region, and the second element would be a half-wave plate that is designed to switch the polarization state of blue light.
The last component is a LC lens with specially designed transmission spectrum, which is effective only for the blue light and red light. The achromatic LC lens system can be achieved by simply stacking these three LC components together. Both achromatic grating and deflector systems can be constructed based on the same principle.
The concept was validated using two different types of light engines: a laser projector and an organic light-emitting diode (OLED) display panel. The image of single LC lens exhibits severe chromatic aberrations which arises from the wavelength dependent optical power of the diffractive optics. The achromatic lens system shows a significantly improved color performance; its chromatic aberrations are dramatically suppressed.
This approach can be extended to other types of diffractive optics by controlling the polarization states properly, which is expected to lead to more compact optical components. It will have widespread applications in MR-enabled smart transportation, smart cities, smart health care, and smart manufacturing.
More information: Zhenyi Luo et al, Achromatic diffractive liquid-crystal optics for virtual reality displays, Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01254-8
Four diamond materials are shown here: “Diamond black” made of polycrystalline nanostructured carbon (top right), the same material before nanostructuring (top left), an intrinsic single crystal (bottom left) and a single crystal doped with boron (bottom right). Credit: A. Chemin/HZB
It sounds like magic: photoelectrodes could convert the greenhouse gas CO2 back into methanol or N2 molecules into valuable fertilizer—using only the energy of sunlight.
An HZB study has now shown that diamond materials are in principle suitable for such photoelectrodes. By combining X-ray spectroscopic techniques at BESSY II with other measurement methods, Tristan Petit’s team has succeeded for the first time in precisely tracking which processes are excited by light as well as the crucial role of the surface of the diamond materials.
At first glance, lab-grown diamond materials have little in common with their namesakes in the jewelry shop. They are often opaque, dark and look not spectacular at all. But even if their looks are unimpressive, they are promising in many different applications, for example in brain implants, quantum sensors and computers, as well as metal-free photoelectrode in photo-electrochemical energy conversion.
They are fully sustainable and made of carbon only, they degrade little in time compared to metal-based photoelectrodes and they can be industrially produced!
Diamond materials are suitable as metal-free photoelectrodes because when excited by light, they can release electrons in water and trigger chemical reactions that are difficult to initiate otherwise. A concrete example is the reduction of CO2 to methanol which turns the greenhouse gas into a valuable fuel. It would also be exciting to use diamond materials to convert N2 into nitrogen fertilizer NH3, using much less energy than the Haber-Bosch process.
However, diamond electrodes oxidize in water and oxidized surfaces, it was assumed, no longer emit electrons into the water. In addition, the bandgap of diamond is in the UV range (at 5.5 eV), so visible light is unlikely to be sufficient to excite electrons. In spite of this expectation, previous studies have shown puzzling emission of electrons from visible light excitation. A new study by Dr. Tristan Petit’s group at HZB now brings new insights and gives cause for hope.
Dr. Arsène Chemin, a postdoctoral researcher in Petit’s team, studied samples of diamond materials produced at the Fraunhofer Institute for Applied Solid State Physics in Freiburg. The samples were engineered to facilitate the CO2 reduction reaction: doped with boron to insure good electrical conductivity and nanostructured, which gives them huge surfaces to increase the emission of charge carriers such as electrons.
Chemin used four X-ray spectroscopic methods at BESSY II to characterize the surface of the sample and the energy needed to excite specific electronic surface states. Then, he used the surface photovoltage measured in a specialized laboratory at HZB to determine which ones of these states are excited and how the charge carriers are displaced in the samples. In complement, he measured the photoemission of electrons of samples either in air or in liquid.
By combining these results, he was able for the first time to draw a comprehensive picture of the processes that take place on the surfaces of the sample after excitation by light.
“Surprisingly, we found almost no difference in the photoemission of charges in liquid, regardless of whether the samples were oxidized or not,” says Chemin. This shows that diamond materials are well suited for use in aqueous solutions. Excitation with visible light is also possible: in the case of the boron-doped samples, violet light (3.5 eV) is sufficient to excite the electrons.
“These results are a great cause for optimism,” says Chemin: “With diamond materials we have a new class of materials that can be explored and widely used.” What’s more, also the methodology of this study is interesting: The combination of these different spectroscopic methods could also lead to new breakthroughs in other photoactive semiconductor materials, the physicist points out.
The work is published in the journal Small Methods.
More information: Arsène Chemin et al, Surface‐Mediated Charge Transfer of Photogenerated Carriers in Diamond, Small Methods (2023). DOI: 10.1002/smtd.202300423
A schematic image of conversion phenomenon from charge current to spin current based on spin Hall effect in Co3Sn2S2 layer. Credit: Takeshi Seki et al
A group of researchers have made a significant breakthrough which could revolutionize next-generation electronics by enabling non-volatility, large-scale integration, low power consumption, high speed, and high reliability in spintronic devices.
Details of their findings were published in the journal Physical Review B on August 25, 2023.
Spintronic devices, represented by magnetic random access memory (MRAM), utilize the magnetization direction of ferromagnetic materials for information storage and rely on spin current, a flow of spin angular momentum, for reading and writing data.
Conventional semiconductor electronics have faced limitations in achieving these qualities.
However, the emergence of three-terminal spintronic devices, which employ separate current paths for writing and reading information, presents a solution with reduced writing errors and increased writing speed. Nevertheless, the challenge of reducing energy consumption during information writing, specifically magnetization switching, remains a critical concern.
A promising method for mitigating energy consumption during information writing is the utilization of the spin Hall effect, where spin angular momentum (spin current) flows transversely to the electric current. The challenge lies in identifying materials that exhibit a significant spin Hall effect, a task that has been clouded by a lack of clear guidelines.
“We turned our attention to a unique compound known as cobalt-tin-sulfur (Co3Sn2S2), which exhibits ferromagnetic properties at low temperatures below 177 K (-96°C) and paramagnetic behavior at room temperature,” explains Yong-Chang Lau and Takeshi Seki, both from the Institute for Materials Research (IMR), Tohoku University and co-authors of the study. “Notably, Co3Sn2S2 is classified as a topological material and exhibits a remarkable anomalous Hall effect when it transitions to a ferromagnetic state due to its distinctive electronic structure.”
Lau, Seki and colleagues employed theoretical calculations to explore the electronic states of both ferromagnetic and paramagnetic Co3Sn2S2, revealing that electron-doping enhances the spin Hall effect. To validate this theoretical prediction, thin films of Co3Sn2S2 partially substituted with nickel (Ni) and indium (In) were synthesized. These experiments demonstrated that Co3Sn2S2 exhibited the most significant anomalous Hall effect, while (Co2Ni)Sn2S2 displayed the most substantial spin Hall effect, aligning closely with the theoretical predictions.
“We uncovered the intricate correlation between the Hall effects, providing a clear path to discovering new spin Hall materials by leveraging existing literature as a guide,” adds Seki. “This will hopefully accelerate the development of ultralow-power-consumption spintronic devices, marking a pivotal step toward the future of electronics.”
More information: Yong-Chang Lau et al, Intercorrelated anomalous Hall and spin Hall effect in kagome-lattice Co3Sn2S2 -based shandite films, Physical Review B (2023). DOI: 10.1103/PhysRevB.108.064429
Cell Schematic. a The location of the quartz fork and LCMN thermometer are shown in relation to the heat exchanger. b Schematic image of the quartz fork with dimensions in millimeters. Credit: Nature Communications (2023). DOI: 10.1038/s41467-023-41422-3
Picture a nanoscale dance floor full of independently moving particles. When things really start to heat up—or, in this case, cool down—particles partner off, but on opposite sides of the space, “dancing” in synch as if telepathically.
In the ultra-pure isotope helium-three (3He), this dance starts at a very specific, very low temperature, when it converts into the superfluid phase (where its superfluid component has no viscosity and thus flows without friction) through a mechanism called pairing. Pairs of particles form over huge atomic distances in three dimensions.
“It’s something like a dance in space,” said Jeevak Parpia, professor of physics in the College of Arts and Sciences (A&S). “The effect of this pairing, called a ‘fluctuation,’ is to scatter other non-paired partners and disrupt the overall transport of momentum.”
These superfluid fluctuation effects were predicted almost 50 years ago, but no one had the instrumentation to see it; now, enabled by a custom thermometer that is accurate at super-low temperatures and sensitive enough to capture this subtle effect, Cornell researchers have observed the phenomenon in experiments—possibly gaining new insight for quantum computing and the physics of the early universe.
“Observation of Suppressed Viscosity in the Normal State of 3He due to Superfluid Fluctuations” was published Sept. 20 in Nature Communications. Parpia led the study, and research was primarily conducted by postdoctoral researcher Yefan Tian and doctoral student Rakin Baten. Eric Smith, Ph.D. ’72, was an essential team member and Erich Mueller, professor of physics (A&S), provided theoretical support.
To observe the minute changes of superfluid fluctuations at ultralow temperatures, the researchers used a tiny thermometer, 1.25 mm in diameter and 1.25 mm long, a homemade device they started to build during the COVID pandemic and are still refining.
“The low noise is essential,” Parpia said. “After all, we are looking for a small effect and if the temperature is ‘blurry’ or noisy then this small upturn [the sign of a superfluid fluctuation] is going to buried in the noise.”
As the only “quantum fluid,” helium is unique, Parpia said. All other elements, when cooled down, undergo phase transitions from liquid to solid; but while helium does change from a gas to the liquid state, the atoms don’t solidify unless a great deal of pressure is applied. This is because each atom’s mass is so small that the motion of the atoms is bigger than the separation of atoms. Even near absolute zero, helium atom components called quasiparticles (also known as excitations) are moving quickly and colliding with each other.
“Fluctuations are signaling that a change is coming, just like a wind gust can signal a storm,” Parpia said. “They occur just above the actual superfluid transition and disrupt the transfer of information. That’s because the quasiparticles pair up and have a very short lifetime, less than a millionth of a second even a few micro-degrees above the superfluid transition.”
A similar pairing mechanism also occurs in superconductors, which conduct charge (electricity) without resistance.
“Once a current is established in a superconductor, for example in a loop, it would flow forever,” Parpia said. “Superfluids are superconductors on steroids. The atoms, not just the electrons, flow without resistance. But unlike electronic superconductors where disorder is almost ubiquitous—it’s very hard to make a superconductor without defects, or ‘dirt,’ if you will—helium-three is ultrapure. So it is the best model system to study some exotic properties.”
Excitations in helium-three may be useful as a platform for quantum computation, Mueller said. A strategy known as “topological quantum computation” relies on the fact that pairs of excitations in certain exotic superconductors, such as those seen in helium-three, are theorized to act as quantum bits (qubits).
“While it has been challenging to find (or create) superconducting devices with the right types of excitations, there are predictions that helium-three could work. The first step is showing that helium-three has these ‘topological’ excitations,” he said. “Characterizing the superfluid fluctuations is an important step towards investigating this possibility.”
There are also suggestions that phase transitions in helium-three are ideal model systems that emulate the physics of the early universe, Parpia said, when energy first started to be differentiated into different forms and different fundamental forces emerged.
“Because the physics of helium is one of extreme purity and ultra-low temperatures, paradoxically that’s what makes it a good model for this ultra-high energy inflationary ‘epoch’ in the early universe,” he said. “How neat would it be if we were able to understand some aspect of the early universe in the lab!”
More information: Rakin N. Baten et al, Observation of suppressed viscosity in the normal state of 3He due to superfluid fluctuations, Nature Communications (2023). DOI: 10.1038/s41467-023-41422-3
PNNL chemist Isaac Arnquist examines ultra-low radiation copper cables specially created for sensitive physics detection experiments. Credit: Andrea Starr | Pacific Northwest National Laboratory
Imagine trying to tune a radio to a single station but instead encountering static noise and interfering signals from your own equipment. That is the challenge facing research teams searching for evidence of extremely rare events that could help understand the origin and nature of matter in the universe. It turns out that when you are trying to tune into some of the universe’s weakest signals, it helps to make your instruments very quiet.
Around the world more than a dozen teams are listening for the pops and electronic sizzle that might mean they have finally tuned into the right channel. These scientists and engineers have gone to extraordinary lengths to shield their experiments from false signals created by cosmic radiation.
Most such experiments are found in very inaccessible places—such as a mile underground in a nickel mine in Sudbury, Ontario, Canada, or in an abandoned gold mine in Lead, South Dakota—to shield them from naturally radioactive elements on Earth. However, one such source of fake signals comes from natural radioactivity in the very electronics that are designed to record potential signals.
Radioactive contaminants, even at concentrations as tiny as one part-per-billion, can mimic the elusive signals that scientists are seeking. Now, a research team at the Department of Energy’s Pacific Northwest National Laboratory, working with Q-Flex Inc., a small business partner in California, has produced electronic cables with ultra-pure materials.
These cables are specially designed and manufactured to have such extremely low levels of the radioactive contaminants that they will not interfere with highly sensitive neutrino and dark matter experiments.
The scientists report in the journal EPJ Techniques and Instrumentation that the cables have applications not only in physics experiments, but they may also be useful to reduce the effect of ionizing radiation interfering with future quantum computers.
“We have pioneered a technique to produce electronic cabling that is a hundred times lower than current commercially available options,” said PNNL principal investigator Richard Saldanha. “This manufacturing approach and product has broad application across any field that is sensitive to the presence of even very low levels of radioactive contaminants.”
An ultra-quiet choreographed ballet
Small amounts of naturally occurring radioactive elements are found everywhere: in rocks, dirt and dust floating in the air. The amount of radiation that they emit is so low that they do not pose any health hazards, but it’s still enough to cause problems for next-generation neutrino and dark matter detectors.
Chemist Isaac Arnquist and post-doctoral researcher Tyler Schlieder examine a sheet of ultra-pure copper cables designed for physics experiments. Credit: Andrea Starr | Pacific Northwest National Laboratory
“We typically need to get a million or sometimes a billion times cleaner than the contamination levels you would find in just a little speck of dirt or dust,” said PNNL chemist Isaac Arnquist, who co-authored the research article and led the measurement team.
For these experiments, Saldanha, Arnquist, and colleagues Maria Laura di Vacri, Nicole Rocco and Tyler Schlieder evaluated the amount of uranium, thorium and potassium at each step of the dozen or so processing steps that ultimately produce a detector cable. The team then developed special cleaning and fabrication techniques to reduce the contamination down to insignificant levels. Working in an ultraclean, dust and contaminant-free laboratory, the researchers meticulously plan out their every move.
“I almost think of us as performance athletes because everything, every movement we make, is extremely thought out. It’s almost like we’re choreographed dancers,” said Arnquist. “When we handle a detector sample material, there’s no wasted extraneous motion or interaction with the sample because that interaction could impart some contamination that limits how well we can measure the materials.”
After several years of work and hundreds of measurements, the resulting cables are now so free of contaminants that they will not impact the operation of next-generation dark matter and neutrino experiments such as DAMIC-M, OSCURA, and nEXO. The research team points out in their study that low-radioactivity cables can increase the sensitivity of the experiments and even allow more flexibility in detector design.
Getting closer to the ‘a-ha’ moment
So, exactly what are the researchers looking for in these experiments? In the case of both dark matter and neutrinoless double beta decay, they are hoping to record extremely rare events that could solve two key mysteries of the universe. Both of these mysteries pose fundamental questions about why the universe looks the way it does.
The galaxies that fill our universe would not have formed without the existence of dark matter. Dark matter makes up around 85% of the matter of the universe, and yet, we have never observed dark matter directly, only its gravitational imprint on the universe. Perhaps more intriguing, the question of why there is matter in the universe at all may hinge on a unique property of the smallest known particles of matter—the neutrino.
Unlike all other fundamental particles, neutrinos could possibly interact as both matter and anti-matter. If true, this could result in an extremely rare nuclear decay called neutrinoless double beta decay. Scientists are building large experiments consisting of many tons of sensitive material with the hope of finding the first evidence of neutrinoless double beta decay within the next decade.
“Every step we take to eliminate interfering radioactivity gets us closer to finding evidence for dark matter or neutrinoless double beta decay,” said Saldanha.
Close up of an ultra-low radiation electronic cable with dozens of conductive circuitry pathways to monitor sensitive physics experiments. This sample cable allows the research team to assess radiopurity after production and cleaning. Credit: Andrea Starr | Pacific Northwest National Laboratory
“These flexible cables have many conductive pathways, which are needed to read out complicated signals,” added Arnquist. “When, say, dark matter interacts with the detector or a neutrinoless double beta decay occurs, it’s going to create an event that needs to be accurately recorded—read out—to make the discovery. We need to put a complex electronic part that is extremely clean of radioactive elements into the heart of the detector.”
“Next generation searches for neutrinoless double beta decay will be among the lowest radioactivity experiments ever constructed,” said David Moore, a Yale University physicist and PNNL collaborator.
“These detectors use such pure materials that even a small amount of normal cabling materials would overwhelm the radioactivity from the entire rest of the detector, so developing ultra-low-background cables to read out such detectors is a major challenge. This recent work from PNNL and Q-Flex is key to enabling these detectors and will reduce the cabling background to a small fraction of what was possible with previous technologies.”
Planning is already underway to upgrade the highly sensitive DAMIC-M dark matter experiment and the new ultra-pure cables are one of the key improvements scheduled for installation in the detector.
“One component that we can’t avoid in our detector are the cables that transmit the signals, which must be of very low radioactivity,” said Alvaro E Chavarria, a physicist at the University of Washington and a collaborator on the DAMIC-M project.
“Prior to this recent PNNL development, the best solution was microcoax cables, which carry too few signals and would have required a significant redesign of our detector. This development is super exciting, since it enables the use of the industry-standard flex-circuit technology for low-background applications.”
Recent research findings by PNNL scientists and other collaborators indicate that the performance of some quantum computing devices can be affected by the presence of trace radioactive contaminants. While radioactivity is not currently what limits the capabilities of existing quantum computers, it is possible that quantum devices of the future might need low-radioactivity cables to enhance their performance.
“We see the potential for these cables to find applications in a wide range of sensitive radiation detectors and perhaps other applications such as quantum computing,” Saldanha said.
More information: Isaac J. Arnquist et al, Ultra-low radioactivity flexible printed cables, EPJ Techniques and Instrumentation (2023). DOI: 10.1140/epjti/s40485-023-00104-6
