Category: Physics

The ability to see invisible structures in our bodies, like the inner workings of cells, or the aggregation of proteins, depends on the quality of one’s microscope. Ever since the first optical microscopes were invented in the 17th century, scientists have pushed for new ways to see more things more clearly, at smaller scales and deeper depths.

Randy Bartels, professor in the Department of Electrical Engineering at Colorado State University, is one of those scientists. He and a team of researchers at CSU and Colorado School of Mines are on a quest to invent some of the world’s most powerful light microscopes—ones that can resolve large swaths of biological material in unimaginable detail.

The name of the game is super–resolution microscopy, which is any optical imaging technique that can resolve things smaller than half the wavelength of light. The discipline was the subject of the 2014 Nobel Prize in Chemistry, and Bartels and others are in a race to keep circumventing that diffraction limit to illuminate biologically important structures inside the body.

Bartels, together with Jeff Squier, a professor of physics at Colorado School of Mines, have theorized a new super–resolution technique that uniquely fuses quantum and classical information derived from light to drastically improve imaging resolution. The math and physics behind their idea, and why they think it will work, is detailed in the journal Intelligent Computing. The paper includes Jeff Field, former director of CSU’s Center for Imaging and Surface Science, as co-author.

Counting photons

Their new computational imaging method works by precisely counting the arrival of photons, or quantum particles of light, emitted from a biological sample. The photons are excited by laser pulses, and by counting them one by one with detectors, a set of quantum and classical images emerge. The researchers then apply an algorithm to produce images that resolve the details of small structures, like cells, over large regions.

“Right now, people can do very high-resolution optical imaging, but it’s kind of like flying over the Rocky Mountains and being able to see all the trees, but not the fine details of the trees,” explained Bartels. Other super–resolution techniques exist that allow someone to zoom way in, say, on an individual leaf, he continued. But to look in fine detail across an entire forest is what Bartels and the team are going for.

“The idea here is to illuminate a large region of many cells, at high resolution, high speed, and across a high volume,” Bartels said.

Bartels has worked with Mines’ Squier for several years. Their research partnership combines Bartels’ expertise in computation with Squier’s expertise in optical engineering, creating custom optics for their shared goals.

“While our paper represents a fusion of classical and quantum physics, it is fair to say it also represents a fusion of ideas from both Mines and CSU,” Squier said. “The fundamental ideas in the paper simply would not have manifested without the close collaboration that has been built up over the past several years by our groups.”

More information: Randy A. Bartels et al, Super-Resolution Imaging by Computationally Fusing Quantum and Classical Optical Information, Intelligent Computing (2022). DOI: 10.34133/icomputing.0003

Provided by Colorado State University 

Researchers have experimentally demonstrated, for the first time, a mechanically flexible silver mesh that is visibly transparent, allows high-quality infrared wireless optical communication and efficiently shields electromagnetic interference in the X band portion of the microwave radio region. Optical communication channels are important to the operation of many devices and are often used for remote sensing and detection.

Electronic devices are now found throughout our homes, on factory floors and in medical facilities. Electromagnetic interference shielding is often used to prevent electromagnetic radiation from these devices from interfering with each other and affecting device performance.

Electromagnetic shielding, which is also used in the military to keep equipment and vehicles hidden from the enemy, can also block the optical communication channels needed for remote sensing, detection or operation of the devices. A shield that can block interference but allow for optical communication channels could help to optimize device performance in a variety of civilian and military settings.

“Many conventional transparent electromagnetic interference shields allow only visible light signals through,” said research team leader Liu Yang from Zhejiang University in China. “However, visible wavelengths are not well suited for optical communication, especially free-space—or wireless—optical communication, because of the huge amount of background noise.”

In the journal Optical Materials Express, the researchers describe their new mesh. They show that when combined with transparent silicone and polyethylene, it can achieve a high average electromagnetic shielding effectiveness of 26.2 dB in the X band with good optical transmittance at a wide range of wavelengths, including those in the infrared.

“We take the advantage of the ultrabroad transparency and low haze of a metallic micromesh to demonstrate efficient electromagnetic shielding, visible transparency and high-quality free-space optical communication,” said Yang. “Sandwiching the mesh between transparent materials improves the chemical stability and mechanical flexibility of the silver mesh while also imparting a self-cleaning quality. These properties will enable our silver mesh to be applied widely both indoors and outdoors, even on corrosive and free-form surfaces.”

A flexible and transparent mesh

The researchers designed the new silver mesh with a very simple structure—a repeating square grid pattern applied to a transparent and flexible polyethylene substrate. The continuous grid structure makes the silver mesh very flexible by releasing stress during bending. Because the transparency of the silver mesh is primarily determined by the opening ratio, a measure of the size of the holes in the mesh, it is independent of the incident light wavelength.

“A large opening ratio, for example, is beneficial for a high, broadband transparency and low haze but is detrimental to high conductivity and thus electromagnetic shielding performance,” said Yang. “Because the physical parameters for our mesh can be easily optimized by changing the grid period, line width and thickness, it is easier to achieve well-balanced optical, electrical and electromagnetic properties compared with what is possible with other kinds of transparent conductive films such as silver nanowire networks, ultrathin metallic films and carbon-based materials.”

To demonstrate their new technology, the researchers fabricated a silver mesh onto a polyethylene substrate. The mesh had a grid period of approximately 150 μm, a grid line width of approximately 6 μm and a thickness that ranged from 59 to 220 nm. This was then covered with a layer of 60-μm thick polydimethylsiloxane. The resulting film showed high transmission for a broad wavelength range from 400 nm to 2000 nm and sheet resistance as low as 7.12 Ω/sq, allowing a high electromagnetic shield effectiveness up to 26.2 dB in the X band. The researchers also showed that the film could shield low-frequency mobile phone signals.

The researchers caution that this work is only a prototype demonstration, so there is much room for improvement. For example, using more conductive materials would improve the electromagnetic shielding effectiveness, and materials that are more transparent and have a lower haze could improve not only the visible transparency but also the free-space optical communication quality.

They are also exploring mid-infrared transparent conductive materials, which would extend the FSO communication to longer wavelengths where atmospheric interference is reduced and higher communication quality can be achieved. For commercialization, the mesh would also have to be more practical to install and less expensive.

More information: Qiyun Lei et al, Broadband transparent and flexible silver mesh for efficient electromagnetic interference shielding and high-quality free-space optical communication, Optical Materials Express (2022). DOI: 10.1364/OME.478830

Journal information: Optical Materials Express 

Provided by Optica 

Approximately 85% of the mass of our galaxy is comprised by dark matter, matter that does not emit, absorb or reflect light and thus cannot be directly observed. While several studies have hinted at or theorized about its composition, it remains one of the greatest unresolved physics problems.

Physicists all over the world have been conducting dark matter searches or trying to come up with new methods to directly observe different dark matter candidates. One hypothetical form of dark matter that has so far eluded detection is dark-photon dark matter.

An intriguing possibility is that dark matter is comprised of dark photons, which resemble photons (i.e., the particles that make up visible light), but interact with charges with feeble strength. These dark photons could theoretically have masses in the milli-electrovolt range, approximately a million times lighter than those of electrons and thus notoriously difficult to detect.

Researchers at Northwestern University, Stanford University and Fermilab have recently introduced a new method that could be used to search for meV dark photons. The validity of method, outlined in a paper published in Physical Review Letters, was demonstrated in a short proof-of-principle trial, which also helped to set new constraints on dark-photon dark matter.

“The idea for our study arose from discussions between experimenters and theorists facilitated by the DOE SQMS Center,” Gabriel Gabrielse, one of the researchers who carried out the study, told Phys.org. “At Northwestern we were looking for BSM applications for an unusual one-electron detector that was background free. We had developed the novel detector to measure the electron electric dipole moment and test the Standard Model’ss most precise prediction.”

After learning about the work carried out by Gabrielse and his colleagues at Northeastern, a team of theoretical physicists at Stanford reached out and pointed out the potential of their detector for conducting meV dark photon searches. This sparked a series of interactions and collaborations between the two research groups, also including Roni Harnik, a theorist at Fermilab.

The new method introduced by the researchers is based on the use of trapped electrons as high-Q resonators for detecting meV dark photon dark matter. Its underlying hypothesis is that when the rest energy of a dark photon matches the energy splitting of the two lowest cyclotron levels, the first state of the electron cyclotron will be excited.

“If a meV dark photon enters the trap in which a single electron is suspended, then the electron can be excited from the ground state to the first excited state of its cyclotron motion,” Gabrielse explained. “There is no background and the single excitation of a single trapped electron can be detected without ambiguity. Failing to see any such excitations for several days allowed us to set a limit on the strength of the dark photon field that passed through, based upon theoretical calculations of the efficiency with which a dark photon could produce such an excitation.”

To demonstrate the practicality of their proposed method, Gabrielse and his colleagues used it to collect an initial measurement, using a single electron. This trial showed that their strategy was background-free for a search that lasted just over 7 days.

The researchers were also able to set a new limit on dark photon dark matter, specifically at 148 GHz (0.6 meV). In the future, their work could pave the way for new studies aimed at evaluating and using their proposed strategy to search for meV dark photons.

“The most notable achievement of our work is the concrete demonstration of an entirely new method for searching for meV dark matter,” Gabrielse added. “We are now planning to do a broad search for meV dark photons in an apparatus that is designed for this. The apparatus in which the demonstration measurement took place was optimized for narrow band electron magnetic moment measurements, while the new apparatus and new ideas we are developing will allow broad searches.”

More information: Xing Fan et al, One-Electron Quantum Cyclotron as a Milli-eV Dark-Photon Detector, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.261801

Journal information: Physical Review Letters 

© 2023 Science X Network

In a new field trial, researchers show that a real-time coherent transceiver prototype can be used for continuous sensing over a 524-km live network aerial fiber wound around high-voltage power cables suspended from outdoor poles.

By monitoring polarization changes in the light traveling through a fiber link, this approach could enable new environmental or current sensing types. It can also be used to improve network integrity by continuously monitoring the health of the fiber link, for example, by detecting increases in fiber length that might indicate a pole is starting to tilt.

Mikael Mazur from Nokia Bell Labs will present the new findings at the Optical Fiber Communication Conference (OFC), which will take place 05—09 March 2023 in San Diego, California, U.S..

“Optical fibers are everywhere, and if we can expand the use of this infrastructure to create a dense worldwide spanning mesh of environmental sensors, these communication systems can play an even bigger role in our daily life,” said Mazur. “Sensing transceivers can prevent service interruption and improve our understanding of the environment by significantly scaling the number of sensors without the cost of a dedicated sensing system. Most importantly, this can be done without any loss in data throughput, enabling full use of the communication system for its intended purpose.”

In the new work, the researchers use a field-programmable gate array coherent transceiver sensing prototype for continuous fiber sensing by using information extracted from coherent digital signal processing (DSP). They used the DSP-based timing recovery module as a time-of-flight sensor.

Using this approach, the researchers continuously monitored a 524-km length of aerial fiber for 70 hours using time-of-flight measurements. They correlated the sensing measurements with temperatures acquired from stations along the network link. The analysis revealed strong oscillations driven by polarization changes over 50 Hz, likely from the Faraday effect induced by the spun fiber. They demonstrated polarization sensing of various wind conditions by filtering out the low-frequency portion of these polarization changes. The results show that coherent transceivers could potentially be used to perform continuous sensing over aerial fibers, making it possible to perform both environmental and network sensing using existing aerial fibers.

“We are just scratching the surface of potential applications and will continues to perform field trials over various networks in different environments,” said Mazur. “Our goal is to better understand how this sensor can be used in future smart cities to improve the resilience of both communication systems and infrastructure, while getting a better understanding of the environment around us. We are also actively looking at algorithms for real-time analytics and autonomous decision-making based on transceiver sensing data, enabling early-warning applications.”

More information: The 2023 Optical Fiber Communication Conference (OFC): www.ofcconference.org/en-us/home/about/

Provided by Optica 

Inspired by the bubbles bacteria create inside their cells, researchers developed a similar system by coating tiny gas vesicles with protein. The resulting bubbles are safe, highly stable, and function as contrast agent in medical applications. They could be used to diagnose, for example, cardiological issues, blood flow, and liver lesions.

Bacteria produce gas vesicles, tiny thin-walled sacs filled with air or fluid, to help them float. This phenomenon has captured the attention of scientists who see potential for similar bubble-based designs in fields like medicine.

A team of researchers at Aalto University’s Department of Applied Physics, led by Professor Robin Ras, have now used the same idea to create a new kind of contrast agent for use in medical applications such as ultrasound imaging. The research was recently published in the Proceedings of the National Academy of Sciences.

Natural materials and biological inspiration

The researchers created bubbles, referred to as giant gas vesicles, ranging from 10 to 100 micrometers in length, and measured their mechanical properties with a technique called micropipette aspiration. The bubbles were coated with proteins called hydrophobins, which come from fungi. In addition, the team developed a theory to better understand the intricacies of compressibility and porosity in micro-scale physics.

“By studying the mechanical properties of gas vesicles and developing our own micropipette technique, we were able to make the bubbles stable enough to withstand pressures like the ones you would find in the human body. The bubbles function as a contrast agent, and potentially could be used to diagnose things like cardiological issues, blood flow, and liver lesions with ultrasound in the future,” says Doctoral Researcher Hedar Al-Terke.

“We have significantly extended the theoretical framework of the pipette aspiration technique. It can now be used to fully characterize the mechanical properties, including porosity, of compressible gas-filled systems such as the hydrophobin-coated bubbles used in this study,” says Research Fellow Grégory Beaune.

The research on giant gas vesicles is part of the team’s focus on researching the medical applications of micro-scale physics.

More information: Hedar H. Al-Terke et al, Compressibility and porosity modulate the mechanical properties of giant gas vesicles, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2211509120

Journal information: Proceedings of the National Academy of Sciences 

Provided by Aalto University 

It’s not at every university that laser pulses powerful enough to burn paper and skin are sent blazing down a hallway. But that’s what happened in UMD’s Energy Research Facility, an unremarkable looking building on the northeast corner of campus. If you visit the utilitarian white and gray hall now, it seems like any other university hall—as long as you don’t peak behind a cork board and spot the metal plate covering a hole in the wall.

But for a handful of nights in 2021, UMD Physics Professor Howard Milchberg and his colleagues transformed the hallway into a laboratory: The shiny surfaces of the doors and a water fountain were covered to avoid potentially blinding reflections; connecting hallways were blocked off with signs, caution tape and special laser-absorbing black curtains; and scientific equipment and cables inhabited normally open walking space.

As members of the team went about their work, a snapping sound warned of the dangerously powerful path the laser blazed down the hall. Sometimes the beam’s journey ended at a white ceramic block, filling the air with louder pops and a metallic tang. Each night, a researcher sat alone at a computer in the adjacent lab with a walkie-talkie and performed requested adjustments to the laser.

Their efforts were to temporarily transfigure thin air into a fiber optic cable—or, more specifically, an air waveguide—that would guide light for tens of meters. Like one of the fiber optic internet cables that provide efficient highways for streams of optical data, an air waveguide prescribes a path for light. These air waveguides have many potential applications related to collecting or transmitting light, such as detecting light emitted by atmospheric pollution, long-range laser communication or even laser weaponry. With an air waveguide, there is no need to unspool solid cable and be concerned with the constraints of gravity; instead, the cable rapidly forms unsupported in the air. In a paper accepted for publication in the journal Physical Review X the team described how they set a record by guiding light in 45-meter-long air waveguides and explained the physics behind their method.

The researchers conducted their record-setting atmospheric alchemy at night to avoid inconveniencing (or zapping) colleagues or unsuspecting students during the workday. They had to get their safety procedures approved before they could repurpose the hallway.

“It was a really unique experience,” says Andrew Goffin, a UMD electrical and computer engineering graduate student who worked on the project and is a lead author on the resulting journal article. “There’s a lot of work that goes into shooting lasers outside the lab that you don’t have to deal with when you’re in the lab—like putting up curtains for eye safety. It was definitely tiring.”

All the work was to see to what lengths they could push the technique. Previously Milchberg’s lab demonstrated that a similar method worked for distances of less than a meter. But the researchers hit a roadblock in extending their experiments to tens of meters: Their lab is too small and moving the laser is impractical. Thus, a hole in the wall and a hallway becoming lab space.

“There were major challenges: the huge scale-up to 50 meters forced us to reconsider the fundamental physics of air waveguide generation, plus wanting to send a high-power laser down a 50-meter-long public hallway naturally triggers major safety issues,” Milchberg says. “Fortunately, we got excellent cooperation from both the physics and from the Maryland environmental safety office!”

Without fiber optic cables or waveguides, a light beam—whether from a laser or a flashlight—will continuously expand as it travels. If allowed to spread unchecked, a beam’s intensity can drop to un-useful levels. Whether you are trying to recreate a science fiction laser blaster or to detect pollutant levels in the atmosphere by pumping them full of energy with a laser and capturing the released light, it pays to ensure efficient, concentrated delivery of the light.

Milchberg’s potential solution to this challenge of keeping light confined is additional light—in the form of ultra-short laser pulses. This project built on previous work from 2014 in which his lab demonstrated that they could use such laser pulses to sculpt waveguides in the air.

The short pulse technique utilizes the ability of a laser to provide such a high intensity along a path, called a filament, that it creates a plasma—a phase of matter where electrons have been torn free from their atoms. This energetic path heats the air, so it expands and leaves a path of low-density air in the laser’s wake. This process resembles a tiny version of lighting and thunder where the lightning bolt’s energy turns the air into a plasma that explosively expands the air, creating the thunderclap; the popping sounds the researchers heard along the beam path were the tiny cousins of thunder.

But these low-density filament paths on their own weren’t what the team needed to guide a laser. The researchers wanted a high-density core (the same as internet fiber optic cables). So, they created an arrangement of multiple low-density tunnels that naturally diffuse and merge into a moat surrounding a denser core of unperturbed air.

The 2014 experiments used a set arrangement of just four laser filaments, but the new experiment took advantage of a novel laser setup that automatically scales up the number of filaments depending on the laser energy; the filaments naturally distribute themselves around a ring.

The researchers showed that the technique could extend the length of the air waveguide, increasing the power they could deliver to a target at the end of the hallway. At the conclusion of the laser’s journey, the waveguide had kept about 20% of the light that otherwise would have been lost from their target area. The distance was about 60 times farther than their record from previous experiments. The team’s calculations suggest that they are not yet near the theoretical limit of the technique, and they say that much higher guiding efficiencies should be easily achievable with the method in the future.

“If we had a longer hallway, our results show that we could have adjusted the laser for a longer waveguide,” says Andrew Tartaro, a UMD physics graduate student who worked on the project and is an author on the paper. “But we got our guide right for the hallway we have.”

The researchers also did shorter eight-meter tests in the lab where they investigated the physics playing out in the process in more detail. For the shorter test they managed to deliver about 60% of the potentially lost light to their target.

The popping sound of the plasma formation was put to practical use in their tests. Besides being an indication of where the beam was, it also provided the researchers with data. They used a line of 64 microphones to measure the length of the waveguide and how strong the waveguide was along its length (more energy going into making the waveguide translates to a louder pop).

The team found that the waveguide lasted for just hundredths of a second before dissipating back into thin air. But that’s eons for the laser bursts the researchers were sending through it: Light can traverse more than 3,000 km in that time.

Based on what the researchers learned from their experiments and simulations, the team is planning experiments to further improve the length and efficiency of their air waveguides. They also plan to guide different colors of light and to investigate if a faster filament pulse repetition rate can produce a waveguide to channel a continuous high-power beam.

“Reaching the 50-meter scale for air waveguides literally blazes the path for even longer waveguides and many applications”, Milchberg says. “Based on new lasers we are soon to get, we have the recipe to extend our guides to one kilometer and beyond.”

More information: Optical guiding in 50-meter-scale air waveguides, Physical Review X (2023). journals.aps.org/prx/accepted/ … e56bdc44a53b2267be80

Journal information: Physical Review X 

Provided by University of Maryland 

Physicists at the University of Bonn have experimentally proven that an important theorem of statistical physics applies to so-called “Bose-Einstein condensates.” Their results now make it possible to measure certain properties of the quantum “superparticles” and deduce system characteristics that would otherwise be difficult to observe. The study has now been published in Physical Review Letters.

Suppose in front of you there is a container filled with an unknown liquid. Your goal is to find out by how much the particles in it (atoms or molecules) move back and forth randomly due to their thermal energy. However, you do not have a microscope with which you could visualize these position fluctuations known as “Brownian motion”.

It turns out you do not need that at all: You can also simply tie an object to a string and pull it through the liquid. The more force you have to apply, the more viscous your liquid. And the more viscous it is, the lesser the particles in the liquid change their position on average. The viscosity at a given temperature can therefore be used to predict the extent of the fluctuations.

The physical law that describes this fundamental relationship is the fluctuation-dissipation theorem. In simple words, it states: The greater the force you need to apply to perturb a system from the outside, the less it will also fluctuate randomly (i.e., statistically) on its own if you leave it alone.

“We have now confirmed the validity of the theorem for a special group of quantum systems for the first time: the Bose-Einstein condensates,” explains Dr. Julian Schmitt from the Institute of Applied Physics at the University of Bonn.

‘Super photons’ made of thousands of light particles

Bose-Einstein condensates are exotic forms of matter that can arise due to a quantum mechanical effect: Under certain conditions, particles, be they atoms, molecules, or even photons (particles that constitute light), become indistinguishable. Many hundreds or thousands of them merge into a single “super particle”—the Bose-Einstein condensate (BEC).

In a liquid at finite temperature, molecules move back and forth at random. The warmer the liquid, the more pronounced are these thermal fluctuations. Bose-Einstein condensates can also fluctuate: The number of condensed particles varies. And this fluctuation also increases with rising temperature.

“If the fluctuation-dissipation theorem applies to BECs, the greater the fluctuation in their particle number, the more sensitively they should respond to an external perturbation,” Schmitt says. “Unfortunately, the number [of] fluctuations in the usually studied BECs in ultracold atomic gases is too small to test this relationship.”

However, the research group of Prof. Dr. Martin Weitz, within which Schmitt is a junior research group leader, works with Bose-Einstein condensates made of photons. And for this system, the limitation does not apply. “We make the photons in our BECs interact with dye molecules,” explains the physicist. When photons interact with dye molecules, it frequently happens that a molecule “swallows” a photon. The dye thereby becomes energetically excited. It can later release this excitation energy by “spitting out” a photon.

Low-energy photons are swallowed less often

“Due to the contact to the dye molecules, the number of photons in our BECs shows large statistical fluctuations,” says the physicist. In addition, the researchers can precisely control the strength of this variation: In the experiment, the photons are trapped between two mirrors, where they are reflected back and forth in a ping-pong game manner.

The distance between the mirrors can be varied. The larger it becomes, the lower the energy of the photons. Since low-energy photons are less likely to excite a dye molecule (so they are swallowed less often), the number of condensed light particles now fluctuates much less.

The Bonn physicists now investigated how the extent of the fluctuation is related to the “response” of the BEC. If the fluctuation-dissipation theorem holds, this sensitivity should decrease as fluctuation decreases.

“In fact, we were able to confirm this effect in our experiments,” emphasizes Schmitt, who is also a member of the Transdisciplinary Research Area (TRA) “Matter” at the University of Bonn and the Cluster of Excellence “ML4Q—Matter and Light for Quantum Computing.”

As with liquids, it is now possible to infer the microscopic properties of Bose-Einstein condensates from macroscopic response parameters that can be more easily measured. “This opens a way to new applications, such as the precise temperature determination in complex photonic systems,” says Schmitt.

More information: Fahri Emre Öztürk et al, Fluctuation-Dissipation Relation for a Bose-Einstein Condensate of Photons, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.033602

Journal information: Physical Review Letters 

Provided by Rheinische Friedrich-Wilhelms-Universität Bonn

Scientists have advanced in discovering how to use ripples in space-time known as gravitational waves to peer back to the beginning of everything we know. The researchers say they can better understand the state of the cosmos shortly after the Big Bang by learning how these ripples in the fabric of the universe flow through planets and the gas between the galaxies.

“We can’t see the early universe directly, but maybe we can see it indirectly if we look at how gravitational waves from that time have affected matter and radiation that we can observe today,” said Deepen Garg, lead author of a paper reporting the results in the Journal of Cosmology and Astroparticle Physics. Garg is a graduate student in the Princeton Program in Plasma Physics, which is based at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL).

Garg and his advisor Ilya Dodin, who is affiliated with both Princeton University and PPPL, adapted this technique from their research into fusion energy, the process powering the sun and stars that scientists are developing to create electricity on Earth without emitting greenhouse gases or producing long-lived radioactive waste. Fusion scientists calculate how electromagnetic waves move through plasma, the soup of electrons and atomic nuclei that fuels fusion facilities known as tokamaks and stellarators.

It turns out that this process resembles the movement of gravitational waves through matter. “We basically put plasma wave machinery to work on a gravitational wave problem,” Garg said.

Gravitational waves, first predicted by Albert Einstein in 1916 as a consequence of his theory of relativity, are disturbances in space-time caused by the movement of very dense objects. They travel at the speed of light and were first detected in 2015 by the Laser Interferometer Gravitational Wave Observatory (LIGO) through detectors in Washington State and Louisiana.

Garg and Dodin created formulas that could theoretically lead gravitational waves to reveal hidden properties about celestial bodies, like stars that are many light years away. As the waves flow through matter, they create light whose characteristics depend on the matter’s density.

A physicist could analyze that light and discover properties about a star millions of light years away. This technique could also lead to discoveries about the smashing together of neutron stars and black holes, ultra-dense remnants of star deaths. They could even potentially reveal information about what was happening during the Big Bang and the early moments of our universe.

The research began without any sense of how important it might become. “I thought this would be a small, six-month project for a graduate student that would involve solving something simple,” Dodin said. “But once we started digging deeper into the topic, we realized that very little was understood about the problem and we could do some very basic theory work here.”

The scientists now plan to use the technique to analyze data in the near future. “We have some formulas now, but getting meaningful results will take more work,” Garg said.

More information: Deepen Garg et al, Gravitational wave modes in matter, Journal of Cosmology and Astroparticle Physics (2022). DOI: 10.1088/1475-7516/2022/08/017

Provided by Princeton Plasma Physics Laboratory

Quantum materials are materials with unique electronic, magnetic or optical properties, which are underpinned by the behavior of electrons at a quantum mechanical level. Studies have showed that interactions between these materials and strong laser fields can elicit exotic electronic states.

In recent years, many physicists have been trying to elicit and better understand these exotic states, using different material platforms. A class of materials that was found to be particularly promising for studying some of these states are monolayer transition metal dichalcogenides.

Monolayer transition metal dichalcogenides are 2D materials that consist in single layers of atoms from a transition metal (e.g., tungsten or molybdenum) and a chalcogen (e.g., sulfur or selenium), which are arranged into a crystal lattice. These materials have been found to offer exciting opportunities for Floquet engineering (a technique to manipulate the properties of materials using lasers) of excitons (quasiparticle electron-hole correlated states).

Researchers at the SLAC National Accelerator Laboratory, Stanford University and University of Rochester have recently demonstrated the Floquet engineering of excitons driven by strong fields in a monolayer transition metal dichalcogenide. Their findings, presented in a paper in Nature Physics, could open new possibilities for the study of excitonic phenomena.

“Our group has been studying strong-field driven processes such as high-harmonic generation (HHG) in 2D-crystals subjected to intense mid-infrared laser fields,” Shambhu Ghimire, one of the researchers who carried out the study, told Phys.org.

“We are very interested to understand the detailed mechanism of the HHG process, and 2D-crystals seem to be a fascinating platform for this, as they are something in between isolated atoms in the gas phase and the bulk crystals. In the gas phase, the process is understood by considering the dynamics of the laser field ionized electron and its recombination to the parent ion.”

When exposed to strong laser fields, 2D crystals can host strongly-driven excitons. In their previous research, Ghimire and his colleagues explored whether driving these quasiparticles with strong laser fields and measuring high harmonics would allow them to better understand the solid-state HHG process.

“While this previous work was the inspiration for our study, we also started measuring the change in absorption on these driven systems and learned more about the non-equilibrium state of the material itself,” Ghimire explained. “Indeed, we find that there are previously not observed absorption features that can be linked to what’s known in the literature as the Floquet states of the materials subjected to strong periodic drives.”

In their experiments, the researchers used high-power ultrafast laser pulses in the mid-infrared wavelength range to monolayer tungsten disulfide (TMDs). The use of these ultrafast pulses allowed them to avoid the sample damage that typically results from strong light-matter interactions.

More specifically, the photon energy of mid-infrared laser pulses is around 0.31 eV, which is significantly below the optical bandgap of monolayer TMDs (~2 eV). Therefore, the team did not expect to observe a particularly sizable generation of charge carriers.

“At the same time, the photon energy in our set up is tunable and can be resonant to exciton energies of the monolayer,” Ghimire said. “To fabricate our material samples, we collaborated with the team of Prof. Fang Liu at Stanford Chemistry. This group has pioneered a new approach to fabricate millimeter scale monolayer samples, which was also a key to the success of these experiments.”

Yuki Kobayashi, a postdoctoral scholar, who is the lead author of the paper said that they unveiled two new mechanisms for creating quantum virtual states in monolayer TMDs. The first of these involves Floquet states, which are attained by mixing the quantum states of materials with external photons, while the second involves the so-called Franz-Keldvsh effect.

“We found that an originally dark exciton state can be optically bright by mixing with single photon, being manifested as a separate absorption signal below the optical bandgap,” Kobayahsi said. “The second mechanism we unveiled is the dynamic Franz-Keldysh effect. This is caused by the external laser field triggering momentum kick to the excitons, leading to universal blue shift of the spectral features. This effect was observed because we applied a high-field laser pulse (~0.3 V/nm) that is strong enough to break apart the electron-hole pair.”

Combining the two mechanisms they unveiled, Kobayashi and his colleagues were able to achieve an energy tuning over 100 meV in their sample of monolayer TMDs. These notable results highlight the huge potential of this monolayer transition metal dichalcogenide as a platform to realize strong-field excitonic phenomena.

“One of the unanswered questions in our work is the real-time response of strong-field excitonic phenomena: how fast can we turn on and off the virtual quantum states?” Ghimire added. “We expect that, by going beyond the perturbative domain, it will be possible to imprint the oscillation patterns of laser carrier waves in the virtual quantum states, approaching the sub-petahertz regime of optical property control.”

More information: Yuki Kobayashi et al, Floquet engineering of strongly driven excitons in monolayer tungsten disulfide, Nature Physics (2023). DOI: 10.1038/s41567-022-01849-9

Hanzhe Liu et al, High-harmonic generation from an atomically thin semiconductor, Nature Physics (2016). DOI: 10.1038/nphys3946

P. B. Corkum, Plasma perspective on strong field multiphoton ionization, Physical Review Letters (2002). DOI: 10.1103/PhysRevLett.71.1994

Shambhu Ghimire et al, High-harmonic generation from solids, Nature Physics (2018). DOI: 10.1038/s41567-018-0315-5

Fang Liu, Mechanical exfoliation of large area 2D materials from vdW crystals, Progress in Surface Science (2021). DOI: 10.1016/j.progsurf.2021.100626

Journal information: Physical Review Letters  , Nature Physics 

© 2023 Science X Network

A birefringence based light modulator that works in the wavelength region of < 350 nm plays a vital role in DUV beam shaping, high density data storage, semiconductor micro-nano processing and photolithography. Actually, a series of DUV birefringent materials, including single crystals of α-BBO, MgF₂, Ca(BO₂)₂, and α-SnF₂, have thus been made and commercially used. However, these birefringent elements have fixed birefringence, limiting their capability of continuous light modulation.

Liquid crystals (LCs) are another kind of birefringent materials, of which birefringence is tunable via the molecular alignment by external electrical or magnetic stimuli. Up to now, the commonly used LCs are mainly based on organic molecules or polymers, which are not stable under DUV light due to photochemical degradation effects. Meanwhile, DUV can also induce free radicals in some organic groups, and initiate their polymerization, which disorders the alignment and the resultant birefringence of LC. Therefore, organic LC cannot modulate DUV light.

In a new paper published in Light Science & Application, three teams of scientists, led by Professor Hui-Ming Cheng and Associate Professor Baofu Ding from Shenzhen Institute of Advanced Technology, CAS, Professor Wei Cai from Xi’an Research Institute of High Technology, Professor Bilu Liu from Tsinghua University, China, cooperatively synthesized two-dimensional (2D) inorganic cobalt-doped titanate (CTO) LC by using a wet chemical method. The 2D LC has large magnetic & optical anisotropy as well as high transmittance of > 70% in the wavelength of 300 ~ 350 nm, which enables the transmitted DUV modulation in a magnetic and portable way (Fig. 1).

Fig. 2 summarizes the desired performance of the 2D CTO LC based DUV modulator, as evidenced by its good reversibility, fast response at the millisecond level, excellent durability and DUV stability.

The magneto-birefringence effect of 2D CTO LC make it applicable in the preparation of flexible DUV birefringent optical hydrogel. By adding a small amount of monomer and photo-initiator into 2D CTO suspension, a DUV birefringent hydrogel was prepared via UV curing during exertion of magnetic field.

Once the hydrogelation is completed, the magnetically aligned 2D CTO nanosheets can be fixed inside the hydrogel and all their long axes parallel each other, even after removal of the magnetic field. The CTO hydrogel can serve as a transparent mechano-optical crystal, through which the DUV light can be in-situ modulated without direction alteration in a mechanical way (Fig. 3). The 2D CTO based hydrogel is the first birefringence-tunable element that can continuously tune the DUV light in a mechanical way.

This work may extend birefringence-tunable optics that are currently used in visible and infrared regions to the DUV region, which is important for high density data storage, semiconductor micro-nano processing and photolithography.

More information: Youan Xu et al, Deep ultraviolet hydrogel based on 2D cobalt-doped titanate, Light: Science & Applications (2023). DOI: 10.1038/s41377-022-00991-6

Journal information: Light: Science & Applications 

Provided by Light Publishing Center, Changchun Institute of Optics, Fine Mechanics And Physics, CAS