Category: Physics

For more than 10 years, Guoliang Huang, the Huber and Helen Croft Chair in Engineering at the University of Missouri, has been investigating the unconventional properties of “metamaterials”—an artificial material that exhibits properties not commonly found in nature as defined by Newton’s laws of motion—in his long-term pursuit of designing an ideal metamaterial.

Huang’s goal is to help control the “elastic” energy waves traveling through larger structures—such as an aircraft—without light and small “metastructures.”

“For many years I’ve been working on the challenge of how to use mathematical mechanics to solve engineering problems,” Huang said. “Conventional methods have many limitations, including size and weight. So, I’ve been exploring how we can find an alternative solution using a lightweight material that’s small but can still control the low-frequency vibration coming from a larger structure, like an aircraft.”

Now, Huang’s one step closer to his goal. In a new study published in the Proceedings of the National Academy of Sciences, Huang and colleagues have developed a prototype metamaterial that uses electrical signals to control both the direction and intensity of energy waves passing through a solid material.

Potential applications of his innovative design include military and commercial uses, such as controlling radar waves by directing them to scan a specific area for objects or managing vibration created by air turbulence from an aircraft in flight.

“This metamaterial has odd mass density,” Huang said. “So, the force and acceleration are not going in the same direction, thereby providing us with an unconventional way to customize the design of an object’s structural dynamics, or properties to challenge Newton’s second law.”

This is the first physical realization of odd mass density, Huang said.

“For instance, this metamaterial could be beneficial to monitor the health of civil structures such as bridges and pipelines as active transducers by helping identify any potential damage that might be hard to see with the human eye.”

More information: Qian Wu et al, Active metamaterials for realizing odd mass density, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2209829120

Journal information: Proceedings of the National Academy of Sciences 

Provided by University of Missouri 

Semicrystalline polymers are solids that are assumed to flow only above their melting temperature. In a new study published in Science Advances, Chien-Hua Tu and a research team at the Max Planck Institute for Polymer Research in Germany and the University of Ioannina Greece confined crystals within nanoscopic cylindrical pores to show the flowing nature of semicrystalline polymers below their melting point, alongside an intermediate state of viscosity to the melt and crystal states.

The capillary process was strong during the phenomenon and dragged the polymer chains into the pores without melting the crystal. The unexpected improvement in flow facilitated polymer processing conditions applicable to low temperatures, suited for use in organic electronics.

Crystalline state

About 2,500 years ago, the philosopher Heraclitus proposed that “everything flows,” and while perfect crystals at zero temperature do not flow, crystalline materials do flow under specific conditions. For example, existing research from about 100 years ago showed that the flow of cast iron in the form of flowing metal grains surrounded by a thin amorphous layer is analogous to an undercooled liquid.

Using molecular dynamics simulations, researchers have confirmed the ideas to further suggest the significance of the complex grain boundary “fluid” on plastic deformation. For instance, the inner core of the Earth is similarly proposed to retain iron in a crystalline state. Furthermore, the core of planets such as Neptune and Uranus are composed of superionic crystalline water, and flow to generate their magnetic field, which may have ultimately led to our own existence.

Crystalline materials that exhibit fluid-like mobilities are known as “superionics” and are important for energy applications. Semicrystalline polymers are solids that do not flow under normal conditions. In this work, Tu and colleagues showed how even the semicrystalline polymers underwent flow. To examine the phenomenon, they used two semicrystalline polymers; poly (ethylene oxide) and poly(ε-caprolactone) with specific molecular characteristics. The materials scientists developed self-ordered nanoporous alumina templates for the study, based on existing literature protocols.

Materials characterization

The scientists examined the thermodynamics, structural and rheological properties of bulk polyethylene oxide materials. And the data confirmed the material film on the alumina template to be in a semicrystalline state. The team observed the domain spacing organization of crystalline lamellae with small-angle X-ray scattering. They used polarizing optical microscopy to study the superstructure of bulk polyethylene oxide with a film slowly cooled from melt to ambient temperature. The outcomes indicated a single spherulitic superstructure for polyethylene oxide, while the structural dynamics of poly(ε-caprolactone) synthesized with a catalyst differed.

The research team carried out a 28-day imbibition (uptake of water that leads to swelling of materials) of the two polymer materials within anodic aluminum oxide templates and observed the samples with scanning electron microscopy and atomic force microscopy to characterize them. In contrast to the relatively smooth appearance of polyethylene oxide, the poly(ε-caprolactone) materials showed abundant grain structures due to diverse morphological origins in intracrystalline diffusion. After studying the surface appearances of materials, the researchers performed nano-infrared microscopy to obtain additional images of the surface topography of the two materials. The outcomes clearly showed the semicrystalline nature of polyethylene oxide. They also addressed the possibility of the capillary force in the experimental setup to be sufficiently high to melt the crystals during flow and noted the viscosity of semicrystalline polymers to be reduced during the experiments.

Mechanisms of imbibition

The mechanisms of fluid uptake and materials swelling known as imbibition from the semicrystalline state relied on the dynamics of its crystalline and amorphous domains. Four processes acted on the amorphous and crystalline regions; segmental relaxation governed the dynamics in the amorphous domain, whereas three other processes affected the crystalline domain to demonstrate intracrystalline chain diffusion for crystal-mobile polymers such as polyethylene oxide.

Since the imbibition of crystals also involved the diffusion of entire crystallites, Tu and the team examined the influence of the molar mass of the polymers on the process of imbibition. The outcomes showed that the molar mass regulated the imbibition speed.

Outlook

In this way, Chien-Hua Tu and colleagues used several imaging methods in materials science, such as scanning electron microscopy, atomic force microscopy, and nano-infrared results to examine how semicrystalline polymers underwent flow within nanopores made of anodic aluminum oxide via capillary action. They measured the viscoelastic behavior of the polymers with a shear rheometer, and the capillary action appeared to drive the polymer adsorption process.

While successful imbibition was a relatively slow process, the capillary force was strong enough to drag polymer crystallites into the nanopores without melting the crystals. The unexpected increase in flow while preserving the polymer crystallites applied to polymer processing at low temperatures. Such phenomenon can lead to cold flow and subsequent bonding of polymers to ceramics or metal under specific conditions to prevent polymer degradation. Such semicrystalline polymers and ferroelectric materials have a variety of applications in organic electronics to affect their electronic and physical properties.

More information: Chien-Hua Tu et al, When crystals flow, Science Advances (2023). DOI: 10.1126/sciadv.adg8865

Journal information: Science Advances 

© 2023 Science X Network

Both the geometric and spectral information of the target are available with the hyperspectral light detection and ranging (LiDAR) in a single measurement. This advanced technique extends the scope of imaging spectroscopy to spectral three-dimensional (3D) sensing.

However, two major geometric radiative effects exist in hyperspectral LiDAR, namely, the distance effect and incidence angle effect, which seriously restrict its quantitative remote sensing applications.

A research team led by Prof. Niu Zheng from the Aerospace Information Research Institute (AIR), Chinese Academy of Sciences (CAS) has proposed correction algorithms while studying the geometric radiative effects existing in hyperspectral LiDAR.

The researchers discovered that the analysis and correction of the distance effect and incidence angle effect can be carried out independently. They proposed a piecewise function model that couples a quadratic function and an exponential decay function to analyze and correct distance effect and developed an improved Poullain algorithm to analyze and correct the incidence angle effect.

The results were published in ISPRS Journal of Photogrammetry and Remote Sensing and IEEE Transactions on Geoscience and Remote Sensing.

They found that distance effect originates from the system itself, and all wavelengths have a unified distance effect function. Based on this, they proposed a piecewise function model that couples a quadratic function and an exponential decay function to analyze and correct distance effect.

For different types of vegetation leaf targets, they usually exhibit different incidence angle effects due to their different surface microscopic physical structures and internal biochemical parameters. This effect is closely related to the bidirectional reflection characteristics of the measured target species under hyperspectral LiDAR conditions.

Therefore, the team pointed out that the more accurate expression of incidence angle effect of hyperspectral LiDAR should be “the incidence angle effect of a certain target under hyperspectral LiDAR.” They developed a new improved Poullain algorithm to correct the incidence angle effect of the target.

Compared with the traditional Lambert cosine law based on the assumption of isotropic scattering and the original Poullain algorithm, this algorithm considers the heterogeneity of the target roughness factor and diffuse reflection coefficient under different incidence angles and wavelengths, which is more in line with the reflection characteristics of natural target echoes.

The results of different vegetation leaf experiments showed that the standard deviations of the correction results were reduced by 30% to 60%, compared with the echo intensity and reflectivity under the standard 0-degree incidence angle.

The algorithm provides an important theoretical basis and technical support for accurate inversion of 3D biochemical parameters of vegetation in the future.

Currently, the research team has completed the design and development of the second-generation hyperspectral LiDAR system with high-speed acquisition capability, which is undergoing performance testing and is expected to be put into use by the end of 2023.

More information: Jie Bai et al, A Novel Algorithm for Leaf Incidence Angle Effect Correction of Hyperspectral LiDAR, IEEE Transactions on Geoscience and Remote Sensing (2021). DOI: 10.1109/TGRS.2021.3070652

Jie Bai et al, An exploration, analysis, and correction of the distance effect on terrestrial hyperspectral LiDAR data, ISPRS Journal of Photogrammetry and Remote Sensing (2023). DOI: 10.1016/j.isprsjprs.2023.03.001

Provided by Chinese Academy of Sciences 

How far along are the quantum technologies? And what do we really mean when we use the word quantum? Senior Adviser Ulrich Busk Hoff has been conducting research into and communicating about quantum physics for several years. Here he provides an overview of a rapidly developing field.

What is quantum technology?

It is the field of technologies in which quantum physics is actively utilized. That is technologies that are only possible using quantum physical phenomena. These are typically phenomena such as superposition, where an object, such as an atom, can assume more than one value or be in more than one place at the same time, and entanglement, where an object, such as an atom, is physically separated from another atom, but where they are nevertheless connected so that an impact of one atom will also affect the other.

Quantum physics is more than 100 years old, so what’s new?

The ideas are indeed old—from the time of Niels Bohr and Einstein—but we have now reached a stage where the theories have been demonstrated and we can begin to exploit them in practice. We can do this because today we can control quantum physical systems such as atoms, electrons, or photons in such a way that they can be used for technological solutions like encryption, sensors, and computers—although most of it is still only possible in laboratories.

Which quantum technology is most mature?

We are quite advanced in encryption and sensors. In the past year, DTU has participated in several demonstration experiments with quantum-encrypted data being sent between two geographical locations.

In quantum sensors, there are already many different types that can measure physical quantities with extreme precision. Some can measure tiny variations in the gravitational field. This can, for example, be utilized in construction works for subsoil mapping before construction, or to predict earthquakes. Other sensors measure magnetic fields from, for example, muscle activity and nerve paths and have great potential in fields such as medical diagnosis. Magnetic field sensors can also be used for military purposes like navigation.

The quantum computer is probably the most immature technology.

Why is there so much hype about the quantum computer?

There are several reasons. What started the hype, and is in some way still driving it, is an algorithm for quantum computers that Peter Shor, an American mathematician and professor at MIT, developed in 1994. Shor’s algorithm makes it possible for a powerful quantum computer to break RSA encryption. This is the encryption that is widely used when we send data on the Internet.

But there is also a clear assumption that quantum computers will eventually be able to handle many other calculations that are impossible using an ordinary computer. Therefore, a huge market potential is predicted for quantum computers. In other words, there is a lot of money to be made for those who can realize the quantum computer.

And the hype is also partly due to the quantum computer being so difficult to develop and to its exploitation of quantum phenomena that are incomprehensible to many. The fascination with the technology itself contributes to the hype.

When will we have quantum computers?

We are still very far from having a fully developed quantum computer. It is still unclear which physical system will constitute the quantum bits that can be utilized quantum-mechanically inside the quantum computer. Some are testing photons, other atoms, or ions—and yet others electrons in superconducting material. In some places, mechanical oscillations are used. Research and development is being conducted within all these platforms worldwide.

Calculations show that it takes a quantum computer of 10–20 million quantum bits to break an RSA encryption. Right now, the largest quantum computer is in the region of 430 quantum bits. So there is still some way to go. So, at the risk of becoming a laughing stock for posterity, I would guess that it will take another 20 years before we have a quantum computer that meets these expectations.

Will I get a quantum computer at home?

I will answer this based on where the technology is today and what we think a quantum computer will be good at—and with this in mind—my guess would be that quantum computers will not be something we will have at home. It will be meant for very specific and large-scale calculations. It will not be a computer we can use to go on Facebook or watch YouTube videos.

I would think that the quantum computer will have a role like HPC (high performance computing), which you can buy access to today if you need to perform large-scale calculations. But I could be completely wrong. Technology often develops completely differently than we predict, so we may all be walking around with a quantum computer in our pockets in 30 years.

Do you need to be interested in quantum physics as a non-physicist?

Quantum phenomena such as superposition and entanglement are a highly fascinating part of nature that can cause wonder and inspire completely new thoughts in us as humans. Instead of racing through the world with blinkers on, it makes us stop up and recognize that there is much more to nature than what immediately meets the eye. It is like the firmament. We could basically be indifferent to black holes and “dark matter,” but I think that it is about fascination, general education, and being aware of what nature actually contains.

Right now, we do not encounter the applications of quantum technology in our everyday lives. This means that when we talk about them, we have to talk about quantum phenomena. They challenge us, because we have no experience with, for example, superposition in our visible world, which is governed by conventional physics, where objects such as a chair can only be in one place in the room.

But I think that when we start using quantum technology, we will stop focusing on the underlying quantum phenomena. This is, in fact, the case with other technologies such as PCs and mobile phones. Most of us do not think about how they work. But we do know how to use them.

Provided by Technical University of Denmark 

Chiral spin liquids are one of the most fascinating phases of matter ever imagined by physicists. These exotic liquids exhibit quasi-particles known as non-Abelian anyons that are neither bosons nor fermions, and whose manipulation could allow for the realization of a universal quantum computer. Despite intense efforts in condensed matter physics, discovering such a phase in nature remains an outstanding challenge at the forefront of modern research.

From a theoretical point of view, chiral spin liquids emerge in a simple model that was imagined by Kitaev in 2006, and which allows researchers to reveal their properties using analytical tools. Remarkably, recent advances in the design of quantum simulators open a possible path for the first experimental realization of the original Kitaev model, hence suggesting that chiral spin liquids (including their exotic quasi-particles) can be studied and manipulated in a highly-controlled experimental environment.

In a new study published in PRX Quantum, BoYe Sun and Nathan Goldman (ULB, Brussels), Monika Aidelsburger (LMU, Munich), and Marin Bukov (MPI-PKS Dresden, Sofia University) propose a realistic implementation of the Kitaev model in quantum simulators.

Based on a precise pulse sequence, their system is shown to host a chiral spin liquid with non-Abelian anyons. The authors describe practical methods to probe the striking properties of these exotic states. In particular, their methods unambiguously reveal the topological heat current that flows on the edge of the system: a hallmark signature of the non-Abelian anyons that emerge on the edge of chiral spin liquids.

This work paves the way for the quantum simulation of chiral spin liquids, offering an appealing alternative to their experimental investigation in quantum materials.

More information: Bo-Ye Sun et al, Engineering and Probing Non-Abelian Chiral Spin Liquids Using Periodically Driven Ultracold Atoms, PRX Quantum (2023). DOI: 10.1103/PRXQuantum.4.020329

Journal information: PRX Quantum 

Provided by Université libre de Bruxelles 

Researchers from QuTech improved the so-called “Andreev spin qubit” in a critical way and believe it can become a prime candidate in the pursuit of a perfect qubit. The new type of qubit is created in a more reliable and intrinsically stable way, compared to previous versions, by combining the advantages of two other types of qubits. The team has published their work in Nature Physics.

Unlike the world of conventional computers, where bits are based on very well established and reliable technologies, the perfect qubit has not been invented yet. Will the quantum computer of the future contain qubits that are based on superconducting transmon qubits, spin qubits in silicon, NV centers in diamond, or perhaps some other quantum phenomenon? Each type of qubit has their own advantages—and disadvantages. One is more stable, the second has a higher fidelity, and others are more easily mass-produced. The perfect qubit does not exist. Yet.

Best of both worlds

In this work, the researchers from QuTech—a collaboration between the Delft University of Technology and TNO—and together with international collaborators made a smart combination of existing techniques to store quantum information.

Marta Pita-Vidal, co-first author explains, “Two of the most promising types are spin qubits in semiconductors and transmon qubits in superconducting circuits. However, each type has its own challenges. For example, spin qubits are small and compatible with current industrial technology, but they struggle with interacting over long distances. On the other hand, transmon qubits can be controlled and read out efficiently over long distances, but they have a built-in speed limit for operations and are relatively large. The researchers in this study aim to harness the advantages of both types of qubits by developing a hybrid architecture that combines them.”

Andreev spin qubits

“In our experiment, we managed to directly manipulate the spin of the qubit using a microwave signal,” says Arno Bargerbos, the other co-first author. “We achieved very high ‘Rabi frequencies,’ which is a measure of how fast they can control the qubit. Next, they embedded this ‘Andreev spin qubit’ within a superconducting transmon qubit which enables fast measurement of the qubit state.”

The researchers characterized the coherence time of the Andreev spin qubit, a measure of how long the qubit can stay alive. They observed that its “longevity” is affected by the magnetic field from the surrounding materials.

“Finally,” says Bargerbos, “we demonstrated the first direct strong coupling between a spin qubit and a superconducting qubit, meaning that they could get the two qubits to interact in a controlled way. This suggests that the Andreev spin qubit can become a key element to interconnect quantum processors based on radically different qubit technologies: semiconducting spin qubits and superconducting qubits.”

Principal investigator Christian Andersen says, “The current Andreev spin qubit is not perfect yet. It still needs to demonstrate multi-qubit operations, which is needed for universal quantum computers. The coherence time is also sub-optimal. That can be improved by using another material. Fortunately, the scalability of the qubits is on par with semiconductor qubits, raising the hope that we can get to the point where making the quantum algorithms becomes the limiting factor and not the quantum hardware.”

More information: Marta Pita-Vidal, Direct manipulation of a superconducting spin qubit strongly coupled to a transmon qubit, Nature Physics (2023). DOI: 10.1038/s41567-023-02071-x. www.nature.com/articles/s41567-023-02071-x

Journal information: Nature Physics 

Provided by QuTech 

Did you know that wires can be used to ionize air to make a loudspeaker? Simply put, it’s possible to generate sound by creating an electric field in a set of parallel wires, aka a plasma transducer, strong enough to ionize the air particles. The charged ions are then accelerated along the magnetic field lines, pushing the residual non-ionized air in a way to produce sound.

If a loudspeaker can generate sound, it can also absorb it.

While this plasma loudspeaker concept is not new, EPFL scientists went ahead and built a demonstration of the plasma transducer, with the aim to study noise reduction. They came up with a new concept, what they call the active “plasmacoustic metalayer” that can be controlled to cancel out noise. Their results are published in Nature Communications.

The scientists were intrigued by the idea of using plasma to reduce noise, since it gets rid of one of the most important aspects of conventional loudspeakers: the membrane. Loudspeakers equipped with membranes, like the ones in your car or at home, are some of the most studied solution for active noise reduction. It’s active because the membrane can be controlled to cancel out different sounds, as opposed to a wall that does the job passively.

The problem with using the conventional loudspeaker as a sound absorber is that its membrane limits the frequency range of operation. For sound absorption, the membrane behaves mechanically, vibrating to cancel out the sound waves in the air. The fact that the membrane is relatively heavy, i.e. the inertia of the membrane, limits its ability to interact efficiently with fast changing sounds or at high frequencies.

“We wanted to reduce the effect of the membrane as much as possible, since it’s heavy. But what can be as light as air? The air itself,” explains Stanislav Sergeev, postdoc at EPFL’s Acoustic Group and first author. “We first ionize the thin layer of air between the electrodes that we call a plasmacoustic metalayer. The same air particles, now electrically charged, can instantaneously respond to external electrical field commands and effectively interact with sound vibrations in the air around the device to cancel them out.”

Sergeev continues, “As expected, the communication between the electrical control system of the plasma and the acoustic environment is much faster than with a membrane.”

Not only is the plasma efficient at high frequencies, but it is also versatile since it can be tuned to work at low frequencies as well. Indeed, the scientists show that the dynamics of thin layers of air plasma can be controlled to interact with sound over deep-subwavelength distances, to actively respond to noise and cancel it out over a broad bandwidth. The fact that their device is active is key, since passive noise reduction technologies are limited in the band of frequencies that can be controlled.

The plasma absorber is also more compact that most conventional solutions. Exploiting the unique physics of plasmacoustic metalayers, the scientists experimentally demonstrate perfect sound absorption: “100% of the incoming sound intensity is absorbed by the metalayer and nothing is reflected back,” says EPFL’s Acoustic Group’s senior scientist Hervé Lissek. They also show tunable acoustic reflection from several Hz to the kHz range, with transparent plasma layers of thicknesses down to only a thousandth of a given wavelength, much smaller than conventional noise reduction solutions.

To give an idea of how much more compact the plasma absorber is, consider a low, audible sound frequency of 20 Hz, where the sonic wavelength is 17m meters long. The plasma layer would only need to be 17 mm thick to absorb the noise, whereas most conventional noise reduction solutions, like absorbing walls, would need to be at least 4 m thick which often limits its feasibility.

“The most fantastic aspect in this concept is that, unlike conventional sound absorbers relying on porous bulk materials or resonant structures, our concept is somehow ethereal. We have unveiled a completely new mechanism of sound absorption, that can be made as thin and light as possible, opening new frontiers in terms of noise control where space and weight matter, especially at low frequencies,” says Hervé Lissek.

EPFL has partnered with Sonexos SA, a Swiss-based audio technology company, to develop cutting-edge active sound absorbers that use the plasmacoustic metalayer concept. Together, they aim to provide novel and efficient solutions for reducing noise in a wide range of applications, including the automotive, consumer, commercial, and industrial sectors.

“This strategic collaboration leverages EPFL’s expertise in material science and acoustics, as well as Sonexos’ proven track record in delivering high-performance audio solutions,” explains Mark Donaldson, CEO and Founder of Sonexos.

More information: Stanislav Sergeev et al, Ultrabroadband sound control with deep-subwavelength plasmacoustic metalayers, Nature Communications (2023). DOI: 10.1038/s41467-023-38522-5

Journal information: Nature Communications 

Provided by Ecole Polytechnique Federale de Lausanne 

Researchers in Singapore have developed the world’s smallest LED (light-emitting diode) that enables the conversion of existing mobile phone cameras into high-resolution microscopes. Smaller than the wavelength of light, the new LED was used to build the world’s smallest holographic microscope, paving the way for existing cameras in everyday devices such as mobile phones to be converted into microscopes via only modifications to the silicon chip and software. This technology also represents a significant step forward in the miniaturization of diagnostics for indoor farmers and sustainable agriculture.

This breakthrough was supplemented by the researchers’ development of a revolutionary neural networking algorithm that is able to reconstruct objects measured by the holographic microscope, thus enabling enhanced examination of microscopic objects such as cells and bacteria without the need for bulky conventional microscopes or additional optics. The research also paves the way for a major advancement in photonics—the building of a powerful on-chip emitter that is smaller than a micrometer, which has long been a challenge in the field.

The light in most photonic chips originates from off-chip sources, which leads to low overall energy efficiency and fundamentally limits the scalability of these chips. To address this issue, researchers have developed on-chip emitters using various materials such as rare-earth-doped glass, Ge-on-Si, and heterogeneously integrated III–V materials. While emitters based on these materials have shown promising device performance, integrating their fabrication processes into standard complementary metal-oxide-semiconductor (CMOS) platforms remains challenging.

While silicon (Si) has shown potential as a candidate material for nanoscale and individually controllable emitters, Si emitters suffer from low quantum efficiency because of the indirect bandgap, and this fundamental disadvantage combined with the limitations set by the available materials and fabrication tools has hindered the realization of a small native Si emitter in CMOS.

In a recently published Nature Communications paper titled “A sub-wavelength Si LED integrated in a CMOS platform,” SMART researchers described their development of the smallest reported Si emitter with a light intensity comparable to that of state-of-the-art Si emitters with much larger emission areas. In a related breakthrough, SMART researchers also unveiled their construction of a novel, untrained deep neural network architecture capable of reconstructing images from a holographic microscope in a paper titled “Simultaneous spectral recovery and CMOS micro-LED holography with an untrained deep neural network” recently published in the journal Optica.

The novel LED developed by SMART researchers is a CMOS-integrated sub-wavelength scale LED at room temperature exhibiting high spatial intensity (102 ± 48 mW/cm²) and possessing the smallest emission area (0.09 ± 0.04 μm²) among all known Si emitters in scientific literature. In order to demonstrate a potential practical application, the researchers then integrated this LED into an in-line, centimeter-scale, all-silicon holographic microscope requiring no lens or pinhole, integral to a field known as lensless holography.

A commonly faced obstacle in lensless holography is computational reconstruction of the imaged object. Traditional reconstruction methods require detailed knowledge of the experimental setup for accurate reconstruction and are sensitive to difficult-to-control variables such as optical aberrations, the presence of noise, and the twin image problem.

The research team also developed a deep neural network architecture to improve the quality of image reconstruction. This novel, untrained deep neural network incorporates total variation regularization for increased contrast and takes into account the wide spectral bandwidth of the source.

Unlike traditional methods of computational reconstruction that require training data, this neural network eliminates the need for training by embedding a physics model within the algorithm. In addition to holographic image reconstruction, the neutral network also offers blind source spectrum recovery from a single diffracted intensity pattern, which marks a groundbreaking departure from all previous supervised learning techniques.

The untrained neural network demonstrated in this study allows researchers to use novel light sources without prior knowledge of the source spectrum or beam profile, such as the novel and smallest known Si LED described above, fabricated via a fully commercial, unmodified bulk CMOS microelectronics.

The researchers envision that this synergetic combination of CMOS micro-LEDs and the neural network can be used in other computational imaging applications, such as a compact microscope for live-cell tracking or spectroscopic imaging of biological tissues such as living plants. This work also demonstrates the feasibility of next-generation on-chip imaging systems. Already, in-line holography microscopes have been employed for a variety of applications, including particle tracking, environmental monitoring, biological sample imaging, and metrology. Further applications include arraying these LEDs in CMOS to generate programmable coherent illumination for more complex systems in the future.

Iksung Kang, lead author of the Optica paper and research assistant at MIT at the time of this research, said, “Our breakthrough represents a proof of concept that could be hugely impactful for numerous applications requiring the use of micro-LEDs. For instance, this LED could be combined into an array for higher levels of illumination needed for larger-scale applications. In addition, due to the low cost and scalability of microelectronics CMOS processes, this can be done without increasing the system’s complexity, cost, or form factor. This enables us to convert, with relative ease, a mobile phone camera into a holographic microscope of this type. Furthermore, control electronics and even the imager could be integrated into the same chip by exploiting the available electronics in the process, thus creating an all-in-one micro-LED that could be transformative for the field.”

“On top of its immense potential in lensless holography, our new LED has a wide range of other possible applications. Because its wavelength is within the minimum absorption window of biological tissues, together with its high intensity and nanoscale emission area, our LED could be ideal for bio-imaging and bio-sensing applications, including near-field microscopy and implantable CMOS devices,” added Rajeev Ram, principal investigator at SMART CAMP and DiSTAP, Professor of Electrical Engineering at MIT and co-author of both papers. “Also, it is possible to integrate this LED with on-chip photodetectors, and it could then find further applications in on-chip communication, NIR proximity sensing, and on-wafer testing of photonics.”

More information: Zheng Li et al, A sub-wavelength Si LED integrated in a CMOS platform, Nature Communications (2023). DOI: 10.1038/s41467-023-36639-1

Iksung Kang et al, Simultaneous spectral recovery and CMOS micro-LED holography with an untrained deep neural network, Optica (2022). DOI: 10.1364/OPTICA.470712

Journal information: Nature Communications  , Optica 

Provided by Singapore-MIT Alliance for Research and Technology 

For the first time, researchers have demonstrated a prototype lidar system that uses quantum detection technology to acquire 3D images while submerged underwater. The high sensitivity of this system could allow it to capture detailed information even in extremely low-light conditions found underwater.

“This technology could be useful for a wide range of applications,” said research team member Aurora Maccarone, a Royal Academy of Engineering research fellow from Heriot-Watt University in the United Kingdom. “For example, it could be used to inspect underwater installations, such as underwater wind farm cables and the submerged structure of the turbines. Underwater lidar can also be used for monitoring or surveying submerged archaeology sites and for security and defense applications.”

Obtaining 3D images through ocean water can be challenging because it is light-limited, and any particles in the water will scatter light and distort the image. However, single-photon detection, which is a quantum-based technique, allows very high penetration and works even in low-light conditions.

In Optics Express, researchers from Heriot-Watt University and the University of Edinburgh describe experiments in which an entire single-photon lidar system was submerged in a large water tank. The new demonstrations bring the technology closer to practical applications compared to the research team’s earlier experiments with underwater single-photon detection, which were performed in carefully controlled laboratory conditions with the optical setup placed outside the water tank and data analysis performed offline.

They also implemented new hardware and software developments that allow the 3D images acquired by the system to be reconstructed in real time.

“This work aims to make quantum detection technologies available for underwater applications, which means that we will be able to image the scene of interest in very low light conditions,” said Maccarone. “This will impact the use of offshore cable and energy installations, which are used by everyone. This technology could also allow monitoring without the presence of humans, which would mean less pollution and a less invasive presence in the marine environment.”

Faster low-light detection

Lidar systems create images by measuring how long it takes laser light to be reflected from objects in the scene and travel back to the system’s receiver, known as the “time of flight.” In the new work, the researchers sought to develop a way to acquire 3D images of targets that are obscured by turbid water and thus not visible to conventional lidar imaging systems.

They designed a lidar system that uses a green pulsed laser source to illuminate the scene of interest. The reflected pulsed illumination is detected by an array of single-photon detectors, which allows ultrafast low light detection and greatly reduces measurement time in photon-starved environments such as highly attenuating water.

“By taking time-of-flight measurements with picosecond timing resolution, we can routinely resolve millimeter details of the targets in the scene,” said Maccarone. “Our approach also allows us to distinguish the photons reflected by the target from those reflected by particles in the water, making it particularly suitable to performing 3D imaging in highly turbid waters where optical scattering can ruin image contrast and resolution.”

The fact that this approach requires thousands of single-photon detectors, all producing many hundreds of events per second, makes it extremely challenging to retrieve and process the data necessary to reconstruct the 3D image in a short time, especially for real-time applications. To solve this problem, the researchers developed algorithms specifically for imaging in highly scattering conditions and applied them in conjunction with widely available graphics processing unit (GPU) hardware.

The new technique builds on some important technological advances. “Heriot-Watt University has a long track record in single‑photon detection techniques and image processing of single-photon data, which allowed us to demonstrate advanced single‑photon imaging in extremely challenging conditions,” said Maccarone.

“The University of Edinburgh has achieved fundamental advances in the design and fabrication of single-photon avalanche diode detector arrays, which allowed us to build compact and robust imaging systems based on quantum detection technologies.”

Underwater testing

After optimizing the optical setup on a laboratory optical bench, the researchers connected the lidar system to a GPU to achieve real-time processing of the data while also implementing a number of image processing approaches for three-dimensional imaging. Once the system was working properly, they moved it to a tank that was 4 meters long, 3 meters wide, and 2 meters deep.

With the system submerged in the water, the researchers added a scattering agent in a controlled manner to make the water more turbid. Experiments at three different turbidity levels demonstrated successful imaging in controlled highly scattering scenarios at distances of 3 meters.

“Single-photon technologies are rapidly developing, and we have demonstrated very promising results in underwater environments,” said Maccarone. “The approach and image processing algorithms could also be used in a wider range of scenarios for improved vision in free space such as in fog, smoke or other obscurants.”

The researchers are now working to reduce the size of the system so that it could be integrated into an underwater vehicle. Through the UK Quantum Technology Hub Network and InnovateUK, the researchers are partnering with industry to make the technology accessible for a range of underwater applications.

More information: Aurora Maccarone et al, Submerged single-photon LiDAR imaging sensor used for real-time 3D scene reconstruction in scattering underwater environments, Optics Express (2023). DOI: 10.1364/OE.487129

Journal information: Optics Express 

Provided by Optica 

Alexandrite laser crystals are well suited for use in Earth observation satellites. They are robust and enable laser systems with a tunable output wavelength. In the European Horizon 2020 project GALACTIC, the partners Laser Zentrum Hannover e.V. (LZH), Optomaterials S.r.l. (Italy) and Altechna (Lithuania) have now succeeded in establishing a solely European supply chain for alexandrite laser crystals, which can be used for space applications.

The Italian partner Optomaterials produces competitive crystals, which the Lithuanian company Altechna provides with a special coating. To achieve the necessary properties for space-applications, Altechna has developed special coating designs and processes based on ion beam and magnetron sputtering processes.

Proven: Space suitability

Scientists at the LZH thoroughly tested the crystals in special laser systems. They have designed these systems with future applications in mind. The demonstrators could lay the foundation for new laser-based measuring instruments. The LZH’s scientists exposed the Alexandrite crystals to proton and gamma radiation and ran them through several temperature cycles typical for space applications.

Before and after these environmental tests, they have characterized the crystals in terms of their transmission properties and laser performance, among other things. Since the environmental tests did not significantly change the measured parameters, the space suitability could be demonstrated. In addition, the researchers showed that the laser-induced damage threshold (LIDT) of the crystals is equal to—or even exceeds—top products on the world market.

The GALACTIC team has thus successfully raised the Technology Readiness Level (TRL) of space-qualified alexandrite crystals from Europe from 4 to 6 and have made the crystals ready for the market.

Special properties for more precise data

Alexandrite crystals have very good thermal conductivity and fracture strength. Therefore, they are suitable for use in high power laser systems and are robust enough to withstand the harsh conditions in space.

Since the crystals can be used to tune the output wavelength of laser systems, they could form the basis of new types of laser-based measuring instruments for Earth observation satellites. Such instruments could be used to collect more precise climate-relevant data on the state of the atmosphere or vegetation.

The findings are published in the journal Optics Express.

More information: S. Unland et al, High-performance cavity-dumped Q-switched Alexandrite laser CW diode-pumped in double-pass configuration, Optics Express (2022). DOI: 10.1364/OE.478628

Journal information: Optics Express 

Provided by Laser Zentrum Hannover e.V.