This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only. At left, the drawing of the LET describes the scientists’ observation, shown in the electroluminescence intensity map at right. The infrared light emitted from the LET reaches the detector, meaning that an electrical signal can be transferred to the detector by light. Credit: Shin-ichi Saito, et al. In one of the early discoveries of the current “silicon electrophotonics era,” scientists from Hitachi, Ltd. in Tokyo have built a light-emitting transistor (LET) that transfers, detects and controls an electrical signal all on a single nanometer-sized chip. Using a silicon-on-insulator (SOI) substrate, the group could optically connect the LET to a detector, resulting in a tiny chip that may integrate a wide range of microelectronics and photonics nano devices. Citation: Light-emitting transistor uses light to transfer an electrical signal (2006, November 1) retrieved 18 August 2019 from https://phys.org/news/2006-11-light-emitting-transistor-electrical.html “With LETs, we could develop an optical interconnection beyond the present copper interconnection,” Shin-Ichi Saito, co-author of the study in Applied Physics Letters, told PhysOrg.com. “If LETs are really integrated on silicon chips, we might reduce power dissipation (since light does not have electrical resistance), as well as RC [Resistive Capacitive] delay: in electrical circuits, the interlayer coupling capacitance reduces the speed of electrical signals, while such a delay might be reduced in optical interconnections.”Similar to a standard field-effect transistor, Saito et al.’s LET takes advantage of some interesting properties of 2D electron and electron hole systems, called “quantum confinement effects.” By reducing the thickness of the crystal silicon down to the nanometer scale, the scientists fabricated an ultra-thin single crystal silicon film, directly connected to the thick silicon electrodes. In such a design, n-type (electrons) and p-type (holes) semiconductors lie next to each other separated by a narrow junction. As the electrons and holes efficiently eliminate each other in a process called “recombination,” photons are emitted; thus electrical signals can be converted to optical signals. This close-up drawing of the LET shows the ultrathin silicon junction acting as a quantum well. After recombination, the electric carriers are confined into a standing wave, greatly increasing the electroluminescence efficiency. Credit: Shin-ichi Saito, et al. Explore further “The key idea is that the 2D conduction band electrons behave just like electrons in direct band-gap semiconductors,” said Saito. “In bulk Si, the conduction band electrons move very fast with a large momentum. However, in the ultra-thin Si, the electrons cannot move perpendicular to the substrate with such a large momentum, simply because that direction is restricted. Many people also consider that this quantum mechanical confinement plays some role for the enhanced luminescence in nano-scale silicon.”To optically interconnect this electrical signal from the LET to a detector–which were electrically isolated but on the same silicon chip–the group applied a forward voltage bias to the LET. The scientists observed the light from the LET to reach the photodetector, and measured the “photocurrent” in the detector to increase with a voltage increase, and decrease when the voltage was turned off (detector limitations caused some current to continue flowing).Although the Hitachi group points out limitations to the present experimental set-up that need to be fixed before applying the principle to marketable technology, they suggest solutions for these problems: for example, reducing the response time of the detector and using waveguides to contain the light on the chip. However, the achievement shows how, using curious phenomena of quantum mechanics, photons, just like electrons, can be manipulated on a silicon chip. Quite possibly, future integrated circuits may use lights instead of currents to enhance performance while reducing power dissipation.“We have just confirmed the basic operation principle for this LET,” said Saito. “The hope is that this is just the beginning of more research; we have lots to do.”Citation: Saito, Shin-ichi, Hisamoto, Digh, Shimizu, Haruka, Hamamura, Hirotaka, Tsuchiya, Ryuta, Matsui, Yuichi, Mine, Toshiyuki, Arai, Tadashi, Sugii, Nobuyuki, Torii, Kazuyoshi, Kimura, Shin’ichiro, and Onai, Takahiro. “Silicon light-emitting transistor for on-chip optical interconnection.” Applied Physics Letters 89, 163504 (2006).By Lisa Zyga, Copyright 2006 PhysOrg.com “With the lateral carrier injections that we used, we can efficiently inject both electrons and holes directly in the quantum confined silicon,” said Saito. “Usually, nano-scale silicon structures are passivated by SiO2, which has huge potential barriers for carriers. Such a limitation does not exist in our device.”In this set-up, the p-n junction consists of a light-emitting diode (LED) made of ultrathin silicon. At a 9nm thickness, the silicon acts as a quantum well, confining the electric carriers to two dimensions, which forms a standing wave consisting of an electron. This confinement serves an especially useful purpose for integrating optical components into silicon circuits, as it enhances the electroluminescence efficiency of the junction. While other LETs–from carbon nanotubes and organic models to semiconductor and nanocrystal devices–have been demonstrated, Saito et al.’s is the first in which recombination occurs along the silicon junction and takes advantage of quantum confinement’s electroluminescence. Decline of entrepreneurship blamed for Japan woes
“We found that a general rule applies to a dynamic system that relies solely on communication (pheromones) and social cooperation,” Oettler told Phys.org. “This system depends on two features. One is routing information that decays over time and needs to be refreshed, thus making the system flexible. And second on behavioral flexibility by the worker ants that carry this information. A path can only be adjusted by worker ants that do not follow this path, but rather take alternative routes (that they advertise), which may be faster (or slower) than the already established path.”Oettler added that the speed with which information decays is crucial to this system, so the results here may be adapted to information routing problems.The researchers noted that Wasmannia worker ants also resemble humans in this way, since humans have also been found to follow Fermat’s principle when forced to cross surfaces with different qualities. One clear example is when a lifeguard has to find the fastest way to traverse both beach and water in order to reach a drowning swimmer. The researchers also noted that several other factors likely influence why ants choose the paths they do. For instance, the results here suggest that ants prefer to move along edges of borders (in real life, for example, ants are often seen walking along the edges of a sidewalk). Landmarks facilitate orientation, and for ants this could mean that lower amounts of pheromone are needed to mark trails when combined with the physical cues. This is one area that the researchers hope to study more in the future.”We have only shown the outcome of this process,” Oettler said. “In the future we want to study the dynamics of the trail pheromones, their active compounds and evaporation rates. We want to perform behavioral tests with synthetic compounds and detect perception thresholds of individual ants. We also want to study the early dynamics of trail formation over time.” Explore further A ‘refracted’ trail of Wasmannia auropunctata workers at the medium border between smooth (white) and rough (green) felt. The position of the food is on the rough felt. The density of workers on the rough felt is higher than on the smooth felt because travel speed is lower. In addition, it appears, although not very obvious, as if the ants on the rough felt ‘float’ on top of the felt hairs, indicating the difficulty of walking on this substrate. Credit: Simon Tragust. This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only. (Phys.org) —Ants have long been known to choose the shortest of several routes to a food source, but what happens when the shortest route is not the fastest? This situation can occur, for example, when ants are forced to travel on two different surfaces, where they can walk faster on one surface than on the other. In a new study, scientists have found that ants behave the same way as light does when traveling through different media: both paths obey Fermat’s principle of least time, taking the fastest route rather than the most direct one. Besides revealing insight into ant communities, the findings could offer inspiration to researchers working on solving complex problems in robotics, logistics, and information technology. More information: Jan Oettler, et al. “Fermat’s Principle of Least Time Predicts Refraction of Ant Trails at Substrate Borders.” PLOS ONE. DOI:10.1371/journal.pone.0059739.g001 http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0059739 The researchers, Jan Oettler, et al., from institutions in Germany, France, and China, have published their paper on using Fermat’s principle of least time to predict ant trails in a recent issue of PLOS ONE. The scientists experimentally studied the behavior of the little fire ant, Wasmannia auropunctata, one of the world’s 100 most invasive species. They collected three colonies, each containing several thousand workers and multiple queens, from different sites in Israel. They put the ants in plastic boxes without food, and then connected each box to its own foraging arena where cockroaches were provided as food in the corner opposite from the entrance. The surface of each foraging arena was split in half, and each half was covered by a different material. The researchers experimented with combinations of three materials that differentially affected the ants’ average walking speed: rough polyester felt (1.73 mm/s), smooth polyester felt (2.97 mm/s), and polyethylene glass (4.89 mm/s). When ants forage, they deposit pheromones to build a trail, and favorable trails become reinforced with increased usage.Given the ants’ average walking speeds on different materials, along with information on the distances across the foraging arena and the angle defining the shortest path (the straight line) between the entrance and the food, the researchers predicted the angle at which the ants would cross from one surface to the other if they chose the fastest path. In accordance with Fermat’s principle of least time, this angle can be thought of as the angle of refraction, similar to how light refracts at a certain angle at the boundary of two media in which it travels at different speeds. Robot ants successfully mimic real colony behavior The researchers calculated the fastest path as predicted by Fermat’s principle of least time (dotted yellow line), which is not the direct path. In this example, the ants can walk faster on Surface 1 than on Surface 2. Credit: Jan Oettler, et al. Journal information: PLoS ONE Copyright 2013 Phys.org All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of Phys.org. The researchers found that the ants’ paths closely matched those predicted by Fermat’s principle of least time and did not follow the most direct route. In other words, the ants accounted for the different walking speeds on the different surfaces and traveled a longer distance on the surface where they could walk faster. Citation: Ants follow Fermat’s principle of least time (2013, April 1) retrieved 18 August 2019 from https://phys.org/news/2013-04-ants-fermat-principle.html
© 2013 Phys.org. All rights reserved. This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only. More information: Sander A. Mann and Erik C. Garnett. “Extreme Light Absorption in Thin Semiconductor Films Wrapped around Metal Nanowires.” Nano Letters. DOI: 10.1021/nl401179h The core-shell hybrid nanowire consists of a metal core wrapped with semiconductor thin films. Three different resonances excited at different wavelengths are shown. (b) The fraction of absorbed above-band gap photons in the silicon shell for a wide variety of configurations. Credit: Mann and Garnett. ©2013 American Chemical Society Now, somewhat counterintuitively, scientists have theoretically found that thin semiconductor films wrapped around metal nanowires have substantially better light absorption properties than solid semiconducting nanowires, despite the fact that they use less semiconducting material. At the same time, the metal core acts as a contact to efficiently extract charge carriers. By confronting the semiconductor thickness trade-off and offering exceptional performance, the nanostructures might become ideal building blocks for inexpensive photovoltaic and solar fuel applications.A paper on the new devices by Sander A. Mann and Erik C. Garnett at the Center for Nanophotonics at FOM Institute AMOLF in Amsterdam, The Netherlands, will be published in a future issue of Nano Letters.”The greatest significance to our work is that we provide a design for nanowire building blocks that incorporates both excellent light trapping properties and a local metal electrode contact (for current extraction),” Garnett told Phys.org. “Silver nanowire networks have already been used as high performance transparent electrodes and we expect that by coating them with thin semiconducting shells we will be able to make high-efficiency solar cells using cheap materials. It has now been observed in a number of papers that nanostructuring a material can increase light absorption even while using less semiconductor material. However, this paper takes the next step and starts thinking about how to design such structures with integrated electrical contacts.” One of the biggest advantages of the design is that it uses very thin semiconducting films while at the same time providing very good light absorption. As mentioned, thick semiconductor layers are needed for good light absorption, but high-quality semiconductor is very costly. This new core-shell geometry opens up a pathway to using cheap, abundant, and environmentally friendly semiconductors that previously were of too low quality for good charge extraction.In semiconductor objects smaller than the wavelength of light, as is the case with most nanowires for photovoltaic purposes, the optical properties are determined primarily by resonances. These resonances enhance absorption the most when they are critically coupled: the loss rates due to absorption in the semiconductor and due to radiative leakage (light escaping the nanowire before being absorbed) are equal. This is often the case near the band gap of the material, where absorption is weak, which leads to the highly counterintuitive result that absorption in the nanowire actually increases when the absorption coefficient decreases.As the scientists explain, in the core-shell geometry, extreme light absorption arises from increasing the number and strength of these resonances. Whereas in horizontal nanowires resonances are always spectrally separated (at different wavelengths), in the core-shell geometry they can overlap. Furthermore, horizontal solid seminconductor nanowires are very polarization-sensitive, but this is undesirable as light from the sun is unpolarized. The core-shell geometry gets rid of this polarization dependence by aligning resonances in both polarizations simultaneously.Overall, by demonstrating that excellent light absorption can be achieved in very thin semiconductor layers, this hybrid nanostructure offers an exciting new path toward realizing inexpensive solar technologies based on abundant and environmentally friendly semiconductors. The researchers plan to fabricate prototypes of the devices soon.”Our immediate plans are to make both single-nanowire and array solar cells based on these core-shell building blocks to verify our calculations experimentally,” Garnett said. Explore further Nanowire solar cells raise efficiency limit (Phys.org) —In photovoltaics, there is generally a trade-off in terms of semiconductor thickness, with thicker semiconductors offering better photon absorption and thinner ones offering higher charge carrier extraction efficiency. In recent years, scientists have begun investigating semiconductor nanowire solar cells, which tackle this tradeoff through morphology-dependent resonances that significantly enhance the absorption compared to a planar film. Journal information: Nano Letters Citation: Hybrid nanostructure with extreme light absorption looks promising for photovoltaics (2013, July 1) retrieved 18 August 2019 from https://phys.org/news/2013-07-hybrid-nanostructure-extreme-absorption-photovoltaics.html
Buying in metal, auto, consumer durable and pharma counters helped the benchmark S&P BSE sensex to rally for the second session in a row by 154 points to end at 27,395.73 amid mixed to positive global trends and renewed buying by foreign funds.A statement given by the Finance Minister Arun Jaitley that the economy is expected to grow “much better” in 2015-16 as compared with the current financial year also aided the sentiment. The Indian economy grew by 5.3 per cent in the September quarter from a year earlier, and is expected to grow 5.5 per cent in the current financial year that ends on March 31. Buying was seen across-the-board as 11 out of 12 sectoral indices closed with gains while only bankex finished with minor losses.Smart rise in Tata Motors, HDFC, RIL, ITC, TCS, SSLT, Sun Pharma, Hero MotoCorp, Hindalco, HUL, Coal India, Tata Steel, Gail India and BHEL mainly supported the sensex rise.Meanwhile, extending losses for the fourth straight session, rupee on Monday declined by 10 paise to log over 13-month closing low of 63.67 against the Greenback following sustained dollar demand from importers.