Members of a University of Arizona Eller College of Management team and a UA alumnus developed a prototype system to detect fake websites. When tested against other existing commercial systems, the team found that its system resulted in effective and more accurate detections of spoof sites – better than a human can.
The team's subsequent article, “Detecting Fake Websites: The Contribution of Statistical Learning Theory" was published last year in an issue of MIS Quarterly, or MISQ. A preeminent peer-reviewed journal in the field of management information systems, MISQ has since been named the article its top paper for 2010.
"Even to get into MISQ is very difficult, and this is probably the first technical paper to receive the Best Paper award," said Hsinchun Chen, the UA Artificial Intelligence Lab director, one of the paper's five authors.
MISQ will formally honor the researchers in Shanghai, China later this year during the International Conference on Information Systems.
"The topic of detecting fake websites and also our computational approach are both considered major contributions. This topic has great relevance to the industry, the society and the citizens in general," said Chen, also the McClelland Professor of Management Information Systems.
"This award is not something just for me, or my lab, but also for our department," he said, adding that the team's eventual goal is technology transfer.
UA alumnus Ahmed Abbasi, now a University of Virginia assistant professor of information technology, is lead author on the paper. Chen served as his dissertation adviser. Other co-authors are UA Eller College's department of management information systems faculty members Zhu Zhang and Jay F. Nunamaker Jr.; and David Zimbra, a doctoral student in the Artificial Intelligence Lab.
For the research, the team used the prototype and several other detection systems to evaluate the authenticity of 900 websites.
It is easy to pick up on a site's authenticity by checking whether the URL contains "http" when it should read "https," when it was last updated, if a security key is missing or if images appear strangely pixelated.
The team found that its system – founded on statistical learning technology, which evaluates a large accumulation of data – was more apt to detect imitation sites and those that were entirely concocted, said Abbasi, who earned his doctoral degree in management information systems from the UA in 2008.
The major difference between the authors’ prototype and the other systems? Their system relied on a tremendously rich set of fraud cues.
The team developed five categories with thousands of cues, finding that the best results were attained when utilizing thousands of highly visible and also deeply embedded cues, such as placement, URL length, the number of links, characters types on the site and how thorough the site's "frequently asked questions" section is detailed, among other features.
The project's origins were born out of the Artificial Intelligence Lab, where Abbasi developed the mathematical formula the team eventually used while working as a project lead and research associate. He continued the work after having taken a faculty position at the University of Wisconsin-Milwaukee.
"It creates a greater awareness for a problem that has been around for a while yet still remains an issue as we increasingly move to the Internet for everything – online banking, online health initiatives and medical information," Abbasi said.
Given the pervasive nature of online phishing scams, being able to readily and frequently detect a site's validity is crucial, Abbasi said, also noting research that indicates people are less than 60 percent accurate in detecting fake sites, and other security issues.
"The problem we're looking at is quite big. Fake websites constitute much of the Internet fraud's multi-billion dollar industry, and that is monetary loss…we can’t even quantify the social ramifications," Abbasi said. "That's the whole motivation. It is so profitable for fraudsters, and it is slipping through the cracks."
Today, Chen and more than one dozen of his collaborators are continuing to investigate fake sites. Meanwhile, Abbasi is undertaking an investigation of peoples' abilities to detect fake sites through a grant funded by the National Science Foundation.
Today, Chen and more than one dozen of his collaborators are continuing to investigate fake sites. Meanwhile, Abbasi is undertaking an investigation of users and peoples' abilities to detect fake sites.
Abbasi said developing better detection systems requires improved statistical learning technology that utilize larger quantities of cues. It also is important to dismiss long-held perceptions about how fake sites might and should appear.
"The idea of protecting from the front level has been around for a while," Abbasi said, adding that companies have begun to employ software that better detects fake sites. "But we are not where we need to be, and there is a lot of potential in future development."
An amazing study authored by professors D. Mark Anderson (University of Montana) and Daniel Rees (University of Colorado) shows that traffic deaths have been reduced in states where medical marijuana is legalized.
The study notes that this is equal to the effect raising the drinking age to 21 had on reducing traffic fatalities.
One key factor is the reduction in alcohol consumption. The study finds that there is a direct correlation between the use of marijuana and a reduction in beer sales, especially in the younger folks aged 20-29.
A drop in beer sales supports the theory that marijuana can act as a substitute for liquor.
The study also finds that marijuana has the inverse effect that alcohol does on drivers. Drivers under the influence of alcohol tend to make rash decisions and risky moves, whereas those under the influence of marijuana tend to slow down, make safer choices, and increase following distances.
"Brain cap" technology being developed at the University of Maryland allows users to turn their thoughts into motion. Associate Professor of Kinesiology José 'Pepe' L. Contreras-Vidal and his team have created a non-invasive, sensor-lined cap with neural interface software that soon could be used to control computers, robotic prosthetic limbs, motorized wheelchairs and even digital avatars.
"We are on track to develop, test and make available to the public- within the next few years -- a safe, reliable, noninvasive brain computer interface that can bring life-changing technology to millions of people whose ability to move has been diminished due to paralysis, stroke or other injury or illness," said Contreras-Vidal of the university's School of Public Health.
The potential and rapid progression of the UMD brain cap technology can be seen in a host of recent developments, including a just published study in the Journal of Neurophysiology, new grants from the National Science Foundation (NSF) and National Institutes of Health, and a growing list of partners that includes the University of Maryland School of Medicine, the Veterans Affairs Maryland Health Care System, the Johns Hopkins University Applied Physics Laboratory, Rice University and Walter Reed Army Medical Center's Integrated Department of Orthopaedics & Rehabilitation.
"We are doing something that few previously thought was possible," said Contreras-Vidal, who is also an affiliate professor in Maryland's Fischell Department of Bioengineering and the university's Neuroscience and Cognitive Science Program. "We use EEG [electroencephalography] to non-invasively read brain waves and translate them into movement commands for computers and other devices.
Peer Reviewed
Contreras-Vidal and his team have published three major papers on their technology over the past 18 months, the latest a just released study in the Journal of Neurophysiology in which they successfully used EEG brain signals to reconstruct the complex 3-D movements of the ankle, knee and hip joints during human treadmill walking. In two earlier studies they showed (1) similar results for 3-D hand movement and (2) that subjects wearing the brain cap could control a computer cursor with their thoughts.
Alessandro Presacco, a second-year doctoral student in Contreras-Vidal's Neural Engineering and Smart Prosthetics Lab, Contreras-Vidal and co-authors write that their Journal of Neurophysiology study indicated "that EEG signals can be used to study the cortical dynamics of walking and to develop brain-machine interfaces aimed at restoring human gait function."
There are other brain computer interface technologies under development, but Contreras-Vidal notes that these competing technologies are either very invasive, requiring electrodes to be implanted directly in the brain, or, if noninvasive, require much more training to use than does UMD's EEG-based, brain cap technology.
Partnering to Help Sufferers of Injury and Stroke
Contreras-Vidal and his team are collaborating on a rapidly growing cadre projects with researchers at other institutions to develop thought-controlled robotic prosthetics that can assist victims of injury and stroke. Their latest partnership is supported by a new $1.2 million NSF grant. Under this grant, Contreras-Vidal's Maryland team is embarking on a four-year project with researchers at Rice University, the University of Michigan and Drexel University to design a prosthetic arm that amputees can control directly with their brains, and which will allow users to feel what their robotic arm touches.
"There's nothing fictional about this," said Rice University co-principal investigator Marcia O'Malley, an associate professor of mechanical engineering. "The investigators on this grant have already demonstrated that much of this is possible. What remains is to bring all of it -- non-invasive neural decoding, direct brain control and [touch] sensory feedback -- together into one device."
In a NIH-supported project underway, Contreras-Vidal and his colleagues are pairing their brain cap's EEG-based technology with a DARPA-funded next-generation robotic arm designed by researchers at the Johns Hopkins Applied Physics Laboratory to function like a normal limb. And the UMD team is developing a new collaboration with the New Zealand's start-up Rexbionics, the developer of a powered lower-limb exoskeleton called Rex that could be used to restore gait after spinal cord injury.
Two of the earliest partnerships formed by Contreras-Vidal and his team are with the University of Maryland School of Medicine in Baltimore and the Veterans Affairs Medical Center in Baltimore. A particular focus of this research is the use of the brain cap technology to help stroke victims whose brain injuries affect their motor-sensory control. Originally funded by a seed grant from the University of Maryland, College Park and the University of Maryland, Baltimore, the work now also is supported by a VA merit grant (anklebot BMI) and an NIH grant (Stroke).
"There is a big push in brain science to understand what exercise does in terms of motor learning or motor retraining of the human brain," says Larry Forrester, an associate professor of physical therapy and rehabilitation science at the University of Maryland School of Medicine.
For the more than a year, Forrester and the UMD team have tracked the neural activity of people on a treadmill doing precise tasks like stepping over dotted lines. The researchers are matching specific brain activity recorded in real time with exact lower-limb movements.
This data could help stroke victims in several ways, Forrester says. One is a prosthetic device, called an "anklebot," or ankle robot, that stores data from a normal human gait and assists partially paralyzed people. People who are less mobile commonly suffer from other health issues such as obesity, diabetes or cardiovascular problems, Forrester says, "so we want to get [stroke survivors] up and moving by whatever means possible."
The second use of the EEG data in stroke victims is more complex, yet offers exciting possibilities. "By decoding the motion of a normal gait," Contreras-Vidal says, "we can then try and teach stroke victims to think in certain ways and match their own EEG signals with the normal signals." This could "retrain" healthy areas of the brain in what is known as neuroplasticity.
One potential method for retraining comes from one of the Maryland research team's newest members, Steve Graff, a first-year bioengineering doctoral student. He envisions a virtual reality game that matches real EEG data with on-screen characters. "It gives us a way to train someone to think the right thoughts to generate movement from digital avatars. If they can do that, then they can generate thoughts to move a device," says Graff, who brings a unique personal perspective to the work. He has congenital muscular dystrophy and uses a motorized wheelchair. The advances he's working on could allow him to use both hands -- to put on a jacket, dial his cell phone or throw a football while operating his chair with his mind.
No Surgery Required
During the past two decades a great deal of progress has been made in the study of direct brain to computer interfaces, most of it through studies using monkeys with electrodes implanted in their brains. However, for use in humans such an invasive approach poses many problems, not the least of which is that most people don't' want holes in their heads and wires attached to their brains. "EEG monitoring of the brain, which has a long, safe history for other applications, has been largely ignored by those working on brain-machine interfaces, because it was thought that the human skull blocked too much of the detailed information on brain activity needed to read thoughts about movement and turn those readings into movement commands for multi-functional high-degree of freedom prosthetics," said Contreras-Vidal. He is among the few who have used EEG, MEG or other sensing technologies to develop non-invasive neural interfaces, and the only one to have demonstrated decoding results comparable to those achieved by researchers using implanted electrodes.
A paper Contreras-Vidal and colleagues published in the Journal of Neuroscience in March 2010 showed the feasibility of Maryland's EEG-based technology to infer multidimensional natural movement from noninvasive measurements of brain activity. In their two latest studies, Contreras-Vidal and his team have further advanced the development of their EEG brain interface technology, and provided powerful new evidence that it can yield brain computer interface results as good as or better than those from invasive studies, while also requiring minimal training to use.
In a paper published in April in the Journal of Neural Engineering, the Maryland team demonstrated that people wearing the EEG brain cap, could after minimal training control a computer cursor with their thoughts and achieve performance levels comparable to those by subjects using invasive implanted electrode brain computer interface systems. Contreras-Vidal and his co-authors write that this study also shows that compared to studies of other noninvasive brain control interface systems, training time with their system was substantially shorter, requiring only a single 40-minute session.
The study, conducted by the University of Glasgow in collaboration with the Glasgow Centre for Population Health, compared the length of telomeres in blood samples taken from 382 Glaswegians from the most and least deprived parts of the city. Telomeres, the tails on the ends of chromosomes, shorten throughout a person’s life and can be used as a measure of the ageing process.
Over a 10-year period, telomeres shortened by 7.7% in people whose household incomes were less than £25,000, but only 0.6% in people with greater incomes. In those living in rented accommodation, telomere length was reduced by 8.7% compared to 2.2% in those who owned their homes. The telomeres of people with the poorest diets were shortened by 7.7%, compared to 1.8% in those with a better diet.
It is hoped that the findings will help to create a test which can be used for faster feedback on the effects of public health improvement measures. Currently, these effects can take decades to become apparent.
However, due to natural variation in telomere length from person to person, the test is only effective at a population level, and will not provide useful information on how long an individual can expect to live.
Dr. Paul Shiels of the University of Glasgow’s Institute of Cancer Sciences, who led this aspect of the research, said: “Glasgow’s population has one of the most extreme socioeconomic gradients in the world, which makes it an ideal place to conduct a study such as this.
“This study is a first for the city in that it provides a link between how adverse social conditions can influence the biology of ageing and hence disease. What we’ve shown is that social status and deprivation play a major part in how quickly people age and develop disease.
“Eating poorly and earning less than average is likely to increase the rate you age, and can lead to increased inflammation and risk for cardiovascular disease, which is endemic in the city.”
The results are published in the journal PLoS one by the Glasgow Centre for Public Health, a consortium of health and local authorities, the Scottish Government and the University of Glasgow.
The way things currently stand in the field of medicine, doctors often have to try out a number of treatments on any one patient, before (hopefully) finding one that works. This wastes both time and medications, and potentially endangers the patients, as they could have negative reactions to some drugs. In the future, however, all that experimenting may not be necessary. The pan-European IT Future of Medicine (ITFoM) project, a consortium of over 25 member organizations, is currently developing a system in which every person would have a computer model of themselves, that incorporated their own genome. Doctors could then run simulations with that model, to see how various courses of treatment would work on the actual person.
Needless to say, technology needs to advance before it becomes relatively easy to map individuals' genomes. That's why ITFoM is currently vying for EUR 1 billion (US$1.5 billion) in funding, from the European Future and Emerging Technologies flagship scheme. It has already received EUR 1.5 million (US$2.2 million) in preliminary funding. Once the 10-year project builds momentum and more organizations join, ITFoM's organizers are predicting that it could become one of the largest collaborative endeavors since the Apollo space program.
Besides genetic data, each person's computer model would incorporate physiological information such as allergies, past and current health issues, and congenital defects - some of the same things that are currently part of their health records. Not only would this allow physicians to virtually test different drugs on specific patients, but they could also asses how changes in things such as diet and exercise might affect them.
Great strides will have to be made in areas such as high-speed data acquisition and evaluation, dynamic storage and processing of that data into mathematical models, and the development of systems that can learn, predict and inform. The implications, however, could be huge.
"The greatest opportunities to improve outcomes in medicine seem likely to come from personalized medicine, the biological sciences are providing the insights required to support informed personalization, and advanced computational techniques are essential for making sense of the data that informs decision making," said Professor Norman Paton, of ITFoM member the University of Manchester. "This is a fantastic opportunity to bring together advances from these three rapidly developing areas to bring about a paradigm shift in medical practice."
"At the heart of this technology is a new generation of high-brightness light-emitting diodes," says Harald Haas from the University of Edinburgh, UK. "Very simply, if the LED is on, you transmit a digital 1, if it's off you transmit a 0," Haas says. "They can be switched on and off very quickly, which gives nice opportunities for transmitting data."
It is possible to encode data in the light by varying the rate at which the LEDs flicker on and off to give different strings of 1s and 0s. The LED intensity is modulated so rapidly that human eyes cannot notice, so the output appears constant.
More sophisticated techniques could dramatically increase VLC data rates. Teams at the University of Oxford and the University of Edinburgh are focusing on parallel data transmission using arrays of LEDs, where each LED transmits a different data stream. Other groups are using mixtures of red, green and blue LEDs to alter the light's frequency, with each frequency encoding a different data channel.
Li-Fi, as it has been dubbed, has already achieved blisteringly high speeds in the lab. Researchers at the Heinrich Hertz Institute in Berlin, Germany, have reached data rates of over 500 megabytes per second using a standard white-light LED. Haas has set up a spin-off firm to sell a consumer VLC transmitter that is due for launch next year. It is capable of transmitting data at 100 MB/s - faster than most UK broadband connections.
Once established, VLC could solve some major communication problems. In 2009, the US Federal Communications Commission warned of a looming spectrum crisis: because our mobile devices are so data-hungry we will soon run out of radio-frequency bandwidth. Li-Fi could free up bandwidth, especially as much of the infrastructure is already in place.
"There are around 14 billion light bulbs worldwide, they just need to be replaced with LED ones that transmit data," says Haas. "We reckon VLC is a factor of ten cheaper than Wi-Fi." Because it uses light rather than radio-frequency signals, VLC could be used safely in aircraft, integrated into medical devices and hospitals where Wi-Fi is banned, or even underwater, where Wi-Fi doesn't work at all.
"The time is right for VLC, I strongly believe that," says Haas, who presented his work at TED Global in Edinburgh last week.
But some sound a cautious note about VLC's prospects. It only works in direct line of sight, for example, although this also makes it harder to intercept than Wi-Fi. "There has been a lot of early hype, and there are some very good applications," says Mark Leeson from the University of Warwick, UK. "But I'm doubtful it's a panacea. This isn't technology without a point, but I don't think it sweeps all before it, either."
The team just unveiled a new photovoltaic energy conversion system that can be powered by heat, the sun’s rays, a hydrocarbon fuel, or a decaying radioisotope. The button-sized power generator that can also run three times longer than a lithium-ion battery of the same weight.
The science behind the device is not necessarily groundbreaking, as engineers have long used the surface of a material to convert heat into precisely tuned wavelengths of light. However MIT’s method to convert light and heat into electricity is much more efficient than previous versions.
Described in the journal Physical Review A, MIT’s breakthrough was enabled by a material with billions of nanoscale pits etched on its surface. When this pitted material absorbs heat, it radiates energy at precisely chosen wavelengths depending on the size of the pits. It is hoped that the technology may one day be used to generate power for spacecraft on long term missions where sunlight may not be available.
“Being able to convert heat from various sources into electricity without moving parts would bring huge benefits,” says Ivan Celanovic, research engineer in MIT’s Institute for Soldier Nanotechnologies (ISN), “especially if we could do it efficiently, relatively inexpensively and on a small scale.” Celanovic went on to say that he believes his team could triple the efficiency of their prototype, adding that “It’s a neat example of how fundamental research in materials can result in new performance that enables a whole spectrum of applications for efficient energy conversion.”
Considering that space firms are looking for new ways to power spacecraft efficiently now that the shuttle fleet has been retired, we imagine NASA will be among the many companies interested in this technology.
They are a daily essential for millions of Britons hoping to ward off ill-health. But despite the millions of pounds spent on vitamin pills, they do nothing for our health, according to a major study.
Researchers spent more than six years following 8,000 people and found that those taking supplements were just as likely to have developed cancer or heart disease as those who took an identical-looking dummy pill.
And when they were questioned on how healthy they felt, there was hardly any difference between the two groups.
Experts said the study – one of the most extensive carried out into vitamin pills – suggested that millions of consumers may be wasting their money on supplements. Many users fall into the category of the ‘worried well’ – healthy adults who believe the pills will insure them against deadly illnesses – according to Catherine Collins, chief dietician at St George’s Hospital in London.
She said: ‘It’s the worried well who are taking these pills to try and protect themselves against Alzheimer’s disease, heart attacks and strokes.
‘But they are wasting their money. This was a large study following people up for a long period of time assessing everything from their mobility and blood pressure to whether they were happy or felt pain.’
Multi-vitamin supplements have become increasingly popular as a quick and easy way of topping up the body’s nutrient levels. But a series of studies have indicated that, for some people, they could actually be harmful.
Two studies published last year suggested supplements could raise the risk of cancer.
One found pills containing vitamin E, ascorbic acid, beta-carotene, selenium and zinc increased the risk of malignant melanoma, the deadliest form of skin cancer, four-fold.
The other discovered women on a daily multi-vitamin pill increased their risk of breast cancer by up to 20 per cent.
While the evidence that vitamins can do harm is still limited, the latest study seems to confirm that many people are at the very least taking them unnecessarily.
A team of French researchers, led by experts at Nancy University, tracked 8,112 volunteers who took either a placebo capsule, or one containing vitamin C, vitamin E, beta-carotene, selenium and zinc, every day for just over six years.
They assessed the state of their health at the beginning and end of the trial, taking a quality of life survey designed to measure everything from mobility and pain to vitality and mental health.
When researchers analysed how many in each group had gone on to develop serious illnesses over the years, they found little difference.
In the supplement group, 30.5 per cent of patients had suffered a major health ‘event’, such as cancer or heart disease.
In the placebo group, the rate was 30.4 per cent.
There were 120 cases of cancer in those taking vitamins, compared to 139 in the placebo group, and 65 heart disease cases, against 57 among the dummy pill users.
In a report on their findings, published in the International Journal of Epidemiology, the researchers said: ‘The perception that supplementation improves general well-being is not supported by this trial.’
Miss Collins said the results of the study ‘reinforce the idea that if you’re worried about your health and start taking multi-vitamins, you will still be worried about it six years later’.
But the Health Supplements Information Service, which is funded by supplements manufacturers, said the finding that vitamins had no impact on how people perceived their health was ‘to be expected’.
Spokeswoman Dr Carrie Ruxton said: ‘The role of vitamin supplements is to prevent deficiencies and make sure people are receiving their recommended levels.
‘They won’t have a measurable impact on how you feel on a day-to-day basis but what they are doing is topping up your recommended levels to the right amount. They are not meant to be a magic bullet.’
Penn Fraser Jillette (born March 5, 1955) is an American magician, comedian, illusionist, juggler, bassist and a best-selling author known for his work with fellow illusionist Teller in the team Penn & Teller, and advocacy of atheism, libertarian philosophy, free-market economics, and scientific skepticism.
Jillette was born in Greenfield, Massachusetts. His mother, Valda R. Jillette (née Parks) (November 8, 1909—January 1, 2000), was a secretary, and his father, Samuel H. Jillette (March 14, 1912—February 14, 1999), worked at Greenfield's Franklin County Jail. Jillette became disenchanted with traditional illusionist acts that presented the craft as authentic magic, such as The Amazing Kreskin on The Tonight Show Starring Johnny Carson. At age eighteen, he saw a show by illusionist James Randi, and became enamored of his approach to magic that openly acknowledged deception as entertainment rather than a mysterious supernatural power. Jillette regularly acknowledges Randi as the one person on the planet he loves the most besides members of his family.
Jillette worked with high school classmate Michael Moschen in developing and performing a juggling act during the years immediately following their 1973 graduation. In 1974, Jillette graduated from Ringling Brothers and Barnum & Bailey Clown College. That same year, he was introduced to Teller by Weir Chrisimer, a mutual friend. The three then formed a three-person act called Asparagus Valley Cultural Society which played in Amherst, Massachusetts and San Francisco, California. In 1981, he and Teller teamed up as Penn & Teller, and went on to do a successful on- and Off Broadway show called "Penn & Teller" that toured nationally.
“We’ve discovered a way to change the three-dimensional structure of a well-established semiconductor material to enable new optical properties while maintaining its very attractive electrical properties,” said Paul Braun, a professor of materials science and engineering and of chemistry who led the research effort.
The team published its advance in the journal Nature Materials.
Photonic crystals are materials that can control or manipulate light in unexpected ways thanks to their unique physical structures. Photonic crystals can induce unusual phenomena and affect photon behavior in ways that traditional optical materials and devices can’t. They are popular materials of study for applications in lasers, solar energy, LEDs, metamaterials and more.
Using an epitaxial approach, researchers developed a 3-D photonic crystal LED, the first such optoelectronic device. | Graphic by Eric Nelson
However, previous attempts at making 3-D photonic crystals have resulted in devices that are only optically active – that is, they can direct light – but not electronically active, so they can’t turn electricity to light or vice versa.
To create a 3-D photonic crystal that is both electronically and optically active, the researchers started with a template of tiny spheres packed together. Then, they deposit gallium arsenide (GaAs), a widely used semiconductor, through the template, filling in the gaps between the spheres.
The GaAs grows as a single crystal from the bottom up, a process called epitaxy. Epitaxy is common in industry to create flat, two-dimensional films of single-crystal semiconductors, but Braun’s group developed a way to apply it to an intricate three-dimensional structure.
“The key discovery here was that we grew single-crystal semiconductor through this complex template,” said Braun, who also is affiliated with the Beckman Institute for Advanced Science and Technology and with the Frederick Seitz Materials Research Laboratory at Illinois. “Gallium arsenide wants to grow as a film on the substrate from the bottom up, but it runs into the template and goes around it. It’s almost as though the template is filling up with water. As long as you keep growing GaAs, it keeps filling the template from the bottom up until you reach the top surface.”
The Illinois team’s photonic crystal has both properties.
“With our approach to fabricating photonic crystals, there’s a lot of potential to optimize electronic and optical properties simultaneously,” said Erik Nelson, a former graduate student in Braun’s lab who now is a postdoctoral researcher at Harvard University. “It gives you the opportunity to control light in ways that are very unique –to control the way it’s emitted and absorbed or how it propagates.”
To create a 3-D photonic crystal that is both electronically and optically active, the researchers started with a template of tiny spheres packed together. Then, they deposit gallium arsenide (GaAs), a widely used semiconductor, through the template, filling in the gaps between the spheres.
The GaAs grows as a single crystal from the bottom up, a process called epitaxy. Epitaxy is common in industry to create flat, two-dimensional films of single-crystal semiconductors, but Braun’s group developed a way to apply it to an intricate three-dimensional structure.
“The key discovery here was that we grew single-crystal semiconductor through this complex template,” said Braun, who also is affiliated with the Beckman Institute for Advanced Science and Technology and with the Frederick Seitz Materials Research Laboratory at Illinois. “Gallium arsenide wants to grow as a film on the substrate from the bottom up, but it runs into the template and goes around it. It’s almost as though the template is filling up with water. As long as you keep growing GaAs, it keeps filling the template from the bottom up until you reach the top surface.”
The epitaxial approach eliminates many of the defects introduced by top-down fabrication methods, a popular pathway for creating 3-D photonic structures. Another advantage is the ease of creating layered heterostructures. For example, a quantum well layer could be introduced into the photonic crystal by partially filling the template with GaAs and then briefly switching the vapor stream to another material.
Once the template is full, the researchers remove the spheres, leaving a complex, porous 3-D structure of single-crystal semiconductor. Then they coat the entire structure with a very thin layer of a semiconductor with a wider bandgap to improve performance and prevent surface recombination.
To test their technique, the group built a 3-D photonic crystal LED – the first such working device.
Now, Braun’s group is working to optimize the structure for specific applications. The LED demonstrates that the concept produces functional devices, but by tweaking the structure or using other semiconductor materials, researchers can improve solar collection or target specific wavelengths for metamaterials applications or low-threshold lasers.
“From this point on, it’s a matter of changing the device geometry to achieve whatever properties you want,” Nelson said. “It really opens up a whole new area of research into extremely efficient or novel energy devices.”
The U.S. Department of Energy and the Army Research Office supported this work. Other Illinois faculty involved in the project are electrical and computer engineering professors James Coleman and Xiuling Li, and materials science and engineering professor John Rogers.
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