Category Archives: light

Graphene-like materials for first smart contact lenses with AR (augmented reality) vision, health monitoring, & content surfing?

A March 6, 2024 XPANCEO news release on EurekAlert (also posted March 11, 2024 on the Graphene Council blog) and distributed by Mindset Consulting announced smart contact lenses devised with graphene-like materials,

XPANCEO, a deep tech company developing the first smart contact lenses with XR vision, health monitoring, and content surfing features, in collaboration with the Nobel laureate Konstantin S. Novoselov (National University of Singapore, University of Manchester) and professor Luis Martin-Moreno (Instituto de Nanociencia y Materiales de Aragon), has announced in Nature Communications a groundbreaking discovery of new properties of rhenium diselenide and rhenium disulfide, enabling novel mode of light-matter interaction with huge potential for integrated photonics, healthcare, and AR. Rhenium disulfide and rhenium diselenide are layered materials belonging to the family of graphene-like materials. Absorption and refraction in these materials have different principal directions, implying six degrees of freedom instead of a maximum of three in classical materials. As a result, rhenium disulfide and rhenium diselenide by themselves allow controlling the light propagation direction without any technological steps required for traditional materials like silicon and titanium dioxide.

The origin of such surprising light-matter interaction of ReS2 and ReSe2 with light is due to the specific symmetry breaking observed in these materials. Symmetry plays a huge role in nature, human life, and material science. For example, almost all living things are built symmetrically. Therefore, in ancient times symmetry was also called harmony, as it was associated with beauty. Physical laws are also closely related to symmetry, such as the laws of conservation of energy and momentum. Violation of symmetry leads to the appearance of new physical effects and radical changes in the properties of materials. In particular, the water-ice phase transition is a consequence of a decrease in the degree of symmetry. In the case of ReS2 and ReSe2, the crystal lattice has the lowest possible degree of symmetry, which leads to the rotation of optical axes – directions of symmetry of optical properties of the material, which was previously observed only for organic materials. As a result, these materials make possible to control the direction of light by changing the wavelength, which opens a unique way for light manipulation in next-generation devices and applications. 

“The discovery of unique properties in anisotropic materials is revolutionizing the fields of nanophotonics and optoelectronics, presenting exciting possibilities. These materials serve as a versatile platform for the advancement of optical devices, such as wavelength-switchable metamaterials, metasurfaces, and waveguides. Among the promising applications is the development of highly efficient biochemical sensors. These sensors have the potential to outperform existing analogs in terms of both sensitivity and cost efficiency. For example, they are anticipated to significantly reduce the expenses associated with hospital blood testing equipment, which is currently quite costly, potentially by several orders of magnitude. This will also allow the detection of dangerous diseases and viruses, such as cancer or COVID, at earlier stages,” says Dr. Valentyn S. Volkov, co-founder and scientific partner at XPANCEO, a scientist with an h-Index of 38 and over 8000 citations in leading international publications.

Beyond the healthcare industry, these novel properties of graphene-like materials can find applications in artificial intelligence and machine learning, facilitating the development of photonic circuits to create a fast and powerful computer suitable for machine learning tasks. A computer based on photonic circuits is a superior solution, transmitting more information per unit of time, and unlike electric currents, photons (light beams) flow across one another without interacting. Furthermore, the new material properties can be utilized in producing smart optics, such as contact lenses or glasses, specifically for advancing AR [augmented reality] features. Leveraging these properties will enhance image coloration and adapt images for individuals with impaired color perception, enabling them to see the full spectrum of colors.

Here’s a link to and a citation for the paper,

Wandering principal optical axes in van der Waals triclinic materials by Georgy A. Ermolaev, Kirill V. Voronin, Adilet N. Toksumakov, Dmitriy V. Grudinin, Ilia M. Fradkin, Arslan Mazitov, Aleksandr S. Slavich, Mikhail K. Tatmyshevskiy, Dmitry I. Yakubovsky, Valentin R. Solovey, Roman V. Kirtaev, Sergey M. Novikov, Elena S. Zhukova, Ivan Kruglov, Andrey A. Vyshnevyy, Denis G. Baranov, Davit A. Ghazaryan, Aleksey V. Arsenin, Luis Martin-Moreno, Valentyn S. Volkov & Kostya S. Novoselov. Nature Communications volume 15, Article number: 1552 (2024) DOI: https://doi.org/10.1038/s41467-024-45266-3 Published: 06 March 2024

This paper is open access.

A graphene joke (of sorts): What did the electron ‘say’ to the phonon in the graphene sandwich?

Unfortunately, there isn’t a punch line but I appreciate the effort to inject a little lightness into the description of a fairly technical achievement, from a February 12, 2024 news item on Nanowerk, Note: A link has been removed,

Electrons carry electrical energy, while vibrational energy is carried by phonons. Understanding how they interact with each other in certain materials, like in a sandwich of two graphene layers, will have implications for future optoelectronic devices.

Key Takeaways

Twisted graphene layers exhibit unique electrical properties.

Electron-phonon interactions crucial for energy loss in graphene.

Discovery of a new physical process involving electron-phonon Umklapp scattering.

Potential implications for ultrafast optoelectronics and quantum applications.

A February 9, 2024 Eindhoven University of Technology (TU/e; Netherlands) press release, which originated the news item, is reproduced here in its entirety, Note: Links have been removed,

Electrons carry electrical energy, while vibrational energy is carried by phonons. Understanding how they interact with each other in certain materials, like in a sandwich of two graphene layers, will have implications for future optoelectronic devices. Recent work has revealed that graphene layers twisted relative to each other by a small ‘magic angle’ can act as perfect insulator or superconductor. But the physics of the electron-phonon interactions are a mystery. As part of a worldwide international collaboration, TU/e researcher Klaas-Jan Tielrooij has led a study on electron-phonon interactions in graphene layers. And they have made a startling discovery.

What did the electron say to the phonon between two layers of graphene?

This might sound like the start of a physics meme with a hilarious punchline to follow. But that’s not the case according to Klaas-Jan Tielrooij. He’s an associate professor at the Department of Applied Physics and Science Education at TU/e and the research lead of the new work published in Science Advances.

“We sought to understand how electrons and phonons ‘talk’ to each other within two twisted graphene layers,” says Tielrooij.

Electrons are the well-known charge and energy carriers associated with electricity, while a phonon is linked to the emergence of vibrations between atoms in an atomic crystal.

“Phonons aren’t particles like electrons though, they’re a quasiparticle. Yet, their interaction with electrons in certain materials and how they affect energy loss in electrons has been a mystery for some time,” notes Tielrooij.

But why would it be interesting to learn more about electron-phonon interactions? “These interactions can have a major effect on the electronic and optoelectronic properties of devices, made from materials like graphene, which we are going to see more of in the future.”

Twistronics: Breakthrough of the Year 2018

Tielrooij and his collaborators, who are based around the world in Spain, Germany, Japan, and the US, decided to study electron-phonon interactions in a very particular case – within two layers of graphene where the layers are ever-so-slightly misaligned.

Graphene is a two-dimensional layer of carbon atoms arranged in a honeycomb lattice that has several impressive properties such as high electrical conductivity, high flexibility, and high thermal conductivity, and it is also nearly transparent.

Back in 2018, the Physics World Breakthrough of the Year award went to Pablo Jarillo-Herrero and colleagues at MIT [Massachusetts Institute of Technology] for their pioneering work on twistronics, where adjacent layers of graphene are rotated very slightly relative to each other to change the electronic properties of the graphene.

Twist and astound!

“Depending on how the layers of graphene are rotated and doped with electrons, contrasting outcomes are possible. For certain dopings, the layers act as an insulator, which prevents the movement of electrons. For other doping, the material behaves as a superconductor – a material with zero resistance that allows the dissipation-less movement of electrons,” says Tielrooij.

Better known as twisted bilayer graphene, these outcomes occur at the so-called magic angle of misalignment, which is just over one degree of rotation. “The misalignment between the layers is tiny, but the possibility for a superconductor or an insulator is an astounding result.”

How electrons lose energy

For their study, Tielrooij and the team wanted to learn more about how electrons lose energy in magic-angle twisted bilayer graphene, or MATBG for short.

To achieve this, they used a material consisting of two sheets of monolayer graphene (each 0.3 nanometers thick), placed on top of each other, and misaligned relative to each other by about one degree.

Then using two optoelectronic measurement techniques, the researchers were able to probe the electron-phonon interactions in detail, and they made some staggering discoveries.

“We observed that the energy vanishes very quickly in the MATBG – it occurs on the picosecond timescale, which is one-millionth of one-millionth of a second!” says Tielrooij.

This observation is much faster than for the case of a single layer of graphene, especially at ultracold temperatures (specifically below -73 degrees Celsius). “At these temperatures, it’s very difficult for electrons to lose energy to phonons, yet it happens in the MATBG.”

Why electrons lose energy

So, why are the electrons losing the energy so quickly through interaction with the phonons? Well, it turns out the researchers have uncovered a whole new physical process.

“The strong electron-phonon interaction is a completely new physical process and involves so-called electron-phonon Umklapp scattering,” adds Hiroaki Ishizuka from Tokyo Institute of Technology in Japan, who developed the theoretical understanding of this process together with Leonid Levitov from Massachusetts Institute of Technology in the US.

Umklapp scattering between phonons is a process that often affects heat transfer in materials, because it enables relatively large amounts of momentum to be transferred between phonons.

“We see the effects of phonon-phonon Umklapp scattering all the time as it affects the ability for (non-metallic) materials at room temperature to conduct heat. Just think of an insulating material on the handle of a pot for example,” says Ishizuka. “However, electron-phonon Umklapp scattering is rare. Here though we have observed for the first time how electrons and phonons interact via Umklapp scattering to dissipate electron energy.”

Challenges solved together

Tielrooij and collaborators may have completed most of the work while he was based in Spain at the Catalan Institute of Nanoscience and Nanotechnology (ICN2), but as Tielrooij notes. “The international collaboration proved pivotal to making this discovery.”

So, how did all the collaborators contribute to the research? Tielrooij: “First, we needed advanced fabrication techniques to make the MATBG samples. But we also needed a deep theoretical understanding of what’s happening in the samples. Added to that, ultrafast optoelectronic measurement setups were required to measure what’s happening in the samples too.”

Tielrooij and the team received the magic-angle twisted samples from Dmitri Efetov’s group at Ludwig-Maximilians-Universität in Munich, who were the first group in Europe able to make such samples and who also performed photomixing measurements, while theoretical work at MIT in the US and at Tokyo Institute of Technology in Japan proved crucial to the success of the research.

At ICN2, Tielrooij and his team members Jake Mehew and Alexander Block used cutting-edge equipment particularly time-resolved photovoltage microscopy to perform their measurements of electron-phonon dynamics in the samples.

The future

So, what does the future look like for these materials then? According to Tielrooij, don’t expect anything too soon.

“As the material is only being studied for a few years, we’re still some way from seeing magic-angle twisted bilayer graphene having an impact on society.”

But there is a great deal to be explored about energy loss in the material.

“Future discoveries could have implications for charge transport dynamics, which could have implications for future ultrafast optoelectronics devices,” says Tielrooij. “In particular, they would be very useful at low temperatures, so that makes the material suitable for space and quantum applications.”

The research from Tielrooij and the international team is a real breakthrough when it comes to how electrons and phonons interact with each other.

But we’ll have to wait a little longer to fully understand the consequences of what the electron said to the phonon in the graphene sandwich.

Illustration showing the control of energy relaxation with twist angle. Image: Authors

Here’s a link to and a citation for the paper,

Ultrafast Umklapp-assisted electron-phonon cooling in magic-angle twisted bilayer graphene by Jake Dudley Mehew, Rafael Luque Merino, Hiroaki Ishizuka, Alexander Block, Jaime Díez Mérida, Andrés Díez Carlón, Kenji Watanabe, Takashi Taniguchi, Leonid S. Levitov, Dmitri K. Efetov, and Klaas-Jan Tielrooij. Science Advances 9 Feb 2024 Vol 10, Issue 6 DOI: 10.1126/sciadv.adj1361

This paper is open access.

Enlightening Morpho butterfly

Apparently, the Morpho butterfly (or blue morpho butterfly) could inspire more balanced lighting, from an October 12, 2023 news item on phys.org,

As you watch Morpho butterflies wobble in flight, shimmering in vivid blue color, you’re witnessing an uncommon form of structural color that researchers are only beginning to use in lighting technologies such as optical diffusers. Furthermore, imparting a self-cleaning capability to such diffusers would minimize soiling and staining and maximize practical utility.

Now, in a study recently published in Advanced Optical Materials, researchers at Osaka University have developed a water-repelling nanostructured light diffuser that surpasses the functionality of other common diffusers. This work might help solve common lighting dilemmas in modern technologies.

Caption: Design and diffused light for the anisotropic (left) and isotropic (right) Morpho-type diffusers. It has high optical functionalities and anti-fouling properties, which until now have not been realized in one device. Credit: K.Yamashita, A.Saito

An October 12, 2023 Osaka University press release (also on EurekAlert), which originated the news item, sheds some light on the subject (sorry! I couldn’t resist),

Standard lighting can eventually become tiring because it’s unevenly illuminating. Thus, many display technologies use optical diffusers to make the light output more uniform. However, conventional optical diffusers reduce the light output, don’t work well for all emitted colors, or require special effort to clean. Morpho butterflies are an inspiration for improved optical diffusers. Their randomly arranged multilayer architecture enables structural color: in this case, selective reflection of blue light over a ≥±40° angle from the direction of illumination. The goal of the present work is to use this inspiration from nature to design a simplified optical diffuser that has both high transmittance and wide angular spread, works for a range of colors without dispersion, cleans by a simple water rinse, and can be shaped with standard nanofabrication tools.

“We create two-dimensional nanopatterns—in common transparent polydimethylsiloxane elastomer—of binary height yet random width, and the two surfaces have different structural scales,” explains Kazuma Yamashita, lead author of the study. “Thus, we report an effective optical diffuser for short- and long-wavelength light.”

The researchers tailored the patterns of the diffuser surfaces to optimize the performance for blue and red light, and their self-cleaning properties. The experimentally measured light transmittance was >93% over the entire visible light spectrum, and the light diffusion was substantial and could be controlled into anisotropic shape: 78° in the x-direction and 16° in the y-direction (similar to values calculated by simulations). Furthermore, the surfaces both strongly repelled water in contact angle and self-cleaning experiments.

“Applying protective cover glass layers on either side of the optical diffuser largely maintains the optical properties, yet protects against scratching,” says Akira Saito, senior author. “The glass minimizes the need for careful handling, and indicates our technology’s utility to daylight-harvesting windows.”

This work emphasizes that studying the natural world can provide insights for improved everyday devices; in this case, lighting technologies for visual displays. The fact that the diffuser consists of a cheap material that essentially cleans itself and can be easily shaped with common tools might inspire other researchers to apply the results of this work to electronics and many other fields.

Here’s a link to and a citation for the paper,

Development of a High-Performance, Anti-Fouling Optical Diffuser Inspired by Morpho Butterfly’s Nanostructure by Kazuma Yamashita, Kana Taniguchi, Takuma Hattori, Yuji Kuwahara, Akira Saito. Advanced Opticla Materials DOI: https://doi.org/10.1002/adom.202301086 First published: 26 July 2023

This paper is open access.

350-year-old mechanical theorem reveals new properties of light waves

Caption: Physicists at Stevens Institute of Technology use a 350-year-old theorem that explains the workings of pendulums and planets to reveal new properties of light waves. Credit: Stevens Institute of Technology

An August 21, 2023 news item on phys.org revisits a 350-year old theorem, Note: Links have been removed,

Since the 17th century, when Isaac Newton and Christiaan Huygens first debated the nature of light, scientists have been puzzling over whether light is best viewed as a wave or a particle—or perhaps, at the quantum level, even both at once. Now, researchers at Stevens Institute of Technology have revealed a new connection between the two perspectives, using a 350-year-old mechanical theorem—ordinarily used to describe the movement of large, physical objects like pendulums and planets—to explain some of the most complex behaviors of light waves.

The work, led by Xiaofeng Qian, assistant professor of physics at Stevens and reported in the August 17 [2023] online issue of Physical Review Research, also proves for the first time that a light wave’s degree of non-quantum entanglement exists in a direct and complementary relationship with its degree of polarization. As one rises, the other falls, enabling the level of entanglement to be inferred directly from the level of polarization, and vice versa. This means that hard-to-measure optical properties such as amplitudes, phases and correlations—perhaps even these of quantum wave systems—can be deduced from something a lot easier to measure: light intensity.

An August 20, 2023 Stevens Institute of Technology news release (also on EurekAlert), which originated the news item, notes the research doesn’t resolve the light waves and light particles conundrum but it does reveal something new about it,,

“We’ve known for over a century that light sometimes behaves like a wave, and sometimes like a particle, but reconciling those two frameworks has proven extremely difficult,” said Qian “Our work doesn’t solve that problem — but it does show that there are profound connections between wave and particle concepts not just at the quantum level, but at the level of classical light-waves and point-mass systems.” 

Qian’s team used a mechanical theorem, originally developed by Huygens in a 1673 book on pendulums, that explains how the energy required to rotate an object varies depending on the object’s mass and the axis around which it turns. “This is a well-established mechanical theorem that explains the workings of physical systems like clocks or prosthetic limbs,” Qian explained. “But we were able to show that it can offer new insights into how light works, too.”  

This 350-year-old theorem describes relationships between masses and their rotational momentum, so how could it be applied to light where there is no mass to measure? Qian’s team interpreted the intensity of a light as the equivalent of a physical object’s mass, then mapped those measurements onto a coordinate system that could be interpreted using Huygens’ mechanical theorem. “Essentially, we found a way to translate an optical system so we could visualize it as a mechanical system, then describe it using well-established physical equations,” explained Qian.

Once the team visualized a light wave as part of a mechanical system, new connections between the wave’s properties immediately became apparent — including the fact that entanglement and polarization stood in a clear relationship with one another.

“This was something that hadn’t been shown before, but that becomes very clear once you map light’s properties onto a mechanical system,” said Qian. “What was once abstract becomes concrete: using mechanical equations, you can literally measure the distance between ‘center of mass’ and other mechanical points to show how different properties of light relate to one another.” 

Clarifying these relationships could have important practical implications, allowing subtle and hard-to-measure properties of optical systems — or even quantum systems — to be deduced from simpler and more robust measurements of light intensity, Qian explained. More speculatively, the team’s findings suggest the possibility of using mechanical systems to simulate and better-understand the strange and complex behaviors of quantum wave systems.

“That still lies ahead of us, but with this first study we’ve shown clearly that by applying mechanical concepts, it’s possible to understand optical systems in an entirely new way,” Qian said. “Ultimately, this research is helping to simplify the way we understand the world, by allowing us to recognize the intrinsic underlying connections between apparently unrelated physical laws.”

Here’s a link to and a citation for the paper,

Bridging coherence optics and classical mechanics: A generic light polarization-entanglement complementary relation by Xiao-Feng Qian and Misagh Izadi. Phys. Rev. Research 5, 033110 Published 17 August 2023

This paper is open access.

Single chip mimics human vision and memory abilities

A June 15, 2023 RMIT University (Australia) press release (also on EurekAlert but published June 14, 2023) announces a neuromorphic (brainlike) computer chip, which mimics human vision and ‘creates’ memories,

Researchers have created a small device that ‘sees’ and creates memories in a similar way to humans, in a promising step towards one day having applications that can make rapid, complex decisions such as in self-driving cars.

The neuromorphic invention is a single chip enabled by a sensing element, doped indium oxide, that’s thousands of times thinner than a human hair and requires no external parts to operate.

RMIT University engineers in Australia led the work, with contributions from researchers at Deakin University and the University of Melbourne.

The team’s research demonstrates a working device that captures, processes and stores visual information. With precise engineering of the doped indium oxide, the device mimics a human eye’s ability to capture light, pre-packages and transmits information like an optical nerve, and stores and classifies it in a memory system like the way our brains can.

Collectively, these functions could enable ultra-fast decision making, the team says.

Team leader Professor Sumeet Walia said the new device can perform all necessary functions – sensing, creating and processing information, and retaining memories – rather than relying on external energy-intensive computation, which prevents real-time decision making.

“Performing all of these functions on one small device had proven to be a big challenge until now,” said Walia from RMIT’s School of Engineering.

“We’ve made real-time decision making a possibility with our invention, because it doesn’t need to process large amounts of irrelevant data and it’s not being slowed down by data transfer to separate processors.”

What did the team achieve and how does the technology work?

The new device was able to demonstrate an ability to retain information for longer periods of time, compared to previously reported devices, without the need for frequent electrical signals to refresh the memory. This ability significantly reduces energy consumption and enhances the device’s performance.

Their findings and analysis are published in Advanced Functional Materials.

First author and RMIT PhD researcher Aishani Mazumder said the human brain used analog processing, which allowed it to process information quickly and efficiently using minimal energy.

“By contrast, digital processing is energy and carbon intensive, and inhibits rapid information gathering and processing,” she said.

“Neuromorphic vision systems are designed to use similar analog processing to the human brain, which can greatly reduce the amount of energy needed to perform complex visual tasks compared with today’s technologies

What are the potential applications?

The team used ultraviolet light as part of their experiments, and are working to expand this technology even further for visible and infrared light – with many possible applications such as bionic vision, autonomous operations in dangerous environments, shelf-life assessments of food and advanced forensics.

“Imagine a self-driving car that can see and recognise objects on the road in the same way that a human driver can or being able to able to rapidly detect and track space junk. This would be possible with neuromorphic vision technology.”

Walia said neuromorphic systems could adapt to new situations over time, becoming more efficient with more experience.

“Traditional computer vision systems – which cannot be miniaturised like neuromorphic technology – are typically programmed with specific rules and can’t adapt as easily,” he said.

“Neuromorphic robots have the potential to run autonomously for long periods, in dangerous situations where workers are exposed to possible cave-ins, explosions and toxic air.”

The human eye has a single retina that captures an entire image, which is then processed by the brain to identify objects, colours and other visual features.

The team’s device mimicked the retina’s capabilities by using single-element image sensors that capture, store and process visual information on one platform, Walia said.

“The human eye is exceptionally adept at responding to changes in the surrounding environment in a faster and much more efficient way than cameras and computers currently can,” he said.

“Taking inspiration from the eye, we have been working for several years on creating a camera that possesses similar abilities, through the process of neuromorphic engineering.” 

Here’s a link to and a citation for the paper,

Long Duration Persistent Photocurrent in 3 nm Thin Doped Indium Oxide for Integrated Light Sensing and In-Sensor Neuromorphic Computation by Aishani Mazumder, Chung Kim Nguyen, Thiha Aung, Mei Xian Low, Md. Ataur Rahman, Salvy P. Russo, Sherif Abdulkader Tawfik, Shifan Wang, James Bullock, Vaishnavi Krishnamurthi. Advanced Functional Materials DOI: https://doi.org/10.1002/adfm.202303641 First published: 14 June 2023

This paper is open access.

Optical memristors and neuromorphic computing

A June 5, 2023 news item on Nanowerk announced a paper which reviews the state-of-the-art of optical memristors, Note: Links have been removed,

AI, machine learning, and ChatGPT may be relatively new buzzwords in the public domain, but developing a computer that functions like the human brain and nervous system – both hardware and software combined – has been a decades-long challenge. Engineers at the University of Pittsburgh are today exploring how optical “memristors” may be a key to developing neuromorphic computing.

Resistors with memory, or memristors, have already demonstrated their versatility in electronics, with applications as computational circuit elements in neuromorphic computing and compact memory elements in high-density data storage. Their unique design has paved the way for in-memory computing and captured significant interest from scientists and engineers alike.

A new review article published in Nature Photonics (“Integrated Optical Memristors”), sheds light on the evolution of this technology—and the work that still needs to be done for it to reach its full potential. Led by Nathan Youngblood, assistant professor of electrical and computer engineering at the University of Pittsburgh Swanson School of Engineering, the article explores the potential of optical devices which are analogs of electronic memristors. This new class of device could play a major role in revolutionizing high-bandwidth neuromorphic computing, machine learning hardware, and artificial intelligence in the optical domain.

A June 2, 2023 University of Pittsburgh news release (also on EurekAlert but published June 5, 2023), which originated the news item, provides more detail,

“Researchers are truly captivated by optical memristors because of their incredible potential in high-bandwidth neuromorphic computing, machine learning hardware, and artificial intelligence,” explained Youngblood. “Imagine merging the incredible advantages of optics with local information processing. It’s like opening the door to a whole new realm of technological possibilities that were previously unimaginable.” 

The review article presents a comprehensive overview of recent progress in this emerging field of photonic integrated circuits. It explores the current state-of-the-art and highlights the potential applications of optical memristors, which combine the benefits of ultrafast, high-bandwidth optical communication with local information processing. However, scalability emerged as the most pressing issue that future research should address. 

“Scaling up in-memory or neuromorphic computing in the optical domain is a huge challenge. Having a technology that is fast, compact, and efficient makes scaling more achievable and would represent a huge step forward,” explained Youngblood. 

“One example of the limitations is that if you were to take phase change materials, which currently have the highest storage density for optical memory, and try to implement a relatively simplistic neural network on-chip, it would take a wafer the size of a laptop to fit all the memory cells needed,” he continued. “Size matters for photonics, and we need to find a way to improve the storage density, energy efficiency, and programming speed to do useful computing at useful scales.”

Using Light to Revolutionize Computing

Optical memristors can revolutionize computing and information processing across several applications. They can enable active trimming of photonic integrated circuits (PICs), allowing for on-chip optical systems to be adjusted and reprogrammed as needed without continuously consuming power. They also offer high-speed data storage and retrieval, promising to accelerate processing, reduce energy consumption, and enable parallel processing. 

Optical memristors can even be used for artificial synapses and brain-inspired architectures. Dynamic memristors with nonvolatile storage and nonlinear output replicate the long-term plasticity of synapses in the brain and pave the way for spiking integrate-and-fire computing architectures.

Research to scale up and improve optical memristor technology could unlock unprecedented possibilities for high-bandwidth neuromorphic computing, machine learning hardware, and artificial intelligence. 

“We looked at a lot of different technologies. The thing we noticed is that we’re still far away from the target of an ideal optical memristor–something that is compact, efficient, fast, and changes the optical properties in a significant manner,” Youngblood said. “We’re still searching for a material or a device that actually meets all these criteria in a single technology in order for it to drive the field forward.”

The publication of “Integrated Optical Memristors” (DOI: 10.1038/s41566-023-01217-w) was published in Nature Photonics and is coauthored by senior author Harish Bhaskaran at the University of Oxford, Wolfram Pernice at Heidelberg University, and Carlos Ríos at the University of Maryland.

Despite including that final paragraph, I’m also providing a link to and a citation for the paper,

Integrated optical memristors by Nathan Youngblood, Carlos A. Ríos Ocampo, Wolfram H. P. Pernice & Harish Bhaskaran. Nature Photonics volume 17, pages 561–572 (2023) DOI: https://doi.org/10.1038/s41566-023-01217-w Published online: 29 May 2023 Issue Date: July 2023

This paper is behind a paywall.

Organic-inorganic nanohybrids: organic ligands attached to colloidal inorganic nanocrystals

A May 24, 2023 news item on phys.org introduces organic-inorganic nanohybrids for optoelectronic devices,

When designing optoelectronic devices, such as solar cells, photocatalysts, and photodetectors, scientists usually prioritize materials that are stable and possess tunable properties. This allows them precise control over optical characteristics of the materials and ensures retention of their properties over time, despite varying environmental conditions.

Organic-inorganic nanohybrids, which are made up of organic ligands attached to the surface of colloidal inorganic nanocrystals via coordinate bonds, are promising in this regard. They are known to exhibit enhanced stability owing to the formation of a protective layer by organic ligands around the reactive inorganic nanocrystal. However, the incorporation of organic ligands has been found to lower the conductivity and photon absorption efficiency of inorganic nanocrystals.

In a breakthrough study on ligand-nanocrystal interactions, researchers from Japan now demonstrate a quasi-reversible displacement of organic ligands on the surface of nanocrystals. Their findings, published in ACS Nano, provide a new perspective to the common belief that the organic ligands are anchored to the surface of the nanocrystals.

A May 22, 2023 Ritsumeikan University (Japan) press release (also on EurekAlert but published May 24, 2023), which originated the news item, provides more detail,

… The research team, led by Professor Yoichi Kobayashi from Ritsumeikan University, Japan, found that the coordination bond between perylene bisimide with a carboxyl group (PBI) and inorganic zinc sulfide (ZnS) nanocrystals can be reversibly displaced by exposing the material to visible light.

Shedding light on this novel behavior of organic-inorganic nanohybrids, Prof. Kobayashi says, “We explored the ligand properties of organic-inorganic nanohybrid systems by using perylene bisimide with a carboxyl group (PBI)-coordinated zinc sulfide (ZnS) NCs (PBI–ZnS) as a model system. Our findings provide the first example of photoinduced displacement of aromatic ligands with semiconductor nanocrystals.”

In their study, the researchers carried out both theoretical analysis and experimental investigations to understand the material’s unique photoinducible characteristics. They first conducted density functional theory calculations to study the structure and orbitals of PBI–ZnS ([PBI-Zn25S31]) in both its ground and first excited states. Next, they performed time-resolved impulsive stimulated Raman spectroscopy to excite the sample with an ultrafast laser. This helped them analyze the corresponding Raman spectrum that revealed the nature of the excited state of PBI–ZnS.

The experimental observations and calculations showed that, upon photoexcitation, an electron is excited from the PBI molecule, and the corresponding “hole”(the vacancy formed due to the absence of the electron) rapidly moves from the aromatic ligand (PBI) to ZnS. This results in a long-lived, negatively-charged PBI ion that is displaced from the surface of the ZnS nanocrystal. Over time, however, the displaced ligands recombine with the surface defects of the ZnS nanocrystal, leading to a quasi-reversible photoinduced displacement of coordinated PBI. Notably, the dynamic behavior of coordinated ligand molecules observed in this study is different from that observed for typical photoinduced charge transfer processes in which the hole typically remains on the donor molecule, enabling it to recombine with the electron quickly.

Explaining the significance of these findings, Prof. Kobayashi says, “The precise understanding of ligand-nanocrystal interaction is important not only for fundamental nanoscience but also for developing advanced photofunctional materials using nanomaterials. These include photocatalysts for the decomposition of persistent chemicals using visible light and photoconductive microcircuit patterning for wearable devices.”

Indeed, the results of this study present a promising avenue for enhancing the tunability and functionality of inorganic materials with aromatic molecules. This, in turn, could significantly impact the field of fundamental nanoscience and photochemistry in the times to come.

Here’s a link to and a citation for the paper,

Quasi-Reversible Photoinduced Displacement of Aromatic Ligands from Semiconductor Nanocrystals by Daisuke Yoshioka, Yusuke Yoneda, I-Ya Chang, Hikaru Kuramochi, Kim Hyeon-Deuk, and Yoichi Kobayashi. ACS Nano 2023, 17, 12, 11309–11317 DOI: https://doi.org/10.1021/acsnano.2c12578 Publication Date:May 9, 2023 Copyright © 2023 American Chemical Society

This paper is behind a paywall.

A nonvolatile photo-memristor

Credit: by Xiao Fu, Tangxin Li, Bin Caid, Jinshui Miao, Gennady N. Panin, Xinyu Ma, Jinjin Wang, Xiaoyong Jiang, Qing Lia, Yi Dong, Chunhui Hao, Juyi Sun, Hangyu Xu, Qixiao Zhao, Mengjia Xia, Bo Song, Fansheng Chen, Xiaoshuang Chen, Wei Lu, Weida Hu

it took a while to get there but the February 13, 2023 news item on phys.org announced research into extending memristors from tunable conductance to reconfigurable photo-response,

In traditional vision systems, the optical information is captured by a frame-based digital camera, and then the digital signal is processed afterwards using machine-learning algorithms. In this scenario, a large amount of data (mostly redundant) has to be transferred from a standalone sensing elements to the processing units, which leads to high latency and power consumption.

To address this problem, much effort has been devoted to developing an efficient approach, where some of the memory and computational tasks are offloaded to sensor elements that can perceive and process the optical signal simultaneously.

In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Weida Hu from School of Physics and Optoelectronic Engineering, Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences, Hangzhou, China, State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, China, and co-workers have developed a non-volatile photo-memristor, in which the reconfigurable responsivity can be modulated by the charge and/or photon flux through it and further stored in the device.

A February 13, 2023 Chinese Academy of Sciences press release, which originated the news item, provided more technical detail about the work,

The non-volatile photo-memristor has a simple two-terminal architecture, in which photoexcited carriers and oxygen-related ions are coupled, leading to a displaced and pinched hysteresis in the current-voltage characteristics. For the first time, non-volatile photo-memristors implement computationally complete logic with photoresponse-stateful operations, for which the same photo-memristor serves as both a logic gate and memory, using photoresponse as a physical state variable instead of light, voltage and memresistance. Polarity reversal of photo-memristors shows great potential for in-memory sensing and computing with feature extraction and image recognition for neuromorphic vision.

The photo-memristor demonstrates tunable short-circuit current in a non-volatile mode under illumination. By mimicking the biological functionalities of the human retina and designing specific device structures, the devices can act as neural network for neuromorphic visual processing and implementation of completely photoresponse-stateful logic operations triggered by electrical and light stimuli together. It can support various kinds of sensing tasks with all-in-one sensing-memory-computing approaches. These scientists summarize the operational principle and feature of their device:

“We design[ed] a two-terminal device with MoS2-xOx and specific graphene for three purposes in one: (1) to provide low barrier energy for the migration of oxygen ions; (2) to perform as geometry-asymmetric metal–semiconductor–metal van der Waals heterostructures with multi-photoresponse states; and (3) as an extension of a memristor, this device not only provides tunable conductance, but also demonstrates reconfigurable photoresponse for reading at zero bias voltage.”

“Moreover, the tunable short-circuit photocurrent and photoresponse can be increased to 889.8 nA and 98.8 mA/W, respectively, which are much higher than that of other reconfigurable phototransistors based on 2D materials. To reverse the channel polarity and obtain a gate-tunable short-circuit photocurrent, the channel semiconductor must be thin enough. Thus, it is difficult to use the thick film needed to absorb enough light to get a large signal. In our case, the mechanism of the two-terminal device rearrangement is based on ion migration, which is not limited by the thickness. We can increase the thickness of the film to absorb more photons and get a large short-circuit photocurrent.” they added.

“This new concept of a two-terminal photo-memristor not only enables all-in-one sensing-memory-computing approaches for neuromorphic vision hardware, but also brings great convenience for high-density integration.” the scientists forecast.

Here’s a link to and a citation for the paper,

Graphene/MoS2−xOx/graphene photomemristor with tunable non-volatile responsivities for neuromorphic vision processing by Xiao Fu, Tangxin Li, Bin Caid, Jinshui Miao, Gennady N. Panin, Xinyu Ma, Jinjin Wang, Xiaoyong Jiang, Qing Lia, Yi Dong, Chunhui Hao, Juyi Sun, Hangyu Xu, Qixiao Zhao, Mengjia Xia, Bo Song, Fansheng Chen, Xiaoshuang Chen, Wei Lu, Weida Hu. Light: Science & Applications volume 12, Article number: 39 (2023) DOI: https://doi.org/10.1038/s41377-023-01079-5 Published: 07 February 2023

This paper is open access.

A brainlike (neuromorphic) camera can go beyond diffraction limit of light

Just when I think I’m getting caught up with my backlog along comes something like this. A February 21, 2023 news item on Nanowerk announces research that combines neuromorphic (brainlike) engineering and nanotechnology, Note: A link has been removed,

In a new study, researchers at the Indian Institute of Science (IISc) show how a brain-inspired image sensor can go beyond the diffraction limit of light to detect miniscule objects such as cellular components or nanoparticles invisible to current microscopes. Their novel technique, which combines optical microscopy with a neuromorphic camera and machine learning algorithms, presents a major step forward in pinpointing objects smaller than 50 nanometers in size.

A February 21, 2023 (?) Indian Institute of Science (IISc) press release (also on EurekAlert), which originated the news item, describes the nature of the task and provides some technical details,

Since the invention of optical microscopes, scientists have strived to surpass a barrier called the diffraction limit, which means that the microscope cannot distinguish between two objects if they are smaller than a certain size (typically 200-300 nanometers). Their efforts have largely focused on either modifying the molecules being imaged, or developing better illumination strategies – some of which led to the 2014 Nobel Prize in Chemistry. “But very few have actually tried to use the detector itself to try and surpass this detection limit,” says Deepak Nair, Associate Professor at the Centre for Neuroscience (CNS), IISc, and corresponding author of the study.  

Measuring roughly 40 mm (height) by 60 mm (width) by 25 mm (diameter), and weighing about 100 grams, the neuromorphic camera used in the study mimics the way the human retina converts light into electrical impulses, and has several advantages over conventional cameras. In a typical camera, each pixel captures the intensity of light falling on it for the entire exposure time that the camera focuses on the object, and all these pixels are pooled together to reconstruct an image of the object. In neuromorphic cameras, each pixel operates independently and asynchronously, generating events or spikes only when there is a change in the intensity of light falling on that pixel. This generates sparse and lower amount of data compared to traditional cameras, which capture every pixel value at a fixed rate, regardless of whether there is any change in the scene. This functioning of a neuromorphic camera is similar to how the human retina works, and allows the camera to “sample” the environment with much higher temporal resolution – because it is not limited by a frame rate like normal cameras – and also perform background suppression.  

“Such neuromorphic cameras have a very high dynamic range (>120 dB), which means that you can go from a very low-light environment to very high-light conditions. The combination of the asynchronous nature, high dynamic range, sparse data, and high temporal resolution of neuromorphic cameras make them well-suited for use in neuromorphic microscopy,” explains Chetan Singh Thakur, Assistant Professor at the Department of Electronic Systems Engineering (DESE), IISc, and co-author. 

In the current study, the group used their neuromorphic camera to pinpoint individual fluorescent beads smaller than the limit of diffraction, by shining laser pulses at both high and low intensities, and measuring the variation in the fluorescence levels. As the intensity increases, the camera captures the signal as an “ON” event, while an “OFF” event is reported when the light intensity decreases. The data from these events were pooled together to reconstruct frames. 

To accurately locate the fluorescent particles within the frames, the team used two methods. The first was a deep learning algorithm, trained on about one and a half million image simulations that closely represented the experimental data, to predict where the centroid of the object could be, explains Rohit Mangalwedhekar, former research intern at CNS and first author of the study. A wavelet segmentation algorithm was also used to determine the centroids of the particles separately for the ON and the OFF events. Combining the predictions from both allowed the team to zero in on the object’s precise location with greater accuracy than existing techniques.  

“In biological processes like self-organisation, you have molecules that are alternating between random or directed movement, or that are immobilised,” explains Nair. “Therefore, you need to have the ability to locate the centre of this molecule with the highest precision possible so that we can understand the thumb rules that allow the self-organisation.” The team was able to closely track the movement of a fluorescent bead moving freely in an aqueous solution using this technique. This approach can, therefore, have widespread applications in precisely tracking and understanding stochastic processes in biology, chemistry and physics.  

Caption: Transformation of cumulative probability density of ON and OFF processes allows localisation below the limit of classical single particle detection. Credit: Mangalwedhekar et al

Here’s a link to and a citation for the paper,

Achieving nanoscale precision using neuromorphic localization microscopy by Rohit Mangalwedhekar, Nivedita Singh, Chetan Singh Thakur, Chandra Sekhar Seelamantula, Mini Jose & Deepak Nair. Nature Nanotechnology volume 18, pages 380–389 (2023) DOI: https://doi.org/10.1038/s41565-022-01291-1 Published online: 23 January 2023 Issue Date: April 2023

This paper is behind a paywall.

Treating cardiac arrhythmia with light: a graphene tattoo

An April 17, 2023 news item on Nanowerk announced research into a graphene cardiac implant/tattoo,

Researchers led by Northwestern University and the University of Texas at Austin (UT) have developed the first cardiac implant made from graphene, a two-dimensional super material with ultra-strong, lightweight and conductive properties.

Similar in appearance to a child’s temporary tattoo, the new graphene “tattoo” implant is thinner than a single strand of hair yet still functions like a classical pacemaker. But unlike current pacemakers and implanted defibrillators, which require hard, rigid materials that are mechanically incompatible with the body, the new device softly melds to the heart to simultaneously sense and treat irregular heartbeats. The implant is thin and flexible enough to conform to the heart’s delicate contours as well as stretchy and strong enough to withstand the dynamic motions of a beating heart.

Caption: Graphene implant on tattoo paper. Credit: Ning Liu/University of Texas at Austin

An April 17, 2023 Northwestern University news release (also on EurekAlert), which originated the news item, provides more detail about the research, graphene, and the difficulties of monitoring a beating heart, Note: Links have been removed,

After implanting the device into a rat model, the researchers demonstrated that the graphene tattoo could successfully sense irregular heart rhythms and then deliver electrical stimulation through a series of pulses without constraining or altering the heart’s natural motions. Even better: The technology also is optically transparent, allowing the researchers to use an external source of optical light to record and stimulate the heart through the device.

The study will be published on Thursday (April 20 [2023]) in the journal Advanced Materials. It marks the thinnest known cardiac implant to date.

“One of the challenges for current pacemakers and defibrillators is that they are difficult to affix onto the surface of the heart,” said Northwestern’s Igor Efimov, the study’s senior author. “Defibrillator electrodes, for example, are essentially coils made of very thick wires. These wires are not flexible, and they break. Rigid interfaces with soft tissues, like the heart, can cause various complications. By contrast, our soft, flexible device is not only unobtrusive but also intimately and seamlessly conforms directly onto the heart to deliver more precise measurements.”

An experimental cardiologist, Efimov is a professor of biomedical engineering at Northwestern’s McCormick School of Engineering and professor of medicine at Northwestern University Feinberg School of Medicine. He co-led the study with Dmitry Kireev, a research associate at UT. Zexu Lin, a Ph.D. candidate in Efimov’s laboratory, is the paper’s first author.

Miracle material

Known as cardiac arrhythmias, heart rhythm disorders occur when the heart beats either too quickly or too slowly. While some cases of arrhythmia are not serious, many cases can lead to heart failure, stroke and even sudden death. In fact, complications related to arrythmia claim about 300,000 lives annually in the United States. Physicians commonly treat arrhythmia with implantable pacemakers and defibrillators that detect abnormal heartbeats and then correct rhythm with electrical stimulation. While these devices are lifesaving, their rigid nature may constrain the heart’s natural motions, injure soft tissues, cause temporary discomfort and induce complications, such as painful swelling, perforations, blood clots, infection and more.

With these challenges in mind, Efimov and his team sought to develop a bio-compatible device ideal for conforming to soft, dynamic tissues. After reviewing multiple materials, the researchers settled on graphene, an atomically thin form of carbon. With its ultra-strong, lightweight structure and superior conductivity, graphene has potential for many applications in high-performance electronics, high-strength materials and energy devices.

“For bio-compatibility reasons, graphene is particularly attractive,” Efimov said. “Carbon is the basis of life, so it’s a safe material that is already used in different clinical applications. It also is flexible and soft, which works well as an interface between electronics and a soft, mechanically active organ.”

Hitting a beating target

At UT, study co-authors Dimitry Kireev and Deji Akinwande were already developing graphene electronic tattoos (GETs) with sensing capabilities. Flexible and weightless, their team’s e-tattoos adhere to the skin to continuously monitor the body’s vital signs, including blood pressure and the electrical activity of the brain, heart and muscles.

But, while the e-tattoos work well on the skin’s surface, Efimov’s team needed to investigate new methods to use these devices inside the body — directly onto the surface of the heart.

“It’s a completely different application scheme,” Efimov said. “Skin is relatively dry and easily accessible. Obviously, the heart is inside the chest, so it’s difficult to access and in a wet environment.”

The researchers developed an entirely new technique to encase the graphene tattoo and adhere it to the surface of a beating heart. First, they encapsulated the graphene inside a flexible, elastic silicone membrane — with a hole punched in it to give access to the interior graphene electrode. Then, they gently placed gold tape (with a thickness of 10 microns) onto the encapsulating layer to serve as an electrical interconnect between the graphene and the external electronics used to measure and stimulate the heart. Finally, they placed it onto the heart. The entire thickness of all layers together measures about 100 microns in total.

The resulting device was stable for 60 days on an actively beating heart at body temperature, which is comparable to the duration of temporary pacemakers used as bridges to permanent pacemakers or rhythm management after surgery or other therapies.

Optical opportunities

Leveraging the device’s transparent nature, Efimov and his team performed optocardiography — using light to track and modulate heart rhythm — in the animal study. Not only does this offer a new way to diagnose and treat heart ailments, the approach also opens new possibilities for optogenetics, a method to control and monitor single cells with light. 

While electrical stimulation can correct a heart’s abnormal rhythm, optical stimulation is more precise. With light, researchers can track specific enzymes as well as interrogate specific heart, muscle or nerve cells.

“We can essentially combine electrical and optical functions into one biointerface,” Efimov said. “Because graphene is optically transparent, we can actually read through it, which gives us a much higher density of readout.”

The University of Texas at Austin issued an April 18, 2023 news release and as you would expect the focus is on their researchers, Note 1: I’ve removed many but not all of the redundancies between the two news releases; Note 2: A link has been removed,

A new cardiac implant made from graphene, a two-dimensional super material with ultra-strong, lightweight and conductive properties, functions like a classic pacemaker with some major improvements.

A team led by researchers from The University of Texas at Austin and Northwestern University developed the implantable derivative from wearable graphene-based electronic tattoo, or e-tattoo – graphene biointerface. The device, detailed in the journal Advanced Materials, marks the thinnest known cardiac implant to date.

“It’s very exciting to take our e-tattoo technology and use it as an implantable device inside the body,” said Dmitry Kireev, a postdoctoral research associate in the lab of professor Deji Akinwande’s lab at UT Austin who co-led the research. “The fact that is much more compatible with the human body, lightweight, and transparent, makes this a more natural solution for people dealing with heart problems.”

Hitting a beating target

At UT Austin, Akinwande and his team had been developing e-tattoos using graphene for several years, with a variety of functions, including monitoring body signals. Flexible and weightless, their team’s e-tattoos adhere to the skin to continuously monitor the body’s vital signs, including blood pressure and the electrical activity of the brain, heart and muscles.

But, while the e-tattoos work well on the skin’s surface, the researchers needed to find new ways to deploy these devices inside the body — directly onto the surface of the heart.

“The conditions inside the body are very different compared to affixing a device to the skin, so we had to re-imagine how we package our e-tattoo technology,” said Akinwande, a professor in the Chandra Family Department of Electrical and Computer Engineering.  

The researchers developed an entirely new technique to encase the graphene tattoo and adhere it to the surface of a beating heart. …

Here’s a link to and a citation for the paper,

Graphene Biointerface for Cardiac Arrhythmia Diagnosis and Treatment by Zexu Lin, Dmitry Kireev, Ning Liu, Shubham Gupta, Jessica LaPiano, Sofian N. Obaid, Zhiyuan Chen, Deji Akinwande, Igor R. Efimov. Advanced Materials Volume 35, Issue 22 June 1, 2023 2212190 DOI: https://doi.org/10.1002/adma.202212190 First published online: 25 March 2023

This paper is open access.