UW-Madison engineers reveal record-setting flexible phototransistor

MADISON, Wis. -- Inspired by mammals' eyes, University of Wisconsin-Madison electrical engineers have created the fastest, most responsive flexible silicon phototransistor ever made.

The innovative phototransistor could improve the performance of myriad products -- ranging from digital cameras, night-vision goggles and smoke detectors to surveillance systems and satellites -- that rely on electronic light sensors. Integrated into a digital camera lens, for example, it could reduce bulkiness and boost both the acquisition speed and quality of video or still photos.

Developed by UW-Madison collaborators Zhenqiang "Jack" Ma, professor of electrical and computer engineering, and research scientist Jung-Hun Seo, the high-performance phototransistor far and away exceeds all previous flexible phototransistor parameters, including sensitivity and response time.

The researchers published details of their advance this week in the journal Advanced Optical Materials.

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Femtoseconds lasers help formation flying

The National Physical Laboratory (NPL) has helped to establish that femtosecond comb lasers can provide accurate measurement of absolute distance in formation flying space missions.

NPL, along with collaborators, produced technical reports for the European Space Agency (ESA). The conclusions demonstrated that the lasers were a suitable method for measurement in such missions.

Formation flying missions involve multiple spacecraft flying between tens and hundreds of metres apart, which autonomously control their position relative to each other. The benefit of such missions is they can gather data in a completely different way to a standard spacecraft – the formation can effectively act as one large sensor.

Measuring absolute distance between the formation spacecraft is critical to mission success. Femtosecond comb lasers are an accurate way of making such measurements. The lasers emit light with very short pulses – each lasting just a few femtoseconds (a femtosecond is one billionth of one millionth of a second). The short pulses allow time of flight measurements to be used to determine distance to a few microns.

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Engineers Ride ‘Rogue’ Laser Waves to Build Better Light Sources

New Technology Presented at World's Largest Optical Communication Conference Produces Better Sources of White Light

WASHINGTON, March 4—A freak wave at sea is a terrifying sight. Seven stories tall, wildly unpredictable, and incredibly destructive, such waves have been known to emerge from calm waters and swallow ships whole. But rogue waves of light -- rare and explosive flare-ups that are mathematically similar to their oceanic counterparts -- have recently been tamed by a group of researchers at the University of California, Los Angeles (UCLA).

UCLA's Daniel Solli, Claus Ropers, and Bahram Jalali are putting rogue light waves to work in order to produce brighter, more stable white light sources, a breakthrough in optics that may pave the way for better clocks, faster cameras, and more powerful radar and communications technologies. Their findings will be presented during the Optical Fiber Communication Conference and Exposition/National Fiber Optic Engineers Conference (OFC/NFOEC), taking place March 22-26 in San Diego.

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Tiny lasers get a notch up

<p>Tiny lasers get a notch up</p>

A new theoretical analysis could help design better microlasers

WASHINGTON, Jan. 22—Tiny disk-shaped lasers as small as a speck of dust could one day beam information through optical computers. Unfortunately, a perfect disk will spray light out, not as a beam, but in all directions. New theoretical results, reported in the Optical Society (OSA) journal Optics Letters, explain how adding a small notch to the disk edge provides a single outlet for laser light to stream out.

To increase the speed of computers and telecommunication networks, researchers are looking to replace electrical currents with beams of light that would originate from small semiconductor lasers. However, shrinking lasers down to a few micrometers in size is not easy. The typical laser builds up its concentrated light beam by bouncing light rays, or modes, back and forth inside a reflective cavity. This linear design is not practical for microlasers. Instead, scientists discovered in 1992 that they could get light amplification by having rays bounce around in a circle inside a small flat disk. These light rays are called "whispering gallery modes" because they are similar to sound waves that travel across a room by skimming along a curved wall or ceiling.

The problem is that a disk is rotationally invariant, so there is no preferred direction for the amplified light to escape. Many microlaser designs end up shooting light out in multiple directions within the plane of the disk. "The experimentalists have a holy grail of unidirectional emission in microlasers," says Martina Hentschel of the Max Planck Institute for the Physics of Complex Systems. In the past few years, some progress has been made with so-called spiral microlasers, which have a tiny notch that resembles the outer opening of a snail shell. Certain experiments have shown that light tends to propagate in a single direction from the notch. But other experiments have not been so lucky. In order to understand these contrasting results, Hentschel and her colleague Tae-Yoon Kwon have performed a systematic study of spiral microlasers using a state-of-the-art theoretical description.

Physicists typically treat the light rays trapped inside a cavity as if they were billiard balls bouncing off walls, Hentschel explains. Some light rays escape, but those rays that just barely graze the inside surface are fully reflected back into the cavity (this being the same effect that channels light beams along optical fibers). Unfortunately, this simple "billiard" model is not sufficient for explaining spiral microlasers, Hentschel says.

Hentschel and Kwon therefore chose a more sophisticated model based on the electromagnetic wave and laser equations. This framework allowed the researchers to control what part of the semiconductor material would be excited, or "pumped," to a light-emitting state. Numerical calculations showed that the two whispering gallery modes inside a spiral cavity—one traveling clockwise, the other counterclockwise—are coupled together, but only one of these modes is able to escape out through the spiral's notch. To maximize this unidirectional emission, the researchers found that the notch size should be roughly twice the wavelength of the light. Moreover, the pumping needs to be confined to the rim of the spiral, specifically the outer 10 percent. These parameters could aid in the design of better-collimated microlasers. "The optimal geometry and boundary pumping is very useful to know for an experimentalist," Hentschel says.

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Scientists demonstrate highly directional semiconductor lasers

CAMBRIDGE, Mass. – July 28, 2008 – Applied scientists at Harvard University in collaboration with researchers from Hamamatsu Photonics in Hamamatsu City, Japan, have demonstrated, for the first time, highly directional semiconductor lasers with a much smaller beam divergence than conventional ones. The innovation opens the door to a wide range of applications in photonics and communications. Harvard University has also filed a broad patent on the invention.

Spearheaded by graduate student Nanfang Yu and Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, all of Harvard's School of Engineering and Applied Sciences (SEAS), and by a team at Hamamatsu Photonics headed by Dr. Hirofumi Kan, General Manager of the Laser Group, the findings were published online in the July 28th issue of Nature Photonics and will appear in the September print issue.

"Our innovation is applicable to edge-emitting as well as surface-emitting semiconductor lasers operating at any wavelength—all the way from visible to telecom ones and beyond," said Capasso. "It is an important first step towards beam engineering of lasers with unprecedented flexibility, tailored for specific applications. In the future, we envision being able to achieve total control of the spatial emission pattern of semiconductor lasers such as a fully collimated beam, small divergence beams in multiple directions, and beams that can be steered over a wide angle."

While semiconductor lasers are widely used in everyday products such as communication devices, optical recording technologies, and laser printers, they suffer from poor directionality. Divergent beams from semiconductor lasers are focused or collimated with lenses that typically require meticulous optical alignment—and in some cases bulky optics.

To get around such conventional limitations, the researchers sculpted a metallic structure, dubbed a plasmonic collimator, consisting of an aperture and a periodic pattern of sub-wavelength grooves, directly on the facet of a quantum cascade laser emitting at a wavelength of ten microns, in the invisible part of the spectrum known as the mid-infrared where the atmosphere is transparent. In so doing, the team was able to dramatically reduce the divergence angle of the beam emerging from the laser from a factor of twenty-five down to just a few degrees in the vertical direction. The laser maintained a high output optical power and could be used for long range chemical sensing in the atmosphere, including homeland security and environmental monitoring, without requiring bulky collimating optics.

"Such an advance could also lead to a wide range of applications at the shorter wavelengths used for optical communications. A very narrow angular spread of the laser beam can greatly reduce the complexity and cost of optical systems by eliminating the need for the lenses to couple light into optical fibers and waveguides," said Dr. Kan.

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Advance brings low-cost, bright LED lighting closer to reality

Researchers at Purdue University have overcome a major obstacle in reducing the cost of "solid state lighting," a technology that could cut electricity consumption by 10 percent if widely adopted.

The technology, called light-emitting diodes, or LEDs, is about four times more efficient than conventional incandescent lights and more environmentally friendly than compact fluorescent bulbs. The LEDs also are expected to be far longer lasting than conventional lighting, lasting perhaps as long as 15 years before burning out.


"The LED technology has the potential of replacing all incandescent and compact fluorescent bulbs, which would have dramatic energy and environmental ramifications," said Timothy D. Sands, the Basil S. Turner Professor of Materials Engineering and Electrical and Computer Engineering.

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Light seems to defy its own speed limit

It's a speed record that is supposed to be impossible to break. Yet two physicists are now claiming they have propelled photons faster than the speed of light. This would be in direct violation of a key tenet of Einstein's special theory of relativity that states that nothing, under any circumstance, can exceed the speed of light.

Günter Nimtz and Alfons Stahlhofen of the University of Koblenz, Germany, have been exploring a phenomenon in quantum optics called photon tunnelling, which occurs when a particle slips across an apparently uncrossable barrier. The pair say they have now tunnelled photons "instantaneously" across a barrier of various sizes, from a few millimetres up to a metre. Their conclusion is that the photons traverse the barrier much faster than the speed of light.

Full story (available after August 18, 2007 from New Scientist)


Aluminum foil lamps outshine incandescent lights

CHAMPAIGN, Ill. -- Researchers at the University of Illinois are developing panels of microcavity plasma lamps that may soon brighten people’s lives. The thin, lightweight panels could be used for residential and commercial lighting, and for certain types of biomedical applications.

“Built of aluminum foil, sapphire and small amounts of gas, the panels are less than 1 millimeter thick, and can hang on a wall like picture frames,” said Gary Eden, a professor of electrical and computer engineering at the U. of I., and corresponding author of a paper describing the microcavity plasma lamps in the June issue of the Journal of Physics D: Applied Physics.

Like conventional fluorescent lights, microcavity plasma lamps are glow-discharges in which atoms of a gas are excited by electrons and radiate light. Unlike fluorescent lights, however, microcavity plasma lamps produce the plasma in microscopic pockets and require no ballast, reflector or heavy metal housing. The panels are lighter, brighter and more efficient than incandescent lights and are expected, with further engineering, to approach or surpass the efficiency of fluorescent lighting.

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Physicists control light at the nanoscale

Physicists in Europe have unveiled a new technique that can control the intensity distribution of laser pulses at dimensions that are much smaller than the wavelength of the laser light. The method combines pulse-shaping techniques with near-field optics and the researchers claim that it is a major step forward in the development of laser-based tools for the manipulation of matter on a very small length scales (Nature 446 301).

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Madison-based Alfalight receives $1.7 million contract


Posted: March 20, 2007

A small, fast-growing Madison company said Tuesday that it won a $1.7 million contract from the Army to help build high-power lasers.

Alfalight Inc. is to use the money to develop very high-power pump blocks, which are power sources for lasers.

The one-year contract is from the Army Research Laboratory in Adelphi, Md., for its scalable, high-efficiency solid-state laser program.

"We expect to develop both usable pump prototypes and provide valuable research results to the Army Research Laboratory upon completion," said Manoj Kanskar, vice president of research and development for Alfalight.

In addition to helping the Army develop lasers, the pump blocks could have uses in commercial material-handling equipment, Alfalight said.

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