Category: Physics

by Advanced Devices & Instrumentation

A single-photon detector (SPD) is sensitive to incidence of individual quanta of light and has many applications in photonics, such as fluorescence measurements, laser ranging, optical time-domain reflectometer, and quantum optics experiments.

Near-infrared SPDs at the telecommunication wavelength of 1550 nm are indispensable for fiber-optic QKD, with choices including cryogenic superconducting nanowire single-photon detectors (SNSPD) and electrically-cooled InGaAs avalanche photodiodes (APDs). Between them, APDs have practical advantages for compactness, low cost, and not requiring ultra-low temperature refrigeration.

Under Geiger mode, APD’s strong capacitive response to sub-nanosecond gating must be rejected through purpose-designed readout circuit so as to enable detection of weak photon-induced avalanches. Rapid gating and readout circuits add challenges to modularization and miniaturization, which is a necessary step to serve a wide range of applications.

A research group has recently developed a novel readout circuit that incorporates a surface acoustic wave (SAW) filter into an asymmetric radio-frequency Mach-Zehnder interferometer, referred to as ultra-narrowband interference circuit (UNIC), and realized exceptional performance for narrow-band rejection of the SPD capacitive response. The work is published in the journal Advanced Devices & Instrumentation.

Thanks to the long group delay of the SAW filter, the UNIC interferometer can produce an ultra-narrrow band rejection with a manufacturing tolerance easily achievable in the RF track lengths.

The UNIC can provide a wide and continuous pass band in the frequency domain and therefore brings little distortion into the avalanche signal. The team reports their development of a standalone InGaAs SPD module that fully integrates driving and readout electronics as well as temperature regulation and compensation.

Its dimension is measured just 8.8×6×2 cm³ and is nearly a factor of 4 smaller in volume than the most compact existing detector module that uses a monolithically integrated readout circuit. Simultaneously, this size reduction does not bring performance deterioration.

The research team uses their previous UNIC techniques for the APD signal readout, but adds an automatic temperature compensation to ensure an optimal performance over a wide ambient temperature range.

With a 1.25 GHz clock input, the module is characterized to have comparable performance to its counterpart built with benchtop equipment. The UNIC-SPD exhibits excellent performance with a net detection efficiency of 30% at an afterpulsing probability of 2.4 % under 3 ns hold-off time. The compact size and state-of-the-art performance allow the UNIC-SPD module a huge potential for single-photon imaging and high-speed quantum key distribution.

More information: Haoyang Wang et al, Progress on Chip-Based Spontaneous Four-Wave Mixing Quantum Light Sources, Advanced Devices & Instrumentation (2023). DOI: 10.34133/adi.0032

by Bob Yirka , Phys.org

Five independent teams of researchers have reviewed the work and claims made by a group at the National Ignition Facility (NIF) who announced in December 2022 that they had achieved the first laser-powered fusion reaction that exceeded “scientific breakeven”—in which more energy was produced by a manmade fusion reaction than consumed by the reaction.

All five teams have confirmed their claims. Three of the teams published their findings and conclusions in the journal Physical Review Letters; the other two teams published papers in the journal Physical Review E.

After many years of effort by multiple teams around the globe, the teams have confirmed that it should be possible to use fusion as a power source. The feat heralds a new era in nuclear fusion research—and possibly power generation.

At its most basic level, nuclear fusion is simple—when light elements are fused into heavier elements, a reaction results in the release of energy. Such reactions account for the energy emitted by stars, including the sun. Prior research has shown that recreating such reactions in a lab setting requires a different environment than that found in stars—higher temperatures are needed, which means using a lot of energy.

That has led to the goal of finding a way to generate fusion reactions that produce more power than is needed to produce them. To achieve that goal, the team at NIF fired lasers at a capsule containing two types of heavy hydrogen. This resulted in the release of X-rays that inundated the fuel, inciting the fusion process. In their groundbreaking experiment, the team at NIF used 2.05 megajoules of energy to power the lasers, and measured 3.15 megajoules of energy from the fusion reaction.

In their reviews, some of the teams conducting an analysis of the experiments note that while the team at NIF has achieved a monumental breakthrough, there is still a lot of work to be done before fusion can be used as a power source. Physicists need to scale up the technique, for example, and the yield needs to be much greater to justify its use in a commercial setting.

But they also found reasons for optimism—they found, for example, that during the experiment, the material in the capsule was unexpectedly reheated due to energy from the fusion reaction to energies higher than that provided by the lasers.

More information: H. Abu-Shawareb et al, Achievement of Target Gain Larger than Unity in an Inertial Fusion Experiment, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.065102

A. L. Kritcher et al, Design of the first fusion experiment to achieve target energy gain G>1, Physical Review E (2024). DOI: 10.1103/PhysRevE.109.025204

O. A. Hurricane et al, Energy Principles of Scientific Breakeven in an Inertial Fusion Experiment, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.065103

A. Pak et al, Observations and properties of the first laboratory fusion experiment to exceed a target gain of unity, Physical Review E (2024). DOI: 10.1103/PhysRevE.109.025203

M. S. Rubery et al, Hohlraum Reheating from Burning NIF Implosions, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.065104

Journal information: Physical Review Letters  , Physical Review E 

© 2024 Science X Network

by Liu Jia, Chinese Academy of Sciences

by Compuscript Ltd

Augmented reality (AR), virtual reality (VR) and mixed reality (MR) have expanded perceptual horizons and ushered in deeper human-digital interactions that transcend the confines of traditional flat panel displays.

This evolution has unlocked a realm of exciting new possibilities, encompassing the metaverse, digital twins and spatial computing, all of which have found widespread applications in diverse fields such as smart education and training, health care, navigation, gaming, entertainment, and smart manufacturing.

For AR, VR and MR displays to become truly wearable for an extended period, there is a pressing need for compact and stylish form factor, low weight and low power consumption. Compared to Fresnel lenses and refractive lenses, polarization-based folded optics, often referred to as pancake optics, have emerged as a pivotal breakthrough for compact and lightweight VR headsets in the past few years, including Apple Vision Pro and Meta Quest 3.

These pancake optics greatly reduce the volume of a VR display, which in turn improves the center of gravity for the headset. However, the employed half-mirror causes considerable optical loss, which limits the maximum efficiency to 25%. Therefore, researchers are working toward a novel optical structure with the same folding capability as the pancake lens, but without the optical loss.

The authors of a new article published in Opto-Electronic Advances have extensively explored light engines, imaging optics, and power consumption of AR, VR and MR displays. A game-changing pancake optics system for reducing the volume of VR and MR displays, while keeping a high efficiency is proposed by this article.

The motivation behind this research is the increasing demand for wearable VR/MR headsets that are not only visually impressive but also comfortable for extended use. Present VR headsets with conventional pancake optics face challenges such as low optical efficiency, which in turn leads to increased thermal effect of the headset and short battery life due to the tremendous optical loss induced by the half mirror.

As depicted in Fig. 1 (a–b), only about 25% of the light (assuming no other loss) from the display panel reaches the observer’s eye. However, if the microdisplay emits unpolarized light, then the maximum optical efficiency is further reduced to 12.5%. The unused light will be either absorbed by the headset, which would increase the thermal effect, or become stray light, which would degrade the image quality.

The novel pancake optics system addresses this challenge by introducing a theoretically lossless design, incorporating a nonreciprocal polarization rotator, also known as Faraday rotator, between reflective polarizers as shown in Fig. 1 (c–d). In such a design, the nonreciprocal polarization rotator plays a critical role in folding the optical paths.

Compared to reciprocal polarization rotator (e.g., half-wave plates), the nonreciprocal polarization rotator rotates the linearly polarized light irrespective of the optical wave’s propagation direction as Fig. 2 depicts. Consequently, a round trip of forward and backward propagations through the nonreciprocal polarization rotator results in a net rotation of 2θ.

Preliminary experiments were conducted with a laser source and a micro-OLED panel to verify its optical efficiency and folding capability as depicted in Fig. 3 (a) and (b–c) respectively. The measured optical efficiency is around 71.5% due to the lack of anti-reflection (AR) coating and non-ideal performance of the employed reflective polarizers.

After using high-performance reflective polarizers and AR coating, the optical efficiency is improved to 93.2%, which is approaching the theoretical prediction. Additionally, four types of possible ghost images are analyzed in this novel pancake optical system. Through identifying the root cause of these ghost images, new methods are proposed to enhance the image contrast ratio. Additionally, a multi-layer structure is proposed to broaden the bandwidth of the Faraday rotator to enable full color displays.

As indicated in Fig. 3 (d–f), a three sequences of nonreciprocal polarization rotators and quarter-wave plates are adequate to achieve a broadband spectral response. Finally, to achieve a large field of view and truly compact formfactor, some possible candidates of thin-film magneto-optics material are analyzed and discussed in the article.

Overall, these demonstrations showcase the potential that such a novel pancake optics system could revolutionize next-generation VR and MR displays with lightweight, compact formfactor, and low power consumption. The pressing need for a thin-film Faraday rotator that is both magnet-free and highly transparent, while possessing a large Verdet constant in the visible region, is expected to inspire the next-round magneto-optic material development in the future.

by Princeton Plasma Physics Laboratory

Artificially intelligent software has been developed to enhance medical treatments that use jets of electrified gas known as plasma. The computer code predicts the chemicals emitted by plasma devices, which can be used to treat cancer, promote healthy tissue growth and sterilize surfaces.

The software learned to predict the cocktail of chemicals coming out of the jet based on data gathered during real-world experiments and using the laws of physics for guidance. This type of artificial intelligence (AI) is known as machine learning because the system learns based on the information provided. The researchers involved in the project published a paper about their code in the Journal of Physics D: Applied Physics.

The plasma studied in the experiments is known as cold atmospheric plasma (CAP). When the CAP jet is turned on, numerous chemical species in the plasma take part in thousands of reactions. These chemicals modify the cells undergoing treatment in different ways, depending on the chemical composition of the jet. While scientists know that CAPs can be used to kill cancer cells, treat wounds and kill bacteria on food, it’s not fully understood why.

“This research is a step toward gaining a deeper understanding of how and why CAP jets work and could also one day be used to refine their use,” said Yevgeny Raitses, a managing principal research physicist at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL).

The project was completed by the Princeton Collaborative Low Temperature Plasma Research Facility (PCRF), a collaboration between researchers at PPPL and the George Washington University (GWU).

PPPL has a growing body of work that combines its 70 years of pioneering plasma research with its expertise in AI to solve societal problems. The Lab’s mission extends beyond using plasma to generate fusion power to its use in fields such as medicine and manufacturing, among others.

The software uses an approach known as a physics-informed neural network (PINN). In a PINN, data is organized into parts called nodes and neurons. The flow of the data mimics the way information is processed in the human brain. Laws of physics are also added to the code.

“Knowing what comes out of the jet is very important. Knowing what comes out accurately is very difficult,” said Sophia Gershman, a lead PPPL research engineer from the PCRF who worked on this collaborative project. The process would require several different devices to collect different kinds of information about the jet.

“In practical studies, it is difficult to go and utilize all of the various technologically advanced diagnostics all at once for each device and for various types of surfaces that we treat,” Gershman explained.

Calculating the chemical composition one nanosecond at a time

Li Lin, a research scientist from GWU and the paper’s primary author, said it’s also difficult to calculate the chemicals in a CAP jet because the interactions need to be considered a nanosecond at a time.

“When you consider that the device is in operation for several minutes, the number of calculations makes the problem more than simply computationally intensive. It’s practically impossible,” Lin said. “Machine learning allows you to bypass the complicated part.”

The project began with a small set of real-world data that was gathered using a technique known as Fourier-transform infrared absorption spectroscopy. The researchers used that small dataset to create a broader set of data. That data was then used to train the neural network using an evolutionary algorithm, which is a type of computer code inspired by nature that searches for the best answers using a survival-of-the-fittest approach.

Several successive batches of data are generated using slightly different approaches, and only the best datasets from each round are carried through to the next round of training until the desired results are achieved.

Ultimately, the team was able to accurately calculate the chemical concentrations, gas temperature, electron temperature and electron concentration of the cold atmospheric plasma jet based on data gathered during real-world experiments.

In a cold atmospheric plasma, the electrons—small, negatively charged particles—can be very hot, though the other particles are close to room temperature. The electrons can be at a low enough concentration that the plasma doesn’t feel hot or burn the skin while still being able to have a significant effect on the targeted cells.

On the path to personalized plasma treatment

Michael Keidar, the A. James Clark Professor of Engineering at GWU and a frequent collaborator with PPPL who also worked on this project, said the long-term goal is to be able to perform these calculations fast enough that the software can automatically adjust the plasma during a procedure to optimize treatment. Keidar is currently working on a prototype of such a “plasma adaptive” device in his lab.

“Ideally, it can be personalized. The way we envision it, you treat the patient, and the response of every patient will be different,” Keidar explained. “So, you can measure the response in real-time and then try to inform, using feedback and machine learning, the right settings in the plasma-producing device.”

More research needs to be done to perfect such a device. For example, this study looked at the CAP jet over time but at only one point in space. Further research would need to broaden the work so it considers multiple points along the jet’s output stream.

The study also looked at the plasma plume in isolation. Future experiments would need to integrate the surfaces treated by the plasma to see how that impacts the chemical composition at the treatment site.

More information: Li Lin et al, Data-driven prediction of the output composition of an atmospheric pressure plasma jet, Journal of Physics D: Applied Physics (2023). DOI: 10.1088/1361-6463/acfcc7

by Compuscript Ltd

Information security has become particularly crucial under the background of the big data era. Optical secret-sharing schemes encrypt information and physically divide it into different shares. Information can only be decrypted by cascading a sufficient number of shares.

These schemes can be widely applied to information encryption and anti-counterfeiting due to high security and rapid information processing capabilities.

Holography is a significant method for optical encryption, and it can also realize holographic multiplexing by using different physical dimensions of light as independent information channels. The metasurface holographic multiplexing technology meets the urgent needs for miniaturization and integration of optical systems.

However, there are significant challenges in building a cascaded optical secret-sharing platform with dynamic tunability and high diffraction efficiency, limited by precise manufacturing requirements and inherent physical properties of materials.

For the realization of low-cost, convenient, high-efficiency, and high-capacity cascaded optical secret sharing schemes, anisotropic structured liquid crystal optoelectronic materials with high diffraction efficiency and voltage-tunable switch features provide a novel approach.

The authors of an article published in Opto-Electronic Advances propose a multi-dimensional multiplexing optical secret-sharing framework with cascaded liquid crystal holograms. In this framework, the polarization state of the incident light and the distance between the liquid crystal holograms are used as the decryption keys of encrypted information.

An error back-propagation neural network with angular spectrum diffraction theory was created, accomplishing the inverse design of complex multi-constraint and multi-layer cascading problems. The multi-dimensional inputs in the encryption process of the network, such as the polarization state of the incident light, the external voltage applied to the cascaded liquid crystal shares and their distances, significantly enhance the security of the secret information. This allows for the ultra-secure transmission of multiple information channels simultaneously, overcoming the limitations of traditional holographic encryption methods.

First, the secret image is hidden in different shares (individual liquid crystal holograms) and can only be decrypted through cascading the shares. Even if one of the shares is stolen, it is impossible to retrieve the final secret information and only an authentication image will be displayed, greatly enhancing the security of the secret-sharing platform.

Second, the multi-dimensional multiplexing technique increases the complexity of the secret keys, enhancing both information security and capacity. Furthermore, the encryption information channels can be further increased by adding more secret shares and utilizing linear polarization state multiplexing. Interestingly, the flexible electric tuning capability of liquid crystal devices effectively heightens the security of the proposed secret-sharing framework. The externally applied voltage can be independently mapped to different secret shares, setting more stringent conditions for information decryption and significantly reducing the possibility of information leakage.

The multiplexing of eight images by controlling the polarization state of the incident light was experimentally demonstrated, the distance between the shares, and applying different voltage states externally to the liquid crystal layers.

In this scheme, the secret information is decomposed and distributed into two mutually constrained liquid crystal holograms. When these two liquid crystal holograms are cascaded together, it is only necessary to adjust the external applied voltage (U_on, U_off) of each liquid crystal share, so that each individual hologram can reconstruct an authentication image (number 2 or 4) at a specific position.

Furthermore, under high modulation efficiency voltage (U_on) for each liquid crystal share, six independent operation images (mathematical symbols) can be decrypted using different secret keys, which include the polarization of the incident light and the distance between the cascaded liquid crystal holograms.

The final encrypted information can be obtained through secondary decoding by performing mathematical operations displayed by different operation images between the authentication images. The mature manufacturing technology of liquid crystal components makes this framework more practical and multifunctional.

With its convenient design, low-cost manufacturing, and ultra-high security, this multi-dimensional multiplexing optical secret sharing scheme has great potential in applications of ultra-high capacity information storage, dynamic holographic display, and multifunctional optical information processing.

More information: Keyao Li et al, Multi-dimensional multiplexing optical secret sharing framework with cascaded liquid crystal holograms, Opto-Electronic Advances (2024). DOI: 10.29026/oea.2024.230121

Provided by Compuscript Ltd

In the realm of materials science, electromagnetic (EM) metamaterials have emerged as a revolutionary class of engineered composites capable of manipulating electromagnetic waves in ways never before possible. Unlike their naturally occurring counterparts, EM metamaterials derive their extraordinary properties from their unique structural arrangements, allowing them to exhibit unattainable electromagnetic characteristics in conventional materials.

One of the most fascinating characteristics of EM metamaterials lies in the realm of zero-index metamaterials (ZIMs). ZIMs possess the remarkable ability to achieve uniform electromagnetic field distribution over arbitrary shape (Figure 1a). This unique property opens many potential applications, from ultra-compact cloaking devices to arbitrarily shaped waveguides and lenses and photonic crystal surface-emitting lasers (Figure 1b).

Despite their immense potential, ZIMs have faced a significant hurdle in their practical implementation. The homogeneity of ZIMs is often limited by the number of unit cells per free-space wavelength. This limitation arises from the low permittivities property of the materials used to construct ZIMs. As a result, ZIMs often require large physical space to achieve their effective electromagnetic properties (Figure 2b).

Researchers have overcome this longstanding challenge in a study published in eLight by developing a highly homogeneous ZIM using a novel combination of high-permittivity materials.

As shown in Figure 3a, by employing SrTiO₃ ceramic pillars embedded in a BaTiO₃ background matrix, they have successfully fabricated a ZIM with an over threefold increase in homogenization level (Figures 2b and 2e), significantly reducing its physical dimensions.

Based on the uniform distribution of the phase of electromagnetic field throughout the ZIM, researchers have demonstrated a high-directivity antenna. By incorporating ZIM in a metallic waveguide (Figure 4a), this antenna has approached the fundamental limitation of directivity in antenna as the aperture size varies from subwavelength regime to a very large scale (Figure 4c).

This breakthrough paves the way for a new era of ZIM-based devices, offering unprecedented performance and compactness. The researchers’ achievement has profound implications for a wide range of fields, including wireless communications, remote sensing, and global positioning systems. Moreover, their work opens up new possibilities for fundamental research in ultracompact waveguides, cloaking devices, and superconducting quantum computing.

More information: Yueyang Liu et al, High-permittivity ceramics enabled highly homogeneous zero-index metamaterials for high-directivity antennas and beyond, eLight (2024). DOI: 10.1186/s43593-023-00059-x

Journal information: eLight 

Provided by TranSpread

by Tohoku University

A collaborative group of researchers has potentially developed a means of controlling spin waves by creating a hexagonal pattern of copper disks on a magnetic insulator. The breakthrough is expected to lead to greater efficiency and miniaturization of communication devices in fields such as artificial intelligence and automation technology.

Details of the study were published in the journal Physical Review Applied on January 30, 2024.

In a magnetic material, the spins of electrons are aligned. When these spins undergo coordinated movement, they generate a kind of ripple in the magnetic order, dubbed spin waves. Spin waves generate little heat and offer an abundance of advantages for next-generation devices.

Implementing spin waves in semiconductor circuits, which conventionally rely on electrical currents, could lessen power consumption and promote high integration. Since spin waves are waves, they tend to propagate in random directions unless controlled by structures and other means. As such, elements capable of generating, propagating, superimposing, and measuring spin waves are being competitively developed worldwide.

“We leveraged the wavelike nature of spin waves to successfully control their propagation directly,” points out Taichi Goto, associate professor at Tohoku University’s Electrical Communication Research Institute, and co-author of the paper. “We did so by first developing an excellent magnetic insulator material called magnetic garnet film, which has low spin wave losses. We then periodically arranged small copper disks with diameters less than 1 mm on this film.”

• Fig. 3. A summary of the results obtained in Figure 2, with the angle of the two-dimensional magnonic crystal on the horizontal axis and the magnonic band gap on the vertical axis. The calculations in (a) and the experiments in (b) are in good agreement, showing a small frequency shift and excellent performance. Credit: Taichi Goto et al
• Fig. 2. Top-view photograph of the fabricated two-dimensional magnonic crystal and the spin wave transmission spectrum at that time. Even when the two-dimensional magnonic crystal is rotated by 5 degrees at a time, it can be seen that the frequency of the magnonic band gap indicated by ▲ remains almost unchanged. This suggests a low angular dependence and the potential for controlling the propagation direction of spin waves. Credit: Taichi Goto et al
• Fig. 3. A summary of the results obtained in Figure 2, with the angle of the two-dimensional magnonic crystal on the horizontal axis and the magnonic band gap on the vertical axis. The calculations in (a) and the experiments in (b) are in good agreement, showing a small frequency shift and excellent performance. Credit: Taichi Goto et al
• Fig. 2. Top-view photograph of the fabricated two-dimensional magnonic crystal and the spin wave transmission spectrum at that time. Even when the two-dimensional magnonic crystal is rotated by 5 degrees at a time, it can be seen that the frequency of the magnonic band gap indicated by ▲ remains almost unchanged. This suggests a low angular dependence and the potential for controlling the propagation direction of spin waves. Credit: Taichi Goto et al

By arranging copper disks in a hexagonal pattern resembling snowflakes, Goto and his colleagues could effectively reflect the spin waves. Furthermore, by rotating the magnonic crystal (shown in Figure 2) and changing the incident angle of spin waves, the researchers revealed that the frequency at which the magnonic band gap occurs remains largely unchanged in the range from 10 to 30 degrees. This suggests the potential for the two-dimensional magnonic crystal to freely control the propagation direction of spin waves.

“To date, there have been no experimental confirmations of changes in the spin wave incident angle for a two-dimensional magnonic crystal comprising a magnetic insulator and copper disks, making this the world’s first report,” says Goto.

Looking ahead, the team hopes to demonstrate the direction control of spin waves using two-dimensional magnonic crystals and to develop functional components that utilize this technology.

by David Nutt, Cornell University

Christopher Parzyck had done everything right. Parzyck, a postdoctoral researcher, had brought his nickelate samples—a newly discovered family of superconductors—to a synchrotron beamline for X-ray scattering experiments. He was measuring his samples, which he’d synthesized with a new method, in the hope of detecting the suspected presence of “charge ordering”—a phenomenon in which electrons self-organize into periodic patterns. The phenomenon has been linked to high-temperature superconductivity.

But there was no significant charge order in his samples. None.

“He came back and said, ‘The better samples didn’t show it,'” said Kyle Shen, the James A. Weeks Professor of Physical Sciences in the College of Arts and Sciences, who oversaw the project. “We were like, ‘Oh, that’s really weird. I don’t understand that.'”

Sometimes scientists get so stumped, they have no choice but to set aside their hypotheses, roll up their sleeves and put on their detective hats. After some extensive sleuthing, Parzyck, Shen and their collaborators realized that they had, in fact, done everything right.

According to findings published Jan. 26 in Nature Materials, Parzyck’s new synthesis method produced nickelates that were so pure, they were free of the flaws that had tainted previous studies of nickelates. The charge order had never existed. They were chasing a phantom.

“Earlier reports said they see this charge ordering, but there were all these inconsistencies,” Shen said. “Chris developed a more controlled way of making these materials that effectively limits the number of defects. Excess oxygen atoms were masquerading as a signature of charge order.”

In recent years, nickelates have been the subject of considerable interest because they are newfound close cousins of the well-known “cuprates,” a family of copper oxide-based superconductors that can have high transition temperatures, upwards of 100 Kelvin, at which point electrical resistance vanishes, whereas for conventional superconductors, such as lead or niobium, their transitions are below 10 Kelvin. High-temperature superconductors are much easier to cool and thus are far more promising for potential future applications.

Ever since cuprates were first discovered in the late 1980s, scientists have sought similar superconducting families that might pinpoint the key qualities that enable high-temperature superconductivity.

“One obvious place to look is nickel, because nickel is right next to copper on the periodic table,” Shen said. “So people thought maybe we can do some material synthesis magic and make nickel-bearing compounds sort of like cuprates. That idea existed 30 years ago. The reason it took so long to realize is it turns out nickelate superconductors are really hard to make.”

Other researchers had synthesized nickelates—which are composed of nickel, oxygen and a rare earth element—by first growing a “precursor” material and then exposing that material to a source of hydrogen and heating them inside a sealed tube. Over the course of a day or so, the hydrogen pulls out roughly a third of the material’s oxygen molecules, which Shen compared to removing blocks in a game of Jenga.

“Synthesizing these materials is a bit of a nightmare,” he said.

Parzyck and Shen devised an alternative technique in which the oxygen is removed by a beam of atomic hydrogen, a process that is commonly used for cleaning semiconductor surfaces, but had never been used for materials synthesis. Atomic hydrogen reduction gives the researchers greater independent control of the amount of hydrogen being applied, in addition to variables such as time and pressure. The process can be completed in minutes, rather than hours or a day.

“Developing the reduction technique was a long and challenging process in-and-of-itself,” Parzyck said. “When I first started out, I tried to apply conditions like those used in traditional calcium hydride reduction—low temperatures for relatively long periods of time—but the sample quality was always low and not very consistent. It wasn’t until I decided to start fresh and go in a completely different direction—opting for higher temperatures for as short of a duration as possible—that I really found some success.”

After their synchrotron experiments failed to show the “resonant scattering peak” that should have signaled the presence of charge ordering, the researchers began varying the amount of oxygen they were stripping out.

“The real breakthrough came when we started measuring the samples which we purposefully prepared to have excess oxygen and saw a very strong, clear response—then we had a viable alternative explanation for the peak’s origin and finally knew we were going in the right direction,” Parzyck said.

To confirm their suspicions, they collaborated with the late Lena Kourkoutis, M.S. ’06, Ph.D. ’09, associate professor of applied and engineering physics, David Muller, the Samuel B. Eckert Professor of Engineering and their doctoral student Lopa Bhatt, who used electron microscopy to directly verify that trace amounts of oxygen in the samples were indeed causing the spurious charge-order signal.

Not only has the team identified a crucial difference between cuprate and nickelate superconductors; they now have a more reliable method for growing cleaner samples that can potentially be used for a wider variety of experiments, with a little less mystery.

by Wayne Gillam, University of Washington

Photonic integrated circuits are an important next-wave technology. These sophisticated microchips hold the potential to substantially decrease costs and increase speed and efficiency for electronic devices across a wide range of application areas, including automotive technology, communications, health care, data storage, and computing for artificial intelligence.

Photonic circuits use photons, fundamental particles of light, to move, store, and access information in much the same way that conventional electronic circuits use electrons for this purpose. Photonic chips are already in use today in advanced fiber-optic communication systems, and they are being developed for implementation in a broad spectrum of near-future technologies, including light detection and ranging, or LiDAR, for autonomous vehicles; light-based sensors for medical devices; 5G and 6G communication networks; and optical and quantum computing.

Given the broad range of existing and future uses for photonic integrated circuits, access to equipment that can fabricate chip designs for study, research and industrial applications is also important. However, today’s nanofabrication facilities cost millions of dollars to construct and are well beyond the reach of many colleges, universities, and research labs.

For those who can access a nanofabrication facility, at least a day must be reserved for the exacting and time-consuming lithographic process used to make these microchips. On top of that, if an error is made in design, or if the chip doesn’t work properly for some other reason, the faulty circuit must be discarded, the design adjusted, and a new chip fabricated. This often results in days or even weeks spent in the cleanroom.

But now, as described in a new paper in Science Advances, a University of Washington-led research team has devised a way to bypass expensive nanofabrication facilities and produce photonic integrated circuits almost anywhere.

The team has developed an innovative method in which these circuits can be written, erased, and modified by a laser writer into a thin film of phase-change material similar to what is used for recordable CDs and DVDs. This new process allows photonic integrated circuits to be constructed and reconfigured in a fraction of the time it would take at a nanofabrication lab.

The multi-university team was led by UW Electrical and Computer Engineering and Physics Professor Mo Li, who is the Department’s associate chair for research, a member of the Institute for Nano-Engineered Systems and the senior author of the paper.

“Photonics technology is on the horizon; therefore, we need to train or educate our students in this field. But for students to study and have hands-on experience with photonic circuits, currently, they need access to a multimillion-dollar facility,” Li said.

“This new technology addresses that problem. Using our method, photonic circuits that previously had to be fabricated in expensive and hard-to-access facilities now can be printed and reconfigured in labs, classrooms, and even garage workshops, by a speedy, low-cost device about the size of a conventional desktop laser printer.”

Benefits for students, researchers and industry

Students aren’t the only ones who stand to benefit from this new way of creating photonic integrated circuits. For researchers, this advance will enable a much quicker turnaround time for prototyping and testing out a new idea before booking valuable time in a nanofabrication facility.

And for industrial applications, a big advantage of this method for producing photonic integrated circuits is reconfigurability. For example, companies could possibly use this technology to create reconfigurable optical connections in data centers, especially in systems that support artificial intelligence and machine learning, which would lead to cost savings and production efficiencies.

Li’s research team included UW ECE graduate student Changming Wu, who is lead author of the paper, and, along with Li, came up with the idea for this novel way of building photonic integrated circuits. UW ECE graduate student Haoqin Deng also contributed to the effort. Their work is the latest result of a six-year line of research at the UW that includes advances in optical computing. It is also a continuation of a productive collaboration with Professors Ichiro Takeuchi and Carlos A. Ríos Ocampo and their students at the University of Maryland.

“Being able to write a whole photonic circuit using only one single step, without a complicated fabrication process, is really exciting. And the fact that we can make any modification to any part of the circuit in our own lab and rewrite and redo it is amazing,” Wu said. “It’s a matter of minutes versus a full day-long process. It’s a huge relief to be able to finish the whole fabrication process within a few minutes instead of what often is several days or even a week.”

Improving performance, building a commercial device

The method the team developed has been proven to work, but it is still an early-stage concept. However, Li has filed a provisional patent application, and he has plans in progress to build a desktop laser writer for photonic integrated circuits. This printer could be sold at an affordable price and distributed widely to research labs and educational institutions around the world. He is also engaging with industry leaders to promote possible applications of this new technology in programmable photonic chips and reconfigurable optical networks.

This laser printer for photonic chips will use a staging system that will move the substrate in a much more precise manner than in a traditional desktop printer. The team will be seeking ways to optimize its performance as they build a prototype. They also will be working on reducing optical loss in the phase-change material they are using through further research in material science and laser writing techniques. This will enable the printer to produce even more detailed and sophisticated circuits than what is currently possible.

Li said that he and his research team were very excited about what lay ahead.

“This technology can create the photonic circuitry you want, but it also can be added onto already-existing electronic circuitry. And because it is reconfigurable and reusable, it just opens many possibilities for students, researchers, and industry,” Li said. “What’s most exciting to me is that we’ll potentially have a huge impact on the field of photonics in disseminating this new tool and technology to the broader research community.”