Etched quantum dots shape up as single photon emitters

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The conventional way to build quantum dots—at NIST and elsewhere—is to grow them like crystals in a solution, but this somewhat haphazard process results in irregular shapes. The new, more precise process was developed by NIST postdoctoral researcher Varun Verma when he was a student at the University of Illinois. Verma uses electron beam lithography and etching to carve quantum dots inside asemiconductorsandwich (called a quantum well) that confines particles in two dimensions. Lithography controls the dot's size and position, while sandwich thickness and composition—as well as dot size—can be used to tune the color of the dot'slightemissions.

Some quantum dots are capable of emitting individual, isolated photons on demand, a crucial trait for quantum information systems that encode information by manipulating single photons. In new work reported inOptics Express, NIST tests demonstrated that the lithographed and etched quantum dots do indeed work as sources of single photons. The tests were performed on dots made of indium gallium arsenide. Dots of various diameters were patterned in specific positions in square arrays. Using a laser to excite individual dots and a photon detector to analyze emissions, NIST researchers found that dots 35 nanometers (nm) wide, for instance, emitted nearly all light at a wavelength of 888.6 nm. The timing pattern indicated that the light was emitted as a train of single photons.

NIST researchers now plan to construct reflective cavities around individual etched dots to guide their light emissions. If each dot can emit most photons perpendicular to the chip surface, more light can be collected to make a more efficient single photon source. Vertical emission has been demonstrated with crystal-grown quantum dots, but these dots can't be positioned or distributed reliably in cavities. Etched dots offer not only precise positioning but also the possibility of making identical dots, which could be used to generate special states of light such as two or morephotonsthat are entangled, a quantum phenomenon that links their properties even at a distance.

Thequantum dotstested in the experiments were made at NIST. A final step was carried out at the University of Illinois, where a crystal layer was grown over the dots to form clean interfaces.

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Breakthrough for photons in the microwave frequency range

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Whenphotonsreleased by a conventional light or radiation source hit a detector, they trigger anelectrical signalcomparable to a single“click” of a Geiger counter, which rattles when radioactive particles strike it. However, unlike optical photons, until now there have been no detectors that can detect single photons at particularly low frequencies, such as the microwave frequency range. The intensity of these microwave photons is much too weak for this. The research group led by ETH Zurich Professor Andreas Wallraff from the Department of Physics has now been able to characterise such low-intensity photons even without any“clicking” detectors by using a special apparatus and method. Physicists need techniques of this kind, for example to research the fundamental principles of quantum mechanics or to enable efficient information transmission in optical data communication.

Detector and emitter in one

In the interdisciplinary collaboration between Wallraff’s group, the Master student Deniz Bozyigit from the Department of Information Technology and Electrical Engineering and Canadian scientists, the researchers integrated a light source that generates single microwave photons onto a microchip. The frequency range of these photons is a few gigahertz, similar to the electromagnetic radiation of mobile phones and microwave ovens. With this innovative arrangement, a single photon is generated with an accuracy controlled to within a few nanoseconds and at a total rate of two million times per second. The scientists also constructed a highly sensitive and efficient measuring device, similar to that used for optical photons, on the same microchip in order to show that this apparatus really does generate only single photons.

Only one way possible

In the case of optical photons, a half-mirrored beam splitter is used for this purpose. The photons can either be reflected or pass through the beam splitter, with equal probability. For each of these two alternatives there is a detector which identifies the reflected or transmitted photon. According to the laws of quantum mechanics, an individual photon generated by a photon source can never be half-reflected and half-transmitted. It must opt for one of the two alternatives. In this case the light is no longer described by its wave properties as in classical physics; the photons behave like particles. If only one of the detectors“clicks”, this proves that the source really does generate only single photons.

Wallraff explains how single photons have now been successfully recorded in this way even in the microwave frequency range:“Until now it was thought that these correlations– characterizing the properties of the photons and how they behave at a beam splitter– could be measured only if the detector detects the photons with a“click”, like a particle. Since the intensity of photons in the microwave frequency range is too weak for this, we amplify the signals of the incoming photons and then measure the amplitude of the field generated by the photons”. To cope with the immense volume of data from these measurements, amounting to more than 1,000 gigabytes per hour, the researchers also needed to develop an innovative system which can process this data in real time. From the statistics thus obtained, and by using the newly developed method, they were able to prove that only a single photon is emitted and, as expected, this took either one or the other of the possible routes.

Very promising for information processing

If it proves possible to manipulate and measure individual photons in the microwave frequency range, this measuring unit could become a component of a quantum computer: single photons strike a beam splitter and transmit information quantum mechanically across an integrated microchip, and are then detected. According to the physicist, this could allow information processing procedures to be carried out more efficiently in the future.

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Researchers fabricate first large-area, full-color quantum dot display

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The researchers, Tae-Ho Kim and coauthors from various institutes in South Korea, have published their study on the first four-inch, full-color quantum dot display in a recent issue ofNature Photonics. The display consists of a film printed with trillions of the tinyquantum dots(an average of 3 trillion per cm²). The quantum dots emit light at a specific wavelength (color) that can be tuned by changing the size of the quantum dots.

Previous attempts to make full-color quantum dot displays have faced challenges in that image quality tended to decrease with the size of the display. To overcome this challenge, the researchers in the current study used a different method for applying the quantum dots to the film’s surface. Instead of spraying the quantum dots onto the film, the researchers created an“ink stamp” out of a patterned silicon wafer. They used the stamp to pick up strips of size-selected quantum dots, and then stamp them onto the substrate. Unlike the spraying methods, this method does not require the use of a solvent, which previously reduced color brightness.

As the results showed, the new quantum dot display has a greater density and uniformity of quantum dots, as well as a brighter picture and higher energy efficiency than previous quantum dot displays. The new display is also flexible, so applications could include roll-up portable displays or flexible lighting applications. The technology could also be used in photovoltaic devices, which would especially benefit from quantum dots’ high energy efficiency.

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Single photon management for quantum computers advanced by NIST

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In principle, quantum computers can perform calculations that are impossible or impractical using conventional computers by taking advantage of the peculiar rules ofquantum mechanics. To do this, they need to operate on things that can be manipulated into specific quantum states. Photons are among the leading contenders.

The new NIST papers address one of the many challenges to a practical quantum computer: the need for a device that produces photons in ready quantities, but only one at a time, and only when the computer's processor is ready to receive them. Just as garbled data will confuse a standard computer, an information-bearingphotonthat enters a quantum processor together with other particles—or when the processor is not expecting it—can ruin a calculation.

The single-photon source has been elusive for nearly two decades, in part because no method of producing these particles individually is ideal."It's a bit like playing a game of whack-a-mole, where solving one problem creates others,"says Alan Migdall of NIST's Optical Technology Division."The best you can do is keep all the issues under control somewhat. You can never get rid of them."

The team's first paper addresses the need to be certain that a photon is indeed coming when the processor is expecting it, and that none show up unexpected. Many kinds of single-photon sources create a pair of photons and send one of them to a detector, which tips off the processor to the fact that the second, information-bearing photon is on its way. But since detectors are not completely accurate, sometimes they miss the"herald"photon—and its twin zips into the processor, gumming up the works.

The team effort, in collaboration with researchers from the Italian metrology laboratory L'Istituto Nazionale di Ricerca Metrologica (INRIM), handled the issue by building a simple gate into the source. When a herald photon reaches the detector, the gate opens, allowing the second photon past."You get a photon when you expect one, and you don't get one when you don't,"Migdall says."It was an obvious solution; others proposed it long ago, we were just the first ones to build it. It makes the single photon source better."

In a second paper, the NIST team describes a photon source to address two other requirements. Quantum computers will need many such sources working in parallel, so sources must be able to be built in large numbers and operate reliably; and so that the computer can tell the photons apart, the sources must create multiple individual photons, but all at different wavelengths. The team outlines a way to create just such a source out of silicon, which has been well-understood by the electronics industry for decades as the material from which standard computer chips are built.

"Ordinarily a particular material can produce only pairs in a specific pair of wavelengths, but our design allows production of photons at a number of regular and distinct wavelengths simultaneously, all from one source,"Migdall says."Because the design is compatible with microfabrication techniques, this accomplishment is the first step in the process of creating sources that are part of integrated circuits, not just prototype computers that work in the hothouse of the lab."

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A guide star lets scientists see deep into human tissue

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Lihong Wang, PhD, the Gene K. Beare Distinguished Professor of Biomedical Engineering at Washington University in St. Louis, has invented a guide star for biomedical rather than celestial imaging, a breakthrough that promises game-changing improvements in biomedical imaging and light therapy.

Wang's guide star is an ultrasound beam that"tags"light that passes through it. When it emerges from thetissue, the tagged light, together with a reference beam, creates ahologram.

When a"reading beam"is then shown back through the hologram, it acts as a time-reversal mirror, creatinglight wavesthat follow their own paths backward through the tissue, coming to a focus at their virtual source, the spot where the ultrasound is focused.

The technique, called time-reversed ultrasonically encoded (TRUE) optical focusing, thus allows the scientist to focus light to a controllable position within tissue.

Wang thinks TRUE will lead to more effective light imaging, sensing, manipulation and therapy, all of which could be a boon medical research, diagnostics, and therapeutics.

In photothermal therapy, for example, scientists have had trouble delivering enough photons to a tumor to heat and kill the cells. So they either have to treat the tumor for a long time or use very strong light to get enough photons to the site, Wang says. But TRUE will allow them to focus light right on thetumor, ideally without losing a single tagged photon to scattering.

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In both cases photons take random paths through tissue. Some are lost (blue) but others (green) will reach the mirror on the other side of the tissue. The mirror is a special phase conjugate mirror that turns the light around and sends it back on its original path, as though time had been reversed. Clever as this is, by itself it isn't very useful because the light scatters again as is backtracks (left). In the new method, called TRUE, ultrasound is focused into the tissue (small black ring). Light passing through the ultrasound field is tagged by it and selectively returned by the mirror to its virtual source, the ultrasound focus (right). Instead of scattering, the light is brought to a focus inside the tissue. Credit: Lihong Wang

"Focusing light into a scattering medium such as tissue has been a dream for years and years, since the beginning of biomedical optics,"Wang says."We couldn't focus beyond say a millimeter, the width of a hair, and now you can focus wherever you wish without any invasive measure."

The new method was published inNature Photonics, which appeared online Jan. 16, and has since been spotlighted by Physics Today (both online and in print) and in aNature PhotonicsBackstage Interview.

The problem

Light is in many ways the ideal form of electromagnetic radiation for imaging and treating biological tissues, but it suffers from an overwhelming drawback. Light photons ricochets off nonuniformities in tissue like a steel ball ricochets off the bumpers of an old-fashioned pinball machine.

This scattering prevents you from seeing even a short distance through tissue; you can't, for example, see the bones in your hand. Light of the right color can penetrate several centimeters into biological tissue, but even with the best current technology, it isn't possible to produce high-resolution images of objects more than a millimeter below the skin with light alone.

Ultrasound's advantages and drawbacks are in many ways complementary to those of light. Ultrasound scattering is a thousand times weaker than optical scattering.

Ultrasound reveals a tissue's density and compressibility, which are often not very revealing. For example, the density of early-stage tumors doesn't differ that much from that of healthy tissue.

Ultrasound tagging

The TRUE technique overcomes these problems by combining for the first time two tricks of biomedical imaging science: ultrasound tagging and time reversal.

Wang had experimented with ultrasound tagging of light in 1994 when he was working at the M.D. Anderson Cancer Center in Houston, Texas. In experiments using a tissue phantom (a model that mimics the opacity of tissue), he focused ultrasound into the phantom from above, and then probed the phantom with a laser beam from the side.

The laser light had only one frequency as it entered the tissue sample, but the ultrasound, which is a pressure wave, changed the tissue's density and the positions of its scattering centers. Light passing through the precise point where the ultrasound was focused acquired different frequency components, a change that"tagged"these photons for further manipulation.

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A conventional mirror (bottom) does not correct the distortion of a wavefront produced by the water-filled bottle in this illustration. A time reversal, or phase conjugating, mirror (top), on the other hand, produces a wavefront that precisely retraces the path of the light, as if time were going backward. It reverses the distortions introduced by the water, producing a perfect image of the tiger. Credit: Wikimedia Commons

By tuning a detector to these frequencies, it is possible to sort photons arriving from one spot (the ultrasound focus) within the tissue and to discard others that have bypassed the ultrasonic beam and carry no information about that spot. The tagged photons can then be used to paint an image of the tissue at the ultrasound focus.

Ultrasound modulation of light allowed Wang to make clearer images of objects in tissue phantoms than could be made with light alone. But this technology selects only photons that have traversed the ultrasound field and cannot focus light.

Time reversal

While Wang was working on ultrasound modulation of optical light, a lab at the Langevin Institute in Paris led by Mathias Fink, was working on time reversal of sound waves.

No law of physics is violated if waves run backward instead of forward. So for every burst of sound (or light) that diverges from a source, there is in theory a set of waves that could precisely retrace the path of the sound back to the source.

To make this happen, however, you need a time-reversal mirror, a device to send the waves backward along exactly the same path by which they arrived. In Fink's experiments, the mirror consisted of a line of transducers that detected arriving sound and fed the signal to a computer.

Each transducer then played back its sound in reverse— in synchrony with the other transducers. This created what is called the conjugate of the original wave, a copy of the wave that traveled backward rather than forward and refocused on the original point source.

The idea of time reversal is so remote from everyday experience it is difficult to grasp, but as Scientific American reported at the time, if you stood in front of Fink's time-reversal"mirror"and said"hello,"you would hear"olleh,"and even more bizarrely, the sound of the"olleh,"instead of spreading throughout the room from the loudspeakers, would converge onto your mouth.

In a 1994 experiment, Fink and his colleagues sent sound through a set of 2000 steel rods immersed in a tank of water. The sound scattered along all the possible paths through the rods, arriving at the transducer array as a chaotic wave. These signals were time-reversed and sent back through the forest of rods, refocusing to a point at the source location.

In effect, time reversal is a way to undo scattering.

Combining the tricks

Wang was aware of the work with time reversal, but at first couldn't see how it might help solve his problem with tissue scattering.

In 2004, Michael Feld, a physicist interested inbiomedical imaging, invited Wang to give a seminar at the Massachusetts Institute of Technology."At dinner we talked about time reversal,"Wang says."Feld was thinking about time reversal, I was thinking about time reversal, and so was another colleague dining with us."

"The trouble was, we couldn't figure out how to use it. You know, if you send light through a piece of tissue, the light will scatter all over the place, and if you capture it and reverse it, sending it back, it will still be scattered all over the place, so it won't concentrate photons."

"And then 13 years after the initial ultrasound-tagging experiments, I suddenly realized I could combine these two techniques.

"If you added ultrasound, then you could focus light into tissue instead of through tissue. Ultrasound tagging lets you reverse and send back only those photons you know are going to converge to a focus in the tissue."

"Ultrasound provides a virtual guide star, and to make optical time reversal effective you need a guide star,"Wang says.

A time-reversal mirror for light

It's much easier to make a time-reversal mirror for ultrasound than for light. Because sound travels slowly, it is easy to record the entire time course of a sound signal and then broadcast that signal in reverse order.

But a light wave arrives so fast it isn't possible to record a time course with sufficient time resolution. No detector will respond fast enough. The solution is to record an interference pattern instead of a time course.

The beam that has gone through the tissue and a reference beam form an interference pattern, which is recorded as a hologram by a special photorefractive crystal.

Then the wavefront is reconstructed by sending a reading beam through the crystal from the direction opposite to that of the reference beam. The reading beam reconstitutes a reversed copy of the original wavefront, one that comes to a focus at the ultrasound focus.

Unlike the usual hologram, the TRUE hologram is dynamic and constantly changing. Thus it is able to compensate for natural motions, such as breathing and the flow of blood, and it adapts instantly when the experimenter moves the ultrasonic focus to a new spot.

More photons to work with

Wang expects the TRUE technique for focusing light within tissue will have many applications, including optical imaging, sensing, manipulation and therapy. He also mentions its likely impact on the emerging field of optogenetics.

In optogenetics, light is used to probe and control living neurons that are expressing light-activatable molecules or structures. Optogenetics may allow the neural circuits of living animals to be probed at the high speeds needed to understand brain information processing.

But until now, optogenetics has suffered from the same limitation that plagues optical methods for studying biological tissues. Areas of the brain near the surface can be stimulated with light sources directly mounted on the skull, but to study deeper areas, optical fibers must be inserted into the brain.

TRUE will allow light to be focused on these deeper areas without invasive procedures, finally achieving the goal of making tissue transparent at optical frequencies.

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Engineers grow nanolasers on silicon, pave way for on-chip photonics

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They describe their work in a paper to be published Feb. 6 in an advanced online issue of the journalNature Photonics.

"Our results impact a broad spectrum of scientific fields, including materials science, transistor technology,laserscience, optoelectronics andoptical physics,"said the study's principal investigator, Connie Chang-Hasnain, UC Berkeley professor of electrical engineering and computer sciences.

The increasing performance demands of electronics have sent researchers in search of better ways to harness the inherent ability oflight particlesto carry far more data thanelectrical signalscan. Optical interconnects are seen as a solution to overcoming the communications bottleneck within and betweencomputer chips.

Becausesilicon, the material that forms the foundation of modern electronics, is extremely deficient at generating light, engineers have turned to another class of materials known as III-V (pronounced"three-five") semiconductors to create light-based components such as light-emitting diodes (LEDs) and lasers.

But the researchers pointed out that marrying III-V with silicon to create a single optoelectronic chip has been problematic. For one, the atomic structures of the two materials are mismatched.

"Growing III-V semiconductor films on silicon is like forcing two incongruent puzzle pieces together,"said study lead author Roger Chen, a UC Berkeley graduate student in electrical engineering and computer sciences."It can be done, but the material gets damaged in the process."

Moreover, the manufacturing industry is set up for the production of silicon-based materials, so for practical reasons, the goal has been to integrate the fabrication of III-V devices into the existing infrastructure, the researchers said.

"Today's massive silicon electronics infrastructure is extremely difficult to change for both economic and technological reasons, so compatibility with silicon fabrication is critical,"said Chang-Hasnain."One problem is that growth of III-V semiconductors has traditionally involved high temperatures– 700 degrees Celsius or more– that would destroy the electronics. Meanwhile, other integration approaches have not been scalable."

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Shown is a schematic (left) and various scanning electron microscope images of nanolasers grown directly on a silicon surface. The achievement could lead to a new class of optoelectronic chips. Credit: Connie Chang-Hasnain Group

The UC Berkeley researchers overcame this limitation by finding a way to grow nanopillars made of indium gallium arsenide, a III-V material, onto asilicon surfaceat the relatively cool temperature of 400 degrees Celsius.

"Working at nanoscale levels has enabled us to grow high quality III-V materials at low temperatures such that silicon electronics can retain their functionality,"said Chen.

The researchers used metal-organic chemical vapor deposition to grow the nanopillars on the silicon."This technique is potentially mass manufacturable, since such a system is already used commercially to make thin film solar cells and light emitting diodes,"said Chang-Hasnain.

Once the nanopillar was made, the researchers showed that it could generate near infrared laser light– a wavelength of about 950 nanometers– at room temperature. The hexagonal geometry dictated by the crystal structure of the nanopillars creates a new, efficient, light-trapping optical cavity. Light circulates up and down the structure in a helical fashion and amplifies via this optical feedback mechanism.

The unique approach of growing nanolasers directly onto silicon could lead to highly efficient silicon photonics, the researchers said. They noted that the miniscule dimensions of the nanopillars– smaller than one wavelength on each side, in some cases– make it possible to pack them into small spaces with the added benefit of consuming very little energy

"Ultimately, this technique may provide a powerful and new avenue for engineering on-chip nanophotonic devices such as lasers, photodetectors, modulators and solar cells,"said Chen.

"This is the first bottom-up integration of III-V nanolasers onto silicon chips using a growth process compatible with the CMOS (complementary metal oxide semiconductor) technology now used to make integrated circuits,"said Chang-Hasnain."This research has the potential to catalyze an optoelectronics revolution in computing, communications, displays and optical signal processing. In the future, we expect to improve the characteristics of these lasers and ultimately control them electronically for a powerful marriage between photonic and electronic devices."

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Scientists study processes using high-intensity ultrashort X-ray pulses

The generation of X-ray flashes that are only a few femtoseconds (quadrillionths of a second) long has been possible for some years. Such flashes can be produced by free-electron lasers (FEL), such as FLASH at the DESY research centre in Hamburg, LCLS in Stanford (USA) and the X-ray laser European XFEL currently under construction. So far, however, experiments only reached time resolutions of typically around one hundred femtoseconds– i.e., two orders of magnitude worse than the actual pulse durations. The problem was to determine precisely when the X-ray pulse arrived at the experiment.

A research group from the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB), DESY, the European XFEL GmbH and the Helmholtz Institute Jena has now found a way to measure the arrival time of the X-ray pulses with a precision of less than ten femtoseconds. The method is based on a so-called cross-correlation.

The new method was developed at the free-electron laser FLASH for so-called pump-probe processes. As an example: a first ultrashort pump pulse triggers a photochemical reaction. A second X-ray radiation pulse takes a“photograph” of how the reaction proceeds. For the first time, researchers are now able to determine exactly at what time the picture produced by the second pulse is created. For this new method, they make use of a side effect of the X-ray pulse generation. Indeed, the electron bunch accelerated in FLASH emits both an X-ray flash and an intense terahertz flash at the same time. The researchers separate the two flashes using a perforated, gold-coated mirror. As both pulses are created at the same time and from the same electron bunch, theterahertzflash can be used as a temporal“marker” of the X-ray flash. Using this method, the researchers were able to determine the time at which the X-ray pulse arrived at the sample with a precision of seven femtoseconds.

The new method can be used at all existing and planned new FEL sources given only very slight modifications. In combination with appropriate experiments, it opens up the possibility to fully exploit the potential of these large-scale facilities. For the first time, phenomena can now be studied withX-rayson the relevant femtosecond time scale– something scientists have long been waiting for.

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Smart lasers could make cancer biopsies painless, help speed new drugs to market

To test forskin cancer, patients today must endure doctors cutting away a sliver of skin, sending thebiopsyto a lab and anxiously awaiting the results. Usinglasermicroscopes that deploy rapid, ultra-short pulses to identifymolecules, doctors may soon have the tools to painlessly scan a patient's troublesome mole and review the results on the spot, said Marcos Dantus.

The results touting this new molecule-selective technology can be found in the current issue ofNature Photonics, which Dantus co-authored with Sunney Xie of Harvard University.

"Smart lasers allow us to selectively excite compounds– even ones with small spectroscopic differences,"said Dantus."We can shape the pulse of the lasers, excite one compound or another based on their vibrational signatures, and this gives us excellent contrast."

In the past, researchers could approach this level of contrast by introducing fluorescent compounds. With the breakthrough using stimulated Raman scattering microscopy, fluorescent markers are unnecessary.

"Label-free molecular imaging has been the holy grail in medicine,"Dantus said."SRS imaging gives greater specificity and the ability to map a particular chemical species in the presence of an interfering species, such as cholesterol in the presence of lipids."

Additional potential applications include allowing researchers to closely examine howcompoundspenetrate skin and hair. Smart lasers also can better identify how drugs penetrate tissue and how drugs and tissue interact, thus mitigating the chances of potential side effects and helping reduce the time required to bring new drugs to market.

Dantus also is using smart laser imaging technology at MSU for detecting traces of hazardous substances from a distance.

"The ability to image with molecular specificity and sensitivity opens a number of applications in medicine as well as in homeland security,"he said.

Collaboration for the paper began when Harvard graduate student Christian Freudiger contacted BioPhotonic Solutions, a high-tech company Dantus launched in 2003 based on his research at MSU. Dantus was not only able to provide the laser pulse shaper Harvard needed to conduct the research, but he also was able to lend his expertise as well as the support of his MSU laboratory, Dantus said.

"I like to say that we enable technology,"he said."Controlling ultrashort pulses, which once required Ph.D. experts, can now be done with push-button simplicity by a small computer-controlled box. This instrument is now being used in the most prestigious research laboratories in the world."

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New center aims to dramatically lower barrier to making silicon photonic chips

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In a ceremony today on the UW campus, Michael Hochberg, a UW assistant professor of electrical engineering; Justin Rattner, chief technology officer atIntelCorp.; Carver Mead, professor emeritus at the California Institute of Technology; and Matt O'Donnell, dean of the UW College of Engineering, kicked off the initiative to support startups and academic researchers.

"We would like the photonics industry, 10 years from now, to function in a way that's very similar to the electronics industry today,"Hochberg said."People building optoelectronic systems will send designs out to an inexpensive, reliable third party for manufacturing, so they can focus on being creative about the design."

The new center aims to create for silicon photonics what Mead and colleague Lynn Conway did for silicon electronics in the 1970s. The"Mead and Conway Revolution"is widely credited with ushering in the current era of integrated circuit computing technology.

Optoelectronics Systems Integration in Silicon, or OpSIS, will offer a service similar to theMetal Oxide SemiconductorImplementation Service, or MOSIS, an organization based at the University of Southern California that helped combine many different circuits, based on Mead and Conway's design principles, onto a single silicon wafer.

"More than 15 years ago, we had a collaboration that allowed my group to design custom integrated circuits that would have been totally impossible for us to do from scratch,"said O'Donnell, who then worked at the University of Michigan."It's now clear that silicon photonics is becoming an integral part of the electronics world, and so it's critical to have that type of capability."

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This is a graphic of a recently built silicon photonic chip measuring 1 cm by 2 cm. At the bottom is a photograph of the chip. Above it, in green, is the original chip design. The rectangle on the upper left shows how layers of silicon (blue), metal (gray) and electrical insulator (orange) combine to create channels, or waveguides, for light to pass through. Credit: A. Spott, University of Washington

The OpSIS project will permit"shuttle runs"in which researchers cut costs by sharingsilicon wafersbetween multiple projects. A single circuit design might use only a few square millimeters. Enabling shuttle runs, Hochberg said, can reduce costs by more than 100 times.

In developing the rules and protocols, Hochberg aims to create a system analogous to Mead and Conway's so that even non-specialists can begin to design and build functioning chips that integrate photonics and electronics.

Creating these rules requires striking a fine balance.

"You want a minimum of rules because people are going to use the technology in ways that you never imagined,"Mead said."You want people to use it in ways that seem crazy."

The emerging field of photonics uses photons, or light, rather than electrons to carry information. Using photons provides a faster, lower-power means for moving data around; a single optical fiber or waveguide can carry many terabits per second of data, tens of thousands of times more than a copper cable does today. Using silicon as the base for the technology eases integration with existing devices and builds on the mature silicon chip manufacturing industry.

Combining photonics and electronics promises to improve radar and sensing technology, and the U.S. Air Force Office of Scientific Research funds Hochberg's UW research. There are also a number of emerging applications for silicon photonics: In the future, Hochberg said, chips that combine electronics and photonics could allow for biological sensors that can test hundreds of blood samples on a single inexpensive chip that combines lasers, sensors and electronics.

"OpSIS will enhance the education of U.S. engineering students, giving them the opportunity to learn the new optical design paradigm,"Intel's Rattner said."The ability to produce such low-costsiliconchips that manipulate photons, instead of electrons, will lead to new inventions and new industries beyond just data communications, including low-cost sensors, new biomedical devices and ultra-fast signal processors."

In August, Hochberg and Tom Baehr-Jones, a UW research scientist in electrical engineering, published aNature Photonicsarticle calling for a foundry forsilicon photonics.

"With such an organization in place, we predict that designing and building photonic-electronicsilicon chipswill constitute a multibillion-dollar industry within the next ten years,"they wrote.

The OPSIS organization already has a half-dozen early users who are participating in so-called"risk runs"that test the protocols now under development.

One early user is John Bowers, a professor of electrical and computer engineering at the University of California, Santa Barbara who has designed a circuit for the first run. While Bowers can build photonics circuits elsewhere, he sees himself as a potential user of the foundry.

"By focusing research of many different groups in one process line, that allows you to advance a library of components and processes faster than any one group could do on its own,"Bowers said."It enables a faster evolution of photonic devices."

Eventually, the UW center plans to offer three runs per year, each of which could accommodate 30 to 40 users. The chips will be built by BAE Systems Inc.

OpSIS will be based at the UW's new Institute for Photonic Integration.

"I'm just rooting for it,"Mead said."It's a wonderful thing and it needs to happen. I might even use it for some of my own research."

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