Tag Archives: infrared

Exploring the fundamental limits of invisibility cloaks

There’s some interesting work on invisibility cloaks coming from the University of Texas at Austin according to a July 6, 2015 news item on Nanowerk,

Researchers in the Cockrell School of Engineering at The University of Texas at Austin have been able to quantify fundamental physical limitations on the performance of cloaking devices, a technology that allows objects to become invisible or undetectable to electromagnetic waves including radio waves, microwaves, infrared and visible light.

A July 5, 2016 University of Texas at Austin news release (also on EurekAlert), which originated the news item, expands on the theme,

The researchers’ theory confirms that it is possible to use cloaks to perfectly hide an object for a specific wavelength, but hiding an object from an illumination containing different wavelengths becomes more challenging as the size of the object increases.

Andrea Alù, an electrical and computer engineering professor and a leading researcher in the area of cloaking technology, along with graduate student Francesco Monticone, created a quantitative framework that now establishes boundaries on the bandwidth capabilities of electromagnetic cloaks for objects of different sizes and composition. As a result, researchers can calculate the expected optimal performance of invisibility devices before designing and developing a specific cloak for an object of interest. …

Cloaks are made from artificial materials, called metamaterials, that have special properties enabling a better control of the incoming wave, and can make an object invisible or transparent. The newly established boundaries apply to cloaks made of passive metamaterials — those that do not draw energy from an external power source.

Understanding the bandwidth and size limitations of cloaking is important to assess the potential of cloaking devices for real-world applications such as communication antennas, biomedical devices and military radars, Alù said. The researchers’ framework shows that the performance of a passive cloak is largely determined by the size of the object to be hidden compared with the wavelength of the incoming wave, and it quantifies how, for shorter wavelengths, cloaking gets drastically more difficult.

For example, it is possible to cloak a medium-size antenna from radio waves over relatively broad bandwidths for clearer communications, but it is essentially impossible to cloak large objects, such as a human body or a military tank, from visible light waves, which are much shorter than radio waves.

“We have shown that it will not be possible to drastically suppress the light scattering of a tank or an airplane for visible frequencies with currently available techniques based on passive materials,” Monticone said. “But for objects comparable in size to the wavelength that excites them (a typical radio-wave antenna, for example, or the tip of some optical microscopy tools), the derived bounds show that you can do something useful, the restrictions become looser, and we can quantify them.”

In addition to providing a practical guide for research on cloaking devices, the researchers believe that the proposed framework can help dispel some of the myths that have been developed around cloaking and its potential to make large objects invisible.
“The question is, ‘Can we make a passive cloak that makes human-scale objects invisible?’ ” Alù said. “It turns out that there are stringent constraints in coating an object with a passive material and making it look as if the object were not there, for an arbitrary incoming wave and observation point.”

Now that bandwidth limits on cloaking are available, researchers can focus on developing practical applications with this technology that get close to these limits.

“If we want to go beyond the performance of passive cloaks, there are other options,” Monticone said. “Our group and others have been exploring active and nonlinear cloaking techniques, for which these limits do not apply. Alternatively, we can aim for looser forms of invisibility, as in cloaking devices that introduce phase delays as light is transmitted through, camouflaging techniques, or other optical tricks that give the impression of transparency, without actually reducing the overall scattering of light.”

Alù’s lab is working on the design of active cloaks that use metamaterials plugged to an external energy source to achieve broader transparency bandwidths.

“Even with active cloaks, Einstein’s theory of relativity fundamentally limits the ultimate performance for invisibility,” Alù said. “Yet, with new concepts and designs, such as active and nonlinear metamaterials, it is possible to move forward in the quest for transparency and invisibility.”

The researchers have prepared a diagram illustrating their work,

The graph shows the trade-off between how much an object can be made transparent (scattering reduction; vertical axis) and the color span (bandwidth; horizontal axis) over which this phenomenon can be achieved. Courtesy: University of Texas at Austin

The graph shows the trade-off between how much an object can be made transparent (scattering reduction; vertical axis) and the color span (bandwidth; horizontal axis) over which this phenomenon can be achieved. Courtesy: University of Texas at Austin

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

Invisibility exposed: physical bounds on passive cloaking by Francesco Monticone and Andrea Alù. Optica Vol. 3, Issue 7, pp. 718-724 (2016) •doi: 10.1364/OPTICA.3.000718

This paper is open access.

Wireless nano for remotely activating neurons

Every once in a while, there’s a piece of research that disconcerts me and this would be one of those pieces. From a May 22, 2014 news item on Nanowerk,

Yang Xiang, PhD, assistant professor of neurobiology at University of Massachusetts Medical School, has received a three-year, $900,000 grant from the Human Frontiers Science Program to lead an international team of scientists, including Gang Han, PhD, assistant professor of biochemistry & molecular pharmacology, in the development and implementation of a new optogenetic platform that can remotely activate neurons inside a free-moving organism.

Using a new class of nanoparticles developed by Dr. Han, Dr. Xiang and colleagues propose to selectively turn on non-image forming photoreceptors (NIFP) inside mice and Drosophila unencumbered by the fiber optic wires used in currently available optogenetic technologies. By wirelessly stimulating these photoreceptors, which are able to sense light even though they don’t generate vision, scientists can better understand their role in regulating physiological functions such as circadian rhythm, sleep and melatonin secretion. The hope is that this new technology can also be used to study the links between other types of neurons, physiology and behavior.

A May 22, 2014 University of Massachusetts Medical School news release by Jim Fessenden, which originated the news item, describes optogenetics and some of its challenges,

“Current optogenetic technologies are limited in their application because they require using ‘wired’ fiber optic implants to deliver blue light to activate neuron activities,” said Xiang. “This is a major technological problem that has become an obstacle to understanding the physiological role NIFP play in animal behavior. If we’re able to overcome this hurdle by using the nanoparticles developed by Dr. Han, it would open the door to more informed investigations of not only NIFP but a wide range of neurons and their effect on behavior.”

In use for only about a decade, optogenetic technology combines techniques from optics and genetics, allowing scientists to precisely control activities of individual neurons using light. By genetically inserting light-activated biological molecules such as channelrhodopsins, a family of proteins found in algae, into neurons, scientists can instantaneously turn them on using beams of blue light with millisecond precision.

A limiting factor to the wider application of this technology, however, is that blue wavelengths are unable to penetrate skin, bone and other tissues deep enough to activate the neurons inside free-moving animals. To overcome this obstacle, current techniques require the insertion of fiber optic wires close enough to the neurons so the light that activates them can be delivered. This technique restricts animal movement and makes it difficult to observe behavioral responses in natural conditions. This fiber optic approach further limits scientists’ ability to study behavior over longer periods of time as the effectiveness of light delivery is relatively short due to scarring.

The news release describes the new technique proposed by Xiang and his associates,

Han has developed an “upconversion nanoparticle” (UCNP) that has the potential to solve the limitations of wired optogenetic techniques. These nanoparticles are capable of absorbing infrared light that can’t be seen and converting it into visible blue light. In contrast to blue light, infrared light is capable of penetrating skin and tissue to a depth of several centimeters. Xiang and Han believe these nanoparticles, tuned to emit blue light, can be inserted into the brain and used as a substitute for traditional fiber optics to wirelessly activate neurons in animals.

The hope is that the nanoparticles will absorb infrared light that passes through the tissue, and convert it to blue light inside the animal. This blue light would then activate the NFIPs. If successful, Xiang and colleagues will be able to observe any changes in animal behavior brought about by activating these non-image forming photoreceptors.

“The nanoparticles act as a kind of relay station,” said Han. “They convert the low-energy red light into a high-energy blue light that can activate the neurons. This technique completely alleviates the need to use intrusive fiber optic wires. It vastly simplifies the technology and expands the potential uses for optogenetics.”

Xiang said, “In many ways, this is the perfect bridge between a technological advancement and an important biological question. With these nanoparticles it’s possible for us to begin answering fundamental neurobiological questions about NIFPs.

“More broadly, it would open up the possibility of using other model organisms, such as Drosophila, that can’t be used with the current wired optogenetic technologies, to investigate and answer important questions about how neural activities regulate behavior.”

Illogical as it is, the idea that neurons could be wirelessly and remotely activated by someone other the owner of those neurons disturbs me even though I know drugs are commonly used to do much the same thing in humans.

In any event, the news release provides this final paragraph about the funding,

HFSP [Human Frontiers Science Program] awards are given to highly innovative teams that demonstrate that they have developed and can test a paradigm-shifting idea that holds promise for the development of new approaches to problems in the life sciences with potential to advance the field of research significantly.

I looked up the HFSP online and found this on the About Us page on the HFSP website,

The Human Frontier Science Program is a program of funding for frontier research in the life sciences. It is implemented by the International Human Frontier Science Program Organization (HFSPO) with its office in Strasbourg.

The members of the HFSPO, the so-called Management Supporting Parties (MSPs) are the contributing countries and the European Union, which contributes on behalf of the non-G7 EU members.

The current MSPs are Australia, Canada, France, Germany, India, Italy, Japan, Republic of Korea, Norway, New Zealand, Switzerland the United Kingdom, the United States of America and the European Union. [emphasis mine]

I was not expecting to find Canada on that list.

Better night vision goggles for the military

I remember a military type, a friend who served as a Canadian peacekeeper (Infantry) in the Balkans, describing night-vision goggles and mentioning they are loud. After all, it’s imaging equipment and that requires a power source or, in this case, a source of noise. The Dec. 29, 2012 news item on Nanowerk about improved imaging for night vision goggles doesn’t mention noise but hopefully, the problem has been addressed or mitigated (assuming this technology is meant to be worn),

Through some key breakthroughs in flexible semiconductors, electrical and computer engineering Professor Zhenqiang “Jack” Ma has created two imaging technologies that have potential applications beyond the 21st century battlefield.

With $750,000 in support from the Air Force Office of Scientific Research (AFOSR), Ma has developed curved night-vision goggles using germanium nanomembranes.

The Dec. 28, 2012 University of Wisconsin-Madison news release, which originated the news item, describes the Air Force project and another night vision project for the US Department of Defense,

Creating night-vision goggles with a curved surface allows a wider field of view for pilots, but requires highly photosensitive materials with mechanical bendability-the silicon used in conventional image sensors doesn’t cut it.

…  Ma’s design employs flexible germanium nanomembranes: a transferrable flexible semiconductor that until now has been too challenging to use in imagers due to a high dark current, the background electrical current that flows through photosensitive materials even when they aren’t exposed to light.

“Because of their higher dark current, the image often comes up much noisier on germanium-based imagers,” says Ma. “We solved that problem.”

Ma’s dark current reduction technology has also been recently licensed to Intel.

In another imaging project, the U.S. Department of Defense has provided Ma with $750,000 in support of development of imagers for military surveillance that span multiple spectra, combining infrared and visible light into a single image.

“The reason they are interested in IR is because visible light can be blocked by clouds, dust, smoke,” says Ma. “IR can go through, so simultaneous visible and IR imaging allows them to see everything.”

Inexpensive silicon makes production of visible light imagers a simple task, but IR relies on materials incompatible with silicon.

The current approach involves a sensor for IR images and a sensor for visible light, combining the two images in post-processing, which requires greater computing power and hardware complexity. Instead, Ma will employ a heterogeneous semiconductor nanomembrane, stacking the two incompatible materials in each pixel of the new imager to layer IR and visible images on top of one another in a single image.

The result will be imagers that can seamlessly shift between IR and visible images, allowing the picture to be richer and more quickly utilized for strategic decisionmaking.

It’s impossible to tell from the description if this particular technology will be worn by foot soldiers or human military personnel but, in the event it will be worn,  it does well to remember that it will need a power source. Interestingly, the average soldier already carries a lot of weight in batteries (up to 35 pounds!) as per my May 9, 2012 posting about energy-harvesting textiles and the military.