Archive for the ‘nanophotonics’ Category

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Bringing home the chilling effects of outer space

Tuesday, April 16th, 2013

They’ve invented a new type of cooling structure at Stanford University (California) which reflects sunlight back into outer space. From the Apr. 16, 2013 news item on Azonano,

A team of researchers at Stanford has designed an entirely new form of cooling structure that cools even when the sun is shining. Such a structure could vastly improve the daylight cooling of buildings, cars and other structures by reflecting sunlight back into the chilly vacuum of space.

The Apr. 15, 2013 Stanford Report by Andrew Myers, which originated the news item, describes the problem the engineers were solving,

The trick, from an engineering standpoint, is twofold. First, the reflector has to reflect as much of the sunlight as possible. Poor reflectors absorb too much sunlight, heating up in the process and defeating the goal of cooling.

The second challenge is that the structure must efficiently radiate heat (from a building, for example) back into space. Thus, the structure must emit thermal radiation very efficiently within a specific wavelength range in which the atmosphere is nearly transparent. Outside this range, the thermal radiation interacts with Earth’s atmosphere. Most people are familiar with this phenomenon. It’s better known as the greenhouse effect – the cause of global climate change.

Here’s the approach they used,

Radiative cooling at nighttime has been studied extensively as a mitigation strategy for climate change, yet peak demand for cooling occurs in the daytime.

“No one had yet been able to surmount the challenges of daytime radiative cooling –of cooling when the sun is shining,” said Eden Rephaeli, a doctoral candidate in Fan’s [Shanhui Fan, a professor of electrical engineering and the paper's senior author] lab and a co-first-author of the paper. “It’s a big hurdle.”

The Stanford team has succeeded where others have come up short by turning to nanostructured photonic materials. These materials can be engineered to enhance or suppress light reflection in certain wavelengths.

“We’ve taken a very different approach compared to previous efforts in this field,” said Aaswath Raman, a doctoral candidate in Fan’s lab and a co-first-author of the paper. “We combine the thermal emitter and solar reflector into one device, making it both higher performance and much more robust and practically relevant. In particular, we’re very excited because this design makes viable both industrial-scale and off-grid applications.”

Using engineered nanophotonic materials, the team was able to strongly suppress how much heat-inducing sunlight the panel absorbs, while it radiates heat very efficiently in the key frequency range necessary to escape Earth’s atmosphere. The material is made of quartz and silicon carbide, both very weak absorbers of sunlight.

This new approach offers both economic and social benefits,

The new device is capable of achieving a net cooling power in excess of 100 watts per square meter. By comparison, today’s standard 10-percent-efficient solar panels generate about the same amount of power. That means Fan’s radiative cooling panels could theoretically be substituted on rooftops where existing solar panels feed electricity to air conditioning systems needed to cool the building.

To put it a different way, a typical one-story, single-family house with just 10 percent of its roof covered by radiative cooling panels could offset 35 percent its entire air conditioning needs during the hottest hours of the summer.

Radiative cooling has another profound advantage over other cooling equipment, such as air conditioners. It is a passive technology. It requires no energy. It has no moving parts. It is easy to maintain. You put it on the roof or the sides of buildings and it starts working immediately.

…

Beyond the commercial implications, Fan and his collaborators foresee a broad potential social impact. Much of the human population on Earth lives in sun-drenched regions huddled around the equator. Electrical demand to drive air conditioners is skyrocketing in these places, presenting an economic and environmental challenge. These areas tend to be poor and the power necessary to drive cooling usually means fossil-fuel power plants that compound the greenhouse gas problem.

“In addition to these regions, we can foresee applications for radiative cooling in off-the-grid areas of the developing world where air conditioning is not even possible at this time. There are large numbers of people who could benefit from such systems,” Fan said.

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

Ultrabroadband Photonic Structures To Achieve High-Performance Daytime Radiative Cooling by Eden Rephaeli, Aaswath Raman, and Shanhui Fan.  Nano Lett. [American Chemical Society Nano Letters], 2013, 13 (4), pp 1457–1461
DOI: 10.1021/nl4004283 Publication Date (Web): March 5, 2013
Copyright © 2013 American Chemical Society

The article is behind a paywall.

For anyone who might be interested in what constitutes hot temperatures, here’s a sampling from the Wikipedia List of weather records (Note: I have removed links and included only countries which experienced temperatures of 43.9 °C or 111 °F or more; I made one exception: Antarctica),

Temperature

Location

Date

North America / On Earth†

 56.7 °C (134 °F) Furnace Creek Ranch (formerly Greenland Ranch), in Death Valley, California, United States 1913-07-10

Canada

 45.0 °C (113 °F) Midale, Yellow Grass, Saskatchewan 1937-07-05

Mexico

 52 °C (125.6 °F) San Luis Rio Colorado, Sonora

Africa

 55.0 °C (131 °F) Kebili, Tunisia 1931-07-07

Algeria

 50.6 °C (123.1 °F) In Salah, Tamanrasset Province 2002-07-12

Benin

 44.5 °C (112 °F) Kandi  ?

Burkina Faso

 47.2 °C (117 °F) Dori  ?

Cameroon

 47.7 °C (117.9 °F) Kousseri  ?

Central African Republic

 45 °C (113 °F) Birao  ?

Chad

 47.6 °C (117.7 °F) Faya-Largeau 2010-06-22

Djibouti

 49.5 °C (121 °F) Tadjourah  ?

Egypt

 50.3 °C (122.6 °F) Kharga  ?

Eritrea

 48 °C (118.4 °F) Massawa  ?

Ethiopia

 48.9 °C (120 °F) Dallol  ?

The Gambia

 45.5 °C (114 °F) Basse Santa Su 2008-?-?

Ghana

 43.9 °C (111 °F) Navrongo  ?

Libya

 50.2 °C (122.4 °F) Zuara 1995-06

Malawi

 45 °C (113 °F) Ngabu, Chikwana  ?

Mali

 48.2 °C (118 °F) Gao  ?

Mauritania

 50.0 °C (122 °F) Akujit  ?

Morocco

 49.6 °C (121.3 °F) Marrakech 2012-07-17

Mozambique

 47.3 °C (117.2 °F) Chibuto 2009-02-03

Namibia

 47.8 °C (118 °F) Noordoewer 2009-02-06

Niger

 48.2 °C (118.8 °F) Bilma 2010-06-23

Nigeria

 46.4 °C (115.5 °F) Yola 2010-04-03

Somalia

 47.8 °C (118 °F) Berbera  ?

South Africa

 50.0 °C (122 °F) Dunbrody, Eastern Cape 1918

Sudan

 49.7 °C (121.5 °F) Dongola 2010-06-25

Swaziland

 46.1 °C (115 °F) Sidvokodvo  ?

Zimbabwe

 45.6 °C (114 °F) Beitbridge,  ?

Asia

 53.6 °C (128.5 °F) Sulaibya, Kuwait 2012-07-31

Bangladesh

 45.1 °C (113.2 °F) Rajshahi 1972-04-30

China

 49.7 °C (118 °F) Ading Lake, Turpan, Xinjiang, China 2008-08-03

India

 50 °C (122 °F) Sri, Ganganagar, Rajasthan Dholpur, Rajasthan  ?

Iraq

 52.0 °C (125.7 °F) Basra, Ali Air Base, Nasiriyah 2010-06-14
2011-08-02

Israel

 53 °C (127.4 °F) Tirat Zvi, Israel 1942-06-21

Myanmar

 47.0 °C (116.6 °F) Myinmu 2010-05-12

Pakistan

 53.5 °C (128.3 °F) Mohenjo-daro, Sindh 2010-05-26

Qatar

 50.4 °C (122.7 °F) Doha 2010-07-14

Saudi Arabia

 52.0 °C (125.6 °F) Jeddah 2010-06-22

Thailand

 44.5 °C (112.1 °F) Uttaradit 1960-04-27

Turkey

 48.8 °C (119.8 °F) Mardin 1993-08-14

Oceania‡

 50.7 °C (123.3 °F) Oodnadatta, South Australia, Australia 1960-01-02

South America

 49.1 °C (120.4 °F) Villa de María, Argentina 1920-01-02

Paraguay

 45 °C (113 °F) Pratts Gill, Boquerón Department 2009-11-14

Uruguay

 44 °C (111.2 °F) Paysandú, Paysandú Department 1943-01-20

Central America and Caribbean Islands

 45 °C (113 °F) Estanzuela, Zacapa Guatemala  ?

Europe

 48.0 °C or 48.5 °C (118.4 °F or 119.3 °F) Athens, Greece or Catenuova, Italy (Catenanuova’s record is disputed) 1977-07-10 or 1999-08-10;

Bosnia and Herzegovina

 46.2 °C (115.16 °F) Mosta (Herzegovina, Federation of Bosnia and Herzegovina) 1900-07-31

Cyprus

 46.6 °C (115.9 °F) Letkoniko, Cyprus 2010-08-01

Italy

 47 °C or 48.5 °C (116.6 or 119.3 °F) Foggia, Apulia or Catenuuova, Sicily (Catenanuova’s record is disputed) 2007-06-25 and 1999-08-10

Macedonia

 45.7 °C(114.26 °F) Demir Kapija, Demir Kapija Municipality 2007-07-24

Portugal

 47.4 °C (117.3 °F) Amarelja, Beja 2003-08-01

Serbia

 44.9 °C (112.8 °F) Smederevska Palanka, Podunavlie Distrrict, 2007-07-24

Spain

 47.2 °C (116.9 °F) Murcia 1994-07-04

Antarctica

 14.6 °C (59 °F) Vanda Station, Scott Coast 1974-01-05

It seems a disproportionate number of these hot temperatures have been recorded since 2000, eh?

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Structure of color

Thursday, February 7th, 2013

AGELESS BRILLIANCE: Although the pigment-derived leaf color of this decades-old specimen of the African perennial Pollia condensata has faded, the fruit still maintains its intense metallic-blue iridescence.COURTESY OF P.J. RUDALL [downloaded from http://www.the-scientist.com/?articles.view/articleNo/34200/title/Color-from-Structure/]

AGELESS BRILLIANCE: Although the pigment-derived leaf color of this decades-old specimen of the African perennial Pollia condensata has faded, the fruit still maintains its intense metallic-blue iridescence.COURTESY OF P.J. RUDALL [downloaded from http://www.the-scientist.com/?articles.view/articleNo/34200/title/Color-from-Structure/]

Hard to believe those berries were collected more than four decades ago, according to Cristina Luiggi in her Feb. 1, 2013 article, Color from Structure, for The Scientist magazine. Her focus is on biological nanostructures and it is a fascinating article which I urge you to read in its entirety if you have the time and this kind of thing interests you. As you can see, the pictures are great.

Here are a few excerpts from the piece,

Colors may be evolution’s most beautiful accident. Spontaneous mutations that perturbed the arrangement of structural components, such as cellulose, collagen, chitin, and keratin, inadvertently created nanoscale landscapes that catch light in the most vibrantly diverse ways—producing iridescent greens, fiery reds, brilliant blues, opalescent whites, glossy silvers, and ebony blacks.

Structural colors, in contrast to those produced by pigments or dyes, arise from the physical interaction of light with biological nanostructures. These color-creating structures likely developed as an important phenotype during the Cambrian explosion more than 500 million years ago, when organisms developed the first eyes and the ability to detect light, color, shade, and contrast. “As soon as you had visual predators, there were organisms that were either trying to distract, avoid, or communicate with those predators using structural coloration,” says Yale University evolutionary ornithologist Richard Prum.

Ever since, structural coloration has evolved multiple times across the tree of life, as a wide range of organisms developed ways to fine-tune the geometry of some of the most abundant (and often colorless) biomaterials on Earth, engineering grooves, pockets, and films that scatter light waves and cause them to interfere with each other in ways we humans happen to find aesthetically pleasing.

Here’s why color derived from structure doesn’t fade, from Luiggi’s article,

Pigments and dyes are molecules that produce colors by the selective absorption and reflection of specific wavelengths of electromagnetic radiation. Structural colors, on the other hand, rely exclusively on the shape of the material and not its chemical properties. While pigments and dyes degrade and their colors fade over time, some types of structural coloration, which rely on the same materials that make up tree bark, insect exoskeletons, and claws or nails, can persist hundreds, thousands, and even millions of years after the death of the organism.

Structural color can be found in a lot of plant life,

Although there are only a handful of known examples of structural colors in fruits, there are plenty to be found in the leaves and petals of plants. Within every family of flowering plants, there is at least one species that displays structural colors.

“The presence of structural colors, especially in flowers, is likely used by pollinators to spot the position of the flower and to recognize it better,” Vignolini [Silvia Vignolini, a physics postdoc at the University of Cambridge] explains. But in some plants, the evolutionary purpose of structural coloration is harder to pin down. The leaves of the low-lying tropical spikemoss Selaginella willdenowii, for example, produce blue-green iridescence when young and growing in the shade, and tend to lose the structural coloration with age and when exposed to high levels of light. The iridescence is achieved by cells in the leaves’ upper epidermis, which contain a few layers of cellulose microfibrils packed with different amounts of water. This ultrastructure is often absent in the leaves of the same species growing in direct sunlight. Researchers hypothesize that the spikemoss turns off its iridescence by changing the water content of the leaves’ cell walls, says Heather Whitney, a research fellow at the University of Bristol who studies iridescence in plants.

This capability is not limited to plants. Insects (jewel beetles and the morpho butterfly are often cited) and fish also have evolved to include structural color as protective or attractive devices, from Luiggi’s article,

The brightest living tissues on the planet are found in fish. Under ideal conditions, for example, the silvery scales of the European sardine and the Atlantic herring can act like near-perfect mirrors—reflecting up to 90 percent of incoming light. It is an irony of nature that these shiniest of structures are not meant to be flaunted, but are intended as camouflage.

“When you’re out in the open water, if you drop down below 10 to 30 meters, in any direction you look, the intensity of light is the same,” explains Nicholas Roberts, a physicist at the University of Bristol who specializes in bio-optics. At that depth, a perfect reflector, or mirror, would seem invisible, because light is equally reflected from all sides and angles.

It will be interesting to see if there’s any future discussion of the giant squid in the context of structural color since, according to very recent research (as per my Feb. 1, 2013 posting), it appears to be covered in gold leaf when observed in its habitat.

Luiggi’s article starts with an ornithologist and circles back in a discussion about the difficulty of creating nanostructures, soft matter condensed physics, and birds,

To create structural colors, organisms must master architecture at the nanoscale—the size of visible-light wavelengths. “But it turns out that biology doesn’t do a good job of creating nanostructures,” Prum says.

Instead, organisms create the initial conditions that allow those nanostructures to grow using self-organizing physical processes. Thus, organisms exploit what’s known as soft condensed matter physics, or “the physics of squishy stuff,” as Prum likes to call it. This relatively new field of physics deals with materials that are right at the boundaries of hard solids, liquids, and gases.

“There’ve been huge advances in this field in the last 30 years which have created rich theories of how structure can arise at the nanoscale,” Prum says. “It has been very applicable to the understanding of how structural colors grow.”

Soft condensed matter physics has been particularly useful in understanding the production of the amorphous nanostructures that imbue the feathers of certain bird species with intensely vibrant hues. The blue color of the male Eastern bluebird (Sialia sialis), for example, is produced by the selective scattering of blue light from a complex nanostructure of b-keratin channels and air pockets in the hairlike branches called feather barbs that give the quill its lift. The size of the air pockets determines the wavelengths that are selectively amplified.

While there’s better understanding of the mechanisms involved in structural color, scientists are a long way from replicating the processes, from the article,

“The three-dimensional nature of the structures themselves is just so complex,” says Vukusic. [physicist Peter Vukusic, a professor of natural photonics at the University of Exeter, UK] “Were it to be a simple periodic system with a regular geometry, you could easily put that into a computer model and run simulations all day. But the problem is that they are never perfectly periodic.”

This article is open access so, as I noted earlier, all you need is the time. As of my Feb. 6, 2013 posting, there was some new research announced about scientists making observations about the structural color in peacock feathers and applying some of those ideas to develop better resolution in e-readers.

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Peacocks and their structural colour inspire better resolution in e-readers

Wednesday, February 6th, 2013

Thank goodness birds, insects, and other denizens of the natural world have not taken to filing patents otherwise we’d be having some serious problems in the courts as I have hinted in previous postings including this March 29, 2012 posting titled, Butterflies give and give … .

This time, it’s the peacock which is sharing its intellectual property as per this Feb. 5, 2013 news item on ScienceDaily,

Now, researchers at the University of Michigan have found a way to lock in so-called structural color, which is made with texture rather than chemicals. A paper on the work is published online in the current edition of the Nature journal Scientific Reports.

In a peacock’s mother-of-pearl tail, precisely arranged hairline grooves reflect light of certain wavelengths. That’s why the resulting colors appear different depending on the movement of the animal or the observer. Imitating this system—minus the rainbow effect—has been a leading approach to developing next-generation reflective displays.

The University of Michigan Feb. 5, 2013 news release, which originated the news item, provides information about potential applications and more details about the science,

The new U-M research could lead to advanced color e-books and electronic paper, as well as other color reflective screens that don’t need their own light to be readable. Reflective displays consume much less power than their backlit cousins in laptops, tablet computers, smartphones and TVs. The technology could also enable leaps in data storage and cryptography. Documents could be marked invisibly to prevent counterfeiting.

Led by Jay Guo, professor of electrical engineering and computer science, the researchers harnessed the ability of light to funnel into nanoscale metallic grooves and get trapped inside. With this approach, they found the reflected hues stay true regardless of the viewer’s angle.

“That’s the magic part of the work,” Guo said. “Light is funneled into the nanocavity, whose width is much, much smaller than the wavelength of the light. And that’s how we can achieve color with resolution beyond the diffraction limit. Also counterintuitive is that longer wavelength light gets trapped in narrower grooves.”

The diffraction limit was long thought to be the smallest point you could focus a beam of light to. Others have broken the limit as well, but the U-M team did so with a simpler technique that also produces stable and relatively easy-to-make color, Guo said.

“Each individual groove—much smaller than the light wavelength—is sufficient to do this function. In a sense, only the green light can fit into the nanogroove of a certain size,” Guo said.

The U-M team determined what size slit would catch what color light. Within the framework of the print industry standard cyan, magenta and yellow color model, the team found that at groove depths of 170 nanometers and spacing of 180 nanometers, a slit 40 nanometers wide can trap red light and reflect a cyan color. A slit 60 nanometers wide can trap green and make magenta. And one 90 nanometers wide traps blue and produces yellow. The visible spectrum spans from about 400 nanometers for violet to 700 nanometers for red.

“With this reflective color, you could view the display in sunlight. It’s very similar to color print,” Guo said.

Particularly interesting (for someone who worked in the graphic arts/printing industry as I did) are the base colours being used to create all the other colours,

To make color on white paper, (which is also a reflective surface), printers arrange pixels of cyan, magenta and yellow in such a way that they appear to our eyes as the colors of the spectrum. [emphasis mine] A display that utilized Guo’s approach would work in a similar way.

To demonstrate their device, the researchers etched nanoscale grooves in a plate of glass with the technique commonly used to make integrated circuits, or computer chips. Then they coated the grooved glass plate with a thin layer of silver. When light—which is a combination of electric and magnetic field components—hits the grooved surface, its electric component creates what’s called a polarization charge at the metal slit surface, boosting the local electric field near the slit. That electric field pulls a particular wavelength of light in.

The base colours in printing are CMYK (cyan, magenta, yellow, black). At least, that was the case when I worked in the graphic arts industry and quick search on the web suggests that standard still holds.(Have I missed a refinement?) In any event, here’s an image that demonstrates how this new colour scale can be used,

University of Michigan researchers created the color in these tiny Olympic rings using precisely-sized nanoscale slits in a glass plate coated with silver. Each ring is about 20 microns, smaller than the width of a human hair. They can produce different colors with different widths of the slits. Yellow is produced with slits that are each 90 nanometers wide. The technique takes advantage of a phenomenon called light funneling that can catch and trap particular wavelengths of light, and it could lead to reflective display screens with colors that stay true regardless of the viewer's angle. Image credit: Jay Guo, College of Engineering

University of Michigan researchers created the color in these tiny Olympic rings using precisely-sized nanoscale slits in a glass plate coated with silver. Each ring is about 20 microns, smaller than the width of a human hair. They can produce different colors with different widths of the slits. Yellow is produced with slits that are each 90 nanometers wide. The technique takes advantage of a phenomenon called light funneling that can catch and trap particular wavelengths of light, and it could lead to reflective display screens with colors that stay true regardless of the viewer’s angle. Image credit: Jay Guo, College of Engineering

You can find more about this work in the ScienceDaily news item, which includes a link to the abstract, or in the University of Michigan news release, which includes more images from the scientists.

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Self-assembling liquid lenses used in optical microscopy to reveal nanoscale objects

Monday, January 21st, 2013

A Jan. 21, 2013 news item on Azonano highlights some research on microscope and self-assembling lenses done at University of California Los Angeles (UCLA),

By using tiny liquid lenses that self-assemble around microscopic objects, a team from UCLA’s Henry Samueli School of Engineering and Applied Science has created an optical microscopy method that allows users to directly see objects more than 1,000 times smaller than the width of a human hair.

Coupled with computer-based computational reconstruction techniques, this portable and cost-effective platform, which has a wide field of view, can detect individual viruses and nanoparticles, making it potentially useful in the diagnosis of diseases in point-of-care settings or areas where medical resources are limited.

The UCLA Jan. 20, 2013 news release, written by Matthew Chin and which originated the news item, explains why another microscopy technique is needed for viewing objects at the nanoscale,

Electron microscopy is one of the current gold standards for viewing nanoscale objects. This technology uses a beam of electrons to outline the shape and structure of nanoscale objects. Other optical imaging–based techniques are used as well, but all of them are relatively bulky, require time for the preparation and analysis of samples, and have a limited field of view — typically smaller than 0.2 square millimeters — which can make viewing particles in a sparse population, such as low concentrations of viruses, challenging.

To overcome these issues, the UCLA team, led by Aydogan Ozcan, an associate professor of electrical engineering and bioengineering, developed the new optical microscopy platform by using nanoscale lenses that stick to the objects that need to be imaged. This lets users see single viruses and other objects in a relatively inexpensive way and allows for the processing of a high volume of samples.
…

At scales smaller than 100 nanometers, optical microscopy becomes a challenge because of its weak light-signal levels. Using a special liquid composition, nanoscale lenses, which are typically thinner than 200 nanometers, self-assemble around objects on a glass substrate.

A simple light source, such as a light-emitting diode (LED), is then used to illuminate the nano-lens object assembly. By utilizing a silicon-based sensor array, which is also found in cell-phone cameras, lens-free holograms of the nanoparticles are detected. The holograms are then rapidly reconstructed with the help of a personal computer to detect single nanoparticles on a glass substrate.

The researchers have used the new technique to create images of single polystyrene nanoparticles, as well as adenoviruses and H1N1 influenza viral particles.

While the technique does not offer the high resolution of electron microscopy, it has a much wider field of view — more than 20 square millimeters — and can be helpful in finding nanoscale objects in samples that are sparsely populated.

Here a citation for and a link to the research article,

Wide-field optical detection of nanoparticles using on-chip microscopy and self-assembled nanolenses by Onur Mudanyali, Euan McLeod, Wei Luo, Alon Greenbaum, Ahmet F. Coskun, Yves Hennequin, Cédric P. Allier, & Aydogan Ozcan. Nature Photonics (2013) doi:10.1038/nphoton.2012.337 Published online: 20 January 2013

The article is behind a paywall.

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

Tuesday, October 16th, 2012

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.

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New paradigm for low power telecommunications

Tuesday, July 17th, 2012

I’m always a sucker for the nonlinear although I’m much more familiar with nonlinear narratives than I am with nonlinear photonics. From the July 15, 2012 news item on EurekAlert,

New research by Columbia Engineering demonstrates remarkable optical nonlinear behavior of graphene that may lead to broad applications in optical interconnects and low-power photonic integrated circuits. With the placement of a sheet of graphene just one-carbon-atom-thick, the researchers transformed the originally passive device into an active one that generated microwave photonic signals and performed parametric wavelength conversion at telecommunication wavelengths.

“We have been able to demonstrate and explain the strong nonlinear response from graphene, which is the key component in this new hybrid device,” says Tingyi Gu, the study’s lead author and a Ph.D. candidate in electrical engineering. “Showing the power-efficiency of this graphene-silicon hybrid photonic chip is an important step forward in building all-optical processing elements that are essential to faster, more efficient, modern telecommunications. And it was really exciting to explore the ‘magic’ of graphene’s amazingly conductive properties and see how graphene can boost optical nonlinearity, a property required for the digital on/off two-state switching and memory.”

Here’s one of the issues that scientists have been grappling with,

Until recently, researchers could only isolate graphene as single crystals with micron-scale dimensions, essentially limiting the material to studies confined within laboratories. “The ability to synthesize large-area films of graphene has the obvious implication of enabling commercial production of these proven graphene-based technologies,” explains James Hone, associate professor of mechanical engineering, whose team provided the high quality graphene for this study. “But large-area films of graphene can also enable the development of novel devices and fundamental scientific studies requiring graphene samples with large dimensions. This work is an exciting example of both—large-area films of graphene enable the fabrication of novel opto-electronic devices, which in turn allow for the study of scientific phenomena.”

Building on the work done by scientists such as Hone,this new group of researchers led by by Chee Wei Wong, professor of mechanical engineering, director of the Center for Integrated Science and Engineering, and Solid-State Science and Engineering at Columbia University, created a new device,

They have engineered a graphene-silicon device whose optical nonlinearity enables the system parameters (such as transmittance and wavelength conversion) to change with the input power level. The researchers also were able to observe that, by optically driving the electronic and thermal response in the silicon chip, they could generate a radio frequency carrier on top of the transmitted laser beam and control its modulation with the laser intensity and color. Using different optical frequencies to tune the radio frequency, they found that the graphene-silicon hybrid chip achieved radio frequency generation with a resonant quality factor more than 50 times lower than what other scientists have achieved in silicon.

“We are excited to have observed four-wave mixing in these graphene-silicon photonic crystal nanocavities,” says Wong. “We generated new optical frequencies through nonlinear mixing of two electromagnetic fields at low operating energies, allowing reduced energy per information bit. This allows the hybrid silicon structure to serve as a platform for all-optical data processing with a compact footprint in dense photonic circuits.”

That bit about the system parameters changing with input levels suggests a biological system responding sensitively to environmental inputs, e.g., when it gets hot, your body tries to cool itself down in a sensitive response to an input. Of course, that fanciful analogy doesn’t extend itself too far since the human body is trying to return to its internal balance point (homeostasis) which isn’t what the Columbia researchers are attempting to do with their device.

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Lumerical and the latest incarnation of its FDTD (finite-difference time-domain) Solution

Friday, July 6th, 2012

Here’s a little bit about Lumerical’s (based in Vancouver, Canada) FDTD Solutions product,

Employing the industry proven finite-difference time-domain (FDTD) method, FDTD Solutions empowers designers to confront the most challenging photonic design problems. Rapid prototyping and highly-accurate simulations reduce reliance upon costly experimental prototypes, leading to a quicker assessment of design concepts and reduced product development costs.

The July 5, 2012 announcement of the FDTD 8.0 release notes this,

Release 8.0 extends the material modeling capabilities of prior versions to include the ability to model liquid crystals and other spatially-varying anisotropic materials, and a Flexible Material Plugin (FMP) framework that enables researchers to model a wide variety of other materials, including nonlinear, magneto-optical, and saturable gain materials.

The new FMP framework enables researchers to describe via computer source code a material’s polarization and magnetization as a function of the electromagnetic field and other physical quantities.  Release 8.0 is expected to contribute to the development of components in the fields of optical switching, integrated magneto-optical polarization control, quantum communications and silicon photonics owing to the wide array of nonlinear effects available in silicon.

…

“We’ve been in discussions with the nonlinear photonics community for years, working to understand their diverse interests and to develop a design environment that can accommodate their needs,”according to Dr. James Pond, Lumerical’s CTO.  “With the release of FDTD Solutions 8.0, researchers can quickly write a few lines of code to incorporate a Kerr medium into their simulation, or painstakingly develop hundreds of lines of code to model a multi-level atomic system.  Either way, when it comes to simulation, with FDTD Solutions 8.0 researchers can focus on developing sophisticated material models while relying on Lumerical for everything else.”

“As an existing user of Lumerical’s software, I am especially pleased by the new nonlinear optics capabilities available in FDTD Solutions 8.0,” according to Professor Robert Boyd of the University of Ottawa in Canada and the University of Rochester in the United States.  “Much of my current research entails the design of photonic structures and devices for use in applications such as all-optical switching.  The new capabilities of Lumerical will conveniently allow my group to treat both the linear and nonlinear properties of these structures using the same numerical platform.”

….

“Our team in the Electronics and Photonics Department is specialized in the area of computational electromagnetics for next-generation active devices in the fields of plasmonics, photonics and nanophotonics” according to Dr. E. P. Li, Department Director of Electronics and Photonics at the Institute for High Performance Computing in Singapore.  “As pioneers in this field, we are interested to start integrating our existing material algorithms into Lumerical’s FDTD framework.  We anticipate that this will make our advanced algorithms more accessible to a broad range of end users.”

There’s more on the FDTD Solutions product page including a video and a free 20-day download of the 8.0 release.  (You can read the full July 5, 2012 news release here.)

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Vancouver (Canada)-based company, Lumerical Solutions, files patent on new optoelectronic simulation software

Thursday, June 14th, 2012

I’m not a huge *fan of patents as per various postings (my Oct. 31, 2011 posting is probably my most overt statement) so I’m not entirely thrilled about this news from Lumerical Solutions, Inc. According to the June 14, 2012 news item on Nanowerk,

Lumerical Solutions, Inc., a global provider of optoelectronic design software, announced the filing of a provisional patent application titled, “System and Method for Transforming a Coordinate System to Simulate an Anisotropic Medium.” The patent application, filed with the US Patent and Trademark Office, describes how the optical response of dispersive, spatially varying anisotropic media can be efficiently simulated on a discretized grid like that employed by finite-difference time-domain (FDTD) or finite-element method (FEM) simulators. The invention disclosed is relevant to a wide array of applications including liquid crystal display (LCD) panels, microdisplays, spatial light modulators, integrated components using liquid crystal on silicon (LCOS) technology like LCOS optical switches, and magneto-optical elements in optical communication and sensing systems.

The company’s June 14, 2012 news release includes this comment from the founder and Chief Technical Office (CTO),

According to Dr. James Pond, the inventor and Lumerical’s Chief Technology Officer, “many next generation opto-electronic products combine complicated materials and nano-scale structure, which is beyond the capabilities of existing simulation tools. Lumerical’s enhanced framework allows designers to accurately simulate everything from liquid crystal displays to OLEDs, and silicon photonics to integrated quantum computing components.”

Lumerical’s new methodology for efficiently simulating anisotropic media is part of a larger effort to allow designers the ability to model the optical response of many different types of materials.  In addition to the disclosed invention, Lumerical has added a material plugin capability which will enable external parties to include complicated material models, such as those required for modelling semiconductor lasers or non-linear optical devices, into FDTD-based simulation projects.

…  According to Chris Koo, an engineer with Samsung, “Lumerical’s latest innovation has established them as the clear leader in the field of optoelectronic device modeling.  Their comprehensive material modeling capabilities paves the way for the development of exciting new technologies.”

I wish the company good luck. Despite my reservations about current patent regimes, I do appreciate that in some situations, it’s best to apply for a patent.

For the curious, here’s a little more (from the company’s About Lumerical page),

By empowering research and product development professionals with high performance optical design software that leverages recent advances in computing technology, Lumerical helps optical designers tackle challenging design goals and meet strict deadlines. Lumerical’s design software solutions are employed in more than 30 countries by global technology leaders like Agilent, ASML, Bosch, Canon, Harris, Northrop Grumman, Olympus, Philips, Samsung, and STMicroelectronics, and prominent research institutions including Caltech, Harvard, Max Planck Institute, MIT, NIST and the Chinese Academy of Sciences.

Our Name

Lu.min.ous (loo’me-nes) adj., full of light, illuminated

Nu.mer.i.cal (noo-mer’i-kel) adj., of or relating to a number or series of numbers

Lu.mer.i.cal (loo-mer’i-kel) – A company that delivers inventive, highly accurate and cost effective design solutions resulting in significant improvements in product development costs and speed-to-market.

* June 15, 2012: I found the error this morning (9:20 am PDT) and added the word ‘fan’.

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Trickster researchers at the University of Maryland and graphene photodetectors

Tuesday, June 5th, 2012

Trickster figures are a feature in mythologies around the world. They’re always mischievous, tricking humans and other beings into doing things they shouldn’t.

Tricksters can be good and/or villainous. For example, Raven in the Pacific Northwest gave us the sun, moon, and stars but stole them in the first place from someone else.

I don’t think the researchers at the University of Maryland have done anything comparable (i.e., stealing) with their graphene discovery but the analogy does amuse me. From the June 3, 2012 news release by Lee Tune,

Researchers at the Center for Nanophysics and Advanced Materials of the University of Maryland have developed a new type of hot electron bolometer a sensitive detector of infrared light, that can be used in a huge range of applications from detection of chemical and biochemical weapons from a distance and use in security imaging technologies such as airport body scanners, to chemical analysis in the laboratory and studying the structure of the universe through new telescopes. [emphasis mine]

Before launching into why I highlighted the part about the universe and the telescopes, here’s the problem the researchers were solving (from the news release),

Most photon detectors are based on semiconductors. Semiconductors are materials which have a range of energies that their electrons are forbidden to occupy, called a “band gap”. The electrons in a semiconductor can absorb photons of light having energies greater than the band gap energy, and this property forms the basis of devices such as photovoltaic cells.

Graphene, a single atom-thick plane of graphite, is unique in that is has a bandgap of exactly zero energy; graphene can therefore absorb photons of any energy. This property makes graphene particularly attractive for absorbing very low energy photons (terahertz and infrared) which pass through most semiconductors. Graphene has another attractive property as a photon absorber: the electrons which absorb the energy are able to retain it efficiently, rather than losing energy to vibrations of the atoms of the material. This same property also leads to extremely low electrical resistance in graphene.

University of Maryland researchers exploited these two properties to devise the hot electron bolometer. It works by measuring the change in the resistance that results from the heating of the electrons as they absorb light.

Normally, graphene’s resistance is almost independent of temperature, unsuitable for a bolometer.

Here’s how the researchers solved the problem (from the news release),

So the Maryland researchers used a special trick: when bilayer graphene is exposed to an electric field it has a small band gap, large enough that its resistance becomes strongly temperature dependent, but small enough to maintain its ability to absorb low energy infrared photons.

The researchers found that their bilayer graphene hot electron bolometer operating at a temperature of 5 Kelvin had comparable sensitivity to existing bolometers operating at similar temperatures, but was more than a thousand times faster.  They extrapolated the performance of the graphene bolometer to lower temperature and found that it may beat all existing technologies.

As usual, there is more work to be done (from the news release),

Some challenges remain. The bilayer graphene bolometer has a higher electrical resistance than similar devices using other materials which may make it difficult to use at high frequencies. Additionally, bilayer graphene absorbs only a few percent of incident light.  But the Maryland researchers are working on ways to get around these difficulties with new device designs, and are confident that a graphene has a bright future as a photo-detecting material.

As for why I highlighted the passage about telescopes and the structure of the universe, our local particle physics laboratory (TRIUMF located in Vancouver, Canada) is hosting the Physics at the Large Hadron Collider (PLHC) conference this week. This is a big deal, from the 7th annual PLHC conference home page (Note: I have removed some links),

PLHC2012 is the seventh conference in the series. The previous conferences in this series were held in Prague (2003), Vienna (2004), Cracow (2006), Split (2008), Hamburg (2010) and Perugia (2011). The conference consists of invited and contributed talks, as well as posters, covering experiment and theory.

Topics at the conference

  • Beauty Physics
  • Heavy Ion Physics
  • Standard Model & Beyond
  • Supersymmetry
  • Higgs Boson

There was a June 3, 2012 public event (mentioned in my May 15, 2012 posting) featuring Rolf Heuer, Director General of CERN (European Particle Physics Laboratory) which houses the Large Hadron Collider and experiments where they are attempting to discern the structure of the universe. (I did attend Heuer’s talk and I think one needs to be more of a physics aficionado than I am.  Thankfully I had watched the Perimeter Institute’s webcast  (What the Higgs is going on?) when the big Higgs Boson announcement was made in December 2012 (mentioned in my Dec. 14, 2012 posting) and that helped.

There is of course an alternate view of the universe and its structure as presented by the story of Raven (from the Wikipedia essay [Note: I have removed a link]),

Raven steals the sun

This is an ancient story told on the Queen Charlotte Islands and includes how Raven helped to bring the Sun, Moon, Stars, Fresh Water, and Fire to the world.

Long ago, near the beginning of the world, Gray Eagle was the guardian of the Sun, Moon and Stars, of fresh water, and of fire. Gray Eagle hated people so much that he kept these things hidden. People lived in darkness, without fire and without fresh water.

Gray Eagle had a beautiful daughter, and Raven fell in love with her. In the beginning, Raven was a snow-white bird, and as a such, he pleased Gray Eagle’s daughter. She invited him to her father’s longhouse.

When Raven saw the Sun, Moon and stars, and fresh water hanging on the sides of Eagle’s lodge, he knew what he should do. He watched for his chance to seize them when no one was looking. He stole all of them, and a brand of fire also, and flew out of the longhouse through the smoke hole. As soon as Raven got outside he hung the Sun up in the sky. It made so much light that he was able to fly far out to an island in the middle of the ocean. When the Sun set, he fastened the Moon up in the sky and hung the stars around in different places. By this new light he kept on flying, carrying with him the fresh water and the brand of fire he had stolen.

He flew back over the land. When he had reached the right place, he dropped all the water he had stolen. It fell to the ground and there became the source of all the fresh-water streams and lakes in the world. Then Raven flew on, holding the brand of fire in his bill. The smoke from the fire blew back over his white feathers and made them black. When his bill began to burn, he had to drop the firebrand. It struck rocks and hid itself within them. That is why, if you strike two stones together, sparks of fire will drop out.

Raven’s feathers never became white again after they were blackened by the smoke from the firebrand. That is why Raven is now a black bird.

While it’s less poetic in tone, there is an image from the University of Maryland illustrating their graphene photodetector,

Electrons in bilayer graphene are heated by a beam of light. Illustration by Loretta Kuo and Michelle Groce, University of Maryland .

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Textiles used as batteries at UC Berkeley; University of Calgary, quantum entanglement and building blocks; Raymor Industries has a nano problem with its shareholders?

Monday, February 15th, 2010

There seems to be a race to get our clothes electrified so we can become portable recharging devices. From the news item on Azonano,

In research that gives literal meaning to the term “power suit,” University of California, Berkeley, engineers have created energy-scavenging nanofibers that could one day be woven into clothing and textiles.

These nano-sized generators have “piezoelectric” properties that allow them to convert into electricity the energy created through mechanical stress, stretches and twists.

“This technology could eventually lead to wearable ‘smart clothes’ that can power hand-held electronics through ordinary body movements,” said Liwei Lin, UC Berkeley professor of mechanical engineering and head of the international research team that developed the fiber nanogenerators.

This announcement is on the heels of a similar announcement (noted in my posting of Jan.22.10 here)  from researchers at the University of Stanford in California.

Meanwhile, scientists at the University of Calgary are playing with construction toys (they use the lego metaphor, which seems quite popular right now). From the news release on the University of Calgary website (thanks to Azonano where I first found notice of the item),

While many of us enjoyed constructing little houses out of toy bricks, this task is much more difficult if the bricks are elementary particles. It is even harder if these are particles of light—photons—which can only exist while flying at an incredible speed and vanish if they touch anything.

Yet a team at the University of Calgary has accomplished exactly that. By manipulating a mysterious quantum property of light known as entanglement, they are able to mount up to two photons on top of one another to construct a variety of quantum states of light—that is, build two-story quantum toy houses of any style and architecture.

The research has just (yesterday, Feb.14.10) been published in Nature Photonics. You can read the abstract (here after you scroll down) but the rest of the article is behind a paywall.

I found something rather odd this morning about Raymor Industries. It’s a Canadian nanotechnology company (their products are based on single-walled carbon nanotubes) traded on the TSX that is currently experiencing difficulty with, at least some, shareholders. From the item on PRNewsWire,

RAYMOR INDUSTRIES INC. (TSX Venture RAR, RAYRF) is a leading Canadian developer of high technology and a producer of advanced materials and nanomaterials for high value-added applications. Raymor holds the exclusive rights to more than 20 patents throughout the world, with other patents pending. Shareholders have formed a group to fight to protect our shareholder rights and prevent the current board of directors from delisting and the eliminating the common shares of the corporation.  The group is called The Raymor Investors Special Action Group.  The group is sending out this communication to get the attention of the 8000 shareholders and advise them that an appeal to the recent January 27, 2010 court ruling has been launched and is underway.  A strong and reasonable chance exists that the appeal can be won.

If you’re curious about the company and its products, you can read more here at their website, although they offer no additional information about the contretemps.

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