Tag Archives: metasurfaces

Nano-clouds change colour and temperature and can provide thermal camouflage

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Caption: The bio-inspired metasurfaces act like a real cloud, enabling daytime cooling, heating and thermal camouflage in a single solution. Credit: Mady Elbahri / Aalto University

While I don’t have a military story for today (Remembrance Day, November 11, 2025), there is the ‘camouflage’ story. A June 30, 2025 news item on Nanowerk announces research into mimicking clouds,

How does a cloud stay cool under direct sunlight –– or seem to vanish in infrared? In nature, phenomena like white cumulus clouds, grey storm systems, and even the hollow hairs of polar bears offer remarkable lessons in balancing temperature, colour and invisibility. Inspired by these atmospheric marvels, researchers have now created a nanoscale ‘cloud’ metasurface capable of dynamically switching between white and grey states — cooling or heating on demand –– all while evading thermal detection.

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A June 27, 2025 Aalto University press release (also on EurekAlert but published June 30, 2025), which originated the news item, further describes research into simulating clouds,\

There is a major global push for passive, energy-efficient thermal management in building materials, wearables, sensors and defence applications. This newly invented system fits perfectly into emerging fields like radiative cooling, adaptive coatings, and thermal heating and thermal camouflage under climate and security pressures.

Much like the transformation from bright cumulus to dark cumulonimbus clouds, this metasurface uses multiple scattering, absorption and polarizonic reflection principles to modulate light and heat. In its ‘white’ state, it strongly backscatters sunlight to enable radiative cooling, while the ‘grey’ state absorbs sunlight efficiently for high-performance heating. Crucially, both states remain ‘invisible’ to infrared sensors due to low mid-infrared emissivity — something no previous surface has achieved.

‘We’ve engineered a nanoscale cloud on every surface. It can tune its colour and temperature like a real cloud — between cooling white and heating grey — while staying hidden from thermal cameras,’ Professor Mady Elbahri from Aalto University explains.

Both white and grey metasurfaces overcome limitations of traditional coatings

Typical white paints cool surfaces by scattering sunlight in all directions, but they still glow in heat vision. This new material works more like a cloud — cooling by bouncing sunlight back and staying hidden from heat sensors.

Conventional white coatings (e.g., titanium dioxide, TiO₂ based) scatter sunlight diffusively, but are only effective in shaded conditions or at night. Their high emissivity in the 8–13 μm range makes them bright in thermal infrared imaging, limiting use in thermal stealth.
‘This new white plasmonic metasurface scatters sunlight through disordered metallic nanostructures while minimising thermal emission — cooling surfaces in full sunlight and remaining thermally camouflaged. This feature makes the innovation groundbreaking,’ says Adel Assad, a PhD student in the group.

Black materials get hot in the sun but also light up thermal cameras as they emit infrared strongly.

‘This grey surface gets hotter than black—but without sending out heat that can be seen by heat sensors. This could be a game-changer for smart textiles, building materials, and camouflage, says Moheb Abdelaziz, a postdoctoral researcher in the group.

Great potential grows from humble beginnings

The research opens new pathways in adaptive surface engineering. Potential applications span from zero-energy building facades that switch between heating and cooling to smart textiles that regulate body temperature without electronics. The discovery also presents opportunities in low-visibility sensors and devices for defence and surveillance.

The next step for the research is to explore dynamic coatings using electrochromic or phase-changing layers for real-time, user-controlled switching between states.

The researchers are proud that the remarkable findings came despite an initial project rejection.

‘With no dedicated funding after initial setbacks, we relied on shared vision and collaboration –– especially with our partners in Germany –– to turn doubt into discovery. It’s proof that science, like clouds, can rise against the odds,’ says Elbahri.

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

Cloud Inspired White and Grey Plasmonic Metasurfaces for Camouflaged Thermal Management by Mhd Adel Assad, Moheb Abdelaziz, Torge Hartig, Thomas Strunskus, Alexander Vahl, Franz Faupel, Mady Elbahri. Advanced Materials DOI: https://doi.org/10.1002/adma.202501080 First published: 06 June 2025

This paper is behind a paywall.

Nanoscale device, which steers & shifts frequency of optical light, could point way to future wireless communication channels

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It seems like there’s never enough memory or enough speed where telecommunication is concerned. According to a July 24, 2023 news item on ScienceDaily announces a new way of transmitting large of amounts of data on earth and in outer space,

It is a scene many of us are familiar with: You’re working on your laptop at the local coffee shop with maybe a half dozen other laptop users — each of you is trying to load websites or stream high-definition videos, and all are craving more bandwidth. Now imagine that each of you had a dedicated wireless channel for communication that was hundreds of times faster than the Wi-Fi we use today, with hundreds of times more bandwidth. That dream may not be far off thanks to the development of metasurfaces — tiny engineered sheets that can reflect and otherwise direct light in desired ways.

In a paper published today [July 24, 2024] in the journal Nature Nanotechnology, a team of Caltech engineers reports building such a metasurface patterned with miniscule tunable antennas capable of reflecting an incoming beam of optical light to create many sidebands, or channels, of different optical frequencies.

“With these metasurfaces, we’ve been able to show that one beam of light comes in, and multiple beams of light go out, each with different optical frequencies and going in different directions,” says Harry Atwater, the Otis Booth Leadership Chair of the Division of Engineering and Applied Science, the Howard Hughes Professor of Applied Physics and Materials Science, and senior author on the new paper. “It’s acting like an entire array of communication channels. And we’ve found a way to do this for free-space signals rather than signals carried on an optical fiber.”

The work points to a promising route for the development of not only a new type of wireless communication channel but also potentially new range-finding technologies and even a novel way to relay larger amounts of data to and from space.

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A July 24, 2024 California Institute of Technology (CalTech) news release (also on EurekAlert) by Kimm Fesenmaier, which originated the news item, delves further into the research,

Going beyond conventional optical elements

Co-lead author on the new paper Prachi Thureja, a graduate student in Atwater’s group, says to understand their work, first consider the word “metasurface.” The root, “meta,” comes from a Greek prefix meaning “beyond.” Metasurfaces are designed to go beyond what we can do with conventional bulky optical elements, such as camera or microscope lenses. The multilayer transistor-like devices are engineered with a carefully selected pattern of nanoscale antennas that can reflect, scatter, or otherwise control light. These flat devices can focus light, in the style of a lens, or reflect it, like a mirror, by strategically designing an array of nanoscale elements that modify the way that light responds.

Much previous work with metasurfaces has focused on creating passive devices that have a single light-directing functionality that is fixed in time. In contrast, Atwater’s group focuses on what are known as active metasurfaces. “Now we can apply an external stimulus, such as an array of different voltages, to these devices and tune between different passive functionalities,” says Jared Sisler, also a graduate student in Atwater’s lab and co-lead author on the paper.

In the latest work, the team describes what they call a space-time metasurface that can reflect light in specific directions and also at particular frequencies (a function of time, since frequency is defined as the number of waves that pass a point per second). This metasurface device, the core of which is just 120 microns wide and 120 microns long, operates in reflection mode at optical frequencies typically used for telecommunications, specifically at 1,530 nanometers. This is thousands of times higher than radio frequencies, which means there is much more available bandwidth.

At radio frequencies, electronics can easily steer a beam of light in different directions. This is routinely accomplished by the radar navigation devices used on airplanes. But there are currently no electronic devices that can do this at the much higher optical frequencies. Therefore, the researchers had to try something different, which was to change the properties of the antennas themselves.

Sisler and Thureja created their metasurface to consist of gold antennas, with an underlying electrically tunable semiconductor layer of indium tin oxide. By applying a known voltage profile across the device, they can locally modulate the density of electrons in the semiconductor layer below each antenna, changing its refractive index (the material’s light-bending ability). “By having the spatial configuration of different voltages across the device, we can then redirect the reflected light at specified angles in real time without the need to swap out any bulky components,” Thureja says.

“We have an incident laser hitting our metasurface at a certain frequency, and we modulate the antennas in time with a high-frequency voltage signal. This generates multiple new frequencies, or sidebands, that are carried by the incident laser light and can be used as high-data-rate channels for sending information. On top of this, we still have spatial control, meaning we can choose where each channel goes in space,” explains Sisler. “We are generating frequencies and steering them in space. That’s the space-time component of this metasurface.”

Looking toward the future

Beyond demonstrating that such a metasurface is capable of splitting and redirecting light at optical frequencies in free space (rather than in optical fibers), the team says the work points to several possible applications. These metasurfaces could be useful in LiDAR applications, the light equivalent of radar, where light is used to capture the depth information from a three-dimensional scene. The ultimate dream is to develop a “universal metasurface” that would create multiple optical channels, each carrying information in different directions in free space.

“If optical metasurfaces become a realizable technology that proliferates, a decade from now you’ll be able to sit in a Starbucks with a bunch of other people on their laptops and instead of each person getting a radio frequency Wi-Fi signal, they will get their own high-fidelity light beam signal,” says Atwater, who is also the director of the Liquid Sunlight Alliance at Caltech. “One metasurface will be able to beam a different frequency to each person.”

The group is collaborating with the Optical Communications Laboratory at JPL, which is working on using optical frequencies rather than radio frequency waves for communicating with space missions because this would enable the ability to send much more data at higher frequencies. “These devices would be perfect for what they’re doing,” says Sisler.

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

Electrically tunable space–time metasurfaces at optical frequencies by Jared Sisler, Prachi Thureja, Meir Y. Grajower, Ruzan Sokhoyan, Ivy Huang & Harry A. Atwater. Nature Nanotechnology (2024) DOI: https://doi.org/10.1038/s41565-024-01728-9 Published: 24 July 2024

This paper is behind a paywall.

Sound absorbing wallpaper inspired by moth wings

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Up close and personal with a moth,

Caption: Antheraea pernyi Credit: University of Bristol

As for how this creature’s wings might provide inspiration for sound absorbing wallpaper, there’s this June 14, 2022 news item on phys.org, Note: A link has been removed,

Experts at the University of Bristol have discovered that the scales on moth wings act as excellent sound absorbers even when placed on an artificial surface.

The researchers, who recently discovered that moth wings offer acoustic protection from bat echolocation calls, have been studying whether their structure could inform better performing sound absorbing panels when not moving in free space.

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A June 15, 2022 University of Bristol press release (also on EurekAlert but published on June 14, 2022), which originated the news item, delves further into the research, Note: Links have been removed,

Bats and moths have been involved in an acoustic arms race between predator and prey ever since bats evolved echolocation some 65 million years ago. Moths are under huge predation pressure from bats and have evolved a plethora of defences in their strive for survival, but it’s the scales, on a moth wing, that hold the key to transforming noise-cancelling technology.

Prof Marc Holderied, of Bristol’s School of Biological Sciences, said: “What we needed to know first, was how well these moth scales would perform if they were in front of an acoustically highly reflective surface, such as a wall.

“We also needed to find out how the mechanisms of absorption might change when the scales were interacting with this surface.”

Prof Holderied and his team tested this by placing small sections of moth wings on an aluminium disc, then systematically tested how orientation of the wing with respect to the incoming sound and the removal of scale layers affected absorption.

Remarkably, they found that moth wings proved to be excellent sound absorbers, even when on top of an acoustical solid substrate, with the wings absorbing as much as 87% of the incoming sound energy. The effect is also broadband and omnidirectional, covering a wide range of frequencies and sound incident angles.

“What is even more impressive is that the wings are doing this whilst being incredibly thin, with the scale layer being only 1/50th of the thickness of the wavelength of the sound that they are absorbing,” explained lead author Dr Thomas Neil. “This extraordinary performance qualifies the moth wing as a natural occurring acoustic absorbing metasurface, a material that has unique properties and capabilities, that are not possible to create using conventional materials.”

The potential to create ultrathin sound absorbing panels has huge implications in building acoustics. As cities get louder, the need for efficient non-intrusive sound mitigation solutions grows. Equally, these lightweight sound absorbing panels could have huge impacts on the travel industry, with any weight saving in planes, cars and trains increasing efficiency in these modes of transport, reducing fuel use and CO2 emissions.  

Now the scientists plan to replicate the sound absorbing performance by designing and building prototypes based on the sound absorbing mechanisms of the moth. The absorption that they have characterised in moth wing scales is all in the ultrasound frequency range, above that which humans can hear. Their next challenge is to design a structure that will work at lower frequencies whilst retaining the same ultrathin architecture employed by the moth.  

Prof Holderied concluded: “Moths are going to inspire the next generation of sound absorbing materials.

“New research has shown that one day it will be possible to adorn the walls of your house with ultrathin sound absorbing wallpaper, using a design that copies the mechanisms that gives moths stealth acoustic camouflage.”

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

Moth wings as sound absorber metasurface by Thomas R. Neil, Zhiyuan Shen, Daniel Robert, Bruce W. Drinkwater and Marc W. Holderied. Proceedings of the Royal Society A Mathematical Physical and Engineering Sciences DOI: https://doi.org/10.1098/rspa.2022.0046 Published:15 June 2022

This paper is open access.

Nanosensors use AI to explore the biomolecular world

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EPFL scientists have developed AI-powered nanosensors that let researchers track various kinds of biological molecules without disturbing them. Courtesy: École polytechnique fédérale de Lausanne (EPFL)

If you look at the big orange dot (representing the nanosensors?), you’ll see those purplish/fuschia objects resemble musical notes (biological molecules?). I think that brainlike object to the left and in light blue is the artificial intelligence (AI) component. (If anyone wants to correct my guesses or identify the bits I can’t, please feel free to add to the Comments for this blog.)

Getting back to my topic, keep the ‘musical notes’ in mind as you read about some of the latest research from l’École polytechnique fédérale de Lausanne (EPFL) in an April 7, 2021 news item on Nanowerk,

The tiny world of biomolecules is rich in fascinating interactions between a plethora of different agents such as intricate nanomachines (proteins), shape-shifting vessels (lipid complexes), chains of vital information (DNA) and energy fuel (carbohydrates). Yet the ways in which biomolecules meet and interact to define the symphony of life is exceedingly complex.

Scientists at the Bionanophotonic Systems Laboratory in EPFL’s School of Engineering have now developed a new biosensor that can be used to observe all major biomolecule classes of the nanoworld without disturbing them. Their innovative technique uses nanotechnology, metasurfaces, infrared light and artificial intelligence.

To each molecule its own melody

In this nano-sized symphony, perfect orchestration makes physiological wonders such as vision and taste possible, while slight dissonances can amplify into horrendous cacophonies leading to pathologies such as cancer and neurodegeneration.

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An April 7, 2021 EPFL press release, which originated the news item, provides more detail,

“Tuning into this tiny world and being able to differentiate between proteins, lipids, nucleic acids and carbohydrates without disturbing their interactions is of fundamental importance for understanding life processes and disease mechanisms,” says Hatice Altug, the head of the Bionanophotonic Systems Laboratory. 

Light, and more specifically infrared light, is at the core of the biosensor developed by Altug’s team. Humans cannot see infrared light, which is beyond the visible light spectrum that ranges from blue to red. However, we can feel it in the form of heat in our bodies, as our molecules vibrate under the infrared light excitation.

Molecules consist of atoms bonded to each other and – depending on the mass of the atoms and the arrangement and stiffness of their bonds – vibrate at specific frequencies. This is similar to the strings on a musical instrument that vibrate at specific frequencies depending on their length. These resonant frequencies are molecule-specific, and they mostly occur in the infrared frequency range of the electromagnetic spectrum. 

“If you imagine audio frequencies instead of infrared frequencies, it’s as if each molecule has its own characteristic melody,” says Aurélian John-Herpin, a doctoral assistant at Altug’s lab and the first author of the publication. “However, tuning into these melodies is very challenging because without amplification, they are mere whispers in a sea of sounds. To make matters worse, their melodies can present very similar motifs making it hard to tell them apart.” 

Metasurfaces and artificial intelligence

The scientists solved these two issues using metasurfaces and AI. Metasurfaces are man-made materials with outstanding light manipulation capabilities at the nano scale, thereby enabling functions beyond what is otherwise seen in nature. Here, their precisely engineered meta-atoms made out of gold nanorods act like amplifiers of light-matter interactions by tapping into the plasmonic excitations resulting from the collective oscillations of free electrons in metals. “In our analogy, these enhanced interactions make the whispered molecule melodies more audible,” says John-Herpin.

AI is a powerful tool that can be fed with more data than humans can handle in the same amount of time and that can quickly develop the ability to recognize complex patterns from the data. John-Herpin explains, “AI can be imagined as a complete beginner musician who listens to the different amplified melodies and develops a perfect ear after just a few minutes and can tell the melodies apart, even when they are played together – like in an orchestra featuring many instruments simultaneously.” 

The first biosensor of its kind

When the scientists’ infrared metasurfaces are augmented with AI, the new sensor can be used to analyze biological assays featuring multiple analytes simultaneously from the major biomolecule classes and resolving their dynamic interactions. 

“We looked in particular at lipid vesicle-based nanoparticles and monitored their breakage through the insertion of a toxin peptide and the subsequent release of vesicle cargos of nucleotides and carbohydrates, as well as the formation of supported lipid bilayer patches on the metasurface,” says Altug.

This pioneering AI-powered, metasurface-based biosensor will open up exciting perspectives for studying and unraveling inherently complex biological processes, such as intercellular communication via exosomesand the interaction of nucleic acids and carbohydrates with proteins in gene regulation and neurodegeneration. 

“We imagine that our technology will have applications in the fields of biology, bioanalytics and pharmacology – from fundamental research and disease diagnostics to drug development,” says Altug. 

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

Infrared Metasurface Augmented by Deep Learning for Monitoring Dynamics between All Major Classes of Biomolecules by Aurelian John‐Herpin, Deepthy Kavungal. Lea von Mücke, Hatice Altug. Advanced Materials Volume 33, Issue 14 April 8, 2021 2006054 DOI: https://doi.org/10.1002/adma.202006054 First published: 22 February 2021

This paper is open access.

US Air Force wants to merge classical and quantum physics

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The US Air Force wants to merge classical and quantum physics for practical purposes according to a May 5, 2014 news item on Azonano,

The Air Force Office of Scientific Research has selected the Harvard School of Engineering and Applied Sciences (SEAS) to lead a multidisciplinary effort that will merge research in classical and quantum physics and accelerate the development of advanced optical technologies.

Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, will lead this Multidisciplinary University Research Initiative [MURI] with a world-class team of collaborators from Harvard, Columbia University, Purdue University, Stanford University, the University of Pennsylvania, Lund University, and the University of Southampton.

The grant is expected to advance physics and materials science in directions that could lead to very sophisticated lenses, communication technologies, quantum information devices, and imaging technologies.

“This is one of the world’s strongest possible teams,” said Capasso. “I am proud to lead this group of people, who are internationally renowned experts in their fields, and I believe we can really break new ground.”

A May 1, 2014 Harvard University School of Engineering and Applied Sciences news release, which originated the news item, provides a description of project focus: nanophotonics and metamaterials along with some details of Capasso’s work in these areas (Note: Links have been removed),

The premise of nanophotonics is that light can interact with matter in unusual ways when the material incorporates tiny metallic or dielectric features that are separated by a distance shorter than the wavelength of the light. Metamaterials are engineered materials that exploit these phenomena, producing strange effects, enabling light to bend unnaturally, twist into a vortex, or disappear entirely. Yet the fabrication of thick, or bulk, metamaterials—that manipulate light as it passes through the material—has proven very challenging.

Recent research by Capasso and others in the field has demonstrated that with the right device structure, the critical manipulations can actually be confined to the very surface of the material—what they have dubbed a “metasurface.” These metasurfaces can impart an instantaneous shift in the phase, amplitude, and polarization of light, effectively controlling optical properties on demand. Importantly, they can be created in the lab using fairly common fabrication techniques.

At Harvard, the research has produced devices like an extremely thin, flat lens, and a material that absorbs 99.75% of infrared light. But, so far, such devices have been built to order—brilliantly suited to a single task, but not tunable.

This project, however,is focused on the future (Note: Links have been removed),

“Can we make a rapidly configurable metasurface so that we can change it in real time and quickly? That’s really a visionary frontier,” said Capasso. “We want to go all the way from the fundamental physics to the material building blocks and then the actual devices, to arrive at some sort of system demonstration.”

The proposed research also goes further. A key thrust of the project involves combining nanophotonics with research in quantum photonics. By exploiting the quantum effects of luminescent atomic impurities in diamond, for example, physicists and engineers have shown that light can be captured, stored, manipulated, and emitted as a controlled stream of single photons. These types of devices are essential building blocks for the realization of secure quantum communication systems and quantum computers. By coupling these quantum systems with metasurfaces—creating so-called quantum metasurfaces—the team believes it is possible to achieve an unprecedented level of control over the emission of photons.

“Just 20 years ago, the notion that photons could be manipulated at the subwavelength scale was thought to be some exotic thing, far fetched and of very limited use,” said Capasso. “But basic research opens up new avenues. In hindsight we know that new discoveries tend to lead to other technology developments in unexpected ways.”

The research team includes experts in theoretical physics, metamaterials, nanophotonic circuitry, quantum devices, plasmonics, nanofabrication, and computational modeling. Co-principal investigator Marko Lončar is the Tiantsai Lin Professor of Electrical Engineering at Harvard SEAS. Co-PI Nanfang Yu, Ph.D. ’09, developed expertise in metasurfaces as a student in Capasso’s Harvard laboratory; he is now an assistant professor of applied physics at Columbia. Additional co-PIs include Alexandra Boltasseva and Vladimir Shalaev at Purdue, Mark Brongersma at Stanford, and Nader Engheta at the University of Pennsylvania. Lars Samuelson (Lund University) and Nikolay Zheludev (University of Southampton) will also participate.

The bulk of the funding will support talented graduate students at the lead institutions.

The project, titled “Active Metasurfaces for Advanced Wavefront Engineering and Waveguiding,” is among 24 planned MURI awards selected from 361 white papers and 88 detailed proposals evaluated by a panel of experts; each award is subject to successful negotiation. The anticipated amount of the Harvard-led grant is up to $6.5 million for three to five years.

For anyone who’s not familiar (that includes me, anyway) with MURI awards, there’s this from Wikipedia (Note: links have been removed),

Multidisciplinary University Research Initiative (MURI) is a basic research program sponsored by the US Department of Defense (DoD). Currently each MURI award is about $1.5 million a year for five years.

I gather that in addition to the Air Force, the Army and the Navy also award MURI funds.