'Microring' device could aid in future optical technologies

ScienceDaily (Oct. 20, 2011) — Researchers at Purdue University and the National Institute of Standards and Technology (NIST) have created a device small enough to fit on a computer chip that converts continuous laser light into numerous ultrashort pulses, a technology that might have applications in more advanced sensors, communications systems and laboratory instruments.

"These pulses repeat at very high rates, corresponding to hundreds of billions of pulses per second," said Andrew Weiner, the Scifres Family Distinguished Professor of Electrical and Computer Engineering.

The tiny "microring resonator" is about 80 micrometers, or the width of a human hair, and is fabricated from silicon nitride, which is compatible with silicon material widely used for electronics. Infrared light from a laser enters the chip through a single optical fiber and is directed by a structure called a waveguide into the microring.

The pulses have many segments corresponding to different frequencies, which are called "comb lines" because they resemble teeth on a comb when represented on a graph.

By precisely controlling the frequency combs, researchers hope to create advanced optical sensors that detect and measure hazardous materials or pollutants, ultrasensitive spectroscopy for laboratory research, and optics-based communications systems that transmit greater volumes of information with better quality while increasing bandwidth. The comb technology also has potential for a generation of high-bandwidth electrical signals with possible applications in wireless communications and radar.

The light originates from a continuous-wave laser, also called a single-frequency laser.

"This is a very common type of laser," Weiner said. "The intensity of this type of laser is constant, not pulsed. But in the microring the light is converted into a comb consisting of many frequencies with very nice equal spacing. The microring comb generator may serve as a competing technology to a special type of laser called a mode-locked laser, which generates many frequencies and short pulses. One advantage of the microrings is that they can be very small."

The laser light undergoes "nonlinear interaction" while inside the microring, generating acomb of new frequencies that is emitted out of the device through another optical fiber.

"The nonlinearity is critical to the generation of the comb," said doctoral student Fahmida Ferdous. "With the nonlinearity we obtain a comb of many frequencies, including the original one, and the rest are new ones generated in the microring."

Findings are detailed in a research paper appearing online this month in the journal Nature Photonics. The paper is scheduled for publication in the Dec. 11 issue.

Although other researchers previously have demonstrated the comb-generation technique, the team is the first to process the frequencies using "optical arbitrary waveform technology," pioneered by Purdue researchers led by Weiner. The researchers were able to control the amplitude and phase of each spectral line, learning that there are two types of combs -- "highly coherent" and "partially coherent" -- opening up new avenues to study the physics of the process.

"In future investigations, the ability to extract the phase of individual comb lines may furnish clues into the physics of the comb-generation process," Ferdous said. "Future work will include efforts to create devices that have the proper frequency for commercial applications."

The silicon-nitride device was fabricated by a team led by Houxun Miao, a researcher at NIST's Center for Nanoscale Science and Technology and the Maryland Nanocenter at the University of Maryland. Some of the work was performed at the Birck Nanotechnology Center in Purdue's Discovery Park, and experiments demonstrating short-pulse generation were performed in Purdue's School of Electrical and Computer Engineering.

The effort at Purdue is funded in part by the National Science Foundation and the Naval Postgraduate School.

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The above story is reprinted from materials provided by Purdue University. The original article was written by Emil Venere.

Note: ScienceDaily reserves the right to edit materials for content and length. For further information, please contact the source cited above.

Journal Reference:

Fahmida Ferdous, Houxun Miao, Daniel E. Leaird, Kartik Srinivasan, Jian Wang, Lei Chen, Leo Tom Varghese, Andrew M. Weiner. Spectral line-by-line pulse shaping of on-chip microresonator frequency combs. Nature Photonics, 2011; DOI: 10.1038/nphoton.2011.255

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

'Microring' device could aid in future optical technologies

ScienceDaily (Oct. 20, 2011) — Researchers at Purdue University and the National Institute of Standards and Technology (NIST) have created a device small enough to fit on a computer chip that converts continuous laser light into numerous ultrashort pulses, a technology that might have applications in more advanced sensors, communications systems and laboratory instruments.

"These pulses repeat at very high rates, corresponding to hundreds of billions of pulses per second," said Andrew Weiner, the Scifres Family Distinguished Professor of Electrical and Computer Engineering.

The tiny "microring resonator" is about 80 micrometers, or the width of a human hair, and is fabricated from silicon nitride, which is compatible with silicon material widely used for electronics. Infrared light from a laser enters the chip through a single optical fiber and is directed by a structure called a waveguide into the microring.

The pulses have many segments corresponding to different frequencies, which are called "comb lines" because they resemble teeth on a comb when represented on a graph.

By precisely controlling the frequency combs, researchers hope to create advanced optical sensors that detect and measure hazardous materials or pollutants, ultrasensitive spectroscopy for laboratory research, and optics-based communications systems that transmit greater volumes of information with better quality while increasing bandwidth. The comb technology also has potential for a generation of high-bandwidth electrical signals with possible applications in wireless communications and radar.

The light originates from a continuous-wave laser, also called a single-frequency laser.

"This is a very common type of laser," Weiner said. "The intensity of this type of laser is constant, not pulsed. But in the microring the light is converted into a comb consisting of many frequencies with very nice equal spacing. The microring comb generator may serve as a competing technology to a special type of laser called a mode-locked laser, which generates many frequencies and short pulses. One advantage of the microrings is that they can be very small."

The laser light undergoes "nonlinear interaction" while inside the microring, generating acomb of new frequencies that is emitted out of the device through another optical fiber.

"The nonlinearity is critical to the generation of the comb," said doctoral student Fahmida Ferdous. "With the nonlinearity we obtain a comb of many frequencies, including the original one, and the rest are new ones generated in the microring."

Findings are detailed in a research paper appearing online this month in the journal Nature Photonics. The paper is scheduled for publication in the Dec. 11 issue.

Although other researchers previously have demonstrated the comb-generation technique, the team is the first to process the frequencies using "optical arbitrary waveform technology," pioneered by Purdue researchers led by Weiner. The researchers were able to control the amplitude and phase of each spectral line, learning that there are two types of combs -- "highly coherent" and "partially coherent" -- opening up new avenues to study the physics of the process.

"In future investigations, the ability to extract the phase of individual comb lines may furnish clues into the physics of the comb-generation process," Ferdous said. "Future work will include efforts to create devices that have the proper frequency for commercial applications."

The silicon-nitride device was fabricated by a team led by Houxun Miao, a researcher at NIST's Center for Nanoscale Science and Technology and the Maryland Nanocenter at the University of Maryland. Some of the work was performed at the Birck Nanotechnology Center in Purdue's Discovery Park, and experiments demonstrating short-pulse generation were performed in Purdue's School of Electrical and Computer Engineering.

The effort at Purdue is funded in part by the National Science Foundation and the Naval Postgraduate School.

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Story Source:

The above story is reprinted from materials provided by Purdue University. The original article was written by Emil Venere.

Note: ScienceDaily reserves the right to edit materials for content and length. For further information, please contact the source cited above.

Journal Reference:

Fahmida Ferdous, Houxun Miao, Daniel E. Leaird, Kartik Srinivasan, Jian Wang, Lei Chen, Leo Tom Varghese, Andrew M. Weiner. Spectral line-by-line pulse shaping of on-chip microresonator frequency combs. Nature Photonics, 2011; DOI: 10.1038/nphoton.2011.255

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

New optical signal processing to satisfy power-hungry, high-speed networks

ScienceDaily (Oct. 10, 2011) — A new all-optical signal processing device to meet the demands of high capacity optical networks and with a wide range of applications including ultrafast optical measurements and sensing has been developed by researchers at the University of Southampton.

The project is part of the European Union Framework 7 PHASORS project which completed earlier this year.

In a paper entitled: Multilevel quantization of optical phase in a novel coherent parametric mixer architecture, which will be published in Nature Photonics on October 9, a team of researchers led by Professor David Richardson at the University of Southampton's Optoelectronics Research Centre (ORC), describes a simple and reconfigurable device created to automatically tune the phase property of ultrafast light signals. This phase quantization function is analogous to the way electronic circuits can adjust electrical signals to ensure their voltage matches the discrete set of values required for digital computing.

According to Professor Richardson at the ORC, this is a significant breakthrough because their new device allows an unprecedented level of control and flexibility in processing light using light, functionality required now that ultra-high speed optical signals can be found everywhere from communication links between microprocessor cores in next generation supercomputers to the sub-sea fibre links spanning continents.

"Today parametric mixers are routinely used for laser wavelength conversion, spectroscopy, interferometry and optical amplification," said Mr Joseph Kakande a PhD student at ORC who undertook most of the research "Conventional parametric mixers when operated in a phase sensitive fashion have for many decades been known to have a two-level response. We have now managed to achieve a multilevel phase response which means that we have demonstrated for the first time, a device that squeezes the classical characteristics of its input light to more than two phase levels."

As an example, the team has already used the device to remove noise picked up by a signal in during transmission in optical fibre at over 100 Gbit/s. In principle, this can be done even faster, at speeds hundreds of times greater than could be done using electronics, and crucially, using less power. The researchers envisage many as yet unknown deployment opportunities, given that controlling the phase of light also finds use in applications spanning enabling ultrasensitive interferometers in the hunt for gravitational waves to facilitating the probing of the inner workings of cells.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Southampton, via AlphaGalileo.

Journal Reference:

Joseph Kakande, Radan Slavík, Francesca Parmigiani, Adonis Bogris, Dimitris Syvridis, Lars Grüner-Nielsen, Richard Phelan, Periklis Petropoulos, David J. Richardson. Multilevel quantization of optical phase in a novel coherent parametric mixer architecture. Nature Photonics, 2011; DOI: 10.1038/nphoton.2011.254

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

New optical signal processing to satisfy power-hungry, high-speed networks

ScienceDaily (Oct. 10, 2011) — A new all-optical signal processing device to meet the demands of high capacity optical networks and with a wide range of applications including ultrafast optical measurements and sensing has been developed by researchers at the University of Southampton.

The project is part of the European Union Framework 7 PHASORS project which completed earlier this year.

In a paper entitled: Multilevel quantization of optical phase in a novel coherent parametric mixer architecture, which will be published in Nature Photonics on October 9, a team of researchers led by Professor David Richardson at the University of Southampton's Optoelectronics Research Centre (ORC), describes a simple and reconfigurable device created to automatically tune the phase property of ultrafast light signals. This phase quantization function is analogous to the way electronic circuits can adjust electrical signals to ensure their voltage matches the discrete set of values required for digital computing.

According to Professor Richardson at the ORC, this is a significant breakthrough because their new device allows an unprecedented level of control and flexibility in processing light using light, functionality required now that ultra-high speed optical signals can be found everywhere from communication links between microprocessor cores in next generation supercomputers to the sub-sea fibre links spanning continents.

"Today parametric mixers are routinely used for laser wavelength conversion, spectroscopy, interferometry and optical amplification," said Mr Joseph Kakande a PhD student at ORC who undertook most of the research "Conventional parametric mixers when operated in a phase sensitive fashion have for many decades been known to have a two-level response. We have now managed to achieve a multilevel phase response which means that we have demonstrated for the first time, a device that squeezes the classical characteristics of its input light to more than two phase levels."

As an example, the team has already used the device to remove noise picked up by a signal in during transmission in optical fibre at over 100 Gbit/s. In principle, this can be done even faster, at speeds hundreds of times greater than could be done using electronics, and crucially, using less power. The researchers envisage many as yet unknown deployment opportunities, given that controlling the phase of light also finds use in applications spanning enabling ultrasensitive interferometers in the hunt for gravitational waves to facilitating the probing of the inner workings of cells.

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Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Southampton, via AlphaGalileo.

Journal Reference:

Joseph Kakande, Radan Slavík, Francesca Parmigiani, Adonis Bogris, Dimitris Syvridis, Lars Grüner-Nielsen, Richard Phelan, Periklis Petropoulos, David J. Richardson. Multilevel quantization of optical phase in a novel coherent parametric mixer architecture. Nature Photonics, 2011; DOI: 10.1038/nphoton.2011.254

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Handling nanoscale particles: 'Next-generation' optical tweezers trap tightly without overheating

ScienceDaily (Sep. 27, 2011) — Engineers at Harvard have created a device that may make it easier to isolate and study tiny particles such as viruses.

Their plasmonic nanotweezers, revealed this month in Nature Communications, use light from a laser to trap nanoscale particles. The new device creates strong forces more efficiently than traditional optical tweezers and eliminates a problem that caused earlier setups to overheat.

"We can get beyond the limitations of conventional optical tweezers, exerting a larger force on a nanoparticle for the same laser power," says principal investigator Ken Crozier, Associate Professor of Electrical Engineering at the Harvard School of Engineering and Applied Sciences (SEAS).

"Until now, overheating has been a major problem with tweezers based on surface plasmons. What we've shown is that you can get beyond that limitation by building a plasmonic nanotweezer with an integrated heat sink."

Optical tweezers have been an essential tool in biophysics for several decades, often used for studying cellular components such as molecular motors. Researchers can trap and manipulate the proteins that whip a flagellum, for example, and measure the force of its swimming motion.

But optical tweezers have drawbacks and limits, so researchers like Crozier are perfecting what might be called the "next-generation" model: plasmonic nanotweezers.

To create conventional optical tweezers, which were invented at Bell Labs in the 1980s, scientists shine a laser through a microscope lens, which focuses it into a very tight spot. The light, which is made up of electromagnetic waves, creates a gradient force at that focused spot that can attract a tiny particle and hold it within the beam for a short period of time -- until random motion, radiation pressure, or other forces knock it out.

The trouble with these optical tweezers is that a lens cannot focus the beam any smaller than half the wavelength of the light. If the targeted particle is much smaller than the focal spot, the trapping will be imprecise.

At the same time, the focal size limit places an upper limit on the gradient force that can be generated. A stronger force is necessary for trapping nanoscale particles, relative to larger, microscopic particles, so conventional optical tweezers must use a very high-powered laser to trap the tiniest targets.

To overcome these problems, researchers in applied physics discovered a few years ago that they could enhance the trapping field by focusing the laser onto an array of nanoscale gold disks. The light excites the electrons at the surface of the metal, creating rapid waves of electromagnetic charge called plasma oscillations, resulting in "hot spots" of enhanced fields around the edges of the disk.

In other researchers' designs, the tiny gold disks were arrayed on a sheet of glass, and the whole setup was submerged in water with the target particles. In tests with those devices, one problem was that the brightest hotspots were at the base of the pillars, partially inside the glass, where the particles could never be trapped. A bigger problem, as Crozier's team discovered, was that unless they kept the laser power very low, the water boiled.

The Harvard team has solved both problems by replacing the glass with a piece of silicon coated in copper and then gold, with raised gold pillars. These materials are much more thermally conductive than glass, so they act as a heat sink.

"The gold, copper, and silicon under the pillars act just like the heat sink attached to the chip in your PC, drawing the heat away," says lead author Kai Wang (Ph.D. '11), who completed the work at SEAS and is now a postdoctoral fellow at the Howard Hughes Medical Institute.

The new device reduces the water heating by about 100-fold and produces hotspots at the top edges of the pillars, where Crozier's team was able to trap polystyrene balls as small as 110 nanometers.

In an unusual twist, the team discovered that they were able to rotate the trapped particles around the pillars by rotating the linear polarizer on the optical table where they conducted the experiments. Going further, they replaced the linear polarizer with a circular one and found that the particle automatically and continuously traveled around the pillar.

As the electromagnetic field circled the pillar, it created an optical force that pushed the particle. Interestingly, despite the fact that the electromagnetic field traveled at about 1014 rotations per second, the balance between the optical force and the fluid drag resulted in a particle velocity of about 5 rotations per second, effectively a terminal velocity.

"This phenomenon seems to be entirely novel," says Crozier. "People have trapped particles before, but they've never done anything like that."

As tools for trapping and manipulating nanoparticles become more advanced, the potential applications in biophysics are extensive. One remaining challenge, however, is the researchers' ability to detect and quantify the motion of such tiny particles.

"It's going to be harder and harder to precisely track the center of the particle when we do these manipulations," says Crozier. "Progress in the realm of sensing tools will need to keep up."

Crozier and Wang's co-authors were Ethan Schonbrun, a former research associate, and Paul Steinvurzel, a former postdoctoral researcher, both from Crozier's lab at SEAS. The work was supported by the National Science Foundation, the Defense Advanced Research Projects Agency, and the U.S. Department of Energy.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Harvard University, via EurekAlert!, a service of AAAS.

Journal Reference:

Kai Wang, Ethan Schonbrun, Paul Steinvurzel, Kenneth B. Crozier. Trapping and rotating nanoparticles using a plasmonic nano-tweezer with an integrated heat sink. Nature Communications, 2011; 2: 469 DOI: 10.1038/ncomms1480

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Nanoscale nonlinear light source optical device can be controlled electronically

ScienceDaily (Sep. 22, 2011) — Not long after the development of the first laser in 1960 scientists discovered that shining a beam through certain crystals produced light of a different color; more specifically, it produced light of exactly twice the frequency of the original. The phenomenon was dubbed second harmonic generation.

The green laser pointers in use today to illustrate presentations are based on this science, but producing such a beautiful emerald beam is no easy feat. The green light begins as an infrared ray that must be first processed through a crystal, various lenses and other optical elements before it can illuminate that PowerPoint on the screen before you.

It was later discovered that applying an electrical field to some crystals produced a similar, though weaker, beam of light. This second discovery, known as EFISH -- for electric-field-induced second harmonic light generation -- has amounted mostly to an interesting bit of scientific knowledge and little more. EFISH devices are big, demanding high-powered lasers, large crystals and thousands of volts of electricity to produce the effect. As a result, they are impractical for all but a few applications.

In a paper published September 22 in Science, engineers from Stanford have demonstrated a new device that shrinks EFISH devices by orders of magnitude to the nanoscale. The result is an ultra-compact light source with both optical and electrical functions. Research implications for the device range from a better understanding of fundamental science to improved data communications.

Spring-loaded electrons

The device is based on the physical forces that bind electrons in orbit around a nucleus.

"It's like a spring," said Mark Brongersma, an associate professor of materials science and engineering at Stanford.

In most cases, when you shine a light on an atom, the added energy will pull the electron away from the positively charged nucleus very predictably, in a linear fashion, so that when the light is turned off and the electron springs back to its original orbit, the energy released is the same as the light that displaced it.

The key phrase here being: "in most cases." When the light source is a high-intensity laser shining on a solid, researchers discovered that the farther the electrons are pulled away from the nuclei the less linearly the light interacts with the atoms.

"In other words, the light-matter interaction becomes nonlinear," said Alok Vasudev, a graduate student and co-author of the paper. "The light you get out is different from the light you put in. Shine a strong near-infrared laser on the crystal and green light exactly twice the frequency emerges."

Engineering possibilities

"Now, Alok and I have taken this knowledge and reduced it to the nanoscale," said the paper's first author, Wenshan Cai, a post-doctoral researcher in Brongersma's lab. "For the first time we have a nonlinear optical device at the nanoscale that has both optical and electrical functionality. And this offers some interesting engineering possibilities."

For many photonic applications, including signal and information processing, it is desirable to electrically manipulate nonlinear light generation. The new device resembles a nanoscale bowtie with two halves of symmetrical gold leaf approaching, but not quite touching, in the center. This thin slit between the two halves is filled with a nonlinear material. The narrowness is critical. It is just 100 nanometers across.

"EFISH requires a huge electrical field. From basic physics we know that the strength of an electric field scales linearly with the applied voltage and inversely with the distance between the electrodes -- smaller distance, stronger field and vice versa," said Brongersma. "So, if you have two electrodes placed extremely close together, as we do in our experiment, it doesn't take many volts to produce a giant electrical field. In fact, it takes just a single volt."

"It is this fundamental science that allows us to shrink the device by orders of magnitude from the human scale to the nanoscale," said Cai.

Enter plasmonics

Brongersma's area of expertise, plasmonics, then enters the scene. Plasmonics is the study of a curious physical phenomenon that occurs when light and metal interact. As photons strike metal they produce waves of energy coursing outward over the surface of the metal, like the ripples when a pebble is dropped in a pond.

Engineers have learned to control the direction of the ripples by patterning the surface of the metal in such a way that almost all of the energy waves are funneled inward toward the slit between the two metallic electrodes.

The light pours into the crevice as if over the edge of a waterfall and there it intensifies, producing light some 80 times stronger than the already intense laser levels from which it came. The researchers next apply a modest voltage to the metal resulting in the tremendous electrical field necessary to produce an EFISH beam.

Practical applications

"This type of device may one day find application in the communications industry," says Brongersma. "Most of the masses of information and social media interaction we send through our data centers, and the future data we will someday create, are saved and transmitted as electrical energy -- ones and zeros."

"Those ones and zeroes are just a switch; one is on, zero is off," said Cai. "As more energy-efficient optical information transport is rapidly gaining in importance, it is not a great leap to see why devices that can convert electrical to optical signals and back are of great value."

For the time being, however, the researchers caution that practical applications remain down the road, but they have created something new.

"It's a great piece of basic science," said Brongersma. "It is work that combines several disciplines -- nonlinear optics, electronics, plasmonics, and nanoscale engineering -- into a really interesting device that could keep us busy for awhile."

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Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Stanford School of Engineering. The original article was written by Andrew Myers.

Journal Reference:

W. Cai, A. P. Vasudev, M. L. Brongersma. Electrically Controlled Nonlinear Generation of Light with Plasmonics. Science, 2011; 333 (6050): 1720 DOI: 10.1126/science.1207858

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Nanoscale nonlinear light source optical device can be controlled electronically

ScienceDaily (Sep. 22, 2011) — Not long after the development of the first laser in 1960 scientists discovered that shining a beam through certain crystals produced light of a different color; more specifically, it produced light of exactly twice the frequency of the original. The phenomenon was dubbed second harmonic generation.

The green laser pointers in use today to illustrate presentations are based on this science, but producing such a beautiful emerald beam is no easy feat. The green light begins as an infrared ray that must be first processed through a crystal, various lenses and other optical elements before it can illuminate that PowerPoint on the screen before you.

It was later discovered that applying an electrical field to some crystals produced a similar, though weaker, beam of light. This second discovery, known as EFISH -- for electric-field-induced second harmonic light generation -- has amounted mostly to an interesting bit of scientific knowledge and little more. EFISH devices are big, demanding high-powered lasers, large crystals and thousands of volts of electricity to produce the effect. As a result, they are impractical for all but a few applications.

In a paper published September 22 in Science, engineers from Stanford have demonstrated a new device that shrinks EFISH devices by orders of magnitude to the nanoscale. The result is an ultra-compact light source with both optical and electrical functions. Research implications for the device range from a better understanding of fundamental science to improved data communications.

Spring-loaded electrons

The device is based on the physical forces that bind electrons in orbit around a nucleus.

"It's like a spring," said Mark Brongersma, an associate professor of materials science and engineering at Stanford.

In most cases, when you shine a light on an atom, the added energy will pull the electron away from the positively charged nucleus very predictably, in a linear fashion, so that when the light is turned off and the electron springs back to its original orbit, the energy released is the same as the light that displaced it.

The key phrase here being: "in most cases." When the light source is a high-intensity laser shining on a solid, researchers discovered that the farther the electrons are pulled away from the nuclei the less linearly the light interacts with the atoms.

"In other words, the light-matter interaction becomes nonlinear," said Alok Vasudev, a graduate student and co-author of the paper. "The light you get out is different from the light you put in. Shine a strong near-infrared laser on the crystal and green light exactly twice the frequency emerges."

Engineering possibilities

"Now, Alok and I have taken this knowledge and reduced it to the nanoscale," said the paper's first author, Wenshan Cai, a post-doctoral researcher in Brongersma's lab. "For the first time we have a nonlinear optical device at the nanoscale that has both optical and electrical functionality. And this offers some interesting engineering possibilities."

For many photonic applications, including signal and information processing, it is desirable to electrically manipulate nonlinear light generation. The new device resembles a nanoscale bowtie with two halves of symmetrical gold leaf approaching, but not quite touching, in the center. This thin slit between the two halves is filled with a nonlinear material. The narrowness is critical. It is just 100 nanometers across.

"EFISH requires a huge electrical field. From basic physics we know that the strength of an electric field scales linearly with the applied voltage and inversely with the distance between the electrodes -- smaller distance, stronger field and vice versa," said Brongersma. "So, if you have two electrodes placed extremely close together, as we do in our experiment, it doesn't take many volts to produce a giant electrical field. In fact, it takes just a single volt."

"It is this fundamental science that allows us to shrink the device by orders of magnitude from the human scale to the nanoscale," said Cai.

Enter plasmonics

Brongersma's area of expertise, plasmonics, then enters the scene. Plasmonics is the study of a curious physical phenomenon that occurs when light and metal interact. As photons strike metal they produce waves of energy coursing outward over the surface of the metal, like the ripples when a pebble is dropped in a pond.

Engineers have learned to control the direction of the ripples by patterning the surface of the metal in such a way that almost all of the energy waves are funneled inward toward the slit between the two metallic electrodes.

The light pours into the crevice as if over the edge of a waterfall and there it intensifies, producing light some 80 times stronger than the already intense laser levels from which it came. The researchers next apply a modest voltage to the metal resulting in the tremendous electrical field necessary to produce an EFISH beam.

Practical applications

"This type of device may one day find application in the communications industry," says Brongersma. "Most of the masses of information and social media interaction we send through our data centers, and the future data we will someday create, are saved and transmitted as electrical energy -- ones and zeros."

"Those ones and zeroes are just a switch; one is on, zero is off," said Cai. "As more energy-efficient optical information transport is rapidly gaining in importance, it is not a great leap to see why devices that can convert electrical to optical signals and back are of great value."

For the time being, however, the researchers caution that practical applications remain down the road, but they have created something new.

"It's a great piece of basic science," said Brongersma. "It is work that combines several disciplines -- nonlinear optics, electronics, plasmonics, and nanoscale engineering -- into a really interesting device that could keep us busy for awhile."

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and Google +1:

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Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Stanford School of Engineering. The original article was written by Andrew Myers.

Journal Reference:

W. Cai, A. P. Vasudev, M. L. Brongersma. Electrically Controlled Nonlinear Generation of Light with Plasmonics. Science, 2011; 333 (6050): 1720 DOI: 10.1126/science.1207858

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Handling nanoscale particles: 'Next-generation' optical tweezers trap tightly without overheating

ScienceDaily (Sep. 27, 2011) — Engineers at Harvard have created a device that may make it easier to isolate and study tiny particles such as viruses.

Their plasmonic nanotweezers, revealed this month in Nature Communications, use light from a laser to trap nanoscale particles. The new device creates strong forces more efficiently than traditional optical tweezers and eliminates a problem that caused earlier setups to overheat.

"We can get beyond the limitations of conventional optical tweezers, exerting a larger force on a nanoparticle for the same laser power," says principal investigator Ken Crozier, Associate Professor of Electrical Engineering at the Harvard School of Engineering and Applied Sciences (SEAS).

"Until now, overheating has been a major problem with tweezers based on surface plasmons. What we've shown is that you can get beyond that limitation by building a plasmonic nanotweezer with an integrated heat sink."

Optical tweezers have been an essential tool in biophysics for several decades, often used for studying cellular components such as molecular motors. Researchers can trap and manipulate the proteins that whip a flagellum, for example, and measure the force of its swimming motion.

But optical tweezers have drawbacks and limits, so researchers like Crozier are perfecting what might be called the "next-generation" model: plasmonic nanotweezers.

To create conventional optical tweezers, which were invented at Bell Labs in the 1980s, scientists shine a laser through a microscope lens, which focuses it into a very tight spot. The light, which is made up of electromagnetic waves, creates a gradient force at that focused spot that can attract a tiny particle and hold it within the beam for a short period of time -- until random motion, radiation pressure, or other forces knock it out.

The trouble with these optical tweezers is that a lens cannot focus the beam any smaller than half the wavelength of the light. If the targeted particle is much smaller than the focal spot, the trapping will be imprecise.

At the same time, the focal size limit places an upper limit on the gradient force that can be generated. A stronger force is necessary for trapping nanoscale particles, relative to larger, microscopic particles, so conventional optical tweezers must use a very high-powered laser to trap the tiniest targets.

To overcome these problems, researchers in applied physics discovered a few years ago that they could enhance the trapping field by focusing the laser onto an array of nanoscale gold disks. The light excites the electrons at the surface of the metal, creating rapid waves of electromagnetic charge called plasma oscillations, resulting in "hot spots" of enhanced fields around the edges of the disk.

In other researchers' designs, the tiny gold disks were arrayed on a sheet of glass, and the whole setup was submerged in water with the target particles. In tests with those devices, one problem was that the brightest hotspots were at the base of the pillars, partially inside the glass, where the particles could never be trapped. A bigger problem, as Crozier's team discovered, was that unless they kept the laser power very low, the water boiled.

The Harvard team has solved both problems by replacing the glass with a piece of silicon coated in copper and then gold, with raised gold pillars. These materials are much more thermally conductive than glass, so they act as a heat sink.

"The gold, copper, and silicon under the pillars act just like the heat sink attached to the chip in your PC, drawing the heat away," says lead author Kai Wang (Ph.D. '11), who completed the work at SEAS and is now a postdoctoral fellow at the Howard Hughes Medical Institute.

The new device reduces the water heating by about 100-fold and produces hotspots at the top edges of the pillars, where Crozier's team was able to trap polystyrene balls as small as 110 nanometers.

In an unusual twist, the team discovered that they were able to rotate the trapped particles around the pillars by rotating the linear polarizer on the optical table where they conducted the experiments. Going further, they replaced the linear polarizer with a circular one and found that the particle automatically and continuously traveled around the pillar.

As the electromagnetic field circled the pillar, it created an optical force that pushed the particle. Interestingly, despite the fact that the electromagnetic field traveled at about 1014 rotations per second, the balance between the optical force and the fluid drag resulted in a particle velocity of about 5 rotations per second, effectively a terminal velocity.

"This phenomenon seems to be entirely novel," says Crozier. "People have trapped particles before, but they've never done anything like that."

As tools for trapping and manipulating nanoparticles become more advanced, the potential applications in biophysics are extensive. One remaining challenge, however, is the researchers' ability to detect and quantify the motion of such tiny particles.

"It's going to be harder and harder to precisely track the center of the particle when we do these manipulations," says Crozier. "Progress in the realm of sensing tools will need to keep up."

Crozier and Wang's co-authors were Ethan Schonbrun, a former research associate, and Paul Steinvurzel, a former postdoctoral researcher, both from Crozier's lab at SEAS. The work was supported by the National Science Foundation, the Defense Advanced Research Projects Agency, and the U.S. Department of Energy.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Harvard University, via EurekAlert!, a service of AAAS.

Journal Reference:

Kai Wang, Ethan Schonbrun, Paul Steinvurzel, Kenneth B. Crozier. Trapping and rotating nanoparticles using a plasmonic nano-tweezer with an integrated heat sink. Nature Communications, 2011; 2: 469 DOI: 10.1038/ncomms1480

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Engineers envision 2-dimensional graphene metamaterials and 1-atom-thick optical devices

ScienceDaily (June 9, 2011) — Two University of Pennsylvania engineers have proposed the possibility of two-dimensional metamaterials. These one-atom- thick metamaterials could be achieved by controlling the conductivity of sheets of graphene, which is a single layer of carbon atoms.

Professor Nader Engheta and graduate student Ashkan Vakil, both of the Department of Electrical and Systems Engineering in Penn's School of Engineering and Applied Science, published their theoretical research in the journal Science.

The study of metamaterials is an interdisciplinary field of science and engineering that has grown considerably in recent years. It is premised on the idea that materials can be designed so that their overall wave qualities rely not only upon the material they are made of but also on the pattern, shape and size of irregularities, known as "inclusions," or "meta-molecules" that are embedded within host media.

"By designing the properties of the inclusions, as well as their shapes and density, you achieve in the bulk property something that may be unusual and not readily available in nature," Engheta said.

These unusual properties generally have to do with manipulating electromagnetic (EM) or acoustic waves; in this case, it is EM waves in the infrared spectrum

Changing the shape, speed and direction of these kinds of waves is a subfield of metamaterials known as "transformation optics" and may find applications in everything from telecommunications to imaging to signal processing.

Engheta and Vakil's research shows how transformation optics might now be achieved using graphene, a lattice of carbon a single atom thick.

Researchers, including many at Penn, have devoted considerable effort into developing new ways to manufacture and manipulate graphene, as its unprecedented conductivity would have many applications in the field of electronics. Engheta and Vakil's interest in graphene, however, is due to its capability to transport and guide EM waves in addition to electrical charges and the fact that its conductivity can be easily altered.

Applying direct voltage to a sheet of graphene, by way of ground plate running parallel to the sheet, changes how conductive the graphene is to EM waves. Varying the voltage or the distance between the ground plate and the graphene alters the conductivity, "just like tuning a knob," Engheta said.

"This allows you to change the conductivity of different segments of a single sheet of graphene differently from each other," he said. And if you can do that, you can navigate and manipulate a wave with those segments. In other words, you can do transformation optics using graphene."

In this marriage between graphene and metamaterials, the different regions of conductivity on the effectively two-dimensional, one-atom-thick sheet function as the physical inclusions present in three-dimensional versions.

The examples Engheta and Vakil have demonstrated with computer models include a sheet of graphene with two areas that have different conductivities, one that can support a wave, and one that cannot. The boundary between the two areas acts as a wall, capable of reflecting a guided EM wave on the graphene much like one would in a three dimensional space.

Another example involves three regions, one that can support a wave surrounded by two that cannot. This produces a "waveguide," which functions like a one-atom-thick fiber optic cable. A third example builds on the waveguide, adding another non-supporting region to split the waveguide into two.

"We can 'tame' the wave so that it moves and bends however we like," Engheta said. "Rather than playing around with the boundary between two media, we're thinking about changes of conductivity across a single sheet of graphene."

Other applications include lensing and the ability to do "flatland" Fourier transforms, a fundamental aspect of signal processing that is found in nearly every piece of technology with audio or visual components.

"This will pave the way to the thinnest optical devices imaginable," Engheta said. "You can't have anything thinner than one atom!"

Support for this research came from U.S. Air Force Office of Scientific Research.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Pennsylvania.

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Engineers envision 2-dimensional graphene metamaterials and 1-atom-thick optical devices

ScienceDaily (June 9, 2011) — Two University of Pennsylvania engineers have proposed the possibility of two-dimensional metamaterials. These one-atom- thick metamaterials could be achieved by controlling the conductivity of sheets of graphene, which is a single layer of carbon atoms.

Professor Nader Engheta and graduate student Ashkan Vakil, both of the Department of Electrical and Systems Engineering in Penn's School of Engineering and Applied Science, published their theoretical research in the journal Science.

The study of metamaterials is an interdisciplinary field of science and engineering that has grown considerably in recent years. It is premised on the idea that materials can be designed so that their overall wave qualities rely not only upon the material they are made of but also on the pattern, shape and size of irregularities, known as "inclusions," or "meta-molecules" that are embedded within host media.

"By designing the properties of the inclusions, as well as their shapes and density, you achieve in the bulk property something that may be unusual and not readily available in nature," Engheta said.

These unusual properties generally have to do with manipulating electromagnetic (EM) or acoustic waves; in this case, it is EM waves in the infrared spectrum

Changing the shape, speed and direction of these kinds of waves is a subfield of metamaterials known as "transformation optics" and may find applications in everything from telecommunications to imaging to signal processing.

Engheta and Vakil's research shows how transformation optics might now be achieved using graphene, a lattice of carbon a single atom thick.

Researchers, including many at Penn, have devoted considerable effort into developing new ways to manufacture and manipulate graphene, as its unprecedented conductivity would have many applications in the field of electronics. Engheta and Vakil's interest in graphene, however, is due to its capability to transport and guide EM waves in addition to electrical charges and the fact that its conductivity can be easily altered.

Applying direct voltage to a sheet of graphene, by way of ground plate running parallel to the sheet, changes how conductive the graphene is to EM waves. Varying the voltage or the distance between the ground plate and the graphene alters the conductivity, "just like tuning a knob," Engheta said.

"This allows you to change the conductivity of different segments of a single sheet of graphene differently from each other," he said. And if you can do that, you can navigate and manipulate a wave with those segments. In other words, you can do transformation optics using graphene."

In this marriage between graphene and metamaterials, the different regions of conductivity on the effectively two-dimensional, one-atom-thick sheet function as the physical inclusions present in three-dimensional versions.

The examples Engheta and Vakil have demonstrated with computer models include a sheet of graphene with two areas that have different conductivities, one that can support a wave, and one that cannot. The boundary between the two areas acts as a wall, capable of reflecting a guided EM wave on the graphene much like one would in a three dimensional space.

Another example involves three regions, one that can support a wave surrounded by two that cannot. This produces a "waveguide," which functions like a one-atom-thick fiber optic cable. A third example builds on the waveguide, adding another non-supporting region to split the waveguide into two.

"We can 'tame' the wave so that it moves and bends however we like," Engheta said. "Rather than playing around with the boundary between two media, we're thinking about changes of conductivity across a single sheet of graphene."

Other applications include lensing and the ability to do "flatland" Fourier transforms, a fundamental aspect of signal processing that is found in nearly every piece of technology with audio or visual components.

"This will pave the way to the thinnest optical devices imaginable," Engheta said. "You can't have anything thinner than one atom!"

Support for this research came from U.S. Air Force Office of Scientific Research.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Pennsylvania.

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Graphene-based polarizer can broaden bandwidth of optical fiber-based telecommunications

ScienceDaily (June 6, 2011) — Researchers at the National University of Singapore have invented a graphene-based polarizer that can broaden the bandwidth of prevailing optical fibre-based telecommunication systems.

The graphene research team, led by Professor Kian Ping Loh at the National University of Singapore, invented an ultra-slim broadband polarizer that uses graphene, a single-atomic-layer crystallized carbon, to convert light beam into polarized light. This is the first experimental demonstration of using graphene as an ultrathin waveguide to couple and modulate light. Light modulation by means of polarization management is vital to avoid signal fading and error in coherent optical communications as well as optical gyroscopes and interferometric sensors.

In principle, the polarizing ability of graphene covers the telecommunication bands from visible to mid-infrared. This means that graphene polarizer can provide all-in-one solution for multiple-channel high-speed optical communications, the researchers said.

The researchers skilfully transferred graphene grown by chemical vapour deposition on the side-polished optical fibre to fabricate the graphene polarizer and measured light polarization at different wavelengths. Unlike polarizers made from thin metal film or semiconductor dielectric, a graphene polarizer has the unique ability to filter out transverse-magnetic-mode and supports transverse-electric-mode surface wave propagation.

The broadband graphene polarizer work was published in the journal Nature Photonics and appeared online on May 29, 2011.

"The results reported in this paper can have a strong impact in the development of graphene-based optical devices for photonic applications…the science behind it is excellent…" says professor Antonio Castro Neto of National University of Singapore.

The Singapore team has earlier pioneered graphene mode-locked lasers in 2009. This work was another breakthrough in bringing graphene photonics a step closer to real applications.

"In the near future, we can envision ultrathin graphene-based photonic circuits with multiple functions of light creation, routing, modulation or detection," said Dr. Qiaoliang Bao, who is the project leader of the pioneering work.

The other Singapore-based partners of this research included teams from Singapore's Nanyang Technological University and Institute of Materials Research and Engineering. One Belgium group from Université Libre de Bruxelles participated in the project and verified the work of the Singapore group.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by National University of Singapore, via EurekAlert!, a service of AAAS.

Journal Reference:

Qiaoliang Bao, Han Zhang, Bing Wang, Zhenhua Ni, Candy Haley Yi Xuan Lim, Yu Wang, Ding Yuan Tang, Kian Ping Loh. Broadband graphene polarizer. Nature Photonics, 2011; DOI: 10.1038/nphoton.2011.102

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Invisibility cloak: Scientists achieve optical invisibility in visible light range of spectrum

ScienceDaily (May 18, 2011) — "Seeing something invisible with your own eyes is an exciting experience," say Joachim Fischer and Tolga Ergin. For about one year, both physicists and members of the team of Professor Martin Wegener at KIT's Center for Functional Nanostructures (CFN) have worked on refining the structure of the Karlsruhe invisibility cloak to such an extent that it is also effective in the visible spectral range.

In invisibility cloaks, light waves are guided by the material such that they leave the invisibility cloak again as if they had never been in contact with the object to be disguised. Consequently, the object is invisible to the observer. The exotic optical properties of the camouflaging material are calculated using complex mathematical tools.

These properties result from a special structuring of the material. It has to be smaller than the wavelength of the light that is to be deflected. For example, the relatively large radio or radar waves require a material "that can be produced using nail scissors," says Wegener. At wavelengths visible to the human eye, materials have to be structured in the nanometer range.

The minute invisibility cloak produced by Fischer and Ergin is smaller than the diameter of a human hair. It makes the curvature of a metal mirror appear flat, as a result of which an object hidden underneath becomes invisible. The metamaterial placed on top of this curvature looks like a stack of wood, but consists of plastic and air. These "logs" have precisely defined thicknesses in the range of 100 nm. Light waves that are normally deflected by the curvature are influenced and guided by these logs such that the reflected light corresponds to that of a flat mirror.

"If we would succeed again in halving the log distance of the invisibility cloak, we would obtain cloaking for the complete visible light spectrum," says Fischer.

Last year, the Wegener team presented the first 3-D invisibility cloak in the journal Science. Until that time, the only invisibility cloaks existed in waveguides and were of practically two-dimensional character. When looking onto the structure from the third dimension, however, the effect disappeared. By means of an accordingly filigree structuring, the Karlsruhe invisibility cloak could be produced for wavelengths from 1500 to 2600 nm. This wavelength range is not visible to the human eye, but plays an important role in telecommunications. The breakthrough was based on the use of the direct laser writing method (DLS) developed by CFN. With the help of this method, it is possible to produce minute 3-D structures with optical properties that do not exist in nature, so-called metamaterials.

In the past year, the KIT scientists continued to improve the already extremely fine direct laser writing method. For this purpose, they used methods that have significantly increased the resolution in microscopy. With this tool, they then succeeded in refining the metamaterial by a factor of two and in producing the first 3-D invisibility cloak for non-polarized visible light in the range of 700 nm. This corresponds to the red color.

"The invisibility cloak now developed is an attractive object demonstrating the fantastic possibilities of the rather new field of transformation optics and metamaterials. The design options that opened up during the last years had not been deemed possible before," emphasizes Ergin. "We expect dramatic improvements of light-based technologies, such as lenses, solar cells, microscopes, objectives, chip production, and data communication."

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Karlsruhe Institute of Technology.

Journal Reference:

J. Fischer, T. Ergin, M. Wegener. Three-dimensional polarization-independent visible-frequency carpet invisibility cloak. Optics Letters, 2011; (in press)

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Toward optical computing in handheld electronics: Graphene optical modulators could lead to ultrafast communications

ScienceDaily (May 9, 2011) — Scientists at the University of California, Berkeley, have demonstrated a new technology for graphene that could break the current speed limits in digital communications.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Other UC Berkeley co-authors of this paper are graduate student Erick Ulin-Avila and post-doctoral researcher Thomas Zentgraf in Zhang's lab; and visiting scholar Baisong Geng and graduate student Long Ju in Wang's lab.

This work was supported through the Center for Scalable and Integrated Nano-Manufacturing (SINAM), an NSF Nanoscale Science and Engineering Center. Funding from the Department of Energy's Basic Energy Science program at Lawrence Berkeley National Laboratory also helped support this research.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of California - Berkeley. The original article was written by Sarah Yang, Media Relations.

Journal Reference:

Ming Liu, Xiaobo Yin, Erick Ulin-Avila, Baisong Geng, Thomas Zentgraf, Long Ju, Feng Wang, Xiang Zhang. A graphene-based broadband optical modulator. Nature, 2011; DOI: 10.1038/nature10067

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Optical microscope without lenses produces high-resolution 3-D images on a chip

ScienceDaily (Apr. 25, 2011) — UCLA researchers have redefined the concept of a microscope by removing the lens to create a system that is small enough to fit in the palm of a hand but powerful enough to create three-dimensional tomographic images of miniscule samples.

The advance, featured in the early online edition of the journal Proceedings of the National Academy of Sciences, represents the first demonstration of lens-free optical tomographic imaging on a chip, a technique capable of producing high-resolution 3-D images of large volumes of microscopic objects.

"This research clearly shows the potential of lens-free computational microscopy," said Aydogan Ozcan, senior author of the research and an associate professor of electrical engineering at UCLA's Henry Samueli School of Engineering and Applied Science. "Wonderful progress has been made in recent years to miniaturize life-sciences tools with microfluidic and lab-on-a-chip technologies, but until now optical microscopy has not kept pace with the miniaturization trend."

An optical imaging system small enough to fit onto an opto-electronic chip provides a variety of benefits. Because of the automation involved in on-chip systems, scientific work could be sped up significantly, which might have a great impact in the fields of cell and developmental biology. In addition, the small size not only has great potential for miniaturizing systems but also leads to cost savings on equipment.

The optical microscope, invented more than 400 years ago, has tended to grow larger and more complex as it has been modified to image ever-smaller objects with better resolution. To address this lack of progress in miniaturization, Ozcan's research group -- with graduate student Serhan Isikman and postdoctoral scholar Waheb Bishara as lead researchers -- developed the new tomographic microscopy platform through the next evolution of a lens-free imaging technology the group created and has been improving for years.

Ozcan, a researcher at the California NanoSystems Institute at UCLA, makes the analogy that a traditional optical microscope is like a huge set of pipes delivering content, in the form of images, to the user. Over years of development, bottlenecks occur that impede further improvement. Even if one part of the system -- that is, one bottleneck -- is improved, other bottlenecks keep that improvement from being fully realized. Not so with the lens-free system, according to Ozcan.

"Lens-free imaging removes the pipes altogether by utilizing an entirely new design," he said.

The system takes advantage of the fact that organic structures, such as cells, are partially transparent. So by shining a light on a sample of cells, the shadows created reveal not only the cells' outlines but details about their sub-cellular structures as well.

"These details can be captured and analyzed if the shadow is directed onto a digital sensor array," Isikman said. "The end result of this process is an image taken without using a lens."

Ozcan envisions this lens-free imaging system as one component in a lab-on-a-chip platform. It could potentially fit beneath a microfluidic chip, a tool for the precise control and manipulation of sub-millimeter biological samples and fluids, and the two tools would operate in tandem, with the microfluidic chip depositing and subsequently removing a sample from the lens-free imager in an automated, or high-throughput, process.

The platform's 3-D images are created by rotating the light source to illuminate the samples from multiple angles. These multiple angles also allow the system to utilize tomography, a powerful imaging technique. Through the use of tomography, the system is able to produce 3-D images without sacrificing resolution.

"The field of view of lens-based microscopes is limited because the lens focuses on a narrow area of a sample," Bishara said. "A lens-free microscope has both a much larger field of view and depth of field because the imaging is done by the digital sensor array and is not constrained by a lens."

The research was funded by grants from the National Science Foundation, the U.S. Office of Naval Research and the National Institutes of Health and was also supported by the Gates Foundation and the Vodafone Americas Foundation.

For more information on the Ozcan research group, visit http://innovate.ee.ucla.edu/.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of California - Los Angeles. The original article was written by Mike Rodewald.

Journal Reference:

S. O. Isikman, W. Bishara, S. Mavandadi, F. W. Yu, S. Feng, R. Lau, A. Ozcan. Lens-free optical tomographic microscope with a large imaging volume on a chip. Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073/pnas.1015638108

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