Category: Physics

by University of Michigan

Silicon is one of the most pervasive functional materials of the modern age, underpinning semiconductor technologies ranging from microelectronics to solar cells. Indeed, silicon transistors enable computing applications from cell phones to supercomputers, while silicon photovoltaics are the most widely deployed solar-cell technology to date.

The U.S. Department of Energy (DOE) reports that nearly 50% of new electric generation capacity in 2022 came from solar cells, and according to the International Energy Agency (IEA), silicon has a 95% market share. Yet despite silicon’s undisputed importance to our modern way of life, many open questions remain about its fundamental physical properties.

In semiconductor devices, the functionality of the material comes from the motion and interactions of subatomic particles such as electrons (which have negative charge) and holes (the absence of an electron from an otherwise occupied state that itself behaves like a positively charged particle), which are called carriers as they “carry” electrical charge through the material.

For example, in a solar cell, the material absorbs incoming light, and the absorbed energy is converted into pairs of electrons and holes. These excited electron and holes then move to opposite ends of the solar cell and generate electricity.

Unfortunately, the electrons and holes can also interact in undesirable ways that convert their energy to heat and limit the efficiency of devices. One such loss mechanism occurs when carriers recombine and convert their energy to heat by interacting with a defect in the material. In many cases this defect-mediated recombination can be reduced by improving the quality of the material.

Other interactions, however, are intrinsic to a material and cannot be eliminated even in perfectly pure samples. Auger-Meitner recombination (AMR), historically known also as Auger recombination, is one such interaction. It is named after Lise Meitner and Pierre Auger, two pioneers of nuclear science who independently discovered this effect in atoms.

The new naming convention of the Auger-Meitner effect recognizes the contributions of Lise Meitner, a female Austrian physicist and the eponym of the Meitnerium chemical element, who independently discovered the process a year prior to Pierre Auger.

In the AMR process in semiconductors, one electron and one hole recombine, transferring their energy to a third carrier. The high-energy carrier can then thermalize or leak out of a device, generating heat and reducing the energy-conversion efficiency or reducing the number of available carriers. Unfortunately, despite decades of research, the specific atomistic mechanisms of AMR in silicon have eluded researchers to this date.

With a new implementation of a computational methodology to accurately calculate AMR rates from first principles—that is using only the physical constants of the universe and the atomic number of silicon as input—Dr. Kyle Bushick and Prof. Emmanouil Kioupakis of Materials Science and Engineering at the University of Michigan have provided the first comprehensive characterization of this important recombination process in silicon. This computational approach is key to gaining a full understanding of the AMR mechanism, because it is a process that does not emit light, making it very difficult to study in the lab.

With the aid of supercomputing resources at the National Energy Research Scientific Computing Center (NERSC) of Lawrence Berkeley National Lab, Bushick and Kioupakis were able to carry out the calculations of AMR in silicon, gaining insights to the behavior of the material at an atomic level.

One reason the AMR process in silicon has not been fully understood is that it includes multiple permutations. On one hand, the excited (third) carrier can either be an electron, or a hole, giving rise to the electron-electron-hole (eeh) and hole-hole-electron (hhe) processes, respectively.

Furthermore, AMR can be both direct, where only the three carriers participate, or phonon-assisted, where one of the carriers interacts with the vibrating atoms (phonons) to transfer additional momentum. While experiments can characterize the combined total AMR rate, parsing out the different contributions from these different components can be much harder. However, by using predictive atomistic calculations, each individual component can be directly computed and characterized.

Although past work had investigated the direct process using such calculations, it was clear that the direct process alone didn’t capture the full experimental picture. By overcoming the added complexity of calculating both the direct and phonon-assisted processes at the same level of theory, many of the unanswered questions about AMR in silicon could be addressed. Additionally, achieving such a detailed understanding of the process then opens the door for finding solutions to reduce the impact of AMR on device efficiency.

In their report, published in Physical Review Letters, Bushick and Kioupakis unequivocally elucidate the importance of the phonon-assisted AMR process in silicon.

“We found that the electron-phonon interactions not only account for the entirety of the hhe process, which was hypothesized in previous works but never conclusively demonstrated, but also for a significant portion of the eeh process, a finding that had been a subject of unresolved debate in the literature,” says Bushick, a recently graduated Ph.D. student of Materials Science and Engineering and a DOE Computational Science Graduate Fellow.

Furthermore, they highlight a potential pathway for altering AMR in silicon by applying strain to the material, a conclusion made possible by their newly implemented methodology.

This work provides a hitherto inaccessible fundamental understanding of an important intrinsic loss mechanism in the world’s most important semiconductor. This understanding, which has eluded scientists for decades, can help design better devices with improved performance by reducing the occurrence of the undesirable AMR process.

Emmanouil Kioupakis, Associate Professor of Materials Science and Engineering and Karl F. and Patricia J. Betz Family Faculty Scholar at the University of Michigan notes, “Ultimately, this work paves the way to understand and mitigate losses in silicon devices such as transistors or solar cells. Considering the size of these industries, even small improvements can lead to massive benefits.”

More information: Kyle Bushick et al, Phonon-Assisted Auger-Meitner Recombination in Silicon from First Principles, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.076902

Journal information: Physical Review Letters 

Provided by University of Michigan 

by Dawn Levy, Oak Ridge National Laboratory

Timothy Gray of the Department of Energy’s Oak Ridge National Laboratory led a study that may have revealed an unexpected change in the shape of an atomic nucleus. The surprise finding could affect our understanding of what holds nuclei together, how protons and neutrons interact and how elements form.

“We used radioactive beams of excited sodium-32 nuclei to test our understanding of nuclear shapes far from stability and found an unexpected result that raises questions about how nuclear shapes evolve,” said Gray, a nuclear physicist. The results are published in Physical Review Letters.

The shapes and energies of atomic nuclei can shift over time between different configurations. Typically, nuclei live as quantum entities that have either spherical or deformed shapes. The former look like basketballs, and the latter resemble American footballs.

How shapes and energy levels relate is a major open question for the scientific community. Nuclear structure models have trouble extrapolating to regions with little experimental data.

For some exotic radioactive nuclei, the shapes predicted by traditional models are the opposite of those observed. Radioactive nuclei that were expected to be spherical in their ground states, or lowest-energy configurations, turned out to be deformed.

What can turn a quantum state on its head?

In principle, the energy of an excited deformed state can drop below that of a spherical ground state, making the spherical shape the high-energy one. Unexpectedly, this role reversal appears to be happening for some exotic nuclei when the natural ratio of neutrons to protons becomes unbalanced. Yet, the post-reversal excited spherical states have never been found. It is as though once the ground state becomes deformed, all the excited states do, too.

Many examples exist of nuclei with spherical ground states and deformed excited states. Similarly, plenty of nuclei have deformed ground states and subsequent excited states that are also deformed—sometimes with different amounts or kinds of deformation. However, nuclei with both deformed ground states and spherical excited states are much more elusive.

Using data collected in 2022 from the first experiment at the Facility for Rare Isotope Beams, or FRIB, a DOE Office of Science user facility at Michigan State University, Gray’s team discovered a long-lived excited state of radioactive sodium-32. The newly observed excited state has an unusually long lifetime of 24 microseconds—about a million times longer than a typical nuclear excited state.

Long-lived excited states are called isomers. A long lifetime indicates that something unanticipated is going on. For example, if the excited state is spherical, a difficulty in returning to a deformed ground state could account for its long life.

The study involved 66 participants from 20 universities and national laboratories. Co-principal investigators came from Lawrence Berkeley National Laboratory, Florida State University, Mississippi State University, the University of Tennessee, Knoxville, and ORNL.

The 2022 experiment that generated the data used for the 2023 result employed the FRIB Decay Station initiator, or FDSi, a modular multidetector system that is extremely sensitive to rare isotope decay signatures.

“FDSi’s versatile combination of detectors shows that the long-lived excited state of sodium-32 is delivered within the FRIB beam and that it then decays internally by emitting gamma rays to the ground state of the same nucleus,” said ORNL’s Mitch Allmond, a co-author of the paper who manages the FDSi project.

To stop FRIB’s highly energetic radioactive beam, which travels at about 50% of the speed of light, an implantation detector built by UT Knoxville was positioned at FDSi’s center. North of the beam line was a gamma-ray detector array called DEGAi, comprising 11 germanium clover-style detectors and 15 fast-timing lanthanum bromide detectors. South of the beam line were 88 modules of a detector called NEXTi to measure time of flight of neutrons emitted in radioactive decay.

A beam of excited sodium-32 nuclei stopped in the detector and decayed to the deformed ground state by emitting gamma rays. Analysis of gamma-ray spectra to discern the time difference between beam implantation and gamma-ray emission revealed how long the excited state existed. The new isomer’s 24-microsecond existence was the longest lifetime seen among isomers with 20 to 28 neutrons that decay by gamma-ray emission. Approximately 1.8% of the sodium-32 nuclei were observed to be the new isomer.

“We can come up with two different models that equally well explain the energies and lifetime that we’ve observed in the experiment,” Gray said.

An experiment with higher beam power is needed to determine whether the excited state in sodium-32 is spherical. If it is, then the state would have six quantized units of angular momentum, which is a quality of a nucleus related to its whole-body rotation or the orbital motion of its individual protons and/or neutrons about the center of mass. However, if the excited state in sodium-32 is deformed, then the state would have zero quantized units of angular momentum.

A planned upgrade to FRIB will provide more power, increasing the number of nuclei in the beam. Data from the more intense beam will enable an experiment that distinguishes between the two possibilities.

“We’d characterize correlations between the angles of two gamma rays that are emitted in a cascade,” Gray said. “The two possibilities have very different angular correlations between the gamma rays. If we have enough statistics, we could disentangle the pattern that reveals a clear answer.”

More information: T. J. Gray et al, Microsecond Isomer at the N=20 Island of Shape Inversion Observed at FRIB, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.242501

Journal information: Physical Review Letters 

Provided by Oak Ridge National Laboratory 

by Princeton University

A Princeton University-led team of scientists has imaged the precise microscopic underpinnings responsible for many quantum phases observed in a material known as magic-angle twisted bilayer graphene (MATBG). This remarkable material, which consists of twisted layers of carbon atoms arranged in a two-dimensional hexagonal pattern, has in recent years been at the forefront of research in physics, especially in condensed matter physics.

For the first time, the researchers were able to specifically capture unprecedentedly precise visualizations of the microscopic behavior of interacting electrons that give rise to the insulating quantum phase of MATBG. Additionally, through the use of novel and innovative theoretical techniques, they were able to interpret and understand these behaviors. Their study is published in the journal Nature.

The amazing properties of twisted bilayer graphene were first discovered in 2018 by Pablo Jarillo-Herrero and his team at the Massachusetts Institute of Technology (MIT). They showed that this material can be superconducting, a state in which electrons flow freely without any resistance. This state is vital to many of our everyday electronics, including magnets for MRIs and particle accelerators as well as in the making of quantum bits (called qubits) that are being used to build quantum computers.

Since that discovery, twisted bilayer graphene has demonstrated many novel quantum physical states, such as insulating, magnetic, and superconducting states, all of which are created by complex interactions of electrons. How and why electrons form insulating states in MATBG has been one of the key unsolved puzzles in the field.

The solution to this puzzle would not only unlock our understanding of both the insulator and the proximate superconductor, but also such behavior shared by many unusual superconductors that scientists seek to understand, including the high-temperature cuprate superconductors.

“MATBG shows a lot of interesting physics in a single material platform－much of which remains to be understood,” said Kevin Nuckolls, the co-lead author of the paper, who earned his Ph.D. in 2023 in Princeton’s physics department and is now a postdoctoral fellow at MIT. “This insulating phase, in which electrons are completely blocked from flowing, has been a real mystery.”

To create the desired quantum effects, researchers place two sheets of graphene on top of each other with the top layer angled slightly. This off-kilter position creates a moiré pattern, which resembles and is named after a common French textile design. Importantly, however, the angle at which the top layer of graphene must be positioned is precisely 1.1 degrees. This is the “magic” angle that produces the quantum effect; that is, this angle induces strange, strongly correlated interactions between the electrons in the graphene sheets.

While physicists have been able to demonstrate different quantum phases in this material, such as the zero-resistance superconducting phase and the insulating phase, there has been very little understanding of why these phases occur in MATBG. Indeed, all previous experiments involving MATBG give good demonstrations of what the system is capable of producing, but not why the system is producing these states.

And that “why” became the basis for the current experiment.

“The general idea of this experiment is that we wanted to ask questions about the origins of these quantum phases—to really understand what exactly are the electrons doing on the graphene atomic scale,” said Nuckolls. “Being able to probe the material microscopically, and to take images of its correlated states—to fingerprint them, effectively—gives us the ability to discern very distinctly and precisely the microscopic origins of some of these phases. Our experiment also helps guide theorists in the search for phases that were not predicted.”

The study is the culmination of two years of work and was achieved by a team from Princeton University and the University of California, Berkeley. The scientists harnessed the power of the scanning tunneling microscope (STM) to probe this very minute realm. This tool relies on a technique called “quantum tunneling,” where electrons are funneled between the sharp metallic tip of the microscope and the sample. The microscope uses this tunneling current rather than light to view the world of electrons on the atomic scale. Measurements of these quantum tunneling events are then translated into high resolution, highly sensitive images of materials.

However, the first step—and perhaps the most crucial step in the experiment’s success—was the creation of what the researchers refer to as a “pristine” sample. The surface of carbon atoms that constituted the twisted bilayer graphene sample had to have no flaws or imperfections.

“The technical breakthrough that made this paper happen was our group’s ability to make the samples so pristine in terms of their cleanliness such that these high-resolution images that you see in the paper were possible,” said Ali Yazdani, the Class of 1909 Professor of Physics and Director of the Center for Complex Materials at Princeton University. “In other words, you have to make one hundred thousand atoms without a single flaw or disorder.”

The actual experiment involved placing the graphene sheets in the correct “magic angle,” at 1.1 degrees. The researchers then positioned the sharp, metallic tip of the STM over the graphene sample and measured the quantum mechanical tunneling current as they moved the tip across the sample.

“Electrons at this quantum scale are not only particles, but they are also waves,” said Ryan Lee, a graduate student in the Department of Physics at Princeton and one of the paper’s co-lead authors. “And essentially, we’re imaging wave-like patterns of electrons, where the exact way that they interfere (with each other) is telling us some very specific information about what is giving rise to the underlying electronic states.”

This information allowed the researchers to make some very incisive interpretations about the quantum phases that were produced by the twisted bilayer graphene. Importantly, the researchers used this information to focus on and solve the long-standing puzzle that for many years has challenged researchers working in this field, namely, the quantum insulating phase that occurs when graphene is tuned to its magic angle.

To help understand this from a theoretical viewpoint, the Princeton researchers collaborated with a team from the University of California-Berkeley, led by physicists B. Andrei Bernevig at Princeton and Michael Zaletel at Berkeley. This team developed a novel and innovative theoretical framework called “local order parameter” analysis to interpret the STM images and understand what the electrons were doing—in other words, how they were interacting—in the insulating phase. What they discovered was that the insulating state occurs because of the strong repulsion between the electrons, on the microscopic level.

“In magic-angle twisted bilayer graphene, the challenge was to model the system,” said Tomohiro Soejima, a graduate student and theorist at U.C. Berkeley and one of the paper’s co-lead authors. “There were many competing theories, and no one knew which one was correct. Our experiment of ‘finger-printing’ was really crucial because that way we could pinpoint the actual electronic interactions that give rise to the insulating phase.”

By using this theoretical framework, the researchers were able, for the first time, to make a measurement of the observed wave functions of the electrons. “The experiment introduces a new way of analyzing quantum microscopy,” said Yazdani.

The researchers suggest the technology—both the imagery and the theoretical framework—can be used in the future to analyze and understand many other quantum phases in MATBG, and ultimately, to help comprehend new and unusual material properties that may be useful for next-generation quantum technological applications.

“Our experiment was a wonderful example of how Mother Nature can be so complicated—can be really confusing—until you have the right framework to look at it, and then you say, ‘oh, that’s what’s happening,'” said Yazdani.

More information: Kevin P. Nuckolls et al, Quantum textures of the many-body wavefunctions in magic-angle graphene, Nature (2023). DOI: 10.1038/s41586-023-06226-x

Journal information: Nature 

Provided by Princeton University 

by Liu Jia, Chinese Academy of Sciences

Microplastics—plastics particles smaller than 5 mm in size—have caused an environmental pollution issue that cannot be ignored by our society. Raman spectroscopy technology, with its non-contact, non-destructive and chemical-specific characteristics, has been widely applied in the field of microplastics detection. However, conventional point confocal Raman techniques are limited to single-point detection, impeding the detection speed.

In a study published in Talanta, a research group led by Prof. Li Bei from the Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS), in collaboration with Prof. Wolfgang Langbein from Cardiff University, proposed a novel line-scan Raman micro-spectroscopy technique for rapid identification of micro- and nanoplastics.

Based on the fundamental principles of confocal Raman spectroscopy, the focused excitation spot transforms from a convergent point into a convergent line with diffraction-limited width. The optical setup employs a conjugate imaging design. In the two-dimensional image recorded by the charge-coupled device (CCD), the vertical dimension maps the vertical dimension of the sample along the excitation line, while the spectrum is dispersed along the horizontal dimension. In this way, a single acquisition provides the spectra for all spatial positions along the excitation line.

Researchers developed a confocal line-scan Raman micro-spectroscopy system, established a preprocessing workflow for line-scan Raman spectral data, and applied the factorization into susceptibilities and concentrations (FSC³) algorithm to obtain Raman hyperspectral images. They employed a concave cylindrical lens to generate the excitation line and improved the uniformity of energy distribution using a Powell lens.

Plastic beads of various sizes were used for size and composition identification. The detection of beads with a diameter of 200 nm, which is smaller than the diffraction limit, was realized, demonstrating the exceptional sensitivity of the line-scan Raman spectroscopy system.

Furthermore, four types of plastic powder samples were used for a large-scale area of 1.2 mm in length and 40 μm in height measurement. Impressively, the imaging time is 20 minutes to obtain a 240,000-pixel Raman image. Compared with point confocal Raman imaging, the line-scan confocal Raman technology increases the imaging speed by two orders of magnitude.

Line-scan Raman micro-spectroscopy offers non-destructive analysis with high sensitivity and high-throughput. By employing appropriate sampling techniques such as filtration or sedimentation, environmental samples from various sources, including water, soil and air, are accessible.

More information: Qingyi Wu et al, Rapid identification of micro and nanoplastics by line scan Raman micro-spectroscopy, Talanta (2023). DOI: 10.1016/j.talanta.2023.125067

Provided by Chinese Academy of Sciences

by Brown University

One of the most common and practically useful experiments in all of fluid dynamics involves holding an object in air or submerging it fully underwater, exposing it to a steady flow to measure its resistance in the form of drag. Studies on drag resistance have led to technological advances in airplane and vehicle design and even advanced our understanding of environmental processes.

That’s much tougher these days. As one of the most thoroughly studied aspects in fluid dynamics, it’s become hard to glean or detail new information on the simple physics of drag resistance from these classic experiments. But a team of engineers led by Brown University scientists managed to do so by bringing this problem to the surface—the water surface, that is.

Described in an new paper in Physical Review Fluids, the researchers created a small river-like channel in the lab and lowered spheres—made of different water repellent materials—into the stream until they were almost fully submerged by the flowing water.

The results from the experiment illustrate the fundamental—and sometimes counterintuitive—mechanics of how drag on a partially submerged object may be several times greater than drag on a fully submerged object made of the same material.

For instance, the researchers—led by Brown engineers Robert Hunt and Daniel Harris—found that drag on the spheres increased the moment they touched the water, no matter how water repellent the sphere material was. Each time, the drag increased substantially more than what was expected and continued to increase as the spheres were lowered, beginning only to drop when the spheres were fully beneath the water.

“There’s this intermediate period where the spheres going into the water are creating the biggest disturbances so that the drag is much stronger than if it were way below the surface,” said Harris, an assistant professor in Brown’s School of Engineering. “We knew the drag would go up as the spheres were lowered because they are blocking more of the steady flow, but the surprising thing was how much it goes up. Then as you keep pushing the sphere deeper, the drag goes back down.”

The study shows drag forces on partially submerged objects can be three or four times greater than on fully submerged objects. The largest drag forces, for instance, were measured just prior to the sphere becoming fully submerged, meaning water is flowing all around it but there’s still a small dry spot sticking out at the surface.

“You might expect how much of the sphere is in the water to correspond with how big the drag is,” said Hunt, a postdoctoral researcher in Harris’ lab and the study’s first author. “If so, then you might naively approximate the drag by saying that if the sphere is almost 100% in the water, the drag is going to be almost the same as if it was fully immersed beneath the surface. What we found is the drag can actually can be much larger than that—and not like 50% but more like 300% or 400%.”

The researchers also found that the sphere’s level of water repellency plays a key role in the drag forces it experiences. This is where things get a bit counterintuitive.

The experiment was done with three spheres that are otherwise identical except one was coated with a superhydrophobic material, making it very repellent to water, while the others were made of materials that are increasingly less water repellent.

Running the experiments, the researchers found that the superhydrophobic coating encountered more drag than the other two spheres. It was a surprise because they expected the opposite.

“Superhydrophobic materials are often proposed to reduce drag, but, in our case, we found that superhydrophobic spheres when almost fully immersed have a much larger drag than the sphere made of any other water repellency,” Hunt said. “In trying to decrease the drag, you might actually increase it substantially.”

The paper explains simple physics is the likely cause.

“The water wants nothing to do with this superhydrophobic sphere so it does anything that it can to, sort of, get out of the way of the sphere,” Harris said. “But what happens is much of it piles up in front of it, so there ends up being a wall of water that the sphere is hitting. Intuitively, you would think the water should slip by more freely. Physics actually conspires against that in this scenario.”

The findings from the paper may one day hold implications for designs and structures that operate at an air and water interface, like small autonomous vehicles. For now, the standalone physics of this basic research is interesting enough as studies on partially submerged objects aren’t as currently well characterized or understood in the field.

“We were surprised no one had made these measurements,” Harris said. “It’s such a simple idea but there’s just a lot of rich physics here.”

The researchers chose spheres as the first three-dimensional objects because of how simple their geometry is. They only have one length scale—the radius. The sphere acts as a starting point to be able to strip the physical mechanics down to its most fundamental principles before moving on to more complicated shapes.

“Starting from the simplest point, we look at what are the physics here and then as a next step we begin to apply our knowledge to more realistic structures, whether it’s emulating a biological structure or looking at manmade propulsive structures,” Harris said.

Hunt and fellow lab member Eli Silver designed the flume apparatus for creating the water stream experiment and programmed the motorized lift that lowers the spheres into the water channel. The work started as a collaboration with Yuri Bazilevs, a professor at Brown’s School of Engineering. It also included researchers from the University of Illinois Urbana-Champagne, who performed computer simulations.

More information: Robert Hunt et al, Drag on a partially immersed sphere at the capillary scale, Physical Review Fluids (2023). DOI: 10.1103/PhysRevFluids.8.084003

Provided by Brown University 

by UCLA Engineering Institute for Technology Advancement

While image denoising algorithms have undergone extensive research and advancements in the past decades, classical denoising techniques often necessitate numerous iterations for their inference, making them less suitable for real-time applications.

The advent of deep neural networks (DNNs) has ushered in a paradigm shift, enabling the development of non-iterative, feed-forward digital image denoising approaches.

These DNN-based methods exhibit remarkable efficacy, achieving real-time performance while maintaining high denoising accuracy. However, these deep learning-based digital denoisers incur a trade-off, demanding high-cost, resource- and power-intensive graphics processing units (GPUs) for operation.

In an article published in Light: Science & Applications, a team of researchers, led by Professors Aydogan Ozcan and Mona Jarrahi from University of California, Los Angeles (UCLA), U.S., and Professor Kaan Akşit from University College London (UCL), UK developed a physical image denoiser comprising spatially engineered diffractive layers to process noisy input images at the speed of light and synthesize denoised images at its output field-of-view without any digital computing.

Following a one-time training on a computer, the resulting visual processor with its passive diffractive layers is fabricated, forming a physical image denoiser that scatters out the optical modes associated with undesired noise or spatial artifacts of the input images.

Through its optimized design, this diffractive visual processor preserves the optical modes representing the desired spatial features of the input images with minimal distortions.

As a result, it instantly synthesizes denoised images within its output field-of-view without the need to digitize, store or transmit an image for a digital processor to act on it. The efficacy of this all-optical image denoising approach was validated by suppressing salt and pepper noise from both intensity- and phase-encoded input images.

Furthermore, this physical image denoising framework was experimentally demonstrated using terahertz radiation and a 3D-fabricated diffractive denoiser.

This all-optical image denoising framework offers several important advantages, such as low power consumption, ultra-high speed, and compact size.

The research team envisions that the success of these all-optical image denoisers can catalyze the development of all-optical visual processors tailored to address various inverse problems in imaging and sensing.

More information: Çağatay Işıl et al, All-optical image denoising using a diffractive visual processor, Light: Science & Applications (2024). DOI: 10.1038/s41377-024-01385-6

Provided by UCLA Engineering Institute for Technology Advancement 

by Tokyo Metropolitan University

Researchers from Tokyo Metropolitan University have created a new superconductor with a chiral crystalline structure by mixing two materials, one with superconductivity but no chirality, another with chirality but no superconductivity.

The new platinum-iridium-zirconium compound transitions to a bulk superconductor below 2.2 K and was observed to have chiral crystalline structure using X-ray diffraction. Their new solid solution approach promises to accelerate the discovery and understanding of new exotic superconducting materials.

Scientists studying superconductivity are on a mission to understand how the exotic nature of superconducting materials arises from their structure, and how we might control the structure to get desirable properties.

Of the many aspects of structure, an interesting recent development is the issue of chirality. Many structures have a “handedness,” that is, they do not look the same in a mirror. An effect of chirality in superconductors is to trigger something called asymmetric spin-orbit coupling (ASOC), an effect that can make superconductors more robust to high magnetic field exposure.

To understand chirality in more depth, however, scientists need more superconductors with a chiral structure to study. The usual route is to search out chiral compounds, check if they are superconducting or not, rinse and repeat: this is very inefficient.

That is why a team from Tokyo Metropolitan University led by Associate Professor Yoshikazu Mizuguchi has introduced an entirely new approach. Instead of combing through lists of compounds, they mixed two compounds with known physical properties, a platinum-zirconium compound with superconductivity but no chirality, and an iridium-zirconium compound with a chiral structure, but no reports of superconductivity. The work is published in the Journal of the American Chemical Society.

By combining elements in a ratio that matches a certain proportion of each compound, they were able to effectively “mix and match” physical properties, coming up with a new material that had both a chiral crystal structure and superconductivity.

• X-ray diffraction patterns at different temperatures (top), and the extracted fraction of chiral compound (bottom) show that the proportion of chiral compound increases at lower temperature. Credit: Tokyo Metropolitan University
• As the proportion of iridium is increased, the proportion of P6122, the chiral component, increases. Credit: Tokyo Metropolitan University
• Superconductivity can be confirmed below an iridium proportion of around x = 0.85 in (Pt_1-xIr_x)₃Zr₅. Credit: Tokyo Metropolitan University
• X-ray diffraction patterns at different temperatures (top), and the extracted fraction of chiral compound (bottom) show that the proportion of chiral compound increases at lower temperature. Credit: Tokyo Metropolitan University
• As the proportion of iridium is increased, the proportion of P6122, the chiral component, increases. Credit: Tokyo Metropolitan University

by US Department of Energy

by Daegu Gyeongbuk Institute of Science and Technology (DGIST)

Programmable photonic integrated circuits (PPICs) process light waves for computation, sensing, and signaling in ways that can be programmed to suit diverse requirements. Researchers at Daegu Gyeongbuk Institute of Science and Technology (DGIST), in South Korea, with collaborators at Korea Advanced Institute of Science and Technology (KAIST), have achieved a major advance in incorporating microelectromechanical systems into PPICs.

Their research has been published in the journal Nature Photonics.

“Programmable photonic processors promise to outperform conventional supercomputers, offering faster, more efficient and massively parallel computing capabilities,” says Sangyoon Han of the DGIST team. He emphasizes that, in addition to the increased speeds achieved by using light instead of electric current, the significant reduction in power consumption and size of PPICs could lead to major advances in artificial intelligence, neural networks, quantum computing, and communications.

The microelectromechanical systems (MEMS) at the heart of the new advance are tiny components that can interconvert optical, electronic, and mechanical changes to perform the variety of communication and mechanical functions needed by an integrated circuit.

The researchers believe they are the first to integrate silicon-based photonic MEMS technologies onto PPIC chips that operate with extremely low power requirements.

“Our innovation has dramatically reduced the power consumption to femtowatt levels, which is over a million times an improvement compared to the previous state of the art,” says Han. The technology can also be built onto chips up to five times smaller than existing options.

One key to the dramatic reduction in power requirements was to move away from the dependence on temperature changes required by the dominant “thermo-optic” systems currently in use. The required tiny mechanical movements are powered by electrostatic forces—the attractions and repulsions between fluctuating electric charges.

The components integrated onto the team’s chips can manipulate a feature of light waves called “phase” and control the coupling between different parallel waveguides, which guide and constrain the light. These are the two most fundamental requirements for building PPICs. These features interact with micromechanical “actuators” (essentially switches) to complete the programmable integrated circuit.

The key to the advance has been to apply innovative concepts to the fabrication of the required silicon-based parts. Crucially, the manufacturing process can be used with conventional silicon wafer technology. This makes it compatible with the large-scale production of photonic chips essential to commercial applications.

The team now plans to refine their technology to build and commercialize a photonic computer that will outperform conventional electronic computers in a wide variety of applications. Han says that examples of specific uses include the crucial inference tasks in artificial intelligence, advanced image processing, and high-bandwidth data transmission.

“We expect to continue to push the boundaries of computational technology, contributing further to the field of photonics and its practical applications in modern technology,” Han concludes.

More information: Dong Uk Kim et al, Programmable photonic arrays based on microelectromechanical elements with femtowatt-level standby power consumption, Nature Photonics (2023). DOI: 10.1038/s41566-023-01327-5

by Chinese Academy of Sciences

Using a simple solvent sieve method, researchers from the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences (CAS) have taken the lead in developing highly efficient and stable perovskite light-emitting diodes (PeLEDs) with record performance.

Their study is published in Nature Photonics.

Perovskites are one of the most promising optoelectronic materials due to their excellent optoelectronic performance and low preparation cost. Compared with traditional organic light-emitting diodes (OLEDs), PeLEDs have a narrower light-emitting spectrum and superior color purity, thus showing great application potential in display and lighting.

However, despite significant progress in efficiency, low operational stability has long limited the practical application of PeLEDs. In particular, a limited understanding of the cause of perovskite instability has greatly hindered the development and commercialization of PeLEDs.

Based on an in-depth analysis of the fine nanostructures of perovskites, the researchers identified the perovskites’ defective low n-phase as the key source of perovskite instability. The low quality of the low n-phase, which contained only one or two layers of lead ions, originated from the rapid and uncontrollable crystallization process.

Inspired by the process of separating sand of different sizes with a sieve, the researchers proposed a solvent sieve method to screen out these undesirable low n-phases.

According to the researchers, the solvent sieve is a combination of polar and nonpolar solvents. The polar solvent acts as a mesh that interacts with perovskites, while the nonpolar solvent acts as a framework that does not affect perovskites. The researchers adjusted the ratio of polar solvents to effectively remove the defective low n-phases.

The PeLEDs based on the sieved perovskites achieved an operating lifetime of more than 5.7 years under normal conditions (luminance of 100 cd/m²), more than 30 times longer than the untreated device. This record lifetime is also the highest value reported to date for green PeLEDs, reaching the fundamental threshold for commercial application.

In addition, these PeLEDs achieved a record high external quantum efficiency (EQE) of 29.5%, significantly improving the efficiency of converting electricity to light.

When exposed to ambient air (50±10% humidity), the device can maintain 75% of its film photoluminescence quantum yield and 80% of its EQE for more than 100 days, thus showing excellent stability.

This solvent sieve method not only significantly improves the luminescence performance and stability of PeLEDs, but also paves the way for the future development and application of perovskites with unique nanostructures and excellent luminescence performance.

More information: Shuo Ding et al, Phase dimensions resolving of efficient and stable perovskite light-emitting diodes at high brightness, Nature Photonics (2024). DOI: 10.1038/s41566-023-01372-0

Provided by Chinese Academy of Sciences