Tag Archives: Mikhail A. Kats

Chameleon materials

Harvard’s School of Engineering and Applied Sciences researchers discovered some unexpected properties when testing a new coating according to an Oct. 22, 2013 news item on Azonano,

Active camouflage has taken a step forward at the Harvard School of Engineering and Applied Sciences (SEAS), with a new coating that intrinsically conceals its own temperature to thermal cameras.

In a laboratory test, a team of applied physicists placed the device on a hot plate and watched it through an infrared camera as the temperature rose. Initially, it behaved as expected, giving off more infrared light as the sample was heated: at 60 degrees Celsius it appeared blue-green to the camera; by 70 degrees it was red and yellow. At 74 degrees it turned a deep red—and then something strange happened. The thermal radiation plummeted. At 80 degrees it looked blue, as if it could be 60 degrees, and at 85 it looked even colder. Moreover, the effect was reversible and repeatable, many times over.

The Oct. 21, 2013 Harvard University news release (also on EurekAlert), which originated the news item, discusses the potential for this discovery and describes the process of discovery in more detail (Note: A link has been removed),

Principal investigator Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at Harvard SEAS, predicts that with only small adjustments the coating could be used as a new type of thermal camouflage or as a kind of encrypted beacon to allow soldiers to covertly communicate their locations in the field.

The secret to the technology lies within a very thin film of vanadium oxide, an unusual material that undergoes dramatic electronic changes when it reaches a particular temperature. At room temperature, for example, pure vanadium oxide is electrically insulating, but at slightly higher temperatures it transitions to a metallic, electrically conductive state. During that transition, the optical properties change, too, which means special temperature-dependent effects—like infrared camouflage—can also be achieved.

The insulator-metal transition has been recognized in vanadium oxide since 1959. However, it is a difficult material to work with: in bulk crystals, the stress of the transition often causes cracks to develop and can shatter the sample. Recent advances in materials synthesis and characterization—especially those by coauthor Shriram Ramanathan, Associate Professor of Materials Science at Harvard SEAS—have allowed the creation of extremely pure samples of thin-film vanadium oxide, enabling a burst of new science and engineering to take off in just the last few years.

“Thanks to these very stable samples that we’re getting from Prof. Ramanathan’s lab, we now know that if we introduce small changes to the material, we can dramatically change the optical phenomena we observe,” explains lead author Mikhail Kats, a graduate student in Capasso’s group at Harvard SEAS. “By introducing impurities or defects in a controlled way via processes known as doping, modifying, or straining the material, it is possible to create a wide range of interesting, important, and predictable behaviors.”

By doping vanadium oxide with tungsten, for example, the transition temperature can be brought down to room temperature, and the range of temperatures over which the strange thermal radiation effect occurs can be widened. Tailoring the material properties like this, with specific outcomes in mind, may enable engineering to advance in new directions.

The researchers say a vehicle coated in vanadium oxide tiles could potentially mimic its environment like a chameleon, appearing invisible to an infrared camera with only very slight adjustments to the tiles’ actual temperature—a far more efficient system than the approaches in use today.

Tuned differently, the material could become a component of a secret beacon, displaying a particular thermal signature on cue to an infrared surveillance camera. Capasso’s team suggests that the material could be engineered to operate at specific wavelengths, enabling simultaneous use by many individually identifiable soldiers.

And, because thermal radiation carries heat, the researchers believe a similar effect could be employed to deliberately speed up or slow down the cooling of structures ranging from houses to satellites.

The Harvard team’s most significant contribution is the discovery that nanoscale structures that appear naturally in the transition region of vanadium oxide can be used to provide a special level of tunability, which can be used to suppress thermal radiation as the temperature rises. The researchers refer to such a spontaneously structured material as a “natural, disordered metamaterial.”

“To artificially create such a useful three-dimensional structure within a material is extremely difficult,” says Capasso. “Here, nature is giving us what we want for free. By taking these natural metamaterials and manipulating them to have all the properties we want, we are opening up a new area of research, a completely new direction of work. We can engineer new devices from the bottom up.”

Here’s an image, from the scientists, illustrating the material’s thermal camouflage (or chameleon) properties,

A new coating intrinsically conceals its own temperature to thermal cameras. (Image courtesy of Mikhail Kats.)

A new coating intrinsically conceals its own temperature to thermal cameras. (Image courtesy of Mikhail Kats.)

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

Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance by Mikhail A. Kats, Romain Blanchard, Shuyan Zhang, Patrice Genevet, Changhyun Ko, Shriram Ramanathan, and Federico Capasso. Phys. Rev. X » Volume 3 » Issue 4  or Phys. Rev. X 3, 041004 (2013) DOI:10.1103/PhysRevX.3.041004

This paper is published in an open access journal according to the Harvard news release,

About Physical Review X

Launched in August 2011, PRX (http://prx.aps.org) is an open-access, peer-reviewed publication of the American Physical Society (www.aps.org), a non-profit membership organization working to advance and diffuse the knowledge of physics through its outstanding research journals, scientific meetings, and education, outreach, advocacy and international activities. APS represents 50,000 members, including physicists in academia, national laboratories and industry in the United States and throughout the world.

A strange state of light

Apparently combining a hologram with subwavelength structures at a scale of just tens of nanometers can lead to ‘strange’ light. From the Aug. 20, 2013 news item on Nanowerk,

Applied physicists at the Harvard School of Engineering and Applied Sciences (SEAS) have demonstrated that they can change the intensity, phase, and polarization of light rays using a hologram-like design decorated with nanoscale structures.

As a proof of principle, the researchers have used it to create an unusual state of light called a radially polarized beam, which—because it can be focused very tightly—is important for applications like high-resolution lithography and for trapping and manipulating tiny particles like viruses.

The Aug. 20, 2013 Harvard University news release by Manny Marone, which originated the news item, further describes the device and the effect (Note: A link has been removed),

This is the first time a single, simple device has been designed to control these three major properties of light at once. (Phase describes how two waves interfere to either strengthen or cancel each other, depending on how their crests and troughs overlap; polarization describes the direction of light vibrations; and the intensity is the brightness.)

“Our lab works on using nanotechnology to play with light,” says Patrice Genevet, a research associate at Harvard SEAS and co-lead author of a paper published this month in Nano Letters. “In this research, we’ve used holography in a novel way, incorporating cutting-edge nanotechnology in the form of subwavelength structures at a scale of just tens of nanometers.” One nanometer equals one billionth of a meter.

Using these novel nanostructured holograms, the Harvard researchers have converted conventional, circularly polarized laser light into radially polarized beams at wavelengths spanning the technologically important visible and near-infrared light spectrum.

“When light is radially polarized, its electromagnetic vibrations oscillate inward and outward from the center of the beam like the spokes of a wheel,” explains Capasso [Federico Capasso, professor of applied physics]. “This unusual beam manifests itself as a very intense ring of light with a dark spot in the center.”

“It is noteworthy,” Capasso points out, “that the same nanostructured holographic plate can be used to create radially polarized light at so many different wavelengths. Radially polarized light can be focused much more tightly than conventionally polarized light, thus enabling many potential applications in microscopy and nanoparticle manipulation.”

The new device resembles a normal hologram grating with an additional, nanostructured pattern carved into it. Visible light, which has a wavelength in the hundreds of nanometers, interacts differently with apertures textured on the ‘nano’ scale than with those on the scale of micrometers or larger. By exploiting these behaviors, the modular interface can bend incoming light to adjust its intensity, phase, and polarization.

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

Nanostructured Holograms for Broadband Manipulation of Vector Beams by Jiao Lin, Patrice Genevet, Mikhail A. Kats, Nicholas Antoniou, and Federico Capasso. Nano Lett., Article ASAP DOI: 10.1021/nl402039y Publication Date (Web): August 5, 2013
Copyright © 2013 American Chemical Society

This article is behind a paywall.

I last wrote about Federico Carpasso’s work in an Oct. 16, 2013 posting, Harvard researchers look deeply into oily puddles as they rethink thin films and optical loss.

Harvard researchers look deeply into oily puddles as they rethink thin films and optical loss

For centuries it was thought that thin-film interference effects, such as those that cause oily pavements to reflect a rainbow of swirling colors, could not occur in opaque materials. Harvard physicists have now discovered that even very “lossy” thin films, if atomically thin, can be tailored to reflect a particular range of dramatic and vivid colors.

from the Oct. 14, 2012 news release on EurekAlert (also available on the Harvard School of Engineering and Applied Sciences [SEAS] news page),

The discovery is the latest to emerge from the laboratory of Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS, whose research group most recently produced ultrathin flat lenses and needle light beams that skim the surface of metals. The common thread in Capasso’s recent work is the manipulation of light at the interface of materials that are engineered at the nano- scale, a field referred to as nanophotonics. Graduate student and lead author Mikhail A. Kats carried that theme into the realm of color.

“In my group, we frequently reexamine old phenomena, where you think everything’s already known,” Capasso says. “If you have perceptive eyes, as many of my students do, you can discover exciting things that have been overlooked. In this particular case there was almost a bias among engineers that if you’re using interference, the waves have to bounce many times, so the material had better be transparent. What Mikhail’s done—and it’s admittedly simple to calculate—is to show that if you use a light-absorbing film like germanium, much thinner than the wavelength of light, then you can still see large interference effects.”

The result is a structure made of only two elements, gold and germanium (or many other possible pairings), that shines in whatever color one chooses.

These are gold films colored with nanometer-thick layers of germanium. Credit: Photo courtesy of Mikhail Kats, Romain Blanchard, and Patrice Genevet

The Oct. 14, 2012 news item on ScienceDaily notes,

“We are all familiar with the phenomenon that you see when there’s a thin film of gasoline on the road on a wet day, and you see all these different colors,” explains Capasso.

Those colors appear because the crests and troughs in the light waves interfere with each other as they pass through the oil into the water below and reflect back up into the air. Some colors (wavelengths) get a boost in brightness (amplitude), while other colors are lost.

That’s essentially the same effect that Capasso and Kats are exploiting, with coauthors Romain Blanchard and Patrice Genevet. The absorbing germanium coating traps certain colors of light while flipping the phase of others so that the crests and troughs of the waves line up closely and reflect one pure, vivid color.

“Instead of trying to minimize optical losses, we use them as an integral part of the design of thin-film coatings,” notes Kats. “In our design, reflection and absorption cooperate to give the maximum effect.”

Most astonishingly, though, a difference of only a few atoms’ thickness across the coating is sufficient to produce the dramatic color shifts. The germanium film is applied through standard manufacturing techniques — lithography and physical vapor deposition, which the researchers compare to stenciling and spray-painting — so with only a minimal amount of material (a thickness between 5 and 20 nanometers), elaborate colored designs can easily be patterned onto any surface, large or small.

“Just by changing the thickness of that film by about 15 atoms, you can change the color,” says Capasso. “It’s remarkable.”

I will never look at another oily puddle the same way again.