NRL Scientists Discover Novel Metamaterial Properties Within Hexagonal Boron Nitride

http://www.photonicsonline.com/doc/nrl-scientists-discover-novel-properties-hexagonal-boron-nitride-0001

U.S. Naval Research Laboratory (NRL) scientists, in collaboration with researchers from the University of Manchester, U.K.; Imperial College, London; University of California San Diego; and the National Institute of Material Science (NIMS), Japan, have demonstrated that confined surface phonon polaritons within hexagonal boron nitride (hBN) exhibit unique metamaterial properties that enable novel nanoscale optical devices for use in optical communications, super-resolution imaging, and improved infrared cameras and detectors.

Metamaterials are artificial composites of various materials designed to exhibit optical properties not anticipated in nature. One such property is hyperbolicity, whereby a material exhibits both metallic- and dielectric-like optical responses simultaneously along different crystal axes. These hyperbolic metamaterials are the basis for many potential applications such as 'hyperlenses,' used for imaging of nanoscale objects not observable using conventional optics.

"Our examination into the characteristics of hBN reveal the first experimental observation of sub-diffractional guided waves confined in all three dimensions, using a natural hyperbolic material," said Joshua Caldwell, Ph.D., Electronics Science and Technology Division, Power Electronics Branch. "This may, in turn, lead to the development of disruptive technologies such as the nanoscale equivalent of an optical fiber due to the volume-bound confinement of sub-diffractional modes within hBN."

Optic phonons, or crystal vibrations that can be excited with infrared light, can also be used to confine light to dimensions much smaller than the wavelength of light, while maintaining record-high efficiencies. These surface phonon polaritons are analogous to electron oscillations in metals or doped-semiconductors, called plasmons, but offer the benefit of low losses and operation in the infrared to terahertz spectral regions.

As a van der Waels crystal—a layered crystal structure similar to graphene or graphite—hBN was demonstrated to be two orders of magnitude more efficient than hyperbolic metamaterials shown to date, says Caldwell. Unlike metallic/dielectric hyperbolic metamaterials, hBN also provides the additional functionality of both types of hyperbolicity, allowing both the in-plane and out-of-plane crystal axes to behave metallic- (reflective) or dielectric-like (transparent) simply by changing the wavelength of the exciting light. This mixing of both types of hyperbolic behavior is to this point unique and allowed the fundamental comparison of antennas within these two regimes.

Using the natural hyperbolic behavior of hBN, the researchers were able to demonstrate that light could also be confined within optical antennas—up to 86 times smaller than the wavelength of light, for instance confinement of 6.8 micrometers of light into a 0.08 micrometer tall antenna—while maintaining record-high efficiencies due to the low-loss nature of the dielectric crystal.

The researchers were able to further demonstrate that the resonance wavelength of the hyperbolic polaritons confined within these antennae was dependent only upon the aspect ratio (height/diameter), and was nominally independent upon the actual size and/or shape—demonstrating that antennas could be defined for a given application simply by controlling this ratio, thereby making them compatible to a wide array of device form-factors. This could enable frequency selective operation and nanophotonic circuits, as well as provide an operational material for mid-infrared imaging of nanoscale objects.

The research team also demonstrated that the resonance response exhibited not a single mode, but four separate series, and according to Caldwell, a change in the wavelength and/or the angle of the incoming light with respect to the sample surface could isolate each series, providing the first complete description of these novel, three-dimensionally confined hyperbolic polariton modes.

Further discoveries found these breakthroughs could have an impact in areas such as enhanced infrared or molecular spectroscopy, improved functionality for nanophotonic circuits and devices for use in infrared cameras, detectors and weapons guidance systems, and tailored thermal emission sources.

The NRL Power Electronics Branch serves as the laboratory's principal resource for the science and technology of solid-state high-power electronic devices. The primary mission of the branch is to conduct research and development (R&D) programs in solid state electronics and related technologies that support U.S. Navy and Department of Defense (DoD) interests and capabilities in the full range of new weapons capabilities enabled by high-power solid state electronic devices. Moreover, the branch serves as the focal point of insertion of power electronics technology into Navy engineering development efforts.

About the U.S. Naval Research Laboratory
The U.S. Naval Research Laboratory is the Navy's full-spectrum corporate laboratory, conducting a broadly based multidisciplinary program of scientific research and advanced technological development. The Laboratory, with a total complement of approximately 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 90 years and continues to meet the complex technological challenges of today's world. For more information, visit http://www.nrl.navy.mil/.

SOURCE: The U.S. Naval Research Laboratory

Growing The Terahertz Transistor

http://www.compoundsemiconductor.net/article/95304-growing-the-terahertz-transistor.html

Breakthrough GaN-on-graphene growth brings Naval Research Laboratories researchers close to hot electron transistors. Compound Semiconductor reports.

Can NRL's low temperature atomic layer epitaxy help researchers grow GaN-on graphene structures for terahertz frequency applications?

Claiming a world first, researchers from the US-based Naval Research Laboratories have successfully grown a high-quality GaN film on graphene, a critical step to developing so-called hot electron transistors for terahertz frequency applications.

Using MOCVD, GaN films were grown on functionalised epitaxial graphene, over an AlN nucleation layer, yielding a crystalline quality on par with GaN grown on sapphire. What's more, lead researcher, Charles Eddy from NRL's Electronics Science and Technology division, says his team can use the same process to grow structures across a four inch wafer, setting the scene for cost-effective device manufacture.

Hot electron transistors (HETs) hold great promise for high-frequency, high-speed current switching applications as during operation, the electrons tunnel ballistically through base and collector structures. However, today's state-of-the-art HETs include a metal or heavily doped semiconductor base layer that impedes electron movement, limiting device performance.

With this in mind, researchers worldwide are turning to graphene as an alternative base layer material. Electrons fly though this so-called wonder material, enabling cut-off frequencies in excess of 1 THz, and pleasingly, researchers have mastered methods to grow a single layer of graphene - epitaxial graphene - on SiC substrates.

But while graphene solves a lot of problems, it also raises new issues. As Eddy puts it: "Graphene, and any other 2D material, has very good in-plane bonds, but not very good out-of-plane bonds. There aren't actually any atomic bonds to connect to if you want to try and grow a material, such as GaN, directly onto a graphene surface."

Indeed, deposit GaN directly onto a graphene surface and individual crystallites form instead of a continous film. And while researchers have modified the surface to circumvent this problem - treatments include plasma-spraying the surface and adding nanostructures - the resulting graphene-GaN interface can inhibit final device performance.

Terahertz designs: the NRL research team is pioneering GaN-on-graphene growth in a bid to fabricate hot electron transistors.

However, a surface modification process, originally pioneered by Eddy's team to integrate high K gate dielectrics with graphene, looks set to solve the problem. So-called fluorine functionalisation involves dosing the graphene surface with xenon difluoride to create semi-ionic carbon-fluorine bonds that provide nucleation sites for III-nitride deposition.

With the nucleation sites in place, Eddy and colleagues then deposit a buffer AlN film onto the functionalised graphene, prior to GaN growth. Aluminium atoms have a higher sticking coefficient than gallium atoms, and in Eddy's words: "'Gallium just isn't as easy to stick as aluminium."

But the surface modification success comes with a hitch. The semi-ionic fluorine bonds can attach to the graphene surface while preserving its structure, but these bonds cannot withstand the high temperatures of conventional MOCVD.

"Ionic bonds would break the bonding in graphene and kill its electronic properties," says Eddy. "However, the [semi-ionic] bonds are very temperature sensitive, so to grow III-nitrides on graphene we really need a low temperature epitaxial growth process."

And as it happens, the NRL researchers had been developing a low temperature process, based on atomic layer epitaxy. Closely related to MOCVD, the process relies on a metalorganic precursor and nitrogen for epitaxial growth.

"Rather than using ammonia that is cracked at the high temperatures of MOCVD to create the nitrogen reactivity you need, we use a nitrogen plasma to provide the reactive nitrogen," says Eddy. "This reacts with the metalorganic precursor at the graphene surface, so all the reactions happen on the surface at a much lower growth temperature."

And so III-nitride growth takes place at only 280ºC, about half the temperature of conventional MOCVD.

Growing structures

With both graphene functionalisation and low temperature epitaxy in hand, Eddy and his team have grown HET structures on 16 mm² functionalised epitaxial graphene sheet. "The 16 mm² sheet was used just for economics but the growth process is readily scaleable," asserts Eddy. "We could grow the structure over a four inch wafer right now."

The researcher also believes initial devices could be just a year away. X-ray diffraction analysis indicates the crystalline quality of the layers is comparable to growth on sapphire, and Eddy reckons his team can do better.

"The lattice mismatch between graphene and AlN is 4.5%, which is significantly smaller than the 13% between AlN and sapphire," he says. "Given this, we should be ultimately getting better crystalline quality."

So now the team is exploring the underlying physics to boost material quality further. "We want to understand the mechanisms with better clarity," says Eddy. "Our work shows it is possible to grow [hot electron transistor] structures using this process, so the question is can we now control it enough to improve materials quality and make better performing devices?"

A New Practical Use For Graphene Will Revolutionise Night Vision Technology

Night vision goggles. Peter Macdiarmid/Getty Imageshttp://www.businessinsider.com.au/a-new-practical-use-for-graphene-will-revolutionise-night-vision-technology-2014-9

Researchers have developed a light detector which could revolutionise chemical sensing and night
vision technology.

In the latest issue of the journal Nature Nanotechnology, a team of researchers at Monash University, the University of Maryland in the US and the US Naval Research Laboratory have created a light detector based on graphene, a single sheet of interconnected carbon atoms.

The detector can detect light over an unusually broad range of wavelengths, including terahertz waves, which are between infrared and microwave radiation where sensitive light detection is most difficult.

Professor Michael Fuhrer at Monash says the research could lead to a generation of light detectors which could see below the surface of walls and other objects.

“We have demonstrated light detection from terahertz to near-infrared frequencies, a range about 100 times larger than the visible spectrum,” Professor Fuhrer says.

“Detection of infrared and terahertz light has numerous uses, from chemical analysis to night vision
goggles and body scanners used in airport security.”

Current applications for terahertz detection are limited, as they need to be kept extremely cold to maintain sensitivity.

Existing detectors that work at room temperature are bulky, slow and expensive.

Professor Fuhrer says the new detector works at room temperature and is already as sensitive as any existing room-temperature detector technology in the terahertz range but is also more than a million times faster.

The device is easily manufactured and could lead to inexpensive infrared cameras or night-vision goggles.

U. S. Navy Awards Advanced Photonix a Development Contract for Imaging Sensor

http://online.wsj.com/article/PR-CO-20140627-908428.html

ANN ARBOR, Mich., June 27, 2014 /PRNewswire/ -- Advanced Photonix(R) (NYSE MKT: API) announced that it has been awarded an advanced development contract for Phase II Terahertz Imaging for Detection of Corrosion and Defects Under Hull Coatings. The contract from the Naval Undersea Warfare Center (NUWC) is valued at approximately $1,000,000 (including all options) for further development of a high-speed imaging system for the detection of corrosion under marine coatings.

Successful completion of the Phase I proof of concept has lead to this award of a Phase II contract to develop a prototype suitable for evaluation. The terahertz imaging system will be designed for in-situ defect detection of commonly used ship hull coatings and rapid, non-destructive detection of corrosion, hull coating delamination, and hull surface gouges recessed into the hull metal through still-attached hull coatings. The prototype will be evaluated on submarine hulls or representative structures. It is expected, that this effort will be followed by preparation of a Phase III development plan to transition the technology into a system that can be deployed by the Navy.

Richard Kurtz, President and CEO of Advanced Photonix, stated, "This is another example of our long and continuing partnership with the Department of Defense. In keeping with their tradition of accelerating the adoption of advanced non-destructive testing technologies, NUWC has invested with us since 2003 to develop terahertz solutions for various applications. The high-speed imaging system developed under this contract will allow the Navy to save maintenance costs in its dockside operations."

About Advanced Photonix, Inc.

Advanced Photonix, Inc.(R) (NYSE MKT: API) is a leading supplier of optoelectronic sensors, devices and instruments used by Test and Measurement, Process Control, Medical, Telecommunication and Homeland Security markets. The company has three product lines: Optosolutions focuses on enabling manufacturers to measure physical properties, including temperature, particular counting, color, and fluorescence for Medical, Homeland Security and Process Control applications. The Terahertz sensor product line is targeted to the Process Control, to enable quality control, and Security markets through nondestructive testing. The T-Gauge(R) sensor can measure subsurface physical properties, like multi-layers thicknesses, density, moisture content, anomaly detection and some chemical features, online and in real time. High-Speed Optical Receiver (HSOR) products are used by the telecommunication market in both telecommunication equipment and in test and measurement equipment utilized in the manufacturing of telecommunication equipment. For more information visit us on the web at www.advancedphotonix.com.

The information contained herein includes forward looking statements that are based on assumptions that management believes to be reasonable but are subject to inherent uncertainties and risks including, but not limited to, unforeseen technological obstacles which may prevent or slow the development and/or manufacture of new products; potential problems with the integration of the acquired company and its technology and possible inability to achieve expected synergies; obstacles to successfully combining product offerings and lack of customer acceptance of such offerings; limited (or slower than anticipated) customer acceptance of new products which have been and are being developed by the Company; and a decline in the general demand for optoelectronic products.

CONTACT: ir@advancedphotonix.com

Logo - http://photos.prnewswire.com/prnh/20130304/LA69982LOGO

SOURCE Advanced Photonix, Inc.

/Web site: http://www.advancedphotonix.com

Metamaterials Make Physics Seem Like Magic

Metamaterials Make Physics Seem Like Magic

David Smith & Yaroslav Urzhumov Duke University Pratt School of Engineering Durham, NC Jeff Wilson NASA Glen Research Center, Cleveland, OH Fabio Alves and Gamani Karunasiri Naval Postgraduate School Monterey, CA

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Metamaterials are manufactured, structured materials that they can interact and manipulate wave phenomena such that objects surrounded by metamaterials are shielded from these waves. In the case of light, metamaterials make these objects ‘invisible’. Researchers throughout the world are applying these materials to many different applications.

At NASA Glen Research Center, physicists are currently working on potential applications for metamaterials, including a “superlens” that would provide much higher resolution than what is possible using natural materials, by utilizing the negative refraction of metamaterials.

Researchers at the Naval Postgraduate School in California are working with metamaterials in a metafilm for terahertz (THz) imaging technologies. Electromagnetic waves in the THz scale are commonly used in airport scanners as they can penetrate non-metallic materials without damaging tissue or DNA. Yet, they require expensive and complex imaging arrangements to combat the fact that most THz waves are absorbed in the air prior to reaching the target. The metafilm must be designed using the appropriate materials and geometry to attain near 100% transmission at the desired frequency.

At the Center for Metamaterials and Integrated Plasmonics (CMIP) at Duke University, led by David R. Smith, researchers are working with Toyota to use metamaterials for a wireless transfer of power in an electrical vehicle. They have accomplished this task by making the distance between the power source and the device disappear with a metamaterial-based lens.

All of these researchers used COMSOL Multiphsyics to implement, verify and optimize the designs. Further, they are using COMSOL to conceptually investigate new uses for metamaterials.

Advanced Photonix/Picometrix secures US Navy contract Terahertz Imaging for Detection of Corrosion and Defects Under Hull Coatings

My note: Thanks for bringing this recent contract to my attention  to IV poster 

bucktailjig05

http://www.navysbirprogram.com/NavySearch/Award/award.aspx?pk=67914101-EB05-49B4-A985-D90642A1679A

Terahertz Imaging for Detection of Corrosion and Defects Under Hull Coatings 

In this Phase I SBIR project we propose to demonstrate the key elements of a practical, field-deployable non-contact time-domain terahertz (TD-THz) system for rapid detection of submarine hull defects such as corrosion and gouges hidden beneath acoustic hull coatings, hull coating material defects, and hull coating adhesion defects. The key feasibility areas addressed will be developing the high-resolution focusing system required for the high contrast detection of flaws on the hull-coating interface buried several inches beneath the surface; and utilizing this focusing lens assembly in a novel TD-THz scanner system which can rapidly image large areas of a submarine hull in a shipyard environment. TD-THz is a non-contact imaging method which has been shown to detect corrosion and volume defects. The ultra-wideband pulsed TD-THz reflection tomography method allows in-situ inspection through hull coatings, since these polymeric coatings transmit in the THz band. The TD-THz pulse reflections can be focused and collected with high numerical aperture lenses, and time-gated at the buried hull-coating interface for precise, high contrast 3D-layer sub-surface imaging

Benefit

Upon successful completion of the Phase I and Phase II project, the proposed TD-THz inspection system will provide the Navy with a novel method of imaging submarine hull coatings and the underlying hull for hull corrosion, coating material defects, and coating adhesion defects. 

U.S. Navy Rides the Terahertz Wave to Next-Gen Electronics

http://cleantechnica.com/2012/05/09/u-s-navy-funds-research-using-graphene-to-tap-terahertz-waves/

MAY 9, 2012 BY TINA CASEY 

The U.S. Navy is behind a push to exploit one of the “hottest” areas of the electromagnetic spectrum, the terahertz band. The Office of Naval Research contributed to a breakthrough project at Lawrence Berkeley National Laboratory last fall with the help of graphene nanoribbons, and just last month a team of ONR-funded researchers at the University of Notre Dame announced another new milestone.

The attraction of the terahertz band

Terahertz waves are situated between the microwave and optical light frequencies, at the “farthest end of the far infrared.” In communications, they could transmit far greater amounts of information than either radio waves or microwaves.

In imaging, terahertz frequencies could lead to the development of diagnostic equipment that avoids the health risks of x-rays.

However, expanding the real-world applications of this part of the spectrum has been stuck for want of a material that can be used to manipulate terahertz waves with precision.

Graphene and terahertz waves

The terahertz worm began to turn in 2004, when a team of researchers in the U.K. literally used sticky tape to lift a one-atom thin sheet of carbon from a chunk of graphite.

Called graphene, the new material possesses outsized strength and unique electrical properties, which have made it the focus for bringing about the next generation of super fast, super small, flexible and even transparent electronic devices.

As Notre Dame researcher Berardi Sensale- Rodriguez explained in a prepared statement:

“A major bottleneck in the promise of THz technology has been the lack of efficient materials and devices that manipulate these energy waves. Having a naturally two-dimensional material with strong and tunable response to THz waves, for example, graphene, gives us the opportunity to design THz devices achieving unprecedented performance.”

Graphene nanoribbons to the rescue

Last fall’s breakthrough at Lawrence Berkeley involved the fabrication of graphene nanoribbons, made by etching patterns into a sheet of carbon atoms laid over a silicon oxide substrate. An overlay of ion gel was used to complete the gated structure of a semiconductor system.

The team was able to “tune” or manipulate the ribbons in to control the movement of electrons. This collective movement, or oscillation, of electrons is referred to as a plasmon.

According to Berkeley research Feng Wang, plasmons can be observed by eye, in the unique glow from medieval-era stained glass which is caused by electrons oscillating on the surface of metal nanoparticles including gold and copper.

A similar effect occurs in graphene but at lower frequencies, which are not visible to the naked eye.

The Berkeley team discovered that altering the width of the graphene nanoribbons will cause the electron waves to “slosh” back and forth at different frequencies, which makes the ribbons absorb different frequencies of light.

The demonstration marked a step along the way to practical, real-world applications partly because the team was able to measure the difference in absorption rates at room temperature, in contrast to other research tracks that require temperatures in the absolute zero range.

The findings of the Notre Dame team, published in mid-April, also involved the development of a practical, room-temperature operation. The team was able to demonstrate proof of concept for a graphene based modulator, building on previous research into the use of an electron gas to manipulate terahertz waves.

The idea of using an electron gas dates all the way back to 2006, so given the pace of research in both the Berkeley and Notre Dame cases a practical graphene/terahertz device is far from bouncing out of the laboratory door and onto retail shelves.

Aside from challenges within the research itself, the commercialization of graphene devices depends on the

development of cost effective methods for fabricating mass quantities of graphene, and sticky tape will only get you so far. At this point there have been some promising developments, but the goal has proved elusive.

Not to worry, though –  the Navy is all over that one, too. Through a separate ONR-funded program, researchers at Rice University are developing a simple, one-step process for creating nanoscale graphene discs.

Image: Some rights reserved by Laineys Repertoire.

Follow me on Twitter: @TinaMCasey.

Source: Clean Technica (http://s.tt/1bhW1)