September 12, 2011
Physics News Highlights of the American Institute of Physics (AIP) contains summaries of interesting research from the AIP journals, notices of upcoming meetings, and other information from the AIP Member Societies. Copies of papers are available to journalists upon request.
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In the race to enhance the efficiency of solar cells, spending the time and effort to get tiny nanowires to line up neatly on the top of ordinary silicon wafers may not be worth the effort. An international team of researchers has for the first time demonstrated that random, haphazardly grown silicon nanowires can significantly boost the power-producing capabilities of solar cells by trapping a broad spectrum of light waves and capturing sunlight streaming in from a wide variety of angles. The nanowires, which are wrapped in a shell of silicon oxide, serve as an antireflective coating on top of the usually shiny silicon wafer. The scraggly tangle captures light ranging in color from red to violet, and the random orientation of the wires means the coating would continue to absorb light even as the angle of the Sun changes throughout the day. The researchers fabricated the jumbled, yet effective, antireflective coating by vaporizing silicon powder and then depositing it on top of a silicon wafer. The process, described in the AIP’s new journal AIP Advances, is relatively inexpensive and could be scaled up for large manufacturing operations. For future work the team plans to create structures that are more ordered to test if the messy arrangement really is better.
Article: “Graded index and randomly oriented core-shell silicon nanowires for broadband and wide angle antireflection” is published in AIP Advances.
Authors: P. Pignalosa (1, 2), H. Lee (3), L. Qiao (3), M. Tseng (4), and Y. Yi (1, 2, 5).
2. Graphene may open the gate to future terahertz technologies
Researchers from the University of Notre Dame in Indiana have harnessed another one of graphene’s remarkable properties to better control a relatively untamed portion of the electromagnetic spectrum: the terahertz band. Terahertz radiation offers tantalizing new opportunities in communications, medical imaging, and chemical detection. Straddling the transition between the highest energy radio waves and the lowest energy infrared light, terahertz waves are notoriously difficult to produce, detect, and modulate. Modulation, or varying the height of the terahertz waves, is particularly important because a modulated signal can carry information and is more versatile for applications such as chemical and biological sensing. Some of today’s most promising terahertz technologies are based on small semiconductor transistor-like structures that are able to modulate a terahertz signal at room temperature, which is a significant advantage over earlier modulators that could only operate at extremely cold temperatures. Unfortunately, these transistor-like devices rely on a thin layer of metal called a “metal gate” to tune the terahertz signal. This metal gate significantly reduces the signal strength and limits how much the signal can be modulated to a lackluster 30 percent. As reported in the AIP’s journal Applied Physics Letters, by replacing the metal gate with a single layer of graphene, the researchers have predicted that the modulation range can be significantly expanded to be in excess of 90 percent. This modulation is controlled by applying a voltage between the graphene and semiconductor. Unlike the metal gate modulator, the graphene design barely diminished the output power of the terahertz energy. Made up of a one-atom-thick sheet of carbon atoms, graphene boasts a host of amazing properties: it’s remarkably strong, a superb thermal insulator, a conductor of electricity, and now a better means to modulate terahertz radiation.
Article: “Unique prospects for graphene-based terahertz modulators” is accepted for publication in Applied Physics Letters.
Authors: Berardi Sensale-Rodriguez (1), Tian Fang (1), Rusen Yan (1), Michelle M. Kelly (1), Debdeep Jena (1), Lei Liu (1), and Huili (Grace) Xing (1).
(1) Department of Electrical Engineering, University of Notre Dame
A simplified mathematical model of the brain’s neural circuitry shows that repetitious, overlapped firing of neurons can lead to the waves of overly synchronized brain activity that may cause the halting movements that are a hallmark of Parkinson’s disease. The model provides a tool in the quest to gain a better understanding of the mechanisms behind this incurable degenerative disorder. Researchers from IUPUI (Indiana University-Purdue University Indianapolis) reduced the complex biology of the basal ganglia, a part of the brain involved in voluntary motor control, down to a key system of two interconnected cells. The cells were linked together in an inhibitory relationship, meaning a signal from one cell would suppress the second cell’s firing. The team ran simulations of the two-cell system while tinkering with the parameters of the model. For example, since levels of the neurotransmitter dopamine decrease in Parkinson’s patients, increasing the inhibitory coupling strength between cells, the team tested how the strength of the inhibitory connection affected the cells’ synchronization. In a paper in the AIP’s journal Chaos, the researchers identified specific ranges of coupling strength most likely to lead to bursts of intermittently synchronized firings. The team also produced squiggly-lined graphs showing how the complex interactions between slow-changing variables such as calcium ion concentration can cause intermittent synchronization of the two cells. Although the model is based on a neural network known to be affected by Parkinson’s disease, the authors believe that their mathematical model might also yield insights into the operation of more generic neural systems.
Article: “Intermittent synchronization in a network of bursting neurons” is accepted for publication in Chaos: An Interdisciplinary Journal of Nonlinear Science.
Authors: Choongseok Park (1) and Leonid L. Rubchinsky (1,2).
(1) Department of Mathematical Sciences and Center for Mathematical Biosciences, Indiana University-Purdue University, Indianapolis
Legend tells of Greek engineer and inventor Archimedes using parabolic mirrors to create “heat rays” to burn the ships attacking Syracuse. Though the underpinnings of that claim are speculative at best, a modern-day team of researchers at the Scientific and Production Association in Uzbekistan has proposed a more scientifically sound method of harnessing parabolic mirrors to drive solar-powered lasers. Small scale analogs of giant reflector telescopes, these proposed ceramic lasers would convert an impressive 35 percent of the Sun’s energy into a laser light, providing a considerable increase in the maximum power produced by current-day solar pumped lasers, which typically achieve only a 1-2 percent efficiency. As outlined in the AIP’s Journal of Renewable and Sustainable Energy, the new solar lasers would concentrate light with a small parabolic mirror 1 meter in diameter that has a focal spot approximately 2-3 centimeters in diameter. The concentrated light would then strike a two-layer ceramic disk known as a Neodymium and Chromium co-doped YAG (yttrium aluminum garnet) laser material. One side of the disk would have a highly reflective coating; the other side would be anti-reflecting. When sunlight penetrates through the ceramic material, it excites the electrons in the material, causing them to emit laser light of a specific wavelength (1.06 micrometers). To control the searing heat produced by the concentrated sunlight, the ceramic disk would be mounted atop a heat sink through which water would be pumped. The laser light would then travel to a prime focus and be reflected back to the ceramic surface before exiting the solar collector at an oblique angle. It’s this “double pass” path that produces the gain in efficiency, enabling a greater fraction of sunlight to be converted into laser light. Potentially, parabolic reflector lasers could be harnessed for the large-scale synthesis of nanoparticles and nanostructures.
Upcoming Conferences of Interest
Physics and Astronomy Degrees at All-time High
The American Institute of Physics (AIP) Statistical Research Center (SRC) published the latest enrollment and degree data for degree-granting U.S. physics and astronomy departments. The SRC report reveals that several physics-related degrees were at an all-time high for the class of 2010, with 380 astronomy bachelor’s degrees awarded, 6,017 physics bachelor’s degrees awarded, and 1,558 physics PhDs awarded. The full reports, which contain detailed department-level enrollment and degree data, are available on the SRC webpage: http://aip.org/statistics/catalog.html.
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