Graphene-Based Optical Modulators Poised to Break Speed Limits in Digital Communications

Engineerblogger
March 2, 2012

In yet another astounding application of the “wonder material” graphene, scientists at the University of California, Berkeley discovered that it makes an excellent active media for optical modulators. Graphene-based modulators are expected to significantly enhance ultrafast optical communication and computing. The team will report on their findings at the Optical Fiber Communication Conference and Exhibition/National Fiber Optic Engineers Conference (OFC/NFOEC) taking place next week in Los Angeles.

Modulators play a vital role in communications due to their switching ability, because this is what controls the speed that data packets can travel through networks. As the speed of data pulses sent out increases, it means that greater volumes of information can be transmitted.

“We demonstrated a graphene-based optical modulator with a broad optical bandwidth (1.35-1.6 µm), a small device footprint (25 µm2), and high operational speed (1.2 GHz at 3dB) under ambient conditions—all of which are essential for optical interconnects for future integrated optoelectronic systems,” says Ming Liu, a post-doctoral researcher working at UC Berkeley’s NSF Nanoscale Science and Engineering Center. “The modulation efficiency of a single layer of a hexagonal carbon atom is already comparable to, if not better than, traditional semiconductor materials, which are orders of magnitude larger in active volume.”

Looking into future applications, graphene-based modulators could be very compact and potentially perform at speeds up to 10 times faster than today’s technology allows. They may someday enable consumers to stream full-length, high-definition, 3-D movies onto their smartphones within mere seconds.

Liu’s talk, “Graphene-based optical modulators,” takes place Tuesday, March 6 at 3:30 p.m. in the Los Angeles Convention Center.

About OFC/NFOEC

For more than 35 years, the Optical Fiber Communication Conference and Exposition/ National Fiber Optic Engineers Conference (OFC/NFOEC) has been the premier destination for converging breakthrough research and innovation in telecommunications, optical networking and, recently, datacom and computing. Uniting service providers, systems companies, enterprise customers, IT businesses and component manufacturers, along with researchers, engineers and development teams, OFC/NFOEC combines dynamic business programming, an exposition of more than 500 companies and cutting-edge peer-reviewed research into one event that showcases the trends and pulse of the entire optical communications industry. OFC/NFOEC is managed by the Optical Society (OSA) and co-sponsored by OSA, the Institute of Electrical and Electronics Engineers/Communications Society (IEEE/ComSoc) and the IEEE Photonics Society. Acting as a non-financial technical co-sponsor is Telcordia Technologies

Source:  Optical Society of America (OSA)

New laser can point the way to new energy harvesting

Engineerblogger
Feb 29, 2012

Ismael Heisler next to a diffractive optic polarisation spectrometer. Credit: EPRSC

New ultrafast laser equipment, capable of generating intense pulses of light as short as a few femtoseconds from the UV to the Infra Red, will help scientists at the University of East Anglia (UEA) measure how energy is transferred from molecule to molecule and point the way to molecular structures for exploiting solar radiation.

Funded by a £466,000 grant from the Engineering and Physical Sciences Research Council, the new laser will be used for 2D electronic spectroscopy experiments that look at the very fastest reactions. By studying how energy transfers in natural and artificial systems such as proteins and molecular materials, researchers will in turn be able to help the design of new nanomachines and solar power collectors.

Steve Meech, Professor of Chemistry at UEA’s said:

"With this equipment we will be able to develop experiments which probe in exquisite detail the link between the efficiency of light driven processes in natural and synthetic systems and the underlying molecular architecture."

2D electronic spectroscopy is in many ways analogous to the much better known 2D Nuclear Magnetic Resonance method. It uses ultra fast visible light pulses to reveal coupling between electronic states whereas NMR uses radio frequency pulses to measure couplings between nuclear spins.

Twenty years ago most ultrafast experiments relied upon amplified dye lasers. These difficult to use and unstable devices severely limited the range of experiments possible. Starting with the discovery of the Titanium Sapphire laser, a whole new family of experiments became possible.

"It is because of the amazing stability and reliability of these modern devices that we can even consider 2D optical experiments, which may take days to run", added Meech.

Lesley Thompson, EPSRC’s Director of Research Base, said:

"The grant for equipment made by our strategic equipment panel will give UEA the tools they need, but EPSRC has also allocated a further £613,000 for staff and collaborations to drive this research forward."

The announcement coincides with the inaugural lecture by Professor Alf Adams at the Royal Society in London, to mark the 25th anniversary of his work on strained quantum well lasers, recently named as one of the Top Ten greatest UK scientific breakthroughs of all time.

The lecture, entitled Semiconductor Lasers TakeThe Strain, is the first in a series named in his honour.

Source: Engineering and Physical Sciences Research Council (EPSRC)

Imaging applications: New Equipment, Sourcing Enhance R&D

Engineerblogger
Feb 16, 2012

Figure 1. Inclusions in the steel of a car frame, as visualized using the Olympus BXiS microscope system and Olympus Stream software. At the center is a Type C/silicate inclusion; at the lower right is a Type D/globular oxide inclusion. Magnification: 100X. All images: Olympus America Inc.

The tools used by industrial R&D departments must evolve quickly to keep up with rapidly changing technology. In the search for more cost-effective materials that maintain safety, automotive suppliers are working on advanced high-strength steels that use less material and reduce overall vehicle weight, while lowering manufacturing costs. Increased production efficiencies and higher performance demands are driving analysis techniques in textiles research. For example, defense contractors use durability analysis that involves measuring the open areas of the weave pattern while developing uniforms that protect soldiers from sun and sand.

To meet stringent specifications, R&D professionals gather material characteristics from testing and conduct measurements that reveal relevant performance information to support decisions about manufactured products. Sometimes, colleagues who are not familiar with the project must access, analyze, and make decisions on the data. All of this occurs in an environment where companies and institutions are cutting costs, using existing resources, and asking R&D engineers and scientists to work more efficiently.

As a result, industrial R&D organizations are changing the way they organize their R&D functions and train staff. Multinational companies are centralizing core R&D functions, but many organizations also are setting up small, entrepreneurial R&D/engineering areas for specialty research. This strategy encourages collaboration among engineers, improves economies of scale in equipment purchase and staffing, and stimulates innovative approaches to design and engineering challenges.

Training practices have been modified to prepare employees who may have specific skill sets but lack advanced degrees to make important contributions to the R&D process. To serve this workforce, tools have become easier to learn and use.

Reporting requirements have also changed. Data must be accessible, usable, and actionable, without the need for advanced interpretation. Previously, an operator would prepare the sample, adjust the microscope, and capture images of the relevant areas of interest, then, create a report with images using a word processing application. Formatting the report takes time; if the image was manipulated or resized, critical parameters such as the scale bar or integral metadata (calibration information, image acquisition data, etc.) could be lost or incorrectly displayed. Advanced imaging technology now allows the image metadata to be recalled and displayed on the image; the scale bar location can be resized and repositioned directly inside the report’s word processing application.

A tool for all users
Companies also have changed how they select and use equipment. Equipment purchased today must be easy to learn, with minimal training. Microscopes and image analysis systems—among the most complex tools used regularly in industrial R&D environments—are good examples of this change. Current tools and equipment must be simple for engineers or scientists to use to their fullest capability, even if the equipment is used only a few times a week. Users should be able to go to the microscope and acquire accurate results every time. Microscope controls and software features should be easy to use. And users should have the confidence that the results are accurate.

Figure 2. Microscope/digital imaging workstations, such as the Olympus BXiS microscope system, can help R&D professionals obtain critical analytical results.

Imaging tools must balance automated features—so users don't have to go through multiple steps to get results—while maintaining flexibility to accommodate differing samples and parameters. Automotive R&D facilities are selecting tools that make it easier and faster to study changes in steel microstructures after exposure to high heat (Figure 1). Non-metallic inclusions embedded in steel during the manufacturing process previously were rated using a manual microscopic inspection protocol and a comparison chart. Now, inclusions can be detected and rated in accordance with international standards as a function of their intensity levels and morphological parameters. Moving this type of analysis to a digital format enables R&D facilities to increase throughput, minimize inconsistencies, and provide better documentation.

Traditionally, textile manufacturers examined the weave and composition of fibrous material under microscopes and captured images. Using an integrated microscope-image analysis system, fabrics can be examined, documented, and quantified. Operators define the objects or open areas to be detected; the system selects the objects and categorizes them by shape and size. While the earlier procedure involved multiple steps and subjective analysis, the new process is reduced to two or three mouse clicks.

As industry has ratcheted up performance demands on R&D organizations, it simultaneously instituted cost-cutting programs. For modern R&D organizations to thrive, they must look at functional reorganization; tools that require less training; reporting systems that deliver information in a readily actionable format; and accurate, reliable tools that maximize the time and efficiency of high-level R&D functions. In addition, integrated solutions—where the microscope, imaging system, instrument control software, and reporting tools all come from one company—can streamline service and training and establish a single-source of responsibility for the system's performance.

Source: R & D Magazine

Engineers Build 'No-Waste' Laser

Engineerblogger
Feb 9, 2012

Postdoctoral researcher Mercedeh Khajavikhan at work in the optics laboratory. Photo Credit: Josh Knoff, UC San Diego Jacobs School of Engineering.

A team of University of California, San Diego researchers has built the smallest room-temperature nanolaser to date, as well as an even more startling device: a highly efficient, “thresholdless” laser that funnels all its photons into lasing, without any waste.

The two new lasers require very low power to operate, an important breakthrough since lasers usually require greater and greater “pump power” to begin lasing as they shrink to nano sizes. The small size and extremely low power of these nanolasers could make them very useful components for future optical circuits packed on to tiny computer chips, Mercedeh Khajavikhan and her UC San Diego Jacobs School of Engineering colleagues report in the Feb. 9 issue of the journal Nature.

They suggest that the thresholdless laser may also help researchers as they develop new metamaterials, artificially structured materials that are already being studied for applications from super-lenses that can be used to see individual viruses or DNA molecules to “cloaking” devices that bend light around an object to make it appear invisible.

All lasers require a certain amount of “pump power” from an outside source to begin emitting a coherent beam of light or “lasing,” explained Yeshaiahu (Shaya) Fainman, a professor in the Department of Electrical and Computer Engineering at UC San Diego and co-author of the new study. A laser’s threshold is the point where this coherent output is greater than any spontaneous emission produced.

The smaller a laser is, the greater the pump power needed to reach the point of lasing. To overcome this problem, the UC San Diego researchers developed a design for the new lasers that uses quantum electrodynamic effects in coaxial nanocavities to alleviate the threshold constraint. Like a coaxial cable hooked up to a television (only at a much smaller scale), the laser cavity consists of a metal rod enclosed by a ring of metal-coated, quantum wells of semiconductor material. Khajavikhan and the rest of the team built the thresholdless laser by modifying the geometry of this cavity.

The new design also allowed them to build the smallest room-temperature, continuous wave laser to date. The new room-temperature nanoscale coaxial laser is more than an order of magnitude smaller than their previous record smallest nanolaser published in Nature Photonics less than two years ago. The whole device is almost half a micron in diameter – by comparison, the period at the end of this sentence is nearly 600 microns wide.

These highly efficient lasers would be useful in augmenting future computing chips with optical communications, where the lasers are used to establish communication links between distant points on the chip. Only a small amount of pump power would be required to reach lasing, reducing the number of photons needed to transmit information, said Fainman.

The nanolaser designs appear to be scalable – meaning that they could be shrunk to even smaller sizes – an extremely important feature that makes it possible to harvest laser light from even smaller nanoscale structures, the researchers note. This feature eventually could make them useful for creating and analyzing metamaterials with structures smaller than the wavelength of light currently emitted by the lasers.

Fainman said other applications for the new lasers could include tiny biochemical sensors or high-resolution displays, but the researchers are still working out the theory behind how these tiny lasers operate. They would also like to find a way to pump the lasers electrically instead of optically.

Source: University of California, San Diego

Managing Heat Transfer at Material Interfaces

Engineerblogger
Feb 9, 2012

Credit: ASME

Managing heat transfer at material interfaces is a major challenge for engineers who work on devices ranging from jet engines to personal electronics to nanoscale transistors. The physical and chemical properties that control thermal transport at interfaces between dissimilar materials are poorly understood. A deeper knowledge of how these properties impact heat transfer could lead to increased performance and energy efficiency in military and commercial materials and devices such as high-power electronics, thermoelectric generators, thermal interface materials, and thermal barrier coatings.

Through a Multidisciplinary University Research Initiative program sponsored by the Air Force Office of Scientific Research (AFOSR), Arlington, VA, Kumar Jata of AFOSR, along with a team of researchers from the University of Michigan, Brown University, Stanford University, and University of California at Santa Cruz, is studying the fundamental processes by which heat is transferred across material interfaces. Their ultimate goal is to develop a set of design rules for custom-engineering interfaces that have specific thermal properties and higher performance.

"Recent progress in nanoscience such as advancements in molecular beam epitaxy and self-assembled monolayers has enabled the precise control of interface physical and chemical structure," says Dr. Kevin Pipe, an associate professor of mechanical engineering at the University of Michigan, Ann Arbor, MI, and leader of the research team. "However, the fundamental physics that link this nanoscale structure with thermal transport is not yet well developed, inhibiting the engineering of interfaces with radically enhanced thermal properties."

Measuring Thermal Resistance

The physical and chemical nature of material interfaces can decrease a composite material's thermal conductivity by scattering the acoustic waves that are the primary carriers of heat in solids. Interface properties that contribute to this scattering include roughness or irregularities and weak chemical bonding, as well as different acoustic properties (acoustic mismatch). This scattering creates a thermal resistance at the interface.

Dr. Pipe's team uses advanced techniques based on ultrafast lasers that emit pulses of light less than 50 fs in duration. When these pulses hit a sample, they generate acoustic waves that scatter off of interfaces within the material. "By examining the echo patterns from these acoustic waves, we learn how they interact with different kinds of interfaces," says Dr. Pipe. The measurements utilize a new optical cavity technique, developed by researchers Dr. Humphrey Maris, professor of physics, and Dr. Arto Nurmikko, professor of engineering and physics, at Brown University, Providence, RI. The technique dramatically enhances the signal-to-noise ratio of the measured echoes.

Abstract acoustic waves. Credit:ASME

The team has further extended this method by combining ultrafast laser pulses with x-ray diffraction through techniques developed by Dr. David Reis, an associate professor of photon science and applied physics at Stanford University, Stanford, CA. In these studies, picosecond x-ray pulses are used to look directly at atomic motion near an interface as heat flows across it. "The x-ray pulses create a stroboscopic movie that allows us to delineate the particular vibrational modes of the atoms that carry heat energy across an interface," he says.

Additional measurement advancements the team has made include a thermal imaging camera with 800-ps time resolution and 250-nm spatial resolution. Developed by Dr. Ali Shakouri, professor of electrical engineering at the University of California at Santa Cruz, CA, the camera allows the researchers to directly map the temperature near an interface on the same spatial and time scales at which heat flows across it.

One particular area that Dr. Pipe and his team have explored is the propagation of heat across interfaces between soft and hard materials, which are commonly found in thermal interface composites. Dr. Pipe and Dr. Max Shtein, associate professor of materials science and engineering at the University of Michigan, have shown that the degree to which organic materials and metals mix at the nanoscale when they form interfaces within a composite can change the composite's thermal conductivity by more than a factor of three. In fact, if very little mixing occurs, the resistance of these interfaces to heat transfer can reduce the thermal conductivity of the composite below that of either constituent.

Controlling Thermal Boundary Resistance

"For some time now, people have been able to measure the thermal boundary resistance of an interface, which quantifies how well heat moves across it," says Dr. Pipe. "However, only recently have people been able to make precise nanoscale modifications to an interface that allow us understand how to control this resistance." These precise modifications are key to generating experimental data that can be compared with fundamental models.

Being able to control thermal boundary resistance is an important capability for either efficiently transferring or blocking heat flow between materials, critical factors in performance and reliability. Inefficient heat flow is a barrier to the development of higher-power lasers and transistors. On the other hand, being able to block heat transfer can dramatically improve the efficiency of thermoelectric energy conversion for compact power sources.

Using their discoveries, Dr. Pipe and his colleagues plan to re-engineer the structure and chemistry of various material interfaces at the nanoscale to better regulate their heat transfer. A broad range of military and commercial products can be enhanced with better thermal interface control, including heat sinks for high-power electronics, thermal barrier coatings for aerospace components, and thermoelectric materials for power generation.

Source: ASME

Cooling semiconductor by laser light

Engineerblogger
Jan 24, 2012

Koji Usami is working in the Quantop laboratories at the Niels Bohr Institute. Photo: Ola J. Joensen

Researchers at the Niels Bohr Institute have combined two worlds – quantum physics and nano physics, and this has led to the discovery of a new method for laser cooling semiconductor membranes. Semiconductors are vital components in solar cells, LEDs and many other electronics, and the efficient cooling of components is important for future quantum computers and ultrasensitive sensors. The new cooling method works quite paradoxically by heating the material! Using lasers, researchers cooled membrane fluctuations to minus 269 degrees C. The results are published in the scientific journal, Nature Physics.

“In experiments, we have succeeded in achieving a new and efficient cooling of a solid material by using lasers. We have produced a semiconductor membrane with a thickness of 160 nanometers and an unprecedented surface area of 1 by 1 millimeter. In the experiments, we let the membrane interact with the laser light in such a way that its mechanical movements affected the light that hit it. We carefully examined the physics and discovered that a certain oscillation mode of the membrane cooled from room temperature down to minus 269 degrees C, which was a result of the complex and fascinating interplay between the movement of the membrane, the properties of the semiconductor and the optical resonances,” explains Koji Usami, associate professor at Quantop at the Niels Bohr Institute.

From gas to solid

Laser cooling of atoms has been practiced for several years in experiments in the quantum optical laboratories of the Quantop research group at the Niels Bohr Institute. Here researchers have cooled gas clouds of cesium atoms down to near absolute zero, minus 273 degrees C, using focused lasers and have created entanglement between two atomic systems. The atomic spin becomes entangled and the two gas clouds have a kind of link, which is due to quantum mechanics. Using quantum optical techniques, they have measured the quantum fluctuations of the atomic spin.

“For some time we have wanted to examine how far you can extend the limits of quantum mechanics – does it also apply to macroscopic materials? It would mean entirely new possibilities for what is called optomechanics, which is the interaction between optical radiation, i.e. light, and a mechanical motion,” explains Professor Eugene Polzik, head of the Center of Excellence Quantop at the Niels Bohr Institute at the University of Copenhagen.

But they had to find the right material to work with.

The experiments are carried out in the Quantop laboratories at the Niels Bohr Institute. The laser light that hits the semiconducting nanomembrane is controlled with a forest of mirrors. Photo: Ola J. Joensen

Lucky coincidence

In 2009, Peter Lodahl (who is today a professor and head of the Quantum Photonic research group at the Niels Bohr Institute) gave a lecture at the Niels Bohr Institute, where he showed a special photonic crystal membrane that was made of the semiconducting material gallium arsenide (GaAs). Eugene Polzik immediately thought that this nanomembrane had many advantageous electronic and optical properties and he suggested to Peter Lodahl’s group that they use this kind of membrane for experiments with optomechanics. But this required quite specific dimensions and after a year of trying they managed to make a suitable one.

“We managed to produce a nanomembrane that is only 160 nanometers thick and with an area of more than 1 square millimetre. The size is enormous, which no one thought it was possible to produce,” explains Assistant Professor Søren Stobbe, who also works at the Niels Bohr Institute.

Koji Usami shows the holder with the semiconductor nanomembrane. The holder measures about one by cm, while the nanomembrane itself has a surface area of 1 by 1 millimeter and a thickness of 160 nanometers. Photo: Ola J. Joensen

Basis for new research

Now a foundation had been created for being able to reconcile quantum mechanics with macroscopic materials to explore the optomechanical effects.

Koji Usami explains that in the experiment they shine the laser light onto the nanomembrane in a vacuum chamber. When the laser light hits the semiconductor membrane, some of the light is reflected and the light is reflected back again via a mirror in the experiment so that the light flies back and forth in this space and forms an optical resonator. Some of the light is absorbed by the membrane and releases free electrons. The electrons decay and thereby heat the membrane and this gives a thermal expansion. In this way the distance between the membrane and the mirror is constantly changed in the form of a fluctuation.

"Changing the distance between the membrane and the mirror leads to a complex and fascinating interplay between the movement of the membrane, the properties of the semiconductor and the optical resonances and you can control the system so as to cool the temperature of the membrane fluctuations. This is a new optomechanical mechanism, which is central to the new discovery. The paradox is that even though the membrane as a whole is getting a little bit warmer, the membrane is cooled at a certain oscillation and the cooling can be controlled with laser light. So it is cooling by warming! We managed to cool the membrane fluctuations to minus 269 degrees C", Koji Usami explains.

“The potential of optomechanics could, for example, pave the way for cooling components in quantum computers. Efficient cooling of mechanical fluctuations of semiconducting nanomembranes by means of light could also lead to the development of new sensors for electric current and mechanical forces. Such cooling in some cases could replace expensive cryogenic cooling, which is used today and could result in extremely sensitive sensors that are only limited by quantum fluctuations,” says Professor Eugene Polzik.

Source: University of Copenhagen

Optical nanoantennas enable efficient multipurpose particle manipulation

Engineerblogger
Jan 13, 2012

Concept art depicting the various potential BNA trapping states

University of Illinois researchers have shown that by tuning the properties of laser light illuminating arrays of metal nanoantennas, these nano-scale structures allow for dexterous optical tweezing as well as size-sorting of particles.

"Nanoantennas are extremely popular right now because they are really good at concentrating optical fields in small areas,” explained Kimani Toussaint, Jr., an assistant professor of mechanical science and engineering at the University of Illinois at Urbana-Champaign. “In this work, we demonstrate for the first time the use of arrays of gold Bowtie Nanoantenna Arrays (BNAs) for multipurpose optical trapping and manipulation of submicrometer- to micrometer-sized objects. We think that this could be a fruitful area to pursue, particularly because of the growing interest in lab-on-a-chip devices.”

According to the researchers, the excellent field enhancement and confinement properties of BNAs enable highly efficient, optical tweezers which permit high-speed manipulation of submicrometer to micrometer-sized objects in aqueous environments using remarkably low-input power densities. These characteristics could be useful for optofluidic applications (e.g., lab-on-a-chip devices), manipulating biological matter with reduced specimen photo damage, formation of optical matter, and basic physics studies of colloidal dynamics.

“In contrast to other plasmonic tweezers, we find that BNAs permit particle trapping, manipulation and sorting utilizing only the optical parameter space, namely, low input power densities, wavelength and polarization,” said Brian Roxworthy, a graduate student in Toussaint’s research group and first author on the paper, “Application of Plasmonic Bowtie Nanoantenna Arrays for Optical Trapping, Stacking, and Sorting,” which appears in the journal Nano Letters.

Using empirically obtained “optical trapping phase diagrams” to achieve the desired trapping response, the researchers demonstrated several types of particle manipulation, including single-beam optical tweezing of single particles over the entire nanoantenna area, single-beam optical tweezing of 2D hexagonal packed particles over the entire nanoantenna area, and optical sorting of particles by size; stacking of submicron to micron-sized particles in 3D.

According to Toussaint, this is the first demonstration of a range of particle manipulation behavior for a given nanoantenna array.
“We actually excite our nanoantennas off resonance, which to our knowledge is a first, and at the right input optical power, we take advantage of thermal effects combined with optical forces to enable tweezing of tens of particles at a time,” Toussaint explained. “We show that very low power densities are required to achieve the aforementioned behavior. For example, we were able to carry out experiments using a standard laser pointer.”

In addition to Toussaint, the lead investigator for the project, co-investigators include Gang Logan Liu, an assistant professor of electrical and computer engineering at Illinois, and former Illinois faculty member Nicholas Fang, who is now at the Massachusetts Institute of Technology.

Source: University of Illinois at Urbana-Champaign

Additional Information:

• The abstract of the report "Application of Plasmonic Bowtie Nanoantenna Arrays for Optical Trapping, Stacking, and Sorting,” which appears in the journal Nano Letters (DOI: 10.1021/nl203811q) on December 30, 2011.

Research could improve laser-manufacturing technique

Engineerblogger
Jan 4, 2012

Mechanical engineering doctoral student Wenqian Hu, who graduated this fall, works on a complex optical setup that is part of research at Purdue University to uncover details about the behavior of ultrafast laser pulses. The technology may have new applications in manufacturing, diagnostics and other research. (Purdue University photo/Mark Simons)

Engineers have discovered details about the behavior of ultrafast laser pulses that may lead to new applications in manufacturing, diagnostics and other research.

Ultrafast laser pulses are used to create features and surface textures in metals, ceramics and other materials for applications including the manufacture of solar cells and biosensors. The lasers pulse at durations of 100 femtoseconds, or quadrillionths of a second, and cause electrons to reach temperatures greater than 60,000 degrees Celsius during the pulse duration. The pulses create precise patterns in a process called "cold ablation," which turns material into a plasma of charged particles.

Images taken with a high-speed camera show tiny mushroom clouds eerily similar in appearance to those created in a nuclear explosion. The clouds expand outward at speeds of 100 to 1,000 times the speed of sound within less than one nanosecond. However, new findings reveal that an earlier cloud forms immediately before the mushroom cloud, and this early plasma interferes with the laser pulses, hindering performance, said Yung Shin, a professor of mechanical engineering and director of Purdue University's Center for Laser-Based Manufacturing.

This series of high-speed images shows how plasma expands when material is exposed to ultrafast laser pulses. Purdue researchers have discovered details that could help to harness the technology for applications in manufacturing, diagnostics and research. (Yung Shin, Purdue University School of Mechanical Engineering)

Finding a way to eliminate the interference caused by the early plasma could open up new applications in manufacturing, materials and chemical processing, machining and advanced sensors to monitor composition, and chemical and atomic reactions on an unprecedented scale, he said.

Researchers used experiments and simulations to study the phenomenon. Research papers about the work were published online Dec. 6 in Applied Physics Letters and in September in the journal Physics of Plasmas. The papers were written by doctoral student Wenqian Hu, Shin and mechanical engineering professor Galen King.

"We found the formation of early plasma has very significant bearing on the use of ultrashort pulse lasers because it partially blocks the laser beam," Shin said. "The early plasma changes the optical properties of air, but the mechanism is still largely unknown."

The researchers studied the early plasma by tracking the movement of millions of individual atoms in the plasma; observing how the laser beam travels in space and interacts with plasma; and using a "laser pump probe shadowgraph," a technique in which one laser ablates a material, producing the early plasma, and a second laser fired perpendicular to the first is used to study the cloud. A series of optical elements and mirrors is used in the shadowgraph technique.


Source: Purdue University

Related Information:

• The Abstract from "Early-stage plasma dynamics with air ionization during ultrashort laser ablation of metal" by Wenqian Hu, Yung C. Shin, and Galen King in the  American Institute of Physics.

'Nanoantennas' show promise in optical innovations

Engineerblogger
Jan 4, 2012

The image in the upper left shows a schematic for an array of gold "plasmonic nanoantennas" able to precisely manipulate light in new ways, a technology that could make possible a range of optical innovations such as more powerful microscopes, telecommunications and computers. At upper right is a scanning electron microscope image of the structures. The figure below shows the experimentally measured refraction angle versus incidence angle for light, demonstrating how the nanoantennas alter the refraction. (Purdue University Birck Nanotechnology Center image)

Researchers have shown how arrays of tiny "plasmonic nanoantennas" are able to precisely manipulate light in new ways that could make possible a range of optical innovations such as more powerful microscopes, telecommunications and computers.

The researchers at Purdue University used the nanoantennas to abruptly change a property of light called its phase. Light is transmitted as waves analogous to waves of water, which have high and low points. The phase defines these high and low points of light.

"By abruptly changing the phase we can dramatically modify how light propagates, and that opens up the possibility of many potential applications," said Vladimir Shalaev, scientific director of nanophotonics at Purdue's Birck Nanotechnology Center and a distinguished professor of electrical and computer engineering.

Findings are described in a paper to be published online Thursday (Dec. 22) in the journal Science.

The new work at Purdue extends findings by researchers led by Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at the Harvard School of Engineering and Applied Sciences. In that work, described in an October Science paper, Harvard researchers modified Snell's law, a long-held formula used to describe how light reflects and refracts, or bends, while passing from one material into another.

"What they pointed out was revolutionary," Shalaev said.

Until now, Snell's law has implied that when light passes from one material to another there are no abrupt phase changes along the interface between the materials. Harvard researchers, however, conducted experiments showing that the phase of light and the propagation direction can be changed dramatically by using new types of structures called metamaterials, which in this case were based on an array of antennas.

The Purdue researchers took the work a step further, creating arrays of nanoantennas and changing the phase and propagation direction of light over a broad range of near-infrared light. The paper was written by doctoral students Xingjie Ni and Naresh K. Emani, principal research scientist Alexander V. Kildishev, assistant professor Alexandra Boltasseva, and Shalaev.

The wavelength size manipulated by the antennas in the Purdue experiment ranges from 1 to 1.9 microns.

"The near infrared, specifically a wavelength of 1.5 microns, is essential for telecommunications," Shalaev said. "Information is transmitted across optical fibers using this wavelength, which makes this innovation potentially practical for advances in telecommunications."

The Harvard researchers predicted how to modify Snell's law and demonstrated the principle at one wavelength.

"We have extended the Harvard team's applications to the near infrared, which is important, and we also showed that it's not a single frequency effect, it's a very broadband effect," Shalaev said. "Having a broadband effect potentially offers a range of technological applications."

The innovation could bring technologies for steering and shaping laser beams for military and communications applications, nanocircuits for computers that use light to process information, and new types of powerful lenses for microscopes.

Critical to the advance is the ability to alter light so that it exhibits "anomalous" behavior: notably, it bends in ways not possible using conventional materials by radically altering its refraction, a process that occurs as electromagnetic waves, including light, bend when passing from one material into another.

Scientists measure this bending of radiation by its "index of refraction." Refraction causes the bent-stick-in-water effect, which occurs when a stick placed in a glass of water appears bent when viewed from the outside. Each material has its own refraction index, which describes how much light will bend in that particular material. All natural materials, such as glass, air and water, have positive refractive indices.

However, the nanoantenna arrays can cause light to bend in a wide range of angles including negative angles of refraction.

"Importantly, such dramatic deviation from the conventional Snell's law governing reflection and refraction occurs when light passes through structures that are actually much thinner than the width of the light's wavelengths, which is not possible using natural materials," Shalaev said. "Also, not only the bending effect, refraction, but also the reflection of light can be dramatically modified by the antenna arrays on the interface, as the experiments showed."

The nanoantennas are V-shaped structures made of gold and formed on top of a silicon layer. They are an example of metamaterials, which typically include so-called plasmonic structures that conduct clouds of electrons called plasmons. The antennas themselves have a width of 40 nanometers, or billionths of a meter, and researchers have demonstrated they are able to transmit light through an ultrathin "plasmonic nanoantenna layer" about 50 times smaller than the wavelength of light it is transmitting.

"This ultrathin layer of plasmonic nanoantennas makes the phase of light change strongly and abruptly, causing light to change its propagation direction, as required by the momentum conservation for light passing through the interface between materials," Shalaev said.

The work has been funded by the U.S. Air Force Office of Scientific Research and the National Science Foundation's Division of Materials Research.

Source: Purdue University.

Optical fiber innovation could make optical computers a ‘snap’

Engineerblogger
Dec 15, 2011

Fig. 1. Propagation of light in a SNAP fiber coupled to a tapered regular optical fiber. Courtesy OFS Laboratories.

Optics and photonics may one day revolutionize computer technology with the promise of light-speed calculations. Storing light as memory, however, requires devices known as microresonators, an emerging technology that cannot yet meet the demands of computing. The solution, described in a paper published today in the Optical Society's (OSA) journal Optics Letters, may lie in combining light's eerie quantum properties with a previously unknown quality of optical fiber.

Researchers from OFS Laboratories in Somerset, N.J., have developed a precise and efficient way to create microresonators by making nanoscale changes to the diameter of normal optical fiber. These narrow sections are able to confine light, sending it on a back-and-forth corkscrew path inside a length of optical fiber and creating a microresonator.

Though trapping light in this so-called "Whispering Gallery" mode is a well-known phenomenon, the researchers have discovered a quick, efficient, and accurate way to manufacture long chains of these new microresonators, all based on a never-before-recognized characteristic of optical fiber. This is a new technology path and an essential step toward designing a practical optical computer, as described in the Optics Letters paper.

"Optical computers, which use light particles—photons—in place of electrons to process and store information, have the potential to be much faster than today's electronic computers," said Misha Sumetsky, a researcher at OFS Laboratories and lead author on the study. "Unfortunately, manufacturing microresonators that meet the demands of optical computing has been a long and, until now, unsuccessful pursuit."

Fig. 2. Illustration of SNAP microresonators formed by nanoscale variation of the optical fiber diameter. Courtesy OFS Laboratories.

Microresonator Design

Designing a practical microresonator has been something of a "Holy Grail" on the path to optical computers. The current microresonator manufacturing technology is based on the well-established process of silicon lithography, which etches extremely precise features onto silicon wafers. For microresonators the most promising design appeared to be a long series of microscopic loops, which bottle up photons in whirling circles and then pass them from one ring to the next. The longer the chain, the longer the signal could be stored as memory. Unfortunately, even the most precise manufacturing processes still produce tiny imperfections in the rings. These bumps on the road slowly weaken the signal, attenuating the light, and allowing the memory held in the buffer to fade away.

Sumetsky and his colleagues at OFS Laboratories, formerly part of the famous Bell Labs, pursued a path that abandoned the silicon wafer in favor of the silica strand of optical fiber.

In conventional applications, optical fiber—a very pure form of glass—uses the fundamental properties of light and refraction to keep light from slipping out and diffusing. The core and cladding of optical fiber have slightly different indexes of refraction, giving the fiber the ability to bend the path of light without causing scattering. Light traveling through the fiber bounces back and forth inside the core, keeping it traveling along the fiber for many kilometers with very little signal loss.

Coaxing Light into a Whispering Gallery

This sends the light careening through the fiber at extremely high speed, but just as cars barreling down the highway sometimes get directed onto "cloverleaf" off ramps, so too can light be coaxed from the fiber and into a spiral path. Unlike cars, however, light doesn't need to slow down on the off ramp.

In this case, the off ramp is created by narrowing the fiber to a small diameter to coax the light out of the core and into a fiber aligned perpendicularly and positioned very close to, or actually touching the first. Because they are so close, and the original fiber narrows down to a mere fraction of its original size, a portion of the light is able to make a literal "quantum leap" to the other fiber. This is an effect known as "evanescent coupling" and it enables an electromagnetic wave – light – to connect (or couple) from one fiber to another.

The light now finds itself not traveling down a straight path but rather racing around the fiber surface in very tight circles. Even though the light maintains its original pace within the glass, because it's really taking the long way around, corkscrewing along the new fiber's surface, it propagates down the fiber at a fraction of its original speed (figure 1).

This special redirection of light is known as the "Whispering Gallery" effect, named after the phenomenon that takes place in certain architectures, such as the St. Paul's Cathedral in London and Grand Central Station in New York, where someone whispering along the wall would hear their whisper coming from behind them as the sound traveled around the edge of the room and returned to its original spot (figure 2).

Optical Fiber Microresonators

Sumetsky and his colleagues were able to vary the optical fiber diameter by several nanometers. They did this with unprecedented precision, on the order of a hundredth of a nanometer.

This dimpling or narrowing of the fiber effectively changes the properties of the Whispering Gallery and has the effect that light traveling along the surface of the fiber would turn around and head back the way it came. If it were traveling between two of these narrowed portions of fiber, the light would continue to resonate back and forth with very little loss of signal. This is, in fact, the microresonator.

These optical fiber microresonators currently are able to retain light two orders of magnitude longer than lithographic microresonators – and the researchers say it's possible to push that number even higher.
If sufficient number of optical fiber microresonators were coupled together, again taking advantage of evanescent coupling, then any information contained in the light pulses could be stored long enough for computational purposes. The researchers have so far been able to couple 10 optical fiber microresonators, an important proof-of-concept step.

Manufacturing is a ‘SNAP'

It's possible to create these nanoscale changes to the radius of the fiber by exploiting a property inherent in the fiber created during manufacturing and discovered at OFS Laboratories several years ago. Optical fiber is made by heating a much thicker rod of glass with a precise chemical makeup and stretching and drawing it out into extremely fine and flexible fibers. When the fiber is drawn out, the process introduces certain tension, and this tension is frozen in, creating a predetermined amount of stress.

The researchers harnessed this fixed stress by directing a laser beam at the fiber to heat it. By raising the fiber's temperature, but keeping it well below the melting point, it was possible to release this intrinsic pressure, changing the diameter and refractive index of the fiber without deforming it any further. As long as the fiber is produced under the same conditions and it is heated below the melting point, the same effect is always achieved. This process enables a technology that the researchers call Surface Nanoscale Axial Photonics (SNAP).

"We heated it to a temperature lower than the melting temperature," said Sumetsky. "This annealing allows us to change the radius in this nanoscale range. In the new system, the accuracy of the fiber radius variation is about 0.1 angstrom – orders of magnitude better than achieved before."

Previous attempts have been made at harnessing optical fiber for microresonators, but these relied on polishing or melting the fiber to change its diameter. This produced very uneven results and could not achieve nanoscale dimensions. To enable evanescent coupling, it's vital that the circumference of the microresonators be controlled to sub-angstrom accuracy. The SNAP process ensures this accuracy and that each microresonator is nearly identical.

This is the crucial point the researchers believe will enable the technology to move from laboratory studies to manufacturing. As long as the optical fiber is produced under the same conditions, it will always produce the same effect when heated, changing its properties in the same precise manner. "We can faithfully reproduce these resonators. There's a real, robust way of fabricating these, and this is the first paper that actually shows that," Sumetsky said.

According to the researchers, it's possible these microresonators could be used in specialized devices in about two to three years. However, their greatest potential may be in pioneering optical computing and in enabling fundamental physics research.

Source: Optical Society (OSA)

Additional Information:  

• Paper: "Surface nanoscale axial photonics: Robust fabrication of high quality factor microresonators," Optics Letters, Vol. 36, Issue 24, pp. 4824-4826 (2011).
  

Laser sensing: Measuring and mapping jet engines

Engineerblogger
Dec 8, 2011

The University of Southampton’s Optoelectronics Research Centre (ORC) is developing lasers, which will allow for a better understanding of the combustion process in jet engines and reduce emissions.

The ORC is working on the £2.7m research project called FLITES (Fibre-Laser Imaging of gas Turbine Exhaust Species). It is funded by the Engineering and Physical Sciences Research Council, led by University of Manchester and aims to develop technology to reduce jet engine emissions.

"FLITES will allow us to map different chemical species and soot in the exhaust plume of aero engines,” said Professor Johan Nilsson at the ORC. "This will create a better understanding of the combustion process in the engine and enable us to optimise it at lower cost - with the increasing interest for substitution of fossil fuels with bio-fuels in the aviation industry, this is particularly important. Currently the cost is too high and the data collected too limited for extensive evaluations of new bio-fuels in aero-engines."

FLITES aims to establish a world-leading capability to map several exhaust species from aeroplanes using tomographic imaging.

The ORC, the University of Manchester, the University of Strathclyde, and commercial partners including Rolls-Royce, Shell, Covesion, Fianium and OptoSci, will work on the four-year study, motivated by lower-cost engine evaluation and monitoring and reduced carbon dioxide emissions and pollution.

It is expected that the research project will enhance turbine-related research and development capacity in both academia and industry by opening up access to exhaust plume chemistry.

It will underpin a new phase of low-net-carbon development that is underway in aviation, based on bio-derived fuels, and which entails extensive research in turbine engineering, turbine combustion, and fuel product formulation.

Source: University of Southampton Optoelectronics Research Centre (ORC)

Better method to produce ultraviolet light beams

Engineerblogger
Dec 06, 2011

In the setup for this experiment, a telecommunication-compatible infrared beam is coupled to the whispering-gallery resonator through a diamond prism and the generated near-infrared, visible, and ultraviolet light are collected by a multi-mode fiber. Photo courtesy of Mona Jarrahi

Existing coherent ultraviolet light sources are power hungry, bulky and expensive. University of Michigan researchers have found a better way to build compact ultraviolet sources with low power consumption that could improve information storage, microscopy and chemical analysis.

A paper on the research is newly published in Optics Express. The research was led by Mona Jarrahi and Tal Carmon, assistant professors in the Department of Electrical Engineering and Computer Science. The experiment was performed by Jeremy Moore and Matthew Tomes, both graduate students in the same department.

The researchers have optimized a type of optical resonator to take an infrared signal from relatively cheap telecommunication-compatible lasers and, using a low-power, nonlinear process, boost it to a higher-energy ultraviolet beam.

Their optical resonator is a millimeter-scale disk with a precisely engineered shape and smooth surface polishing to encourage the input beam to gain power as it circulates inside the resonator.

"We optimized the structure to achieve high gain over a broad range of optical wavelengths," Jarrahi said. "This allows us to make low-cost, wavelength-tunable ultraviolet sources using low-infrared power levels."

The researchers used their resonator to generate the fourth harmonic of the infrared beam they started with.

Like the harmonic distortions you get from new sound frequencies when you crank up a loudspeaker, engineers can generate harmonics of light by using the right materials. By pushing light beams through a nonlinear medium, they can coax out offshoot beams that are double, or in this case, quadruple the frequency and energy of the input beam, and one-quarter of the original wavelength.

Lasers get progressively more difficult to generate and more inefficient, as engineers aim for shorter wavelengths, the researchers said.

"As we go from green to blue, the efficiency of the laser goes down. Going to UV lasers is even harder," Jarrahi said. "This principle was first suggested by Einstein and is the reason why green laser pointers do not actually contain a green laser. It is actually a red laser and its wavelength is divided by two to become green light."

Ultraviolet light sources have applications in chemical detection, crisper medical imaging and finer lithography for more sophisticated integrated circuits and greater computer memory capacity.

Source:  University of Michigan

Additional Information:

• The paper is titled "Continuous-wave ultraviolet emission through fourth-harmonic generation in a whispering-gallery resonator." The research was funded by the National Science Foundation and the Air Force Office of Scientific Research.

New material can enhance energy, computer, lighting technologies

Engineerblogger
Nov 16, 2011

The image, called a “ball-and-stick modal,” illustrates the crystal structure of the new erbium crystal compound developed at ASU’s Nanophotonics Lab. The four different colors represent the four elements that were combined to produce the new material.

Arizona State University researchers have created a new compound crystal material that promises to help produce advances in a range of scientific and technological pursuits.

ASU electrical engineering professor Cun-Zheng Ning says the material, called erbium chloride silicate, can be used to develop the next generations of computers, improve the capabilities of the Internet, increase the efficiency of silicon-based photovoltaic cells to convert sunlight into electrical energy, and enhance the quality of solid-state lighting and sensor technology.

Ning’s research team of team of students and post-doctoral degree assistants help synthesize the new compound in ASU’s Nanophotonics Lab in the School of Electrical, Computer and Energy Engineering, one of the university’s Ira A. Fulton Schools of Engineering.

The lab’s erbium research is supported by the U.S. Army Research Office and U.S. Air Force Office of Scientific Research. Details about the new compound are reported in the Optical Materials Express on the website of the Optical Society of America.

The breakthrough involves the first-ever synthesis of a new erbium compound in the form of a single-crystal nanowire, which has superior properties compared to erbium compounds in other forms.

Erbium is one of the most important members of the rare earth family in the periodic table of chemical elements. It emits photons in the wavelength range of 1.5 micrometers, which are used in the optical fibers essential to high-quality performance of the Internet and telephones.

Erbium is used in doping optical fibers to amplify the signal of the Internet and telephones in telecommunications systems. Doping is the term used to describe the process of inserting low concentrations of various elements into other substances as a way to alter the electrical or optical properties of the substances to produce desired results. The elements used in such processes are referred to as dopants.

“Since we could not dope as many erbium atoms in a fiber as we wish, fibers had to be very long to be useful for amplifying an Internet signal. This makes integrating Internet communications and computing on a chip very difficult,” Ning explains.

“With the new erbium compound, 1,000 times more erbium atoms are contained in the compound. This means many devices can be integrated into a chip-scale system,” he says. “Thus the new compound materials containing erbium can be integrated with silicon to combine computing and communication functionalities on the same inexpensive silicon platform to increase the speed of computing and Internet operation at the same time.”

Erbium materials can also be used to increase the energy-conversion efficiency of silicon solar cells.

Silicon does not absorb solar radiation with wavelengths longer than 1.1 microns, which results in waste of energy – making solar cells less efficient.

Erbium materials can remedy the situation by converting two or more photons carrying small amounts of energy into one photon that is carrying a larger amount of energy. The single, more powerful photon can then be absorbed by silicon, thus increasing the efficiency of solar cells.

Erbium materials also help absorb ultraviolet light from the sun and convert it into photons carrying small amounts of energy, which can then be more efficiently converted into electricity by silicon cells. This color-conversion function of turning ultraviolet light into other visible colors of light is also important in generating white light for solid-state lighting devices.

While erbium’s importance is well-recognized, producing erbium materials of high quality has been challenging, Ning says.

The standard approach is to introduce erbium as a dopant into various host materials, such as silicon oxide, silicon, and many other crystals and glasses.

“One big problem has been that we have not been able to introduce enough erbium atoms into crystals and glasses without degrading optical quality, because too many of these kinds of dopants would cluster, which lowers the optical quality,” he says.

What is unique about the new erbium material synthesized by Ning’s group is that erbium is no longer randomly introduced as a dopant. Instead, erbium is part of a uniform compound and the number of erbium atoms is a factor of 1,000 more than the maximum amount that can be introduced in other erbium-doped materials.

Increasing the number of erbium atoms provides more optical activity to produce stronger lighting. It also enhances the conversion of different colors of light into white light to produce higher-quality solid-state lighting and enables solar cells to more efficiently convert sunlight in electrical energy.

In addition, since erbium atoms are organized in a periodic array, they do not cluster in this new compound. The fact that the material has been produced in a high-quality single-crystal form makes the optical quality superior to the other doped materials, Ning says.

Like many scientific discoveries, the synthesis of this new erbium material was made somewhat by accident.

“Similar to what other researchers are doing, we were originally trying to dope erbium into silicon nanowires. But the characteristics demonstrated by the material surprised us,” he says. “We got a new material. We did not know what it was, and there was no published document that described it. It took us more than a year to finally realize we got a new single-crystal material no one else had produced.”

Ning and his team are now trying to use the new erbium compound for various applications, such as increasing silicon solar cell efficiency and making miniaturized optical amplifiers for chip-scale photonic systems for computers and high-speed Internet.

“Most importantly,” he says, “there are many things we have yet to learn about what can be achieved with use of the material. Our preliminary studies of its characteristics show it has many amazing properties and superior optical quality. More exciting discoveries are waiting to be made.”

Source: Arizona State University

Additional Information:  

• The study "Single-crystal erbium chloride silicate nanowires as a Si-compatible light emission material in communication wavelength" by Anlian Pan, Leijun Yin, Zhicheng Liu, Minghua Sun, Ruibin Liu, Patricia L. Nichols, Yanguo Wang, and C. Z. Ning in The Optical Society (OSA).

• To read more on Ning's nanophotonics group.

Using Light, Researchers Convert 2-D Patterns Into 3-D Objects

Engineerblogger
Nov 11, 2011

The new technique can be used to create a variety of objects, such as cubes or pyramids, without ever having to physically touch the material.



Researchers from North Carolina State University have developed a simple way to convert two-dimensional patterns into three-dimensional (3-D) objects using only light.

“This is a novel application of existing materials, and has potential for rapid, high-volume manufacturing processes or packaging applications,” says Dr. Michael Dickey, an assistant professor of chemical and biomolecular engineering at NC State and co-author of a paper describing the research.

The process is remarkably simple. Researchers take a pre-stressed plastic sheet and run it through a conventional inkjet printer to print bold black lines on the material. The material is then cut into a desired pattern and placed under an infrared light, such as a heat lamp.

The bold black lines absorb more energy than the rest of the material, causing the plastic to contract – creating a hinge that folds the sheets into 3-D shapes. This technique can be used to create a variety of objects, such as cubes or pyramids, without ever having to physically touch the material. The technique is compatible with commercial printing techniques, such as screen printing, roll-to-roll printing, and inkjet printing, that are inexpensive and high-throughput but inherently 2-D.

By varying the width of the black lines, or hinges, researchers are able to change how far each hinge folds. For example, they can create a hinge that folds 90 degrees for a cube, or a hinge that folds 120 degrees for a pyramid. The wider the hinge, the further it folds. Wider hinges also fold faster, because there is more surface area to absorb energy.

“You can also pattern the lines on either side of the material,” Dickey says, “which causes the hinges to fold in different directions. This allows you to create more complex structures.”

The researchers developed a computer-based model to explain how the process works. There were two key findings. First, the surface temperature of the hinge must exceed the glass transition temperature of the material, which is the point at which the material begins to soften. Second, the heat has to be localized to the hinge in order to have fast and effective folding. If all of the material is heated to the glass transition temperature, no folding will occur.

“This finding stems from work we were doing on shape memory polymers, in part to satisfy our own curiosity. As it turns out, it works incredibly well,” Dickey says.

Source: North Carolina State University


Additional Information:

• The paper, “Self-folding of polymer sheets using local light absorption,” was published Nov. 10 in the journal Soft Matter, and was co-authored by Dickey; NC State Celanese Professor of Chemical and Biomolecular Engineering Jan Genzer; NC State Ph.D. student Ying Liu; and NC State undergraduate Julie Boyles.

Manufacturing microscale medical devices for faster tissue engineering

Engineerblogger
Nov 2, 2011

Figure 1. This image, taken by a scanning electron microscope, shows 16 micro-Venus structures that were produced simultaneously by a 16-beam two-photon polymerization system. The width of the entire array is less than the diameter of a typical human hair. The 16 structures were produced in approximately 45 seconds. Biomedical Optics Express.

In the emerging field of tissue engineering, scientists encourage cells to grow on carefully designed support scaffolds. The ultimate goal is to create living structures that might one day be used to replace lost or damaged tissue, but the manufacture of appropriately detailed scaffolds presents a significant challenge that has kept most tissue engineering applications confined to the research lab. Now a team of researchers from the Laser Zentrum Hannover (LZH) eV Institute in Hannover, Germany, and the Joint Department of Biomedical Engineering at the University of North Carolina at Chapel Hill and North Carolina State University have modified a manufacturing technique called two-photon polymerization (2PP) to create finely detailed structures such as tissue scaffolds more quickly and efficiently than was previously possible. The new technique, which the team describes in a paper published  in the open-access journal Biomedical Optics Express, could help pave the way to more wide-spread clinical use of microscale medical devices.

Many important biological functions take place on the microscopic level and as medical research advances into this Lilliputian realm, scientists have turned to precise techniques such as 2PP to create the tiny tools necessary to manipulate cells and other miniscule structures. In current-generation 2PP technology, a laser pulse that lasts approximately one quadrillionth of a second sends a burst of energy into unset resin, causing the molecules around the pulse to fuse together into two adjoining cone shapes. By focusing on multiple points in succession, 2PP can build up complex 3D structures, cone-shaped block by cone-shaped block.

2PP can be used to manufacture devices from a wide range of base materials and does not require extreme temperatures, harsh chemicals, or cleanroom facilities, but its main drawback is long fabrication times. Like in a tiled mosaic, small 2PP building blocks can create a richly detailed design, but if you want a large structure, like a tissue scaffold that could mimic natural body parts, it can take a long time to lay all the pieces together.

"Blood vessel networks can be several centimeters in length, but walls of the smallest branches (capillaries) are only a few micrometers thick. The same applies with any tissue. Many tissues may be large, but they all have important features on the microscale," says team member Shaun Gittard of the LZH. The team notes that using conventional 2PP to manufacture the tissue scaffolds for such structures could be prohibitively slow. They address the problem by using a computer-controlled hologram to split the 2PP laser into multiple beams, creating up to 16 different focus points that can work simultaneously.

"As an example, take the time for fabricating a single layered, 1-millimeter square with 100 nanometer resolution," the authors write. "With conventional single-focus 2PP at one millimeter per second, the fabrication time would be 2 hours and 47 minutes. In contrast, with 16 foci this same area could be scanned in merely 10 minutes." Or, in other words, many foci make light work.

The team first tested their multiple foci system by creating 16 miniature Venus statues, each so small as to be invisible to the human eye (see figure 1). "The Venus is kind of a logo of our research group," says Gittard. "We have used it as a familiar demonstration structure for various fabrication techniques."

Figure 2. Multifoci two-photon polymerization can be used to produce tissue engineering scaffolds made up of neighboring cylinders. With a conventional, single-focus system, only one cylinder at a time can be created (images a and c), but using the multifoci system, four cylinders (images b and d) were created simultaneously. The outer diameter of each cylinder is 35 micrometers. Biomedical Optics Express.

Figure 3. In this image, a multi-layer tissue scaffold created with single-focus two-photon polymerization ("2PP") (a) is shown beside one created with multifoci two-photon polymerization (b). The two scaffolds are structurally similar, but the one created with multifoci 2PP was completed in approximately one-fourth the time. In image c, cells from the inner lining of bovine blood vessels are shown growing on the multifoci-created scaffold. Biomedical Optics Express.

In addition to replicas of classic Greek artwork, the team also used the new technique to manufacture cylindrical tissue scaffolds (see figures 2 and 3) and an array of microneedles. Less than a half millimeter wide, rocket-shaped microneedles can be used to provide painless injections or take blood samples, notes Gittard (see figure 4). "One of the biggest promises in the future is real-time, pain-free glucose sensing and insulin delivery for treating diabetes," he says.

Figure 4. This array of microneedles was created, four needles at a time, by a four-beam two-photon polymerization system. The scale at the bottom shows a length of 1 millimeter. Biomedical Optics Express.

For now the team has only used the multiple beams to create multiple copies of the same structure. Their next goal is to use the system to produce one large, complex 3-D structure, which is a more complicated task since it requires moving the relative placement of the different foci during the fabrication process, Gittard says.

"The ability to produce large-scale devices with sub-micron features is exciting, as many cell features, such as organelles, are on this size scale," the authors write. Gittard explains that such detailed features could be used to control cell attachment and alignment, which is important since cell orientation affects function in a number of tissues, such as blood vessels, nerves, bone, and muscle.

Source: The Optical Society

Additional Information:

• Paper: "Fabrication of microscale medical devices by two-photon polymerization with multiple foci via a spatial light modulator," Shaun D. Gittard, Alexander Nguyen, Kotaro Obata, Anastasia Koroleva, Roger J. Narayan, Boris N. Chichkov, Biomedical Optics Express, Volume 2, Issue 11, pp. 3167-3178 (2011).

High-quality white light produced by four-color diode laser source

Engineerblogger
Oct 27, 2011

Sandia researcher Jeff Tsao examines the set-up used to test diode lasers as an alternative to LED lighting. Skeptics felt laser light would be too harsh to be acceptable. Research by Tsao and colleagues suggests the skeptics were wrong. (Photo by Randy Montoya).


The human eye is as comfortable with white light generated by diode lasers as with that produced by increasingly popular light-emitting diodes (LEDs), according to tests conceived at Sandia National Laboratories.

Both technologies pass electrical current through material to generate light, but the simpler LED emits lights only through spontaneous emission. Diode lasers bounce light back and forth internally before releasing it.

The finding is important because LEDs — widely accepted as more efficient and hardier replacements for century-old tungsten incandescent bulb technology — lose efficiency at electrical currents above 0.5 amps. However, the efficiency of a sister technology — the diode laser — improves at higher currents, providing even more light than LEDs at higher amperages.

“What we showed is that diode lasers are a worthy path to pursue for lighting,” said Sandia researcher Jeff Tsao, who proposed the comparative experiment. “Before these tests, our research in this direction was stopped before it could get started. The typical response was, ‘Are you kidding? The color rendering quality of white light produced by diode lasers would be terrible.’ So finally it seemed like, in order to go further, one really had to answer this very basic question first.”

Little research had been done on diode lasers for lighting because of a widespread assumption that human eyes would find laser-based white light unpleasant. It would comprise four extremely narrow-band wavelengths — blue, red, green, and yellow — and would be very different from sunlight, for example, which blends a wide spectrum of wavelengths with no gaps in between. Diode laser light is also ten times narrower than that emitted by LEDs.

The tests — a kind of high-tech market research — took place at the University of New Mexico’s Center for High Technology Materials. Forty volunteers were seated, one by one, before two near-identical scenes of fruit in bowls, housed in adjacent chambers. Each bowl was randomly illuminated by warm, cool, or neutral white LEDs, by a tungsten-filament incandescent light bulb, or by a combination of four lasers (blue, red, green, yellow) tuned so their combination produced a white light.

The experiment proceeded like an optometrist’s exam: the subjects were asked: Do you prefer the left picture, or the right? All right, how about now?

In the test setup, similar bowls of fruit were placed in a lightbox with a divider in the middle. In this photo, the bowl on one side was illuminated by a diode laser light and the other was lit by a standard incandescent bulb. The aesthetic quality of diode laser lighting (left bowl) compares favorably with standard incandescent lighting (right). (Photo by Randy Montoya).


The viewers were not told which source provided the illumination. They were instructed merely to choose the lit scene with which they felt most comfortable. The pairs were presented in random order to ensure that neither sequence nor tester preconceptions played roles in subject choices, but only the lighting itself. The computer program was written, and the set created, by Alexander Neumann, a UNM doctoral student of CHTM director Steve Brueck.

Each participant, selected from a variety of age groups, was asked to choose 80 times between the two changing alternatives, a procedure that took ten to twenty minutes, said Sandia scientist Jonathan Wierer, who helped plan, calibrate and execute the experiments. Five results were excluded when the participants proved to be color-blind. The result was that there was a statistically significant preference for the diode-laser-based white light over the warm and cool LED-based white light, Wierer said, but no statistically significant preference between the diode-laser-based and either the neutral LED-based or incandescent white light.

The results probably won’t start a California gold rush of lighting fabricators into diode lasers, said Tsao, but they may open a formerly ignored line of research. Diode lasers are slightly more expensive to fabricate than LEDs because their substrates must have fewer defects than those used for LEDs. Still, he said, such substrates are likely to become more available in the future because they improve LED performance as well.

Also, while blue diode lasers have good enough performance that the automaker BMW is planning their use in its vehicles’ next-generation white headlights, performance of red diode lasers is not as good, and yellow and green have a ways to go before they are efficient enough for commercial lighting opportunities.

Four laser beams — yellow, blue, green and red — converge to produce a pleasantly warm white light. Results suggest that diode-based lighting could be an attractive alternative to increasingly popular LED lighting, themselves an alternative to compact-florescent lights and incandescent bulbs. (Photo by Randy Montoya).


Still, says Tsao, a competition wouldn’t have to be all or nothing. Instead, he said, a cooperative approach might use blue and red diode lasers with yellow and green LEDs. Or blue diode lasers could be used to illuminate phosphors — the technique currently used by fluorescent lights and the current generation of LED-based white light — to create desirable shades of light.

The result makes possible still further efficiencies for the multibillion dollar lighting industry. The so-called ‘‘smart beams’’ can be adjusted on site for personalized color renderings for health reasons and, because they are directional, also can provide illumination precisely where it’s wanted.

The research was published in the July 1, Optics Express. This work was conducted as part of the Solid-State Lighting Science Energy Frontier Research Center, funded by the U.S. DOE Office of Science.

Source: Sandia National Laboratory

Engineers bring new meaning to the force of light

Engineerblogger
Oct 26, 2011

Yale University engineers recently demonstrated that nanomechanical resonators can operate at much higher amplitudes than previously thought. The results represent an advance in optomechanics, in which the force of light is used to control mechanical devices, and could have implications for future communications and sensing technologies.


New research by engineers at the Yale School of Engineering & Applied Science demonstrates that nanomechanical resonators can operate at much higher amplitudes than previously thought. The results represent an advance in optomechanics, in which the force of light is used to control mechanical devices. The findings could have implications for future communications and sensing technologies.

“We can flip a tiny switch with light,” said Hong Tang, associate professor of electrical engineering at Yale and the principal investigator of a new paper appearing online Oct. 23 in the journal Nature Nanotechnology.

Amplitude refers to vibration range. Achieving high-amplitudes in traditional nanoscale mechanical systems has proven difficult because reducing a resonator’s dimensions generally limits how much the resonator can move. Tang’s team shows a way of overcoming the performance limitations of conventional systems.

The operating principle is similar to the laser cooling technique used in atomic physics. “One can control the motion of a mechanical structure, amplify or cool its vibrations, just by controlling the wavelength of laser light,” said Mahmood Bagheri, the postdoctoral associate in Tang’s lab who is the paper’s lead author.

Tang and his research team also demonstrate in the paper that a tiny silicon structure within an optomechanical system can effectively store information without the aid of steady power supply — thus serving as a mechanical memory device.

Among other benefits, optomechancial memory devices can withstand harsher environments than electronic or magnetic memory devices, without losing data. Future technologies containing similar high-amplitude optomechanical resonators might be less sensitive to environmental conditions, such as variations in temperature and radiation. At the same time, high-amplitude resonators might enable more accurate and robust measuring devices.

Source: Yale University