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Showing posts with label Nanotechnology. Show all posts
Showing posts with label Nanotechnology. Show all posts

Thursday, 21 October 2021

Shrinking quantum key distribution technology to a semiconductor chip


Toshiba Europe Ltd today announced it has developed the world's first chip-based quantum key distribution (QKD) system. This advance will enable the mass manufacture of quantum security technology, bringing its application to a much wider range of scenarios including to Internet of Things (IoT) solutions.

QKD addresses the demand for cryptography which will remain secure from attack by the supercomputers of tomorrow. In particular, a large-scale quantum computer will be able to efficiently solve the difficult mathematical problems that are the basis of the public key cryptography widely used today for secure communications and e-commerce. In contrast, the protocols used for quantum cryptography can be proven secure from first principles and will not be vulnerable to attack by a quantum computer, or indeed any computer in the future.

The QKD market is expected to grow to approximately $20 billion worldwide in financial year 2035. Large quantum-secured fiber networks are currently under construction in Europe and South-East Asia, and there are plans to launch satellites that can extend the networks to a global scale. In October 2020, Toshiba released two products for fiber-based QKD, which are based on discrete optical components. Together with project partners, Toshiba has implemented quantum-secured metro networks and long-distance fiber optic backbone links in the UK, Europe, US and Japan. 

Manufacturing advances

For quantum cryptography to become as ubiquitous as the algorithmic cryptography we use today, it is important that the size, weight and power consumption are further reduced. This is especially true for extending QKD and quantum random number generators (QRNG) into new domains such as the last-mile connection to the customer or IoT. The development of chip-based solutions is essential to enabling mass market applications, which will be integral to the realization of a quantum-ready economy.

Toshiba has developed techniques for shrinking the optical circuits used for QKD and QRNG into tiny semiconductor chips. These are not only much smaller and lighter than their fiber optic counterparts, but also consume less power. Most significantly, many can be fabricated in parallel on the same semiconductor wafer using standard techniques used within the semiconductor industry, allowing them to be manufactured in much larger numbers. For example, the quantum transmitter chips developed by Toshiba measure just 2x6mm, allowing several hundred chips to be produced simultaneously on a wafer. 

Andrew Shields, Head of Quantum Technology at Toshiba Europe, remarked, "Photonic integration will allow us to manufacture quantum security devices in volume in a highly repeatable fashion. It will enable the production of quantum products in a smaller form factor, and subsequently allow the roll out of QKD into a larger fraction of the telecom and datacom network."

Taro Shimada, Corporate Senior Vice President and Chief Digital Officer of Toshiba Corporation comments, "Toshiba has invested in quantum technology R&D in the UK for over two decades. This latest advancement is highly significant, as it will allow us to manufacture and deliver QKD in much larger quantities. It is an important milestone towards our vision of building a platform for quantum-safe communications based upon ubiquitous quantum security devices." 

The details of the advancement are published in the journal Nature Photonics.

Technical Summary

QKD systems typically comprise a complex fiber-optic circuit, integrating discrete components, such as lasers, electro-optic modulators, beam-splitters and fiber couplers.  As these components are relatively bulky and expensive, the purpose of this work was to develop a QKD system in which the fiber-optic circuit and devices are written in millimeter scale semiconductor chips.

Toshiba has developed the first complete QKD prototype in which quantum photonic chips of different functionality are deployed. Random bits for preparing and measuring the qubits are produced in quantum random number generator (QRNG) chips and converted in real-time into high-speed modulation patterns for the chip-based QKD transmitter (QTx) and receiver (QRx) using field-programmable gate arrays (FPGAs).  Photons are detected using fast-gated single photon detectors. Sifting, photon statistics evaluation, time synchronization and phase stabilization are done via a 10 Gb/s optical link between the FPGA cores, enabling autonomous operation over extended periods of time. As part of the demonstration, the chip QKD system was interfaced with a commercial encryptor, allowing secure data transfer with a bit rate up to 100 Gb/s. 

To promote integration into conventional communication infrastructures, the QKD units are assembled in compact 1U rackmount cases. The QRx and QTx chips are packaged into C-form-factor-pluggable-2 (CFP2) modules, a widespread form-factor in coherent optical communications, to ensure forward compatibility of the system with successive QKD chip generations, making it easily upgradeable. Off-the-shelf 10 Gb/s small-form-factor pluggable (SFP) modules are used for the public communication channels.

Taofiq Paraiso, lead author of the Nature Photonics paper describing the chip-scale QKD system, says that "we are witnessing with photonic integrated circuits a similar revolution to that which occurred with electronic circuits. PICs are continuously serving more and more diverse applications. Of course, the requirements for quantum PICs are more stringent than for conventional applications, but this work shows that a fully deployable chip-based QKD system is now attainable, marking the end of an important challenge for quantum technologies. This opens a wide-range of perspectives for the deployment of compact, plug-and-play quantum devices that will certainly strongly impact our society."

Reference: 

Taofiq Paraïso, A photonic integrated quantum secure communication system, Nature Photonics (2021). DOI: 10.1038/s41566-021-00873-0.

A rapid mechanism for muscle self-repair independent of stem cells


Muscle is known to regenerate through a complex process that involves several steps and relies on stem cells. Now, a new study led by researchers at UPF, Centro Nacional de Investigationes Cardiovasculares (CNIC), CIBERNED and Instituto de Medicina Molecular João Lobo Antunes (iMM, Portugal), published on 15 October in the journal Science, describes a new mechanism for muscle repair after physiological damage relying on the rearrangement of muscle fibre nuclei, and independently of muscle stem cells. This protective mechanism paves the way to a broader understanding of muscle repair in physiology and disease.

Skeletal muscle tissue, the organ responsible for locomotion, is formed by cells (fibres) that have more than one nucleus, an almost unique feature in our body. Despite the plasticity of these fibres, their contraction can be associated with muscle damage. William Roman, first author of the study and researcher at UPF, explains: “Even in physiological conditions, regeneration is vital for muscle to endure the mechanical stress of contraction, which often leads to cellular damage”. Although muscle regeneration has been investigated in depth in recent decades, most studies centred on mechanisms involving several cells, including muscle stem cells, which are required when extensive muscle damage occurs”.

“In this study we found an alternative mechanism of muscle tissue repair that is muscle-fibre autonomous”, says Pura Muñoz-Cánoves, ICREA professor and principal investigator at UPF and the CNIC, and study leader. Researchers (including Antonio Serrano (UPF) and Mari Carmen Gómez-Cabrera (University of Valencia and INCLIVA) used different in vitro models of injury and models of exercise in mice and humans to observe that upon injury, nuclei are attracted to the damage site, accelerating the repair of the contractile units. Next, the team dissected the molecular mechanism of this observation: “Our experiments with muscle cells in the laboratory showed that the movement of nuclei to injury sites resulted in the local delivery of mRNA molecules. These mRNA molecules are translated into proteins at the site of injury to act as building blocks for muscle repair”, explains William Roman. “This muscle fibre self-repair process occurs rapidly both in mice and in humans after exercise-induced muscle injury, and thus represents a time- and energy-efficient protective mechanism for the repair of minor lesions”, adds Pura Muñoz-Cánoves.

In addition to its implications for muscle research, this study also introduces more general concepts for cell biology, such as the movement of nuclei to injury sites. “One of the most fascinating things about these cells is the movement during the development of their nuclei, the biggest organelles inside the cell, but the reasons why nuclei move are largely unknown. Now, we have shown a functional relevance for this phenomenon in adulthood during cell repair and regeneration”, says Edgar R. Gomes, group leader at the Instituto de Medicina Molecular and a professor at the Faculty of Medicine at the University of Lisbon, who co-led the study.

On the importance of these discoveries, Pura Muñoz-Cánoves, Antonio Serrano and Mari Carmen Gómez-Cabrera agree that: “This finding constitutes an important advance in the understanding of muscle biology, in physiology (including exercise physiology) and muscle dysfunction”.

Reference: 

Roman W, Pinheiro H, Pimentel MR, et al. Muscle repair after physiological damage relies on nuclear migration for cellular reconstruction. Science. 2021;374(6565):355-359. doi: 10.1126/science.abe5620

Friday, 1 October 2021

State-of-the-Art Imaging Reveals Live Deep Brain Neurons in Unprecedented Detail


A pioneering technique developed by the Prevedel Group at EMBL allows neuroscientists to observe live neurons deep inside the brain – or any other cell hidden within an opaque tissue. The technique is based on two state-of-the-art microscopy methods, three-photon microscopy and adaptive optics. The paper reporting on this advancement was published on 30th September 2021 in Nature Methods.

Until the development of the new technique, it was challenging for neuroscientists to observe astrocytes generating calcium waves in deep layers of the cortex, or to visualise any other neural cells in the hippocampus, a region deep in the brain that is responsible for spatial memory and navigation. The phenomenon takes place regularly in the brains of all live mammals. By developing the new technique, Lina Streich from the Prevedel Group and her collaborators were able to capture the fine details of these versatile cells at unprecedented high resolution. The international team included researchers from Germany, Austria, Argentina, China, France, the USA, India, and Jordan.

In the neurosciences, brain tissues are observed mostly in small model organisms or in ex vivo samples that need to be sliced up to be observed – both of which represent non-physiological conditions. Normal brain cell activity takes place only in live animals, but the “mouse brain is a highly scattering tissue,” said Robert Prevedel. “In these brains, light cannot be focused very easily, because it interacts with the cellular components. This limits how deep you can generate a crisp image, and it makes it very difficult to focus on small structures deep inside the brain with traditional techniques.”

Thanks to Streich, a former PhD student in the lab who worked for more than four years to overcome this problem, scientists can now peer further into tissues.

“With traditional fluorescence brain microscopy techniques, two photons are absorbed by the fluorescence molecule each time, and you can make sure that the excitement caused by the radiation is confined to a small volume,” explained Prevedel, a physicist by training. “But the further the photons travel, the more likely they are lost due to scattering.” One way to overcome this is to increase the wavelength of the exciting photons towards the infrared, which ensures enough radiation energy to be absorbed by the fluorophore. In addition, using three photons instead of two allows to obtain crisper images deep inside the brain. But another challenge remains: making sure that the photons are focused, so that the whole image is not blurry.

This is where the second technique used by Streich and her team is important. Adaptive optics is used regularly in astronomy – and indeed it was crucial for Roger Penrose, Reinhard Genzel and Andrea Ghez to obtain the Nobel Prize in Physics in 2020 for their discovery of black holes. Astrophysicists use deformable, computer-controlled mirrors to correct in real time for the distortion in the light wave front caused by atmospheric turbulence. In Prevedel’s lab, the distortion is caused by the scattering inhomogeneous tissue, but the principle and the technology are very similar. “We also use an actively controlled deformable mirror, which is capable of optimising the wave fronts to allow the light to converge and focus even deep inside the brain,” explained Prevedel. “We developed a custom approach to make it fast enough to use on live cells in the brain,” added Streich. To reduce the invasiveness of the technique, the team also minimised the number of measurements needed to achieve high-quality images.

“This is the first time these techniques have been combined,” said Streich, “and thanks to them, we were able to show the deepest in vivo images of live neurons at high resolution.” The scientists, who worked in collaboration with colleagues from EMBL Rome and the University of Heidelberg, even visualised the dendrites and axons that connect the neurons in the hippocampus, while leaving the brain completely intact.

“This is a leap towards developing more advanced non-invasive techniques to study live tissues,” Streich said. Although the technique was developed for use on a mouse brain, it is easily applicable to any opaque tissue. “Besides the obvious advantage of being able to study biological tissues without the need to sacrifice the animals or to remove overlaying tissue, this new technique opens the way to study an animal longitudinally, that is, from the onset of a disease to the end. This will give scientists a powerful instrument to better understand how diseases develop in tissues and organs.”

Reference:

Streich L, Boffi JC, Wang L, et al. High-resolution structural and functional deep brain imaging using adaptive optics three-photon microscopy. Nat Methods. Published online September 30, 2021:1-6. doi:10.1038/s41592-021-01257-6

Monday, 20 September 2021

Engineers create light-emitting plants that can be charged repeatedly


Using specialized nanoparticles embedded in plant leaves, MIT engineers have created a light-emitting plant that can be charged by an LED. After 10 seconds of charging, plants glow brightly for several minutes, and they can be recharged repeatedly.

These plants can produce light that is 10 times brighter than the first generation of glowing plants that the research group reported in 2017.

"We wanted to create a light-emitting plant with particles that will absorb light, store some of it, and emit it gradually," says Michael Strano, the Carbon P. Dubbs Professor of Chemical Engineering at MIT and the senior author of the new study. "This is a big step toward plant-based lighting."

"Creating ambient light with the renewable chemical energy of living plants is a bold idea," says Sheila Kennedy, a professor of architecture at MIT and an author of the paper who has worked with Strano's group on plant-based lighting. "It represents a fundamental shift in how we think about living plants and electrical energy for lighting."

The particles can also boost the light production of any other type of light-emitting plant, including those Strano's lab originally developed. Those plants use nanoparticles containing the enzyme luciferase, which is found in fireflies, to produce light. The ability to mix and match functional nanoparticles inserted into a living plant to produce new functional properties is an example of the emerging field of "plant nanobionics."

Pavlo Gordiichuk, a former MIT postdoc, is the lead author of the new paper, which appears in Science Advances.

Light capacitor

Strano's lab has been working for several years in the new field of plant nanobionics, which aims to give plants novel features by embedding them with different types of nanoparticles. Their first generation of light-emitting plants contained nanoparticles that carry luciferase and luciferin, which work together to give fireflies their glow. Using these particles, the researchers generated watercress plants that could emit dim light, about one-thousandth the amount needed to read by, for a few hours.

In the new study, Strano and his colleagues wanted to create components that could extend the duration of the light and make it brighter. They came up with the idea of using a capacitor, which is a part of an electrical circuit that can store electricity and release it when needed. In the case of glowing plants, a light capacitor can be used to store light in the form of photons, then gradually release it over time.

To create their "light capacitor," the researchers decided to use a type of material known as a phosphor. These materials can absorb either visible or ultraviolet light and then slowly release it as a phosphorescent glow. The researchers used a compound called strontium aluminate, which can be formed into nanoparticles, as their phosphor. Before embedding them in plants, the researchers coated the particles in silica, which protects the plant from damage.

The particles, which are several hundred nanometers in diameter, can be infused into the plants through the stomata—small pores located on the surfaces of leaves. The particles accumulate in a spongy layer called the mesophyll, where they form a thin film. A major conclusion of the new study is that the mesophyll of a living plant can be made to display these photonic particles without hurting the plant or sacrificing lighting properties, the researchers say.

This film can absorb photons either from sunlight or an LED. The researchers showed that after 10 seconds of blue LED exposure, their plants could emit light for about an hour. The light was brightest for the first five minutes and then gradually diminished. The plants can be continually recharged for at least two weeks, as the team demonstrated during an experimental exhibition at the Smithsonian Institute of Design in 2019.

"We need to have an intense light, delivered as one pulse for a few seconds, and that can charge it," Gordiichuk says. "We also showed that we can use big lenses, such as a Fresnel lens, to transfer our amplified light a distance more than one meter. This is a good step toward creating lighting at a scale that people could use."

"The Plant Properties exhibition at the Smithsonian demonstrated a future vision where lighting infrastructure from living plants is an integral part of the spaces where people work and live," Kennedy says. "If living plants could be the starting point of advanced technology, plants might replace our current unsustainable urban electrical lighting grid for the mutual benefit of all plant-dependent species—including people."

Large-scale illumination

The MIT researchers found that the "light capacitor" approach can work in many different plant species, including basil, watercress, and tobacco, the researchers found. They also showed that they could illuminate the leaves of a plant called the Thailand elephant ear, which can be more than a foot wide—a size that could make the plants useful as an outdoor lighting source.

The researchers also investigated whether the nanoparticles interfere with normal plant function. They found that over a 10-day period, the plants were able to photosynthesize normally and to evaporate water through their stomata. Once the experiments were over, the researchers were able to extract about 60 percent of the phosphors from plants and reuse them in another plant.

Researchers in Strano's lab are now working on combining the phosphor light capacitor particles with the luciferase nanoparticles that they used in their 2017 study, in hopes that combining the two technologies will produce plants that can produce even brighter light, for longer periods of time.

Reference: 

Pavlo Gordiichuk et al, Augmenting the living plant mesophyll into a photonic capacitor, Science Advances (2021). DOI: 10.1126/sciadv.abe9733

Wednesday, 8 September 2021

Researchers watch bacteria repair damaged DNA in real time


How the cell can mend broken DNA using another DNA copy as template has puzzled researchers for years. How is it possible to find the correct sequences in the busy interior of the cell? Researchers from Uppsala university have now discovered the solution; it is easier to find a rope than a ball if you are blindfolded.

When a DNA molecule breaks in two, the fate of the cell is threatened. From the perspective of a bacterium, fixing the break quickly is a matter of life and death. But to mend the DNA without introducing mistakes in the sequence is challenging; the repair machinery needs to find a template. The process of healing broken DNA using a template from a sister chromosome is known as homologous recombination and is well described in the literature.

However, the description usually disregards the daunting task of finding the matching template among all the other genome sequences. The chromosome is a complex structure with several million base pairs of genetic code and it is quite clear that simple diffusion in 3D would not be sufficiently fast by a long shot. But then, how is it done? This has been the mystery of homologous recombination for 50 years. From previous studies, it is clear that the molecule RecA is involved and important in the search process, but, up until now, this has been the limit of our understanding of this process.

Using a CRISPR-based technique

Now, a group of Uppsala researchers headed by Professor Johan Elf has finally found the solution to this search enigma. In a study that is published in Nature, they use a CRISPR-based technique to make controlled DNA breaks in bacteria. By growing the cells in a microfluidic culture chip and tracking labeled RecA molecules with fluorescence microscopy, the researchers can image the homologous recombination process from start to finish.

“The microfluidic culture chip allows us to follow the fate of thousands of individual bacteria simultaneously and to control CRISPR-induced DNA breaks in time. It is very precise, almost like having a pair of tiny DNA scissors,” says Jakub Wiktor, one of the researchers behind the study.

The label on RecA together with fluorescent markers on the DNA allows the researchers to follow every step of the process accurately; for example, they conclude that the whole repair is finished in 15 minutes, on average, and that the template is located in about nine.

Rearranging RecA to form thin filaments

Using microscopy, Elf and his team investigate the fate of the break site and its homologous copy in real-time. They also find that the cell responds by rearranging RecA to form thin filaments that span the length of the cell.

“We can see the formation of a thin, flexible structure that protrudes from the break site just after the DNA damage. Since the DNA ends are incorporated into this fiber, it is sufficient that any part of the filament finds the precious template and thus the search is theoretically reduced from three to two dimensions. Our model suggests that this is the key to fast and successful homology repair,” says Arvid Gynnå, who has worked on the project throughout his PhD studies.

Might help us understand the causes of tumor growth

Going from a 3D to a 2D search is indeed a considerable improvement regarding the probability of finding the homologous sequence quickly enough, or in fact, at all. As the Japanese mathematician, Shizuo Kakutani put it: “A drunk man will find his way home, but a drunk bird may be lost forever”. With these words, he tried to explain a curious fact; an object that explores a 2D surface by a random walk will sooner or later find its way back to its starting point while in a 3D space, it is likely that it will never return “home”.

The Uppsala researchers performed their study in the model organism E. coli, but the process of homology repair is nearly identical for higher organisms such as ourselves, or doves for that matter. DNA damage occurs frequently in our bodies, and without the ability to heal broken DNA, we would be extremely vulnerable to, for example, UV light and reactive oxygen species, and more likely to develop cancer. In fact, most oncogenes are related to DNA repair and the new mechanistic insights might help us understand the causes of tumor growth.

Reference:

Wiktor, J., Gynnå, A.H., Leroy, P. et al. RecA finds homologous DNA by reduced dimensionality search. Nature (2021). DOI: 10.1038/s41586-021-03877-6

Friday, 13 August 2021

Study suggests ‘memories’ can be stored in synthetic brain cells


Three researchers from the Physics Laboratory of the École Normale Supérieure in Paris, France, recreated in a model the electrical charge transport system that characterizes a nerve cell.

The experiment involved placing an aqueous electrolyte, similar to the one that fills neurons, in a very fine and almost two-dimensional space, and the desired effect was the imitation of intracellular functions associated with memory.

Most of the memory artificial resistor systems, known in professional parlance as ‘memristors’, use electrons as charge carriers, but an ion solution can also transmit charge. And that’s precisely the way neurons work, scientists argue in a published article last Friday in the journal Science. In his tests, that liquid was confined between two layers of graphite barely spaced from each other by a ten-billionth of a meter.

In general, under the effects of an electric field, ions assemble into elongated groups – ones worm-like structures– and show slow dynamics and voltage typical for intracellular transmission, the researchers observed. This phenomenon promotes the ‘memristor’ effect, and the team believes that it can be used to build an artificial neuron.

Lydéric Bocquet, study co-author, think That this is the first time it has been possible to incorporate to a physical model the neural transmission channels, which are the basis of brain activity. He also explains that the team opted for an almost two-dimensional environment, very rare for nature, because in two dimensions the particles tend to react more strongly than in three and exhibit different properties.

To generate an action potential in a real, living brain, a neuron lets in a group of positive ions, attracted by other negatively charged ions. The electrical potential, or voltage, passes through the cell membrane and causes ‘doors’ known as ion channels open in the cell. Meanwhile, the possibility of activating them with the accumulated electrical charge implies that it reaches a peak before the entry of the ions and returns to normal a few milliseconds later. The signal is then transmitted to other cells, which allows information to travel through the brain.

Scientists suppose that this mechanism could serve, probably in the distant future, to develop computers as ‘energy efficient’ as brain tissue and, more immediately, to help scientists better understand how the brain processes information. The tests carried out are part of the attempts to develop an artificial synapse, that is, the connection that transmits electrical signals between two neurons or from one neuron to another cell.

Reference: 

Modeling of emergent memory and voltage spiking in ionic transport through angstrom-scale slits” by Paul Robin, Nikita Kavokine and Lydéric Bocquet, 6 August 2021, Science. DOI: 10.1126/science.abf7923

Sunday, 8 August 2021

Translation Software Enables Efficient Storage of Massive Amounts of Data in DNA Molecules


ADS Codex translates binary data into nucleotides that can be sequenced in molecules as files for later retrieval, bringing potential cost savings and compact ‘cold storage.’

In support of a major collaborative project to store massive amounts of data in DNA molecules, a Los Alamos National Laboratory–led team has developed a key enabling technology that translates digital binary files into the four-letter genetic alphabet needed for molecular storage.

“Our software, the Adaptive DNA Storage Codec (ADS Codex), translates data files from what a computer understands into what biology understands,” said Latchesar Ionkov, a computer scientist at Los Alamos and principal investigator on the project. “It’s like translating from English to Chinese, only harder.”

“Our software, the Adaptive DNA Storage Codec (ADS Codex), translates data files from what a computer understands into what biology understands.” — Latchesar Ionkov

The work is key part of the Intelligence Advanced Research Projects Activity (IARPA) Molecular Information Storage (MIST) program to bring cheaper, bigger, longer-lasting storage to big-data operations in government and the private sector. The short-term goal of MIST is to write 1 terabyte—a trillion bytes—and read 10 terabytes within 24 hours for $1,000. Other teams are refining the writing (DNA synthesis) and retrieval (DNA sequencing) components of the initiative, while Los Alamos is working on coding and decoding.

“DNA offers a promising solution compared to tape, the prevailing method of cold storage, which is a technology dating to 1951,” said Bradley Settlemyer, a storage systems researcher and systems programmer specializing in high-performance computing at Los Alamos. “DNA storage could disrupt the way we think about archival storage, because the data retention is so long and the data density so high. You could store all of YouTube in your refrigerator, instead of in acres and acres of data centers. But researchers first have to clear a few daunting technological hurdles related to integrating different technologies.”

Not lost in translation

Compared to the traditional long-term storage method that uses pizza-sized reels of magnetic tape, DNA storage is potentially less expensive, far more physically compact, more energy efficient, and longer lasting—DNA survives for hundreds of years and doesn’t require maintenance. Files stored in DNA also can be very easily copied for negligible cost.

DNA’s storage density is staggering. Consider this: humanity will generate an estimated 33 zettabytes by 2025—that’s 3.3 followed by 22 zeroes. All that information would fit into a ping pong ball, with room to spare. The Library of Congress has about 74 terabytes, or 74 million million bytes, of information—6,000 such libraries would fit in a DNA archive the size of a poppy seed. Facebook’s 300 petabytes (300,000 terabytes) could be stored in a half poppy seed.

Encoding a binary file into a molecule is done by DNA synthesis. A fairly well understood technology, synthesis organizes the building blocks of DNA into various arrangements, which are indicated by sequences of the letters A, C, G, and T. They are the basis of all DNA code, providing the instructions for building every living thing on earth.

The Los Alamos team’s ADS Codex tells exactly how to translate the binary data—all 0s and 1s—into sequences of four letter-combinations of A, C, G, and T. The Codex also handles the decoding back into binary. DNA can be synthesized by several methods, and ADS Codex can accommodate them all. The Los Alamos team has completed a version 1.0 of ADS Codex and in November 2021 plans to use it to evaluate the storage and retrieval systems developed by the other MIST teams.

Unfortunately, DNA synthesis sometimes makes mistakes in the coding, so ADS Codex addresses two big obstacles to creating DNA data files.

First, compared to traditional digital systems, the error rates while writing to molecular storage are very high, so the team had to figure out new strategies for error correction. Second, errors in DNA storage arise from a different source than they do in the digital world, making the errors trickier to correct.

“On a digital hard disk, binary errors occur when a 0 flips to a 1, or vice versa, but with DNA, you have more problems that come from insertion and deletion errors,” Ionkov said. “You’re writing A, C, G, and T, but sometimes you try to write A, and nothing appears, so the sequence of letters shifts to the left, or it types AAA. Normal error correction codes don’t work well with that.”

ADS Codex adds additional information called error detection codes that can be used to validate the data. When the software converts the data back to binary, it tests if the codes match. If they don’t, ACOMA tries removing or adding nucleotides until the verification succeeds.

Smart scale-up

Large warehouses contain today’s largest data centers, with storage at the exabyte scale—that’s a trillion million bytes or more. Costing billions to build, power, and run, this type of digitally based data centers may not be the best option as the need for data storage continues to grow exponentially.

Long-term storage with cheaper media is important for the national security mission of Los Alamos and others. “At Los Alamos, we have some of the oldest digital-only data and largest stores of data, starting from the 1940s,” Settlemyer said. “It still has tremendous value. Because we keep data forever, we’ve been at the tip of the spear for a long time when it comes to finding a cold-storage solution.”

Settlemyer said DNA storage has the potential to be a disruptive technology because it crosses between fields ripe with innovation. The MIST project is stimulating a new coalition among legacy storage vendors who make tape, DNA synthesis companies, DNA sequencing companies, and high-performance computing organizations like Los Alamos that are driving computers into ever-larger-scale regimes of science-based simulations that yield mind-boggling amounts of data that must be analyzed.

Deeper dive into DNA

When most people think of DNA, they think of life, not computers. But DNA is itself a four-letter code for passing along information about an organism. DNA molecules are made from four types of bases, or nucleotides, each identified by a letter: adenine (A), thymine (T), guanine (G), and cytosine (C).

These bases wrap in a twisted chain around each other—the familiar double helix—to form the molecule. The arrangement of these letters into sequences creates a code that tells an organism how to form. The complete set of DNA molecules makes up the genome—the blueprint of your body. 

By synthesizing DNA molecules—making them from scratch—researchers have found they can specify, or write, long strings of the letters A, C, G, and T and then read those sequences back. The process is analogous to how a computer stores information using 0s and 1s. The method has been proven to work, but reading and writing the DNA-encoded files currently takes a long time, Ionkov said.

“Appending a single nucleotide to DNA is very slow. It takes a minute,” Ionkov said. “Imagine writing a file to a hard drive taking more than a decade. So that problem is solved by going massively parallel. You write tens of millions of molecules simultaneously to speed it up.”

While various companies are working on different ways of synthesizing to address this problem, ADS Codex can be adapted to every approach.

Funding for ADS Codex was provided by the Intelligence Advanced Research Projects Activity (IARPA), a research agency within the Office of the Director of National Intelligence.

Source: Link

Friday, 30 July 2021

Scientists Find Way to Navigate a Heavy Uphill Climb With Tiny Motors That Behave Like Rock Climbers


A team of scientists has uncovered how heavy, motorized objects climb steep slopes — a newly discovered mechanism that also mimics how rock climbers navigate inclines.

The findings, which appear in the journal Soft Matter, stem from a series of experiments in which motorized objects were placed in liquid and then moved up tilted surfaces.

“These ‘micro-swimmers’ are about 20 times heavier than the fluid they swim in, but they were able to climb steep slopes that are almost vertical,” explains Jun Zhang, one of the paper’s authors and a professor of physics and mathematics at New York University’s Courant Institute of Mathematical Sciences and NYU Shanghai.

The work enhances our understanding of “gravitaxis” — directional movement in response to gravity. The phenomenon is a vital consideration in not only engineering, but also in medicine and pharmaceutical development. It explains, in part, how bacteria move through the body and provides insights into ways to create more effective drug-delivery mechanisms.

In the Soft Matter research, the scientists created swimmers, or nanorods, whose length is roughly 1/40th the width of a strand of human hair. These motorized swimmers were tasked with moving up an inclined surface while immersed in a liquid solution within a walled container. The swimmers were composed of two types of metal — gold and rhodium as well as gold and platinum — a makeup that gave them unbalanced densities given the varying weights of these metals.

The swimmers’ composition, liquid environment, and juxtaposition of surfaces enabled them to move upward, despite their significant weight.

“These motors reorient themselves upward against gravity thanks to their density imbalance — much like a seesaw reorients itself in response to the movement and weight of its riders,” adds Michael Shelley, a professor at the Courant Institute and director of the Flatiron Institute’s Center for Computational Biology. “A hydrodynamic effect amplifies this movement — swimming next to a wall yields a bigger torque in repositioning the motors’ bodies upwards. This is important because the microscopic world is noisy — for the motor it’s always two steps up and one step down — and the bigger torque improves their ability to move vertically.”

In previous work, published in Physical Review Letters, Zhang, Shelley, and their colleagues created “nano-motors” in uncovering an effective means of movement against currents. The new research expands on these findings by revealing how heavy objects can move up steeply inclined surfaces, offering the promise of even more sophisticated maneuvers.

“Now that these micro-swimmers are able to climb very steep slopes against gravity, we can look toward developing even more difficult assignments,” observes Zhang. “Future, advanced motors will be designed to reach targeted locations and to perform designated functions.”

Reference: 

Metallic microswimmers driven up the wall by gravity by Quentin Brosseau, Florencio Balboa Usabiaga, Enkeleida Lushi, Yang Wu, Leif Ristroph, Michael D. Ward, Michael J. Shelley and Jun Zhangaef, 11 June 2021, Soft Matter.

Thursday, 29 July 2021

Nanostructures Enable Record High-Harmonic Generation From Ultra-Intense Laser Pulses


Cornell researchers have developed nanostructures that enable record-breaking conversion of laser pulses into high-harmonic generation, paving the way for new scientific tools for high-resolution imaging and studying physical processes that occur at the scale of an attosecond – one quintillionth of a second.

High-harmonic generation has long been used to merge photons from a pulsing laser into one, ultrashort photon with much higher energy, producing extreme ultraviolet light and X-rays used for a variety of scientific purposes. Traditionally, gases have been used as sources of harmonics, but a research team led by Gennady Shvets, professor of applied and engineering physics in the College of Engineering, has shown that engineered nanostructures have a bright future for this application.

The research is detailed in the paper “Generation of Even and Odd High Harmonics in Resonant Metasurfaces Using Single and Multiple Ultra-Intense Laser Pulses,” published on July 7, 2021, in Nature Communications. Maxim Shcherbakov, who conducted the research as a Cornell postdoctoral associate before becoming an assistant professor at the University of California, Irvine, is the lead author.

The nanostructures created by the team make up an ultrathin resonant gallium-phosphide metasurface that overcomes many of the usual problems associated with high-harmonic generation in gases and other solids. The gallium-phosphide material permits harmonics of all orders without reabsorbing them, and the specialized structure can interact with the laser pulse’s entire light spectrum.

“Achieving this required engineering of the metasurface’s structure using full-wave simulations,” Shcherbakov said. “We carefully selected the parameters of the gallium-phosphide particles to fulfill this condition, and then it took a custom nanofabrication flow to bring it to light.”

The result is nanostructures capable of generating both even and odd harmonics – a limitation of most other harmonic materials – covering a wide range of photon energies between 1.3 and 3 electron volts. The record-breaking conversion efficiency enables scientists to observe molecular and electronic dynamics within a material with just one laser shot, helping to preserve samples that may otherwise be degraded by multiple high-powered shots.

The study is the first to observe high-harmonic generated radiation from a single laser pulse, which allowed the metasurface to withstand high powers – five to 10 times higher than previously shown in other metasurfaces.

“It opens up new opportunities to study matter at ultrahigh fields, a regime not readily accessible before,” Shcherbakov said. “With our method, we envision that people can study materials beyond metasurfaces, including but not limited to crystals, 2D materials, single atoms, artificial atomic lattices and other quantum systems.”

Now that the research team has demonstrated the advantages of using nanostructures for high-harmonic generation, it hopes to improve high-harmonic devices and facilities by stacking the nanostructures together to replace a solid-state source, such as crystals.

Reference: 

Generation of even and odd high harmonics in resonant metasurfaces using single and multiple ultra-intense laser pulses by Maxim R. Shcherbakov, Haizhong Zhang, Michael Tripepi, Giovanni Sartorello, Noah Talisa, Abdallah AlShafey, Zhiyuan Fan, Justin Twardowski, Leonid A. Krivitsky, Arseniy I. Kuznetsov, Enam Chowdhury and Gennady Shvets, 7 July 2021, Nature Communications.

Monday, 26 July 2021

Main Attraction: Scientists Create World’s Thinnest Magnet – Just One Atom Thick!


A one-atom-thin 2D magnet developed by Berkeley Lab and UC Berkeley could advance new applications in computing and electronics.

The development of an ultrathin magnet that operates at room temperature could lead to new applications in computing and electronics – such as high-density, compact spintronic memory devices – and new tools for the study of quantum physics.

The ultrathin magnet, which was recently reported in the journal Nature Communications, could make big advances in next-gen memory devices, computing, spintronics, and quantum physics. It was discovered by scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley.

“We’re the first to make a room-temperature 2D magnet that is chemically stable under ambient conditions,” said senior author Jie Yao, a faculty scientist in Berkeley Lab’s Materials Sciences Division and associate professor of materials science and engineering at UC Berkeley.

“This discovery is exciting because it not only makes 2D magnetism possible at room temperature, but it also uncovers a new mechanism to realize 2D magnetic materials,” added Rui Chen, a UC Berkeley graduate student in the Yao Research Group and lead author on the study.

The magnetic component of today’s memory devices is typically made of magnetic thin films. But at the atomic level, these materials are still three-dimensional – hundreds or thousands of atoms thick. For decades, researchers have searched for ways to make thinner and smaller 2D magnets and thus enable data to be stored at a much higher density.

Previous achievements in the field of 2D magnetic materials have brought promising results. But these early 2D magnets lose their magnetism and become chemically unstable at room temperature.

“State-of-the-art 2D magnets need very low temperatures to function. But for practical reasons, a data center needs to run at room temperature,” Yao said. “Our 2D magnet is not only the first that operates at room temperature or higher, but it is also the first magnet to reach the true 2D limit: It’s as thin as a single atom!”

The researchers say that their discovery will also enable new opportunities to study quantum physics. “It opens up every single atom for examination, which may reveal how quantum physics governs each single magnetic atom and the interactions between them,” Yao said.

The making of a 2D magnet that can take the heat
The researchers synthesized the new 2D magnet – called a cobalt-doped van der Waals zinc-oxide magnet – from a solution of graphene oxide, zinc, and cobalt.

Just a few hours of baking in a conventional lab oven transformed the mixture into a single atomic layer of zinc-oxide with a smattering of cobalt atoms sandwiched between layers of graphene.

In a final step, the graphene is burned away, leaving behind just a single atomic layer of cobalt-doped zinc-oxide.

“With our material, there are no major obstacles for industry to adopt our solution-based method,” said Yao. “It’s potentially scalable for mass production at lower costs.”

To confirm that the resulting 2D film is just one atom thick, Yao and his team conducted scanning electron microscopy experiments at Berkeley Lab’s Molecular Foundry to identify the material’s morphology, and transmission electron microscopy (TEM) imaging to probe the material atom by atom.

X-ray experiments at Berkeley Lab’s Advanced Light Source characterized the 2D material’s magnetic parameters under high temperature.

Additional X-ray experiments at SLAC National Accelerator Laboratory’s Stanford Synchrotron Radiation Lightsource verified the electronic and crystal structures of the synthesized 2D magnets. And at Argonne National Laboratory’s Center for Nanoscale Materials, the researchers employed TEM to image the 2D material’s crystal structure and chemical composition.

The researchers found that the graphene-zinc-oxide system becomes weakly magnetic with a 5-6% concentration of cobalt atoms. Increasing the concentration of cobalt atoms to about 12% results in a very strong magnet.

To their surprise, a concentration of cobalt atoms exceeding 15% shifts the 2D magnet into an exotic quantum state of “frustration,” whereby different magnetic states within the 2D system are in competition with each other.

And unlike previous 2D magnets, which lose their magnetism at room temperature or above, the researchers found that the new 2D magnet not only works at room temperature but also at 100 degrees Celsius (212 degrees Fahrenheit).

“Our 2D magnetic system shows a distinct mechanism compared to previous 2D magnets,” said Chen. “And we think this unique mechanism is due to the free electrons in zinc oxide.”

True north: Free electrons keep magnetic atoms on track
When you command your computer to save a file, that information is stored as a series of ones and zeroes in the computer’s magnetic memory, such as the magnetic hard drive or a flash memory.

And like all magnets, magnetic memory devices contain microscopic magnets with two poles – north and south, the orientations of which follow the direction of an external magnetic field. Data is written or encoded when these tiny magnets are flipped to the desired directions.

According to Chen, zinc oxide’s free electrons could act as an intermediary that ensures the  magnetic cobalt atoms in the new 2D device continue pointing in the same direction – and thus stay magnetic – even when the host, in this case the semiconductor zinc oxide, is a nonmagnetic material.

“Free electrons are constituents of electric currents. They move in the same direction to conduct electricity,” Yao added, comparing the movement of free electrons in metals and semiconductors to the flow of water molecules in a stream of water.

The new material – which can be bent into almost any shape without breaking, and is a million times thinner than a sheet of paper – could help advance the application of spin electronics or spintronics, a new technology that uses the orientation of an electron’s spin rather than its charge to encode data. “Our 2D magnet may enable the formation of ultra-compact spintronic devices to engineer the spins of the electrons,” Chen said.

“I believe that the discovery of this new, robust, truly two-dimensional magnet at room temperature is a genuine breakthrough,” said co-author Robert Birgeneau, a faculty senior scientist in Berkeley Lab’s Materials Sciences Division and professor of physics at UC Berkeley who co-led the study.

“Our results are even better than what we expected, which is really exciting. Most of the time in science, experiments can be very challenging,” Yao said. “But when you finally realize something new, it’s always very fulfilling.”

Reference: 

Tunable room-temperature ferromagnetism in Co-doped two-dimensional van der Waals ZnO by Rui Chen, Fuchuan Luo, Yuzi Liu, Yu Song, Yu Dong, Shan Wu, Jinhua Cao, Fuyi Yang, Alpha N’Diaye, Padraic Shafer, Yin Liu, Shuai Lou, Junwei Huang, Xiang Chen, Zixuan Fang, Qingjun Wang, Dafei Jin, Ran Cheng, Hongtao Yuan, Robert J. Birgeneau and Jie Yao, 25 June 2021, Nature Communications. DOI: 10.1038/s41467-021-24247-w

Sunday, 23 May 2021

Researchers see atoms at record resolution


In 2018, Cornell researchers built a high-powered detector that, in combination with an algorithm-driven process called ptychography, set a world record by tripling the resolution of a state-of-the-art electron microscope.

As successful as it was, that approach had a weakness. It only worked with ultrathin samples that were a few atoms thick. Anything thicker would cause the electrons to scatter in ways that could not be disentangled.


Now a team, again led by David Muller, the Samuel B. Eckert Professor of Engineering, has bested its own record by a factor of two with an electron microscope pixel array detector (EMPAD) that incorporates even more sophisticated 3D reconstruction algorithms.

The resolution is so fine-tuned, the only blurring that remains is the thermal jiggling of the atoms themselves.

The group's paper, "Electron Ptychography Achieves Atomic-Resolution Limits Set by Lattice Vibrations," published May 20 in Science. The paper's lead author is postdoctoral researcher Zhen Chen.

"This doesn't just set a new record," Muller said. "It's reached a regime which is effectively going to be an ultimate limit for resolution. We basically can now figure out where the atoms are in a very easy way. This opens up a whole lot of new measurement possibilities of things we've wanted to do for a very long time. It also solves a long-standing problem—undoing the multiple scattering of the beam in the sample, which Hans Bethe laid out in 1928—that has blocked us from doing this in the past."

Ptychography works by scanning overlapping scattering patterns from a material sample and looking for changes in the overlapping region.

"We're chasing speckle patterns that look a lot like those laser-pointer patterns that cats are equally fascinated by," Muller said. "By seeing how the pattern changes, we are able to compute the shape of the object that caused the pattern."

The detector is slightly defocused, blurring the beam, in order to capture the widest range of data possible. This data is then reconstructed via complex algorithms, resulting in an ultraprecise image with picometer (one-trillionth of a meter) precision.

"With these new algorithms, we're now able to correct for all the blurring of our microscope to the point that the largest blurring factor we have left is the fact that the atoms themselves are wobbling, because that's what happens to atoms at finite temperature," Muller said. "When we talk about temperature, what we're actually measuring is the average speed of how much the atoms are jiggling."

The researchers could possibly top their record again by using a material that consists of heavier atoms, which wobble less, or by cooling down the sample. But even at zero temperature, atoms still have quantum fluctuations, so the improvement would not be very large.

This latest form of electron ptychography will enable scientists to locate individual atoms in all three dimensions when they might be otherwise hidden using other imaging methods. Researchers will also be able to find impurity atoms in unusual configurations and image them and their vibrations, one at a time. This could be particularly helpful in imaging semiconductors, catalysts and quantum materials—including those used in quantum computing—as well as for analyzing atoms at the boundaries where materials are joined together.

The imaging method could also be applied to thick biological cells or tissues, or even the synapse connections in the brain—what Muller refers to as "connectomics on demand."

While the method is time-consuming and computationally demanding, it could be made more efficient with more powerful computers in conjunction with machine learning and faster detectors.

"We want to apply this to everything we do," said Muller, who co-directs the Kavli Institute at Cornell for Nanoscale Science and co-chairs the Nanoscale Science and Microsystems Engineering (NEXT Nano) Task Force, part of Cornell's Radical Collaboration initiative. "Until now, we've all been wearing really bad glasses. And now we actually have a really good pair. Why wouldn't you want to take off the old glasses, put on the new ones, and use them all the time?"


Reference:

Electron ptychography achieves atomic-resolution limits set by lattice vibrations. Science, 21 May 2021: DOI: 10.1126/science.abg2533

Monday, 3 May 2021

New Brain-Like Computing Device Simulates Human Learning


Researchers have developed a brain-like computing device that is capable of learning by association.

Similar to how famed physiologist Ivan Pavlov conditioned dogs to associate a bell with food, researchers at Northwestern Engineering and the University of Hong Kong successfully conditioned their circuit to associate light with pressure.

The research was published April 30 in the journal Nature Communications.


The device’s secret lies within its novel organic, electrochemical “synaptic transistors,” which simultaneously process and store information just like the human brain. The researchers demonstrated that the transistor can mimic the short-term and long-term plasticity of synapses in the human brain, building on memories to learn over time.

With its brain-like ability, the novel transistor and circuit could potentially overcome the limitations of traditional computing, including their energy-sapping hardware and limited ability to perform multiple tasks at the same time. The brain-like device also has higher fault tolerance, continuing to operate smoothly even when some components fail.

“Although the modern computer is outstanding, the human brain can easily outperform it in some complex and unstructured tasks, such as pattern recognition, motor control and multisensory integration,” said Northwestern’s Jonathan Rivnay, a senior author of the study. “This is thanks to the plasticity of the synapse, which is the basic building block of the brain’s computational power. These synapses enable the brain to work in a highly parallel, fault tolerant and energy-efficient manner. In our work, we demonstrate an organic, plastic transistor that mimics key functions of a biological synapse.”

Rivnay is an assistant professor of biomedical engineering at Northwestern’s McCormick School of Engineering. He co-led the study with Paddy Chan, an associate professor of mechanical engineering at the University of Hong Kong. Xudong Ji, a postdoctoral researcher in Rivnay’s group, is the paper’s first author.

Problems with conventional computing

Conventional, digital computing systems have separate processing and storage units, causing data-intensive tasks to consume large amounts of energy. Inspired by the combined computing and storage process in the human brain, researchers, in recent years, have sought to develop computers that operate more like the human brain, with arrays of devices that function like a network of neurons.

“The way our current computer systems work is that memory and logic are physically separated,” Ji said. “You perform computation and send that information to a memory unit. Then every time you want to retrieve that information, you have to recall it. If we can bring those two separate functions together, we can save space and save on energy costs.”

Currently, the memory resistor, or “memristor,” is the most well-developed technology that can perform combined processing and memory function, but memristors suffer from energy-costly switching and less biocompatibility. These drawbacks led researchers to the synaptic transistor — especially the organic electrochemical synaptic transistor, which operates with low voltages, continuously tunable memory and high compatibility for biological applications. Still, challenges exist.

“Even high-performing organic electrochemical synaptic transistors require the write operation to be decoupled from the read operation,” Rivnay said. “So if you want to retain memory, you have to disconnect it from the write process, which can further complicate integration into circuits or systems.”

How the synaptic transistor works

To overcome these challenges, the Northwestern Engineering and University of Hong Kong team optimized a conductive, plastic material within the organic, electrochemical transistor that can trap ions. In the brain, a synapse is a structure through which a neuron can transmit signals to another neuron, using small molecules called neurotransmitters. In the synaptic transistor, ions behave similarly to neurotransmitters, sending signals between terminals to form an artificial synapse. By retaining stored data from trapped ions, the transistor remembers previous activities, developing long-term plasticity.

The researchers demonstrated their device’s synaptic behavior by connecting single synaptic transistors into a neuromorphic circuit to simulate associative learning. They integrated pressure and light sensors into the circuit and trained the circuit to associate the two unrelated physical inputs (pressure and light) with one another.

Perhaps the most famous example of associative learning is Pavlov’s dog, which naturally drooled when it encountered food. After conditioning the dog to associate a bell ring with food, the dog also began drooling when it heard the sound of a bell. For the neuromorphic circuit, the researchers activated a voltage by applying pressure with a finger press. To condition the circuit to associate light with pressure, the researchers first applied pulsed light from an LED lightbulb and then immediately applied pressure. In this scenario, the pressure is the food and the light is the bell. The device’s corresponding sensors detected both inputs.

After one training cycle, the circuit made an initial connection between light and pressure. After five training cycles, the circuit significantly associated light with pressure. Light, alone, was able to trigger a signal, or “unconditioned response.”

Future applications

Because the synaptic circuit is made of soft polymers, like a plastic, it can be readily fabricated on flexible sheets and easily integrated into soft, wearable electronics, smart robotics, and implantable devices that directly interface with living tissue and even the brain.

“While our application is a proof of concept, our proposed circuit can be further extended to include more sensory inputs and integrated with other electronics to enable on-site, low-power computation,” Rivnay said. “Because it is compatible with biological environments, the device can directly interface with living tissue, which is critical for next-generation bioelectronics.”


Reference:

Xudong Ji, Bryan D. Paulsen, Gary K. K. Chik, Ruiheng Wu, Yuyang Yin, Paddy K. L. Chan, Jonathan Rivnay. Mimicking associative learning using an ion-trapping non-volatile synaptic organic electrochemical transistor. Nature Communications, 2021; 12 (1) DOI: 10.1038/s41467-021-22680-5

Thursday, 22 April 2021

Scientists capture image of an electron's orbit within an exciton


In a world-first, researchers from the Okinawa Institute of Science and Technology Graduate University (OIST) have captured an image showing the internal orbits, or spatial distribution, of particles in an exciton -- a goal that had eluded scientists for almost a century.


Excitons are excited states of matter found within semiconductors -- a class of materials that are key to many modern technological devices, such as solar cells, LEDs, lasers and smartphones.

"Excitons are really unique and interesting particles; they are electrically neutral which means they behave very differently within materials from other particles like electrons. Their presence can really change the way a material responds to light," said Dr. Michael Man, co-first author and staff scientist in the OIST Femtosecond Spectroscopy Unit. "This work draws us closer to fully understanding the nature of excitons."

Excitons are formed when semiconductors absorb photons of light, which causes negatively charged electrons to jump from a lower energy level to a higher energy level. This leaves behind positively charged empty spaces, called holes, in the lower energy level. The oppositely charged electrons and holes attract and they start to orbit each other, which creates the excitons.

Excitons are crucially important within semiconductors, but so far, scientists have only been able to detect and measure them in limited ways. One issue lies with their fragility -- it takes relatively little energy to break the exciton apart into free electrons and holes. Furthermore, they are fleeting in nature -- in some materials, excitons are extinguished in about a few thousandths of a billionth of a second after they form, when the excited electrons "fall" back into the holes.

"Scientists first discovered excitons around 90 years ago," said Professor Keshav Dani, senior author and head of the Femtosecond Spectroscopy Unit at OIST. "But up until very recently, one could generally access only the optical signatures of excitons -- for example, the light emitted by an exciton when extinguished. Other aspects of their nature, such as their momentum, and how the electron and the hole orbit each other, could only be described theoretically."

However, in December 2020, scientists in the OIST Femtosecond Spectroscopy Unit published a paper in Science describing a revolutionary technique for measuring the momentum of the electrons within the excitons.

Now, reporting on 21st April in Science Advances, the team used the technique to capture the first ever image that shows the distribution of an electron around the hole inside an exciton.

The researchers first generated excitons by sending a laser pulse of light at a two-dimensional semiconductor -- a recently discovered class of materials that are only a few atoms in thickness and harbor more robust excitons.

After the excitons were formed, the team used a laser beam with ultra-high energy photons to break apart the excitons and kick the electrons right out of the material, into the vacuum space within an electron microscope.

The electron microscope measured the angle and energy of the electrons as they flew out of the material. From this information, the scientists were able to determine the initial momentum of the electron when it was bound to a hole within the exciton.

"The technique has some similarities to the collider experiments of high-energy physics, where particles are smashed together with intense amounts of energy, breaking them open. By measuring the trajectories of the smaller internal particles produced in the collision, scientists can start to piece together the internal structure of the original intact particles," said Professor Dani. "Here, we are doing something similar -- we are using extreme ultraviolet light photons to break apart excitons and measuring the trajectories of the electrons to picture what's inside."

"This was no mean feat," continued Professor Dani. "The measurements had to be done with extreme care -- at low temperature and low intensity to avoid heating up the excitons. It took a few days to acquire a single image."

Ultimately, the team succeeded in measuring the exciton's wavefunction, which gives the probability of where the electron is likely to be located around the hole.

"This work is an important advancement in the field," said Dr. Julien Madeo, co-first author and staff scientist in the OIST Femtosecond Spectroscopy Unit. "Being able to visualize the internal orbits of particles as they form larger composite particles could allow us to understand, measure and ultimately control the composite particles in unprecedented ways. This could allow us to create new quantum states of matter and technology based on these concepts."


Reference:

Michael K. L. Man, Julien Madéo, Chakradhar Sahoo, Kaichen Xie, Marshall Campbell, Vivek Pareek, Arka Karmakar, E Laine Wong, Abdullah Al-Mahboob, Nicholas S. Chan, David R. Bacon, Xing Zhu, Mohamed M. M. Abdelrasoul, Xiaoqin Li, Tony F. Heinz, Felipe H. da Jornada, Ting Cao, Keshav M. Dani. Experimental measurement of the intrinsic excitonic wave function. Science Advances, 2021; 7 (17): eabg0192 DOI: 10.1126/sciadv.abg0192