Betabiotics: antibiotics of the future

An alarming number of infectious diseases are becoming increasingly difficult to treat with existing antibacterial drugs. This is due primarily to the rate at which bacteria becomes resistant to standard antibiotics. As a result there is a growing concern that an 'antibiotic crisis' is looming.

CSIRO scientists have identified and patented a new antibacterial drug target named 'Beta' that may address this problem.

This was followed by the development of an assay that helps researchers identify molecules that interfere with the normal function of Beta.

These new drug candidates offer a starting point for researchers and industry to develop new classes of antibiotics with improved resistance profiles.

Invisibility Made Easier

In the past year, the media have been abuzz with talk of an exotic class of materials, called metamaterials, that could be used to make flat and distortion-free lenses, powerful microscopes, and even cloaking devices that make objects invisible. But versions of the materials suitable for practical applications have been difficult to make. Now researchers at Princeton University have demonstrated metamaterials that are both higher performing and much easier to manufacture, perhaps bringing these applications closer to reality.

"It's quite an important step," says Igor Smolyaninov, a research scientist at the University of Maryland who works with metamaterials. "It's much less expensive than anything else that people are doing."

Light passing from one ordinary material into another bends slightly--think of how a straight stick in water looks bent--but light passing into a metamaterial bends in the opposite direction. Metamaterials thus have what's called a negative index of refraction. A lens made from such a material wouldn't have to be curved. (It's the curvature of an ordinary lens that enables it to focus incoming light.) Metamaterials could also be used to route electromagnetic waves around an object, rendering it invisible. Already, researchers have demonstrated a cloaking device that makes objects invisible to microwaves, and others have created materials that negatively refract electromagnetic waves in the visible part of the electromagnetic spectrum. But until now, metamaterials have had to be patterned with intricate shapes smaller than the wavelength of light they're meant to manipulate. Consequently, materials that work with light of microscopic wavelengths, such as infrared and visible light, have been difficult to make. Because of the way they produce negative refraction, existing metamaterials have also had a strong tendency to absorb light, making them impractical for use in optics.

The materials developed at Princeton retain the property of negative refraction, yet they're much easier to make. Rather than requiring intricate structures, such as the split rings used in the microwave cloaking device, the materials can be made simply by stacking up extremely thin layers of semiconductor material. What's more, that stacking can be done by the same tools now used to make semiconductor materials for lasers used in telecommunications, says Claire Gmachl, the Princeton researcher who led the work. The new materials consist of alternating layers of indium gallium arsenide and aluminum indium arsenide, and they're tuned to work in the infrared region of the spectrum.

Like other metamaterials, the new materials affect light differently than ordinary materials do because they are made of structures significantly smaller than the wavelength of the light passing through them. In this case, however, it is the layers of semiconductors themselves that are thinner than the wavelength of light. Consequently, a wave passing through the material encounters multiple layers at once, responding to them as if they were a single material with properties quite unlike those of either semiconductor in isolation.

What makes the new materials different from previous metamaterials is that rather than changing two aspects of the way light moves, they change only one. If light is thought of as a wave, the wave front is perpendicular to the direction the light is moving. Imagine an ocean wave crashing ashore: it's moving in just one direction, but the wave front is a huge wall of water. Previous metamaterials changed the direction of light beams passing through them, and the wave front remained perpendicular to the direction of the beam. In the new materials, the light beam changes direction, but the wave fronts don't, giving the impression that they are slipping to the side rather than moving forward. (See image below.)

When a light beam moves through an ordinary material, it moves in the same direction the light waves are facing (top part of image). When a light beam enters a new type of "metamaterial," it changes direction, but the waves remain facing the same way, seeming to slip sideways (see bottom half of image). This image is from a computer simulation.
Credit: Anthony Hoffman, Princeton University

The overall effect on the direction of the light beam is the same as in the earlier metamaterial, but the new materials are simpler to create, and they absorb far less light, making them more attractive for use in optics.

The first application the Princeton researchers are developing is a flat lens for chemical-sensing devices, an application for which materials that work with infrared light are particularly well suited. Gmachl says that the current optical setups for such devices are bulky because they use conventional lenses. "The first application would be using that material to miniaturize optical setups" by replacing curved lenses with flat ones, she says.

Another early application could be in night-vision devices, which also work with infrared wavelengths. "For people who want to improve night-vision devices, this could be quite interesting," Smolyaninov says.

Terabyte Storage for Cell Phones

A new type of memory technology could lead to thumb drives or digital-camera memory cards that store a terabyte of information--more than most hard drives hold today. The first examples of the new technology, which could also slash energy consumption by more than 99 percent, could be on the market within 18 months.

"It's a radically new technology," says Michael Kozicki, a professor of electrical engineering at the Arizona State University, whose group is one of several working on a version of the new memory. "If it really works as well as everybody thinks it could, it could genuinely revolutionize the memory and storage industry."

The new type of memory, called programmable-metallization-cell (PMC) memory, or nano-ionic memory, has been under development at the Arizona State University and at companies such as Sony and IBM. It's one of a new generation of experimental technologies that are bidding to replace hard drives, the nonvolatile "flash" memory used in portable electronics, and the dynamic random-access memory (DRAM) in personal computers. The first ionic-memory prototypes were far too slow for practical use. But recently, researchers have demonstrated that materials structured at the nanoscale could yield ionic-memory devices that are much faster. Nano-ionic memory is significantly faster than flash memory, and the speed of some experimental cells has rivaled that of DRAM, which is orders of magnitude faster than flash.

The memory could also prove easy to make. Recently, the Arizona group published work demonstrating that nano-ionic memory can be made from materials conventionally used in computer memory chips and microprocessors. That could make it easier to integrate with existing technologies, and it would mean less retooling at factories, which would appeal to manufacturers.

Another reason that ionic memory is attractive is that it uses extremely low voltages, so it could consume as little as a thousandth as much energy as flash memory. In theory, it could also achieve much higher storage densities--bits of information per unit of surface area--than current technologies can.

These attractions are largely the result of a new mechanism for storing information. Flash memory stores bits of information as electrical charge, but the smaller the memory cells that hold the bits, the less charge they can hold, and the less reliable they become. The new memory stores information by rearranging atoms to form stable, and potentially extremely small, memory cells. What's more, each cell could potentially store multiple bits of information, and the cells can be layered on top of each other, increasing the memory's storage density to the point that it might rival that of the densest form of memory today: hard drives.

Each memory cell consists of a solid electrolyte sandwiched between two metal electrodes. The electrolyte is a glasslike material that contains metal ions. Ordinarily, the electrolyte resists the flow of electrons. But when a voltage is applied to the electrodes, electrons bind to the metal ions, forming metal atoms that cluster together. These atoms form a virus-sized filament that bridges the electrodes, providing a path along which electrical current can flow. Reversing the voltage causes the wire to "dissolve," Kozicki says. The highly resistive state of the electrolyte and the other, low-resistance, state can be used to represent zeroes and ones. Because the metal filament stays in place until it's erased, nano-ionic memory is nonvolatile, meaning that it doesn't require energy to hold on to information, just to read it or write it.

A thumb drive that stored a terabyte of information, however, would have to take advantage of two other characteristics of nano-ionic memory, Kozicki says. First, it would have to store more than one bit of information per memory cell. Once the wire inside the cell forms, it's possible to apply a voltage again, causing more atoms to form, thickening the wire and further decreasing resistance. Successive jolts will thicken the wire yet more, and the different states of resistance could be used to store multiple bits of information per wire.

What's more, this type of memory can be stacked up in layers, since it's not necessary for each cell to be in contact with a base layer of silicon, as is the case with some other types of memory. Combining multiple bits per cell with multiple layers could make it possible to form extraordinarily dense memory, Kozicki says.

William Gallagher, a senior manager for exploratory nonvolatile-memory research at IBM Research, says that nano-ionic memory is one of several promising next-generation memory technologies. These include MRAM, which stores information using magnetic fields, and phase-change memory, which stores information in a way similar to that used to store bits on DVDs. Gallagher says that ionic memory's competitors have a head start on it. MRAM chips are already sold for some special applications, such as devices that will be exposed to harsh environments. But MRAM may also prove better for high-speed memory applications than as a replacement for flash, so it may not compete directly with nano-ionic memory. Samsung, however, could be selling a phase-change-based flash-replacement memory within a year.

Still, nano-ionic memory may not be far behind. A few companies have licensed nano-ionic-memory technology developed at the Arizona State University. These include Qimonda, based in Germany; Micron Technologies, based in Boise, ID; and a Bay Area stealth-mode startup. The startup is well on the way to producing its first memory devices, which Kozicki says could be available within 18 months. These first chips, however, won't rival hard drives in memory density, he says.

The new technology could nevertheless have difficulty winning wide adoption. Flash-type memory continues to improve and may do so for a few more generations of products. Also, the best nano-ionic-memory prototypes have been made from materials that aren't used in conventional microchips, so manufacturing could be costly, at least initially. Kozicki's group recently demonstrated that ionic memory can be built from a combination of silicon dioxide and copper--materials that are compatible with conventional manufacturing. But these materials do not perform as well, which could make them less attractive than alternatives such as phase-change memory. For the new type of memory to succeed, it may be necessary to convince manufacturers to switch to new materials.

The World's Smallest Radio

Researchers have fashioned the world's tiniest radio out of a carbon nanotube. The nanotube, placed between two electrodes, combines the roles of all the major electrical components in a radio, including the tuner and amplifier. It can tune in to a radio signal and play the audio through an external speaker.

While the practical application of the radio is uncertain, it could be used in biological and environmental sensors. Researchers are now developing microelectromechanical (MEMS) sensors to measure blood sugar levels or cancer markers in the body. Instead of researchers using a stamp-size radio-frequency identification tag, a nanotube radio could be packaged with the MEMS-based sensor and injected directly into the bloodstream, says Alex Zettl, an experimental physicist at the University of California, Berkeley, who is leading the development of the nanotube radio. Once in the body, the radio could provide wireless communication between the tiny biological sensors and an external monitor. To do that, however, the nanotube radio would have to work as a transmitter. Right now, it is only configured as a receiver, but Zettl says that "the same physics would work as a transmitter."

The nanotube radio works differently than a conventional radio does. Conventional radios have four main functional parts: antenna, tuner, amplifier, and demodulator. Radio waves falling on a radio antenna create electric currents at different frequencies. When someone selects a radio station, the tuner filters out all but one of the frequencies. Transistors amplify the signal, while a demodulator, typically a rectifier or a diode, separates the data--the music or other audio--that has been encoded on a "carrier" electromagnetic wave.

Zettl's team used one carbon nanotube for all these functions. Because of their unique electrical properties, carbon nanotubes have been previously used to make electronic components such as diodes, transistors, and rectifiers. "It was a revelation that all of this could be built into the same [nanotube]," Zettl says.

The nanotube is grown sticking out from a tungsten surface, which acts as a negative electrode. The tip of the carbon nanotube is also negatively charged. A vacuum separates the nanotube from a positive copper electrode. The researchers use an external battery to apply a voltage between the two electrodes. Electrons jump out from the negative nanotube tip to the positive electrode, creating what is called a field emission current.

Zettl explains that the "nanotube does not act as an antenna in the conventional sense." That is, instead of picking up electromagnetic waves electrically, it picks them up mechanically. This happens because of the nanotube's natural resonance frequency. As soon as it encounters radio waves that match the frequency, the nanotube starts vibrating in step with the waves, effectively tuning in only to that radio signal. The nanotube's vibrations change the field emission current, and the mechanical vibrations are converted into an electrical signal. An external battery powers the field emission current and amplifies the radio signal. The field emission is naturally asymmetrical--it allows current to flow only in one direction, just like the diodes and rectifiers used in demodulators. So the nanotube also acts as a demodulator and detects the music encoded onto the carrier wave.

To tune to a different radio station, the researchers change the resonance frequency of the nanotube. They do this by changing the voltage applied across the electrodes. "It's like tuning a guitar string," Zettl says. "The electric field pulls on the nanotube." With the same nanotube, the researchers can cover the entire FM radio band.

Cees Dekker, a nanotube researcher at the Delft University of Technology, in the Netherlands, calls the new radio "an appealing demonstration that very simple devices can be used for everyday [tools]." Whether or not the device is used for sensors remains to be seen, he says, but for now, the simple demonstration is a good start.

Announcing the world's first 40G silicon laser modulator!

In this blog, I would like to share with you our recent breakthrough in Silicon Photonics research at Photonics Technology Lab of Intel, a laser modulator that encodes optical data at 40 billion bits per second. Here I am holding a packaged device:

As you may know, a photonic integrated circuit (PIC) could provide a cost-effective solution for optical communication and future optical interconnects in computing industry. PICs on silicon platforms have attracted particular interest because of silicon’s low cost and high volume manufacturability. Competition in this arena is intense as many players in both academia and industry have been aggressively pursuing research into completely integrated CMOS photonics. The DARPA-initiated Electronic & Photonic Integrated Circuits (EPIC) program has also been supporting several Universities and startups to develop capabilities in this area.

One of the key components needed for silicon PICs is the high-speed silicon optical modulator, which is used to encode data on optical beam. Today’s commercially available optical modulators at 10 Gbps are based on more exotic electro-optic materials such as lithium niobate and III-V compound semiconductors. These devices have deployed at speeds up to 40 Gbps. Our goal to achieve similar performance in silicon has been very challenging, because crystalline silicon does not exhibit the linear electro-optic (Pockels) effect used to modulate light in these materials. Engineers are forced to rely on the free-carrier plasma dispersion effect, in which silicon’s refractive index is changed when the density of free carriers (electrons/holes) is varied, to modulate light in silicon.

In 2004, we published in Nature the first silicon modular to reach gigahertz speeds, 50x times faster than previous attempts in silicon. Since then, we scaled the device to 10Gbps, brining silicon modulation speed to a level comparable to most commercial devices. In January 2007, we designed and fabricated a new type of silicon optical modulator scalable to >>10 Gbps and demonstrated data transmission at 30 Gbps (see Optics Express, 22 January 2007, pp. 660-668). The modulator still relies on the free-carrier effect, but its high speed is the result of a unique device design with traveling-wave drive scheme.

This is the new chip . With a similar device configuration, the modulator performance has been further improved by better device packaging to reduce the parasitic effect, better traveling-wave electrode with lower RF attenuation, and better modulator termination circuitry. In the conference of Integrated Photonics and Nanophotonics Research and Applications, Salt Lake City, Utah, July 9-11, 2007, I presented our world record results in a silicon modulator to a small group of scientists. We have finally reached the goal of data transmission at 40 Gbps speed, matching the fastest devices deployed today using other materials.

The Intel modulator is based on a Mach-Zehnder interferometer with a reverse-biased pn junction in each of the arms (Figure 1a). When a reverse voltage is applied to the junction, free carriers – electrons and holes resulting from the n- and p-dopants – are pulled out of the junction, changing its refractive index via the free-carrier effect. The intensity of the light transmitted through the Mach-Zehnder interferometer is modulated by modulating the phase difference between the interferometer’s two arms. This modulation can be very fast, because free carriers can be swept out of the junction with a time of approximately 7 ps. The modulator speed is thus limited by the parasitic effects such as RC time constant limit.

To minimize the RC constant limitation, Intel researchers adopted a traveling-wave drive scheme allowing electrical and optical signal co-propagation along the waveguide. The traveling-wave electrode which is based on a coplanar waveguide was designed to match the velocity for both optical and electrical signals, while keeping the RF attenuation small. To operate the traveling-wave modulator, the RF signal is fed into the transmission line using a commercially available driver from the optical input side and the transmission line is terminated with an external resistor (see Fig. 1a). After packaging the modulator on a printed circuit board, the researchers demonstrated that the modulator has a 3 dB bandwidth of ~30 GHz (Fig. 2a) and data transmission capability up to 40 Gbps (Fig. 2b).

The high-speed silicon modulator could find use in various future applications. For example, a highly integrated silicon photonic circuit may provide a cost effective solution for the future optical interconnects within computers and other devices. With the demonstration of the 40 Gbps silicon modulator and the electrically pumped hybrid silicon laser, it will become possible to integrate multiple devices on a single chip (Fig. 3) that can transmit terabits of aggregate data per second in the near future – truly enabling tera-scale computing.