From Lawrence Berkeley National Lab: “Dance, Electron, Dance: Scientists Use Light to Choreograph Electronic Motion in 2D Materials”

From Lawrence Berkeley National Lab

June 26, 2020
Theresa Duque
tnduque@lbl.gov
(510) 424-2866

Study led by Berkeley Lab, UC Berkeley could advance understanding of electron interactions for quantum devices.

Microscope image of the TMD moiré superlattice device. (Credit: Emma Regan/Berkeley Lab)

A team of scientists led by the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley has demonstrated a powerful new technique that uses light to measure how electrons move and interact within materials. With this technique, the researchers observed exotic states of matter in stacks of atomically thin semiconductors called transition metal dichalcogenide (TMD) moiré superlattices.

Their study, which was published in the journal Nature, is the first to prove that interactions between electrons play a significant role in how charge flows in TMD moiré superlattices.

“Moiré superlattices provide a unique method for introducing exotic electronic behavior in materials where they don’t typically exist,” said lead author Emma Regan, a doctoral researcher in Berkeley Lab’s Materials Sciences Division and the UC Berkeley physics department. “Understanding and engineering electronic behavior in quantum materials may provide new approaches for electronic devices in the future.”

In most materials, electrons move fast and rarely interact. But in previous studies, other researchers have shown that a moiré superlattice – which creates an energy landscape for electrons – can slow the electrons down enough that they feel interactions between each other.

“We suspected that these electron-electron interactions in TMD moiré superlattices are very strong – even stronger than what you would find in stacks of graphene,” said Regan.

Typically, physicists investigate electron-electron interactions by attaching wires to a material and measuring how easily electrical current flows. But in stacks of TMDs, electrons don’t flow easily between the wires and the material, which makes it difficult to understand how the electrons interact.

So the researchers turned to light instead.

The research team, led by senior author Feng Wang, fabricated the TMD moiré superlattice from atomically thin layers of tungsten diselenide and tungsten disulfide – two common semiconductors known for their ability to efficiently absorb and emit light. They then formed a device just 25 nanometers (25 billionths of a meter) thick by sandwiching the tungsten diselenide/tungsten disulfide moiré superlattice between boron nitride and graphene.

In Wang’s ultrafast nano-optics lab, the researchers shone lasers on the TMD device to observe how electrons flowed in the superlattice as they varied the number of electrons injected into the material. Wang is a faculty scientist in Berkeley Lab’s Materials Sciences Division and professor of physics at UC Berkeley.

Using light to choreograph electron spin

Electrons resting in the moire superlattice at different electron densities. Wigner crystal states are shown left and center. Typical insulating state is shown right. (Credit: Emma Regan/Berkeley Lab)

As expected, the researchers uncovered evidence of very strong electron-electron interactions in the TMD moiré superlattice device.

In one experiment, for example, the device suddenly became electrically insulating – the electrons stopped moving – when they added enough electrons to fill each unit cell in the moiré superlattice.

This behavior is common in a material with strong electron-electron interactions, Regan said. “Since the electrons interact strongly, they prefer not to sit at the same position because this will increase their energy. If all of the unit cells are already occupied, then the electrons stop moving around,” she explained.

So Regan and co-authors were surprised to see similar insulating behavior in the TMD moiré superlattice device when there were fewer electrons in the material, and not all the superlattice unit cells were occupied.

“Electron interactions were so strong in the TMD moiré superlattice that electrons also avoided sitting on neighboring sites,” she said. “These states are called generalized Wigner crystal states and haven’t been seen in any other moiré superlattice system.”

TMDs have a unique property where different polarizations of light can excite electrons to spin up or spin down, so the researchers used a laser to inject electrons with “spin up” or “spin down” into the material, probing their behavior with a second laser. “Direct optical access to the electron spin is special because it helps us understand the details of these exotic states,” Regan said.

“This study is very exciting because we were able to demonstrate strong electron-electron interactions in TMD moiré superlattices, which also have fascinating and useful optical properties,” she added. “This work weds traditional correlated electron physics with 2D TMD materials – two communities that usually don’t overlap.”

The researchers hope to further develop their technique to take optical measurements of electron spin at tiny scales of distance and timing.

Researchers from the Kavli Institute at Cornell for Nanoscale Science; Huazhong University of Science and Technology, and the University of the Chinese Academy of Sciences, China; Arizona State University; Lund University, Sweden; and the National Institute for Materials Science, Japan, also contributed to the study.

The work was supported by the DOE Office of Science. Additional funding was provided by the U.S. Army Research Office.

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

LBNL campus

LBNL Molecular Foundry

Bringing Science Solutions to the World
In the world of science, Lawrence Berkeley National Laboratory (Berkeley Lab) is synonymous with “excellence.” Thirteen Nobel prizes are associated with Berkeley Lab. Seventy Lab scientists are members of the National Academy of Sciences (NAS), one of the highest honors for a scientist in the United States. Thirteen of our scientists have won the National Medal of Science, our nation’s highest award for lifetime achievement in fields of scientific research. Eighteen of our engineers have been elected to the National Academy of Engineering, and three of our scientists have been elected into the Institute of Medicine. In addition, Berkeley Lab has trained thousands of university science and engineering students who are advancing technological innovations across the nation and around the world.

Berkeley Lab is a member of the national laboratory system supported by the U.S. Department of Energy through its Office of Science. It is managed by the University of California (UC) and is charged with conducting unclassified research across a wide range of scientific disciplines. Located on a 202-acre site in the hills above the UC Berkeley campus that offers spectacular views of the San Francisco Bay, Berkeley Lab employs approximately 3,232 scientists, engineers and support staff. The Lab’s total costs for FY 2014 were $785 million. A recent study estimates the Laboratory’s overall economic impact through direct, indirect and induced spending on the nine counties that make up the San Francisco Bay Area to be nearly $700 million annually. The Lab was also responsible for creating 5,600 jobs locally and 12,000 nationally. The overall economic impact on the national economy is estimated at $1.6 billion a year. Technologies developed at Berkeley Lab have generated billions of dollars in revenues, and thousands of jobs. Savings as a result of Berkeley Lab developments in lighting and windows, and other energy-efficient technologies, have also been in the billions of dollars.

Berkeley Lab was founded in 1931 by Ernest Orlando Lawrence, a UC Berkeley physicist who won the 1939 Nobel Prize in physics for his invention of the cyclotron, a circular particle accelerator that opened the door to high-energy physics. It was Lawrence’s belief that scientific research is best done through teams of individuals with different fields of expertise, working together. His teamwork concept is a Berkeley Lab legacy that continues today.

A U.S. Department of Energy National Laboratory Operated by the University of California.

From SLAC National Accelerator Lab: “In a first, researchers use ultrafast ‘electron camera’ to learn about molecules in liquid samples”

From SLAC National Accelerator Lab

April 9, 2020
Ali Sundermier

This new technology could enable future insights into chemical and biological processes that occur in solution, such as vision, catalysis and photosynthesis.

High-speed “electron cameras” can detect tiny molecular movements in a material by scattering a powerful beam of electrons off a sample. Until recently, researchers had only used this technique to study gases and solids. But some of the most important biological and chemical processes, in particular the conversion of light into energy, happen in molecules in a solution.

Now, researchers have applied this technique, ultrafast electron diffraction, to molecules in liquid samples. They developed a method to create 100-nanometer thick liquid jets–about 1,000 times thinner than the width of a human hair–that enable them to get clear diffraction patterns from electrons. In the future, this method could allow them to explore light-driven processes such as vision, catalysis, photosynthesis and DNA damage caused by UV rays.

The team, which included researchers from the Department of Energy’s SLAC National Accelerator Laboratory, Stanford University and the University of Nebraska-Lincoln (UNL), published their results in Structural Dynamics in March.

“This research is a huge breakthrough in the field of ultrafast electron diffraction,” says Xijie Wang, director of the MeV-UED instrument, who co-authored the paper. “Being able to study biological and chemical systems in their natural environment is a valuable tool that opens up a new window for the future.”

Stop-motion movies

Liquid jets have long been used to deliver samples at X-ray lasers such as SLAC’s Linac Coherent Light Source (LCLS) [below], providing valuable information about ultrafast processes as they occur in their natural environment.

SLAC’s ultrafast “electron camera,” MeV-UED, uses high-energy electron beams to complement the range of structural information collected at LCLS.

SLAC Megaelectronvolt Ultrafast Electron Diffraction Instrument: MeV-UED

Here, scientists begin by exciting a sample with laser light, kicking off the processes they hope to study. Next they blast the sample with a short pulse of electrons with high energy, measured in millions of electronvolts (MeV), to look inside, generating snapshots of its shifting atomic structure that can be strung together into a stop-motion movie of the light-induced structural changes in the sample.

Looking into the kaleidoscope

The tiny wavelengths of these high-energy electrons allow scientists to take high-resolution snapshots, offering insight into processes such as proton transfer and hydrogen-bond breaking that are difficult to study with other methods. But applying this technique to liquid samples has proven challenging.

“Since electrons don’t penetrate samples as easily as X-rays,” says Kathryn Ledbetter, a graduate student at the Stanford PULSE Institute who coauthored the paper, “applying this technique to liquids has been a longstanding challenge in the field.”

If the sample is too thick, the electrons can get stuck and scatter multiple times, producing a wild mix of patterns that’s difficult to glean information from, like looking through a kaleidoscope. In this new study, the team overcame those challenges through the use of MeV electrons and a gas-accelerated thin liquid sheet jet. As the electrons break through the jet, they scatter only once, producing a clean pattern that’s much easier to reconstruct. The team also designed a chamber that housed the liquid jet and monitored the interaction between the sample and the electron beam.

‘Another tool in the ultrafast toolbox’

This paper sets the stage for upcoming research that investigates questions such as what happens when hydrogen bonds break or when molecules absorb UV radiation. As a next step, SLAC researchers are upgrading the MeV-UED facility and developing a new generation of direct electron detectors that will greatly expand the scientific reach of this technique.

“We’d like this to be another tool in the toolbox of researchers trying to learn about liquids and light-driven reactions,” says Pedro Nunes, a postdoctoral researcher at UNL who led the research. “We want to show the community that what was once believed to be far-fetched is not only possible, but capable of running smoothly enough to watch structural changes unfold in real time.”

UED-MeV and LCLS are DOE Office of Science user facilities. The project was funded by the Office of Science.

See the full article here .

five-ways-keep-your-child-safe-school-shootings
Please help promote STEM in your local schools.

Stem Education Coalition

SLAC National Accelerator Lab

SLAC/LCLS

SLAC/LCLS II projected view

SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.

SSRL and LCLS are DOE Office of Science user facilities.

From University of Sussex via phys.org: “Researchers combine lasers and terahertz waves in camera that sees ‘unseen’ detail”

From University of Sussex

via

phys.org

February 18, 2020

The time-resolved nonlinear ghost imaging camera uses a nonlinear crystal to convert standard laser light to terahertz patterns, allowing the reconstruction of complex samples using a single terahertz pixel. Credit: University of Sussex

A team of physicists at the University of Sussex has successfully developed the first nonlinear camera capable of capturing high-resolution images of the interior of solid objects using terahertz (THz) radiation.

Led by Professor Marco Peccianti of the Emergent Photonics (EPic) Lab, Luana Olivieri, Dr. Juan S. Totero Gongora and a team of research students built a new type of THz camera capable of detecting THz electromagnetic waves with unprecedented accuracy.

Images produced using THz radiation are called ‘hyperspectral’ because the image consists of pixels, each one containing the electromagnetic signature of the object in that point.

Lying between microwaves and infrared in the electromagnetic spectrum, THz radiation easily penetrates materials like paper, clothes and plastic in the same way X-rays do, but without being harmful. It is safe to use with even the most delicate biological samples. THz imaging makes it possible to ‘see’ the molecular composition of objects and distinguish between different materials—such as sugar and cocaine, for example.

Explaining the significance of their achievement, Prof Peccianti said: “The core challenge in THz cameras is not about collecting an image, but it is about preserving the objects spectral fingerprint that can be easily corrupted by your technique. This is where the importance of our achievement lies. The fingerprint of all the details of the image is preserved in such a way that we can investigate the nature of the object in full detail. ”

Artistic rendering of the terahertz field transmitted by an abstract object. Credit: University of Sussex

Until now, cameras capable of capturing a hyperspectral image preserving all the fine details revealed by THz radiation had not been considered possible.

The EPic Lab team used a single-pixel camera to image sample objects with patterns of THz light. The prototype they built can detect how the object alters different patterns of THz light. By combining this information with the shape of each original pattern, the camera reveals the image of an object as well as its chemical composition.

Sources of THz radiation are very faint and hyperspectral imaging had, until now, limited fidelity. To overcome this, The Sussex team shone a standard laser onto a unique non-linear material capable of converting visible light to THz. The prototype camera creates THz electromagnetic waves very close to the sample, similar to how a microscope works. As THz waves can travel right through an object without affecting it, the resulting images reveal the shape and composition of objects in three dimensions.

Dr. Totero Gongora said: “This is a major step forward because we have demonstrated that all the possibilities explored in our previous theoretical research are not only feasible, but our camera works even better than we expected. While building our device, we discovered several ways to optimise the imaging process and now the technology is stable and works well. The next phase of our research will be in speeding up the image reconstruction process and taking us closer to applying THz cameras to real-world applications; like airport security, intelligent car sensors, quality control in manufacturing and even scanners to detect health problems like skin cancer.”

Science paper:
Luana Olivieri et al. Hyperspectral terahertz microscopy via nonlinear ghost imaging, Optica (2020)

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

About Science X in 100 words

Science X™ is a leading web-based science, research and technology news service which covers a full range of topics. These include physics, earth science, medicine, nanotechnology, electronics, space, biology, chemistry, computer sciences, engineering, mathematics and other sciences and technologies. Launched in 2004 (Physorg.com), Science X’s readership has grown steadily to include 5 million scientists, researchers, and engineers every month. Science X publishes approximately 200 quality articles every day, offering some of the most comprehensive coverage of sci-tech developments world-wide. Science X community members enjoy access to many personalized features such as social networking, a personal home page set-up, article comments and ranking, the ability to save favorite articles, a daily newsletter, and other options.
Mission 12 reasons for reading daily news on Science X Organization Key editors and writersinclude 1.75 million scientists, researchers, and engineers every month. Phys.org publishes approximately 100 quality articles every day, offering some of the most comprehensive coverage of sci-tech developments world-wide. Quancast 2009 includes Phys.org in its list of the Global Top 2,000 Websites. Phys.org community members enjoy access to many personalized features such as social networking, a personal home page set-up, RSS/XML feeds, article comments and ranking, the ability to save favorite articles, a daily newsletter, and other options.

The University of Sussex is a public research university located in Falmer, Sussex, England. Its campus is surrounded by the South Downs National Park and it is a short distance away from central Brighton. The University received its Royal Charter in August 1961, the first of the plate glass university generation, and was a founding member of the 1994 Group of research-intensive universities.

More than a third of its students are enrolled in postgraduate programs and approximately a third of staff are from outside the United Kingdom. Sussex has a diverse community of over 17,000 students, with around one in three being foreign students, and over 1000 academics, representing over 140 different nationalities. The annual income of the institution for 2016–17 was £286.1 million with an expenditure of £270.4 million. In 2017, over 32,000 students applied to the University of Sussex, with around 5,000 joining the institution.

The Times Higher Education World University Rankings 2018 placed Sussex 147th in the world overall, 39th in the world for Social Sciences and 49th globally for Business and Law studies. Sussex is particularly known for its Humanities and Social Sciences departments, with its Development studies program being placed at number 1 globally in the QS World University Ranking.

Sussex counts 5 Nobel Prize winners, 15 Fellows of the Royal Society, 9 Fellows of the British Academy, 24 fellows of the Academy of Social Sciences and a winner of the Crafoord Prize among its faculty. By 2011, many of its faculty members had also received the Royal Society of Literature Prize, the Order of the British Empire and the Bancroft Prize. Alumni include heads of states, diplomats, politicians, eminent scientists and activists.

From NIST: “NIST’s Light-Sensing Camera May Help Detect Extraterrestrial Life, Dark Matter”

From NIST

November 19, 2019
Laura Ost
laura.ost@nist.gov
(303) 497-4880

Credit: V. Verma/NIST

Researchers at the National Institute of Standards and Technology (NIST) have made one of the highest-performance cameras ever composed of sensors that count single photons, or particles of light.

With more than 1,000 sensors, or pixels, NIST’s camera may be useful in future space-based telescopes searching for chemical signs of life on other planets, and in new instruments designed to search for the elusive “dark matter” believed to constitute most of the “stuff” in the universe.

Described in Optics Express, the camera consists of sensors made from superconducting nanowires, which can detect single photons. They are among the best photon counters in terms of speed, efficiency, and range of color sensitivity. A NIST team used these detectors to demonstrate Einstein’s “spooky action at a distance,” for example.

The nanowire detectors also have the lowest dark count rates of any type of photon sensor, meaning they don’t count false signals caused by noise rather than photons. This feature is especially useful for dark-matter searches and space-based astronomy. But cameras with more pixels and larger physical dimensions than previously available are required for these applications, and they also need to detect light at the far end of the infrared band, with longer wavelengths than currently practical.

NIST’s camera is small in physical size, a square measuring 1.6 millimeters on a side, but packed with 1,024 sensors (32 columns by 32 rows) to make high-resolution images. The main challenge was to find a way to collate and obtain results from so many detectors without overheating. The researchers extended a “readout” architecture they previously demonstrated with a smaller camera of 64 sensors that adds up data from the rows and columns, a step toward meeting the requirements of the National Aeronautics and Space Administration (NASA).

“My primary motivation for making the camera is NASA’s Origins Space Telescope project, which is looking into using these arrays for analyzing the chemical composition of planets orbiting stars outside of our solar system,” NIST electronics engineer Varun Verma said. Each chemical element in the planet’s atmosphere would absorb a unique set of colors, he pointed out.

“The idea is to look at the absorption spectra of light passing through the edge of an exoplanet’s atmosphere as it transits in front of its parent star,” Verma explained. “The absorption signatures tell you about the elements in the atmosphere, particularly those that might give rise to life, such as water, oxygen and carbon dioxide. The signatures for these elements are in the mid- to far-infrared spectrum, and large-area single-photon counting detector arrays don’t yet exist for that region of the spectrum, so we received a small amount of funding from NASA to see if we could help solve that problem.”

Verma and colleagues achieved high fabrication success, with 99.5% of the sensors working properly. But detector efficiency at the desired wavelength is low. Boosting efficiency is the next challenge. The researchers also hope to make even bigger cameras, perhaps with a million sensors.

Other applications are also possible. For example, the NIST cameras may help find dark matter. Researchers around the world have been unable to find so-called weakly interacting massive particles (WIMPs) and are considering looking for dark matter with lower energy and mass. Superconducting nanowire detectors offer promise for counting emissions of rare, low-energy dark matter and discriminating real signals from background noise.

The new camera was made in a complicated process at NIST’s Microfabrication Facility in Boulder, Colorado. The detectors are fabricated on silicon wafers diced into chips. The nanowires, made of an alloy of tungsten and silicon, are about 3.5 millimeters long, 180 nanometers (nm) wide and 3 nm thick. The wiring is made of superconducting niobium.

The camera performance was measured by the Jet Propulsion Laboratory (JPL) at the California Institute of Technology in Pasadena, California. JPL has the necessary electronics due to its work on deep space optical communications.

The work was supported by NASA and the Defense Advanced Research Projects Agency.

See the full article here.

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

NIST Campus, Gaitherberg, MD, USA

NIST Mission, Vision, Core Competencies, and Core Values

NIST’s mission

To promote U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology in ways that enhance economic security and improve our quality of life.
NIST’s vision

NIST will be the world’s leader in creating critical measurement solutions and promoting equitable standards. Our efforts stimulate innovation, foster industrial competitiveness, and improve the quality of life.
NIST’s core competencies

Measurement science
Rigorous traceability
Development and use of standards

NIST’s core values

NIST is an organization with strong values, reflected both in our history and our current work. NIST leadership and staff will uphold these values to ensure a high performing environment that is safe and respectful of all.

Perseverance: We take the long view, planning the future with scientific knowledge and imagination to ensure continued impact and relevance for our stakeholders.
Integrity: We are ethical, honest, independent, and provide an objective perspective.
Inclusivity: We work collaboratively to harness the diversity of people and ideas, both inside and outside of NIST, to attain the best solutions to multidisciplinary challenges.
Excellence: We apply rigor and critical thinking to achieve world-class results and continuous improvement in everything we do.

From École Polytechnique Fédérale de Lausanne: “Making and controlling crystals of light”

From École Polytechnique Fédérale de Lausanne

09.09.19
Maxim Karpov
Nik Papageorgiou

EPFL scientists have shown how light inside optical on-chip microresonators can be crystallized in a form of periodic pulse trains that can boost the performance of optical communication links or endow ultrafast LiDAR with sub-micron precision.

Optical microresonators convert laser light into ultrashort pulses travelling around the resonator’s circumference. These pulses, called “dissipative Kerr solitons”, can propagate in the microresonator maintaining their shape.

When solitons exit the microresonator, the output light takes the form of a pulse train – a series of repeating pulses with fixed intervals. In this case, the repetition rate of the pulses is determined by the microresonator size. Smaller sizes enable pulse trains with high repetition rates, reaching hundreds of gigahertz in frequency. These can be used to boost the performance of optical communication links or become a core technology for ultrafast LiDAR with sub-micron precision.

Exciting though it is, this technology suffers from what scientists call “light-bending losses” – loss of light caused by structural bends in its path. A well-known problem in fiber optics, light-bending loss also means that the size of microresonators cannot drop below a few tens of microns. This therefore limits the maximum repetition rates we can achieve for pulses.

Publishing in Nature Physics, researchers from the lab of Tobias J. Kippenberg at EPFL have now found a way to bypass this limitation and uncouple the pulse repetition rate from the microresonator size by generating multiple solitons in a single microresonator.

The scientists discovered a way of seeding the microresonator with the maximum possible number of dissipative Kerr solitons with precisely equal spacing between them. This new formation of light can be thought of as an optical analogue to atomic chains in crystalline solids, and so the researchers called them “perfect soliton crystals” (PSCs).

Due to interferometric enhancement and the high number of optical pulses, PSCs coherently multiply the performance of the resulting pulse train – not just its repetition rate, but also its power.

The researchers also investigated the dynamics of PSC formations. Despite their highly organized structure, they seem to be closely linked to optical chaos, a phenomenon caused by light instabilities in optical microresonators, which is also common for semiconductor-based and fiber laser systems.

“Our findings allow the generation of optical pulse trains with ultra-high repetition rates with several terahertz, using regular microresonators,” says researcher Maxim Karpov. “These can be used for multiple applications in spectroscopy, distance measurements, and as a source of low-noise terahertz radiation with a chip-size footprint.” Meanwhile, the new understanding of soliton dynamics in optical microresonators and the behavior of PSCs opens up new avenues into the fundamental physics of soliton ensembles in nonlinear systems.

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

EPFL campus

EPFL is Europe’s most cosmopolitan technical university. It receives students, professors and staff from over 120 nationalities. With both a Swiss and international calling, it is therefore guided by a constant wish to open up; its missions of teaching, research and partnership impact various circles: universities and engineering schools, developing and emerging countries, secondary schools and gymnasiums, industry and economy, political circles and the general public.

From Stanford University: “Stanford researchers design a light-trapping, color-converting crystal”

From Stanford University

August 7, 2019

Taylor Kubota
Stanford News Service
(650) 724-7707
tkubota@stanford.edu

Researchers propose a microscopic structure that changes laser light from infrared to green and traps both wavelengths of light to improve efficiency of that transformation. This type of structure could help advance telecommunication and computing technologies. (Image credit: Getty Images)

Five years ago, Stanford postdoctoral scholar Momchil Minkov encountered a puzzle that he was impatient to solve. At the heart of his field of nonlinear optics are devices that change light from one color to another – a process important for many technologies within telecommunications, computing and laser-based equipment and science. But Minkov wanted a device that also traps both colors of light, a complex feat that could vastly improve the efficiency of this light-changing process – and he wanted it to be microscopic.

“I was first exposed to this problem by Dario Gerace from the University of Pavia in Italy, while I was doing my PhD in Switzerland. I tried to work on it then but it’s very hard,” Minkov said. “It has been in the back of my mind ever since. Occasionally, I would mention it to someone in my field and they would say it was near-impossible.”

In order to prove the near-impossible was still possible, Minkov and Shanhui Fan, professor of electrical engineering at Stanford, developed guidelines for creating a crystal structure with an unconventional two-part form. The details of their solution were published Aug. 6 in Optica, with Gerace as co-author. Now, the team is beginning to build its theorized structure for experimental testing.

An illustration of the researchers’ design. The holes in this microscopic slab structure are arranged and resized in order to control and hold two wavelengths of light. The scale bar on this image is 2 micrometers, or two millionths of a meter. (Image credit: Momchil Minkov)

A recipe for confining light

Anyone who’s encountered a green laser pointer has seen nonlinear optics in action. Inside that laser pointer, a crystal structure converts laser light from infrared to green. (Green laser light is easier for people to see but components to make green-only lasers are less common.) This research aims to enact a similar wavelength-halving conversion but in a much smaller space, which could lead to a large improvement in energy efficiency due to complex interactions between the light beams.

The team’s goal was to force the coexistence of the two laser beams using a photonic crystal cavity, which can focus light in a microscopic volume. However, existing photonic crystal cavities usually only confine one wavelength of light and their structures are highly customized to accommodate that one wavelength.

So instead of making one uniform structure to do it all, these researchers devised a structure that combines two different ways to confine light, one to hold onto the infrared light and another to hold the green, all still contained within one tiny crystal.

“Having different methods for containing each light turned out to be easier than using one mechanism for both frequencies and, in some sense, it’s completely different from what people thought they needed to do in order to accomplish this feat,” Fan said.

After ironing out the details of their two-part structure, the researchers produced a list of four conditions, which should guide colleagues in building a photonic crystal cavity capable of holding two very different wavelengths of light. Their result reads more like a recipe than a schematic because light-manipulating structures are useful for so many tasks and technologies that designs for them have to be flexible.

“We have a general recipe that says, ‘Tell me what your material is and I’ll tell you the rules you need to follow to get a photonic crystal cavity that’s pretty small and confines light at both frequencies,’” Minkov said.

Computers and curiosity

If telecommunications channels were a highway, flipping between different wavelengths of light would equal a quick lane change to avoid a slowdown – and one structure that holds multiple channels means a faster flip. Nonlinear optics is also important for quantum computers because calculations in these computers rely on the creation of entangled particles, which can be formed through the opposite process that occurs in the Fan lab crystal – creating twinned red particles of light from one green particle of light.

Envisioning possible applications of their work helps these researchers choose what they’ll study. But they are also motivated by their desire for a good challenge and the intricate strangeness of their science.

“Basically, we work with a slab structure with holes and by arranging these holes, we can control and hold light,” Fan said. “We move and resize these little holes by billionths of a meter and that marks the difference between success and failure. It’s very strange and endlessly fascinating.”

These researchers will soon be facing off with these intricacies in the lab, as they are beginning to build their photonic crystal cavity for experimental testing.

See the full article here .

five-ways-keep-your-child-safe-school-shootings
Please help promote STEM in your local schools.

Stem Education Coalition

Stanford University campus. No image credit

Stanford University

Leland and Jane Stanford founded the University to “promote the public welfare by exercising an influence on behalf of humanity and civilization.” Stanford opened its doors in 1891, and more than a century later, it remains dedicated to finding solutions to the great challenges of the day and to preparing our students for leadership in today’s complex world. Stanford, is an American private research university located in Stanford, California on an 8,180-acre (3,310 ha) campus near Palo Alto. Since 1952, more than 54 Stanford faculty, staff, and alumni have won the Nobel Prize, including 19 current faculty members

From MIT News: “Teaching artificial intelligence to create visuals with more common sense”

From MIT News

July 1, 2019
Adam Conner-Simons | MIT CSAIL

The GANpaint system developed at MIT can easily add features to an existing image. At left, the original photo of a kitchen; at right, the same kitchen with the addition of a window. Co-author Jun-Yan Zhu believes better understanding of GANs will help researchers be able to better stamp out fakery: “This understanding may potentially help us detect fake images more easily.”

GANpaint Studio general interface

An MIT/IBM system could help artists and designers make quick tweaks to visuals while also helping researchers identify “fake” images.

David Bau, a PhD student at MIT’s Computer Science and Artificial Intelligence Lab (CSAIL), describes the project as one of the first times computer scientists have been able to actually “paint with the neurons” of a neural network — specifically, a popular type of network called a generative adversarial network (GAN).

Available online as an interactive demo, GANpaint Studio allows a user to upload an image of their choosing and modify multiple aspects of its appearance, from changing the size of objects to adding completely new items like trees and buildings.

Boon for designers

Spearheaded by MIT professor Antonio Torralba as part of the MIT-IBM Watson AI Lab he directs, the project has vast potential applications. Designers and artists could use it to make quicker tweaks to their visuals. Adapting the system to video clips would enable computer-graphics editors to quickly compose specific arrangements of objects needed for a particular shot. (Imagine, for example, if a director filmed a full scene with actors but forgot to include an object in the background that’s important to the plot.)

GANpaint Studio could also be used to improve and debug other GANs that are being developed, by analyzing them for “artifact” units that need to be removed. In a world where opaque AI tools have made image manipulation easier than ever, it could help researchers better understand neural networks and their underlying structures.

“Right now, machine learning systems are these black boxes that we don’t always know how to improve, kind of like those old TV sets that you have to fix by hitting them on the side,” says Bau, lead author on a related paper about the system with a team overseen by Torralba. “This research suggests that, while it might be scary to open up the TV and take a look at all the wires, there’s going to be a lot of meaningful information in there.”

One unexpected discovery is that the system actually seems to have learned some simple rules about the relationships between objects. It somehow knows not to put something somewhere it doesn’t belong, like a window in the sky, and it also creates different visuals in different contexts. For example, if there are two different buildings in an image and the system is asked to add doors to both, it doesn’t simply add identical doors — they may ultimately look quite different from each other.

“All drawing apps will follow user instructions, but ours might decide not to draw anything if the user commands to put an object in an impossible location,” says Torralba. “It’s a drawing tool with a strong personality, and it opens a window that allows us to understand how GANs learn to represent the visual world.”

GANs are sets of neural networks developed to compete against each other. In this case, one network is a generator focused on creating realistic images, and the second is a discriminator whose goal is to not be fooled by the generator. Every time the discriminator ‘catches’ the generator, it has to expose the internal reasoning for the decision, which allows the generator to continuously get better.

“It’s truly mind-blowing to see how this work enables us to directly see that GANs actually learn something that’s beginning to look a bit like common sense,” says Jaakko Lehtinen, an associate professor at Finland’s Aalto University who was not involved in the project. “I see this ability as a crucial steppingstone to having autonomous systems that can actually function in the human world, which is infinite, complex and ever-changing.”

Stamping out unwanted “fake” images

The team’s goal has been to give people more control over GAN networks. But they recognize that with increased power comes the potential for abuse, like using such technologies to doctor photos. Co-author Jun-Yan Zhu says that he believes that better understanding GANs — and the kinds of mistakes they make — will help researchers be able to better stamp out fakery.

“You need to know your opponent before you can defend against it,” says Zhu, a postdoc at CSAIL. “This understanding may potentially help us detect fake images more easily.”

To develop the system, the team first identified units inside the GAN that correlate with particular types of objects, like trees. It then tested these units individually to see if getting rid of them would cause certain objects to disappear or appear. Importantly, they also identified the units that cause visual errors (artifacts) and worked to remove them to increase the overall quality of the image.

“Whenever GANs generate terribly unrealistic images, the cause of these mistakes has previously been a mystery,” says co-author Hendrik Strobelt, a research scientist at IBM. “We found that these mistakes are triggered by specific sets of neurons that we can silence to improve the quality of the image.”

Bau, Strobelt, Torralba and Zhu co-wrote the paper with former CSAIL PhD student Bolei Zhou, postdoctoral associate Jonas Wulff, and undergraduate student William Peebles. They will present it next month at the SIGGRAPH conference in Los Angeles. “This system opens a door into a better understanding of GAN models, and that’s going to help us do whatever kind of research we need to do with GANs,” says Lehtinen.

See the full article here .

five-ways-keep-your-child-safe-school-shootings
Please help promote STEM in your local schools.

Stem Education Coalition

The mission of MIT is to advance knowledge and educate students in science, technology, and other areas of scholarship that will best serve the nation and the world in the twenty-first century. We seek to develop in each member of the MIT community the ability and passion to work wisely, creatively, and effectively for the betterment of humankind.

From Michigan Technical University: “Better Microring Sensors for Optical Applications”

From Michigan Technical University

May 10, 2019
Kelley Christensen

An exceptional surface-based sensor. The microring resonator is coupled to a waveguide with an end mirror that partially reflects light, which in turn enhances the sensitivity. Image Credit: Ramy El-Ganainy and Qi Zhong

Tweaking the design of microring sensors enhances their sensitivity without adding more implementation complexity.

Optical sensing is one of the most important applications of light science. It plays crucial roles in astronomy, environmental science, industry and medical diagnoses.

Despite the variety of schemes used for optical sensing, they all share the same principle: The quantity to be measured must leave a “fingerprint” on the optical response of the system. The fingerprint can be its transmission, reflection or absorption. The stronger these effects are, the stronger the response of the system.

While this works well at the macroscopic level, measuring tiny, microscopic quantities that induce weak response is a challenging task. Researchers have developed techniques to overcome this difficulty and improve the sensitivity of their devices. Some of these techniques, which rely on complex quantum optics concepts and implementations, have indeed proved useful, such as in sensing gravitational waves in the LIGO project.

Others, which are based on trapping light in tiny boxes called optical resonators, have succeeded in detecting micro-particles and relatively large biological components.

Nonetheless, the ability to detect small nano-particles and eventually single molecules remains a challenge. Current attempts focus on a special type of light trapping devices called microring or microtoroid resonators — these enhance the interaction between light and the molecule to be detected. The sensitivity of these devices, however, is limited by their fundamental physics.

In their article “Sensing with Exceptional Surfaces in Order to Combine Sensitivity with Robustness” published in Physical Review Letters, physicists and engineers from Michigan Technological University, Pennsylvania State University and the University of Central Florida propose a new type of sensor. They are based on the new notion of exceptional surfaces: surfaces that consist of exceptional points.

Exceptional Points for Exceptionally Sensitive Detection

In order to understand the meaning of exceptional points, consider an imaginary violin with only two strings. In general, such a violin can produce just two different tones — a situation that corresponds to a conventional optical resonator. If the vibration of one string can alter the vibration of the other string in a way that the sound and the elastic oscillations create only one tone and one collective string motion, the system has an exceptional point.

A physical system that exhibits an exceptional point is very fragile. In other words, any small perturbation will dramatically alter its behavior. The feature makes the system highly sensitive to tiny signals.

“Despite this promise, the same enhanced sensitivity of exceptional point-based sensors is also their Achilles heel: These devices are very sensitive to unavoidable fabrication errors and undesired environmental variations,” said Ramy El-Ganainy, associate professor of physics, adding that the sensitivity necessitated clever tuning tricks in previous experimental demonstrations.

“Our current proposal alleviates most of these problems by introducing a new system that has the same enhanced sensitivity reported in previous work, while at the same time robust against the majority of the uncontrivable experimental uncertainty,” said Qi Zhong, lead author on the paper and a graduate student who is currently working towards his doctorate degree at Michigan Tech.

Though the design of microring sensors continues to be refined, researchers are hopeful that by improving the devices, seemingly tiny optical observations will have large effects.

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

Michigan Technological University (http://www.mtu.edu) is a leading public research university developing new technologies and preparing students to create the future for a prosperous and sustainable world. Michigan Tech offers more than 130 undergraduate and graduate degree programs in engineering; forest resources; computing; technology; business; economics; natural, physical and environmental sciences; arts; humanities; and social sciences.
The College of Sciences and Arts (CSA) fills one of the most important roles on the Michigan Tech campus. We play a part in the education of every student who comes through our doors. We take pride in offering essential foundational courses in the natural sciences and mathematics, as well as the social sciences and humanities—courses that underpin every major on campus. With twelve departments, 28 majors, 30-or-so specializations, and more than 50 minors, CSA has carefully developed programs to suit many interests and skill sets. From sound design and audio technology to actuarial science, applied cognitive science and human factors to rhetoric and technical communication, the college offers many unique programs.

From University of Cambridge: “Physicists get thousands of semiconductor nuclei to do ‘quantum dances’ in unison”

From University of Cambridge

22 Feb 2019
Communications office

Theoretical ESR spectrum buildup as a function of two-photon detuning δ and drive time τ, for a Rabi frequency of Ω = 3.3 MHz on the central transition. Credit: University of Cambridge.

A team of Cambridge researchers have found a way to control the sea of nuclei in semiconductor quantum dots so they can operate as a quantum memory device.

Quantum dots are crystals made up of thousands of atoms, and each of these atoms interacts magnetically with the trapped electron. If left alone to its own devices, this interaction of the electron with the nuclear spins, limits the usefulness of the electron as a quantum bit – a qubit.

Led by Professor Mete Atatüre from Cambridge’s Cavendish Laboratory, the researchers are exploiting the laws of quantum physics and optics to investigate computing, sensing or communication applications.

“Quantum dots offer an ideal interface, as mediated by light, to a system where the dynamics of individual interacting spins could be controlled and exploited,” said Atatüre, who is a Fellow of St John’s College. “Because the nuclei randomly ‘steal’ information from the electron they have traditionally been an annoyance, but we have shown we can harness them as a resource.”

The Cambridge team found a way to exploit the interaction between the electron and the thousands of nuclei using lasers to ‘cool’ the nuclei to less than 1 milliKelvin, or a thousandth of a degree above the absolute zero temperature. They then showed they can control and manipulate the thousands of nuclei as if they form a single body in unison, like a second qubit. This proves the nuclei in the quantum dot can exchange information with the electron qubit and can be used to store quantum information as a memory device. The results are reported in the journal Science.

Quantum computing aims to harness fundamental concepts of quantum physics, such as entanglement and superposition principle, to outperform current approaches to computing and could revolutionise technology, business and research. Just like classical computers, quantum computers need a processor, memory, and a bus to transport the information backwards and forwards. The processor is a qubit which can be an electron trapped in a quantum dot, the bus is a single photon that these quantum dots generate and are ideal for exchanging information. But the missing link for quantum dots is quantum memory.

Atatüre said: “Instead of talking to individual nuclear spins, we worked on accessing collective spin waves by lasers. This is like a stadium where you don’t need to worry about who raises their hands in the Mexican wave going round, as long as there is one collective wave because they all dance in unison.

“We then went on to show that these spin waves have quantum coherence. This was the missing piece of the jigsaw and we now have everything needed to build a dedicated quantum memory for every qubit.”

In quantum technologies, the photon, the qubit and the memory need to interact with each other in a controlled way. This is mostly realised by interfacing different physical systems to form a single hybrid unit which can be inefficient. The researchers have been able to show that in quantum dots, the memory element is automatically there with every single qubit.

Dr Dorian Gangloff, one of the first authors of the paper [Science] and a Fellow at St John’s, said the discovery will renew interest in these types of semiconductor quantum dots. Dr Gangloff explained: “This is a Holy Grail breakthrough for quantum dot research – both for quantum memory and fundamental research; we now have the tools to study dynamics of complex systems in the spirit of quantum simulation.”

The long term opportunities of this work could be seen in the field of quantum computing. Last month, IBM launched the world’s first commercial quantum computer, and the Chief Executive of Microsoft has said quantum computing has the potential to ‘radically reshape the world’.

Gangloff said: “The impact of the qubit could be half a century away but the power of disruptive technology is that it is hard to conceive of the problems we might open up – you can try to think of it as known unknowns but at some point you get into new territory. We don’t yet know the kind of problems it will help to solve which is very exciting.”

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

The University of Cambridge (abbreviated as Cantab in post-nominal letters) is a collegiate public research university in Cambridge, England. Founded in 1209, Cambridge is the second-oldest university in the English-speaking world and the world’s fourth-oldest surviving university. It grew out of an association of scholars who left the University of Oxford after a dispute with townsfolk. The two ancient universities share many common features and are often jointly referred to as “Oxbridge”.

Cambridge is formed from a variety of institutions which include 31 constituent colleges and over 100 academic departments organised into six schools. The university occupies buildings throughout the town, many of which are of historical importance. The colleges are self-governing institutions founded as integral parts of the university. In the year ended 31 July 2014, the university had a total income of £1.51 billion, of which £371 million was from research grants and contracts. The central university and colleges have a combined endowment of around £4.9 billion, the largest of any university outside the United States. Cambridge is a member of many associations and forms part of the “golden triangle” of leading English universities and Cambridge University Health Partners, an academic health science centre. The university is closely linked with the development of the high-tech business cluster known as “Silicon Fen”.

From Caltech: “‘Folded’ Optical Devices Manipulate Light in a New Way”

From Caltech

10/30/2018

Robert Perkins
(626) 395-1862
rperkins@caltech.edu

An array of 11 metasurface-based optical spectrometers, pictured here before the final fabrication step. Each spectrometer is composed of three metasurfaces that disperse and focus light with different wavelengths to different points. Credit: Faraon Lab/Caltech

The future of optics

The next generation of electronic devices, ranging from personal health monitors and augmented reality headsets to sensitive scientific instruments that would only be found in a laboratory, will likely incorporate components that use metasurface optics, according to Andrei Faraon, professor of applied physics in Caltech’s Division of Engineering and Applied Science. Metasurface optics manipulate light similarly to how a lens might—bending, focusing, or reflecting it—but do so in a finely controllable way using carefully designed microscopic structures on an otherwise flat surface. That makes them both compact and finely tunable, attractive qualities for electronic devices. However, engineers will need to overcome several challenges to make them widespread.

The problem

Most optical systems require more than a single metasurface to function properly. In metasurface-based optical systems, most of the total volume inside the device is just free space through which light propagates between different elements. The need for this free space makes the overall device difficult to scale down, while integrating and aligning multiple metasurfaces into a single device can be complicated and expensive.

The invention

To overcome this limitation, the Faraon group has introduced a technology called “folded metasurface optics,” which is a way of printing multiple types of metasurfaces onto either side of a substrate, like glass. In this way, the substrate itself becomes the propagation space for the light. As a proof of concept, the team used the technique to build a spectrometer, which is a scientific instrument for splitting light into different colors, or wavelengths, and measuring their corresponding intensities. (Spectrometers are used in a variety of fields; for example, in astronomy they are used to determine the chemical makeup of stars based on the light they emit.) The spectrometer built by Faraon’s team is 1 millimeter thick and is composed of three reflective metasurfaces placed next to each other that split and reflect light, and ultimately focus it onto a detector array. It was fabricated at the Kavli Nanoscience Institute, and its design is described in a paper published by Nature Communications on October 10.

What it could be used for

A compact spectrometer like the one developed by Faraon’s group has a variety of uses, including as a noninvasive blood-glucose measuring system that could be invaluable for diabetes patients. The platform uses multiple metasurface elements that are fabricated in a single step, so, in general, it provides a potential path toward complex but inexpensive optical systems.

The details

The paper is titled “Compact folded metasurface spectrometer.” Co-authors include Caltech graduate students MohammadSadegh Faraji-Dana (MS ’18), Ehsan Arbabi (MS ’17), Seyedeh Mahsa Kamali (MS ’17), and Hyounghan Kwon (MS ’18), and Amir Arbabi of the University of Massachusetts Amherst. This research was supported by Samsung Electronics, the National Sciences and Engineering Research Council of Canada, and the U.S. Department of Energy.

See the full article here .

five-ways-keep-your-child-safe-school-shootings
Please help promote STEM in your local schools.

Stem Education Coalition

The California Institute of Technology (commonly referred to as Caltech) is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering. Its 124-acre (50 ha) primary campus is located approximately 11 mi (18 km) northeast of downtown Los Angeles. “The mission of the California Institute of Technology is to expand human knowledge and benefit society through research integrated with education. We investigate the most challenging, fundamental problems in science and technology in a singularly collegial, interdisciplinary atmosphere, while educating outstanding students to become creative members of society.”

Caltech campus