DISCOVERING HOW ROMULAN CLOAKING TECHNOLOGY WORKS THROUGH MATH

FROM:  THE NATIONAL SCIENCE FOUNDATION
Hidden from view
Mathematicians formulate equations, bend light and figure out how to hide things

The idea of cloaking and rendering something invisible hit the small screen in 1966 when a Romulan Bird of Prey made an unseen, surprise attack on the Starship Enterprise on Star Trek. Not only did it make for a good storyline, it likely inspired budding scientists, offering a window of technology's potential.

Today, between illusionists who make the Statue of Liberty disappear to Harry Potter's invisibility cloak that not only hides him from view but also protects him from spells, pop culture has embraced the idea of hiding behind force fields and magical materials. And not too surprisingly, National Science Foundation (NSF)-funded mathematicians, scientists and engineers are equally fascinated and looking at how and if they can transform science fiction into, well, just science.

"Cloaking is about detection and rendering something--and the cloak itself--not detectable or seen," said Michael Weinstein, an NSF-funded mathematician at Columbia University. "An object is seen when waves are bounced off it and observed by a detector."

In recent years, researchers have developed new ways in which light can move around and even through a physical object, making it invisible to parts of the electromagnetic spectrum and undetectable by sensors. Additionally, mathematicians, theoretical physicists and engineers are exploring how and whether it's feasible to cloak against other waves besides light waves. In fact, they are investigating sound waves, sea waves, seismic waves and electromagnetic waves including microwaves, infrared light, radio and television signals.

Successful outcomes have far-reaching results--like protecting deep-water oil rigs from earthquakes and vulnerable beaches from tsunamis.

Uncloaking cloaking's math and science history

Partial differential equations, coordinate invariance, wave equations--when you start talking to researchers about cloaking, it soon starts sounding a lot like math. And that's because at the very heart of this scientific question lies a mathematical one.

"There are very nice mathematical problems associated with this, and some of the ideas are mathematically very, very simple," said Michael Vogelius, NSF's division director for mathematical sciences and whose own research at Rutgers University has contributed significantly to this field. "But that doesn't mean they are simple to implement. In transformation cloaking the materials with the desired cloaking properties are found by singular or nearly singular change of variables in the energy expression--these material coefficients are sometimes referred to as the push-forward (or pull-back) of the original background. Basically, mathematicians ask, 'what do the equations have to look like to get this effect?' The thing that will be very hard--and is very hard--is to build these materials. They are singular in all kinds of ways."

That is why throughout cloaking research history, mathematicians, theoretical physicists and engineers have looked at the problem together.

According to Graeme Milton, an NSF-funded mathematician at the University of Utah, cloaking's start is rooted in math.

"Mathematicians and theoretical physicists basically had the idea independently for transformation-based cloaking," he said, adding that other mathematicians along the way--including himself--have taken the same wave equations and developed them further.

Milton and his collaborators created superlens cloaking, where cloaking occurs near lenses with capabilities far greater than traditional ones, and active exterior cloaking, where cloaking is created by active devices, and the cloak does not completely surround the object.

While cloaking has made considerable theoretical strides, its triumphs have been fairly limited for those awaiting real-world applications.

"Essentially, all the cloaking that has been done successfully in experiment involves a fixed frequency or small band of frequencies," Weinstein said. "So, it's a bit like--suppose you detect things by shining a light on them, and we all agree you're only allowed to shine blue light. I can construct a cloak that will conceal it under blue light, but if you vary the color--that is the wavelength--of the probing light, it will then be detectable. So far, we are unable to cloak something that is invisible to all colors. And because white light is composed of a broad spectrum of colors, no one has come near to making things truly undetectable."

Even with those limitations, there have been distinct milestones in cloaking research.

One of the best examples is actually widely available but probably not commonly thought of as cloaking technology, yet it applies the same sort of math. It involves sounds waves.

"Noise-cancelling headphones are basically cloaking the sounds from outside so they don't reach your ears," Milton said. "Active cloaking is very much along these same lines."

In 2006, as Milton published a key paper that expanded on the superlens cloaking he developed more than a decade earlier, a group of Duke University physicists created the first-ever microwave invisibility cloak using specially engineered "metamaterials," which can manipulate wavelengths, such as light, in a way that naturally occurring materials cannot do alone. However, it only cloaked microwaves and only in two dimensions.

And in 2014, a group in France actually did some experiments with a company to drill 5-meter-deep holes in strategic locations that would modify the earth's density and then measure effectiveness in cloaking. The experiments man-made vibrations that were at a given frequency, not earthquakes. They were able to deflect the seismic waves, showing some possibility to develop this application further.

"Science needs to figure out how to cloak against multiple frequencies before there can be any 'real' cloaking, however," Milton said. "Earthquakes and tsunamis involve a mixture of frequencies, so they are particularly challenging problems."

Passive and active approaches to cloaking research

To understand cloaking, one must first understand where the idea comes from.

When light encounters an object, it is either reflected, refracted or absorbed. Reflection means light waves bounce off an object, like a mirror. Refraction bends light waves, much like looking at a straw in a glass of water seems to break the straw into two pieces. When waves are absorbed, they are stopped, neither bouncing back nor transmitting through the object--although perhaps heating it. Objects which absorb light appear opaque or dark. These interactions between light and objects are what allow us to see those objects.

For cloaking to occur, light must be tricked into doing unusual things that reduce our ability to "see" or detect the object. Mathematicians look for how to control the flow of waves, using wave equations to characterize their behavior. Wave equations are an example of partial differential equation (PDE); PDEs are the language of the fundamental laws of physics. (Just this year, John Nash and Louis Nirenberg received the prestigious Abel Prize for their work in partial differential equations. Their contributions have had a major impact on how mathematicians analyze the PDEs used to understand phenomena such as cloaking.)

"All wave phenomena are predictable from these wave equations--at least in principle," Weinstein said. "That is, light waves, sound waves, elastic waves, quantum waves, gravitational waves. But the problem is that these equations are not so easily solved, so one tries to come up with guiding principles, useful approximations and rules of thumb. Coming to the question of cloaking, there's a mathematical property of wave equations, governing, for example, light, called coordinate invariance. That's basically a way of saying that you can change coordinates and perspectives of viewing the object, and the equations themselves don't change their essential form. By exploiting this idea of coordinate invariance, scientists have come up with prescriptions for optical properties that can cloak arbitrary objects."

In 2009, Milton and colleagues first introduced exterior active cloaking. Scientists in this field describe their research as involving either active or passive cloaking. Active cloaking uses devices that actively generate electromagnetic fields that distort waves. Passive cloaking employs metamaterials that passively shield objects from electromagnetic waves rather than intervening.

"The term 'metamaterial' is a bit deceptive," Weinstein noted. "Metamaterials are roughly composite materials. You take a bunch of building blocks, made from naturally occurring materials, and put them together in interesting ways to create some emergent property--some collective property of this novel arrangement not in naturally occurring materials. That new collective material is a metamaterial. But it's more like a device that actually interacts actively with waves moving through it."

With new metamaterial designs come new cloaking capabilities. NSF-funded engineer Andrea Alù won NSF's Waterman award in 2015 for creating metamaterials that can cloak a three-dimensional object. He and his team developed two methods--plasmonic cloaking and mantle cloaking--that take advantage of different light-scattering effects to hide an object.

Weinstein is exploring, through his research on the partial differential equations governing light, electromagnetism, sound, etc.--different ways of controlling the flow of energy, cloaking being one example, by using novel media such as metamaterials. Vogelius is known for bringing credibility to the transformation optics that serve as a backbone to cloaking broadly.

Where's my invisibility cloak?

But most fans of stealthful space ships, submarines and cloaks will still wonder: how close are we to really having any of this technology?

"I think that from the perspective of lay people, the most misunderstood thing is thinking this technology is right around the corner," Milton said. "Realistic Harry Potter cloaks are still a long way off."

Unfortunately, addressing multi-frequency cloaking will take time.

"What I do see is a merging of mathematical, physical and engineering principles to more effectively enable isolation of objects from harmful environments--there will be movement in that direction," Weinstein said. "Also, there will be important experimental advances resulting from attempts to achieve what is only theoretically possible at this time."

In the meantime, these mathematicians often look at other issues--sometimes similar ones that offer the potential to rethink their approaches.

"Right now, we're working on the opposite sort of problem--on the limitations to cloaking," Milton said. "Cloaking is just one of the many avenues I work on. Honestly, it's always stimulating to explore the limitations of what's possible and what's impossible."

-- Ivy F. Kupec
Investigators
Andrea Alu
Graeme Milton
Michael Vogelius
Michael Weinstein
Related Institutions/Organizations
Rutgers University
University of Utah
Columbia University
University of Texas at Austin

NSF TOUTS FUNDING OF LASER RESEARCH

FROM:  NATIONAL SCIENCE FOUNDATION
On the road to ubiquity
NSF support of laser research
When the National Science Foundation (NSF) was founded in 1950, the laser didn't exist. Some 65 years later, the technology is ubiquitous.

As a tool, the laser has stretched the imaginations of countless scientists and engineers, making possible everything from stunning images of celestial bodies to high-speed communications. Once described as a "solution looking for a problem," the laser powered and pulsed its way into nearly every aspect of modern life.

For its part, NSF funding enabled research that has translated into meaningful applications in manufacturing, communications, life sciences, defense and medicine. NSF also has played a critical role in training the scientists and engineers who perform this research.

"We enable them [young researchers] at the beginning of their academic careers to get funding and take the next big step," said Lawrence Goldberg, senior engineering adviser in NSF's Directorate for Engineering.

Getting started

During the late 1950s and throughout the 1960s, major industrial laboratories run by companies such as Hughes Aircraft Company, AT&T and General Electric supported laser research as did the Department of Defense. These efforts developed all kinds of lasers--gas, solid-state (based on solid materials), semiconductor (based on electronics), dye and excimer (based on reactive gases).

Like the first computers, early lasers were often room-size, requiring massive tables that held multiple mirrors, tubes and electronic components. Inefficient and power-hungry, these monoliths challenged even the most dedicated researchers. Undaunted, they refined components and techniques required for smooth operation.

As the 1960s ended, funding for industrial labs began to shrink as companies scaled back or eliminated their fundamental research and laser development programs. To fill the void, the federal government and emerging laser industry looked to universities and NSF.

Despite budget cuts in the 1970s, NSF funded a range of projects that helped improve all aspects of laser performance, from beam shaping and pulse rate to energy consumption. Support also contributed to developing new materials essential for continued progress toward new kinds of lasers. As efficiency improved, researchers began considering how to apply the technology.

Charge of the lightwave

One area in particular, data transmission, gained momentum as the 1980s progressed. NSF's Lightwave Technology Program in its engineering directorate was critical not only because the research it funded fueled the Internet, mobile devices and other high-bandwidth communications applications, but also because many of the laser advances in this field drove progress in other disciplines.

An important example of this crossover is optical coherence tomography (OCT). Used in the late 1980s in telecommunications to find faults in miniature optical waveguides and optical fibers, this imaging technique was adapted by biomedical researchers in the early 1990s to noninvasively image microscopic structures in the eye. The imaging modality is now commonly used in ophthalmology to image the retina. NSF continues to fund OCT research.

As laser technology matured through the 1990s, applications became more abundant. Lasers made their way to the factory floor (to cut, weld and drill) and the ocean floor (to boost signals in transatlantic communications). The continued miniaturization of lasers and the advent of optical fibers radically altered medical diagnostics as well as surgery.

Focus on multidisciplinary research

In 1996, NSF released its first solicitation solely targeting multidisciplinary optical science and engineering. The initiative awarded $13.5 million via 18 three-year awards. Grantees were selected from 76 proposals and 627 pre-proposals. Over a dozen NSF program areas participated.

The press release announcing the awards described optical science and engineering as "an 'enabling' technology" and went on to explain that "for such a sweeping field, the broad approach...emphasizing collaboration between disciplines, is particularly effective. By coordinating program efforts, the NSF has encouraged cross-disciplinary linkages that could lead to major findings, sometimes in seemingly unrelated areas that could have solid scientific as well as economic benefits."

"There is an advantage in supporting groups that can bring together the right people," said Denise Caldwell, director of NSF's Division of Physics. In one such case, she says NSF's support of the Center for Ultrafast Optical Science (CUOS) at the University of Michigan led to advances in multiple areas including manufacturing, telecommunications and precision surgery.

During the 1990s, CUOS scientists were developing ultrafast lasers. As they explored femtosecond lasers--ones with pulses one quadrillionth of a second--they discovered that femtosecond lasers drilled cleaner holes than picosecond lasers—ones with pulses one trillionth of a second.

Although they transferred the technology to the Ford Motor Company, a young physician at the university heard about the capability and contacted the center. The collaboration between the clinician and CUOS researchers led to IntraLASIK technology used by ophthalmologists for cornea surgery as well as a spin-off company, Intralase (funded with an NSF Small Business Innovative Research grant).

More recently, NSF support of the Engineering Research Center for Extreme Ultraviolet Science and Technology at Colorado State University has given rise to the development of compact ultrafast laser sources in the extreme UV and X-ray spectral regions.

This work is significant because these lasers will now be more widely available to researchers, diminishing the need for access to a large source like a synchrotron. Compact ultrafast sources are opening up entirely new fields of study such as attosecond dynamics, which enables scientists to follow the motion of electrons within molecules and materials.

Identifying new research directions

NSF's ability to foster collaborations within the scientific community has also enabled it to identify new avenues for research. As laser technology matured in the late 1980s, some researchers began to consider the interaction of laser light with biological material. Sensing this movement, NSF began funding research in this area.

"Optics has been a primary force in fostering this interface," Caldwell said.

One researcher who saw NSF taking the lead in pushing the frontiers of light-matter interactions was University of Michigan researcher Duncan Steel.

At the time, Steel continued pursuing quantum electronics research while using lasers to enable imaging and spectroscopy of single molecules in their natural environment. Steel and his colleagues were among the first to optically study how molecular self-assembly of proteins affects the neurotoxicity of Alzheimer's disease.

"New classes of light sources and dramatic changes in detectors and smart imaging opened up new options in biomedical research," Steel said. "NSF took the initiative to establish a highly interdisciplinary direction that then enabled many people to pursue emergent ideas that were rapidly evolving. That's one of NSF's biggest legacies--to create new opportunities that enable scientists and engineers to create and follow new directions for a field."

Roadmaps for the future

In the mid-1990s, the optical science and engineering research community expressed considerable interest in developing a report to describe the impact the field was having on national needs as well as recommend new areas that might benefit from focused effort from the community.

As a result, in 1998, the National Research Council (NRC) published Harnessing Light: Optical Science and Engineering for the 21st Century. Funded by the Defense Advanced Research Projects Agency, NSF and the National Institute of Standards and Technology (NIST), the 331-page report was the first comprehensive look at the laser juggernaut.

That report was significant because, for the first time, the community focused a lens on its R&D in language that was accessible to the public and to policymakers. It also laid the groundwork for subsequent reports. Fifteen years after Harnessing Light, NSF was the lead funding agency for another NRC report, Optics and Photonics: Essential Technologies for Our Nation.

Widely disseminated through the community's professional societies, the Optics and Photonics report led to a 2014 report by the National Science and Technology Council's Fast Track Action Committee on Optics and Photonics, Building a Brighter Future with Optics and Photonics.

The committee, comprised of 14 federal agencies and co-chaired by NSF and NIST, would identify cross-cutting areas of optics and photonics research that, through interagency collaboration, could benefit the nation. It also was also set to prioritize these research areas for possible federal investment and set long-term goals for federal optics and photonics research.

Developing a long-term, integrated approach

One of the recommendations from the NRC Optics and Photonics report was creation of a national photonics initiative to identify critical technical priorities for long-term federal R&D funding. To develop these priorities, the initiative would draw on researchers from industry, universities and the government as well as policymakers. Their charge: Provide a more integrated approach to industrial and federal optics and photonics R&D spending.

In just a year, the National Photonics Initiative was formed through the cooperative efforts of the Optical Society of America, SPIE--the international society for optics and photonics, the IEEE Photonics Society, the Laser Institute of America and the American Physical Society Division of Laser Science. One of the first fruits of this forward-looking initiative is a technology roadmap for President Obama's BRAIN Initiative.

To assess NSF's own programs and consider future directions, the agency formed an optics and photonics working group in 2013 to develop a roadmap to "lay the groundwork for major advances in scientific understanding and creation of high-impact technologies for the next decade and beyond."

The working group, led by Goldberg and Charles Ying, from NSF's Division of Materials Research, inventoried NSF's annual investment in optics and photonics. Their assessment showed that NSF invests about $300 million each year in the field.

They also identified opportunities for future growth and investment in photonic/electronic integration, biophotonics, the quantum domain, extreme UV and X-ray, manufacturing and crosscutting education and international activities.

As a next step, NSF also formed an optics and photonics advisory subcommittee for its Mathematical and Physical Sciences Directorate Advisory Committee. In its final report released in July 2014, the subcommittee identified seven research areas that could benefit from additional funding, including nanophotonics, new imaging modalities and optics and photonics for astronomy and astrophysics.

That same month, NSF released a "Dear Colleague Letter" to demonstrate the foundation's growing interest in optics and photonics and to stimulate a pool of proposals, cross-disciplinary in nature, that could help define new research directions.

And so the laser, once itself the main focus of research, takes its place as a device that extends our ability to see.

"To see is a fundamental human drive," Caldwell said. "If I want to understand a thing, I want to see it. The laser is a very special source of light with incredible capabilities."

-- Susan Reiss, National Science Foundation
-- Ivy F. Kupec, (703) 292-8796 ikupec@nsf.gov
Investigators
John Nees
Jorge Rocca
Duncan Steel
Tibor Juhasz
Carmen Menoni
David Attwood
Henry Kapteyn
Herbert Winful
Steven Yalisove
Margaret Murnane

LANL SAYS NEW TECHNOLOGY COULD TRANSFORM OPTICS

 

Photograph of an ultrathin (72 µm thick) metamaterial sample. Image courtesy Los Alamos National Laboratory

FROM: LOS ALAMOS NATIONAL LABORATORY
Metamaterial Flexible Sheets Could Transform Optics
New design flattens bulky optical devices
LOS ALAMOS, N.M., June 5, 2013—New ultrathin, planar, lightweight, and broadband polarimetric photonic devices and optics could result from recent research by a team of Los Alamos National Laboratory scientists. The advances would boost security screening systems, infrared thermal cameras, energy harvesting, and radar systems.

This development is a key step toward replacing bulky conventional optics with flexible sheets that are about the thickness of a human hair and weighing a fraction of an ounce. The advance is in the design of artificially created materials, called metamaterials, that give scientists new levels of control over light wavelengths.

The research was reported online in Science magazine, "Terahertz Metamaterials for Linear Polarization Conversion and Anomalous Refraction." The team demonstrated broadband, high-performance linear polarization conversion using ultrathin planar metamaterials, enabling possible applications in the terahertz (THz) frequency regime. Their design can be scaled to other frequency ranges from the microwave through infrared.

Polarization is one of the basic properties of electromagnetic waves, describing the direction of the electric field oscillation, and thus conveying valuable information in signal transmission and sensitive measurements.

"Conventional methods for advanced polarization control impose very demanding requirements on material properties and fabrication methods, but they attain only limited performance," said Hou-Tong Chen, the senior researcher on the project.

Metamaterial-based polarimetric devices are particularly attractive in the terahertz frequency range due to the lack of suitable natural materials for THz applications. Currently available designs suffer from either very limited bandwidth or high losses. The Los Alamos designs further enable the near-perfect realization of the generalized laws of reflection/refraction. According to the researchers, this can be exploited to make flat lenses, prisms, and other optical elements in a fashion very different from the curved, conventional designs that we use in our daily life.

The Los Alamos National Laboratory Directed Research and Development (LDRD) program funded a portion of the research. Part of the work was performed at the Center for Integrated Nanotechnologies (CINT).

Reference: ‘Terahertz Metamaterials for Linear Polarization Conversion and Anomalous Refraction," Science, published online in Science Express, May 16, DOI: 10.1126/science.1235399, by Nathaniel K. Grady, Jane E. Heyes, Dibakar Roy Chowdhury, Yong Zeng, Matthew T. Reiten, Abul K. Azad, Antoinette J. Taylor, Diego A. R. Dalvit and Hou-Tong Chen of Los Alamos National Laboratory.