Interaction between light and sound in nanoscale waveguide

February 16, 2015 Ghent University Scientists have demonstrated interaction between light and sound in a nanoscale area. Their findings elucidate the physics of light-matter coupling at these scales – and pave the way for enhanced signal processing on mass-producible silicon photonic chips. In the last decade, the field of silicon photonics has gained increasing attention as a key driver of lab-on-a-chip biosensors and of faster-than-electronics communication between computer chips. The technology builds on tiny structures known as silicon photonic wires, which are roughly a hundred times narrower than a typical human hair. These nanowires carry optical signals from one point to another at the speed of light. They are fabricated with the same technological toolset as electronic circuitry. Fundamentally, the wires work only because light moves slower in the silicon core than in the surrounding air and glass. Both light (left) and sound (right) are trapped in a nanoscale silicon core. Scientists from Ghent University and imec announce today that they demonstrated interaction between light and sound in a nanoscale area. Their findings elucidate the physics of light-matter coupling at these scales -- and pave the way for enhanced signal processing on mass-producible silicon photonic chips. In the last decade, the field of silicon photonics has gained increasing attention as a key driver of lab-on-a-chip biosensors and of faster-than-electronics communication between computer chips. The technology builds on tiny structures known as silicon photonic wires, which are roughly a hundred times narrower than a typical human hair. These nanowires carry optical signals from one point to another at the speed of light. They are fabricated with the same technological toolset as electronic circuitry. Fundamentally, the wires work only because light moves slower in the silicon core than in the surrounding air and glass. Thus, the light is trapped inside the wire by the phenomenon of total internal reflection. Simply confining light is one thing, but manipulating it is another. The issue is that one light beam cannot easily change the properties of another. This is where light-matter interaction comes into the picture: it allows some photons to control other photons. Publishing in Nature Photonics, researchers from the Photonics Research Group of Ghent University and imec report on a peculiar type of light-matter interaction. They managed to confine not only light but also sound to the silicon nanowires. The sound oscillates ten billion times per second: far more rapid than human ears can hear. They realized that the sound cannot be trapped in the wire by total internal reflection. Unlike light, sound moves faster in the silicon core than in the surrounding air and glass. Thus, the scientists sculpted the environment of the core to make sure any vibrational wave trying to escape it would actually bounce back. Doing so, they confined both light and sound to the same nanoscale waveguide core -- a world's first observation. Trapped in that incredibly small area, the light and vibrations strongly influence each other: light generates sound and sound shifts the color of light, a process known as stimulated Brillouin scattering. The scientists exploited this interaction to amplify specific colors of light. They anticipate this demonstration to open up new ways to manipulate optical information. For instance, light pulses could be converted into sonic pulses and back into light -- thereby implementing much-needed delay lines. Further, the researchers expect that similar techniques can be applied to even smaller entities such as viruses and DNA. These particles have unique acoustic vibrations that may be used to probe their global structure. Story Source: The above story is based on materials provided by Ghent University. Note: Materials may be edited for content and length. Journal Reference: Raphaël Van Laer, Bart Kuyken, Dries Van Thourhout, Roel Baets. Interaction between light and highly confined hypersound in a silicon photonic nanowire. Nature Photonics, 2015; DOI: 10.1038/nphoton.2015.11

Bipolar disorder discovery at the nano level

 

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A nano-sized discovery by Northwestern Medicine® scientists helps explain how bipolar disorder affects the brain and could one day lead to new drug therapies to treat the mental illness.

Scientists used a new super-resolution imaging method -- the same method recognized with the 2014 Nobel Prize in chemistry -- to peer deep into brain tissue from mice with bipolar-like behaviors. In the synapses (where communication between brain cells occurs), they discovered tiny "nanodomain" structures with concentrated levels of ANK3 -- the gene most strongly associated with bipolar disorder risk. ANK3 is coding for the protein ankyrin-G.

"We knew that ankyrin-G played an important role in bipolar disease, but we didn't know how," said Northwestern Medicine scientist Peter Penzes, corresponding author of the paper. "Through this imaging method we found the gene formed in nanodomain structures in the synapses, and we determined that these structures control or regulate the behavior of synapses."

Penzes is a professor in physiology and psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine. The results were published Oct. 22 in the journal Neuron.

High-profile cases, including actress Catherine Zeta-Jones and politician Jesse Jackson, Jr., have brought attention to bipolar disorder. The illness causes unusual shifts in mood, energy, activity levels and the ability to carry out day-to-day tasks. About 3 percent of Americans experience bipolar disorder symptoms, and there is no cure.

Recent large-scale human genetic studies have shown that genes can contribute to disease risk along with stress and other environmental factors. However, how these risk genes affect the brain is not known.

This is the first time any psychiatric risk gene has been analyzed at such a detailed level of resolution. As explained in the paper, Penzes used the Nikon Structured Illumination Super-resolution Microscope to study a mouse model of bipolar disorder. The microscope realizes resolution of up to 115 nanometers. To put that size in perspective, a nanometer is one-tenth of a micron, and there are 25,400 microns in one inch. Very few of these microscopes exist worldwide.

"There is important information about genes and diseases that can only been seen at this level of resolution," Penzes said. "We provide a neurobiological explanation of the function of the leading risk gene, and this might provide insight into the abnormalities in bipolar disorder."

The biological framework presented in this paper could be used in human studies of bipolar disorder in the future, with the goal of developing therapeutic approaches to target these genes.

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Story Source:

The above story is based on materials provided by Northwestern University. The original article was written by Erin White. Note: Materials may be edited for content and length.

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Journal Reference:

1. Katharine R. Smith, Katherine J. Kopeikina, Jessica M. Fawcett-Patel, Katherine Leaderbrand, Ruoqi Gao, Britta Schürmann, Kristoffer Myczek, Jelena Radulovic, Geoffrey T. Swanson, Peter Penzes. Psychiatric Risk Factor ANK3/Ankyrin-G Nanodomains Regulate the Structure and Function of Glutamatergic Synapses. Neuron, 2014; 84 (2): 399 DOI: 10.1016/j.neuron.2014.10.010

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Nanoelectronics: Edgy look at 2-D molybdenum disulfide

 

A new SHG imaging technique allows rapid and all-optical determination of the crystal orientations of 2D semiconductor membranes at a large scale, providing the knowledge needed to use these materials in nanoelectronic devices.

The drive to develop ultrasmall and ultrafast electronic devices using a single atomic layer of semiconductors, such as transition metal dichalcogenides, has received a significant boost. Researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) have recorded the first observations of a strong nonlinear optical resonance along the edges of a single layer of molybdenum disulfide. The existence of these edge states is key to the use of molybdenum disulfide in nanoelectronics, as well as a catalyst for the hydrogen evolution reaction in fuel cells, desulfurization and other chemical reactions.

"We observed strong nonlinear optical resonances at the edges of a two-dimensional crystal of molybdenum disulfide" says Xiang Zhang, a faculty scientist with Berkeley Lab's Materials Sciences Division who led this study. "These one-dimensional edge states are the result of electronic structure changes and may enable novel nanoelectronics and photonic devices. These edges have also long been suspected to be the active sites for the electrocatalytic hydrogen evolution reaction in energy applications. We also discovered extraordinary second harmonic light generation properties that may be used for the in situ monitoring of electronic changes and chemical reactions that occur at the one-dimensional atomic edges."

Zhang, who also holds the Ernest S. Kuh Endowed Chair Professor at the University of California (UC) Berkeley and directs the National Science Foundation's Nano-scale Science and Engineering Center, is the corresponding author of a paper in Science describing this research. The paper is titled "Edge Nonlinear Optics on a MoS2 Atomic Monolayer." Co-authors are Xiaobo Yin, Ziliang Ye, Daniel Chenet, Yu Ye, Kevin O'Brien and James Hone.

Emerging two-dimensional semiconductors are prized in the electronics industry for their superior energy efficiency and capacity to carry much higher current densities than silicon. Only a single molecule thick, they are well-suited for integrated optoelectronic devices. Until recently, graphene has been the unchallenged superstar of 2D materials, but today there is considerable attention focused on 2D semiconducting crystals that consist of a single layer of transition metal atoms, such as molybdenum, tungsten or niobium, sandwiched between two layers of chalcogen atoms, such as sulfur or selenium. Featuring the same flat hexagonal "honeycombed" structure as graphene and many of the same electrical advantages, these transition metal dichalcogenides, unlike graphene, have direct energy bandgaps. This facilitates their application in transistors and other electronic devices, particularly light-emitting diodes.

Full realization of the vast potential of transition metal dichalcogenides will only come with a better understanding of the domain orientations of their crystal structures that give rise to their exceptional properties. Until now, however, experimental imaging of these three-atom-thick structures and their edges have been limited to scanning tunneling microscopy and transmission electron microscopy, technologies that are often difficult to use. Nonlinear optics at the crystal edges and boundaries enabled Zhang and his collaborators to develop a new imaging technique based on second-harmonic generation (SHG) light emissions that can easily capture the crystal structures and grain orientations with an optical microscope.

"Our nonlinear optical imaging technique is a non-invasive, fast, easy metrologic approach to the study of 2D atomic materials," says Xiaobo Yin, the lead author of the Science paper and a former member of Zhang's research group who is now on the faculty at the University of Colorado, Boulder. "We don't need to prepare the sample on any special substrate or vacuum environment, and the measurement won't perturb the sample during the imaging process. This advantage allows for in-situ measurements under many practical conditions. Furthermore, our imaging technique is an ultrafast measurement that can provide critical dynamic information, and its instrumentation is far less complicated and less expensive compared with scanning tunneling microscopy and transmission electron microscopy."

For the SHG imaging of molybdenum disulfide, Zhang and his collaborators illuminated sample membranes that are only three atoms thick with ultrafast pulses of infrared light. The nonlinear optical properties of the samples yielded a strong SHG response in the form of visible light that is both tunable and coherent. The resulting SHG-generated images enabled the researchers to detect "structural discontinuities" or edges along the 2D crystals only a few atoms wide where the translational symmetry of the crystal was broken.

"By analyzing the polarized components of the SHG signals, we were able to map the crystal orientation of the molybdenum disulfide atomic membrane," says Ziliang Ye, the co-lead author of the paper and current member of Zhang's research group. "This allowed us to capture a complete map of the crystal grain structures, color-coded according to crystal orientation. We now have a real-time, non-invasive tool that allows us explore the structural, optical, and electronic properties of 2D atomic layers of transition metal dichalcogenides over a large area."

This research was supported by the DOE Office of Science through the Energy Frontier Research Center program, and by the U.S. Air Force Office of Scientific Research Multidisciplinary University Research Initiative.

Nanoparticle Opens the Door to Clean-Energy Alternatives

 

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A transmission-electron microscope image of a collection of quasi-spherical nickel phosphide nanoparticles. A team led by Raymond Schaak of Penn State University has found that these nanoparticles can catalyze an important chemical reaction that generates hydrogen from water. Credit: Eric Popczun, Penn State University

13 June 2013 — Cheaper clean-energy technologies could be made possible thanks to a new discovery. Led by Raymond Schaak, a professor of chemistry at Penn State University, research team members have found that an important chemical reaction that generates hydrogen from water is effectively triggered -- or catalyzed -- by a nanoparticle composed of nickel and phosphorus, two inexpensive elements that are abundant on Earth. The results of the research will be published in the Journal of the American Chemical Society.

Schaak explained that the purpose of the nickel phosphide nanoparticle is to help produce hydrogen from water, which is a process that is important for many energy-production technologies, including fuel cells and solar cells. "Water is an ideal fuel, because it is cheap and abundant, but we need to be able to extract hydrogen from it," Schaak said. Hydrogen has a high energy density and is a great energy carrier, Schaak explained, but it requires energy to produce. To make its production practical, scientists have been hunting for a way to trigger the required chemical reactions with an inexpensive catalyst. Schaak noted that this feat is accomplished very well by platinum but, because platinum is expensive and relatively rare, he and his team have been searching for alternative materials. "There were some predictions that nickel phosphide might be a good candidate, and we had already been working with nickel phosphide nanoparticles for several years," Schaak said. "It turns out that nanoparticles of nickel phosphide are indeed active for producing hydrogen and are comparable to the best known alternatives to platinum."

Image showing hydrogen gas bubbling off of the surface of a nickel phosphide crystal. A team led by Raymond Schaak of Penn State University is studying nanoparticles made from nickel phosphide as a means to create cleaner energy technologies. Credit: Eric Popczun, Penn State University

To create the nickel phosphide nanoparticles, team members began with metal salts that are commercially available. They then dissolved these salts in solvents, added other chemical ingredients, and heated the solution to allow the nanoparticles to form. The researchers were able create a nanoparticle that was quasi-spherical -- not a perfect sphere, but spherical with many flat, exposed edges. "The small size of the nanoparticles creates a high surface area, and the exposed edges means that a large number of sites are available to catalyze the chemical reaction that produces hydrogen," Schaak explained.

The next step was for team members at the California Institute of Technology to test the nanoparticles' performance in catalyzing the necessary chemical reactions. Led by Nathan S. Lewis, the George L. Argyros Professor of Chemistry at the California Institute of Technology, the researchers performed these tests by placing the nanoparticles onto a sheet of titanium foil and immersing that sheet in a solution of sulfuric acid. Next, the researchers applied a voltage and measured the current produced. They found that, not only were the chemical reactions happening as they had hoped, they also were happening with a high degree of efficacy.

"Nanoparticle technology has already started to open the door to cheaper and cleaner energy that is also efficient and useful," Schaak said. "The goal now is to further improve the performance of these nanoparticles and to understand what makes them function the way they do. Also, our team members believe that our success with nickel phosphide can pave the way toward the discovery of other new catalysts that also are comprised of Earth-abundant materials. Insights from this discovery may lead to even better catalysts in the future."

In addition to Schaak and Lewis, other researchers who contributed to this study include Eric J. Popczun, Carlos G. Read, Adam J. Biacchi, and Alex M. Wiltrout from Penn State; and James R. McKone from the California Institute of Technology.

Marvin Cohen describes his role in determining the properties of matter, core to nanoscience.

 

Marvin Cohen describes his role in determining the properties of matter, core to nanoscience. Marvin L. Cohen, 2001 National Medal of Science medalist, discusses how his research career developed, including the integration of computing, which led to his creation and application of quantum theory for explaining and predicting the properties of materials. This is core to semiconductor physics and nanoscience, today a burgeoning field.