A University at Buffalo-led research team has developed a technique for “seeing through” a stack of graphene sheets to identify and describe the electronic properties of each individual sheet — even when the sheets are covering each other up.
The method involves shooting a beam of infrared light at the stack, and measuring how the light wave’s direction of oscillation changes as it bounces off the layers within.
When a magnetic field is applied and increased, different types of graphene alter the direction of polarization, in different ways. A graphene layer stacked neatly on top of another will have a different effect on polarization than a graphene layer that is messily stacked.
“By measuring the polarization of reflected light from graphene in a magnetic field and using new analysis techniques, we have developed an ultrasensitive fingerprinting tool that is capable of identifying and characterizing different graphene multilayers,” said John Cerne, PhD, UB associate professor of physics, who led the project.
The technique allows the researchers to examine dozens of individual layers within a stack.
Graphene, a nanomaterial that consists of a single layer of carbon atoms, has generated huge interest due to its remarkable fundamental properties and technological applications. It’s lightweight but also one of the world’s strongest materials.
But Cerne’s new research looks at graphene’s electronic properties, which change as sheets of the material are stacked on top of one another. The findings appeared Nov. 5 in Scientific Reports, an online, open-access journal produced by the publishers of Nature.
The study showed that absorption and emission patterns change when a magnetic field is applied, which means that scientists can turn the polarization of light on and off either by applying a magnetic field to graphene layers or, more quickly, by applying a voltage that sends electrons flowing through the graphene.
“Applying a voltage would allow for fast modulation, which opens up the possibility for new optical devices using graphene for communications, imaging and signal processing,” said first author Chase T. Ellis, a former graduate research assistant at UB and current postdoctoral fellow at the Naval Research Laboratory.
Abstract of Scientific Reports paper
The remarkable electronic properties of graphene strongly depend on the thickness and geometry of graphene stacks. This wide range of electronic tunability is of fundamental interest and has many applications in newly proposed devices. Using the mid-infrared, magneto-optical Kerr effect, we detect and identify over 18 interband cyclotron resonances (CR) that are associated with ABA and ABC stacked multilayers as well as monolayers that coexist in graphene that is epitaxially grown on 4H-SiC. Moreover, the magnetic field and photon energy dependence of these features enable us to explore the band structure, electron-hole band asymmetries, and mechanisms that activate a CR response in the Kerr effect for various multilayers that coexist in a single sample. Surprisingly, we find that the magnitude of monolayer Kerr effect CRs is not temperature dependent. This unexpected result reveals new questions about the underlying physics that makes such an effect possible.
By exploiting flaws in nanoscale diamond fragments, researchers say they have created precise quantum sensors in a biocompatible material.
Nanoscopic thermal and magnetic field detectors that could be inserted into living cells could enhance our understanding of everything from chemical reactions within single cells to signaling in neural networks and the origin of magnetism in novel materials.
Atomic impurities in natural diamond structure give rise to the color seen in rare and coveted pink, blue and yellow diamonds. But these impurities are also a major research focus in emerging areas of quantum physics.
One such defect, the nitrogen-vacancy center (NVC), consists of a gap in the crystal lattice next to a nitrogen atom. This system tightly traps electrons whose spin states can be manipulated with extreme precision.
Electron coherence — the extent to which the spins of these particles can sustain their quantum mechanical properties — has been achieved to high levels in the NVCs of large “bulk” diamonds, with coherence times of an entire second in certain conditions — the longest yet seen in any solid material.
However in nanodiamonds — nanometer sized crystals that can be produced by milling conventional diamond — any acceptable degree of coherence has, until now, proved elusive.
Nanodiamonds offer the potential for both extraordinarily precise resolution because they can be positioned at the nanoscale, and biocompatibility — they have can be inserted into living cells. But without high levels of coherence in their NVCs to carry information, these unique nanodiamond benefits cannot be utilized.
By observing the spin dynamics in nanodiamond NVCs, researchers at Cambridge‘s Cavendish Laboratory, have now identified that it is the concentration of nitrogen impurities that impacts coherence, rather than interactions with spins on the crystal surface.
By controlling the dynamics of these nitrogen impurities separately, they have increased NVC coherence times to a record 0.07 milliseconds longer than any previous report, an order of magnitude, putting nanodiamonds back in play as an extremely promising material for quantum sensing.
“Our results unleash the potential of the smallest magnetic field and temperature detector in the world,” said Helena Knowles, a researcher on the study. “Nanodiamond NVCs can sense the change of such features within a few tens of nanometers; no other sensor has ever had this spatial resolution under ambient conditions. We now have both high spin coherence and spatial resolution, crucial for various quantum technologies.”`
Dr Dhiren Kara, who also worked on the study, points out that the nanodiamond’s biocompatibility can also provide noninvasive optical access to magnetic changes within a living cell — essentially the ability to perform MRI and detect, for instance, a cell’s reaction to a drug in real time.
“We may also be able to answer some key questions in material science, such as magnetic ordering at the edges of graphene or the origin of magnetism in oxide materials,” Kara said.
Dr Mete Atature, director of the research, added: “The pursuit of simultaneous high NVC coherence and high spatial resolution, and the fact that nanodiamonds couldn’t deliver on this promise until now, has required researchers to invest in alternative means including advanced nanofabrication techniques, which tends to be both expensive and low-yield.”
Abstract of Nature Materials paper
Nitrogen-vacancy (NV) centres in diamond are attractive for research straddling quantum information science, nanoscale magnetometry and thermometry. Whereas ultrapure bulk diamond NVs sustain the longest spin coherence times among optically accessible spins, nanodiamond NVs exhibit persistently poor spin coherence. Here we introduce high-purity nanodiamonds accommodating record-long NV coherence times, >60 μs, observed through universal dynamical decoupling. We show that the main contribution to decoherence comes from nearby nitrogen impurities rather than surface states. We protect the NV spin free precession, essential to d.c. magnetometry, by driving solely these impurities into the motional narrowing regime. This extends the NV free induction decay time from 440 ns, longer than that in type Ib bulk diamond, to 1.27 μs, which is comparable to that in type IIa (impurity-free) diamond. These properties allow the simultaneous exploitation of both high sensitivity and nanometre resolution in diamond-based emergent quantum technologies.
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EPFL scientists have shown how to achieve a dramatic increase in the capacity of optical fibers by reducing the amount of space required between the pulses of light that transport data.
Optical fibers carry data in the form of pulses of light over distances of thousands of miles at high speeds. But their capacity is limited, because the pulses of light need to be lined up one after the other in the fiber with a minimum distance between them so the signals don’t interfere with each other.
EPFL researchers have come up with a method for fitting pulses together within the fibers, thereby reducing the space between pulses. Their approach, published in Nature Communications (open access), opens the door to a ten-fold increase in throughput in telecommunications systems.
Extending exponential growth of fiber-optic capacity
“Since it appeared in the 1970s, the data capacity of fiber optics has increased by a factor of ten every four years, driven by a constant stream of new technologies,” says Camille Brès, of the Photonics Systems Laboratory (PHOSL). “But for the last few years we’ve reached a bottleneck, and scientists all over the world are trying to break through.”
There have been several different approaches to the problem of supplying more throughput to respond to growing consumer demand, but they often require changes to the fibers themselves. That would entail pulling out and replacing the existing infrastructure.
The problem with this system is that the volume of data transmitted at one time can’t be increased. If the pulses get too close together, they no longer deliver the data reliably.
The EPFL team took a different approach: they noticed that changes in the shape of the pulses could limit the interference. Instead of replacing the entire optical fiber network, only the transmitters would need to be changed.
Their breakthrough is based on a method that can produce what are known as “Nyquist sinc pulses” almost perfectly. “These pulses have a shape that’s more pointed, making it possible to fit them together,” says Brès. “There is of course some interference, but not at the locations where we actually read the data.” The EPFL team used a simple laser and modulator to generate a pulse that is more than 99% perfect.
Simple lasers are generally made up essentially of just one optical frequency, with a very narrow spectrum. However, a laser can be subtly modulated (using a device called a modulator) so that it has other frequencies. The result is a pulse with a larger spectrum. The problem is that the pulse’s main frequency generally still tends to be stronger than the others. This means the spectrum won’t have the rectangular shape needed. For that, each frequency in the pulse needs to be of the same intensity.
So the team made a series of subtle adjustments based on a concept known as a “frequency comb” and succeeded in generating pulses with almost perfectly rectangular spectrum. This constitutes a breakthrough, the researchers say, since the team has succeeded in producing the long-sought-after Nyquist sinc pulses.
The technology is already mature, as well as 100% optic and relatively cheap. In addition, it appears that it could fit on a simple chip.
Abstract of Nature Communications paper
Sinc-shaped Nyquist pulses possess a rectangular spectrum, enabling data to be encoded in a minimum spectral bandwidth and satisfying by essence the Nyquist criterion of zero inter-symbol interference (ISI). This property makes them very attractive for communication systems since data transmission rates can be maximized while the bandwidth usage is minimized. However, most of the pulse-shaping methods reported so far have remained rather complex and none has led to ideal sinc pulses. Here a method to produce sinc-shaped Nyquist pulses of very high quality is proposed based on the direct synthesis of a rectangular-shaped and phase-locked frequency comb. The method is highly flexible and can be easily integrated in communication systems, potentially offering a substantial increase in data transmission rates. Further, the high quality and wide tunability of the reported sinc-shaped pulses can also bring benefits to many other fields, such as microwave photonics, light storage and all-optical sampling.
Researchers at Chongqing University in China have adapted capsule endoscopy to allow for detecting tiny quantities of “occult” blood for screening of early-stage gastric cancer.
The data is automatically transmitted to an external monitoring device in real time for diagnosis by a physician.
The non-invasive Gastric Occult Blood (GOB) capsule, which carries inside a detector, power supply, and wireless transmitter, is encased in non-toxic, acid-safe polycarbonate. The device has a detection limit of 6 micrograms per liter of fluid. Laboratory tests have demonstrated its simplicity and reliability, the researchers say.
Once its task is complete, the tiny pill-like device would be disposed of through the usual elimination route without harm to the stomach or intestine. This approach avoids the uncomfortable, risky retrieval of such a device via the oral route.
Occult bleeding is usually first identified in patients who have given a stool sample in which blood is found (the fecal occult blood test), but that test can’t identify the source of such blood — intestine or stomach.
The researchers — Hongying Liu, Panpan Qiao, Xueli Wu, Lan Zhu, Xitian Pi, and Xiaolin Zheng, with the Key Laboratory of Biorheological Science and Technology of Ministry of Education, Bioengineering College — say the device is likely to prove safe to use and less invasive than other endoscopic technology and devices for occult-blood testing.
The researchers now plan to take the patent-pending device to clinical safety testing, and then to patients.Funding was provided by the National Natural Science Foundation of China and the Fundamental Research Funds for the Central Universities.
Full disclosure: I am a consultant to Chongqing University, but I have no connection to this research. — Amara D. Angelica, Editor, KurzweilAI
Abstract of International Journal of Biomedical Engineering and Technology paper
Based on capsule endoscopy and Occult Blood (OB) detection theory, an automatic detection capsule system for Gastric Occult Blood (GOB) was proposed. This system utilised the non-invasive deglutible capsule to automatically identify the GOB information and transmitted the detection result to the external device which could display the detection result and identify the OB status with a particular algorithm. Subjects or doctors could discriminate whether OB existed by observing the external device. This paper designed the automatic detection capsule of GOB and the detecting sensor based on collaurum immune theory. We did in vitro experiments to testify the system and acquired the detecting result image information. Experimental results indicated that the effective detection concentration range of the OB sensor was 6-1800 μg/ml. With simplicity and reliability, this system provides a new idea for the screening of early stage gastric cancer.
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