Category: Physics

In a study that confirms its promise as the next-generation semiconductor material, UC Santa Barbara researchers have directly visualized the photocarrier transport properties of cubic boron arsenide single crystals.

“We were able to visualize how the charge moves in our sample,” said Bolin Liao, an assistant professor of mechanical engineering in the College of Engineering. Using the only scanning ultrafast electron microscopy (SUEM) setup in operation at a U.S. university, he and his team were able to make “movies” of the generation and transport processes of a photoexcited charge in this relatively little-studied III-V semiconductor material, which has recently been recognized as having extraordinary electrical and thermal properties. In the process, they found another beneficial property that adds to the material’s potential as the next great semiconductor.

Their research, conducted in collaboration with physics professor Zhifeng Ren’s group at the University of Houston, who specialize in fabricating high-quality single crystals of cubic boron arsenide, appears in the journal Matter.

‘Ringing the bell’

Boron arsenide is being eyed as a potential candidate to replace silicon, the computer world’s staple semiconductor material, due to its promising performance. For one thing, with an improved charge mobility over silicon, it easily conducts current (electrons and their positively charged counterpart, “holes”). However, unlike silicon, it also conducts heat with ease.

“This material actually has 10 times higher thermal conductivity than silicon,” Liao said. This heat conducting—and releasing—ability is particularly important as electronic components become smaller and more densely packed, and pooled heat threatens the devices’ performance, he explained.

“As your cellphones become more powerful, you want to be able to dissipate the heat, otherwise you have efficiency and safety issues,” he said. “Thermal management has been a challenge for a lot of microelectronic devices.”

What gives rise to the high thermal conductivity of this material, it turns out, can also lead to interesting transport properties of photocarriers, which are the charges excited by light, for example, in a solar cell. If experimentally verified, this would indicate that cubic boron arsenide can also be a promising material for photovoltaic and light detection applications. Direct measurement of photocarrier transport in cubic boron arsenide, however, has been challenging due to the small size of available high-quality samples.

The research team’s study combines two feats: The crystal growth skills of the University of Houston team, and the imaging prowess at UC Santa Barbara. Combining the abilities of the scanning electron microscope and femtosecond ultrafast lasers, the UCSB team built what is essentially an extremely fast, exceptionally high-resolution camera.

“Electron microscopes have very good spatial resolution—they can resolve single atoms with their sub-nanometer spatial resolution—but they’re typically very slow,” Liao said, noting this makes them excellent for capturing static images.

“With our technique, we couple this very high spatial resolution with an ultrafast laser, which acts as a very fast shutter, for extremely high time resolution,” Liao continued. “We’re talking about one picosecond—a millionth of a millionth of a second. So we can make movies of these microscopic energy and charge transport processes.” Originally invented at Caltech, the method was further developed and improved at UCSB from scratch and now is the only operational SUEM setup at an American university.

“What happens is that we have one pulse of this laser that excites the sample,” explained graduate student researcher Usama Choudhry, the lead author of the Matter paper. “You can think of it like ringing a bell; it’s a loud noise that slowly diminishes over time.” As they “ring the bell,” he explained, a second laser pulse is focused onto a photocathode (“electron gun”) to generate a short electron pulse to image the sample. They then scan the electron pulse over time to gain a full picture of the ring. “Just by taking a lot of these scans, you can get a movie of how the electrons and holes get excited and eventually go back to normal,” he said.

Among the things they observed while exciting their sample and watching the electrons return to their original state is how long the “hot” electrons persist.

“We found, surprisingly, the ‘hot’ electrons excited by light in this material can persist for much longer times than in conventional semiconductors,” Liao said. These “hot” carriers were seen to persist for more that 200 picoseconds, a property that is related to the same feature that is responsible for the material’s high thermal conductivity. This ability to host “hot” electrons for significantly longer amounts of time has important implications.

“For example, when you excite the electrons in a typical solar cell with light, not every electron has the same amount of energy,” Choudhry explained. “The high-energy electrons have a very short lifetime, and the low-energy electrons have a very long lifetime.” When it comes to harvesting the energy from a typical solar cell, he continued, only the low-energy electrons are efficiently being collected; the high-energy ones tend to lose their energy rapidly as heat. Because of the persistence of the high-energy carriers, if this material was used as a solar cell, more energy could efficiently be harvested from it.

With boron arsenide beating silicon in three relevant areas—charge mobility, thermal conductivity and hot photocarrier transport time—it has the potential to become the electronics world’s next state-of-the-art material. However, it still faces significant hurdles—fabrication of high-quality crystals in large quantities—before it can compete with silicon, enormous amounts of which can be manufactured relatively cheaply and with high quality. But Liao doesn’t see too much of a problem.

“Silicon is now routinely available because of years of investment; people started developing silicon around the 1930s and ’40s,” he said. “I think once people recognize the potential of this material, there will be more effort put into finding ways to grow and use it. UCSB is actually uniquely positioned for this challenge with strong expertise in semiconductor development.”

An international group of researchers from Technical University of Denmark (DTU) and Chalmers University of Technology in Gothenburg, Sweden have achieved dizzying data transmission speeds and are the first in the world to transmit more than 1 petabit per second (Pbit/s) using only a single laser and a single optical chip.

1 petabit corresponds to 1 million gigabits.

In the experiment, the researchers succeeded in transmitting 1.8 Pbit/s, which corresponds to twice the total global Internet traffic. And only carried by the light from one optical source. The light source is a custom-designed optical chip, which can use the light from a single infrared laser to create a rainbow spectrum of many colors, i.e., many frequencies. Thus, the one frequency (color) of a single laser can be multiplied into hundreds of frequencies (colors) in a single chip.

All the colors are fixed at a specific frequency distance from each other—just like the teeth on a comb—which is why it is called a frequency comb. Each color (or frequency) can then be isolated and used to imprint data. The frequencies can then be reassembled and sent over an optical fiber, thus transmitting data. Even a huge volume of data, as the researchers have discovered.

One single laser can replace thousands

The experimental demonstration showed that a single chip could easily carry 1.8 Pbit/s, which—with contemporary state-of-the-art commercial equipment—would otherwise require more than 1,000 lasers.

Victor Torres Company, professor at Chalmers University of Technology, is head of the research group that has developed and manufactured the chip.

“What is special about this chip is that it produces a frequency comb with ideal characteristics for fiber-optical communications—it has high optical power and covers a broad bandwidth within the spectral region that is interesting for advanced optical communications,” says Victor Torres Company.

Interestingly enough, the chip was not optimized for this particular application.

“In fact, some of the characteristic parameters were achieved by coincidence and not by design,” says Victor Torres Company. “However, with efforts in my team, we are now capable to reverse engineer the process and achieve with high reproducibility microcombs for target applications in telecommunications.”

Enormous potential for scaling

In addition, the researchers created a computational model to examine theoretically the fundamental potential for data transmission with a single chip identical to the one used in the experiment. The calculations showed enormous potential for scaling up the solution.

Professor Leif Katsuo Oxenløwe, Head of the Center of Excellence for Silicon Photonics for Optical Communications (SPOC) at DTU, says:

“Our calculations show that—with the single chip made by Chalmers University of Technology, and a single laser—we will be able to transmit up to 100 Pbit/s. The reason for this is that our solution is scalable—both in terms of creating many frequencies and in terms of splitting the frequency comb into many spatial copies and then optically amplifying them, and using them as parallel sources with which we can transmit data. Although the comb copies must be amplified, we do not lose the qualities of the comb, which we utilize for spectrally efficient data transmission.”

This is how you pack light with data

Packing light with data is known as modulation. Here, the wave properties of light are utilized such as:

• Amplitude (the height/strength of the waves)
• Phase (the “rhythm” of the waves, where it is possible to make a shift so that a wave arrives either a little earlier or a little later than expected)
• Polarization (the directions in which the waves spread).

By changing these properties, you create signals. The signals can be translated into either ones or zeros—and thus utilized as data signals.

Reduces Internet power consumption

The researchers’ solution bodes well for the future power consumption of the Internet.

“In other words, our solution provides a potential for replacing hundreds of thousands of the lasers located at Internet hubs and data centers, all of which guzzle power and generate heat. We have an opportunity to contribute to achieving an Internet that leaves a smaller climate footprint,” says Leif Katsuo Oxenløwe.

Even though the researchers have broken the petabit barrier for a single laser source and a single chip in their demonstration, there is still some development work ahead before the solution can be implemented in our current communication systems, according to Leif Katsuo Oxenløwe.

“All over the world, work is being done to integrate the laser source in the optical chip, and we’re working on that as well. The more components we can integrate in the chip, the more efficient the whole transmitter will be, i.e., laser, comb-creating chip, data modulators, and any amplifier elements. It will be an extremely efficient optical transmitter of data signals,” says Leif Katsuo Oxenløwe.

The research is published in Nature Photonics.

Words like “tough” or “rugged” are rarely associated with a quantum inertial sensor. The remarkable scientific instrument can measure motion a thousand times more accurately than the devices that help navigate today’s missiles, aircraft and drones. But its delicate, table-sized array of components that includes a complex laser and vacuum system has largely kept the technology grounded and confined to the controlled settings of a lab.

Jongmin Lee wants to change that.

The atomic physicist is part of a team at Sandia that envisions quantum inertial sensors as revolutionary, onboard navigational aids. If the team can reengineer the sensor into a compact, rugged device, the technology could safely guide vehicles where GPS signals are jammed or lost.

In a major milestone toward realizing their vision, the team has successfully built a cold-atom interferometer, a core component of quantum sensors, designed to be much smaller and tougher than typical lab setups. The team describes their prototype in the academic journal Nature Communications, showing how to integrate several normally separated components into a single monolithic structure. In doing so, they reduced the key components of a system that existed on a large optical table down to a sturdy package roughly the size of a shoebox.

“Very high sensitivity has been demonstrated in the lab, but the practical matters are, for real-world application, that people need to shrink down the size, weight and power, and then overcome various issues in a dynamic environment,” Jongmin said.

The paper also describes a roadmap for further miniaturizing the system using technologies under development.

The prototype, funded by Sandia’s Laboratory Directed Research and Development program, demonstrates significant strides toward moving advanced navigation tech out of the lab and into vehicles on the ground, underground, in the air and even in space.

Ultrasensitive measurements drive navigational power

As a jet does a barrel roll through the sky, current onboard navigation tech can measure the aircraft’s tilts and turns and accelerations to calculate its position without GPS, for a time. Small measurement errors gradually push a vehicle off course unless it periodically syncs with the satellites, Jongmin said.

Quantum sensing would operate in the same way, but the much better accuracy would mean onboard navigation wouldn’t need to cross-check its calculations as often, reducing reliance on satellite systems.

Roger Ding, a postdoctoral researcher who worked on the project, said, “In principle, there are no manufacturing variations and calibrations,” compared to conventional sensors that can change over time and need to be recalibrated.

Aaron Ison, the lead engineer on the project, said to prepare the atom interferometer for a dynamic environment, he and his team used materials proven in extreme environments. Additionally, parts that are normally separate and freestanding were integrated together and fixed in place or were built with manual lockout mechanisms.

“A monolithic structure having as few bolted interfaces as possible was key to creating a more rugged atom interferometer structure,” Aaron said.

Furthermore, the team used industry-standard calculations called finite element analysis to predict that any deformation of the system in conventional environments would fall within required allowances. Sandia has not conducted mechanical stress tests or field tests on the new design, so further research is needed to measure the device’s strength.

“The overall small, compact design naturally leads towards a stiffer more robust structure,” Aaron said.

Photonics light the way to a more miniaturized system

Most modern atom interferometry experiments use a system of lasers mounted to a large optical table for stability reasons, Roger said. Sandia’s device is comparatively compact, but the team has already come up with further design improvements to make the quantum sensors much smaller using integrated photonic technologies.

“There are tens to hundreds of elements that can be placed on a chip smaller than a penny,” said Peter Schwindt, the principal investigator on the project and an expert in quantum sensing.

Photonic devices, such as a laser or optical fiber, use light to perform useful work and integrated devices include many different elements. Photonics are used widely in telecommunications, and ongoing research is making them smaller and more versatile.

With further improvements, Peter thinks the space an interferometer needs could be as little as a few liters. His dream is to make one the size of a soda can.

In their paper, the Sandia team outlines a future design in which most of their laser setup is replaced by a single photonic integrated circuit, about eight millimeters on each side. Integrating the optical components into a circuit would not only make an atom interferometer smaller, it would also make it more rugged by fixing the components in place.

While the team can’t do this yet, many of the photonic technologies they need are currently in development at Sandia.

“This is a viable path to highly miniaturized systems,” Roger said.

Meanwhile, Jongmin said integrated photonic circuits would likely lower costs and improve scalability for future manufacturing.

“Sandia has shown an ambitious vision for the future of quantum sensing in navigation,” Jongmin said.

Quark–gluon plasma is an extremely hot and dense state of matter in which the elementary constituents—quarks and gluons—are not confined inside composite particles called hadrons, as they are in the protons and neutrons that make up the nuclei of atoms. Thought to have existed in the early universe, this special phase of matter can be recreated at the Large Hadron Collider (LHC) in collisions between lead nuclei.

A new analysis from the international ALICE collaboration at the LHC investigates how different bound states of a charm quark and its antimatter counterpart, also produced in these collisions, are affected by quark–gluon plasma. The results open new avenues for studying the strong interaction—one of the four fundamental forces of nature—in the extreme temperature and density conditions of quark–gluon plasma.

Bound states of a charm quark and a charm antiquark, known as charmonia or hidden-charm particles, are held together by the strong interaction and are excellent probes of quark–gluon plasma. In the plasma, their production is suppressed due to “screening” by the large number of quarks and gluons present in this form of matter.

The screening, and thus the suppression, increases with the temperature of the plasma and is expected to affect different charmonia to varying degrees. For example, the production of the ψ(2S) state, which is ten times more weakly bound and 20% more massive than the J/ψ state, is expected to be more suppressed than that of the J/ψ state.

This hierarchical suppression is not the only fate of charmonia in quark–gluon plasma. The large number of charm quarks and antiquarks in the plasma—up to about a hundred in head-on collisions—also gives rise to a mechanism, called recombination, that forms new charmonia and counters the suppression to a certain extent.

This process is expected to depend on the type and momentum of the charmonia, with the more weakly bound charmonia possibly being produced through recombination later in the evolution of the plasma, and charmonia with the lowest (transverse) momentum having the highest recombination rate.

Previous studies, which used data from CERN’s Super Proton Synchrotron and subsequently from the LHC, have shown that the production of the ψ(2S) state is indeed more suppressed than that of the J/ψ. ALICE has also previously provided evidence of the recombination mechanism in J/ψ production. But, until now, no studies of ψ(2S) production at low particle momentum had been precise enough to provide conclusive results in this momentum regime, preventing a complete picture of ψ(2S) production from being obtained.

The ALICE collaboration has now reported the first measurements of ψ(2S) production down to zero transverse momentum, based on lead–lead collision data from the LHC collected in 2015 and 2018.

The measurements show that, regardless of particle momentum, the ψ(2S) state is suppressed about two times more than the J/ψ. This is the first time that a clear hierarchy in suppression has been observed for the total production of charmonia at the LHC. A similar observation was previously reported by the LHC collaborations for bound states of a bottom quark and its antiquark.

When further studied as a function of particle momentum, the ψ(2S) suppression is seen to be reduced towards lower momentum. This feature, which was previously observed by ALICE for the J/ψ state, is a signature of the recombination process.

Future higher-precision studies of these and other charmonia using data from LHC Run 3, which started in July, may lead to a definitive understanding of the modification of hidden-charm particles and, as a result, of the strong interaction that holds them together, in the extreme environment of quark–gluon plasma.

Spectrometers are widely used throughout industry and research to detect and analyze light. Spectrometers measure the spectrum of light—its strength at different wavelengths, like the colors in a rainbow—and are an essential tool for identifying and analyzing specimens and materials. Integrated on-chip spectrometers would be of great benefit to a variety of technologies, including quality inspection platforms, security sensors, biomedical analyzers, health care systems, environmental monitoring tools, and space telescopes.

An international research team led by researchers at Aalto University has developed high-sensitivity spectrometers with high wavelength accuracy, high spectral resolution, and broad operation bandwidth, using only a single microchip-sized detector. The research behind this new ultra-miniaturized spectrometer was published today in the journal Science.

“Our single-detector spectrometer is an all-in-one device. We designed this optoelectronic-lab-on-a-chip with artificial intelligence replacing conventional hardware, such as optical and mechanical components. Therefore, our computational spectrometer does not require separate bulky components or array designs to disperse and filter light. It can achieve a high resolution comparable to benchtop systems but in a much smaller package,” says Postdoctoral Researcher Hoon Hahn Yoon.

“With our spectrometer, we can measure light intensity at each wavelength beyond the visible spectrum using a device at our fingertips. The device is entirely electrically controllable, so it has enormous potential for scalability and integration. Integrating it directly into portable devices such as smartphones and drones could advance our daily lives. Imagine that the next generation of our smartphone cameras could be fitted with hyperspectral cameras that outperform color cameras,” he adds.

Shrinking computational spectrometers is essential for their use in chips and implantable applications. Professor Zhipei Sun, the head of the research team, says, “Conventional spectrometers are bulky because they need optical and mechanical components, so their on-chip applications are limited. There is an emerging demand in this field to improve the performance and usability of spectrometers. From this point of view, miniaturized spectrometers are very important to offer high performance and new functions in all fields of science and industry.”

Professor Pertti Hakonen adds that “Finland and Aalto have invested in photonics research in recent years. For example, there has been great support from the Academy of Finland’s Center of Excellence on quantum technology, Flagship on Photonics Research and Innovation, InstituteQ, and the Otanano Infrastructure. Our new spectrometer is a clear demonstration of the success of these collaborative efforts. I believe that with further improvements in resolution and efficiency, these spectrometers could provide new tools for quantum information processing.”