New semiconductor nanowire laser technology could kill viruses, improve DVDs

A team led by a professor at the University of California, Riverside Bourns College of Engineering has made a discovery in semiconductor nanowire laser technology that could potentially do everything from kill viruses to increase storage capacity of DVDs.

Ultraviolet semiconductor diode lasers are widely used in data processing, information storage and biology. Their applications have been limited, however, by size, cost and power. The current generation of ultraviolet lasers is based on a material called gallium nitride, but Jianlin Liu, a professor of electrical engineering, and his colleagues have made a breakthrough in zinc oxide nanowire waveguide lasers, which can offer smaller sizes, lower costs, higher powers and shorter wavelengths.

Until now, zinc oxide nanowires couldn't be used in real world light emission applications because of the lack of p-type, or positive type, material needed by all semiconductors. Liu solved that problem by doping the zinc oxide nanowires with antimony, a metalloid element, to create the p-type material.

The p-type zinc oxide nanowires were connected with n-type, or negative type, zinc oxide material to form a device called p-n junction diode. Powered by a battery, highly directional laser light emits only from the ends of the nanowires.

"People in the zinc oxide research community throughout the world have been trying hard to achieve this for the past decade," Liu said. "This discovery is likely to stimulate the whole field to push the technology further."

Liu's findings have been published in the July issue of Nature Nanotechnology. Co-authors are: Sheng Chu, Guoping Wang, Jieying Kong, Lin Li and Jingjian Ren, all graduate students at UC Riverside; Weihang Zhou, a student at Fudan University in China; Leonid Chernyak, a professor of physics at the University of Central Florida; Yuqing Lin, a graduate student at the University of Central Florida; and Jianze Zhao, a visiting student from Dalian University of Technology in China.

The discovery could have a wide-range of impacts.

For information storage, the zinc oxide nanowire lasers could be used to read and process much denser data on storage media such as DVDs because the ultraviolet has shorter wavelength than other lights, such as red. For example, a DVD that would store two hours of music could store four or six hours using the new type of laser.

For biology and medical therapeutics, the ultra-small laser light beam from a nanowire laser can penetrate a living cell, or excite or change its function from a bad cell to a good cell. The light could also be used to purify drinking water.

For photonics, the ultraviolet light could provide superfast data processing and transmission. Reliable small ultraviolet semiconductor diode lasers may help develop ultraviolet wireless communication technology, which is potentially better than state-of-the-art infrared communication technologies used in various electronic information systems.

While Liu and the students in his laboratory have demonstrated the p-type doping of zinc oxide and electrically powered nanowire waveguide lasing in the ultraviolet range, he said more work still needs to be done with the stability and reliability of the p-type material.

Provided by University of California - Riverside (news : web)

Why 'event cloaks' could be the key to the ultimate bank heist

In this month's special issue of Physics World, which examines the science and applications of invisibility, Martin McCall and Paul Kinsler of Imperial College London describe a new type of invisibility cloak that does not just hide objects ? but events.

Using the ultimate bank heist as an example, McCall and Kinsler explain how a thief could, in principle, use an "event cloak" to steal money from a safe, without even the CCTV surveillance cameras being aware.

The burglar would somehow need to split all the light approaching the safe into two parts: "before" and "after", with the "before" part sped up and the "after" part slowed down.

This would create a brief period of darkness during which the burglar could enter the scene and steal the money, being careful to close the safe door before they leave.

With the safe-cracker gone, the process of speeding up and slowing down the light would then be reversed, leading to an apparently untouched scene once again.

Robbing a bank is, of course, only an example to illustrate the principle of what an event cloak could do. As McCall and Kinsler explain, a more likely application of a full-size event cloak would be to control the flow of signals in an optical routing system, where one may need to process simultaneous uninterrupted signals at the same time.

For these aspirations to become a reality, suitable materials need to be developed that can manipulate the light to speed it up or slow it down. The consensus seems to be that a set of parallel, artificially structured "metamaterial" layers would be needed, each containing an array of tiny metallic elements that can be controlled to dynamically adjust the speed of light passing through.

If a high-performance, macroscopic-size, fully functional space?time cloak could be developed, one potential "party trick" could be a Star Trek-type transporter, in which a person could appear to instantly relocate from one point to another.

Although no-one has yet tried to build a space?time cloak in the lab, McCall and Kinsler argue that "there is no obvious reason why such a cloak could not be achieved quite soon, perhaps even within a few years".

More information: Physics World: http://www.physicsworld.com/

Also in this month's special issue on invisibility.

• Invisibility is now a reality, but scientists are not satisfied and are still searching for the holy grail ? a cloak of invisibility that can hide macroscale objects from view at any angle, using unpolarized visible light. Wenshan Cai and Vladimir Shalaev map out the road ahead on this mission.
  
  
• The quest to achieve true invisibility has inspired ambitious goals beyond merely cloaking visible light. Ulf Leonhardt takes a light-hearted look at the top five possible spin-offs from invisibility science.
  
  
• Tales of invisible people and magical invisibility-bestowing objects have thrilled us for millennia. Sidney Perkowitz reveals how these myths and fantasies are now becoming a reality.

Provided by Institute of Physics (news : web)

Mantis shrimp eye could improve high-definition DVDs, holographic technology

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Image of a Peacock Mantis Shrimp. Credit: Jenny Huang

(PhysOrg.com) -- The eye of the peacock mantis shrimp has led an international team of researchers to develop a two-part waveplate that could improve CD, DVD, blu-ray and holographic technology, creating even higher definition and larger storage density.

Peacock mantis shrimp are one of only a few animal species that can see circularly polarized light -- like the light used to create 3-D movies. Some researchers believe the mantis shrimp's eyes are better over the entire visual spectrum than any man-made waveplates.

A waveplate is a transparent slab that can alter the polarization of light because it is birefringent -- exhibits double refraction. The mineral calcite, which is sometimes used as a waveplate, is birefringent. This print viewed through a calcite lens appears as doubled and slightly offset letters.

"We want to change the polarization without affecting the amount of light that gets through," said Akhlesh Lakhtakia, Charles Godfrey Binder Professor of Engineering Science and Mechanics, Penn State. " We want both transmittance and changing polarization to occur quite independent of frequency. In other words, we do not want to affect the color."

Waveplates restore polarization in devices that require only one polarization of light, but lose polarization within the process. They can also separate light into separate polarizations to carry specific information and can filter light for optical devices.

Typically, waveplates are made from minerals like quartz, calcite or birefringent polymers. In some cases, to create the range and transparency required, two different materials are stacked or joined, but this type of construction sometimes delaminates, coming apart at the seams.

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Schematic of periodically multilayered material showing vertical and slanted layers of nanorods.

A team of engineers from the National Taipei University of Technology and Lakhtakia developed a method to produce periodically multilayered materials, similar to the lens in the peacock mantis shrimp, that are suitable for waveplates in the visual light spectrum and cannot delaminate because they are manufactured as one piece. The researchers report their work in the current online issue of Nature Communications.

The research team's waveplate is made of two layers of nanorods that are structurally similar to those in the eye of the peacock mantis shrimp. The researchers begin with tantalum pentoxide, but deposit the two layers using different deposition processes creating a multilayered thin film. One method produces a layer of needle-like nanorods that are all parallel to each other and all slanted in the same direction. The second method produces nanorods that are also parallel to each other but are upright.

"The two separate layers are needed so that we can play off one against the other to achieve the desiredpolarization without significantly reducing transmittance over a broad range of frequencies," said Lakhtakia.

The waveplates consist of a layer of slanted nanorods sandwiched between two layers of upright nanorods. Multiple sandwiches are then stacked to make the required waveplate. Because the size of the nanorods is much less than the wavelengths of visible light, the waveplate is birefringent. Because of the two materials, the waveplate can polarize or repolarize light over the visual spectrum.

The researchers report that "the fabrication technique of the periodically multilayered structures is a workhorse technique in the thin-film industry, does not require expensive lithographyequipment and is compatible with ? technology commonplace in electronics and optoelectronics industries."

Provided by Pennsylvania State University (news : web)

Lasers could produce much sought-after band gaps in graphene

 

This scheme shows graphene illuminated by a laser. By interacting with electrons in graphene, the laser field can produce band gaps in the graphene's electronic structure. Image credit: Luis E. F. Foa Torres.

One of the most often-cited features of graphene ? the two-dimensional crystal lattice made of carbon ? is its unique electronic properties. Many of these electronic properties make graphene attractive as a material for carbon-based electronics. But also among these properties is that graphene is a gapless semiconductor. As physicist Luis Foa Torres explains, the fact that graphene does not have a band gap is its Achilles? heel, which makes it difficult to be incorporated into electronic devices.

?In semiconductors, there is an energy region called the band gap where there are no electronic states available,? Foa Torres, at the National University of Córdoba in Córdoba, Argentina, told PhysOrg.com. ?One says that the carrier density there is zero. If you have your device connected between two electrodes and it has no states available, then the electric current through it can be made very small. A zero-gap semiconductor, also called a gapless semiconductor, is a material where the density of electronic states vanishes at a single point. This is the case for graphene, where the pi and pi* bands touch at a single point, the so-called Dirac point. In practice, they behave as not having a gap at all.

?Lacking a band gap means that graphene cannot be ?switched-off,?? he explained. ?Having ?on? and ?off? currents may encode information such as the 1?s and 0?s necessary for computing and is crucial for active electronic devices such as switches and transistors. This is the reason why having no band gap is one of the main drawbacks hindering many applications of this outstanding material.?

In a new study, Foa Torres and his coauthors have addressed this problem. By analyzing the way that a laser field interacts with electrons in graphene, the researchers have predicted that shining a mid-infrared laser on graphene can produce band gaps in its electronic structure. Further, the researchers predict that the band gaps could be tuned by controlling the laser polarization. As Foa Torres explained, the key to how polarized light "opens up" band gaps in graphene involves electrons interacting with the laser field.

?Imagine an electron moving, say from left to right, into a region illuminated by the laser field,? he said. ?Then what happens is that the electron interacts with the radiation by absorbing or emitting photons. This interaction leads to the electron being reflected or backscattered, as it would have hit a wall: the band gap. In contrast with usual band gaps, this one is dynamically produced by the laser.?

By showing that a laser field could be used to tune the electronic structure of graphene, the study has both fundamental implications and technological applications.

?The interplay between the peculiar electronic structure of graphene and the laser may help to induce exotic states of matter such as topological insulators, materials that are insulators in the bulk but show robust conduction in the surface,? Foa Torres said. ?On the other hand, from an applied point of view, I consider that these laser-induced band gaps could open an avenue for a new breed of optoelectronic devices, devices that transduce optical into electrical signals.?

For Foa Torres and his coauthors, the next crucial step is experimental verification.

?Experimental verification of our findings is one of the main driving forces of our project,? he said. ?With the aim of paving the way for experimentalists to be able to verify them, we have performed a very fine tuning of parameters such as laser frequency, amplitude, etc. During the last months we received very valuable feedback from top-level experimental groups in the US and Spain that are interested in our proposal. As always, there are surely still some issues to solve before it turns into reality but one should go step by step. The door is now open, we are just entering a promising terra incognita.?

 

'Nanowire' measurements could improve computer memory

 

In this schematic image (top) and transmission electron micrograph, a silicon nanowire is shown surrounded by a stack of thin layers of material called dielectrics, which store electrical charge. NIST scientists determined the best arrangement for this dielectric stack for the optimal construction of silicon nanowire-based memory devices. Credit: Schematic Zhu, GMU. TEM Bonevich, NIST.

A recent study at the National Institute of Standards and Technology may have revealed the optimal characteristics for a new type of computer memory now under development. The work, performed in collaboration with researchers from George Mason University (GMU), aims to optimize nanowire-based charge-trapping memory devices, potentially illuminating the path to creating portable computers and cell phones that can operate for days between charging sessions.

The nascent technology is based on silicon formed into tiny wires, approximately 20 nanometers in diameter. These "nanowires" form the basis of memory that is non-volatile, holding its contents even while the power is off?just like the flash memory in USB thumb drives and many mp3 players. Such nanowire devices are being studied extensively as the possible basis for next-generation computer memory because they hold the promise to store information faster and at lower voltage.

Nanowire memory devices also hold an additional advantage over flash memory, which despite its uses is unsuitable for one of the most crucial memory banks in a computer: the local cache memory in the central processor.

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"Cache memory stores the information a microprocessor is using for the task immediately at hand," says NIST physicist Curt Richter. "It has to operate very quickly, and flash memory just isn't fast enough. If we can find a fast, non-volatile form of memory to replace what chips currently use as cache memory, computing devices could gain even more freedom from power outlets?and we think we've found the best way to help silicon nanowires do the job."

While the research team is by no means the only lab group in the world working on nanowires, they took advantage of NIST's talents at measurement to determine the best way to design charge-trapping memory devices based on nanowires, which must be surrounded by thin layers of material called dielectrics that store electrical charge. By using a combination of software modeling and electrical device characterization, the NIST and GMU team explored a wide range of structures for the dielectrics. Based on the understanding they gained, Richter says, an optimal device can be designed.

"These findings create a platform for experimenters around the world to further investigate the nanowire-based approach to high-performance non-volatile memory," says Qiliang Li, assistant professor of Electrical and Computer Engineering at GMU. "We are optimistic that nanowire-based memory is now closer to real application."

Graphene optical modulators could lead to ultrafast communications

 

This is a schematic illustration of the graphene-based optical modulator. A layer of graphene (black fishnet) is placed on top of a silicon waveguide (blue), which is used as an optical fiber to guide light. Electrical signals are sent in from the side of the graphene to alter the amount of photons the graphene absorbs. Credit: Graphic by Ming Liu, UC Berkeley

Scientists at the University of California, Berkeley, have demonstrated a new technology for graphene that could break the current speed limits in digital communications.

The team of researchers, led by UC Berkeley engineering professor Xiang Zhang, built a tiny optical device that uses graphene, a one-atom-thick layer of crystallized carbon, to switch light on and off. This switching ability is the fundamental characteristic of a network modulator, which controls the speed at which data packets are transmitted. The faster the data pulses are sent out, the greater the volume of information that can be sent. Graphene-based modulators could soon allow consumers to stream full-length, high-definition, 3-D movies onto a smartphone in a matter of seconds, the researchers said.

"This is the world's smallest optical modulator, and the modulator in data communications is the heart of speed control," said Zhang, who directs a National Science Foundation (NSF) Nanoscale Science and Engineering Center at UC Berkeley. "Graphene enables us to make modulators that are incredibly compact and that potentially perform at speeds up to ten times faster than current technology allows. This new technology will significantly enhance our capabilities in ultrafast optical communication and computing."

In this latest work, described in the May 8 advanced online publication of the journal Nature, researchers were able to tune the graphene electrically to absorb light in wavelengths used in data communication. This advance adds yet another advantage to graphene, which has gained a reputation as a wonder material since 2004 when it was first extracted from graphite, the same element in pencil lead. That achievement earned University of Manchester scientists Andre Geim and Konstantin Novoselov the Nobel Prize in Physics last year.

Zhang worked with fellow faculty member Feng Wang, an assistant professor of physics and head of the Ultrafast Nano-Optics Group at UC Berkeley. Both Zhang and Wang are faculty scientists at Lawrence Berkeley National Laboratory's Materials Science Division.

"The impact of this technology will be far-reaching," said Wang. "In addition to high-speed operations, graphene-based modulators could lead to unconventional applications due to graphene's flexibility and ease in integration with different kinds of materials. Graphene can also be used to modulate new frequency ranges, such as mid-infrared light, that are widely used in molecular sensing."

Graphene is the thinnest, strongest crystalline material yet known. It can be stretched like rubber, and it has the added benefit of being an excellent conductor of heat and electricity. This last quality of graphene makes it a particularly attractive material for electronics.

"Graphene is compatible with silicon technology and is very cheap to make," said Ming Liu, post-doctoral researcher in Zhang's lab and co-lead author of the study. "Researchers in Korea last year have already produced 30-inch sheets of it. Moreover, very little graphene is required for use as a modulator. The graphite in a pencil can provide enough graphene to fabricate 1 billion optical modulators."

It is the behavior of photons and electrons in graphene that first caught the attention of the UC Berkeley researchers.

Shown is a scanning electron microscope (SEM) image magnifying the key structures of the graphene-based optical modulator. (Colors were added to enhance the contrast). Gold (Au) and platinum (Pt) electrodes are used to apply electrical charges to the sheet of graphene, shown in blue, placed on top of the silicon (Si) waveguide, shown in red. The voltage can control the graphene's transparency, effectively turning the setup into an optical modulator that can turn light on and off. (Ming Liu image)

The researchers found that the energy of the electrons, referred to as its Fermi level, can be easily altered depending upon the voltage applied to the material. The graphene's Fermi level in turn determines if the light is absorbed or not.

When a sufficient negative voltage is applied, electrons are drawn out of the graphene and are no longer available to absorb photons. The light is "switched on" because the graphene becomes totally transparent as the photons pass through.

Graphene is also transparent at certain positive voltages because, in that situation, the electrons become packed so tightly that they cannot absorb the photons.

The researchers found a sweet spot in the middle where there is just enough voltage applied so the electrons can prevent the photons from passing, effectively switching the light "off."

"If graphene were a hallway, and electrons were people, you could say that, when the hall is empty, there's no one around to stop the photons," said Xiaobo Yin, co-lead author of the Nature paper and a research scientist in Zhang's lab. "In the other extreme, when the hall is too crowded, people can't move and are ineffective in blocking the photons. It's in between these two scenarios that the electrons are allowed to interact with and absorb the photons, and the graphene becomes opaque."

In their experiment, the researchers layered graphene on top of a silicon waveguide to fabricate optical modulators. The researchers were able to achieve a modulation speed of 1 gigahertz, but they noted that the speed could theoretically reach as high as 500 gigahertz for a single modulator.

While components based upon optics have many advantages over those that use electricity, including the ability to carry denser packets of data more quickly, attempts to create optical interconnects that fit neatly onto a computer chip have been hampered by the relatively large amount of space required in photonics.

Light waves are less agile in tight spaces than their electrical counterparts, the researchers noted, so photon-based applications have been primarily confined to large-scale devices, such as fiber optic lines.

"Electrons can easily make an L-shaped turn because the wavelengths in which they operate are small," said Zhang. "Light wavelengths are generally bigger, so they need more space to maneuver. It's like turning a long, stretch limo instead of a motorcycle around a corner. That's why optics require bulky mirrors to control their movements. Scaling down the optical device also makes it faster because the single atomic layer of graphene can significantly reduce the capacitance ? the ability to hold an electric charge ? which often hinders device speed."

Graphene-based modulators could overcome the space barrier of optical devices, the researchers said. They successfully shrunk a graphene-based optical modulator down to a relatively tiny 25 square microns, a size roughly 400 times smaller than a human hair. The footprint of a typical commercial modulator can be as large as a few square millimeters.

Even at such a small size, graphene packs a punch in bandwidth capability. Graphene can absorb a broad spectrum of light, ranging over thousands of nanometers from ultraviolet to infrared wavelengths. This allows graphene to carry more data than current state-of-the-art modulators, which operate at a bandwidth of up to 10 nanometers, the researchers said.

"Graphene-based modulators not only offer an increase in modulation speed, they can enable greater amounts of data packed into each pulse," said Zhang. "Instead of broadband, we will have 'extremeband.' What we see here and going forward with graphene-based modulators are tremendous improvements, not only in consumer electronics, but in any field that is now limited by data transmission speeds, including bioinformatics and weather forecasting. We hope to see industrial applications of this new device in the next few years."

Quantum sensor tracked in human cells could aid drug discovery

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The image illustrates the quantum measurement carried out on a single atom quantum sensor in a living human HeLa cell. The atom sensor is encased in a nanodiamond particle and is controlled by external microwaves and laser light, and tracked by its emission of red light. The information gleaned is of a quantum nature, where the states of the atom exist in two quantum states at the same time prior to measurement. The measurement and control of the atom sensor provides information about the nanoscale environment of the cell and the motion and orientation of the nanoparticle, which could be used in the development of new drugs and delivery systems for nanomedicine. Credit: David Haworth

Groundbreaking research has shown a quantum atom has been tracked inside a living human cell and may lead to improvements in the testing and development of new drugs.

Professor Lloyd Hollenberg from the University of Melbourne?s School of Physics who led the research said it is the first time a single atom encased in nanodiamond has been used as a sensor to explore the nanoscale environment inside a living human cell.

?It is exciting to see how the atom experiences the biological environment at the nanoscale,? he said.

?This research paves the way towards a new class of quantum sensors used for biological research into the development of new drugs and nanomedicine.?

The sensor is capable of detecting biological processes at a molecular level, such as the regulation of chemicals in and out of the cell, which is critical in understanding how drugs work.

The paper has been published in the journal Nature Nanotechnology.

Funded by the ARC Centre of Excellence for Quantum Computation and Communication Technology, the research was conducted by a cross-disciplinary team from the University of Melbourne?s Physics, Chemistry, and Chemical and Biomolecular Engineering departments.

The researchers developed state of the art technology to control and manipulate the atom in the nanodiamond before inserting it into the human cells in the lab.

Biologist Dr Yan Yan of the University?s Department of Chemical and Biomolecular Engineering who works in the field of nanomedicine, said the sensor provides critical information about the movement of the nanodiamonds inside the living cell.

?This is important for the new field of nanomedicine where drug delivery is dependant on the uptake of similar sized nanoparticles into the cell.?

Quantum physicist and PhD student Liam McGuinness from the University?s School of Physics said that monitoring the atomic sensor in a living cell was a significant achievement. ?Previously, these atomic level quantum measurements could only be achieved under carefully controlled conditions of a physics lab,? he said.

It is hoped in the next few years, that following these proof of principle experiments, the researchers will be able to develop the technology and provide a new set of tools for drug discovery and nanomedicine.