Terahertz pulse drives strontium titanate into hidden ferroelectric phase

Left: The initial configuration of STO, in which a central titanium ion (Ti4+) is surrounded by oxygen ions (red) and strontium ions (gray, Sr2+); there is no dipole moment and the crystal is paraelectric. Center: A THz pulse drives the soft lattice vibrational mode, causing positive and negative ions to move in different directions as shown by the arrows. Right: The resulting ferroelectric crystalline phase with lower-symmetry geometry and a dipole moment (μ). Credit: Science

By Kendra Redmond
https://www.cambridge.org/core/journals/mrs-bulletin/news/terahertz-pulse-drives-strontium-titanate-into-hidden-ferroelectric-phase

With an intense pulse of terahertz radiation, researchers from the Massachusetts Institute of Technology (MIT) and the University of Pennsylvania (Penn) have induced an ultrafast phase transition in a metal oxide. As reported in a recent issue of Science, the transition reveals a hidden phase in which the ferroelectric crystal structure displays different properties than in other phases of the material, suggesting a path toward the collective, coherent control of materials structures. 

The physical properties of a solid are largely controlled by the collective vibrations (phonons) of its crystal lattice. Ferroelectric structural phase transitions are usually associated with the so-called “soft” vibrational mode, which induces a new lattice structure that has reduced crystal symmetry and leads to electric polarization.  

A decade ago, a collaboration led by Keith Nelson at MIT and Andrew Rappe at Penn published a theoretical study suggesting that in some materials, terahertz-frequency (THz) radiation could excite a resonance in the soft mode of the crystal that would move the ions in the lattice from their position in one ferroelectric domain structure to their position in another, changing the direction of the ferroelectric polarization. 

In this new research, Nelson, Rappe, and their colleagues studied whether they could directly drive the soft mode in strontium titanate (SrTiO₃, STO) to activate a hidden ferroelectric phase. Hidden phases are metastable collective states of matter not usually accessible on a material’s equilibrium phase diagram. They are of special interest because hidden phases in conventional materials sometimes give rise to exotic physical properties. 

STO is a dielectric material with a cubic perovskite structure at room temperature. Unlike many perovskites, STO does not transition to a ferroelectric material at any point on its equilibrium phase diagram. Even at its critical temperature of 36 K, the material is paraelectric because quantum fluctuations prevent long-range ordering. As a result, the researchers call STO “a textbook example” of a material in the quantum paraelectric (QPE) phase. 

In an experiment at MIT led by then-graduate student Xian Li, the research team sent a single-cycle THz pump pulse through STO, followed by an optical pulse that probed the material’s response. They repeated this process for several different temperatures and THz field strengths. Spectroscopic analyses revealed characteristic signals of lower crystal symmetry, as well as nonlinear increases in dipole ordering and phonon amplitude as a function of field strength. These results demonstrate a QPE-to-ferroelectric phase transition. The new phase was maintained for about 10 ps. 

A complementary theoretical investigation by the Penn colleagues explored whether a single pulse could induce ferroelectricity in STO. The researchers ran a molecular dynamics simulation in which a rapid electric field pulse was applied to a supercell consisting of many units of the lattice. After running the simulation over a range of field strengths, they found that a THz pulse on the order of 200 kV/cm or greater can stimulate a soft mode response that drives ions to new positions in a ferroelectric lattice structure. This work also revealed how other vibrational modes adapt to the soft mode change, stabilizing the hidden polar phase. 

This research “highlights the unique capability of light to selectively deform a material lattice through vibrational resonances,” says Andrea Cavalleri, director of the Max Planck Institute (MPI) for the Structure and Dynamics of Matter and a professor at the University of Oxford. Cavalleri is familiar with this approach, as he recently led a separate research effort at MPI to influence the electric polarization of STO. The MPI team irradiated STO with a mid-infrared pulse over a range of temperatures and frequencies. They also saw signs of lattice deformation, signs that were most pronounced when the pulse was resonant with the highest-frequency vibrational mode of STO. Follow-up experiments suggested that the IR pulse induced a transition to a metastable ferroelectric phase that persisted for several hours. These results were published alongside the MIT-Penn results in Science. 

“[B]y driving a specific lattice deformation with a single-cycle terahertz pulse, Li et al. have shown that a ferroelectric order forms on ultrafast timescales. Gaining control of technologically relevant properties such as ferroelectricity on short timescales could open up new strategies for next-generation high-speed devices,” says Cavalleri. Rappe agrees. “These studies launch the age of ultrafast reconfiguration of nonlinear optics, paving the way for rapidly reconfigurable optical devices,” he says. 

The MIT-Penn team plans to explore applied facets of this research going forward. “We’ve dreamed for many years about coherent control over collective material structure,” says Nelson. “It is becoming possible to use THz fields to control crystal lattice structure, ferroelectric order, magnetic order, and electronic state (such as insulating or metallic). Next must be control over combinations of these properties in complex materials like high-temperature superconductors, in which changes in the properties are strongly coupled to each other,” he says. 

Read the abstract in Science.

OT-SpectroscopyNOW blog-Phosphorene: Two-dimensional Raman

 Phosphorene: Two-dimensional Raman

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http://www.spectroscopynow.com/raman/details/highlight/14de3301024/Last-Months-Most-Accessed-Feature-Phosphorene-Two-dimensional-Raman.html

Flat out phosphorus

Raman spectroscopy and transmission electron microscopy have been used by an international team to investigate the phosphorus analogue of graphene, the two-dimensional phosphane, known as phosphorene.

Phosphorene has potential applications in a new class of semiconducting transistor for that perennial aspiration, the ever faster and more powerful computer of the future. Unfortunately, while phosphorene can conduct electrons its ability to do so is anisotropic, meaning it depends on which way you orient it relative to the system as to whether it does so or not. Thus, a quick and simple way to determine the orientation of the material was needed for experimental setups and now, a team comprising researchers from the Massachusetts Institute of Technology, the Rensselaer Polytechnic Institute (RPI) in Troy, New York state, Tohoku University in Japan, Oak Ridge National Laboratory, Tennessee and the University of Pennsylvania, has done just that. There approach accurately determines orientation by examining the interaction between light and electrons within phosphorene or other thin layers of black phosphorus.

Calculated approach

Materials scientists have been studying phosphorene intently since it was first isolated in 2014. RPI's Vincent Meunier and his team confirmed the structure of phosphorene that same year. "This is a really interesting material because, depending on which direction you do things, you have completely different properties," explains Meunier, a phenomenon that might of course be exploited in devices. "But because it's such a new material, it's essential that we begin to understand and predict its intrinsic properties."

Meunier and colleagues have now built on the theoretical modelling and prediction of the properties of phosphorene using Rensselaer's supercomputer in the Center for Computational Innovations (CCI). On the basis of their calculations, they have home in on certain features of this novel material that will ultimately help physicists and materials scientists better understand it and thence technologists make use of those properties.

Writing in the journal ACS Nano Letters, the team initially set out to refine an existing technique for determining the orientation of the crystal using Raman spectroscopy. The team were reviewing their Raman data and spotted a few unexplained inconsistencies. So, they next turned to obtaining images of the orientation of their crystalline samples using Transmission Electron Microscopy (TEM), and lined these up with the "images" gleaned from the Raman results. As a topographic technique, TEM offers a definitive determination of the orientation of a crystal, but takes a lot more effort than recording a Raman spectrum. Nevertheless, the comparison revealed that electron-phonon interactions alone did not account for the orientation of the crystal. And the reason why led the way to yet another anisotropy of phosphorene - that of interactions between photons of light and electrons in the crystal.

Intrinsic anisotropy

The Raman spectrum should be intrinsic to the material and thus show the anisotropy of phosphorene. "But, it turns out that if you shine the light in different directions, you get different results, because the interaction between the light and the electrons in the material - the electron-photon interaction - is also anisotropic, but in a non-commensurate way," explains Meunier. The team suspected that phosphorene was anisotropic with respect to electron-photon interactions, but hadn't quite anticipated the significance of the property. "Usually electron-photon anisotropy doesn’t make such a big difference, but here, because we have such a particular chemistry on the surface and such a strong anisotropy, it's one of those materials where it makes a huge difference," Meunier adds.

Fundamentally, the discovery reveals a limitation in what current interpretation of Raman spectra can achieve in studying these materials. "It turns out that it's not so easy to use Raman vibrations to find out the direction of the crystal," Meunier explains. "But, and this is the beautiful thing, what we found is that the electron-photon interaction (which can be measured by recording the amount of light absorbed) - the interaction between the electrons and the laser - is a good predictor of the direction. Now you can really predict how the material will behave as a function of excitement with an outside stimulus."

Meunier worked with Mildred Dresselhaus of the Massachusetts Institute of Technology, as well as colleagues at Tohoku University in Japan, Oak Ridge National Laboratory, Tennessee and the University of Pennsylvania.

Researchers develop technique for making light-bending 'raspberry-like metamolecules'

These raspberry-like metamolecules react to light’s magnetic field as a loop of wire does to an oscillating magnet.

 http://phys.org/news/2015-03-technique-light-bending-raspberry-like-metamolecules.html#jCp

by Evan Lerner

The field of metamaterials is all about making structures that have physical properties that aren't found in nature. Predicting what kinds of structures would have those traits is one challenge; physically fabricating them is quite another, as they often require precise arrangement of constituent materials on the smallest scales.

Researchers at the University of Pennsylvania have now devised a way of mass-producing metamaterials that exhibit magnetic resonancein optical frequencies. Called "raspberry-like metamolecules" due to their unique shape, these nanoscale structures could be used as building blocks for metamaterials that could scatter light as if they had magnetic properties, which could be relevant to applications in optical processing and signal handling. These raspberry-like metamolecules react to light's magnetic field as a loop of wire does to an oscillating magnet.

This ability stems from the precise arrangements of the raspberry-like metamolecule's "drupelets," which are composed ofgold nanoparticles. These drupelets need to be as close as possible without touching so as not to "short circuit" the optical electric fields around them. Through a carefully designed chemical process that coated each drupelet with an insulating surfactant, the Penn team was able to space these nanoparticles an average distance of just two nanometers apart.

And because the assembly of the nanoparticle drupelets and the surfactant coating can be done in a single step, vast quantities of these raspberry-like metamolecules can be fabricated at once, rather than being painstakingly assembled one at a time.

The research was conducted by lead author Zhaoxia Qian, who recently graduated with a doctorate in chemistry from Penn's School of Arts & Sciences; Nader Engheta, the H. Nedwill Ramsey Professor of Electrical and Systems Engineering in Penn's School of Engineering and Applied Science; Zahra Fakhraai, assistant professor of chemistry in Penn Arts & Sciences; and So-Jung Park, formerly associate professor of the Department of Chemistry, now professor of chemistry at South Korea's Ewha Womans University. Also contributing were Simon Hastings, who recently graduated with a doctorate in physics, and chemistry graduate student Chen Li, along with research specialist Brian Edwards and visiting undergraduate student Christine K. McGinn, both of Electrical and Systems Engineering

It was published in the journal ACS Nano.

If one takes a loop of wire and passes a magnet up and down through the center, the resulting oscillating magnetic field drives electrons around the wire, producing electrical current in the wire. That principle is in play in every generator, which has magnets that oscillate at around 50 hertz, or 50 times a second. But what if this principle could be extended into optical frequencies, on the order of 500 terahertz? Rather than generating electricity, the loop would be able to manipulate visible light.

"There are no known materials that have magnetic properties in optical frequencies," Fahkraai said. "If you could fabricate structures like this, they could be building blocks for metamaterials that could scatter light as if they had magnetic properties."

Engheta predicted that such a structure was possible in 2006, and in the intervening years other research groups have physically produced metamaterials that exhibit this trait. Such structures were mostly painstakingly constructed rings of metal nanoparticles, spaced on a flat surface such that electrons couldn't actually move between them.

"Because the metal doesn't touch," Engheta said, "the electrons can only oscillate within individual particles and can't move from one nanoparticle to its neighbor. This is known as a displacement current. It's like doing the wave in a stadium; no one fan is moving from their seat, but the wave moves around in a circle."

A raspberry-like configuration, where nanoparticles are spherically clustered around a core, rather than a ring, would be even better, as a cross-section of the raspberry acts like a ring of nanoparticles no matter which direction the magnetic field is applied. Other researchers have begun moving from mechanical assembly techniques toward the chemical self-assembly of such structures but have hit roadblocks.

The Penn team's approach solves the problems by adopting a synthetic approach.

"People have tried to make these kinds of structures in solution before, typically by assembling pre-synthesized nanoparticles," Qian said, "but it is hard to achieve high density of nanoparticles packing through that route."

"In our case," Park said, "we generate closely packed nanoparticle clusters by a synthetic approach where the nanoparticle growth and assembly occurs simultaneously. A challenge in such synthetic approach is that growing nanoparticles tend to form a fused shell. In our method, we use a special surfactant that forms a molecularly-thin, but tightly protecting, layer around the nanoparticles, which keeps them from touching each other."

The Penn team's synthetic method reduces some of the complexity that otherwise comes with making these raspberry-like metamolecules.

"It's like making a stew," Engheta said. "You throw everything into one pot."

The ingredients to the stew are polystyrene spheres decorated with small silver seed particles, silver nitrate, gold salts and reducing agents that break up those salts and allow the gold atoms to form nanoparticles. All of these ingredients are placed into a growth formula containing the insulating surfactant, which forms a thin layer on the exterior of the growing gold nanoparticles, cushioning them from each other.

Further research on the surfactant chemistry will enable the team to reduce the distance between the nanoparticles even more, to further strengthen the magnetic properties of the raspberry-like metamolecules. That trait is critical to the structures' abilities to manipulate light and thus be used in optical devices.

"If you want to make inductors at optical frequencies," Fahkraai said, "you need something that can respond at very high frequencies. The closer we can make the nanoparticles, the stronger we can make the scattering of light due to magnetic effects."

 Explore further: Researchers find easy way to deposit metal nanoparticles on a surface using tape

More information: ACS Nano, dx.doi.org/10.1021/nn5050678

Journal reference: ACS Nano  

Provided by University of Pennsylvania  

SPIE Announces 2015 Award Recipients For Achievements In Optics And Photonics

http://www.photonicsonline.com/doc/spie-announces-award-recipients-achievements-optics-photonics-0001

Gold Medal of the Society goes to University of Pennsylvania professor Nader Engheta

Winners of awards recognizing outstanding technical achievements and service to the society have been announced by SPIE, the international society for optics and photonics. The annual awards program recognizes both individual and team technical accomplishments.

Winners of prestigious annual awards have been announced by the Awards Committee of SPIE, the international society for optics and photonics. The awards recognize outstanding individual and team technical accomplishments and meritorious service to the Society.

Award winners for 2015 are:

Gold Medal of the Society: Nader Engheta, University of Pennsylvania, for his transformative and groundbreaking contributions to optical engineering of metamaterials and nanoscale plasmonics, metamaterial-based optical nano circuits, and biologically-inspired optical imaging. The Gold Medal is the highest honor bestowed by SPIE.

Britton Chance Biomedical Optics Award: Lihong Wang, Washington University in St. Louis, for his pioneering technical contributions and visionary leadership in the development and application of photo-acoustic tomography, photoacoustic microscopy and photon transport modeling.

A.E. Conrady Award: Richard C. Juergens, Raytheon Missile Systems, recognizing him as a leading authority in optical system design, optical component fabrication and testing, and training and mentoring of optical engineers, and instrumental in developing optimization techniques and tolerancing methods for optical designs.

Dennis Gabor Award: Kazuyoshi Itoh, Osaka University, for his eminent contribution to the development of incoherent holography and nonlinear optical microscopy through your pioneering work on coherence-based multispectral and 3D imaging, and nonlinear optical imaging and manipulations of biological and inorganic industrial materials.

George W. Goddard Award: Grady H. Tuell, Georgia Tech Research Institute, recognizing his foundational research and development in bathymetric lidar and data fusion; and his efforts to further advance airborne LIDAR remote sensing in other ways including real-time calculation of total propagated positioning error.

G.G. Stokes Award: Aristide Dogariu, CREOL, University of Central Florida, for his development of new theoretical concepts and innovative methods and techniques for understanding and measuring polarization properties of light-matter interaction.

Chandra S. Vikram Award in Optical Metrology: Guillermo H. Kaufmann, Istituto de Física Rosario (CONICET-UNR) for his contributions to speckle metrology and its applications in material science, experimental mechanics and nondestructive testing, and also for the development of novel fringe analysis methods.

Frits Zernike Award in Microlithography: Ralph R. Dammel, AZ Electronics Materials, for his significant contributions to the development of photoresist, anti-reflective coating, and directed self-assembly materials for semiconductor microlithography.

SPIE Early Career Achievement Award – Academic: Miriam Serena Vitiello, recognizing her outstanding results in research on semiconductor laser sources and electronic high frequency nanodetectors which have opened new frontiers in the Terahertz photonics and optoelectronics fields.

SPIE Early Career Achievement Award – Industry: Alan Lee, LongWave Photonics LLC, recognizing his pioneering research on stand-off distance real-time THz imaging. The locking-in differential imaging proposed in his work formed the basic working principle of several commercial THz imagers/cameras.

SPIE Educator Award: Virendra Mahajan, recognizing his sharing of knowledge in the area of optical imaging, aberrations, and wavefront analysis through his voluntary teaching of students and professionals and the writing of five excellent books.

SPIE Technology Achievement Award: Keith B. Doyle, MIT Lincoln Laboratory, for his outstanding contributions to integrated analysis of optical systems, incorporating in this analysis elements of optical, thermal, and structural engineering.

For future awards, members of the photonics community may nominate colleagues to recognize their outstanding achievements. Nominations may be made through 1 October of any given year and are considered active for three years from the submission date. Instructions and nomination forms are at www.spie.org/x1164.xml.

About SPIE
SPIE is the international society for optics and photonics, a not-for-profit organization founded in 1955 to advance light-based technologies. The Society serves nearly 256,000 constituents from approximately 155 countries, offering conferences, continuing education, books, journals, and a digital library in support of interdisciplinary information exchange, professional networking, and patent precedent. SPIE provided more than $3.4M in support of education and outreach programs in 2014.

SOURCE: SPIE

Research demonstrates various possibilities for controlling light in the terahertz frequency range

        

                Detail of a researcher working in the laboratory Mario Sorolla. Credit: Teralab at the UPNA
http://phys.org/news/2014-09-possibilities-terahertz-frequency-range.html

The Journal of Optics has devoted the front page of its special edition on Mid-infrared and THz Photonics to the work produced by the NUP/UPNA-Public University of Navarre researchers Víctor Pacheco-Peña, Víctor Torres, Miguel Beruete and Miguel Navarro-Cía, together with Nader Engheta (University of Pennsylvania). In their research they have proposed various devices capable of redirecting electromagnetic waves with efficiency levels close to 100%.

To explain what their work consists of they have put forward the following example: "If we shine a torch on a wall in which we have made a hole, experience tells us that the bigger the hole is, the greater the amount of light that will pass through to the other side. However, if we fill the hole with an ENZ metamaterial, something that appears to defy logic happens: the smaller the hole is, the greater the amount of light that passes through. This phenomenon has a tremendous practical implication because it opens up new ways of miniaturising numerous components and for light control."

Metamaterials are artificial materials with properties that go beyond those of natural means. To understand how they work, we can take a look at nature itself: while natural elements acquire their physical properties from the atoms that form them and the way in which they are ordered, metamaterials use natural means, like small metal fragments that fit together like parts of a Meccano model to artificially synthesise properties that are impossible to find otherwise. Initially put forward to control electromagnetic radiation, right now their use has become widespread and has extended to other areas like mechanical waves (sound, for example).

The piece of work referred to above proposes various compact devices comprising rectangular metal tubes with extremely narrow openings of dimensions designed in such a way that they are capable of redirecting the electromagnetic waves with levels of efficiency close to 100%. These gaps are capable of imitating an ENZ (Epsilon Near Zero, which means permittivity close to zero) metamaterial so that it is not necessary to "fill them" with anything in order to obtain amazing results.

Amazing properties

Among the electromagnetic metamaterials, the above-mentioned ENZ ones make it possible to achieve the super coupling of the light, the tunnel effect and the confining of energy in tiny spaces. "Going back to the first example," say the authors, "super coupling means that all the light will be transferred from one side of the wall to the other through any shape of hole we want to make; tunnel effect refers to light passing through a hole of any length, no matter how long we want to make it; and the confining of energy is due to the fact that the light is transferred even through very small holes, so the energy inside the hole is squeezed enormously."

This work has shown theoretically and by means of simulations how beam steerers and power splitters work for terahertz waves, and is of tremendous importance in view of their huge potential in sectors like security, biomedical engineering, pharmacy, space, etc. Right now, the authors of this piece of research are working to confirm the study through experimental means. In this respect, they stress that "this constitutes another milestone in an initiative of an international nature that has been going on for nearly four years."

 Explore further: Invisibility cloaks closer thanks to 'digital metamaterials'

More information: Pacheco-Peña V., Torres V., Beruete M., Navarro-Cía M., Nader Engheta. 2014. "Near-zero (ENZ) graded index quasi-optical devices: steering and splitting millimeter waves". Journal of Optics, 16: 094009. DOI: 10.1088/2040-8978/16/9/094009

OT-Invisibility cloaks closer thanks to 'digital metamaterials'

    Now you see him … Eric Tastad/Flickr, CC BY-NC-SA

  

by Penny Orbell

http://phys.org/news/2014-09-invisibility-cloaks-closer-digital-metamaterials.html

The concept of "digital metamaterials" – a simple way of designing metamaterials with bizarre optical properties that could hasten the development of devices such as invisibility cloaks and superlenses – is reported in a paper published today in Nature Materials.
Metamaterials are artificially engineered out of microscopic subunits – such as glass, metal or plastic – arranged in a repeating fashion. Once assembled, these metamaterials possess unique properties, such as interacting with light in unusual ways, which aren't often seen in natural materials.

"The idea behind metamaterials is to mimic the way atoms interact with light, but with artificial structures much smaller than the wavelength of light itself," said Boris Kuhlmey, associate professor of photonics and optics at the University of Sydney.

"This way, optical properties are no longer restricted to those of the constituent materials, and can be designed almost arbitrarily."

The material world goes digital

The researchers of the Nature Materials paper, from the University of Pennsylvania, were inspired to develop digital metamaterials by the binary numeral system of Boolean algebra.

The binary system is used internally by most digital electronic devices, such as computers and smartphones. Complex digital devices have their digital information simply encoded as a string of 1s and 0s called "bits".

The proposed method for digital metamaterials is a simplified way of building metamaterials, yet still allows for complex and diverse properties to be achieved.

"The beauty of the new method is its simplicity," said Min Gu, professor of optoelectronics at Swinburne University of Technology.

Through the use of simulations in two-dimensional space, the researchers explored the possibility of creating metamaterials with only two specially chosen component parts, called metamaterial bits – analogous to the 1 and 0 "bits" of binary computer code. The arrangement of metamaterial bits represents the "digitising" of metamaterials.

In their study, the researchers chose to use nano-sized pieces of silver and silica (glass) as their repeating metamaterial bits. These are materials that interact with light in very different ways on an individual level. Once they were "digitised", the resulting metamaterial had its own unique properties, very different to those of its constituent parts.

"The components of the material work together to generate effects or give rise to phenomena that you wouldn't observe if they weren't arranged together in 3D (or in this case, 2D) space as an ordered assembly," said Tiffany Walsh, professor of bionanotechnology at Deakin University.

Sourcing material parts in order to achieve unusual properties of a metamaterial can be time consuming and expensive. This new way of thinking about the design of metamaterials may allow researchers to produce the optical properties they want from the metamaterial using only two component parts.

"What this [research] really does is put a new spin on the idea that with only two set materials arranged with the right portions – one metal, one insulator, here silver and silica – almost any optical property can be achieved," said Associate Professor Kuhlmey.

Professor Walsh said: "This is like the concept of turning sound waves from analog into digital – and they've pushed it into a new realm of physics.

"They've been able to take the permittivity – the response of the material when it's exposed to radiation – and digitised this. They've turned it into something that is more readily manipulated."

Waves and matter collide

One of the key applications for metamaterials lies in their ability to manipulate light.

"We already have knowledge about how to manipulate radiation (such as light) – we can use lenses, like a magnifying glass, for example, which focus light down on a spot; we can use mirrors to reflect light and change its direction," Professor Walsh said.

"But what these [metamaterials] can do is something more sophisticated: they're able to bend light, to scatter it, to manipulate it in unusual ways."

Using their digital method, the researchers showed that it is possible to create certain metamaterials with very low permittivity, which are rarely found in nature. Having control over these properties may open doors to more advanced technological applications, such as invisibility cloaking devices.

"It would be interesting in future to see if such a digital design method can facilitate the construction of optical, or invisibility, cloaks," said Professor Gu.

"With varying changes of silver/glass ratios (structured at the nanoscale) it is then in principle possible to make flat lenses and other tiny optical elements," Associate Professor Kuhlmey said.

"The authors […] showed in simulations that nano-patterned glass/silver structures can then bend light, which is also the principle behind invisibility cloaking."

He added that fabricating the proposed structures would be challenging but not impossible.

"[It would] require structuring glass and metal with a precision of a few atoms in thickness only – but thinking of metamaterials as binary structures may help devise new nano-patterning lithography (printing) techniques that take advantage of this," he said.

 Explore further: Array of perfect nanometer-scale spheres that can control the flow of visible light may find use in invisibility cloaks

Journal reference: Nature Materials  

SpectroscopyNOW.com-Stroke monitoring: NIR gets in the patient's head

Last Month's Most Accessed Feature: Stroke monitoring: NIR gets in the patient's head

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• Published: May 7, 2014
• Categories: Infrared Spectroscopy
• http://www.spectroscopynow.com/ir/details/highlight/13a3020348d/Last-Months-Most-Accessed-Feature-Stroke-monitoring-NIR-gets-in-the-patients-hea.html

Stroke heads-up

A near-infrared monitoring device has been developed by scientists at the University of Pennsylvania, Philadelphia. The system could be used for bedside monitoring of cerebral blood flow in ischemic stroke patients with a view to ensuring optimal treatment and improved outcomes in a potentially debilitating and often lethal condition.

Most patients admitted to hospital with an acute ischemic stroke are kept supine for at least 24 hours in an effort to increase cerebral blood flow to vulnerable brain regions surrounding the tissues damaged by the stroke. Now, researchers have used a device designed and patented by researchers at the University of Pennsylvania to non-invasively and continuously monitor CBF in such patients. The team from Penn Medicine and the Department of Physics & Astronomy are discovering exactly how "head-of-bed" positioning affects blood flow and thus outcome for stroke patients. Their initial findings suggest that for about three-quarters of patients a flat head-of-bed position increased blood flow to the damaged hemisphere, but for a proportion, there was a quite paradoxical response in that optimal blood flow was seen if their head were at an elevated angle.

"HOB positioning is just one of several interventions commonly used in the treatment of stroke patients to increase CBF," Detre told SpectroscopyNow. "Others include giving fluids and withholding antihypertensive agents.  All of these interventions can theoretically improve CBF and therefore stroke outcomes, but there is really little data to show whether they work or in whom. We chose to look at HOB because a HOB position change is easy to study at the bedside in a short period of time."

Insult and injury

Stroke, also known as cerebrovascular insult (CVI), occurs when blood supply to the brain is disrupted acutely. A blockage (thrombosis, arterial embolism), or a haemorrhage in the brain or intracranial blood vessels is the usual cause, although the latter is not considered in this present research. Damage to the brain usually results and commonly leaves patients that survive with speech problems or paralysis down one side of their bodies, depending on which parts of the brain are affected. It can also cause cognitive deficit and problems with vision.

Rapid and appropriate treatment can minimise the effects of stroke. As such, an optical and non-invasive device for monitoring of blood flow at the patient's bedside is proving to be a boon to healthcare workers treating such patients. The key technology development is a near-infrared probe that is simply placed on the surface of the patient's head, the probe emits NIR and detects the reflections from within the brain, these fluctuate depending on the flow of red blood cells through the tissues within. This diffuse correlation spectroscopy (DCS) matches blood flow precisely, the team says, much more so than Transcranial Doppler (TCD), ultrasound scanning, which uses acoustic waves to quantify blood flow in the larger arteries supplying the brain.

"This study illustrates the potential of using advanced technology to make individualized treatment decisions in real time" explains senior author John Detre. "While, on average, our findings support current guidelines to lay patients flat following stroke, they also suggest that for some stroke patients, lying flat may be either unnecessary or even harmful. Future studies examining clinical outcomes after stroke and using optical CBF measurements to guide management will be needed to confirm this."

Improving outcomes at a stroke

Arjun Yodh adds that, "We believe that these optical CBF measurements are detecting brain tissue blood flow of local microvasculature that might differ due to injury." The team showed that for all patients, blood flow was reduced by 9 percent to the brain hemisphere with the stroke damage if the head-of-bed was elevated 15 degrees, but 17 percent lower when elevated to 30 degrees. But, in 29 percent of the patients, CBF improved with head elevation although no clinical nor radiological differences predicted this unexpected response.

"Our study suggests that it would be impossible for stroke clinicians to know whether HOB flat is optimal without actually measuring the response," stroke neurologist Michael Mullen explains. This may also be true for other clinical interventions such as administration of fluids, withholding antihypertensive therapies, or using medications to raise blood pressure. "The ability to measure cerebral blood flow continuously has tremendous potential and may one day allow clinicians to individualize therapy for each patient," he adds.

"We are currently using the technology to evaluate the effects of an intravenous saline bolus (fluids)," Detre told us. "Other future technical work includes improving the probes further, and expanding the number of probes.  For this study we only had two probes - one over each frontal lobe. Obviously it would be good to have more coverage over the brain."

Related Links

Stroke, 2014, online: "Optical Bedside Monitoring of Cerebral Blood Flow in Acute Ischemic Stroke Patients During Head-of-Bed Manipulation"

Article by David Bradley

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The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Nanoplasmonic 'whispering gallery' breaks emission time record in semiconductors

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http://www.physorg.com/news/2011-07-nanoplasmonic-gallery-emission-semiconductors.html
Renaissance architects demonstrated their understanding of geometry and physics when they built whispering galleries into their cathedrals. These circular chambers were designed to amplify and direct sound waves so that, when standing in the right spot, a whisper could be heard from across the room. Now, scientists at the University of Pennsylvania have applied the same principle on the nanoscale to drastically reduce emission lifetime, a key property of semiconductors, which can lead to the development of new ultrafast photonic devices.
The research was conducted by associate professor Ritesh Agarwal, postdoctoral fellows Chang-Hee Cho and Sung-Wook Nam and graduate student Carlos O. Aspetti, all of the Department of Materials Science and Engineering in Penn's School of Engineering and Applied Science. Michael E. Turk and James M. Kikkawa of the Department of Physics and Astronomy in the School of Arts and Sciences also contributed to the study.
Their research was published in the journal Nature Materials.
"When you excite a semiconductor, then it takes a few nanoseconds to get back to the ground state accompanied by emission of light," Agarwal said. "That's the emission lifetime. It's roughly the amount of time the light is on, and hence is the amount of time it takes for it to be ready to be turned on again.

"The previous state of the art was taking a nanowire, just like ours, and laying it on a metal surface," Agarwal said. "We curved the metal surface around the wire, making a complete nanoscale plasmonic cavity and the whispering gallery effect."
For certain nanowire sizes, the silver coating creates pockets of resonance and hence highly confined electromagnetic fields within the nanostructure. Emission lifetime can then be engineered by precisely controlling high intensity electromagnetic fields inside the light-emitting medium, which is the cadmium sulfide core.
To reach an emission lifetime measured in femtoseconds, the researchers needed to optimally balance this high-confinement electromagnetic field with an appropriate "quality factor," the measurement of how good a cavity is at storing energy. To complicate matters, quality factor and confinement have an inverse relationship; the higher the quality-factor a cavity has the bigger it is and the smaller its confinement. However, by opting for a reasonable quality factor, the researchers could vastly increase the confinement of the electric field inside the nanowire by using resonant surface plasmons and get the record-breaking emission lifetime.
This many-orders-of-magnitude improvement could find a home in a variety of applications such as LEDs, detectors and other nanophotonic devices with novel properties.
"Plasmonic computers could make good use of these nanowires," Cho said. "We could increase modulation speed into the terahertz range whereas electronic computers are limited to a few gigahertz range."
"The same physics governs emission and absorption, so these nanowires could also be used for increasing efficiency of absorption in solar cells," Agarwal said.
Provided by University of Pennsylvania (news : web)

"If you're making a modulator, something that switches back and forth, you're limited by this time constant. What we've done is reduced it to less than a picosecond. It's more than a thousand times faster than anything currently available."
In semiconductors, the excited state is when energy is present in the system, and the ground state is when there is none. Normally, the semiconductor must first "cool down" in the excited state, releasing energy as heat, before "jumping" back to the ground state, releasing the remaining energy as light. The Penn team's semiconductor nanowires, however, can jump directly from a high-energy excited state to the ground, all but eliminating the cool-down period.
The advancement in emission lifetime is due to the unique construction of the team's nanowires. At their core, they are cadmium sulfide, a common nanowire material. But they are also wrapped in a buffer layer of silicon dioxide, and, critically, an outer layer of silver. The silver coating supports what are known as surface plasmons, unique waves that are a combination of oscillating metal electrons and of light. These surface plasmons are highly confined to the surface the silicon dioxide and silver layers meet.