Research from Washington University in St. Louis suggests that organelles in the eukaryotic cell grow in random bursts from a limiting pool of building blocks. Credit: Shutterstock
Eukaryotic cells—the ones that make up most life as we know it, including all animals, plants and fungi—are highly structured objects.
These cells assemble and maintain their own smaller, internal bits: the membrane-bound organelles like nuclei, which store genetic information, or mitochondria, which produce chemical energy. But much remains to be learned about how they organize themselves into these spatial compartments.
Physicists at Washington University in St. Louis conducted new experiments that show that eukaryotic cells can robustly control average fluctuations in organelle size. By demonstrating that organelle sizes obey a universal scaling relationship that the scientists predict theoretically, their new framework suggests that organelles grow in random bursts from a limiting pool of building blocks.
The study was published Jan. 6 in Physical Review Letters.
“In our work, we suggest that the steps by which organelles are grown—far from being an orderly ‘brick-by-brick’ assembly—occur in stochastic bursts,” said Shankar Mukherji, assistant professor of physics in Arts & Sciences.
“Such bursts fundamentally limit the precision with which organelle size is controlled but also maintain noise in organelle size within a narrow window,” Mukherji said. “Burstlike growth provides a general biophysical mechanism by which cells can maintain, on average, reliable yet plastic organelle sizes.”
Organelles must be flexible enough to allow cells to grow or shrink them as environments demand. Still, the size of organelles must be maintained within certain limits. Biologists have previously identified certain molecular factors that regulate organelle sizes, but this study provides new insights into the quantitative principles underlying organelle size control.
While this study used budding yeast as a model organism, the team is excited to explore how these assembly mechanisms are utilized across different species and cell types. Mukherji said that they plan to examine what these patterns of robustness can teach us about how to harness organelle assembly for bioengineering applications and how to spot defects in organelle biogenesis in the context of disease.
“The pattern of organelle size robustness is shared between budding yeast and human iPS cells,” Mukherji said. “The underlying molecular mechanisms producing these bursts are yet to be fully elucidated and are likely to be organelle-specific and potentially species-specific.”
More information: Kiandokht Panjtan Amiri et al, Robustness and Universality in Organelle Size Control, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.018401
ITER, under construction at Saint-Paul-les-Durance in southern France, aims at emulating the Sun, which fuses particles together to release energy.
An international project in nuclear fusion may face “years” of delays, its boss has told AFP, weeks after scientists in the United States announced a breakthrough in their own quest for the coveted goal.
The International Thermonuclear Experimental Reactor (ITER) project seeks to prove the feasibility of fusion as a large-scale and carbon-free source of energy.
Installed at a site in southern France, the decades-old initiative has a long history of technical challenges and cost overruns.
Fusion entails forcing together the nuclei of light atomic elements in a super-heated plasma, held by powerful magnetic forces in a doughnut-shaped chamber called a tokamak.
The idea is that fusing the particles together from isotopes of hydrogen—which can be extracted from seawater—will create a safer and almost inexhaustible form of energy compared with splitting atoms from uranium or plutonium.
ITER’S previously-stated goal was to create the plasma by 2025.
But that deadline will have to be postponed, Pietro Barabaschi—who in September became the project’s director-general—told AFP during a visit to the facility.
The date “wasn’t realistic in the first place,” even before two major problems surfaced, Barabaschi said.
One problem, he said, was wrong sizes for the joints of blocks to be welded together for the installation’s 19-by-11-metre (62-by-36-feet) chamber.
The second was traces of corrosion in a thermal shield designed to protect the outside world from the enormous heat created during nuclear fusion.
Fixing the problems “is not a question of weeks, but months, even years,” Barabaschi said.
A new timetable is to be worked out by the end of this year, he said, including some modification to contain the expected cost overrun, and to meet the French nuclear safety agency’s security requirements.
Barabaschi said he hoped ITER would be able to make up for the delays as it prepares to enter the full phase, currently scheduled for 2035.
On December 13, US researchers working separately from ITER announced an important technical breakthrough.
Scientists at the Lawrence Livermore National Laboratory (LLNL) in California said they had used the world’s largest laser to create, for the first time, a fusion reaction generating more energy than it took to produce.
“Some competition is healthy in any environment,” Barabaschi said about the success.
“If tomorrow somebody found another breakthrough that would make my work redundant, I would be very happy,” he added.
ITER was set in motion after a 1985 summit between US president Ronald Reagan and Soviet leader Mikhail Gorbachev.
Its seven partners are China, the European Union, India, Japan, South Korea, Russia and the United States.
Russia still participates in ITER despite the start of the Ukraine conflict.
In November it dispatched one of six giant magnets needed for the top part of the tokamak.
Plasmoid behavior of pure hydrogen and hydrogen mixed with 5 % neon. In this experiment, a new Thomson Scattering (TS) diagnostic system operating at (an unprecedented rate of) 20 kHz was used to (i) measure the density of the plasmoid at the moment it passed through the observation region, and (ii) identify its position, which verified the theoretical predictions. Credit: National Institute for Fusion Science
At ITER—the world’s largest experimental fusion reactor, currently under construction in France through international cooperation—the abrupt termination of magnetic confinement of a high temperature plasma through a so-called “disruption” poses a major open issue. As a countermeasure, disruption mitigation techniques, which allow to forcibly cool the plasma when signs of plasma instabilities are detected, are a subject of intensive research worldwide.
Now, a team of Japanese researchers from National Institutes for Quantum Science and Technology (QST) and National Institute for Fusion Science (NIFS) of National Institute of National Sciences (NINS) found that by adding approximately 5% neon to a hydrogen ice pellet, it is possible to cool the plasma more deeply below its surface and hence more effectively than when pure hydrogen ice pellets are injected.
Using theoretical models and experimental measurements with advanced diagnostics at Large Helical Device owned by NIFS, the researchers clarified the dynamics of the dense plasmoid that forms around the ice pellet and identified the physical mechanisms responsible for the successful enhancement of the performance of the forced cooling system, which is indispensable for carrying out the experiments at ITER. These results will contribute to the establishment of plasma control technologies for future fusion reactors. The team’s report was made available online in Physical Review Letters.
The construction of the world’s largest experimental fusion reactor, ITER, is underway in France through international cooperation. At ITER, experiments will be conducted to generate 500 MW fusion energy by maintaining the “burning state” of the hydrogen isotope plasma at more than 100 million degrees. One of the major obstacles to the success of those experiments is a phenomenon called “disruption” during which the magnetic field configuration used to confine the plasma collapses due to magnetohydrodynamic instabilities.
Disruption causes the high-temperature plasma to flow into the inner surface of the containing vessel, resulting in structural damage that, in turn, may cause delays in the experimental schedule and higher cost. Although the machine and the operating conditions of ITER have been carefully designed to avoid disruption, uncertainties remain and for a number of experiments so that a dedicated machine protection strategy is required as a safeguard.
A promising solution to this problem is a technique called “disruption mitigation,” which forcibly cools the plasma at the stage where first signs of instabilities that may cause a disruption are detected, thereby preventing damage to plasma-facing material components. As a baseline strategy, researchers are developing a method using ice pellets of hydrogen frozen at temperatures below 10 Kelvin and injecting it into a high-temperature plasma.
The injected ice melts from the surface and evaporates and ionizes owing to heating by the ambient high-temperature plasma, forming a layer of low-temperature, high-density plasma (hereafter referred to as a “plasmoid”) around the ice. Such a low-temperature, high-density plasmoid mixes with the main plasma, whose temperature is reduced in the process. However, in recent experiments, it has become clear that when pure hydrogen ice is used, the plasmoid is ejected before it can mix with the target plasma, making it ineffective for cooling the high-temperature plasma deeper below the surface.
This ejection was attributed to the high pressure of the plasmoid. Qualitatively, a plasma confined in a donut-shaped magnetic field tends to expand outward in proportion to the pressure. Plasmoids, which are formed by the melting and the ionization of hydrogen ice, are cold but very dense. Because temperature equilibration is much faster than density equilibration, the plasmoid pressure rises above that of the hot target plasma. The consequence is that the plasmoid becomes polarized and experiences drift motion across the magnetic field, so that it propagates outward before being able to fully mix with the hot target plasma.
A solution to this problem was proposed from theoretical analysis: model calculations predicted that by mixing a small amount of neon into hydrogen, the pressure of the plasmoid could be reduced. Neon freezes at a temperature of approximately 20 Kelvin and produces strong line radiation in the plasmoid. Therefore, if the neon is mixed with hydrogen ice before injection, part of the heating energy can be emitted as photon energy.
To demonstrate such a beneficial effect of using a hydrogen-neon mixture, a series of experiments was conducted in the Large Helical Device (LHD) located in Toki, Japan. For many years, the LHD has operated a device called the “solid hydrogen pellet injector” with high reliability, which injects ice pellets with a diameter of approximately 3 mm at the speed of 1100 m/s. Due to the system’s high reliability, it is possible to inject hydrogen ice into the plasma with a temporal precision of 1 ms, which allows measurement of the plasma temperature and density just after the injected ice melts.
Recently, the world’s highest time resolution for Thomson Scattering (TS) of 20 kHz was achieved in the LHD system using new laser technology. Using this system, the research team has captured the evolution of plasmoids. They found that, as predicted by theoretical calculations, plasmoid ejection was suppressed when hydrogen ice was doped with approximately 5 % neon, in stark contrast to the case where pure hydrogen ice was injected. In addition, the experiments confirmed that the neon plays a useful role in the effective cooling of the plasma.
The results of this study show for the first time that the injection of hydrogen ice pellets doped with a small amount of neon into a high-temperature plasma is useful to effectively cool the deep core region of the plasma by suppressing plasmoid ejection. This effect of neon doping is not only interesting as a new experimental phenomenon, but also supports the development of the baseline strategy of disruption mitigation in ITER. The design review of the ITER disruption mitigation system is scheduled for 2023, and the present results will help improve the performance of the system.
More information: A. Matsuyama et al, Enhanced Material Assimilation in a Toroidal Plasma Using Mixed H2+Ne Pellet Injection and Implications to ITER, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.255001
Future versions of the new photonic circuits will feature low-loss waveguides—the channels through which the single photons travel–some 3 meters long but tightly coiled to fit on a chip. The long waveguides will allow researchers to more precisely choose the time intervals (Δt) when photons exit different channels to rendezvous at a particular location. Credit: NIST
The ability to transmit and manipulate, with minimal loss, the smallest unit of light—the photon—plays a pivotal role in optical communications as well as designs for quantum computers that would use light rather than electric charges to store and carry information.
Now, researchers at the National Institute of Standards and Technology (NIST) and their colleagues have connected, on a single microchip, quantum dots—artificial atoms that generate individual photons rapidly and on-demand when illuminated by a laser—with miniature circuits that can guide the light without significant loss of intensity.
To create the ultra-low-loss circuits, the researchers fabricated silicon-nitride waveguides—the channels through which the photons traveled—and buried them in silicon dioxide. The channels were wide but shallow, a geometry that reduced the likelihood that photons would scatter out of the waveguides. Encapsulating the waveguides in silicon dioxide also helped to reduce scattering.
The scientists reported that their prototype circuits have a loss of intensity equal to only one percent of similar circuits—also using quantum dots—that were fabricated by other teams.
Ultimately, devices that incorporate this new chip technology could take advantage of the strange properties of quantum mechanics to perform complex computations that classical (non-quantum) circuits may not be capable of doing.
Illustration shows some of the steps in creating the new ultra-low-loss photonic circuit on a chip. A microprobe lifts a gallium arsenide device containing a quantum dot—artificial atoms that generate single photons—from one chip. Then the probe places the quantum-dot device atop a low-loss silver-nitride waveguide built on another chip. Credit: S. Kelley/NIST
For instance, according to the laws of quantum mechanics, a single photon has a probability of residing in two different places, such as two different waveguides, at the same time. Those probabilities can be used to store information; an individual photon can act as a quantum bit, or qubit, which carries much more information than the binary bit of a classical computer, which is limited to a value of 0 or 1.
To perform operations necessary to solve computational problems, these photon qubits—all of which travel at the same speed and are indistinguishable from each other—must simultaneously arrive at specific processing nodes in the circuit. That poses a challenge because photons originating from different locations—and traveling along different waveguides—across the circuit may lie at significantly different distances from processing points. To ensure simultaneous arrival, photons emitted closer to the designated destination must delay their journey, giving those that lie in more distant waveguides a head start.
The circuit devised by NIST researchers including Ashish Chanana and Marcelo Davanco, along with an international team of colleagues, allows for significant time delays because it employs waveguides of various lengths that can store photons for relatively long periods of time. For instance, the researchers calculate that a 3-meter-long waveguide (tightly coiled so its diameter on a chip is only a few millimeters) would have a 50 percent probability of transmitting a photon with a time delay of 20 nanoseconds (billionths of a second). By comparison, previous devices, developed by other teams and operating under similar conditions, were limited to inducing time delays only one one-hundredth as long.
The longer delay times achieved with the new circuit are also important for operations in which photons from one or more quantum dots need to arrive at a specific location at equally spaced time intervals. In addition, the low-loss quantum-dot circuit could dramatically increase the number of single photons available for carrying quantum information on a chip, enabling larger, speedier, and more reliable computational and information-processing systems
The scientists, who include researchers from the University of California, Santa Barbara (UCSB), the Massachusetts Institute of Technology (MIT), the Korea Institute of Science and Technology and the University of São Paulo in Brazil, reported their findings December 11 in Nature Communications.
Laser light shining on the quantum dots triggers them to produce a series of single photons that travel through the silicon nitride waveguide. Credit: S. Kelley/NIST
The hybrid circuit consists of two components, each initially built on a separate chip. One, a gallium arsenide semiconductor device designed and fabricated at NIST, hosts the quantum dots and directly funnels the single photons they generate into a second device—a low-loss silicon nitride waveguide developed at UCSB.
To marry the two components, researchers at MIT first used the fine metal tip of a pick-and-place microprobe, acting like a miniature crowbar, to pry the gallium arsenide device from the chip built at NIST. They then placed it atop the silicon nitride circuit on the other chip.
The researchers face several challenges before the hybrid circuit can be routinely employed in a photonic device. At present, only about 6 percent of the individual photons generated by the quantum dots can be funneled into the circuit. However, simulations suggest that if the team changes the angle at which the photons are funneled, in tandem with improvements in the positioning and orientation of the quantum dots, the rate could rise above 80 percent.
Another issue is that the quantum dots do not always emit single photons at exactly the same wavelength, a requirement for creating the indistinguishable photons necessary for the quantum computational operations. The team is exploring several strategies, including applying a constant electric field to the dots, that may alleviate that problem.
More information: Ashish Chanana et al, Ultra-low loss quantum photonic circuits integrated with single quantum emitters, Nature Communications (2022). DOI: 10.1038/s41467-022-35332-z
A display that projects holographic images that change when in contact with water has been developed. This new technology increases the possibility of commercialization as it can infinitely imprint holographic images.
A POSTECH research team led by Professor Junsuk Rho (Department of Mechanical Engineering and Department of Chemical Engineering) and Ph.D. candidates Byoungsu Ko, Younghwan Yang, Jaekyung Kim, and Dr. Trevon Badloe has developed a technology for a humidity-responsive display that changes in brightness and color depending on the degree of humidity.
The team first successfully realized holographic images with tunable brightness using polyvinyl alcohol (PVA). This material is so flexible that it is usually used for liquid glue or slime and one of its distinctive properties is that it swells as humidity increases. A holographic image that is clear at a low degree of humidity gradually becomes unclear as humidity increases.
The team additionally developed a display on which structural colors can be discretionally tuned. A blue image at low humidity turns red as humidity increases. If humidity is fine-tuned, all RGB colors may be expressed, in addition to the two colors.
This study also draws attention to the team’s success in using the single-step nanoimprinting technique to print the images. It is notable that images can be vividly expressed even on a flexible substrate. In addition, as a single pixel of this display—which reaches 700 nm (1nm = 1/1 billion m)—is smaller than those of currently commercialized displays, it is anticipated to become the core technology for nanostructured displays.
Humidity responsive nanoscale pixel with color tunability. Credit: POSTECH
The findings from the study have received significant attention as the newly developed technology may be employed to security labels for authentication against counterfeits, including food items like whisky, currency bills, or passports.
The team has been working with Korea Minting and Security Printing Corporation (KOMSCO) to apply the optics-based future security technology to actual products. Subsequently, this technology is expected to be applied to the development of a hydrogel macromolecule-based display that responds to external stimuli such as heat, acidity (pH), and fine-dust pollution.
These findings on the brightness and color tunability of holographic images were published in Nature Communications and Advanced Science, respectively.
More information: Byoungsu Ko et al, Tunable metasurfaces via the humidity responsive swelling of single-step imprinted polyvinyl alcohol nanostructures, Nature Communications (2022). DOI: 10.1038/s41467-022-32987-6
Byoungsu Ko et al, Humidity‐Responsive RGB‐Pixels via Swelling of 3D Nanoimprinted Polyvinyl Alcohol, Advanced Science (2022). DOI: 10.1002/advs.202204469
Researchers used high-speed cameras equipped with microscope lenses alongside laser-induced cavitation to document the tiny bubbles, which typically boast the size of a single millimeter and last just one-tenth of a millisecond. Credit: Texas A&M Engineering
Researchers embarked on a multidisciplinary project to determine how cavitation bubbles within micro- or nano-structures could mitigate surface erosion and enhance the efficiency in microfluidic mixing devices, often used to quickly and effectively mix multiple samples.
Potential applications of the research findings include the creation of more efficient and resilient pumping machinery and implementation in portable, high-precision biological tests currently reserved only for laboratory settings. The research was recently published in Scientific Reports.
The project was led by Dr. Guillermo Aguilar, James and Ada Forsyth Professor and department head in the J. Mike Walker ’66 Department of Mechanical Engineering at Texas A&M University.
Cavitation—the rapid formation and collapse of vapor bubbles in a liquid—is a widely studied field. This project sought to better understand the basic science of cavitation dynamics while also identifying potential applications.
“Although cavitation has been studied extensively, the interaction of cavitation bubbles and jets with micro- or nano-structures and shockwaves are still an active area of research,” Aguilar said. “This study may also help us better understand and further develop new technologies such as erosion mitigation through surface micropatterning and development of efficient microfluidic mixing devices.”
The methods enabled the team to trap air pockets in a microstructure surface and demonstrate how these air pockets could serve to greatly diminish the erosion typically caused by cavitation phenomena mechanisms. Credit: Texas A&M Engineering
Researchers used high-speed cameras equipped with microscope lenses alongside laser-induced cavitation to document the tiny bubbles, which typically boast the size of a single millimeter and last just one-tenth of a millisecond. Additionally, the project used multiple different lasers to serve various purposes throughout the research process, including a femtosecond laser to create the micropatterning in the target surface, a nano-second laser to induce cavitation and a continuous wave laser to perform particle tracking.
The methods enabled the team to trap air pockets in a microstructure surface and demonstrate how these air pockets could serve to greatly diminish the erosion typically caused by cavitation phenomena mechanisms. At the same time, the collapse of cavitation bubbles near micro- and nano-patterned surfaces enhanced the mixing of the contiguous fluid.
“We believe that this work has the potential to be the starting point for developing applications in microfluidics and erosion mitigation,” Aguilar said. “In the future, we could have commercial microfluidic devices that use this technique for in-situ, high-precision biological tests, which are currently restricted to laboratory environments. We also believe that this technique can be implemented to allow pumping machinery to work more efficiently and last longer, which would translate into cost reduction.”
One of the major challenges for this project came in the preparation. While assembling the team, a multitude of expertise in various disciplines was needed to effectively execute the project—a task for which Aguilar said mechanical engineers are well equipped.
“This can be described as a multidisciplinary project as not only fluids but also optics, photonics and material science,” Aguilar said. “As mechanical engineers, we have the broad knowledge base to tackle complex problems like this. Thus, this kind of multidisciplinary research relies heavily on teamwork.”
More information: Vicente Robles et al, The effect of scalable PDMS gas-entrapping microstructures on the dynamics of a single cavitation bubble, Scientific Reports (2022). DOI: 10.1038/s41598-022-24746-w
Far-field properties of light from isotropic light sources near the topological junction metasurface. (A) Far-field radiation pattern |Ey|2 from an array of isotropic emitters that are evenly placed inside the device region from x = −5 to 5 μm at z = 0. (B and C) Normalized localization envelope function f(x) and its spatial Fourier transform F(kx) for the emission angle θ in free space, respectively. (D) Position–wave vector uncertainty relation of the leaky JR state according to bandgap size g. Credit: Science Advances, doi: 10.1126/sciadv.add8349
Nanophotonic light emitters are compact and versatile devices with wide-ranging applications in applied physics. In a new report now published on Science Advances, Ki Young Lee and a research team in physics and engineering in China and the UK, proposed to develop a topological beam emitter structure of a submicron-footprint size and high efficiency, with adaptable beam shaping capacity.
The proposed device facilitated a highly desirable and efficient microlight emitter to detect a variety of applications including displays, solid-state light detection, optical interconnects and telecommunications.
Photonic topological phenomena
Topological interface states have remarkably high robustness against environmental disturbances with unique physical properties. Many researchers in mathematics and photonics have extensively investigated the photonic topological phenomena due to their promise across telecommunications, data processing and sensor applications.
In this study, Lee and colleagues explored novel far-field optical properties associated with non-Hermitian topological photonics. They showed how a topological junction metasurface of two guided-mode resonance gratings can serve as efficient submicron scale light emitters with high quantum efficiency and adaptable beam-shaping capacity.
During the experiments, the team used a junction containing two distinct guided mode resonant gratings directly adjacent to each other in the absence of an aperture. In such structures, the leaky Jackiw-Rebbi (JR) state at the junction; which corresponds to a historically important relativistic model—emitted a narrow beam of light. The process was driven by cavity-quantum electrodynamics coupling and electromagnetic funneling effects. The team explored a fundamental theory of topological beam emission and conducted rigorous numerical analyses during the study.
Fundamental properties of the leaky JR state in a topological junction metasurface. (A) Schematic of a topological junction consisting of two different thin-film subwavelength gratings. (B) Angle-dependent reflection spectra for the left unit cell in the topological phase (left), the right unit cell in the trivial phase (right), and their junction (middle). (C) Electric field amplitude Ey of the leaky JR state at wavelength λJR = 633 nm. We use the finite element method (Comsol Multiphysics) for this calculation. Credit: Science Advances, doi: 10.1126/sciadv.add8349
Leakage radiation from a Jackiw-Rebbi (JR) state
Lee et al. explored the leaky JR state localized at a photonic topological junction metasurface, where the structure maintained a high-index film. Under specific conditions, the first-order diffraction from the JR state led to beam leakage radiation towards the surrounding background, enabling characteristic features of leakage radiation to be gathered during the study.
Based on the narrow-beam emission associated with the leaky JR state, the team investigated the emission properties of light sources near the topological junction. They used the finite element method to calculate the radiation pattern, which showed a narrow beam emitted in the optical far-field. The team next disclosed the possibility of designing an appropriate structure, where two grating regions would have identical Dirac mass to achieve ideal symmetry of the emitted beam.
During these experiments, the narrow beam emission from isotropic light sources followed the exact diffraction properties of radiation leakage from the JR state. The team also considered external sources for the proposed beaming effect, which they accomplished by introducing modifications to the experimental setup, including a reduced index contrast and vertically coupled multilayer waveguides, among other modifications.
Electromagnetic funneling and Purcell enhancement of internal sources. (A) Optical power-flow (time-average Poynting vector 〈S〉t; red arrows) distribution for a topological junction with bandgap size g = 40 nm in reference to the near-field intensity distribution (gray-level density). (B and C) Optical power flow excited by a single isotropic source 1 and 2 μm away from the junction (xP = 1 and 2 μm), respectively. (D) Source-position (xP)–dependent far-field intensity distribution on an observation plane 3.5 μm above the grating surface as a function of xP. (E) Purcell factor distribution in comparison with the near-field intensity distribution associated with the JR state. Credit: Science Advances, doi: 10.1126/sciadv.add8349Flat-top beam generation by Dirac mass control. (A) Dirac mass m(x) distribution for flat-top beam generation. It has three plateaus at m′ = −0.634, 0, and +0.635 μm−1, and associated JR state intensity profiles and emitted beam profiles are plotted together for references. (B) Electric field intensity |Ey|2 pattern from the structure design based on the Dirac mass distribution in (A). The device structure in this simulation has three grating regions of different fill factors at F = 0.264, 0.46, and 0.7, corresponding to the three Dirac mass plateaus. Credit: Science Advances, doi: 10.1126/sciadv.add8349
Adaptable beam shaping
The concept of beam shaping is important for many general applications of light sources. The described topological beaming effect provides a possibility of regulating the beam shape directly from the source. The scientists described the Dirac mass distribution required to generate the expected beam profile.
For instance, to generate a flat top beam, a zero Dirac mass region can be extended—across the desired width, and around the junction of the device. The outcomes of guided mode resonance Dirac mass regulation can thereby efficiently facilitate beam shaping applications.
Outlook
In this way, Ki Young Lee and colleagues proposed a topological junction metasurface for efficient beam emission. They simulated the characteristic field distributions of a leaky Jackiw-Rabbi state at the junction to achieve efficient light beaming from internal emitters by integrating cavity-quantum electrodynamics coupling with electromagnetic funneling effects.
The proposed architecture is significant to the creation of efficient micro-light emitters for strong localization, high quantum efficiency and adaptable beam shaping capacity. These properties are significant for numerous applications, including the development of display pixels, laser machining and telecommunications applications. The proposed devices are also capable of functioning as efficient optical detectors due to their scope of acting as time-reversed emitters, in principle. The scientists propose further optimizations of the study outcomes to develop new optical effects and concomitant device applications to surpass any existing technical limits.
More information: Ki Young Lee et al, Topological beaming of light, Science Advances (2022). DOI: 10.1126/sciadv.add8349
Alexander Cerjan et al, Experimental realization of a Weyl exceptional ring, Nature Photonics (2019). DOI: 10.1038/s41566-019-0453-z
Photograph of a soap film hanging on a frame constituted of a thermocouple probe. The radius of the soap film in this picture is R = 6 mm . Credit: Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.268001
A team of researchers at Université Paris-Saclay, CNRS, has discovered that the film that makes up ordinary soap bubbles is cooler than the surrounding air. In their paper published in the journal Physical Review Letters, the group describes experiments they conducted with soap bubbles.
Bubbles exist in a wide variety of environments, from beer glasses to clothes and dish washers to crests on waves. They even exist in tiny environments, like in the space between human teeth. A lot of research has been done with bubbles, much of it focused on controlling them during industrial processes. But there is still more to be learned, apparently, as the researchers in Orsay recently discovered something new about them—their films are cooler than the air surrounding them.
As with many discoveries in science, the researchers did not set out to make such a discovery; they were studying stability of bubbles, and while doing so, happened to use equipment that allowed them to measure the temperature of the bubble film, finding that it was cooler than the ambient air for all the bubbles they tested.
In their work, the researchers created bubbles using ordinary dish soap, water and glycerol. After discovering a temperature difference, the team refocused their efforts to learn more. They tried changing the temperature of the air, the humidity level and also the proportions of the ingredients used to make the bubbles. They found that they were able to make bubbles that were up to 8 degrees Celsius cooler than the air around them. They also found that changing the amount of glycerol impacted the temperature of the resulting bubbles—more of it yielded higher temperatures.
The researchers suggest the cooler films could be the result of evaporation as the bubbles form. They noted also that as the bubbles persisted, their films slowly grew warmer, eventually matching the ambient air temperature. They suggest the large temperature differences they found with some bubbles might have an impact on bubble stability, and conclude that more work is required to find out why the films are cooler and if it might be a useful attribute.
Researchers at the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS) have conducted a study on the separation route of germanium-68 and successfully prepared a germanium-68/gallium-68 generator.
Published in Applied Radiation and Isotopes, the study lays a good foundation for producing the generator-based medical isotope gallium-68 in China.
Positron emission computed tomography (PET) is an advanced imaging technique used in the clinical field of nuclear medicine. The medical isotope fluorine-18 is widely used in PET for detection of cancer. Usually, fluorine-18 is produced by an onsite cyclotron in hospitals.
As the application of gallium-68 in nuclear medicine is growing quickly on a global scale, obtaining gallium-68 directly from the germanium-68/gallium-68 generator has received much attention. More importantly, the generator could be used for a long time because of the long half-life period (about 271 days) of the mother nuclide germanium-68.
The quantity and purity of germanium-68 is of great importance in producing gallium-68. In this study, the researchers from IMP reported an efficient method to produce and separate germanium-68. They prepared the Ga-Ni alloy target with large area by electroplating. And the content of gallium-68 in the Ga-Ni layer could attain 78% by weight. The Ga-Ni target was bombarded using a proton beam for 10 hours. After “cooling” for a long period, they applied an automated system equipped with double-column chromatography to separate germanium-68 with recovery of about 70%.
Then, the researchers used separated germanium-68 to assemble the SnO2 germanium-68/gallium-68 generator. They collected gallium-68 in about 70% yields with high radioactive concentration. The eluted gallium-68 has a radionuclidic purity of > 99.9% and a radiochemical purity of > 99%, which satisfies the requirement of European Pharmacopeia.
The techniques presented in this study are expected to be applied in production of germanium-68/gallium-68 generators, facilitating the development of gallium-68 radiopharmaceuticals in China.
More information: Jieru Wang et al, Production of medical isotope 68Ge based on a novel chromatography separation technique and assembling of 68Ge/68Ga generator, Applied Radiation and Isotopes (2022). DOI: 10.1016/j.apradiso.2022.110599
Illustration of an active element in the new integrated THz photonics platform, with the laser waveguide embedded in a low-loss BCB polymer and covered with an extended top metallization. The inset shows an electron microscope image (SEM) of a fabricated device that features improved dispersion, RF and thermal properties and can be co-integrated with various passive elements on the same photonic chip. Credit: Senica, U., Forrer, A., Olariu, T. et al.
Integrated photonics extensively uses on-chip optical elements such as sources, splitters, modulators, and high-confinement waveguides embedded in a planar platform to efficiently process and route optical signals. There is a growing interest in integrated Mid-IR and THz photonics for telecommunications and sensing. In the THz frequency range, a prominent candidate for source integration is the THz quantum cascade laser.
Recent advances in the high-temperature operation of these devices, combined with their frequency agility and the possibility to operate as frequency combs and high-speed detectors, make them highly appealing as crucial building blocks for THz photonics. Some of the previous approaches to THz integration include hybrid plasmonic waveguides, monolithically integrated THz transceivers, coupled cavity devices, and, most recently, devices integrated on silicon.
In a new paper published in Light: Science & Applications, a team of scientists led by doctoral student Urban Senica and Professor Giacomo Scalari from ETH Zurich developed a novel integrated photonic platform.
In more complex photonic systems, some crucial features for laser integration are reducing electrical consumption and the efficient coupling to low-loss passive waveguides. The researchers proposed a new platform for integrated THz photonics that allows signal propagation with passive elements and coherent source integration for broadband sensing and telecommunications.
They leveraged the presence of a common metallic ground plane, which was chosen to demonstrate the integration of several active and passive THz photonic components onto the same semiconductor platform. This approach allows for efficient signal processing at THz and R.F. frequencies.
The researchers focused on broadband and frequency comb devices. They highlighted improved performance in several crucial figures of merit, such as dispersion, R.F., and thermal properties. Their model demonstrated the co-integration of active and passive elements on the same photonic chip. The basic building block is a high-performance planarized double-metal waveguide with an extended top metallization. A similar waveguide has already proven to be very efficient for THz and microwave applications.
In the context of THz frequency combs, the researchers showed that control over transverse modes is essential to obtain a regular and flat-top comb spectrum, mainly with a reduction of the ridge lateral dimensions. However, the width cannot be arbitrarily small since the waveguides are connected by wire bonding directly on the top metallic cladding. It inherently limits the effective ridge width to the dimensions of the bonding wire patch. It makes devices with ridges of 50 µm or below challenging to contact and prone to failure. Bonding directly on the active region can introduce defects, increasing the waveguide losses and non-intentionally selecting specific modes, potentially compromising the long-term performance of the device and its spectral characteristics.
These issues are solved within the team’s planarized platform. Placing the bonding wires on top of the extended top metallization over the passive, BCB-covered area prevents the formation of any defects or local hotspots on top of the active region. It enables the fabrication of very narrow waveguides well below the bonding wire size.
The narrow waveguide width can be employed as an efficient selection mechanism for the fundamental transversal lasing mode and is also beneficial for heat dissipation and high-temperature continuous wave (C.W.) operation. Moreover, the extended contact facilitates a lateral heat flow and eases the heat extraction as in a radiator scheme. It results in an improved measured maximum operating temperature of the planarized devices.
More information: Urban Senica et al, Planarized THz quantum cascade lasers for broadband coherent photonics, Light: Science & Applications (2022). DOI: 10.1038/s41377-022-01058-2
