Category: Physics

by University of Ottawa

Researchers at the University of Ottawa, in collaboration with Danilo Zia and Fabio Sciarrino from the Sapienza University of Rome, recently demonstrated a novel technique that allows the visualization of the wave function of two entangled photons, the elementary particles that constitute light, in real-time.

Using the analogy of a pair of shoes, the concept of entanglement can be likened to selecting a shoe at random. The moment you identify one shoe, the nature of the other (whether it is the left or right shoe) is instantly discerned, regardless of its location in the universe. However, the intriguing factor is the inherent uncertainty associated with the identification process until the exact moment of observation.

The wave function, a central tenet in quantum mechanics, provides a comprehensive understanding of a particle’s quantum state. For instance, in the shoe example, the “wave function” of the shoe could carry information such as left or right, the size, the color, and so on.

More precisely, the wave function enables quantum scientists to predict the probable outcomes of various measurements on a quantum entity, e.g. position, velocity, etc.

This predictive capability is invaluable, especially in the rapidly progressing field of quantum technology, where knowing a quantum state which is generated or input in a quantum computer will allow to test the computer itself. Moreover, quantum states used in quantum computing are extremely complex, involving many entities that may exhibit strong non-local correlations (entanglement).

Knowing the wave function of such a quantum system is a challenging task—this is also known as quantum state tomography or quantum tomography in short. With the standard approaches (based on the so-called projective operations), a full tomography requires large number of measurements that rapidly increases with the system’s complexity (dimensionality).

Previous experiments conducted with this approach by the research group showed that characterizing or measuring the high-dimensional quantum state of two entangled photons can take hours or even days. Moreover, the result’s quality is highly sensitive to noise and depends on the complexity of the experimental setup.

The projective measurement approach to quantum tomography can be thought of as looking at the shadows of a high-dimensional object projected on different walls from independent directions. All a researcher can see is the shadows, and from them, they can infer the shape (state) of the full object. For instance, in CT scan (computed tomography scan), the information of a 3D object can thus be reconstructed from a set of 2D images.

In classical optics, however, there is another way to reconstruct a 3D object. This is called digital holography, and is based on recording a single image, called interferogram, obtained by interfering the light scattered by the object with a reference light.

The team, led byEbrahim Karimi, Canada Research Chair in Structured Quantum Waves, co-director of uOttawa Nexus for Quantum Technologies (NexQT) research institute and associate professor in the Faculty of Science, extended this concept to the case of two photons.

Reconstructing a biphoton state requires superimposing it with a presumably well-known quantum state, and then analyzing the spatial distribution of the positions where two photons arrive simultaneously. Imaging the simultaneous arrival of two photons is known as a coincidence image. These photons may come from the reference source or the unknown source. Quantum mechanics states that the source of the photons cannot be identified.

This results in an interference pattern that can be used to reconstruct the unknown wave function. This experiment was made possible by an advanced camera that records events with nanosecond resolution on each pixel.

Dr. Alessio D’Errico, a postdoctoral fellow at the University of Ottawa and one of the co-authors of the paper, highlighted the immense advantages of this innovative approach, “This method is exponentially faster than previous techniques, requiring only minutes or seconds instead of days. Importantly, the detection time is not influenced by the system’s complexity—a solution to the long-standing scalability challenge in projective tomography.”

The impact of this research goes beyond just the academic community. It has the potential to accelerate quantum technology advancements, such as improving quantum state characterization, quantum communication, and developing new quantum imaging techniques.

The study “Interferometric imaging of amplitude and phase of spatial biphoton states” was published in Nature Photonics.

More information: Danilo Zia et al, Interferometric imaging of amplitude and phase of spatial biphoton states, Nature Photonics (2023). DOI: 10.1038/s41566-023-01272-3

Journal information: Nature Photonics 

Provided by University of Ottawa 

by American Institute of Physics

Millions of people around the world experience some form of hearing loss, resulting in negative impacts to their health and quality of life. Treatments exist in the form of hearing aids and cochlear implants, but these assistive devices cannot replace the full functionality of human hearing and remain inaccessible for most people. Auditory experiences, such as speech and music, are affected the most.

In The Journal of the Acoustical Society of America, researchers from the University of Oldenburg studied the impact of hearing loss on subjects’ enjoyment of different music mixes.

Modern pop or rock music is built from several individual tracks, such as vocals, instruments, and synthesized sounds, all recorded separately and mixed to make the final product. To cater to listener preferences, this mixing might entail raising or lowering the volume of one of the tracks or amplifying the high- or low-frequency sounds.

“Mixing is tailored to suit the needs of normal-hearing listeners,” said author Kai Siedenburg. “We wanted to explore whether there are actually differences in mixing preferences between normal-hearing and hard-of-hearing listeners.”

To do this, the researchers played different music mixes to listeners with and without hearing loss. They found that those with hearing loss preferred louder lead vocals, higher frequencies, and sparser mixes with fewer frequencies overall.

“Generally, hard-of-hearing listeners have reduced frequency selectivity and impaired level perception,” said author Aravindan Benjamin. “They tend to prefer louder levels of lead vocals compared to normal listeners.”

Previous research from the group has found that music steadily shifted to quieter vocals and louder instrumentals leading up to 1975 and has remained there, meaning today’s music may be less accessible to those with hearing loss.

Use of hearing aids can remedy these issues to a degree, but they are not available to many people with hearing loss and come with their own set of problems. Some users might prefer to adjust their music with software rather than listen to the default mix through hearing aids.

“Getting good headphones, for example, and then playing around with the equalization might be a better approach than trying to squeeze everything through the hardware of the hearing aid,” said Siedenburg.

Ultimately, the biggest difference must come from the production side. Sound engineers with access to the individual tracks can make a big difference by making their work more accessible to millions of listeners.

“One approach could be to offer a couple of different mixes, one for the general public and one for people who are moderately hard of hearing,” said Siedenburg. “Certain adjustments to the mix might help to cater to the needs of this group of people in a better way.”

More information: Exploring level- and spectrum-based music mixing transforms for hearing-impaired listeners, The Journal of the Acoustical Society of America (2023). DOI: 10.1121/10.0020269

Journal information: Journal of the Acoustical Society of America 

Provided by American Institute of Physics 

Researchers at the Racah Institute of Physics, Hebrew University of Jerusalem, have made a discovery that challenges the conventional understanding of fracture mechanics. The team, led by Dr. Meng Wang, Dr. Songlin Shi, and Prof. Jay Fineberg, has experimentally demonstrated the existence of “supershear” tensile cracks that exceed classical speed limits and transition to near-supersonic velocities. Their paper is published in the journal Science.

Traditionally, brittle materials have been observed to fail through the rapid propagation of cracks. Classical fracture mechanics describes the motion of tensile cracks that release elastic energy within a localized zone at their tips, limiting their speed to the Rayleigh wave speed (CR). However, the recent findings by the Hebrew University researchers indicate a paradigm shift in this understanding.

Utilizing brittle neo-Hookean materials in their experiments, the team identified the occurrence of “supershear” tensile cracks that smoothly accelerate beyond the classical speed limit of CR. Surprisingly, these cracks were observed to surpass the shear wave speed (cS) as well. In certain cases, the velocities of these supershear cracks approached dilatation wave speeds, presenting phenomena previously unobserved in classical fracture mechanics.

One of the most remarkable aspects of the discovery is the observation that supershear dynamics are governed by different principles than those guiding classical cracks. This non-classical mode of tensile fracture is not a random occurrence; rather, it is excited at critical strain levels that depend on the material properties.

“This finding represents a fundamental shift in our understanding of the fracture process in brittle materials,” commented Prof. Jay Fineberg, the corresponding author of the research. “By demonstrating the existence of supershear tensile cracks and their ability to exceed classical speed limits, we have opened up new avenues for studying fracture mechanics and its applications.”

The implications of this research extend beyond the realm of physics. By showing that tensile cracks can surpass their classical speed limits, the researchers have paved the way for a new understanding of fracture mechanics.

More information: Jay Fineberg et al, Tensile cracks can shatter classical speed limits, Science (2023). DOI: 10.1126/science.adg7693. www.science.org/doi/10.1126/science.adg7693

Journal information: Science 

Provided by Hebrew University of Jerusalem 

Researchers have demonstrated the ability to grow high-quality thin films of a recently discovered superconductor material called potassium tantalate (KTaO₃). The researchers also discovered that the material retains its superconductive characteristics even when exposed to extremely high magnetic fields.

A superconductor is a material that can carry electricity without any resistance—meaning none of the energy is dissipated as heat, for example. Superconductive materials hold promise for making a variety of more efficient technologies, such as faster computer components and more energy-efficient power devices. However, the field faces significant challenges. For example, many superconductive materials lose their superconductivity when exposed to magnetic fields, which limits their potential applications.

“Our work here is important because not only have we demonstrated how to fabricate high quality KTaO₃, but we have also shown that the material is capable of withstanding substantial magnetic fields without losing its desirable properties,” says Kaveh Ahadi, corresponding author of three papers on the work and an assistant professor of materials science and engineering at North Carolina State University.

“Specifically, we found that KTaO₃ retains superconductivity even when exposed to magnetic fields up to 25 Tesla. This fundamental work is a necessary step toward the development of any potential applications for the material.”

The researchers were able to “grow” KTaO₃ using a technique called molecular beam epitaxy, which effectively creates two-dimensional (2D) thin films of the material on a substrate by laying molecule-thin layers on top of one another with atomic-level precision. The resulting thin films have extremely high quality, meaning the molecular structure of the material has very few defects.

“These high-quality thin films are an ideal platform for studying the intrinsic properties of this materials system,” Ahadi says.

One such characterization study revealed that KTaO₃ thin films remained superconductive when exposed to magnetic fields of up to 25 Tesla. To put that in context, the only place in the United States capable of generating a 25 Tesla magnetic field is the National High Magnetic Field Laboratory, which is where Ahadi and his collaborators tested the material.

“The research community is still in the early stages of understanding the superconductivity in KTaO₃, much less identifying applications for the material,” Ahadi says. “Our work here not only identifies one attractive quality that sets it apart from other 2D superconductors, but provides a blueprint for how we can create KTaO₃ thin films that are well suited for performing the research necessary to understand intrinsic properties of this materials system.”

The research is covered in three journal articles. Most recently, the paper “Enhanced Critical Field of Superconductivity at an Oxide Interface” was published July 27 in Nano Letters. Co-first authors of that paper are Athby Al-Tawhid, a postdoctoral researcher at NC State; and Samuel Poage, a Ph.D. student at NC State.

A second characterization article, “Anisotropic superconductivity at KTaO₃(111) interfaces,” was published earlier this year in Science Advances. The journal article covering the ability to grow KTaO₃ using molecular beam epitaxy, “Molecular beam epitaxy of KTaO₃,” was published earlier this year in the Journal of Vacuum Science & Technology A.

More information: Athby H. Al-Tawhid et al, Enhanced Critical Field of Superconductivity at an Oxide Interface, Nano Letters (2023). DOI: 10.1021/acs.nanolett.3c01571

Ethan G. Arnault et al, Anisotropic superconductivity at KTaO 3 (111) interfaces, Science Advances (2023). DOI: 10.1126/sciadv.adf1414

Tobias Schwaigert et al, Molecular beam epitaxy of KTaO3, Journal of Vacuum Science & Technology A (2023). DOI: 10.1116/6.0002223

Journal information: Science Advances  , Nano Letters 

Provided by North Carolina State University 

Nearly every material, whether it is solid, liquid, or gas, expands when its temperature goes up and contracts when its temperature goes down. This property, called thermal expansion, makes a hot air balloon float, and the phenomenon has been harnessed to create thermostats that automatically turn a home furnace on and off. Railroads, bridges, and buildings are designed with this property in mind, and they are given room to expand without buckling or breaking on a hot day.

Thermal expansion occurs because a material’s atoms vibrate more as its temperature increases. The more its atoms vibrate, the more they push away from their neighboring atoms. As the space between the atoms increases, the density of the material decreases and its overall size increases.

There are a few exceptions, but by and large, materials conform strictly to this principle. There is, however, a class of metal alloys called Invars (think “invariable”), that stubbornly refuse to change in size and density over a large range of temperatures.

“It’s almost unheard of to find metals that don’t expand,” says Stefan Lohaus, a graduate student in materials science and lead author of the new paper. “But in 1895, a physicist discovered by accident that if you combine iron and nickel, each of which has positive thermal expansion, in a certain proportion, you get this material with very unusual behavior.”

That anomalous behavior makes these alloys useful in applications where extreme precision is required, such as in the manufacture of parts for clocks, telescopes, and other fine instruments. Until now, no one knew why Invars behave this way. In a new paper titled “Thermodynamic explanation of the Invar effect,” published in Nature Physics, researchers from the lab of Brent Fultz, the Barbara and Stanley R. Rawn, Jr., Professor of Materials Science and Applied Physics, say they have figured out the secret to at least one Invar’s steadiness.

For over 150 years, scientists have known that thermal expansion is related to entropy, a central concept in thermodynamics. Entropy is a measure of the disorder, such as positions of atoms, in a system. As temperature increases, so does the entropy of a system. This is universally true, so an Invar’s unusual behavior must be explained through something counteracting that expansion.

Lohaus says it had been long suspected that this behavior was somehow related to magnetism because only certain alloys that are ferromagnetic (capable of being magnetized) behave as invars.

“We decided to look at that because we have this very neat experimental setup that can measure both magnetism and atomic vibrations,” Lohaus says. “It was a perfect system for this.”

Since the magnetic properties of a material are the result of its electrons’ so-called spin state— a quantum measure of angular momentum that can be either “up” or “down”—any magnetic effect counteracting the material’s expected expansion must be due to the activity of its electrons.

The relationship between entropy, thermal expansion, and pressure, known as the “Maxwell relations” is often presented as a textbook curiosity, but the Caltech group found a way to use it to independently measure the thermal expansion caused by magnetism and by atom vibrations. The experiments were done at the Advanced Photon Source, a source of synchrotron X-rays at the Argonne National Laboratory in Illinois, by measuring the vibrational spectra and magnetism of small samples of Invar at pressures within a diamond anvil cell.

The measurements showed a delicate cancelation of the thermal expansion from atom vibrations and from magnetism. Both changed with temperature and pressure, but in a way that maintained their balance. Using a newly developed accurate theoretical approach, collaborators on this work showed how this balance was helped by interactions between vibrations and magnetism, such as where the frequencies of atom vibrations are altered by magnetism. Such coupling between vibrations and magnetism could be useful for understanding thermal expansion in other magnetic materials, as well for developing materials for magnetic refrigeration.

The experimental setup consisted of a diamond anvil cell, which is essentially two precisely ground diamond tips between which samples of materials can be tightly squeezed. In this case, a small piece of Invar alloy was squeezed at a pressure of 200,000 atmospheres. The researchers passed a powerful beam of X-rays through the alloy, and during that process the X-rays interacted with the vibrations (phonons) of its atoms. That interaction changed the amount of energy carried by the X-rays, allowing the researchers to measure how much the atoms were vibrating.

They also placed sensors around the diamond anvil cell that can detect interference patterns created by the spin state of the electrons belonging to the sample’s atoms.

The team used their experimental setup to observe both the atomic vibrations of an Invar sample and the spin state of its electrons as they increased the sample’s temperature. At cooler temperatures, more of the Invar’s electrons shared the same spin state, causing them to move farther apart and push their parent atoms farther apart as well.

As the temperature of the Invar rose, the spin state of some of those electrons increasingly flipped. As a result, the electrons became more comfortable cozying up to their neighboring electrons. Typically, this would cause the Invar to contract as it warmed up. But here, the Invar’s atoms were also vibrating more and taking up more room. The contraction due to changing spin states and the atomic vibration expansion counteracted each other, and the Invar stayed the same size.

“This is exciting because this has been a problem in science for over a hundred years or so,” Lohaus says. “There are literally thousands of publications trying to show how magnetism causes contraction, but there was no holistic explanation of the Invar effect.”

Co-authors are graduate students in materials science Pedro Guzman and Camille M. Bernal-Choban, visitor in applied physics and materials science Claire N. Saunders, Guoyin Shen of the Argonne National Laboratory, Olle Hellman of the Weizmann Institute of Science, David Broido and Matthew Heine of Boston College, and Fultz.

More information: S. H. Lohaus et al, A thermodynamic explanation of the Invar effect, Nature Physics (2023). DOI: 10.1038/s41567-023-02142-z

Journal information: Nature Physics 

Provided by California Institute of Technology 

Researchers at Leipzig University have developed a highly efficient method to investigate systems with long-range interactions that were previously puzzling to experts. These systems can be gases or even solid materials such as magnets whose atoms interact not only with their neighbors but also far beyond.

Professor Wolfhard Janke and his team of researchers use Monte Carlo computer simulations for this purpose. This stochastic process, named after the Monte Carlo casino, generates random system states from which the desired properties of the system can be determined. In this way, Monte Carlo simulations provide deep insights into the physics of phase transitions.

The researchers have developed a new algorithm that can perform these simulations in a matter of days, which would have taken centuries using conventional methods. They have published their new findings in the journal Physical Review X.

A physical system is in equilibrium when its macroscopic properties such as pressure or temperature do not change over time. Nonequilibrium processes occur when environmental changes push a system out of equilibrium and the system then seeks a new state of equilibrium.

“These processes are increasingly becoming the focus of attention for statistical physicists worldwide. While a large number of studies have analyzed numerous aspects of nonequilibrium processes for systems with short-range interactions, we are only just beginning to understand the role of long-range interactions in such processes,” explains Janke.

The curse of long-range interactions

For short-range systems whose components interact only with their short-range neighbors, the number of operations needed to calculate the evolution of the entire system over time increases linearly with the number of components it contains. For long-range interacting systems, the interaction with all other components, even distant ones, must be included for each component. As the size of the system grows, the runtime increases quadratically.

A team of scientists led by Professor Janke has now succeeded in reducing this algorithmic complexity by restructuring the algorithm and using a clever combination of suitable data structures. In the case of large systems, this leads to a massive reduction in the required computing time and allows completely new questions to be investigated.

New horizons opened

The article shows how the new method can be efficiently applied to nonequilibrium processes in systems with long-range interactions. One example describes spontaneous ordering processes in an initially disordered “hot” system, in which following an abrupt temperature drop ordered domains grow with time until an ordered equilibrium state is reached.

From our daily lives, we know that when we take a hot shower and there is a cold window nearby, droplets will form on the window. The hot steam cools down quickly and the droplets get larger. A related example are processes with controlled slower cooling rates, where the formation of vortices and other structures is of particular interest as these play an important role in cosmology and in solid state physics.

In addition, researchers at the Institute of Theoretical Physics have already successfully applied the algorithm to the process of phase separation, in which, for example, two types of particles spontaneously separate. Such nonequilibrium processes play a fundamental role both in industrial applications and in the functioning of cells in biological systems. These examples illustrate the wide range of application scenarios that this methodological advance offers for basic research and practical applications.

Computer simulations form the third pillar of modern physics, alongside experiments and analytical approaches. A large number of issues in physics can only be approached approximately or not at all with analytical methods. With an experimental approach, certain issues are often difficult to access and require complex experimental set-ups, sometimes lasting years. Computer simulations have therefore contributed significantly to the understanding of a broad spectrum of physical systems in recent decades.

More information: Fabio Müller et al, Fast, Hierarchical, and Adaptive Algorithm for Metropolis Monte Carlo Simulations of Long-Range Interacting Systems, Physical Review X (2023). DOI: 10.1103/PhysRevX.13.031006

Journal information: Physical Review X 

Provided by Leipzig University 

A team of physicists affiliated with several institutions in South Korea is claiming to have created the elusive room-temperature/ambient-pressure superconducting material. Their work has not yet been peer reviewed. They have posted two papers on the arXiv preprint server.

Scientists around the world have been trying for more than a century to find a type of material that would conduct electricity without resistance—discovery of such a material would revolutionize the electricity business because it would mean that electricity would no longer be lost to heat dissipation as it moves along power lines. It would also revolutionize the electronics business because engineers would no longer have to worry about heat dissipation causing problems in devices.

In their two papers, the research team describes the new material, which they call LK-99, and how it was created. It was made, they report, by a solid-state reaction between lanarkite (Pb₂SO₅) and copper phosphide (Cu₃P). The reaction, they claim, transformed the mixture into a dark gray, superconductive material.

In their papers, the team claims to have measured samples of LK-99 as electricity was applied and found its sensitivity fell to near zero. They also claim that in testing its magnetism, it exhibited the Meissner effect—another test of superconductivity. In such a test, a sample should levitate when placed on a magnet. The team has provided a video of the material partially levitating. They claim that the levitation was only partial because of impurities in their material.

The papers by the research team have generated much excitement and skepticism in the science community. There have been other instances of researchers claiming to have found room-temperature/ambient-pressure superconductors over the past several years—all have failed to live up to their claims. The researchers on this new effort have responded to such skepticism by suggesting that others repeat their efforts to test their findings.

If their claims turn out to be true, the team in Korea will have made one of the biggest breakthroughs in physics history, no doubt leading to revolutionary changes in electronics and certainly Nobel medals for all those involved.

More information: Sukbae Lee et al, The First Room-Temperature Ambient-Pressure Superconductor, arXiv (2023). DOI: 10.48550/arxiv.2307.12008

Sukbae Lee et al, Superconductor Pb_10-xCu_x(PO₄)_6O showing levitation at room temperature and atmospheric pressure and mechanism, arXiv (2023). DOI: 10.48550/arxiv.2307.12037

Journal information: arXiv 

© 2023 Science X Network

Acoustic emission (AE) monitoring is used to reveal the interaction mechanisms in pulsed laser processing of float glass. Circular ablated pits and irregular shaped cracks are formed on the float glass’s upper surface by the pulsed laser dotting. By analyzing the AE signals, the intensity of laser ablation can be assessed, and the formation of large crack can be extracted.

A recent study proves that it is feasible to apply AE monitoring to study the pulsed laser dotting process of float glass, and provides an alternative for monitoring study of pulsed laser processing of other brittle materials.

Researchers led by Prof. Yu Huang at Huazhong University of Science and Technology (HUST), China, are interested in laser fine processing technology and equipment. Their research focuses on short pulsed/ultrafast laser processing of various hard-to-process materials, such as glass, ceramics, and composite materials.

The work, titled “Revealing the interaction mechanism of pulsed laser processing with the application of acoustic emission,” was published on Frontiers of Optoelectronics on June 14, 2023.

By optimizing the laser machining quality, exploring the interaction mechanism, and monitoring the machining process, researchers have established a more complete stereoscopic research system, further realizing the relevant laser machining equipment manufacturing.

More information: Weinan Liu et al, Revealing the interaction mechanism of pulsed laser processing with the application of acoustic emission, Frontiers of Optoelectronics (2023). DOI: 10.1007/s12200-023-00070-7

Provided by Frontiers Journals

Dr. Carlo Cafaro, SUNY Poly faculty in the Department of Mathematics and Physics, has collaborated with Dr. Paul M. Alsing, Principal Research Physicist at the Air Force Research Laboratory in Rome, NY, on work published in The European Physical Journal Plus.

The tutorial paper, titled, “Bures and Sjöqvist Metrics over Thermal State Manifolds for Spin Qubits and Superconducting Flux Qubits,” in which Cafaro is lead author, is a useful and relatively simple theoretical piece of work. It combines concepts of quantum physics with elements of differential geometry to clarify in simple terms the differences between two important metrics for mixed quantum states of great use in quantum information science.

The interplay among differential geometry, statistical physics, and quantum information science has been increasingly gaining theoretical interest in recent years.

In this paper, Cafaro and Alsing present an explicit analysis of the Bures and Sjöqvist metrics over the manifolds of thermal states for specific spin qubit and the superconducting flux qubit Hamiltonian models. While the two metrics equally reduce to the Fubini-Study metric in the asymptotic limiting case of the inverse temperature approaching infinity for both Hamiltonian models, they observe that the two metrics are generally different when departing from the zero-temperature limit.

Cafaro and Alsing discuss this discrepancy in the case of the superconducting flux Hamiltonian model.

They conclude the two metrics differ in the presence of a non-classical behavior specified by the noncommutativity of neighboring mixed quantum states. Such a noncommutativity, in turn, is quantified by the two metrics in different manners. Finally, Cafaro and Alsing briefly discuss possible observable consequences of this discrepancy between the two metrics when using them to predict critical and/or complex behavior of physical systems of interest in quantum information science.

More information: Carlo Cafaro et al, Bures and Sjöqvist metrics over thermal state manifolds for spin qubits and superconducting flux qubits, The European Physical Journal Plus (2023). DOI: 10.1140/epjp/s13360-023-04267-9

Provided by Colleges of Nanoscale Science and Engineering

The interactions between light and matter are a central research focus in the field of photonics. Resonant cavities with high quality factors (Q) are capable to confine light effectively and exhibit ultra-long radiation lifetimes, making them essential for applications such as lasers, modulators, nonlinear optics, and quantum computing.

Traditional methods for light confinement involve microring resonators, Bragg microcavities, photonic crystals, and so on. Bound states in the continuum (BICs) are unique nonradiative modes that exist within the radiation continuum, above the light line. However, their intrinsic optical fields can still be confined within the structure without leaking into free space, thereby exhibiting an infinite radiative Q.

BICs provide a generalized approach to achieve extremely high-Q resonant cavities, offering a powerful mechanism for enhancing the light-matter interactions. Over the past few decades, BICs have been established in various photonic structures, and the fundamental physical mechanisms have been greatly explored.

In the past few years, abundant articles have reported various applications of BICs in different areas. While there are also several review articles on photonic BICs providing guidance and summarizing progress in recent years, the perspectives of BICs in terahertz photonics were overlooked.

Recently, Prof. Longqing Cong’s team at Southern University of Science and Technology published an online review article entitled “Recent advances and perspective of photonic bound states in the continuum” in the journal Ultrafast Science. This article provides a perspective of BICs on applications in terahertz photonics after summarizing the most recent results and interesting applications of photonic BICs, which will update the literature library in this rapidly developing field.

The review starts by discussing interpretations of BICs from two perspectives, namely, the far-field interference of multipoles and the near-field properties of topological charges. Recent works on manipulating the far-field radiation properties of BICs through engineering topological charges are then highlighted.

Subsequently, the most recent developments in applications are categorized into chiral light and vortex beam generation, harmonics generation, sensors, and lasing. Finally, a comprehensive overview of the current progress of BICs in terahertz regime is summarized, and their potential applications in terahertz generation, detection, modulation, sensing, and isolation are envisioned.

The recent advancements in BICs, both in theory and applications, have profound implications for engineering resonances in photonic devices. As the field of photonics continues to expand in industrial applications, BIC-enabled photonics is expected to remain a highly active research area, driving further progress not only in the classical optical regime but also in quantum photonics.

More information: Guizhen Xu et al, Recent Advances and Perspective of Photonic Bound States in the Continuum, Ultrafast Science (2023). DOI: 10.34133/ultrafastscience.0033

Provided by Ultrafast Science