Researchers squeeze light into nanoscale devices and circuits

https://www.nanowerk.com/nanotechnology-news/newsid=50272.php

(Nanowerk News) As electronic devices and circuits shrink into the nanoscale, the ability to transfer data on a chip, at low power with little energy loss, is becoming a critical challenge. Over the past decade, squeezing light into tiny devices and circuits has been a major goal of nanophotonics researchers.

Electronic oscillations at the surface of metals, known as surface plasmon polaritons or plasmons for short, have become an intense area of focus. Plasmons are hybrids of light (photons) and electrons in a metal. If researchers can harness this nanolight, they will be able to improve sensing, subwavelength waveguiding, and optical transmission of signals.

Columbia investigators have made a major breakthrough in this research, with their invention of a novel “home-built” cryogenic near-field optical microscope that has enabled them to directly image, for the first time, the propagation and dynamics of graphene plasmons at variable temperatures down to negative 250 degrees Celsius. The study was published online today in Nature ("Fundamental limits to graphene plasmonics").

The best pictorial representation of a surface plasmon polariton is in terms of a “ripple” of electron density on the surface of graphene sample. (Image: Dimitri Basov/Columbia University)

“Our temperature-dependent study now gives us direct physical insight into the fundamental physics of plasmon propagation in graphene,” says Dimitri N. Basov, professor of physics at Columbia University, who led the study together with colleagues Cory Dean (physics) and James Hone (mechanical engineering, Columbia Engineering). “This insight was impossible to attain in previous nanoimaging studies done at room temperature. We were particularly surprised at discovering, after many years of failed attempts to get anywhere close, that compact nanolight can travel along the surface of graphene for distances of many tens of microns without unwanted scattering. The physics limiting the travel range of nanolight is a fundamental finding of our study and may lead to new applications in sensors, imaging, and signal processing.”

Basov, Dean, and Hone bring together years of experience in working with graphene, the one-atom-thick material that is one of the most promising candidates for novel photonic materials. Graphene’s optical properties are readily tunable and can be altered at ultrafast time scales. However, implementing nanolight without introducing unwanted dissipation in graphene has been very difficult to achieve.

The Columbia researchers developed a practical approach to confining light to the nanoscale. They knew they could form plasmon-polaritons, or resonant modes, in the graphene that propagate through the material as hybrid excitations of light and mobile electrons. These plasmon-polariton modes can confine the energy of electromagnetic radiation, or light, down to the nanoscale. The challenge was how to visualize these waves with ultra-high spatial resolution, so that they could study the performance of plasmonic modes at varying temperatures.

Alexander S. McLeod, a postdoctoral research scientist in the Basov Nano-optics Laboratory, built a unique microscope that enabled the team to explore the plasmon-polariton waves at high resolution while they cooled the graphene to cryogenic temperatures. Lowering the temperatures allowed them to “turn off” various scattering, or dissipation, mechanisms, one after another, as they cooled down their samples and learned which mechanisms were relevant.

“Now that our novel nanoimaging capabilities are deployed to low temperatures, we can see directly the unmitigated wave propagation of collective light-and-charge excitations within graphene,” says McLeod, co-lead author of the study with Guangxin Ni, also a postdoctoral research scientist in Basov’s lab. “Often times in physics, as in life, seeing truly is believing! The record-breaking travel range of these waves shows they’re destined to take on a life of their own, funneling signals and information back and forth inside next-generation optical devices.”

The study is the first to demonstrate the fundamental limitations for the propagation of plasmon polariton waves in graphene. The team found that graphene plasmons propagate ballistically, across tens of micrometers, throughout the tiny device. These plasmon modes are confined within a volume of space hundreds, if not thousands, of times smaller than that occupied by freely propagating light.

Plasmons in graphene can be tuned and controlled via an external electric field, which gives graphene a big advantage over conventional plasmonic media such as metal surfaces, which are inherently non-tunable. Moreover, the lifetimes of plasmon waves in graphene are now found to exceed those in metals by a factor of 10 to a 100, while propagating over comparably longer distances. These features offer enormous advantages for graphene as a plasmonic medium in next-generation opto-electronic circuits.

“Our results establish that graphene ranks among the best candidate materials for infrared plasmonics, with applications in imaging, sensing, and nano-scale manipulation of light,” says Hone. “Furthermore, our findings reveal the fundamental physics of processes that limit propagation of plasmon waves in graphene. This monumental insight will guide future efforts in nanostructure engineering, which may be able to remove the remaining roadblocks for long-range travel of versatile nanoconfined light within future optical devices.”

The current study is the beginning of a series of low-temperature investigations focused on controlling and manipulating confined plasmons in nanoscale optoelectronic graphene devices. The team is now using low-temperature nanoimaging to explore novel plasmonics effects such as electrically-induced plasmonic reflection and modulation, topological chiral plasmons, and also superconducting plasmonics in the very recently discovered “magic angle” system of twisted bilayer graphene.

Source: By Holly Evarts, Columbia University

A chip-scale broadband optical system that can sense molecules in the mid-infrared

https://www.nanowerk.com/nanotechnology-news/newsid=50270.php

(Nanowerk News) Researchers at Columbia Engineering have demonstrated, for the first time, a chip-based dual-comb spectrometer in the mid-infrared range, that requires no moving parts and can acquire spectra in less than 2 microseconds. The system, which consists of two mutually coherent, low-noise, microresonator-based frequency combs spanning 2600 nm to 4100 nm, could lead to the development of a spectroscopy lab-on-a-chip for real-time sensing on the nanosecond time scale.

“Our results show the broadest optical bandwidth demonstrated for dual-comb spectroscopy on an integrated platform,” said Alexander Gaeta, David M. Rickey Professor of Applied Physics and of Materials Science and senior author of the study, published in Nature Communications("Silicon-chip-based mid-infrared dual-comb spectroscopy").

Schematic of silicon microresonator generating a frequency comb that samples molecules for chemical identification. (Image: Alexander Gaeta/Columbia Engineering)

Creating a spectroscopic sensing device on a chip that can realize real-time, high-throughput detection of trace molecules has been challenging. A few months ago, teams led by Gaeta and Michal Lipson, Higgins Professor of Electrical Engineering, were the first to miniaturize dual-frequency combs by putting two frequency comb generators on a single millimeter-sized chip. They have been working on broadening the frequency span of the dual combs, and on increasing the resolution of the spectrometer by tuning the lines of the comb.

In this current study, the researchers focused on the mid-infrared (mid-IR) range, which, because its strong molecular absorption is typically 10 to 1,000 times greater than those in the visible or near-infrared, is ideal for detecting trace molecules. The mid-IR range effectively covers the “fingerprint” of many molecules.

The team performed mid-IR dual-comb spectroscopy using two silicon nanophotonic devices as microresonators. Their integrated devices enabled the direct generation of broadband mid-infrared light and fast acquisition speeds for characterizing molecular absorption.

“Our work is a critical advance for chip-based dual-comb spectroscopy for liquid/solid phase studies,” said Mengjie Yu, lead author of the paper and a PhD student in Gaeta’s lab. “Our chip-scale broadband optical system, essentially a photonic lab-on-a-chip, is well-suited for identification of chemical species and could find a wide range of applications in chemistry, biomedicine, material science, and industrial process control.”

Integrated metasurface converts wavelengths in a waveguide

Researchers at Harvard and Columbia develop system to convert one wavelength of light into another without phase-matching.

http://optics.org/news/9/2/9

A fabricated device showing four phased antenna arrays consisting of silicon nano-rods.

Light at different wavelengths travels at different speeds through a given material but in order for light to be converted between wavelengths, it needs to have the same momentum or phase. A US-based research partnership working on this problem says that “one of the biggest challenges in developing integrated photonic circuits is controlling the momentum of light.”

Many devices have already been designed to momentum-match or phase-match light at various points as it passes throughout an integrated circuit but the team is asking the question: what if the phase-matching process could be circumvented all together in certain cases?

The researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences, together with collaborators from the Fu Foundation School of Engineering and Applied Science at Columbia University, New York, have developed a system to convert one wavelength of light into another without the need to phase-match. The research has been published in Nature Communications.

Single wavelength

“For any wavelength conversion process to be efficient, it has to be carefully designed to phase match, and it only works at a single wavelength,” commented Marko Lončar, the Tiantsai Lin Professor of Electrical Engineering at SEAS and senior author of the paper. “The devices shown in our work, in contrast, do not need to satisfy the phase-matching requirement, and can convert light in a broad color range.”

The new converter relies on a metasurface, consisting of an array of silicon nanostructures, integrated into a lithium niobate waveguide. The light passes through waveguide, interacting with the nanostructures along the way. The array of nanostructures act like a TV antenna — receiving the optical signal, manipulating its momentum and re-emitting it back into the waveguide.

“Unlike most metasurfaces, where light travels perpendicularly to the metasurface, here light interacts with the metasurface while being confined inside a waveguide,” said Cheng Wang, co-first author of the paper and postdoctoral fellow at SEAS. “In this way, we take advantage of both the momentum control from the metasurface and a long interaction distance.”

Scanning electron microscope (SEM) image of the fabricated device.

Double the frequency

The researchers have demonstrated that they could double the frequency of a wavelength, converting near infrared colors to red, with high efficiency over a broad bandwidth. In previous research, the team demonstrated that they could also control and convert the polarization and mode of a guided wave using a similar structure.

“The integrated metasurface is distinct from other phase-matching mechanisms in that it provides a unidirectional optical momentum to couple optical energy from one to another color components — while inhibiting the inverse process — which is critical for realizing broadband nonlinear conversion,” said Nanfang Yu, assistant professor of applied physics at Columbia and a co-senior author of the paper. “Future work will demonstrate broadband integrated photonic devices based on metasurfaces for realizing other functions such as optical modulation.”

This research was co-first authored by Zhaoyi Li, and co-authored by Myoung-Hwan Kim, Xiao Xiong, Xi-Feng Ren, Guang-Can Guo, Nanfang Yu and Marko Lončar. It was supported by the National Science Foundation, the Air Force Office of Scientific Research and Defense Advanced Research Projects Agency Young Faculty Award.

Presentation/Abstract Columbia University-Is Terahertz a Communication Waste Land or a Vibrant Frontier?

Electrical Engineering Distinguished Lectures

Monday, March 30, 2015 - 3:00pm - 4:00pm

Columbia University Morningside Campus CEPSR 414, Davis Auditorium

http://events.columbia.edu/event/electrical-engineering-distinguished-lectures-77668

Is Terahertz a Communication Waste Land or a Vibrant Frontier?

ABSTRACT The infamous Terahertz Gap represents frequency spectrum that ranges from 0.3 to 3THz (or 300 to 3000GHz). It lies between traditional microwave and infrared domains but remains untouchable via either electronic or photonic means. The conventional transit-time-limited electronic devices can hardly operate even at its lowest frequency; the band-gap-limited photonic devices on the other hand can only operate beyond its highest frequency. Since wavelengths range from 1000 to 100 μm, Terahertz signals tend to behave quasi-optically and are potentially instrumental for a wide range of scientific and industrial applications. Those include high-data rate, short distance and secured wireless & wireline communications, telemetric and remote sensing based on high-resolution radar, spectrometer and imagers for intelligent traffic/landing control, safety/security screening and bio-medical/food/drug sensing, and analysis and controls. In this talk, we will discuss fundamental & technical challenges involved in building Terahertz systems and progress made recently at UCLA to overcome electronic/photonic barriers for realizing highly integrated (sub)-mm-Wave and Terahertz systems.   

BIO Mau-Chung Frank Chang, Professor University of California, Los Angeles Host: Professor Harish Krishnaswamy Electrical Engineering Distinguished Lectures Dr. Frank Chang is currently the Wintek Chair Professor and Chairman of the Electrical Engineering Department at UCLA. Before joining UCLA in 1997, he was the Assistant Director and Department Manager of the High Speed Electronics Laboratory at the Rockwell Science Center, Thousand Oaks, California (1983-1997). Throughout his career, his research has primarily focused on developing high-speed semiconductor devices and circuits for high-frequency and mixed-signal communication, radar, interconnect and imaging systems. He was elected to the National Academy of Engineering in 2008 and Academia Sinica (Taiwan, ROC) in 2012. He is an IEEE Fellow and received the IEEE David Sarnoff Award in 2006 for developing and commercializing GaAs HBT power amplifiers for modern wireless communication systems (especially for cell phones).

Is Terahertz a Communication Waste Land or a Vibrant Frontier?

March 30, 2015
3:00-4:00pm
Davis Auditorium, CEPSR
Speaker: Mau-Chung Frank Chang, Professor/Chairman, Electrical Engineering Department, University of California, Los Angeles

http://www.ee.columbia.edu/terahertz-communication-waste-land-or-vibrant-frontier

Abstract

The infamous “Terahertz Gap” represents frequency spectrum that ranges from 0.3 to 3THz (or 300 to 3000GHz). It lies between traditional microwave and infrared domains but remains “untouchable” via either electronic or photonic means. The conventional “transit-time-limited” electronic devices can hardly operate even at its lowest frequency; the “band-gap-limited” pho- tonic devices on the other hand can only operate beyond its highest fre- quency. Since wavelengths range from 1000 to 100 μm, Terahertz signals tend to behave quasi-optically and are potentially instrumental for a wide range of scientific and industrial applications. Those include high-data rate, short distance and secured wireless & wireline communications, telemetric and remote sensing based on high-resolution radar, spectrometer and im- agers for intelligent traffic/landing control, safety/security screening and bio-medical/food/drug sensing, and analysis and controls. In this talk, we will discuss fundamental & technical challenges involved in building Tera- hertz systems and progress made recently at UCLA to overcome electronic/photonic barriers for realizing highly integrated (sub)-mm-Wave and Terahertz systems.

Speaker Bio

Dr. Frank Chang is currently the Wintek Chair Professor and Chairman of the Electrical Engineering Department at UCLA. Before joining UCLA in 1997, he was the Assistant Di- rector and Department Manager of the High Speed Electronics Laboratory at the Rock- well Science Center, Thousand Oaks, California (1983-1997). Throughout his career, his research has primarily focused on developing high-speed semiconductor devices and circuits for high-frequency and mixed-signal communication, radar, interconnect and im- aging systems. He was elected to the National Academy of Engineering in 2008 and Aca- demia Sinica (Taiwan, ROC) in 2012. He is an IEEE Fellow and received the IEEE David Sarnoff Award in 2006 for developing and commercializing GaAs HBT power amplifiers for modern wireless communication systems (especially for cell phones).

OT-Measuring the mass of 'massless' electrons

Hosang Yoon, Ph.D. '14, and Prof. Donhee Ham have measured the collective mass of electrons in graphene. (Photo by Eliza Grinnell, SEAS Communications.)


http://www.seas.harvard.edu/news/2014/06/measuring-mass-of-massless-electrons

TAMING GRAPHENE, HARVARD-LED RESEARCHERS SUCCESSFULLY MEASURE COLLECTIVE MASS OF ‘MASSLESS’ ELECTRONS IN MOTION

Cambridge, Mass. – June 23, 2014 – Individual electrons in graphene are massless, but when they move together, it’s a different story.

Graphene, a one-atom-thick carbon sheet, has taken the world of physics by storm—in part, because its electrons behave as massless particles. Yet these electrons seem to have dual personalities. Phenomena observed in the field of graphene plasmonics suggest that when the electrons move collectively, they must exhibit mass.

After two years of effort, researchers led by Donhee Ham, Gordon McKay Professor of Electrical Engineering and Applied Physics at the Harvard School of Engineering and Applied Sciences (SEAS), and his student Hosang Yoon, Ph.D.’14, have successfully measured the collective mass of ‘massless’ electrons in motion in graphene.

By shedding light on the fundamental kinetic properties of electrons in graphene, this research may also provide a basis for the creation of miniaturized circuits with tiny, graphene-based components.

The results of Ham and Yoon’s complex measurements, performed in collaboration with other experts at Columbia University and the National Institute for Materials Science in Japan, have been published online in Nature Nanotechnology.

“Graphene is a unique material because, effectively, individual graphene electrons act as though they have no mass. What that means is that the individual electrons always move at a constant velocity,” explains Ham. “But suppose we apply a force, like an electric field. The velocity of the individual electrons still remains constant, but collectively, they accelerate and their total energy increases—just like entities with mass. It’s quite interesting.”

Without this mass, the field of graphene plasmonics cannot work, so Ham’s team knew it had to be there—but until now, no one had accurately measured it.

“One of the greatest contributions of this work is that it is actually an extremely difficult measurement,” says Ham.

Prof. Donhee Ham and his student Hosang Yoon in the laboratory at the Harvard School of Engineering and Applied Sciences. (Photo by Eliza Grinnell, SEAS Communications.)

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As Newton’s second law dictates, a force applied to a mass must generate acceleration. Yoon and Ham knew that if they could apply an electric field to a graphene sample and measure the electrons’ resulting collective acceleration, they could then use that data to calculate the collective mass.

But the graphene samples used in past experiments were replete with imperfections and impurities—places where a carbon atom was missing or had been replaced by something different. In those past experiments, electrons would accelerate but very quickly scatter as they collided with the impurities and imperfections.

“The scattering time was so short in those studies that you could never see the acceleration directly,” says Ham.

To overcome the scattering problem, several smart changes were necessary.

First, Ham and Yoon joined forces with Philip Kim, a physics professor at Columbia who will join the Harvard faculty on July 1 as Professor of Physics and of Applied Physics. A Harvard graduate (Ph.D. ’99), Kim is well known for his pioneering fundamental studies of graphene and his expertise in fabricating high-quality graphene samples. The team was now able to reduce the number of impurities and imperfections by sandwiching the graphene between layers of hexagonal boron nitride, an insulating material with a similar atomic structure. By also collaborating with James Hone, a professor of mechanical engineering at Columbia, they designed a better way to connect electrical signal lines to the sandwiched graphene. And Yoon and Ham applied an electric field at a microwave frequency, which allows for the direct measurement of the electrons’ collective acceleration in the form of a phase delay in the current.

“By doing all this, we translated the situation from completely impossible to being at the verge of either seeing the acceleration or not,” says Ham. “However, the difficulty was still very daunting, and Hosang [Yoon] made it all possible by performing very fine and subtle microwave engineering and measurements—a formidable piece of experimentation.”

“To me, it was a victorious moment that finally justified a long-term effort, going through multiple trials and errors,” says Yoon, lead author of the paper in Nature Nanotechnology. “Until then, I wasn’t even sure if the experiment would really be possible, so it was like a ‘through darkness comes light’ moment.”

A schematic of the experimental setup. Ham and Yoon measured the change in phase of a microwave signal sent through the graphene. (Image courtesy of Hosang Yoon, Harvard SEAS.)

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Collective mass is a key aspect of explaining plasmonic behaviors in graphene. By demonstrating that graphene electrons exhibit a collective mass and by measuring its value accurately, Yoon says, “We think it will help people to understand and design more sophisticated plasmonic devices with graphene.”

The team’s experiments also revealed that, in graphene, kinetic inductance (the electrical manifestation of collective mass) is several orders of magnitude larger than another, far more commonly exploited property called magnetic inductance. This is important in the push toward smaller and smaller electronic circuitry—the main theme of modern integrated circuits—because it means the same level of inductance can be achieved in a far smaller area. Furthermore, Ham and Yoon say that this miniature graphene-based kinetic inductor could enable the creation of a solid-state voltage-controlled inductor, complementary to the widely used voltage-controlled capacitor. It could be used to substantially increase the frequency tuning range of electronic circuits, which is an important function in communication applications.

For now, the challenge remains to improve the quality of graphene samples so that the detrimental effects of electron scattering can be further reduced.

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Hosang Yoon is lead author of the paper in Nature Nanotechnology, with corresponding authors Donhee Ham at Harvard SEAS and Philip Kim at Columbia. Additional coauthors include Columbia professor James Hone, Columbia graduate students Carlos Forsythe and Lei Wang; Nikolaos Tombros, a former member of the Kim lab at Columbia, now at the University of Groningen in the Netherlands; Kenji Watanabe, chief researchers in optoelectronic materials at the National Institute for Materials Science (NIMS) in Japan; and Takashi Taniguchi, group leader in the Ultra-high Pressure Processes Group at NIMS.

This research was supported by the Air Force Office of Scientific Research, the Office of Naval Research, the National Science Foundation, and the Samsung Advanced Institute of Technology and its Global Research Opportunity program. Additional support was provided by the Nano Material Technology Development Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning; the Columbia Optics and Quantum Electronics IGERT; and the Netherlands Organisation for Scientific Research.