Researchers have created a new “plasmonic oxide material” that could make possible devices for optical communications that are at least 10 times faster than conventional technologies.
In optical communications, laser pulses are used to transmit information along fiber-optic cables for telephone service, the Internet and cable television.
Researchers at Purdue University have shown how an optical material made of aluminum-doped zinc oxide (AZO) is able to modulate – or change – how much light is reflected by 40 percent while requiring less power than other “all-optical” semiconductor devices.
“Low power is important because if you want to operate very fast – and we show the potential for up to a terahertz or more – then you need low energy dissipation,” said doctoral student Nathaniel Kinsey. “Otherwise, your material would heat up and melt when you start pushing it really fast. All-optical means that unlike conventional technologies we don’t use any electrical signals to control the system. Both the data stream and the control signals are optical pulses.”
Being able to modulate the amount of light reflected is necessary for potential industrial applications such as data transmission.
“We can engineer the film to provide either a decrease or an increase in reflection, whatever is needed for the particular application,” said Kinsey, working with a team of researchers led by Alexandra Boltasseva, an associate professor of electrical and computer engineering, and Vladimir M. Shalaev, scientific director of nanophotonics at Purdue’s Birck Nanotechnology Center and a distinguished professor of electrical and computer engineering. “You can use either an increase or a decrease in the reflection to encode data. It just depends on what you are trying to do. This change in the reflection also results in a change in the transmission.”
Findings were detailed in a research paper appearing in July in the journal Optica, published by the Optical Society of America.
The material has been shown to work in the near-infrared range of the spectrum, which is used in optical communications, and it is compatible with the complementary metal–oxide–semiconductor (CMOS) manufacturing process used to construct integrated circuits. Such a technology could bring devices that process high-speed optical communications.
The researchers have proposed creating an “all optical plasmonic modulator using CMOS-compatible materials,” or an optical transistor.
In electronics, silicon-based transistors are critical building blocks that switch power and amplify signals. An optical transistor could perform a similar role for light instead of electricity, bringing far faster systems than now possible.
The Optica paper, featured on the cover of the journal, was authored by Kinsey, graduate students Clayton DeVault and Jongbum Kim; visiting scholar Marcello Ferrera from Heriot-Watt University in Edinburgh, Scotland; Shalaev and Boltasseva.
Exposing the material to a pulsing laser light causes electrons to move from one energy level called the valence band to a higher energy level called the conduction band. As the electrons move to the conduction band they leave behind “holes” in the valance band, and eventually the electrons recombine with these holes.
The switching speed of transistors is limited by how fast it takes conventional semiconductors such as silicon to complete this cycle of light to be absorbed, excite electrons, produce holes and then recombine.
“So what we would like to do is drastically speed this up,” Kinsey said.
This cycle takes about 350 femtoseconds to complete in the new AZO films, which is roughly 5,000 times faster than crystalline silicon and so fleeting that light travels only about 100 microns, or roughly the thickness of a sheet of paper, in that time.
“We were surprised that it was this fast,” Kinsey said.
The increase in speed could translate into devices at least 10 times faster than conventional silicon-based electronics.
The AZO films are said to be “Epsilon-near-zero,” meaning the refractive index is near zero, a quality found normally in metals and new “metamaterials,” which contain features, patterns or elements that enable unprecedented control of light by harnessing clouds of electrons called surface plasmons. Unlike natural materials, metamaterials are able to reduce the index of refraction to less than one or less than zero. Refraction occurs as electromagnetic waves, including light, bend when passing from one material into another. Each material has its own refraction index, which describes how much light will bend in that particular material and defines how much the speed of light slows down while passing through a material.
The pulsing laser light changes the AZO’s index of refraction, which, in turn, modulates the amount of reflection and could make higher performance possible.
“If you are operating in the range where your refractive index is low then you can have an enhanced effect, so enhanced reflection change and enhanced transmission change,” he said.
The researchers “doped” zinc oxide with aluminum, meaning the zinc oxide is impregnated with aluminum atoms to alter the material’s optical properties. Doping the zinc oxide causes it to behave like a metal at certain wavelengths and like a dielectric at other wavelengths.
A new low-temperature fabrication process is critical to the material’s properties and for its CMOS compatibility.
“For industrial applications you can’t go to really high fabrication temperatures because that damages underlying material on the chip or device,” Kinsey said. “An interesting thing about these materials is that by changing factors like the processing temperature you can drastically change the properties of the films. They can be metallic or they can be very much dielectric.”
The AZO also makes it possible to “tune” the optical properties of metamaterials, an advance that could hasten their commercialization, Boltasseva said.
The ongoing research is based at Purdue’s Birck Nanotechnology Center and is funded by the Air Force Office of Scientific Research, a Marie Curie Outgoing International Fellowship, the National Science Foundation, and the Office of Naval Research.
Source: Purdue University
Published on 31st July 2015
Slavery was abolished more than 150 years ago, but its effects are still felt today in K-12 education in the South, according to a new Rice University study, “How the Legacy of Slavery and Racial Composition Shape Public School Enrollment in the American South.”
“Our results suggest that the legacy of slavery contributes to black-white education disparities through greater public-private school racial segregation,” said Heather O’Connell, co-author and postdoctoral fellow at Rice’s Kinder Institute for Urban Research.
Using regression analysis to explain differences in the degree of attendance disparities across most counties in the South, researchers found a correlation between historical geographic slave concentration and modern day K-12 school segregation. An increase in slave concentration is related to greater underrepresentation of white students in public schools.
Overall, the proportion of black students in a county who are enrolled in public schools is an average of 17 percent higher than white students. But that gap in public school attendance is even larger where slaves were more heavily concentrated, increasing by just over 1 percentage point with every 10 percentage-point increase in slave concentration.
Soon after slavery was abolished, the former slaves quickly organized schools, according to the study. However, white resistance was substantial. Several other separation tactics were employed along the way, but the construction of private schools was the most recent action taken to maintain a segregated school system.
“As blacks began to enter the local white public schools, private schools cropped up seemingly overnight,” the researchers said.
Private schools are important for explaining contemporary school segregation. The study found that having more private schools in a county is related to a greater underrepresentation of white students in public schools. But this relationship doesn’t explain why slavery still matters for public-private school segregation.
The researchers found the same is true when considering another important county characteristic — the relative size of the black population. Generally speaking, a larger concentration of black students is related to increased separation of white students. The findings of this study support this “white flight” argument, but add another dimension.
“Whites are increasingly not enrolled in public schools in counties with higher black concentrations and are instead increasingly enrolled in private schools,” the researchers said. But O’Connell said that the role of black population concentration plays out primarily in states where slavery was most strongly rooted.
The study found that the black population concentration relationship only holds in the original Confederate States, or Deep South: Alabama, Florida, Georgia, Louisiana, Mississippi, South Carolina and Texas.
Even with this added dimension, the researchers were unable to identify tangible county characteristics that explain why slave concentration from 1860 is related to contemporary school enrollment patterns. The relationship is complex, but the authors urge greater attention to how slavery relates to contemporary racial disparities.
“Understanding the role of our slavery history provides insight into the structural foundations supporting this segregation, which might be valuable to efforts to reverse dangerous trends in school resegregation that have been increasing across the South over the last few decades,” O’Connell said.
Source: Rice University
Published on 26th June 2015
In some patients, aggressive cancers can become resistant to chemotherapy and radiation treatments. In a paper published in the journal Nature Communications, University of California, San Diego School of Medicine researchers identified a pathway that causes the resistance and a new therapeutic drug that targets this pathway.
New technology developed by UC Berkeley bioengineers promises to make a workhorse lab tool cheaper, more portable and many times faster by accelerating the heating and cooling of genetic samples with the switch of a light.
Artist’s rendering of photonic PCR on a chip using light to rapidly heat and cool electrons at the surface of a thin film of gold. This method yields gene amplification results in mere minutes, and promises to transform point-of-care diagnostics in fields as diverse as medicine, food security and evolutionary biology. (Image courtesy of Luke Lee’s BioPOETS lab)
turbocharged thermal cycling, described in a paper published July 31 in the journal Light: Science & Application, greatly expands the clinical and research applications of the polymerase chain reaction (PCR) test, with results ready in minutes instead of an hour or more.
The PCR test, which amplifies a single copy of a DNA sequence to produce thousands to millions of copies, has become vital in genomics applications, ranging from cloning research to forensic analysis to paternity tests. PCR is used in the early diagnosis of hereditary and infectious diseases, and for analysis of ancient DNA samples of mummies and mammoths.
The huge impact of the PCR test in modern science was recognized in 1993 with a Nobel Prize in Chemistry for its inventors, Kary Mullis and Michael Smith.
Using light-emitting diodes, or LEDs, the UC Berkeley researchers were able to heat electrons at the interface of thin films of gold and a DNA solution. They clocked the speed of heating the solution at around 55 degrees Fahrenheit per second. The rate of cooling was equally impressive, coming in at about 43.9 degrees per second.
“PCR is powerful, and it is widely used in many fields, but existing PCR systems are relatively slow,” said study senior author Luke Lee, a professor of bioengineering. “It is usually done in a lab because the conventional heater used for this test requires a lot of power and is expensive. Because it takes an hour or longer to complete each test, it is not practical for use for point-of-care diagnostics. Our system can generate results within minutes.”
The slowdown in conventional PCR tests comes from the time it takes to heat and cool the DNA solution. The PCR test requires repeated temperature changes – an average of 30 thermal cycles at three different temperatures – to amplify the genetic sequence, a process that involves breaking up the double-stranded DNA and binding the single strand with a matching primer. With each heating-cooling cycle, the amount of the DNA sample is doubled.
To pick up the pace of this thermal cycling, Lee and his team of researchers took advantage of plasmonics, or the interaction between light and free electrons on a metal’s surface. When exposed to light, the free electrons get excited and begin to oscillate, generating heat. Once the light is off, the oscillations and the heating stop.
Gold, it turns out, is a popular metal for this plasmonic photothermal heating because it is so efficient at absorbing light. It has the added benefit of being inert to biological systems, so it can be used in biomedical applications.
For their experiments, the researchers used thin films of gold that were 120 nanometers thick, or about the width of a rabies virus. The gold was deposited onto a plastic chip with microfluidic wells to hold the PCR mixture with the DNA sample.
The light source was an array of off-the-shelf LEDs positioned beneath the PCR wells. The peak wavelength of the blue LED light was 450 nanometers, tuned to get the most efficient light-to-heat conversion.
The researchers were able to cycle from 131 degrees to 203 degrees Fahrenheit 30 times in less than five minutes.
They tested the ability of the photonic PCR system to amplify a sample of DNA, and found that the results compared well with conventional PCR tests.
“This photonic PCR system is fast, sensitive and low-cost,” said Lee, who is also co-director of the Berkeley Sensor and Actuator Center. “It can be integrated into an ultrafast genomic diagnostic chip, which we are developing for practical use in the field. Because this technology yields point-of-care results, we can use this in a wide range of settings, from rural Africa to a hospital ER.”
Source: University of California – Berkeley.
Published on 31st July 2015