Category: Physics

Femtosecond pulsed lasers—which emit light in ultrafast bursts lasting a millionth of a billionth of a second—are powerful tools used in a range of applications from medicine and manufacturing, to sensing and precision measurements of space and time. Today, these lasers are typically expensive table-top systems, which limits their use in applications that have size and power consumption restrictions.

An on-chip femtosecond pulse source would unlock new applications in quantum and optical computing, astronomy, optical communications and beyond. However, it’s been a challenge to integrate tunable and highly efficient pulsed lasers onto chips.

Now, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a high-performance, on-chip femtosecond pulse source using a tool that seems straight out of science fiction: a time lens.

The research is published in Nature.

“Pulsed lasers that produce high-intensity, short pulses consisting of many colors of light have remained large,” said Marko Lončar, the Tiantsai Lin Professor of Electrical Engineering at SEAS and senior author of the study.

“To make these sources more practical, we decided to shrink a well-known approach, used to realize conventional—and large—femtosecond sources, leveraging a state of the art integrated photonics platform that we have developed. Importantly, our chips are made using microfabrication techniques like those used to make computer chips, which ensures not only reduced cost and size, but also improved performance and reliability of our femtosecond sources.”

Traditional lenses, like contact lenses or those found in magnifying glasses and microscopes, bend rays of light coming from different directions by altering their phase so that they hit the same location in space—the focal point.

Time lenses, on the other hand, “bend” light beams in similar ways—but they alter the phase of light beams in time rather than space. In this way, different colors of light, which travel at different speeds, are re-timed so that they each hit the focal plane at the same time.

Imagine a car race, in which each color of light is a different car. First, the time lens staggers the leave time of each car, then sets their speed so they arrive at the finish line at the same time.

To generate femtosecond pulses, the team’s device uses a series of optical waveguides, couplers, modulators and optical grating on the lithium niobate platform pioneered by Lončar’s lab.

The team starts by passing a continuous-wave, single-color laser beam through an amplitude modulator that controls the amount of light going through the time-lens, a function similar to an aperture in a conventional lens. The light then propagates through the “bendy” part of the lens, a phase modulator in this case, where a frequency comb of different colors is generated. Going back to the car analogy, the phase modulator creates and then releases the cars of different colors at different starting times.

Then the final component of the laser comes in—a fishbone grating along the waveguide. The grating changes the speed of the different colors of light to bring them all in line with each other, neck and neck in the race, so that they hit the finish line (or focal plane) at the same time

Because the device controls how fast different wavelengths travel and when they hit the focal plane, it effectively transforms the continuous, single color laser beam into a broadband, high-intensity pulse source that can produce ultra-fast, 520 femtosecond bursts.

The device is highly tunable, integrated onto a 2cm by 4mm chip and, because of lithium niobate’s electro-optical properties, requires significantly reduced power than table-top products.

“We’ve shown that integrated photonics offers simultaneous improvements in energy consumption and size,” said Mengjie Yu, a former postdoctoral fellow at SEAS and first author of the study.

“There’s no tradeoff here; you save energy at the same time you save space. You just get better performance as the device gets smaller and more integrated. Just imagine—in the future we can carry around femtosecond pulse lasers in our pockets to sense how fresh fruit is or track our well-being in real time, or in our cars to do distance measurement.”

Next, the team aims to explore some of the applications for both the laser itself and the time lens technology, including in lensing systems like telescopes as well as in ultrafast signal processing and quantum networking.

More information: Mengjie Yu et al, Integrated femtosecond pulse generator on thin-film lithium niobate, Nature (2022). DOI: 10.1038/s41586-022-05345-1

Journal information: Nature 

Provided by Harvard John A. Paulson School of Engineering and Applied Sciences 

A group of scientists from Aalto University, IQM Quantum Computers, and VTT Technical Research Center have discovered a new superconducting qubit, the unimon, to increase the accuracy of quantum computations. The team has achieved the first quantum logic gates with unimons at 99.9% fidelity—a major milestone on the quest to build commercially useful quantum computers. This research was just published in the journal Nature Communications.

Of all the different approaches to build useful quantum computers, superconducting qubits are in the lead. However, the qubit designs and techniques currently used do not yet provide high enough performance for practical applications. In this noisy intermediate-scale quantum (NISQ) era, the complexity of the implementable quantum computations is mostly limited by errors in single- and two-qubit quantum gates. The quantum computations need to become more accurate to be useful.

“Our aim is to build quantum computers which deliver an advantage in solving real-world problems. Our announcement today is an important milestone for IQM, and a significant achievement to build better superconducting quantum computers,” said Professor Mikko Möttönen, joint Professor of Quantum Technology at Aalto University and VTT, and also a Co-Founder and Chief Scientist at IQM Quantum Computers, who was leading the research.

Today, Aalto, IQM and VTT have introduced a new superconducting-qubit type, the unimon, which unites in a single circuit the desired properties of increased anharmonicity, full insensitivity to dc charge noise, reduced sensitivity to magnetic noise, and a simple structure consisting only of a single Josephson junction in a resonator. The team achieved fidelities from 99.8% to 99.9% for 13-nanoseconds-long single-qubit gates on three different unimon qubits.

“Because of the higher anharmonicity, or non-linearity, than in transmons, we can operate the unimons faster, leading to fewer errors per operation,” said Eric Hyyppä who is working on his Ph.D. at IQM.

To experimentally demonstrate the unimon, the scientists designed and fabricated chips, each of which consisted of three unimon qubits. They used niobium as the superconducting material apart from the Josephson junctions, in which the superconducting leads were fabricated using aluminum.

The team measured the unimon qubit to have a relatively high anharmonicity while requiring only a single Josephson junction without any superinductors, and bearing protection against noise. The geometric inductance of the unimon has the potential for higher predictability and yield than the junction-array-based superinductors in conventional fluxonium or quarton qubits.

“Unimons are so simple and yet have many advantages over transmons. The fact that the very first unimon ever made worked this well, gives plenty of room for optimization and major breakthroughs. As next steps, we should optimize the design for even higher noise protection and demonstrate two-qubit gates,” added Prof. Möttönen.

“We aim for further improvements in the design, materials, and gate time of the unimon to break the 99.99% fidelity target for useful quantum advantage with noisy systems and efficient quantum error correction. This is a very exciting day for quantum computing,” concluded Prof. Möttönen.

More information: Eric Hyyppä et al, Unimon qubit, Nature Communications (2022). DOI: 10.1038/s41467-022-34614-w

Journal information: Nature Communications 

Provided by Aalto University 

Teams of astrophysicists worldwide are trying to observe different possible types of dark matter (DM), hypothetical matter in the universe that does not emit, absorb or reflect light and would thus be very difficult to detect. Fermionic DM, however, which would be made of fermions, has so far been primarily explored theoretically.

The PandaX Collaboration, a large consortium of researchers in China involved in the PandaX-4T experiment, has recently carried out a study aimed at extending the sensitive mass window for experiments aimed at directly detecting fermionic DM from above GeV to MeV or even keV ranges.

The team recently published two papers in Physical Review Letters outlining the results of the two searches for the absorption of fermionic DM using data gathered as part of the Panda X-4T experiment, a large-scale research effort aimed at detecting DM using a dual-phase time projection chamber (TPC) in China.

“With a massive DM converted to a massless neutrino, the DM mass is absorbed and converted to the kinetic energy of the neutrino and most importantly the recoiled electron or nuclear targets,” Prof. Shao-Feng Ge, one of the researchers who carried out the study, told Phys.org.

“With efficient mass conversion to energy, according to the Einstein relation E = mc², even keV (MeV) DM can deposit a large enough recoil energy in the recoil electron (nuclei).”

The idea of observing light fermionic DM by detecting the recoil energy resulting from the absorption of its mass first emerged a few years ago and has since been explored by different groups of theoretical physicists. While these studies offered valuable theoretical predictions, these predictions had so far never been tested experimentally.

“Past phenomenological papers established the basic features of this unique channel for fermionic DM r searches,” Prof. Ge explained. “The PandaX collaboration worked hard to first search for the predicted signals using real data.”

Theoretical studies predict that in nuclear absorption reactions, the mass of DM is converted into kinetic energy that charges the outgoing neutrino and nucleus. This energy, known as “nuclear recoil energy,” should be approximately proportional to the square of the DM mass, resulting in a unique mono-energetic spectrum. In their first study, the PandaX-4T collaboration tried to detect the energy resulting from the absorption of fermionic DM by nuclei.

“This mono-energetic spectrum is dramatically different from the traditional elastic scattering spectrum and has not been searched dedicatedly before in the DM direct detection experiment,” Dr. Yi Tao, another researcher involved in the study, told Phys.org. “As part of this PandaX-4T search, we performed dedicated studies on the nuclear recoil energy reconstruction and then compared simulation and neutron calibration data.”

The researchers found that there was a good consistency between the data collected by their dual-phase time projection chamber (TPC) and their detector response theoretical model. More specifically, the signal region they scanned corresponded to nuclear recoil energy up to 100 keV, which covers the DM mass parameter from 30 MeV/c² to 125 MeV/c².

In a similar way to nuclear absorption processes, electronic absorption processes are also predicted to be sensitive to light DM, but in a different mass range. In fact, electronic absorption processes imply the conversion of a hypothetical fermionic DM particle’s static mass into the kinetic energy of electrons, creating a free electron.

Theoretically, fermionic DM should thus induce electronic recoiling signals in liquid xenon detectors that could be experimentally detected. In their second study, the PandaX-4T collaboration searched for this other potential trace of fermionic DM.

Electrons are much lighter than nuclei and thus easier to be ejected during absorption processes. Therefore, electronic absorption searches can be sensitive to the sub-MeV mass range.

“In addition, unlike nuclear recoiling signals where quite a bit of energy is quenched into heat and cannot be detected in a liquid xenon detector, most of the electronic recoiling energy is detectable,” Dr. Dan Zhang, another researcher who carried out the study, told Phys.org.

“For more detailed theoretical models, different hypothetical six-dimensional operators in the four-fermion process (fermionic DM + electron -> electron + neutrino) have been studied with an effective field theory approach. It turns out electronic absorption signals will be similar regardless of operators in the direct detection experiments, but the interpretations on the couplings are quite different, and the comparison with other cosmological and astrophysical observations are also different.”

The search for sub-MeV fermionic DM absorbed by electrons carried out by Dr. Zhang and the rest of the PandaX-4T collaboration did not lead to the detection of any significant signals over the expected background. Nonetheless, the team was able to set the strongest limits on the axial-vector and vector interactions for DMs with a mass of several tens keV/c², which surpass the existing astronomy and cosmology constraints for such light fermion DMs.

“About two years ago, XENON1T reported a low-energy excess, which could be interpreted as an electronic absorption of 60 keV/c² fermionic DM according to phenomenological studies,” Dr. Zhang said. “This possibility is now challenged by our data.”

The recent searches performed by the PandaX-4T collaboration highlight the potential of nuclear absorption and electronic absorption processes as channels to search for light mass DM. In the future, they could inspire other astrophysics collaborations worldwide to perform similar searches.

“Once any excess is observed, the energy of the excess would indicate the mass of DM,” Prof. Ning Zhou, another researcher involved in the study, told Phys.org. “For this channel, we obtained model-independent constraints on the sub-GeV DM-nucleon scattering cross section and probe down to the 10^-50 cm² region for 35 MeV/c² DM mass, for the first time. In addition, we study a UV-complete model with Z’ mediator, which brings together the cosmology constraint, the collider constraint, and our limit from direct detection.”

So far, the Panda X-4T collaboration successfully set new limits for experiments aimed at directly detecting fermionic DM. As their experiment is ongoing and is thus still collecting data, the team will soon be conducting additional searches for elusive, light DM.

“The data we reported is equivalent to exposing a 600-kg liquid xenon target for one year to the illumination of this hypothetical DM,” Prof. Jianglai Liu, Spokesperson for the PandaX Collaboration, told Phys.org. “When PandaX-4T concludes in 2025, we anticipate a cumulative exposure of 10 times greater. We also expect to obtain a more precise understanding of our detector to the nuclear recoil and electronic recoil signals via thorough calibrations and we are excited to see how the story unfolds in the future.”

More information: Linhui Gu et al, First Search for the Absorption of Fermionic Dark Matter with the PandaX-4T Experiment, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.161803

Dan Zhang et al, Search for Light Fermionic Dark Matter Absorption on Electrons in PandaX-4T, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.161804

Jeff A. Dror et al, Directly Detecting Signals from Absorption of Fermionic Dark Matter, Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.124.181301

Jeff A. Dror et al, Absorption of sub-MeV fermionic dark matter by electron targets, Physical Review D (2021). DOI: 10.1103/PhysRevD.103.035001

Jeff A. Dror et al, Erratum: Absorption of sub-MeV fermionic dark matter by electron targets [Phys. Rev. D 103 , 035001 (2021)], Physical Review D (2022). DOI: 10.1103/PhysRevD.105.119903

Jeff A. Dror et al, Absorption of fermionic dark matter by nuclear targets, Journal of High Energy Physics (2020). DOI: 10.1007/JHEP02(2020)134

Shao-Feng Ge et al, Revisiting the fermionic dark matter absorption on electron target, Journal of High Energy Physics (2022). DOI: 10.1007/JHEP05(2022)191

Journal information: Physical Review Letters  , Physical Review D 

Spin glasses are alloys formed by noble metals in which a small amount of iron is dissolved. Although they do not exist in nature and have few applications, they have nevertheless been the focus of interest of statistical physicists for some 50 years. Studies of spin glasses were crucial for Giorgio Parisi’s 2021 Nobel Prize in Physics.

The scientific interest of spin glasses lies in the fact that they are an example of a complex system whose elements interact with each other in a way that is sometimes cooperative and sometimes adversarial. The mathematics developed to understand their behavior can be applied to problems arising in a variety of disciplines, from ecology to machine learning, not to mention economics.

Spin glasses are magnetic systems, that is, systems in which individual elements, the spins, behave like small magnets. Their peculiarity is the co-presence of ferromagnetic-type bonds, which tend to align the spins, with antiferromagnetic-type bonds, which tend to orient them in opposite directions.

This causes lower-energy configurations to exhibit residual frustration: it is not possible to find an arrangement of spins that satisfies all bonds. The frustrated configurations are also clustered in a huge (exponential!) number of possible equilibria. This is in stark contrast to what happens in purely ferromagnetic systems, where at low temperature only two states are admissible (spin aligned “up” or spin aligned “down”).

To make an analogy with an ecosystem, having a high number of equilibria indicates a resilient ecosystem, able to cope, for example, with the disappearance of a species, through a limited number of rearrangements. A low equilibrium number describes a fragile system, which requires numerous and complicated rearrangements to return to equilibrium and can, therefore, be seriously damaged, if not destroyed, by relatively small perturbations.

This phenomenology has been well elucidated and mathematically described in systems living in infinite dimension, so-called mean-field systems, the solution to which was provided by Parisi in 1979 and then better understood in subsequent years with the help of Marc Mézard (now a full professor at Bocconi) and Michelangelo Virasoro.

“One of the most debated issues,” as Carlo Lucibello, Assistant Professor in the Department of Computing Sciences and co-author, with Parisi and others, of a paper just published in Physical Review Letters explains, “is to what extent mean-field phenomenology applies in low dimensionality.”

For we know that in dimension 1, that is, on one spin chain, the system is always in a paramagnetic phase, so by lowering the temperature there are no transitions either to a spin glass phase with its many equilibria or to a simple ferromagnetic phase.

“There is a so-called critical upper dimension,” Lucibello says, “above which the mean-field theory applies, allowing us to predict the exponents governing the transition. At the moment, however, no one can say for sure what this dimension is (5, 6, or a non-integer number?) and what happens below it.”

The paper just published by Lucibello and co-authors introduces a new mathematical technique for analyzing finite-dimensional systems. The new theory predicts a critical higher dimension of 8, so we can reasonably conclude that spin glasses in our three-dimensional world are unlikely to be described by a mean-field theory and that there is still a lot of work to do in this branch of theoretical physics.

More information: Maria Chiara Angelini et al, Unexpected Upper Critical Dimension for Spin Glass Models in a Field Predicted by the Loop Expansion around the Bethe Solution at Zero Temperature, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.128.075702

Journal information: Physical Review Letters 

There’s an awkward, irksome problem with our understanding of nature’s laws which physicists have been trying to explain for decades. It’s about electromagnetism, the law of how atoms and light interact, which explains everything from why you don’t fall through the floor to why the sky is blue.

Our theory of electromagnetism is arguably the best physical theory humans have ever made—but it has no answer for why electromagnetism is as strong as it is. Only experiments can tell you electromagnetism’s strength, which is measured by a number called α (aka alpha, or the fine-structure constant).

The American physicist Richard Feynman, who helped come up with the theory, called this “one of the greatest damn mysteries of physics” and urged physicists to “put this number up on their wall and worry about it.”

In research just published in Science, we decided to test whether α is the same in different places within our galaxy by studying stars that are almost identical twins of our sun. If α is different in different places, it might help us find the ultimate theory, not just of electromagnetism, but of all nature’s laws together—the “theory of everything.”

We want to break our favorite theory

Physicists really want one thing: a situation where our current understanding of physics breaks down. New physics. A signal that cannot be explained by current theories. A sign-post for the theory of everything.

To find it, they might wait deep underground in a gold mine for particles of dark matter to collide with a special crystal. Or they might carefully tend the world’s best atomic clocks for years to see if they tell slightly different time. Or smash protons together at (nearly) the speed of light in the 27-km ring of the Large Hadron Collider.

The trouble is, it’s hard to know where to look. Our current theories can’t guide us.

Of course, we look in laboratories on Earth, where it’s easiest to search thoroughly and most precisely. But that’s a bit like the drunk only searching for his lost keys under a lamp-post when, actually, he might have lost them on the other side of the road, somewhere in a dark corner.

Stars are terrible, but sometimes terribly similar

We decided to look beyond Earth, beyond our solar system, to see if stars which are nearly identical twins of our sun produce the same rainbow of colors. Atoms in the atmospheres of stars absorb some of the light struggling outwards from the nuclear furnaces in their cores.

Only certain colors are absorbed, leaving dark lines in the rainbow. Those absorbed colors are determined by α—so measuring the dark lines very carefully also lets us measure α.

The problem is, the atmospheres of stars are moving—boiling, spinning, looping, burping—and this shifts the lines. The shifts spoil any comparison with the same lines in laboratories on Earth, and hence any chance of measuring α. Stars, it seems, are terrible places to test electromagnetism.

But we wondered: if you find stars that are very similar—twins of each other—maybe their dark, absorbed colors are similar as well. So instead of comparing stars to laboratories on Earth, we compared twins of our sun to each other.

A new test with solar twins

Our team of student, postdoctoral and senior researchers, at Swinburne University of Technology and the University of New South Wales, measured the spacing between pairs of absorption lines in our sun and 16 “solar twins”—stars almost indistinguishable from our sun.

The rainbows from these stars were observed on the 3.6-meter European Southern Observatory (ESO) telescope in Chile. While not the largest telescope in the world, the light it collects is fed into probably the best-controlled, best-understood spectrograph: HARPS. This separates the light into its colors, revealing the detailed pattern of dark lines.

HARPS spends much of its time observing sun-like stars to search for planets. Handily, this provided a treasure trove of exactly the data we needed.

From these exquisite spectra, we have shown that α was the same in the 17 solar twins to an astonishing precision: just 50 parts per billion. That’s like comparing your height to the circumference of Earth. It’s the most precise astronomical test of α ever performed.

Unfortunately, our new measurements didn’t break our favorite theory. But the stars we’ve studied are all relatively nearby, only up to 160 light years away.

What’s next?

We’ve recently identified new solar twins much further away, about half way to the center of our Milky Way galaxy.

In this region, there should be a much higher concentration of dark matter—an elusive substance astronomers believe lurks throughout the galaxy and beyond. Like α, we know precious little about dark matter, and some theoretical physicists suggest the inner parts of our galaxy might be just the dark corner we should search for connections between these two “damn mysteries of physics.”

If we can observe these much more distant suns with the largest optical telescopes, maybe we’ll find the keys to the universe.

More information: Michael T. Murphy et al, A limit on variations in the fine-structure constant from spectra of nearby Sun-like stars, Science (2022). DOI: 10.1126/science.abi9232

Journal information: Science 

This article is republished from The Conversation under a Creative Commons license. Read the original article.

When 2D layered materials are made thinner (i.e., at the atomic scale), their properties can dramatically change, sometimes resulting in the emergence of entirely new features and in the loss of others. While new or emerging properties can be very advantageous for the development of new technologies, retaining some of the material’s original properties is often equally important.

Researchers at Tsinghua University, the Chinese Academy of Sciences and the Frontier Science Center for Quantum Information have recently been able to realize tailored Ising superconductivity in a sample of intercalated bulk niobium diselenide (NbSe₂), a characteristic of bulk NbSe₂ that is typically compromised in atomically thin layers. The methods they used, outlined in a paper published in Nature Physics, could pave the way towards the fabrication of 2D thin-layered superconducting materials.

“Atomically thin 2D materials exhibit interesting properties that are often distinct from their bulk materials, which consist of hundreds and thousands of layers,” Shuyun Zhou, one of the researchers who carried out the study, told Phys.org. “However, atomically thin films/flakes are difficult to fabricate, and the emerging new properties are sometimes achieved by sacrificing some other important properties.”

Zhou and his colleagues have been trying to identify experimental methods to achieve novel properties comparable to atomically thin samples without losing any vital material properties for some years now. In their recent study, they specifically evaluated the effectiveness of electrochemical intercalation, a valuable strategy for tuning the electronic properties of layered solid materials.

“The bulk material is immersed in the ionic liquid, which consists of cations and anions,” Zhou explained. “Such ionic liquids have been widely used for injecting electrons into few-layer samples, while the ions remain in the liquid. We have found out that by applying a larger negative voltage, the large-size organic cations can be driven into the van der Waals gap (the empty space between the active layers, NbSe₂ layers in this case), forming hybrid materials.”

In their experiments, Zhou and his colleagues found that intercalation is an effective strategy for controlling both the dimensionality and carrier concentration of their NbSe₂ layered sample. Using this strategy, they were able to attain a tailored Ising superconductivity that exceeded both that observed in bulk NbSe₂ crystals and monolayer NbSe₂ samples, but in an intercalated bulk NbSe₂ sample.

Essentially, intercalation strategies consist in the immersion of a bulk material in an ionic liquid and the subsequent application of electrical voltage. This process prompts an increase in the spacing between a bulk layered material’s active layers, reducing interactions between them.

“Although the intercalated NbSe₂ material still consists of many layers, its properties behave quite similarly to those of monolayer NbSe₂ samples,” Zhou said. “Specifically, the intercalated material’s superconductivity can survive under a large in-plane magnetic field, but the superconducting transition temperature is higher than monolayer NbSe₂. In addition, the cations can transfer charges to the active layers and act as protecting layers, making the hybrid material stable in the air.”

While Zhou and his colleagues specifically used their intercalation-based strategy to broaden the properties of a layered 2D NbSe₂ sample, the exact same strategy could also be applied to a wide range of layered materials to achieve properties comparable to those of monolayer versions of these materials, or even better. So far, this method has enabled tailored Ising superconductivity in NbSe₂, enhanced superconductivity in Weyl semimetal MoTe₂ and semiconducting-to-superconducting transition in SnSe₂.

“Our intercalation method is quite generic and can be readily extended to a large variety of layered materials and a large selection of ionic liquids with different cations,” Zhou added. “Therefore, our work provides an important pathway for creating hybrid materials with tunable functionalities possibly exceeding the bulk crystals and monolayer samples. Besides superconductors, we would like to apply this strategy to many other layered materials to obtain more intriguing properties. We expect that thanks to intercalation, intriguing properties exceeding both bulk crystals and monolayer samples will soon be enabled in a growing number of layered materials.”

More information: Haoxiong Zhang et al, Tailored Ising superconductivity in intercalated bulk NbSe2, Nature Physics (2022). DOI: 10.1038/s41567-022-01778-7

Journal information: Nature Physics 

© 2022 Science X Network

Today, astronomers seek to observe the faintest and most distant objects possible. Extremely Large Telescopes (ELTs), with apertures in the order of several dozen meters, are the next generation facilities to do so. However, building larger telescopes is only one part of the equation. The other part is the capability of detecting the gathered photons in the most efficient way possible.

This is where making all other optical components in astronomical instruments more efficient becomes crucial. One essential component used in modern astronomical science is the diffraction grating. Its role is to spatially spread incoming light into its constituent frequencies, similar to how a glass prism does.

Thanks to a precisely engineered structure that leverages the wave-like nature of photons, diffraction gratings can separate light of different wavelengths with very high resolution. When coupled with a telescope and a spectrometer, gratings allow scientists to analyze the spectral properties of celestial bodies.

Motivated by the somewhat stagnant progress made in grating technology over the past decade, researchers Hanshin Lee of the University of Texas at Austin and Menelaos K. Poutous of the University of North Carolina at Charlotte, focused on a completely different way of fabricating diffraction gratings.

In their paper recently published in the Journal of Astronomical Telescopes, Instruments, and Systems, they report their success on manufacturing proof-of-concept high-efficiency diffraction gratings using reactive ion-plasma etching (RIPLE), a plasma-based manufacturing technology normally used for semiconductors.

Put simply, the RIPLE process used in this study involves “drawing” (using a high-precision electron beam) the desired grating pattern onto a chrome masking layer placed atop a quartz substrate. The grating pattern is then carved directly onto the quartz substrate using chemically reactive plasma; the chrome mask acts as a shield and the plasma only eats away at the exposed regions.

After fine tuning various parameters of the process through theoretical calculations, simulations, and experimental trial and error, the researchers managed to produce first-order diffraction gratings with very precise nano-scale structures. This translated to a near-theoretical unpolarized diffraction efficiency, reaching 94.3% at its peak and staying over 70% across a wavelength range broader than 200 nm.

“This type of performance has been only rarely achieved in diffraction gratings used for astronomy, where every bit of efficiency gain really matters due to photon starvation,” said Lee.

Another advantage of using the RIPLE process to produce diffraction gratings is that the grating structure is embedded directly into the glass substrate, which means that they share the same material characteristics.

“Our gratings can be very robust optically, thermally, and mechanically, which makes them ideal for harsh environments, such as those found in space observatories and cryogenic systems,” said Poutous, “This allows for their application in a broad range of scientific and engineering spectroscopic measurements.”

Overall, the results of this study showcase the potential of the RIPLE process to revolutionize the way in which diffraction gratings are fabricated. The researchers are optimistic about the future use of such high-efficiency gratings in the upcoming era of ground-based ELTs with apertures over of 30 meters. With any luck, these gratings will be instrumental for astronomers to observe extremely faint objects far out in space in upcoming years.

More information: Hanshin Lee et al, Reactive ion plasma etched surface relief gratings for low/medium/high resolution spectroscopy in astronomy, Journal of Astronomical Telescopes, Instruments, and Systems (2022). DOI: 10.1117/1.JATIS.8.4.045002

Chirality is the breaking of reflection and inversion symmetries. Simply put, it is when an object’s mirror images cannot be superimposed over each other. A common example are your two hands—while mirror images of each other, they can never overlap. Chirality appears at all levels in nature and is ubiquitous.

In addition to static chirality, chirality can also occur due to dynamic motion including rotation. With this in mind, we can distinguish true and false chirality. A system is truly chiral if—when translating—space inversion does not equate to time reversal combined with a proper spatial rotation.

Phonons are quanta (or small packets) of energy associated with the vibration of atoms in a crystal lattice. Recently, phonons with chiral properties have been theorized and experimentally discovered in two-dimensional (2D) materials such as tungsten diselenide. The discovered chiral phonons are rotating—yet not propagating—atomic motions. But, truly chiral phonons would be atomic motions that are both rotating and propagating, and these have never been observed in three-dimensional (3D) bulk systems.

Now, a team of researchers led by scientists from Tokyo Institute of Technology (Tokyo Tech) has identified truly chiral phonons, both theoretically and experimentally. Their work is published in Nature Physics. The team, led by Professor Takuya Satoh of the Department of Physics at Tokyo Tech, observed the chiral phonons in cinnabar (α-HgS). This was achieved using a combination of first-principles calculations and an experimental technique called circularly polarized Raman scattering.

“Chiral structures can be probed using chiral techniques. So, using circularly polarized light, which has its own handedness (i.e., right-handed or left-handedness), is critical. Dynamic chiral structures can be mapped using pseudo-angular momentum (PAM). Pseudo-momentum and PAM originate from the phase factors acquired by discrete translation and rotation symmetry operations, respectively,” explains Professor Satoh.

The researchers’ novel experimental approach also allowed them to probe the fundamental traits of PAM. They found that the law of the conservation of PAM—one of the key laws of physics—holds between circularly polarized photons and chiral phonons.

“Our work also provides an optical method to identify the handedness of chiral materials using PAM. Namely, we can determine the handedness of materials with better resolution than X-ray diffraction (XRD) can achieve. Moreover, XRD requires a large-enough crystal, is invasive, and can be destructive. Circularly polarized Raman scattering, on the other hand, allowed us to determine the chirality of structures XRD could not, in a non-contact and non-destructive manner,” concludes Professor Satoh.

This study is the first to identify truly chiral phonons in 3D materials, which are clearly distinct from those seen previously in 2D hexagonal systems. The knowledge gained here could drive new research into developing ways for transferring the PAM from photons to electron spins via propagating chiral phonons in future devices. Furthermore, this approach enables the determination of the true chirality of a crystal in an improved manner, providing a new critical tool for experimentalists’ and researchers.

More information: Kyosuke Ishito et al, Truly chiral phonons in α-HgS, Nature Physics (2022). DOI: 10.1038/s41567-022-01790-x

Journal information: Nature Physics 

Provided by Tokyo Institute of Technology 

On July 5, the LHC roared to life for its third run after three years of continual improvements to the machine as well as to the experiments’ detectors and analysis tools, and immediately reached a record energy of 13.6 TeV. Just three weeks later, the compact muon solenoid (CMS) collaboration was ready for its physics data-taking period.

The CMS collaboration recently presented its first Run 3 physics results of the production rate of pairs of the heaviest elementary particle, the top quark. In just one week, from July 28 to 3August 3, the CMS collaboration collected data equivalent to almost 12% of the data set that had been required for the Higgs boson discovery in 2012.

Before Run 3 began, it was hoped—and has now been confirmed—that it would be possible to gather such a vast amount of data in a very short time. It took physicists two years to collect the data used to announce the Higgs boson discovery in 2012. But now, thanks to developments in data acquisition and selection systems and to the unprecedented speed of the analyses, the Run 3 data can now be analyzed in almost real time.

Due to the high number of top-quark pairs created at the LHC, physics analysis can start with even a small amount of data. The production rate of this heavy system of particles has been enhanced by about 10% thanks to the collision energy increase from 13 TeV in Run 2 to 13.6 TeV in Run 3.

The CMS results, which agree with the Standard Model prediction, are important because precise measurements of top-quark properties provide, among other things, crucial input for various searches for new phenomena in Run 3. Because of its high mass, the top quark decays immediately to a b quark and a W boson, which is also an unstable particle. The decay products leave traces as they pass through the detector, making it possible to observe them and to test the detector performance.

Precision measurements of the Standard Model are an essential part of the Run 3 program, as any significant deviation could hint at new physics. The measurement of top-quark pair production rate is only the first step into the unexplored territory of the new energy regime, where answers to fundamental physics questions may be found.

Provided by CERN 

Terahertz light, radiation in the far-infrared part of the emission spectrum, is currently not fully exploited in technology, although it shows great potential for many applications in sensing, homeland security screening, and future (sixth generation) mobile networks.

Indeed, this radiation is harmless due to its small photon energy, but it can penetrate many materials (such as skin, packaging, etc.). In the last decade, a number of research groups have focused their attention on identifying techniques and materials to efficiently generate THz electromagnetic waves: among them is the wonder material graphene, which, however, does not provide the desired results. In particular, the generated terahertz output power is limited.

Better performance has now been achieved by topological insulators (TIs)—quantum materials that behave as insulators in the bulk while exhibiting conductive properties on the surface—according to a paper recently published in Light: Science & Applications.

This study was carried out by members of the ICN2 Ultrafast Dynamics in Nanoscale Systems Group, led by Dr. Klaas-Jan Tielrooij, and of the High-field THz Driven Phenomena Group at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR, Germany), led by Dr. Sergey Kovalev, in collaboration with researchers from the ICN2 Physics and Engineering of Nanodevices Group, headed by ICREA Prof. Sergio O. Valenzuela, from the School of Physics and Astronomy of the University of Manchester (UK), and the Physics Institute of the University of Würzburg (Germany). The experiments were performed at the TELBE THz facility in Dresden.

Earlier studies had shown that materials which host electrons with zero effective mass enable efficient generation of terahertz harmonics, including the aforementioned graphene and topological insulators. The phenomenon of harmonic generation occurs when photons of the same frequency and energy interact non-linearly with matter, leading to the emission of photons whose energy is a multiple of that of the incident ones. This can be exploited, for example, to upconvert electronically generated signals in the high GHz regime into signals in the THz regime.

Dr. Tielrooij and colleagues investigated the behavior of two topological insulators—the prototypical Bi₂Se₃ and Bi₂Te₃—in direct comparison with a reference graphene sample.

They observed that, while the maximum power of the harmonics generated in graphene is limited by saturation effects (which arise at high incident powers), in these quantum materials it continued to increase with the incident fundamental power. The performed experiments revealed an improvement in generated output power by orders of magnitude over graphene, approaching the milliwatt regime.

This significant divergence in behavior is due to the fact that topological insulators can rely on a highly efficient cooling mechanism, in which the massless charges on the surface dissipate their electronic heat to those in the rest of the thin film. In other words, bulk electrons lend a helping hand to the surface-state electrons by sinking electronic heat.

The highest output power for the terahertz third-harmonic –i.e. radiation with three times the same energy– was achieved in a metamaterial that contained a topological insulator film together with a metallic grating –consisting of metal strips separated by gaps on the surface of the material.

“In this work we were able to demonstrate that the saturation effect occurring in graphene is much less present in topological insulators. This occurs thanks to a novel cooling mechanism between surface and bulk electrons of topological insulators,” explains Dr. Klaas-Jan Tielrooij, first author of the paper. “These quantum metamaterials thus bring nonlinear terahertz photonics technology a big step closer.”

Sergey Kovalev, last author of the paper, adds that “the obtained results furthermore offer interesting possibilities towards studying the quantum properties of these materials with prospects towards quantum technologies.”

More information: Klaas-Jan Tielrooij et al, Milliwatt terahertz harmonic generation from topological insulator metamaterials, Light: Science & Applications (2022). DOI: 10.1038/s41377-022-01008-y

Journal information: Light: Science & Applications 

Provided by Chinese Academy of Sciences