Electronic ‘Crowd Behavior’ Revealed in Semiconductors
Physicists at JILA have confirmed subtle “collective behavior” among electronic structures in semiconductors, research that may help improve the design of optoelectronic devices.
In the first image (#1, showing new experimental data), matching large peaks in the foreground, showing energy intensity ranging from low in blue to high in red, indicate that pairs of large electronic particles called excitons are oscillating in concert as they absorb ultrafast laser light and emit energy at various frequencies.
The data match new theoretical models accounting for all electronic properties of semiconductors (image #2) much better than older theoretical models (image #3).
*Move your mouse over an image to stop the page flipping temporarily.
Credit: JILA and University of Marburg
View hi-resolution copy of: Image 1, Image 2, Image 3.
Like crowds of people, microscopic particles can act in concert under the right conditions. By exposing crowd behavior at the atomic scale, scientists discover new states and properties of matter. Now, ultrafast lasers have revealed a previously unseen type of collective electronic behavior in semiconductors, which may help in the design of optoelectronic devices. The work at JILA, a joint venture of the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder, is described in a new paper in the Proceedings of the National Academy of Sciences.*
Design of optoelectronic devices, like the semiconductor diode lasers used in telecommunications, currently involves a lot of trial and error. A designer trying to use basic theory to calculate the characteristics of a new diode laser will be off by a significant amount because of subtle interactions in the semiconductor that could not be detected until recently. To shed light on these interactions, the JILA team used a highly sensitive and increasingly popular method of manipulating laser light energy and phase (the point in time when a single light wave begins) to reveal the collective behavior of electronic particles that shift the phase of any deflected light. Their work is an adaptation of a technique that was developed years ago by other researchers to probe correlations between spinning nuclei as an indicator of molecular structure (and led to a Nobel prize).
In the latest JILA experiments, a sample made of thin layers of gallium arsenide was hit with a continuous series of three near-infrared laser pulses lasting just 100 femtoseconds each. Trillions of electronic structures called excitons were formed. Excitons are large, fluffy particles consisting of excited electrons and the “holes” they left behind as they jumped to higher-energy vibration patterns. By tinkering with the laser tuning—the frequency and orientation of the electric field—and analyzing how the semiconductor altered the intensity and phase of the light, the researchers identified a subtle coupling between pairs of excitons with different energy levels, or electron masses. The experimental data matched advanced theoretical calculations of the electronic properties of semiconductors, confirming the importance of the collective exciton behavior—and dramatically demonstrated the superiority of those calculations over simpler models of semiconductor behavior (see graphic).
The work may help researchers better predict optoelectronic device characteristics, not only the magnitude of the emissions signals but also the phase, which is especially significant in optics. Authors of the paper include a NIST collaborator and theorists from Philipps University in Marburg, Germany. The JILA research is supported in part by the U.S. Department of Energy.
* T. Zhang, I. Kuznetsova, T. Meier, X. Li, R.P. Mirin, P. Thomas and S.T. Cundiff. Polarization-dependent optical two-dimensional Fourier transform spectroscopy of semiconductors. Proceedings of the National Academy of Sciences. To be posted online the week of July 9.
Bend It Like NIST: Nanoscale Soccer Debuts at RoboCup
Microchip with nanosoccer fields of play. The glass microchip on the left measures 3 centimeters across – slightly more than the diameter of a quarter on the right – and is divided into 16 nanosoccer playing fields.
Photomicrograph of field of play with defenders in place. The nanobots must navigate around the stationary defenders to go from goal to goal.
Imagine a mechanical Pelé or David Beckham six times smaller than an amoeba playing with a “soccer ball” no wider than a human hair on a field that can fit on a grain of rice. Purely science fiction? Not anymore.
This weekend, the National Institute of Standards and Technology (NIST) is hosting the first ever nanoscale soccer games at the 2007 RoboCup in Atlanta, Ga.
RoboCup is an annual international competition designed to foster innovations and advances in artificial intelligence and intelligent robotics by using the game of soccer as a testing ground. NIST hopes that a competition between the smallest robots in RoboCup history will show the feasibility and accessibility of technologies for fabricating MicroElectroMechanical Systems (MEMS), tiny mechanical devices that are built onto semiconductor chips and are measured in micrometers (millionth of a meter).
The 2007 RoboCup features six competition leagues: Four-Legged, Humanoid, Middle Size, Small Size, Simulation and Rescue Robot. RoboCup and NIST are jointly organizing this year’s nanosoccer competition as a demonstration event with plans for it to become the Nanogram League in 2008. Five teams are entered in the Nanogram Demonstration Competition: two from Carnegie Mellon University (Pittsburgh, Pa.), and one each from the U.S. Naval Academy (Annapolis, Md.), the Swiss Federal Institute of Technology (Zurich, Switzerland) and Simon Fraser University (Burnaby, British Columbia, Canada).
The soccer nanobots (nanoscale robots) operate under an optical microscope, are controlled by remote electronics using visual feedback and are viewed on a monitor. Three of the robots move in response to different electrical signals transmitted across the microchip playing field while the other two are maneuvered by changing magnetic fields. They are manufactured from materials such as aluminum, nickel, gold, silicon and chromium.
While they are a few tens of micrometers to a few hundred micrometers long, the robots are considered “nanoscale” because their masses range from a few nanograms to a few hundred nanograms.
To win the RoboCup nanosoccer competition, a nanobot must be fast, agile and capable of manipulating objects. These abilities will be tested in three events: a two-millimeter dash in which each nanobot seeks the best time for a goal-to-goal sprint across the playing field; a slalom drill where the path between goals is blocked by “defenders” (polymer posts) and a ball handling drill that requires robots to “dribble” as many “nanoballs” (microdisks with the diameter of a human hair) as possible into the goal within a 3-minute period.
A team of scientists and engineers at the Commerce Department’s National Institute of Standards and Technology (NIST) that is investigating the collapse of New York City’s World Trade Center 7 (WTC 7) building expects to release its draft report for public comment by the end of the year. WTC 7 was a 47-story office building adjacent to the WTC towers (WTC 1 and 2) that collapsed following the terrorist attacks of Sept. 11, 2001. WTC 7 collapsed later that afternoon.
NIST’s investigation of WTC 7 includes an extremely complex analysis that incorporates detailed information about the building’s structure and construction, as well as data about fires, damage sustained from falling WTC 1 debris and other technical factors to determine its probable collapse sequence.
“We are proceeding as quickly as possible while rigorously testing and evaluating a wide range of scenarios to reach the most definitive conclusion possible,” said Shyam Sunder, WTC lead investigator for NIST. “The WTC 7 investigation is in some respects just as challenging, if not more so, than the study of the towers. However, the current study does benefit greatly from the significant technological advances achieved and lessons learned from our work on the towers.”
The NIST investigation team initially worked simultaneously on both the WTC towers and WTC 7 collapses. In June 2004, the team shifted to full-time study of the towers to develop needed simulation methods and other research tools and to expedite completion of the WTC towers report. Work resumed on the WTC 7 study in October 2005.
The current NIST working collapse hypothesis for WTC 7 is described in the June 2004 Progress Report on the Federal Building and Fire Safety Investigation of the World Trade Center Disaster (Volume 1, page 17, as well as Appendix L), as follows:
An initial local failure occurred at the lower floors (below floor 13) of the building due to fire and/or debris induced structural damage of a critical column (the initiating event) which supported a large span floor bay with an area of about 2,000 square feet.
Vertical progression of the initial local failure occurred up to the east penthouse, as the large floor bays were unable to redistribute the loads, bringing down the interior structure below the east penthouse.
Horizontal progression of the failure across the lower floors (in the region of floors 5 and 7, that were much thicker than the rest of the floors), triggered by damage due to the vertical failure, resulting in a disproportionate collapse of the entire structure.
This hypothesis may be supported or modified, or new hypotheses may be developed, through the course of the continuing investigation. NIST also is considering whether hypothetical blast events could have played a role in initiating the collapse. While NIST has found no evidence of a blast or controlled demolition event, NIST would like to determine the magnitude of hypothetical blast scenarios that could have led to the structural failure of one or more critical elements.
Updated information with the specific date for the public release of the NIST team’s draft report will be posted on the WTC investigation Web site, http://wtc.nist.gov.
The latest trends in biometrics research and development, as well as the application of biometrics technologies in many important government programs and commercial settings, will be the topic of the Biometric Consortium Conference 2007 on Sept. 11-13, in Baltimore, Md. Sponsored and supported by 17 federal and private organizations, including the National Institute of Standards and Technology (NIST), this year’s conference is expected to attract about 1,000 attendees and features 100 speakers; 60 representatives of local, state and federal agencies; and at least 75 exhibitors.
John Marburger III, director of the Office of Science and Technology Policy, Executive Office of the President, is one of this years’ keynote speakers.
Topics on the agenda include biometric-based government efforts and initiatves, including solutions for homeland security applications such as airport security, travel documents, visas and border control; prevention of ID theft; advanced biometric systems; biometric interoperability and performance; national and international standards development efforts and adoption of biometric standards; research and evaluation of biometric technologies, commercial applications, user requirements and technological issues; and security of biometrics in government and commercial implementations and initiatives.
August Meeting to Address Chemical Regulation Issues
The challenge of balancing the social and economic impacts of chemical controls and regulations will be the subject of a two-day meeting next month in Baltimore, Md. “Action and Reaction: Developing a sustainable approach to emerging chemical issues,” sponsored by the American National Standards Institute (ANSI), will bring together representatives from industry, government, the scientific and technical communities, and other affected groups on August 9 and 10, 2007, to discuss recent developments in chemicals regulation and the impacts on chemical security, waste management, energy efficiencies, and other environmental and production issues.
Manufacturers are dealing with an increasing number of regulations covering every aspect of the industry, from production to end-of-use disposal and recycling. Most recently, a new European Union regulation on Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) went into effect on June 1. The ANSI meeting will include a keynote address by John Marburger, director of the President’s Office of Science and Technology Policy; panel discussions and breakout sessions to identify existing programs, initiatives, and applicable standards and compliance programs; and produce a conference report that will outline resources and recommendations for coordinated strategies and action plans. The National Institute of Standards and Technology (NIST) is one of several co-sponsors of the meeting.
Conference Seeks Strategy on Bio, IT Standards for Health Care
The National Institute of Standards and Technology (NIST), in partnership with the Biotechnology Council and the Commerce Department’s Technology Administration, is sponsoring a one-day conference on the potential of bio and information technologies to improve healthcare delivery over the next 25 years. The role of measurements and standards in enabling broad-based advances in the use of biomeasurements, computational analysis and information technology in the nation’s healthcare system will be discussed. The conference on Economic Strategy for Health Care through Bio and Information Standards and Technologies will be held in Gaithersburg, Md., on Sept. 25, 2007.
Healthcare spending accounts for nearly a fifth of the U.S. economy—the costs are expected to exceed $4 trillion a year by 2015. Rapidly developing advances at the intersection of measurement science and biology and information technology have the potential to curb the escalation in healthcare costs by shifting resources from the diagnosis and treatment of disease to the more cost-effective strategy of preventing disease in the first place, and more effective delivery of health care when necessary. However, breakthroughs in both measurement technologies for bioinformatics and information standards to assure the interoperability of bioinformatics data across different healthcare industry information systems are needed to enable commercial acceptance of these technologies.
The NIST conference is designed to begin a dialogue to develop a strategic plan for the nation to address the growing need for new technologies to help avert the impending economic crisis in health care and to improve the quality of disease prevention and treatment. For more details, go to www.itl.nist.gov/Healthcare/conf/intro.htm
