World's Smallest Semiconductor Laser Heralds New Era In Optical Science

Researchers at the University of California, Berkeley, have reached a new milestone in laser physics by creating the world's smallest semiconductor laser, capable of generating visible light in a space smaller than a single protein molecule.

This breakthrough, described in an advanced online publication of the journal Nature on Sunday, Aug. 30, breaks new ground in the field of optics. The UC Berkeley team not only successfully squeezed light into such a tight space, but found a novel way to keep that light energy from dissipating as it moved along, thereby achieving laser action.

"This work shatters traditional notions of laser limits, and makes a major advance toward applications in the biomedical, communications and computing fields," says Xiang Zhang, professor of mechanical engineering and director of UC Berkeley's Nanoscale Science and Engineering Center, which is funded by the National Science Foundation (NSF), and head of the research team behind this work.

The achievement helps enable the development of such innovations as nanolasers that can probe, manipulate and characterize DNA molecules; optics-based telecommunications many times faster than current technology; and optical computing in which light replaces electronic circuitry with a corresponding leap in speed and processing power.

While it is traditionally accepted that an electromagnetic wave -- including laser light -- cannot be focused beyond the size of half its wavelength, research teams around the world have found a way to compress light down to dozens of nanometers by binding it to the electrons that oscillate collectively at the surface of metals. This interaction between light and oscillating electrons is known as surface plasmons.

Scientists have been racing to construct surface plasmon lasers that can sustain and utilize these tiny optical excitations. However, the resistance inherent in metals causes these surface plasmons to dissipate almost immediately after being generated, posing a critical challenge to achieving the buildup of the electromagnetic field necessary for lasing.

Zhang and his research team took a novel approach to stem the loss of light energy by pairing a cadmium sulfide nanowire -- 1,000 times thinner than a human hair -- with a silver surface separated by an insulating gap of only 5 nanometers, the size of a single protein molecule. In this structure, the gap region stores light within an area 20 times smaller than its wavelength. Because light energy is largely stored in this tiny non-metallic gap, loss is significantly diminished.

With the loss finally under control through this unique "hybrid" design, the researchers could then work on amplifying the light.

"When you are working at such small scales, you do not have much space to play around with," says Rupert Oulton, the research associate in Zhang's lab who first theorized this approach last year and the study's co-lead author. "In our design, the nanowire acts as both a confinement mechanism and an amplifier. It's pulling double duty."

Trapping and sustaining light in radically tight quarters creates such extreme conditions that the very interaction of light and matter is strongly altered, the study authors explained. An increase in the spontaneous emission rate of light is a telltale sign of this altered interaction; in this study, the researchers measured a six-fold increase in the spontaneous emission rate of light in a gap size of 5 nanometers.

Recently, researchers from Norfolk State University reported lasing action of gold spheres in a dye-filled, glasslike shell immersed in a solution. The dye coupled to the gold spheres could generate surface plasmons when exposed to light.

The UC Berkeley researchers used semiconductor materials and fabrication technologies that are commonly employed in modern electronics manufacturing. By engineering hybrid surface plasmons in the tiny gap between semiconductors and metals, they were able to sustain the strongly confined light long enough that its oscillations stabilized into the coherent state that is a key characteristic of a laser.

"What is particularly exciting about the plasmonic lasers we demonstrated here is that they are solid state and fully compatible with semiconductor manufacturing, so they can be electrically pumped and fully integrated at chip-scale," says Volker Sorger, a Ph.D. student in Zhang's lab and study co-lead author.

"Plasmon lasers represent an exciting class of coherent light sources capable of extremely small confinement," says Zhang. "This work can bridge the worlds of electronics and optics at truly molecular length scales."

Scientists hope to eventually shrink light down to the size of an electron's wavelength, which is about a nanometer, or one-billionth of a meter, so that the two can work together on equal footing.

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Even Healthy Lungs Labor At Acceptable Ozone Levels

Ozone exposure, even at levels deemed safe by current clean air standards, can have a significant and negative effect on lung function, according to researchers at the University of California Davis.

"The National Ambient Air Quality Standard (NAAQS) for ozone was recently revised to set lower limits for ozone concentrations. Our research indicates that the threshold for decrements in ozone-induced lung function in healthy young subjects is below this standard," says Edward Schelegle, of the University of California Davis. "Specifically, we found that 6.6 hours exposure to mean ozone concentrations as low as 70 parts per billion have a significant negative effect on lung function, even though the current NAAQS standards allow ozone concentrations to be up to 75 parts per billion (ppb) over an eight-hour period."

The results we published in the August 1 issue of the American Thoracic Society's American Journal of Respiratory and Critical Care Medicine.

To test whether mean ozone concentrations above and below the new standard induce lung function decrements and to further study the time-course of these decrements, Schelegle and colleagues recruited 31 healthy nonsmoking individuals to participate in 6.6-hour sessions during which they were exposed to ozone at 60, 70, 80 or 87 ppb or filtered air while undergoing six 50-minute bouts of moderate exercise followed by 10-minute breaks. A 35-minute lunch break separated the third and fourth bouts of exercise.

Lung function for each subject was assessed before, during and after exposure, and each individual answered a questionnaire evaluating their subjective symptoms. Of the four levels of ozone concentration tested, Dr. Schelegle and colleagues found significant decrements in both lung function and subjective respiratory symptoms at 70 ppb and above, beginning at 5.6 hours after exposure.

"These data tells us that even at levels currently below the air quality standard, healthy people may experience decreased lung function after just a few hours of moderate to light exercise such as bicycling or walking," says Schelegle. "While these changes were fully reversible within several hours, these findings highlight the need to study susceptible individuals, such as asthmatics, at similar ozone concentrations and durations of exposure. These studies are needed to better understand the acute rise in hospitalizations that often occur in conjunction with high-ozone periods."

The study also supports the previously reported smooth dose-response curve associated with ozone. That is, the higher the level of ozone, the greater the decrease in lung function. However, the healthy subjects in the study showed a marked individual variability in their responses to ozone, with a few exhibiting strong sensitivity to ozone concentrations. What causes some individuals to respond strongly while others do not is still unknown.

"Schelegle and colleagues do not, nor did they seek to explain the determinants of susceptibility in young, healthy adults," says James Brown, of the U.S. Environmental Protection Agency, in an accompanying editorial. "Only with continued research efforts will we be able to better characterize the susceptibility in some healthy individuals, to the effects of short-term ozone exposures."

Schelegle also notes the need for further research to further elucidate the precise mechanisms that determine an individual's ozone responsiveness in both healthy and susceptible populations. "Understanding how these mechanisms change with repeated daily exposures is critical, especially as ambient ozone levels are often elevated several days in a row," Schelegle says.

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China's Eye on the Internet

The "Great Firewall of China," used by the government of the People's Republic of China to block users from reaching content it finds objectionable, is actually a "panopticon" that encourages self-censorship through the perception that users are being watched, rather than a true firewall, according to researchers at University of California Davis and the University of New Mexico.

The researchers are developing an automated tool, called ConceptDoppler, to act as a weather report on changes in Internet censorship in China. ConceptDoppler uses mathematical techniques to cluster words by meaning and identify keywords that are likely to be blacklisted.

Many countries carry out some form of Internet censorship. Most rely on systems that block specific Web sites or Web addresses, says Earl Barr, a graduate student in computer science at UC Davis who is an author on the paper. China takes a different approach by filtering Web content for specific keywords and selectively blocking Web pages.

In 2006, a team at the University of Cambridge, England, discovered that when the Chinese system detects a banned word in data traveling across the network, it sends a series of three "reset" commands to both the source and the destination. These "resets" effectively break the connection. But they also allow researchers to test words and see which ones are censored.

Barr, along with Jed Crandall, a recent UC Davis graduate who is now an assistant professor of computer science at the School of Engineering, University of New Mexico; UC Davis graduate students Daniel Zinn and Michael Byrd; and independent researcher Rich East sent messages to Internet addresses within China containing a variety of different words that might be subject to censorship.

If China's censorship system were a true firewall, most blocking would take place at the border with the rest of the Internet, Barr said. But the researchers found that some messages passed through several routers before being blocked.

A firewall would also block all mentions of a banned word or phrase, but banned words reached their destinations on about 28 percent of the tested paths, Byrd said. Filtering was particularly erratic at times of heavy Internet use.

The words used to probe the Chinese Internet were not selected at random.

"If we simply bombarded the Great Firewall with random words, we would waste resources and time," Zinn says.

The researchers took the Chinese version of Wikipedia, extracted individual words and used a mathematical technique called latent semantic analysis to work out the relationships between different words. If one of the words was censored within China, they could look up which other closely related words are likely to be blocked as well.

Examples of words tested by the researchers and found to be banned included references to the Falun Gong movement and the protest movements of 1989; Nazi Germany and other historical events; and general concepts related to democracy and political protest.

"Imagine you want to remove the history of the Wounded Knee massacre from the Library of Congress," Crandall says. "You could remove 'Bury My Heart at Wounded Knee' and a few other selected books, or you could remove every book in the entire library that contains the word 'massacre.'"

By analogy, Chinese Internet censorship based on keyword filtering is the equivalent of the latter -- and indeed, the keyword "massacre" (in Chinese) is on the blacklist.

Because it filters ideas rather than specific Web sites, keyword filtering stops people from using proxy servers or "mirror" Web sites to evade censorship. But because it is not completely effective all the time, it probably acts partly by encouraging self-censorship, Barr says. When users within China see that certain words, ideas and concepts are blocked most of the time, they might assume that they should avoid those topics.

The original panopticon was a prison design developed by the English philosopher Jeremy Bentham in the 18th century. Bentham proposed that a central observer would be able to watch all the prisoners, while the prisoners would not know when they were being watched.

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University Launches Study on Prescription Drug Addiction Treatment

The University of California, San Francisco (UCSF) is launching a new study to evaluate treatments for addiction to prescription painkillers and has openings for patients to enroll.

This is the first large-scale study to assess whether addiction to opioid painkillers, such as Vicodin and OxyContin, can effectively be treated with drug treatments currently used for heroin addiction.

The study is part of a national effort involving 11 clinical research centers to evaluate such therapies. Known as the Prescription Opiate Addiction Treatment Study, or POATS, it is being led by the National Drug Abuse Treatment Clinical Trials Network, under the National Institute of Drug Abuse (NIDA). UCSF is the only study site in Northern California.

The research is in response to the growing national problem of prescription drug abuse that has resulted in higher emergency room admissions and potentially devastating impacts on millions of Americans and their families, according to Stephen Dominy, MD, director of the Division of Substance Abuse and Addiction Medicine at San Francisco General Hospital Medical Center, who is co-leading the UCSF portion of the study.

“The abuse of prescription opiates has become a very serious problem in our society, but until now, there have been no large-scale studies to evaluate how to treat those addictions,” Dominy said. “This study hopes to assess whether current opiate dependence therapies are effective, as well as the role of counseling in treatment outcomes.”

An estimated 2.2 million Americans aged 12 or older start using prescription pain relievers each year for non-medical uses, surpassing the number of new marijuana users (2.1 million), according to the 2005 National Survey on Drug Use and Health. In that survey, more than 6 million Americans reported using prescription drugs for non-medical uses in the previous month, which is more than the number abusing cocaine, heroin, hallucinogens and inhalants, combined.

Those users, however, seem to fit a very different profile from traditional patients in heroin dependence programs, according to Yong Song, co-principal investigator for the UCSF site study and an assistant clinical professor of psychiatry in the UCSF School of Medicine. These users tend to be younger, he said, with fewer other dependency issues, such as alcohol or cocaine, and often come from a middle-class background.

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Study of China One-Child Policy Reveals Complexity, Effectiveness

In what is being called the first systematic examination of China’s fertility policy and practice reveals that, despite government exemptions in rural areas, 63 percent of Chinese couples are strictly limited to one child. Furthermore, the policy has proven remarkably effective, with actual birth rates decreasing nearly to the mandated levels.

The study, which involved researchers in the United States and China, is the first to use data on fertility policy and population growth collected from 420 Chinese prefectures (districts comparable to U.S. counties).

“We want to clear up confusion about the one-child policy,” says Wang Feng, sociology professor at University of California Irvine and a lead author of the study. “Despite what some say, the policy has not been ‘relaxed’ over the years.”

Published in the current issue of the journal Population and Development Review, the study reveals the complexity of the one-child policy. For example, it details the kinds of exceptions within prefectures for couples who give birth to a girl first, and for parents who themselves come from a one-child family.

“The system of exemptions resembles the American tax code in its complexity,” Wang says. “But this does not change the fact that the one-child policy applies without exception to a significant majority of Chinese couples.”

China’s average mandated fertility rate, accounting for the variety of exceptions across the country, is 1.47 children per couple, Wang and his collaborators find, and their analysis of census data shows the actual fertility rate is about 1.5 children per couple.

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NASA Mission Launches to Study Geomagnetic Substorms

NASA says its THEMIS mission successfully launched Saturday, Feb. 17, at 6:01 p.m. EST from Pad 17-B at Cape Canaveral, Florida.

THEMIS stands for the Time History of Events and Macroscale Interactions during Substorms. It is NASA's first five-satellite mission launched aboard a single rocket. The spacecraft separated from the launch vehicle approximately 73 minutes after liftoff. By 8:07 p.m. EST, mission operators at the University of California, Berkeley, commanded and received signals from all five spacecraft, confirming nominal separation status, NASA says.

The mission will help resolve the mystery of what triggers geomagnetic substorms. Substorms are atmospheric events visible in the Northern Hemisphere as a sudden brightening of the Northern Lights, or aurora borealis. The findings from the mission may help protect commercial satellites and humans in space from the adverse effects of particle radiation, the space agency says.

THEMIS' satellite constellation will line up along the sun-Earth line, collect coordinated measurements, and observe substorms during the two-year mission. Data collected from the five identical probes will help pinpoint where and when substorms begin, a feat impossible with any previous single-satellite mission, NASA says.

"The THEMIS mission will make a breakthrough in our understanding of how Earth's magnetosphere stores and releases energy from the sun and also will demonstrate the tremendous potential that constellation missions have for space exploration," says Vassilis Angelopoulos, THEMIS principal investigator at the University of California, Berkeley. "THEMIS' unique alignments also will answer how the sun-Earth interaction is affected by Earth's bow shock, and how 'killer electrons' at Earth's radiation belts are accelerated."

The Mission Operations Center at the University of California, Berkeley, will monitor the health and status of the five satellites. Instrument scientists will turn on and characterize the instruments over the next 30 days. The center will then assign each spacecraft a target orbit within the THEMIS constellation based on its performance. Mission operators will direct the spacecraft to their final orbits in mid-September, NASA says.

During the mission the five THEMIS satellites will observe an estimated 30 substorms in process. At the same time, 20 ground observatories in Alaska and Canada will time the aurora and space currents. The relative timing between the five spacecraft and ground observations underneath them will help scientists determine the elusive substorm trigger mechanism, NASA says.

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