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Ultra-Sensitive Magnetic Sensors Don't Need Ultra Cold

Newly developed magnetic sensors not only perform better than most standard commercial devices, but also can operate at temperatures well above absolute zero, say UK researchers.

Superconducting quantum interference devices, or SQUIDs, can detect minute magnetic fields, making them useful for applications such as analyzing brain activity, medical imaging, and oil prospecting. SQUIDs work by converting magnetic flux—a measure of magnetic intensity—into a voltage.

The most sensitive commercial magnetic sensors are single SQUIDs that need to be kept at 4.2 Kelvin. Such incredibly cold temperatures, within a hair’s breadth of absolute zero, require expensive and difficult to handle liquid helium.

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Zero-Index Metamaterials Open New Possibilities for Optical Chips

One class of metamaterials, with a refraction index of zero [pdf], was first created in 2013 and quickly caught the interest of scientists because of the materials’ unique properties. Such a material, when irradiated with light, behaves in a peculiar way, explains Eric Mazur, who leads a team of researchers investigating metamaterials at  Harvard University in Cambridge, MA.  For example, light directed at a planar slab can only pass when its incident angle is exactly 90 degrees. Says Mazur:

The light sets up a response in the material so that combined with the incident electromagnetic field the field in the material has the same phase throughout the material—not unlike the wheels of a carriage in a movie can appear at rest due to the interplay of the frequency of rotation and the frequency of the movie. This complete coherence in space, results in light with infinite wavelength and infinite phase velocity.

Since the phase of the light is the same in all of the metamaterial, it looks like the sinusoidal field has a wavelength that is streched to infinity and the phase propagates instantaneously. These two properties allow the light to be controlled in an unprecedented way in the very small space available on optical chips; it can travel through exteremely narrow channels or waveguides, and go around sharp corners without losing energy. 

Now Mazur and his team at Harvard, working with researchers at Peking University in Beijing, report in the 19 October online edition of Nature Photonics the creation of an on-chip metamaterial with a refractive index of zero.

Fabricating such a metamaterial on a chip allows integration with other nanofabrication techniques for on-chip light manipulation, says Mazur.  He and his collaborators created a metamaterial layer consisting of silicon pillars embedded in a polymer matrix and covered on both sides with gold film, deposited on a silicon substrate.

Such a zero-index metamaterial layer can fill in many of the potholes engineers might otherwise face on the road to future photonic chips. One of those bumps in the road is is the coupling of light into the small structures—smaller than the diffraction limit of light—on optical chips. The team devised the concept of a "super coupler” [pdf] in which light is transported through zero-index materials that can deal with small sizes and sharp angles. This will reduce the size of optical connections, and eliminate losses in the transmission of light, says Yang Li, a member of Mazur's research team. 

“If we put a zero-index metamaterial into a waveguide made of mirrors, we can achieve a high-efficiency transmission, regardless of the length, squeeze, shape, twisting, or bending of the waveguide. These are phenomena that we are not able to achieve in the microwave and optical regimes by using regular waveguides,” says Li. A demonstation of a super coupler is next on the books, says Li. 

A second application is phase matching in nonlinear optics, which is the study of phenomena in matter caused by light that are not proportional to the intensity of the light. Optical processing will require separate light beams to interact with each other. Two light beams can only interact with each other by nonlinear processes, and only when the momentum of the outgoing photons matches the momentum of incoming photons, says Mazur. 

“Zero-index materials make this particularly easy because the momentum vector of light in a zero-index material is zero. This relaxes some of the constraints on nonlinear optical processes at the nanoscale,” says Mazur.

Applications in optical quantum computers may be promising because all the light emitters in a zero-index material must oscillate in phase. “Take quantum emitters, such as erbium ions, and you can have them entangled over much larger distances that you can have in any other type of environmemt,” says Mazur. 

Artificial Intelligence Outperforms Human Data Scientists

Artificial intelligence may be poised to ease the shortage of data scientists who build models that explain and predict patterns in the ocean of “Big Data” representing today’s world. An MIT startup’s computer software has proved capable of building better predictive models than the majority of human researchers it competed against in several recent data science contests. 

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Light Where the Sun Don't Shine

An optical fiber made mostly of water could be used to zap tumors with light-activated drugs, continuously monitor a patient’s health, or trigger genetically modified cells with a laser beam, according to the scientist developing the fiber.

“We can reinvent optical components with biologically compatible materials,” says Seok-Hyun (Andy) Yun, an associate professor of biomedical optics at the Wellman Center for Photomedicine at Massachusetts General Hospital and Harvard Medical School. He presented the results of his recent work at the 2015 IEEE Photonics Conference in Reston, Va., earlier this month.

Yun and his colleagues have created a thin optical fiber from a hydrogel made of polyethylene glycol-diacrylate. They pour a solution of the material into a thin, transparent tube then cure it with ultraviolet light. Once they’ve formed the fiber, they coat it with alginate, a natural polymer derived from brown seaweed and commonly used in wound-healing dressings. The coating acts as a cladding similar to that used in typical silica fibers; the difference between the index of refraction of the two materials keeps the light traveling down the fiber instead of escaping through its transparent surface.

The fiber can carry blue-green laser light (wavelengths around 492 nanometers) for about 10 centimeters, a distance sufficient to reach from the surface of the skin to various organs. Visible light can normally only penetrate tissue to depths of a few millimeters, limiting the use of optical techniques to natural cavities such as the gastrointestinal tract.

Because the hydrogel is 80 to 90 percent water and very porous, it’s easy to dope it with various drugs that could be delivered to where they’re needed and then turned on with a beam of light. The fiber, which could carry molecules that would act as sensors for glucose, pH levels, or various biomarkers, would fluoresce in the presence of a target molecule. Yun’s team has even shown that they can infuse the fibers with genetically modified cells that stimulate insulin production in mice when the fibers are hit with a laser beam.

Clinical use of the fiber would probably require approval of the U.S. Food and Drug Administration (FDA), Yun says, but he’s using materials on the FDA’s list of items generally recognized as safe. One of the ingredients, for example, is the same one used to make biodegradable sutures.

Yun says his fiber has two advantages over conventional silica fibers. It’s much more flexible. “You can insert it like a needle into the body, with less worry about fiber breakages,” he says. And it’s biocompatible, so it could be left in for days or weeks with no immune reaction. One use he imagines is long-term monitoring of organ transplants. And even if a fiber were to get stuck, it would eventually be harmlessly absorbed by the body. It’s possible, by manipulating the chemistry of the fiber, to make it biodegrade in less than an hour or have it last for months, says Yun.


European Laser Facility Opens in Prague

A new laser research center in Europe, which will house the most powerful laser in the world, officially opened on 19 October.

The new ELI (Extreme Light Infrastructure) Beamlines facility, located in Prague, will be the world's first international laser facility when completed in 2018. The strongest of the four lasers housed there will reach intensities 10 times as great as any currently achievable; it will have a peak power of 10 petawatts, or 10 million billion (1015) watts.

The ELI center’s lasers will be used for research into material sciences, medicine, biology, chemistry, pharmaceuticals, astrophysics, and nuclear physics. Researchers noted that laser-driven particle acceleration could ultimately find use in cancer treatments and extremely compact electron-positron colliders. It is also hoped that the center will be the site of discoveries leading to improved contrast and resolution in medical X-ray imaging. That, in turn, could lead to early detection of small tumors. Research into plasma physics at the complex might also shed light on phenomena such as controlled nuclear fusion, ultra-high energy cosmic rays, and the Hawking radiation given off by black holes.

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Bright Blue PHOLEDs Almost Ready for TV

A new energy-efficient organic LED (OLED) that glows a deep blue is finally close to meeting the most stringent U.S. video display brightness requirements, researchers say.

OLEDs have enabled a new generation of bright, high-quality, low-cost, power-efficient, flexible, lightweight flat panel displays. Each pixel in an OLED display typically consists of red, green, and blue OLEDs that shine with different brightnesses to produce any desired color.

Phosphorescent OLEDs (PHOLEDs) use only one quarter the energy of conventional OLEDs. Green and red PHOLEDs are already used in smartphones and TVs, leading to longer battery lives and lower electricity bills, but developing the kind of bright deep blue PHOLEDs needed for video displays has proven challenging.

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Computer Count of Huge Crowds Now Possible

Getting a headcount of crowds numbering in the hundreds of thousands need not strain human eyes any longer. New software has carried out the first automated crowd count on that scale ever by analyzing aerial photographs of a huge demonstrationa time-saving innovation that could eventually help save lives and prevent disasters.

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The Slightly Bizarre Fantasies of the 2015 Electrolux Design Lab Challenge

Every year, Electrolux asks design students to stretch their already alarmingly flexible imaginations and develop concepts around a theme that's intended to “raise questions about what design will be like in the future.” This year's theme is “Healthy Happy Kids.” And as per usual, the concepts the students dreamed up are mostly (or entirely) unconstrained by things that are often inconvenient to designers, like the laws of physics. Let's take a look at the top six finalists to see how reality will inevitably fail to live up to the future of design.

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Bummer: No Evidence That Anti-Depression Apps Really Work

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England’s publicly funded health care system, the National Health Service (NHS), has endorsed more than a dozen depression treatment apps, but there’s no proof that most of them actually work, according to a report published this week in the journal Evidence Based Mental Health. The authors of the report examined each of the fourteen depression apps the NHS lists in its app library and found that only two of them had been clinically validated using standard  metrics. 

Depression apps are computer-based programs that help people monitor their symptoms, provide education, and sometimes offer coping skills and therapy. But many of the NHS-endorsed apps “seemed a bit sketchy” or made strange recommendations, says Simon Leigh, an author of the paper and a senior health economist at Lifecode Solutions in Liverpool, UK. “I think it was rather dangerous of the NHS to endorse them without having the information.” 

Leigh’s study is the latest in a slew of reports finding that mental health apps generally lack rigorous testing. Recent research papers on apps that treat bipolar disorder, eating disorders and post-traumatic stress disorder (PTSD) found a similar dearth of evidence validating app developers’ claims. 

“Mobile health apps are increasing at an amazing rate, and most don’t have any empirical validation,” says Stephen Schueller, a clinical psychologist at Northwestern University who specializes in internet and mobile interventions for depression, and was not involved in the NHS study. “The last time I looked at the literature, there were five or six randomized controlled trials of an app for depression, and none of those apps were available on a public app store, and none ran on an iPhone or Android operating system. So anything that I would want to touch as a consumer has not been validated in a randomized controlled trial”—the gold standard of clinical research, he says. 

The gap between consumer health technology and science to support it is nothing new, of course. Computerized systems that help physicians make clinical decisions fail two-thirds of the time, according to a paper published in June. And a report published last month by IMS Institute for Healthcare Informatics found that more than 165,000 mobile health apps are now available to consumers—twice as many as two years ago—severely outpacing the mechanisms by which physicians can assess them. Fifty percent of them have “limited functionality,” according to the report. 

The fact that thousands of mental health apps may be bunk may not come as a surprise to many consumers. The trouble comes when an influential national health system like NHS puts its stamp of approval on an app without requiring standard mental health tests to be applied to it, says Leigh. Metrics the NHS commonly uses to accredit other mental health treatments include the Generalized Anxiety Disorder 7 (GAD-7) and the Patient Health Questionnaire-9 (PHQ-9). Only two of the apps listed on the NHS library—Big White Wall and Moodscope—used such metrics, Leigh’s study found.

The potential harm of a depression app that doesn’t work is that it could compound feelings of anxiety and lack of motivation. “I’m untreatable. I’m a failure. These are common symptoms of depression,” says Schueller. “Lack of motivation is another big one, so if a person gets motivated enough to download an app, and then it doesn’t work, we might have missed that window and they might not get motivated again,” he says. “But I’m not terribly worried that these apps might actually cause problems for a person,” he says.  

NHS does not claim to have officially accredited the apps in its library, but “their badge is plastered all over them,” says Leigh. “And the NHS badge connotes an implicit level of quality.” The NHS says it chooses apps based on three criteria: that the apps are relevant to people in England, that they use information from a trusted source, and that they comply with legislation on appropriate use of data.

The NHS did not return Spectrum’s request for comment by press time. The agency has noted on its website that its health app library, which began in 2013, was a pilot project, and is scheduled to close this week. A new list of online mental health services has appeared on the NHS Choices website.

Other reports have criticized the NHS apps library as well. Imperial College London last month published a studying finding that many of the agency’s endorsed apps sent unencrypted personal and medical details over the internet.  

The US Food and Drug Administration (FDA) so far hasn’t taken much of an active stance on validating mental health apps. The agency published guidance in February this year, noting that apps that are intended to help with coping skills for people with depression and other psychiatric conditions may be subject to FDA oversight. One developer of cognitive health software, Akili Brain Interactive, plans to approach the FDA for approval of its therapeutic video games. The FDA in 2010 approved the first prescription-only diabetes app, called BlueStar

A Particle Accelerator the Size of a Sewing Needle

An international research team has demonstrated a high-performance particle accelerator the size of a 1-millimeter mechanical pencil replacement lead.

Tens of thousands of particle accelerators around the world are used for more than physics research. They are also used to manufacture semiconductors, probe new materials, illuminate too-fast-to-follow chemical reactions, treat cancer, strengthen polymers, sterilize medical devices, and even to make diamonds green and pearls black.

A key accelerator parameter is the acceleration gradient, the energy (measured in mega electron volts, MeV) gained per meter of travel. The amount of energy the accelerator can pump into a cluster of particles, electrons, for example, thus becomes a function of the device’s gradient and length. And cost, of course, increases with physical size of the accelerator.

Thus, conventional linear accelerators, with acceleration gradients around 30–50 MeV/m, can grow as big as the 3,073-meter-long Stanford Linear Accelerator in Menlo Park, Calif., housed in what may be the world’s longest building. These machines accelerate charged particles using either a pulse of radio frequency radiation or a wakefield (using high energy “bunches” of electrons to blast a tunnel through plasma; when the tunnel collapses back on itself, following particles accelerate by riding the charged wake of the collapsing front). RF accelerators can reach energies of a few tens of mega electron volts before the RF energy itself begins to destabilize the mechanism in what’s called plasma breakdown. In wakefield approaches, balancing the skittish plasma bubble requires terawatt or petawatt lasers, tricky micromachinging, and femtosecond laser timing.

In Nature Communications, researchers describe an alternative: a compact device that uses pulses of terahertz (THz) radiation. The research group includes scientists from the Massachusetts Institute of Technology (MIT), the University of Toronto, and the Deutsches Electronen Synchrotron (DESY, the German Electron Syncrotron), the Center for Free-Electron Laser Science (CFEL), the Max Planck Institute for Structure and Dynamics, and the University of Hamburg (all in Hamburg, Germany). It was led by Franz Kärtner, who is affiliated with MIT, CFEL, and DESY.

“Terahertz frequencies provide the best of both worlds,” the group writes. “On one hand, the wavelength is long enough that we can fabricate waveguides with conventional machining techniques, provide accurate timing, and accommodate a significant amount of charge per bunch [of electrons]… On the other hand, the frequency is high enough that the plasma breakdown threshold for surface electric fields increases….”

The terahertz approach also allows them to use readily available picoseconds lasers.

The accelerator itself is a quartz capillary about 1.5 centimeters long and 940 micrometers in diameter, sheathed in a copper jacket. The quartz walls are 270 μm thick, leaving a central vacuum 400 μm in diameter.

In operation, a 0.45 THz pulse is radially polarized bounced off a mirror to enter at one end (call it the right end) of quartz tube. As the pulse traveled down the tube, electrons are injected at 60 keV through a pinhole at the left end. When the terahertz pulse reflects off the left wall (around the injection pinhole) it catches the electrons, accelerating them back towards the right.

In the initial experiments, the electrons could ride the wave for just 3 mm before the wave started to spread out. That short ride, however, boosted their energy to 67 keV. A back of the envelope calculation translates this modest energy gain into an acceleration gradient over 2 MeV/m.

“This is not a particularly large acceleration, but the experiment demonstrates that the principle does work in practice,” explains co-author Arya Fallahi of CFEL. “The theory indicates that we should be able to achieve an accelerating gradient of up to one gigavolt per meter.”

Or, as the paper itself concludes, “This proof-of-principle terahertz linear accelerator demonstrates the potential for an all-optical acceleration scheme that can be readily integrated into small-scale laboratories providing users with electron beams that will enable new experiments in ultrafast electron diffraction and X-ray production.”


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